FOR THE BUDDING ENGINEERS AND UPSC CANDIDATES ---------------------------------------------------------------------------- Chapter – 1: PROPERTIES OF FLUID Q.1 What is the difference between Fluid Mechanics and Hydraulics? Ans: Fluid Mechanics deals with all kinds of Fluid. It deals with compressible, incompressible, highly viscous and low viscous fluid, but Hydraulics deals with only water and water is an incompressible fluid. Q.2 What is the difference between Fluid and Solid from the view point of Engineering? Ans: Solid can sustain tension; compression and shear but fluid can sustain only compression (equivalent to pressure). Fluid cannot sustain tension and shear. Q.3 What is flow? Ans: When shear force is applied to a fluid then the fluid becomes incapable to sustain shear, so the free surface undergoes continuous deformation, and that is called flow. Q.4 If we gently place a needle over the free surface of water it will not sink. Why? Ans: Due to surface tension force the needle will not sink into the water. Q.5 Give the example of a line force, which is also a property of fluid. Ans: Surface tension force is a line force, its unit is Ns / m2 or Kg m /s and the dimension is MLT-1. Q.6. Comment on the initial shape of a body. The initial shape pf a particle may be chosen arbitrarily. It is considered as a triangular prism or as a parallelopiped for the caertesian co-ordinates, segment of a cylinder for a cylindrical system and asegment of a sphere for a spherical system. Q.7 What type of function the mass density should be? For a fluid continuum mass density must be a continuous function. Q.8. How do you co-relate Hooke’s Law of Elasticity for Solids and Newton’s Law of Viscosity for Fluids? In Hooke’s Law Stress and Strain are directly proportional and in the second case Shear Stress and rate of strain or velocity gradient is directly proportional. In the first case constant of proportionality is the modulus of Elasticity andd in the second case constant of proportionality is the coefficient of Dynamic Viscosity. Q.9.Deine the concept of Viscosity. It is a measure of the internal fluid friction which causes resistance to flow. The origibn of viscosity is due to cohesion and molecular momentum transfer. These two factors add up to provide the resistance to shearing reckoned by the property of fluid. Q.10. Why in gases viscosity is directly proportional to the temperature? In gases cohesive forces are small and the molecular momentum transfer predominates which increases with a rise in the temperature causing a rise in the viscosity. Q.11.Do you think the viscosity given for water at any temperature is absolutely correct? No, the pure distilled water referred may contain about one part of heavy water in every6500 part. The viscosity of heavy water is 1.25 times than the water taken for usual consideration. Q.12.What is the surface tension at the critical temperature of a liquid? At the critical temperature of a liquid the surface tension becomes zero and the boundary between the liqwuid and its vapour vanishes. Q.13.How the surface tension of water can be increased or decreased? The surface tension of water is considerably reduced by adding a small quantity of an organic solute such as soap or detergent. The surface tension of water increases if a salt e.g. Sodium Chloride is dissolved in it. Q.14. How you can co-relate the angle and capillary action? Acute angle of contact corresponds with capillary rise and obtuse angle of contact with capillary depression. Q.15.What is the difference between vaporization and boiling? When vapour pressure is less than the pressure above a liquid surface at a particular temperature then vapourization takes place and when vapour pressure is equal to the pressure above a liquid then boiling takes place. Q.16. Why do we use Mercury, and not Gasoline in the barometer? The saturation vapour pressure of mercury at 20° C is about 1.3 x 10-6 m of mercury which speaks of its stability at vacuum in barometer. On the contrary the saturation vapour pressure for gasoline at 20° C is about 4.3 m of Gasoline which means that much greater pressure must be maintained to keep gasoline in liquid state. Q.17. What do you mean by lapse rate? The rate of change of temperature with altitude is called lapse rate. The lapse rate for international standard atmosphere is taken as 6.5 K/km which means that, from the sea level upto the tropopause the temperature drops linearly at the rate of 6.5 degree Kelvin per kilometer altitude. Q.18.What is ideal fluid. A fluid is said to be ideal if it is assumed as both inviscid and incompressible. Q.19. Chapter-2: FLUID STATICS Q.1 As we go from the free surface to the downward direction within an incompressible liquid gradually pressure increases. Is it increasing linearly or non-linearly? Ans: The pressure intensity increases linearly, because p = * h. Q.2 A tank is placed over a concrete column having height 19m. The height of free surface of water in the tank is 1m. So the height of free surface of water from the ground level is 20m. Now if U tube is connected from the top of the tank then what will be the rise of liquid in the other limb of U-tube? Assume the liquid as an ideal fluid and assume the U-tube is just touching the ground tangentially. Ans: Since the liquid is assumed as an ideal fluid for that no loss will be there. So in the other column of U-tube the water will raise up to the same height i.e. 20m. Because when the water is just coming near to the G.L. then its pressure is 20m of water column. For that it will rise up to 20m. Q.3 What is the very basic difference between absolute pressure and gauge pressure? Ans: The very basic difference is that absolute pressure cannot be negative. It can be zero or positive. Though practically absolute pressure cannot be zero. But gauge pressure can be positive, negative or zero. Because when absolute pressure is more than atmospheric pressure then gauge pressure is positive, when absolute pressure is equal to atmospheric pressure then gauge pressure is zero, and when absolute pressure is less than atmospheric pressure than partial vacuum is created and the gauge pressure becomes negative. Q.4 If you take a piezometer having 1mm diameter can you measure accurately the positive gauge pressure of liquids of low magnitude? Ans: We cannot measure the positive gauge pressure accurately, because due to very small diameter capillary action will take place and result will be inaccurate. Q.5 If you take a piezometer having 30cm diameter can you measure the positive gauge pressure accurately? Ans: We cannot measure the positive gauge pressure accurately, with a piezometer having 30cm diameter. Because due to very big diameter a major portion of the flow will be transferred to the piezometer, so result will be inaccurate. Q.6. Whart are the conditions where Pascal’s Law is Valid. i) For a Fluid at rest, whether a fluid is viscous or inviscid, compressible or incompressible ii) For a liquid in solid-body movement, either at constant linear acceleration or at constant rotational velocity in a container, here again there is no relative motion between different fluid layers iii) For the flow of an inviscid fluid and for ideal fluid flow where dynamic viscosity is zero corresponds to the absence of shear strees. Q.7. Why it is more convenient to deal with piezometric head than with pressure and datum heads individually. i) It is usually the sum of the pressure head and datum head that is of interest to us in calculations. ii) It is the sum of two that is easily measurable by employing simple piezometers i.e. vertical tubes at different points in a fluid flow. iii) The sum of the two remains constant at all points in a fluid at rest or in motion at constant velocity when referred to a datum level. Chapter – 3: BUOYANCY AND FLOATATION Q.1 What is the typical speciality of a buoyant force? Ans: Buoyant force will always act in a vertical direction, that is the speciality and it will act always vertically upward. Q.2 Can you say that buoyant force acting on a body is an algebraic function of the height of the body submerged within the liquid? Ans: Yes. Q.3 One kilo cotton is there and one-kilo iron is there, which will weigh more if the weight is taken in the air? Ans: One-kilo iron will weigh more, because, Net weight = Total weight - Buoyant Force. Since the cotton occupies a very high volume for that the buoyant force acting on the volume of iron will be much less as compared to the buoyant force acting on the cotton. Total weight for both iron and cotton will be 1 Kg * 9.81 m / s2 = 9.81 N. Since the buoyant force acting on cotton is more than buoyant force acting on iron, for that net weight of cotton will be less than iron. So apparently when we will weigh 1-kilo iron and 1 kilo cotton in air then 1 kilo iron will weigh more due to the higher buoyant force acting on 1 kilo cotton. Q.4 If a plastic mug is kept inverted at the bottom of a plastic bucket full of water it will immediately come to the water surface but if a plastic mug is kept inverted at the bed of a river what will happen? Ans: It will not come to the water surface when it will be kept inverted at the bed of a river, because due to very high downward pressure of the liquid it will not come to the surface of the water. Q.5 When any bubble is created within water then it comes from bottom to top. Why? Ans: When the bubble is created then the self-weight of the bubble becomes negligible as compared to the buoyant force exerting over it. Since the vertically upward buoyant force is much higher than the vertically downward weight force for that the bubble will go upward. Chapter – 4:LIQUIDS IN RELATIVE EQUILLIBRIUM Q.1 There is a cylinder, which is partially filled with water. When the cylinder partially filled with water rotates about a vertical axis, along which line the pressure will vary? Ans: The pressure will vary along the radius. Q.2 When a cylinder is partially filled with water and rotated about a vertical axis then the pressure will vary along the radius. What will be the pressure at the center? Ans: At the center the pressure will be zero. Q.3 An oil tanker filled up to the brim is moving with zero acceleration. Will there be any spilling of liquid? Ans: There will be no spilling of liquid, because here the acceleration ‘’ is equal to zero, so tan = / g = 0 / g = 0. Since amount of spilling is a direct algebraic function of ‘tan’ and tan is zero for that amount of spilling is zero. Q.4 A container filled by water is coming vertically downward with an acceleration, which is equal to ‘g’ of that place. What is the difference of pressure between atmosphere and pressure of water at the bottom of the container? Ans: Here the pressure difference is zero, because we know; P - Pa = h (1 - / g) Here ‘P’ is the pressure within liquid (absolute pressure) and (P - Pa) is the gauge pressure at any point within the liquid. Now here = -g (since it is coming downward for that negative sign is used). So here gauge pressure at any point within the liquid will be; (P - Pa) = h (1 - g / g) = 0 Since the gauge pressure at any point within the liquid is zero, for that at any point within the liquid the pressure which will act, that is nothing but atmospheric pressure. ‘h’ is the depth of water column in the container. So the difference of pressure between the atmospheric pressure and the pressure at the bottom of the container will be zero. Since the downward acceleration is equal to ‘g’ for that everywhere atmosphere pressure will act. Same is the case with a freely falling jet of water in atmosphere. Q.5 A cylinder filled by liquid is rotating with an increasing acceleration about an vertical axis Z-Z. A time will come when there will be no acceleration and the cylinder will attain a constant angular speed . Will there be any shear stress in the liquid mass when the angular speed will be constant? Ans: Wherever the angular speed will be constant the liquid mass will attain a steady state and it will be in equilibrium and it will rotate as a solid mass with the container at the same angular velocity. Therefore there will be no shear stress in the liquid mass. Chapter – 5: BASIC HYDRODYNAMICS Q.1 Is Hydrodynamics valid for real fluid? Ans: No. Hydrodynamics is valid only for ideal fluid flow. Q.2 Is velocity potential is a measurable physical quantity like temperature or electric potential? Ans: No. We cannot measure velocity potential, because velocity potential is a mathematical tool to solve the problems. It is a fictitious continuous function (x, y, z). Q.3 What is the basic difference between velocity potential and stream function? Ans: Velocity potential or - function can exist only for irrotational flow and it is a function of three co-ordinates. = f (x, y, z). But stream function can exist both for rotational and irrotational flow and it is two-dimensional. Either = f 1 (x, y) or f2 (y, z) or f3 (z, x). Q.4 Whenever there is a flow of ideal fluid the flow is irrotational, but whenever there is an irrotational flow is it necessary for the fluid to be ideal? Ans: It is not true. Because irrotational flow can exist both for ideal and real fluid. As for example when water passes through the drain whole of a washbasin there free vortex is formed and free vortex flow is nothing but an irrotational flow. But water is a real fluid. So for real fluid flow also the flow can be irrotational. Q.5 Whenever we express the component of velocity in x, y and z direction we write; u = - / x, v = - / y, and w = - / z. What is the significance of the minus sign? Ans: Velocity potential decreases in the direction of fluid flow, more precisely speaking if we consider x-axis then when we express the component of velocity u then we write; u = - / x, it indicates that as we go in the positive direction of x, gradually the value of decreases. So the gradient is negative and if we make the angle of that line with the positive direction of x then the tangent of that angle will be negative, So tan value of that angle will be also negative. That means the velocity potential decreases in the direction of flow. Q.6. What do you mean by axisymmetric flow? A flow is called axisymmetric if the velocity profile is symmetrical about the axis of symmetry. In other words, the velocity profile is the same at different diametral planes drawn through the passage. In other words the flow is invariant in the circumferencial i.e.’θ’ direction. Q.7. What is the reason which rules out the possibility of applying the Lagrangian approach. Difficulty in tracing the lump of the fluid rules out the possibility of applying the Lagrangian approach. Q.8 Give a classic example of the various combinations of steady, unsteady and uniform and non-uniform flow. The flow through the hose of a fire engine can be taken. When the pumping unit is started the flow in the hose is unsteady; unsteady but uniform at the nozzle exit and unsteady and non-uniform between the hose and the nozzle. After the flow establishes, it is said to be steady, steady and uniform at the nozzle exit and steady and non-uniform between the hose and the nozzle. Q.9Define stream line in a special way. Streamlines are therefore equivalent to an instantaneous snap-shot indicating the directions of velocity. Q.10 Can a single point belong to two streamlines. No. Q.11.Define pathline. A pathline in a fluid flow is the trajectory of a fluid particle, say P1as it advances with the passage of time, say from initial time ti to final time tf.Pathline are therefore history lines of individual fluid particles overa span of time. Q.12 Is it possible to have free vortex flow at the origin? Free vortex flow at the origin ‘O’ is impossible because it demands an infinite velocity at this point. In practice the flow near the origin must be rotational. I.e. forced vortex so as to make the peripherial velocity distribution, shown dotted, dropping to zero at the origin. Pl refer page 146 of KLK. Q.13.All the flownets should correspond to which things. i) Possible flow ii) Irrotational flow iii) Instantaneous condition iv) One,two-dimensional or axisymmetric flow. Q.14Do you think that circulation depends on the radius of the circular streamline? No, the basic equation of circulation shows that the circulation doesn’t depend upon the radius of the circular streamline. Q.15.What are the general classifications of purely circulatory motion. i) By the way of irrotational and free vortex ii) By the way of rotational and forced vortex Q.16What are the statements originated from Kelvin’s Theorem without any proof. i) Circulation along a closed contour remains constant with the passage of time ii) A flow, irrotational at some time, must continue to be irrotational thereafter. Q.17.Write down some very special characteristics of stream function and velocity potential. For a fluid flow to be physically possible in two dimensions, the stream function must exist and for a flow to be irrotational the velocity potential must exist. Q.18. Chapter – 6: FLUID DYNAMICS Q.1 From which principle Bernoulli’s equation is derived? Ans: Bernoulli’s equation is derived from principle of conservation of energy. Q.2 For an incompressible fluid Bernoulli’s equation becomes p / + z + v2 / 2g = Constant. Here ‘Z’ is called the potential head. Explain the term potential head with logic. Ans: If we consider a fluid particle having mass ‘m’ over the datum line at a height ‘z’ then w.r.t. the datum the potential energy of the fluid particle becomes ‘mgz’. Now if we consider the potential energy per unit weight then it becomes mgz / mg = z. That is the explanation of potential head. Q.3 Why the divergent cone of the Venturimeter is kept longer than the convergent cone? Ans: To avoid separation of flow the divergent cone is kept longer, though practically we cannot avoid separation of flow, only it can be delayed and reduced. Q.4 What is the concept of separation of flow? Ans: Whenever there will be a diverging c/s area of flow, then separation of fluid will take place. Let us take the example of fluid flow within a Venturimeter. Now when the fluid will flow through the convergent cone then gradually the area will be reducing and velocity will be increasing (from continuity equation) and pressure will be reducing (from Bernoulli’s equation) and pressure gradient will be negative and cross sectional area of flow will be gradually reduced, but when the fluid comes from throat to the divergent cone then due to expansion the fluid i.e. the stream tube cannot get attached to the whole inner surface area of the divergent cone, for that the stream tube separates from the inner wall of the divergent cone and in diverging cone gradually area increases, velocity decreases and pressure increases and the pressure gradient will be positive. Positive pressure gradient is all the time considered as an adverse pressure gradient. Q.5 There is a Venturimeter, the length of the convergent cone is 20cm and length of the divergent cone is 20km. Water is flowing through it. Will the water mass be able to flow the whole 20km length of the divergent cone? Ans: No, because in the divergent cone gradually c/s area will increase, velocity will decrease and pressure will be increased and the pressure gradient will be positive. The water will flow with some total energy and due to various types of losses the total energy will be gradually reduced and due to losses before reaching the end point of the diverging cone the total energy will be zero and water will be stopped. Q.6.How can you co-relate Euler’s Equation and Navier-Stokes Equation. Euler’s equation is a special case of Navier Stokes equation. Q.7. What are the speciality of pressure distribution for a rectangular sluice gate? It is noticed that the pressure distribution is nearly hydrostatic on the upper part of the gate because of the velocity of flow in that region is very low. The pressure distribution in the lower part of the gate is far from hydrostatic due to large velocity in this part. The pressure distribution at the lower tip of the gate is atmospheric because it is exposed to the atmosphere. Q.8. Apart from the integration of Euler’s equation what is the other way of getting Bernoulli’s Equation. It can be obtained by the degeneration of the steady flow energy equation. Q.9 What are the prerequisite to choose control volume correctly. i) The control volume should enclose al the fluid between the inlet and outlet i.e. between the stations where the conditions are known. ii) The control surface should be drawn normal to inlet and exit flows so that calculations are simpler and shear stress doesn’t come to the picture. iii) The control surface should come across the supports of fixtures to include the external reactions on the control volume which can then be calculated. iv) The control surface should not just surround the working fluid, the reaction of the container would enter which can be evaluated. v) The control volume for which our analysis applies in inertial or non-accelerating, therefore care should be taken to take it stationary or moving with constant rectilinear velocity only. Q.10 What is the meaning of stagnation pressure .It is nothing but the summation of static pressure and dynamic pressure. Q.11.Define the working pattern of a Flow-Nozzle. A flow-nozzle is a device in which the contraction of area is brought about by nozzle only. One of the pressure tappings is provided at a distance of one diameter upstream the nozzle plate and the other at the nozzle exit. Q.12. Define the workingf pattern of an Orificemeter. An Orificemeter is a circular plate with an orifice at the centre. The flow adjusts itself such that it contracts until a point downstream the orifice plate and then expands to fill the passage. One of the pressure tappings is provided at a distance one one diameter upstream the orifice plate and the other at a distance of half a orifice diameter downstream the plate. Q.13.What are the approximate values of Cd obtained for Venturimeter, Flow-Nozzle and Orificemeter. 0.95-0.98, 0.70-0.80 and 0.60-0.65 Q.14.What is the most significant difference between a Weir and a Notch. The only difference between a weir and a rectangular Notch is that a weir runs full all the way across the channel whereas a rectangular notch may be as wide as the channel. Q.15. What are the logic behind the use of rectangular notch and V-notch. A V-Notch is preferred where the discharge is low. For heacy discharge the rectangular notch is preferred because of the incremental head is more for an increment in discharge in rectangular notch than a V- notch. Q.16.What is LASER Anemometry? LASER is an acronym for Light Amplification by Stimulated Emission of Radiation . It is a device for producing a powerful monochromatic beam of light. There are gaseous, liquid and solid-state lasers. Chapter – 7: FLOW MEASUREMENT Q.1 When the liquid approaches the orifice it tends to contract. Why? Ans: The streamlines unable to take a sharp curvature, so the out coming jet becomes contracted. Q.2 What is the basic difference between small orifice and large orifice? Ans: In small orifice the depth of orifice is very small, for that for our calculation we can take the velocity which is acting at the central line, and we assume at the upper layer and at the lower layer velocity is the same, but in case of a large orifice we can’t do so. In that case we consider the depth of orifice and consider both the height H1 (surface to top of orifice) and H2 (surface to bottom of orifice) for discharge calculation. Q.3 What is the basic difference between orifice and mouthpiece? Ans: Mouthpiece is a short tube fitted to a circular orifice provided in a tank or reservoir. By fitting a mouthpiece the discharge through an orifice may be increased. Q.4 Why it is more economic to use convergent-divergent mouthpiece than to cylindrical mouthpiece? Ans: In cylindrical mouthpiece firstly the jet of water contracts up to the vena-contracta then again it diverges and loss of energy takes place, but in convergent-divergent mouthpiece it is made to conform to the shape of the jet up to vena-contracta so that the loss of energy may be eliminated. Q.5 What is the difference between the crest of a notch and crest of a weir? Ans: In case of notch the bottom edge is called the crest, but in case of weir the top edge is called the crest. Chapter – 8: FLOW THROUGH PIPES Q.1 Why the pipes cross-section is circular? Ans: Pipe section becomes circular, because it is economic. As for example if we take four different geometric figures (square, rectangle, isosceles triangle and circle) having same cross sectional area (each having area one square meter) then though for every cross section the area becomes same but for all of them perimeter will be different and in case of circular c/s the perimeter becomes minimum. So for circular c/s minimum amount of steel sheet is required for production, second thing is that since there is no sharp bend or sharp corner in circular c/s for that the loss becomes less as compared to any other geometric section. So from all points of view circular c/s is the best cross section for a pipe. Q.2 What is critical velocity in a pipe? Ans: For a given temperature in a given pipe the velocity at which the flow changes from laminar to turbulent is known as critical velocity. Q.3 Sometimes we fit nozzle at the end of a pipe. What is the function of it? Ans: Nozzle is nothing but a gradually converging short tube, which is fitted at the outlet end of a pipe for converting the total energy of the flowing water into velocity energy or kinetic energy. Where high velocity flow in required there we use nozzle, as for example for extinguishing fire, fire hose is used and at the end of the hose nozzle is fitted. Q.4 Whenever a siphon operation is done why do we take a limited value of the summit height? Ans: We know in a siphon operation the portion of the pipe which lies over the hydraulic grade line has negative pressures and as we go upward gradually the pressure decreases and negative gauge pressure increases, so automatically at the summit the pressure becomes minimum. Regarding one thing we should take very much care that is the negative pressure at the summit should not be lower than the vapour pressure of the fluid, which is flowing through the pipe. If the negative pressure goes below the vapour pressure of the flowing fluid, then the phenomenon of cavitation will start and complete or partial blockage of the flow will take place. Due to this cavitation either discontinuity of flow will be there or the flow will be totally stopped. Q.5 When a pipeline will suffer negative pressure? Ans: When a pipeline will go above of its hydraulic grade line then the portion, which is above of the Hydraulic Grade Line, will suffer negative pressure and the negative pressure is directly proportional to the distance between the point under consideration and the Hydraulic Grade Line. Q.6. There is sudden enlargement and tapered enlargement. Where the loss will be less. In tapered enlargement. Q.7. Chapter – 9:BOUNDARY LAYER FLOW Q.1 Can we get Boundary layer in case of ideal fluid flow? Ans: No. We cannot get Boundary layer in case of ideal fluid flow, because ‘no slip’ condition does not exist in ideal fluid flow, but the ‘no slip’ condition is the backbone for formation of Boundary layer. But ‘no slip’ condition can be achieved only in case of real fluid flow. Q.2 Why the thickness of Boundary layer becomes very small? Ans: Boundary layer exist in case of real fluid flow, in real fluid flow at the boundary the fluid molecules stick to the surface and get the velocity of the surface. If the surface is at rest then fluid velocity at the boundary becomes zero, and if the surface is in motion then the molecules, which are attached to the surface, get the velocity of the surface. In the normal direction of flow gradually the velocity increases and within a very small distance the fluid velocity becomes 99% of the free stream velocity, the velocity gradient becomes very high. So 99% velocity can be achieved within a very small distance and 99% velocity can be obtained only on the boundary layer. For this reason thickness of Boundary Layer becomes very small. Q.3 What is the nature of flow within the boundary layer? Ans: Within the boundary layer the flow is highly viscous, so it is obviously a laminar flow. Within the Boundary Layer one layer slides over another layer and the streamlines are parallel. Q.4 Why outside the boundary layer we generally assume that the flow is irrotational? Ans: We know that ideal flow is always irrotational. Now we know velocity of fluid at the boundary layer is 99% of the free stream velocity and within the boundary layer the velocity is less than 99% of the free stream velocity. Outside the boundary layer the velocity is more than 99% of the free stream velocity. Let us assume that the free stream velocity is ‘U’. Now if we go from boundary layer to the surface at rest then velocity will change from 0.99U to zero within a very small distance. The velocity gradient within the boundary layer is very high due to very high viscous flow. But outside the boundary layer velocity changes from 0.99U to U and velocity gradient becomes very small due to very small viscous effect, for that safely we can apply the formulae of potential flow outside the boundary layer and for that easily we can assume that the fluid flow outside the Boundary Layer is ideal. Q.5 Why the boundary layer thickness gradually increases from leading edge to the trailing edge? Ans: As the fluid passes over the plate, the action of viscous shear retards more and more fluid in the lateral direction. For that the thickness of the boundary layer increases along the plate in the downstream direction. Q.6. When boundary layer is drawn over a flat plate it is generally observed that the boundary layer touches the leading edge of the plate. Is it correct? It is not correct. It creates an ambiguity. Mathematically it is wrong. If the boundary layer touches the leading edge then it creates an ambiguity. At a time it shows 99% of the free stream velocity and zero velocity due to no-slip condition. Actually there should be a very small gap between the leading edge and the starting of the boundary layer. Q.7. Why do people prefer nylon mosquito net than cotton mosquito net? Due the fibres in the cotton the velocity profile becomes slick as compared to the nylon mosquito net. That is why we prefer nylon mosquito net. Q.8.What is a basic difference between laminar boundary layer and turbulent boundary layer? The velocity gradient at the wall in a turbulent boundary layer is larger than that in a laminar layer. Q.9.Within a pipe can the boundary layer exceed the radius of the pipe? The boundary layer can grow only upto the cetre line of the pipe and the boundary layer thickness can’t exceed the radius of the pipe. When once it happens, it continues with an unchanged profile and is said to constitute a fully developed flow. Q.10.What is a simple difference between laminar boundary layer and turbulent boundary layer? In laminar boundary layer δ is proportional to x0.5and in turbulent boundary layer x0.8 Q.11 Based on the dependence of δ upon Reynolds Number, comment when the thickness of boundary layer is likely to be more. i) At a point downstream the flow than at a point upstream of it ii) For a lower free stream velocity ‘V’ than for a higher free stream velocity iii) For a non-viscous fluid than for a less viscous fluid iv) For a lower density fluid Q.12 What is the significance of positive pressure gradient? Whenever the pressure gradient is positive the critical Reynolds Number is lower than that for no pressure gradient. Q.13. Comment on the growth of the Boundary Layer. The rate of Boundary Layer Growth is faster for a flow under adverse pressure gradient than that for a flow under favourable pressure gradient. For example the boundary layer growth is more rapid for flow over the convex shaped (upper) surface of an aerofoil than the flow over a flat plate. The rate of growth is higher for flow through a diverging passage than that in a constant area passage or in a converging passage. Q.14. What is the Blockage Correction Factor? The blockage correction factor is defined as the rate of effective area to the actual area. Q.15.How the eddy size and the wave number is co-related? Turbulent motion is characterized by eddies of different sizes and wave numbers. Large eddies have smaller wave numbers. Q.16Differentiate between laminar boundary layer and turbulent boundary layer. A laminar layer tends to separate off the surface more readily than a turbulent layer. Q.17.How to differentiate attached flow, Flow at the verge of separation and separated flow? For attached flow the velocity gradient velocity gradient is positive, for flow at the verge of separation velocity gradient is zero, and for the separated flow the velocity gradient is negative. Q.18. What are the various types of flow coming out of a water tap? If the tap is slightly open, the flow commences laminar but, after a certain distance, the laminar nature of the flow breaks down and it becomes turbulent. If the tap is opened further, the transition may be brought about earlier. At higher openings, the flow may get turbulent and continue to be so. Chapter – 10: SIMILITUDE AND DIMENSIONAL ANALYSIS Q.1 What is the basic difference between dimension and unit? Ans: For any variable there can be more than one unit but dimension for that variable will be unique. As for example if you take length as the variable then it can have many units like cm, mm, km, light year etc. but it will have unique dimension and that is ‘L’. Q.2 What do you mean by dimensionally homogeneous equation? Ans: Every equation contains two sides; one is the left hand side and second is the right hand side. Now if we take an equation and if we analyze the dimension of both the sides and if we get same dimension for both the sides then that equation is called dimensionally homogeneous equation. As for example we can take the equation: v2 = u2 + 2gh Here the dimension of v2 = L2T-2; dimension of u2 is L2T-2; Dimension of 2.g.h is L / T2. L = L2T-2 So it is clear that both the sides are having the same dimension, so it is a dimensionally homogeneous equation. Q.3 What do you mean by dimensional non-homogeneity? Ans: There are some equations in which if we analyze the dimension of both the sides then in both the sides the dimensions will not be equal. As for example we can mention the well-known Manning’s formula which is used in open channel flow. Manning’s formula is V = 1 / n * R2/3 * S1/2 Here V is the mean velocity of flow in the channel; n = roughness coefficient; R = Hydraulic radius; S = Bed slope (actually ‘S’ is the energy line slope, but for uniform flow the energy line slope and the bed slope are equal). If ‘n’ is assumed to be a dimensionless parameter then if we analyze both the sides then the dimension of left side will be ‘L / T’ and dimension of right side will come ‘L2/3’. But L / T L2/3. So this is the example of dimensional non-homogeneity. Q.4 Can you mention the name of two quantities which are totally different from each other, but which have same dimension after analysis? Ans: As for example we can mention the name of two quantities 1) Work done 2) Moment of a force. Unit of work = unit of force X unit of displacement Therefore, we can say, unit of work = N * m = Kg. m / s2 * m =Kg.m2 /s2 Therefore, dimension of work becomes ML2T-2. Similarly, Unit of moment of a force = unit of force * unit of perpendicular distance. So unit of moment of a force = N * m Or, Unit of moment of a force = Kg. m / s2 * m So the dimension of moment of a force after analysis becomes ML2T-2. So the quantities, work and moment of a force having the same dimension ML2T-2 ,though they are different. Q.5 In Buckingham’s theorem can we replace any - term by any power of that - term? Ans: Yes. We can do it. As for example can be replace by 1/2or . Chapter – 11: LAMINAR FLOW Q.1 If laminar flow takes place through a circular pipe what are the geometric shapes of the lamina of the fluid? Ans: The lamina is depending upon the shape of the boundary of the passage through which it flows, so here are the laminar will be concentric cylindrical sheets. Q.2 Generally all of us possess an idea that if in pipe flow Reynolds’s number becomes more than 4000, then the flow becomes turbulent, is it always true? Ans: No. This is always not true. By very high precision if we maintain the flow then even at Reynolds’s number equal to 40000 the flow can be laminar. Q.3 It is observed that low-density fluids are more prone to laminar flow than the high-density fluid. What is the reason behind it? Ans: The exchange of momentum between different layers and effectiveness of mixing is less in low-density fluids than denser fluids, for that low density fluids are more prone to laminar flow. Q.4 We see that in a pipe if laminar flow takes place then the velocity distribution is parabolic, but if turbulent flow takes place in that pipe then the velocity distribution diagram flattens. What is the reason behind it? Ans: In laminar flow the velocity gradient is very high and difference of velocity between two layers is much more than the difference of velocities between two layers in turbulent flow. More precisely speaking in turbulent flow the velocity distribution at any cross section is much more uniform than the laminar flow, and this uniformity is responsible for the flattened nature of the velocity profile. Q.5 What is the difference between upper critical Reynolds number and lower critical Reynolds number? Ans: The Reynolds number up to which, the laminar flow, can be maintained, is called upper critical Reynolds number. Reynolds number up to which, the flow should be reduced to change the flow from turbulent to laminar is called lower critical Reynolds number. Q.6.What are the most likely conditions for a flow to be laminar? A flow is said to be laminar if the velocity ‘V’ is low, the diameter ‘d’ is small, the mass density is low and the coefficient of dynamic viscosity is high. Q.7. Chapter – 12: TURBULENT FLOW Q.1 In a liquid flow through a pipe as the Reynolds number gradually increases the velocity profile gradually becomes flatter. What is the reason behind it? Ans: As the Reynolds number continuously increases the flow becomes more and more turbulent, and as the flow becomes more and more turbulent the intermingling of molecules between different layers increases and gradually velocity distribution becomes more and more uniform, for that the velocity profile gradually flattens with the gradual increase of Reynolds number. Q.2 We see, as the Reynolds number increases the velocity profile becomes flatter and flatter. Now if Reynolds number becomes very high can the velocity profile be rectangular? Ans: Reynolds number can be assumed as high as possible, but velocity profile will never be rectangular. Because the fluid under consideration is real and for real fluid obviously at the pipe boundary the velocity will be zero due to ‘no slip’ condition. So the velocity profile will be flatter and flatter as the Reynolds number increases, but never it can be rectangular due to boundary friction. Q.3 How the variation for friction factor for turbulent flow takes place in smooth pipe? Ans: When turbulent flow takes place in smooth pipe then friction factor only depends on Reynolds number and in that case it is independent of the relative roughness ‘ε’ (ε =k / d) because in smooth pipe the roughness protrusions are so small they become submerged within the laminar sub-layer. So in this case friction factor ‘f’ is expressed as a function of Reynolds number only and it is written as; f = 0.316 / (Re)1/4 Q.4 What is the classification of turbulence? Ans: Turbulence can be classified into two divisions: a) Wall Turbulence - The turbulence generated by the viscous effect for the presence of a solid wall is known as the wall turbulence, example of wall turbulence is flow through a pipe. b) Free Turbulence - In free turbulence solid wall is absent. The turbulent by the fluid flowing at different velocities is known as free turbulence. Turbulent mixing of submerged jet is a good example of free turbulence. Q.5 What is turbulence? Ans: Turbulence is a random phenomenon where the different quantities related to the flow vary randomly with time and space. Turbulence may appear in the flow around a solid object or it can appear in the flow through a passage under certain circumstances. Chapter – 13: FLOW AROUND IMMERSED BODIES Q.1 If an object moves from left to right what will be the direction of drag on it? Ans: We know drag force always acts against the direction of motion, since there the direction of movement is from left to right for that the direction of drag will be from right to left. Q.2 What is necessary for inducing lift on an object? Ans: For inducing lift on an object it is necessary that the shape of the object should be such that the velocity distribution around the object is unsymmetrical. For unsymmetrical velocity distribution on any object in one side velocity will be high and on the other side the velocity will be low. For lift the velocity in the lower side should be low. If velocity below the object becomes low then from Bernoulli’s equation we come to know that pressure below the object will be high and lift will take place. Q.3 What do you mean by stream lined body? Ans: Streamlined body is such a body in which the boundary layer separation is delayed to the greatest extent. Truly speaking separation of Boundary layer can’t be avoided; it can only be delayed. If the body is a streamlined body then the separation can be delayed for the maximum amount of time. So in streamlined bodies separation of boundary layer takes place at the rear most part and for that the wake size becomes small and loss of energy due to eddy is minimum. Q.4 What is the nature of boundary layer flow over a sufficiently large flat plate? Ans: Starting from the leading edge for a small length we will get laminar boundary layer and after that through out the flat plate there will be turbulent boundary layer accompanied by a laminar sub-layer. Q.5 What are the factors, which govern the profile drag in case of an aerofoil? Ans: The shape and orientation of the aerofoil govern the profile drag of an aerofoil. Q.6.Can lift be produced without Drag? The production of the lift requires assyemtry of flow about the direction of the free stream, whereas drag must be produced under all circumstances. It is possible to produce drag without lift, but it is not possible to produce lift withiot drag. Q.7. What are the classification of drags. REFER PAGE 349 and 350 OF K.L.KUMAR Q8. Do you think all the submerged bodies will suffer equal amount of compressibility? The effect of compressibility is different for different bodies. A cylinder would experience much more compressibility drag than a sharp nosed projectile. Q.9. What is the basic difference between surface waves and gravitational waves? Surface waves are set up and exhibit oscillations of the liquid, mixing and froth formation with the air and consequent energy dissipation. Gravitational waves are formed ahead of, on the sides and at the rear ends of boats, ships, ocean-liner and other sea going vessels. Q.10. Define a stream lined body. A body is said to be streamlined if, whwen placed in a flow, thev surfaces of the body tend to coincide with stream surfaces. The streamlines therefore conform with the boundaries of the body. Q.11. Do you think streamlining is a function of velocity and the direction? Yes, a body may be streamlined at low velocity but may may not be so at high velocity. A body may be streamlined at when placed in a particular direction but may not be so when placed in other direction. Q.12.How do you co-relate the flight of a bird and the lift to drag ratio? Since the drag on a streamlined body is low, the lift/drag ratio is high. Flight of birds is attributed to the production of high lift/drag ratio by virtue of wings attached to its bodies. Q.13.What is the basic difference between a streamlined body and a bluff body from the technical point of view? A streamlined body is characterized by high skin-friction drag to form drag ratio, a bluff body is characterized by high form drag to skin-friction drag ratio. Q.14.FIG 9.3- (c) page 352-353 of K.L.KUMAR Q.15.What are the reasons due to which an early separation may be delayed. i) By increasing the Reynolds number of flow which may be achieved by increasing the velocity of free stream. ii) By increasing the initial level of turbulence in the free stream iii) By roughening a part of the leading face of the cylinder iv) By providing a trip-wire in laminar boundary layer region. Q.16.Should there be drag force if a symmetrical body is placed in a free stream along the axis of symmetry. No Q. 17.When do we consider the flow over an aerofoil as two-dimensional? The flow over an aerofoil is two dimensional if its aspect ratio is infinite. Practically the aspect ratio is large because the end flow effects may then be neglected. Q.18 What is the critical angle of attack? The angle of attack at which the lift force is maximum is called the critical angle of attack. Q.19.What is the terminal or equilibrium state? At the terminal or equilibrium state the upward drag force would just balance the downward weight force and the net force acting on the body would be zero. Q.20 Can the velocity of a falling body can be more than the terminal velocity? The velocity of a falling body can’t be more than the terminal velocity. In other words the terminal velocity is the maximum velocity of a freely falling body in an environment. Q.21.If a coin and a feather are dropped together, then comment on their terminal velocities. If a coin and a feather were dropped from a big height the terminal velocity of the coin will be much more than the feather because the drag on the coin is per unit weight is much less as compared to the drag per unit weight on the feather. Q.22. For ideal fluid flow what is the pressure distribution over a cylinder. It is clear from the following table Position Pressure Coefficients Comment 0 1 Stagnation Point 30 0 Zero Point 90 -3 Least Pressure 150 0 Zero Pressure 180 1 Stagnation Point Q.23 Is there any flow which is having similarity with a stationary Tornado? Yes, The flow obtained by superimposing a free vortex on a sink flow bears an interesting similarity with a stationary tornado. Q.24. What is profile drag? The component of the total force along the direction of flow or the direction of motion of the body is called the profile drag or simply the drag. Q.25.Write regarding separation and stagnation point. FIG- 11.2 NATARAJAN At the nose of the body is a stagnation point where the pressure is maximum. Thereafter the pressure decreases along the boundary up to the point T and then starts to increase as the flow proceeds beyond T. From S to T the pressure gradient is negative and the decreasing pressure aids to accelerate the fluid particles in the boundary layer acceleration helps to oppose the retarding effect of the viscous shear and hence we may say that the negative pressure gradient is favourable pressure gradient. Beyond T the pressure gradient is positive and is called adverse pressure gradient. It produces complications in the flow. In this region the flow near the wall must not only negotiate an upgrade in pressure but also must overcome the viscous shear. On the other hand the flow near the edge of the boundary layer need only to manage the unfavourable pressure gradient snce the opposing shear there is vanishingly small. Consequently the flow near the wall is retarded more rapidly and eventually get reversed. The flow is then said to separate and the point of separation is the point where this reversal takes place.At the point of the separation the slope of the velocity profile is zero. Q.26.Write some speciality of separation. Sepoaration may occur in both laminar and turbulent flow. Angulasr boundaries have the separation points fixed at the point of siscontinuity. In smoothly curved boundaries the point of separation advances upstream with increasing Re and suddenly shifts downstream when the boundary layer becomes turbulent.Surface roughness is also a factor to be considered while talking of separation. For a given shape and roughness of the boundary , the tendency of the flow to separate depends upon the magnitude of the adverse pressure gradient.The greater the adverse pressure gradient sooner is the separation. Q.27.Co-relatwe separation and radius of curvature of a body. Generally separation is associated with a boundary of small radius of curvature. However as an exception one may find the straight boundaries of as diverging diffuser is more prone to separation unless the angle of divergence is small. Q.28. Is the complete mathematical solution of Boundary Layer is there? The mathematical solution of the boundary layer foe even simple curved boundaries like the cylinder are still unknown to us. Q.29.Generally which drag is predominant in most of the cases? Often the dominant part of the total drag is the pressure drag which is largely due to the wake and its size. Q.30.What is your comment on the pressure at the separation point. The pressure at the separation point is bound to be low, because separation normally initiates at a point of low pressure, followed by an upgrade in the pressure gradient. Q.31.Write about streamlining. The point of separations which determine the size of the wake governs the pressure drag. When the shape of the bosdy is such that the separation points are pushed for downstream resulting in a small wake. The pressure drag is bound to be very small. Such bodies are known as streamlined bodies have their drag mainly due to skin friction. As a matter of fact a body is streamlined to minimise the drag. This is achieved by providing smooth changes in curvature to avoid separation and by trying to obtain a laminar boundary layer as long as possible. On the other hand, a blunt or bluff body allows the flow to separate over a wide area resulting in a higher value of form drag and a lesser value of skin friction drag. The extreme case of the bluff body is a fl;at plate placed normal to the direction of flow. Flow separates at the edges and the drag becomes completely pressure drag. FIG- 11.4 (NAT…) Q.32. What is the reason behind the singing of telephone wires. The alternate formation and detachment of vortices from the boundary result in a periodic shifting of the low pressure zone in the wake, thereby producing a fluctuating drag and transverse force. The effect of the fluctuating force is set the body in vibration. The singing of telephone wires and power cables and howling of winds are the classic examples of this phenomena. If the velocity of flow is such that the frequency of vortex shedding (or vibration) is near the natural frequency of the cylinder, a resonant condition may reach. Due to this occurrence of resonance any engineering structure like chimney may collapse. Q.33.In the laboratory how the conditions of infinite length is created? It is created by preventing the flow around the ends of the cylinder. Q.34.How the CD varies with Re? When the boundary layer begins to separate with increase in Re the pressure drag comes into existence and the slope of the curve begins to decrease. For cylinder the drag coefficient reaches a minimum of about 0.95 at Re value around2000 and slightly rises to 1.2 at Re value 5000.Re equal to 5000 corresponds to the end of laminar condition but not the boundary layer. Chapter – 14: UNIFORM FLOW IN OPEN CHANNELS Q.1 What is the main difference between open channel flow and pipe flow? Ans: The main difference between the open channel flow and pipe flow is that, open channel flow takes place in a sloping bed due to gravity force and the whole free surface suffers atmospheric pressure everywhere, but in case of pipe flow it is a closed conduit flow. So there can be no free surface. So pipe flow takes place due to difference of pressure between two sections. Q.2 If a pipe flows in a partially filled condition; can we call it an open channel flow? Ans: Yes. The very basic specialty of open channel flow is that, it must have a free surface. Now when a pipe flows in a partially filled condition then it possesses a free surface, so it can be called an open channel flow. Q.3 If we insert a piezometer within an open channel will there be any rise of water within the piezometer? Ans: In case of pipe flow if we join the free surface of water within different piezometers then we will get the hydraulic grade line, but in open channel flow everywhere over the free surface atmospheric pressure acts and the free surface itself is the hydraulic grade line in case of an open channel flow. For this reason there will be no rise of water within the piezometer. Q.4 In an open channel why we don’t get the maximum velocity of flow at the free surface? Ans: Due to the surface tension of liquid-air interface (the free surface) resistance is offered to the flow, apart from that wind velocity is also a secondary factor. If wind blows strongly from upstream to downstream then it affects the velocity profile; so due to this reasons we don’t get maximum velocity at the free surface. Generally maximum velocity is obtained at 0.6H depth, where ‘H’ is the depth of flow in the open channel. Q.5 What is the specialty of uniform flow in an open channel? Ans: For the uniform flow in an open channel the depth of flow becomes constant. Chapter – 15: NON-UNIFORM FLOW IN OPEN CHANNELS Q.1 Why we don’t get ‘H1’ type of curve in case of non-uniform open channel flow? Ans: Actually with respect to every type of slope there can be three types of flow profile and for that we should get three type of depth relationship; i) y > yn > yc or y > yc > yn ii) yn > y > yc or yc > y > yn iii) yn > yc > y or yc > yn > y But in case of horizontal channel since the bed slope becomes zero for that yn becomes infinite, since yn itself is infinite for that reason there can be no depth ‘y’ which will be more than infinite. For that in case of horizontal channel the depth relation y > yn > yc does not exist. For this reason question of H1 curve does not arise. Q.2 Why we don’t get C2 type of curve in case of non-uniform flow in open channel? Ans: We know there can be five types of slopes in an open channel and generally for each case i.e. for each type of slope we get three profiles and the depth relation for them are: i) y > yn > yc or y > yc > yn ii) yn > y > yc or yc > y > yn iii) yn > yc > y or yc > yn > y So for case two either yn > y > yc or yc > y > yn but in case of critical flow since yn is equal to yc for that the value of ‘y’ between yn and yc does not arise. For that the second condition does not exist and for that in critical slope we don’t get the second condition and for that reason C2 curve does not come to the picture. Q.3 Why we don’t get A1 type of curve in case of non-uniform flow in an open channel? Ans: In case of adverse slope the channel slope itself becomes negative, for that the normal depth yn becomes imaginary, and if the normal depth yn itself becomes imaginary then there can be no depth ‘y’ which is more than that. So the depth relation y > yn > yc does not come to the picture and for that reason we don’t get A1 type of curve. Q.4 Why we are unable to find the length of the hydraulic jump analytically? Ans: Due to some practical complications, which arise from the general instability of hydraulic jump, it is not possible to find the length of hydraulic jump analytically. It is very difficult to predict the actual sections where the hydraulic jump begins and ends. Q.5 What are the factors for which uniform flow changes to non-uniform flow in an open channel? Ans: There are several factors behind this. One or more factors can be responsible for the non-uniform flow in open channel. If there is a change in shape and rise of the channel c/s then non-uniform flow can take place. If there is a change in channel slope then also there can be non-uniform flow. If there is a presence of obstruction such as weir then also we can have a non-uniform flow. At the last we can say if the frictional force at the boundary changes then we can get non-uniform flow. Q.6. What is the relation between Chezy’s ‘C’ and Manning’s ‘n’. C = 1/n R1/6 Q.7. What are the basic causes of Hydraulic Jump. If an incoming flow is supercritical and obstruction is placed in its passage or the slope of the bed provided is inadequate FOR A STABLE SUPERCRITICAL FLOW TO PROCEED THEN THE FLOW MAY TRANSFORM ABRUPTLY from supercritical to subcritical through a hydraulic jump. Q.8. Is it possible to have the prejump Frode Number less than one and post jump Froude Number more than one for a Hydraulic Jump. If this happens then it requires an increase of available energy showing a decrease in entropy of a system which is in violation of the second law of Thermodynamics. Q.9. State a classic relation between the prejump and post-jump Froude Number. The higher is the value of pre-jump Froude Number the lower is the post-jump Froude Number. Q.10.How the Hydraulic Jump is used in mixing of two different liquids in industry. The Hydraulic Jump is formed in the primary liquid and secondary liquid is poured from the top. Q.11. CHAPTER:- 16- FLUID MACHINES Q.1. What are the various types of Fluid Machines. A fluid machine which operates to convert fluid power into shaft power is called turbine or an expander, a fluid machine which operates to convert shaft power into fluid power is called pump, a fan, a blower or a compressor. Q.2. Based on various types of fluid how the turbines can be classified? Fluid Types of Turbine Water Hydraulic Turbine Steam Steam Turbine Freon Vapour Turbine Gas or Air Gas Turbine Wind Wind Mills Q.3. Based on various types of fluid how the pumps can be classified? Fluid Type of Pumps Water and other liquids Pumps Air and other gases (with slight pressure rise) Fans and Propellers Air and other gases (with higher pressure rise) Blowers and Compressors
Posted on: Thu, 04 Dec 2014 09:23:57 +0000
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