Question: A wind turbine with two or four

> What is the effect of wing tip vortices (the air circulation from the lower part of the wings to the upper part) on the drag and the lift?

> What is the difference between streamlined and bluff bodies? Is a tennis ball a streamlined or bluff body?

> Air is flowing past a spherical ball. Is the lift exerted on the ball zero or nonzero? Answer the same question if the ball is spinning.

> Air is flowing past a symmetrical airfoil at an angle of attack of 5°. Is the (a) lift and (b) drag acting on the airfoil zero or nonzero?

> Why is the contribution of viscous effects to lift usually negligible for airfoils?

> Why are flaps used at the leading and trailing edges of the wings of large aircraft during takeoff and landing? Can an aircraft take off or land without them?

> Both the lift and the drag of an airfoil increase with an increase in the angle of attack. In general, which increases at a higher rate, the lift or the drag?

> At winter design conditions, a house is projected to lose heat at a rate of 60,000 Btu/h. The internal heat gain from people, lights, and appliances is estimated to be 6000 Btu/h. If this house is to be heated by electric resistance heaters, determine th

> Air is flowing past a symmetrical airfoil at zero angle of attack. Is the (a) lift and (b) drag acting on the airfoil zero or nonzero?

> What is stall? What causes an airfoil to stall? Why are commercial aircraft not allowed to fly at conditions near stall?

> Dust particles of diameter 0.06 mm and density 1.6 g/cm3 are unsettled during high winds and rise to a height of 200 m by the time things calm down. Estimate how long it takes for the dust particles to fall back to the ground in still air at 1 atm and

> A person extends his uncovered arms into the windy air outside at 1 atm and 60°F and 25 mi/h in order to feel nature closely. Treating the arm as a 2-ft-long and 4-in-diameter cylinder, determine the combined drag force on both arms.

> A 0.12-mm-diameter dust particle whose density is 2.1 g/cm3 is observed to be suspended in the air at 1 atm and 20°C at a fixed point. Estimate the updraft velocity of air motion at that location. Assume Stokes’ law to be applicable. Is this a valid assu

> What is the difference between the upstream velocity and the free-stream velocity? For what types of flow are these two velocities equal to each other?

> A 2-m-long, 0.2-m-diameter cylindrical pine log (density = 513 kg/m3) is suspended by a crane in the horizontal position. The log is subjected to normal winds of 40 km/h at 5°C and 88 kPa. Disregarding the weight of the cable and its drag, det

> A 1.2-in-outer-diameter pipe is to span a river at a 115-ft-wide section while being completely immersed in water. The average flow velocity of the water is 8 ft/s, and its temperature is 70°F. Determine the drag force exerted on the pipe by the river.

> Consider 0.8-cm-diameter hail that is falling freely in atmospheric air at 1 atm and 5°C. Determine the terminal velocity of the hail. Take the density of hail to be 910 kg/m3.

> A long 5-cm-diameter steam pipe passes through some area open to the wind. Determine the drag force acting on the pipe per unit of its length when the air is at 1 atm and 10°C and the wind is blowing across the pipe at a speed of 50 km/h.

> A vertical piston–cylinder device contains water and is being heated on top of a range. During the process, 65 Btu of heat is transferred to the water, and heat losses from the side walls amount to 8 Btu. The piston rises as a result of evaporation, and

> A 5-mm-diameter electrical transmission line is exposed to windy air. Determine the drag force exerted on a 160-m-long section of the wire during a windy day when the air is at 1 atm and 15°C and the wind is blowing across the transmission line at 50 km/

> In flow over cylinders, why does the drag coefficient suddenly drop when the boundary layer becomes turbulent? Isn’t turbulence supposed to increase the drag coefficient instead of decreasing it?

> Why is flow separation in flow over cylinders delayed when the boundary layer is turbulent?

> In flow over bluff bodies such as a cylinder, how does the pressure drag differ from the friction drag?

> The forming section of a plastics plant puts out a continuous sheet of plastic that is 1.2 m wide and 2 mm thick at a rate of 18 m/min. The sheet is subjected to airflow at a velocity of 4 m/s on both top and bottom surfaces normal to the direction of mo

> The weight of a thin flat plate 50 cm × 50 cm in size is balanced by a counterweight that has a mass of 2 kg, as shown in Fig. P15–50. Now a fan is turned on, and air at 1 atm and 25°C flows downward over both s

> Define the planform area of a body subjected to external flow. When is it appropriate to use the planform area in drag and lift calculations?

> Repeat Prob. 15–48 for water. Data from Prob. 15-48: Air at 25°C and 1 atm is flowing over a long flat plate with a velocity of 8 m/s. Determine the distance from the leading edge of the plate where the flow becomes turbulent, and the thickness of the b

> Air at 25°C and 1 atm is flowing over a long flat plate with a velocity of 8 m/s. Determine the distance from the leading edge of the plate where the flow becomes turbulent, and the thickness of the boundary layer at that location.

> Reconsider Prob. 15–46E. Using appropriate software, investigate the effect of truck speed on the total drag force acting on the top and side surfaces and the power required to overcome it. Let the truck speed vary from 0 to 100 mi/h in

> How are heat, internal energy, and thermal energy related to each other?

> Consider a refrigeration truck traveling at 70 mi/h at a location where the air is at 1 atm and 80°F. The refrigerated compartment of the truck can be considered to be a 9-ft-wide, 8-ft-high, and 20 ft-long rectangular box. Assuming the airflo

> Consider laminar flow of a fluid over a flat plate. Now the free-stream velocity of the fluid is tripled. Determine the change in the drag force on the plate. Assume the flow remains laminar.

> Air at 70°F flows over a 10-ft-long flat plate at 25 ft/s. Determine the local friction coefficient at intervals of 1 ft, and plot the results against the distance from the leading edge.

> The local atmospheric pressure in Denver, Colorado, (elevation 1610 m) is 83.4 kPa. Air at this pressure and at 25°C flows with a velocity of 9 m/s over a 2.5-m × 5-m flat plate. Determine the drag force acting on the top surface of the plate if the air

> Light oil at 75°F flows over a 17-ft-long flat plate with a free-stream velocity of 5 ft/s. Determine the total drag force per unit width of the plate.

> How is the average friction coefficient determined in flow over a flat plate?

> What fluid property is responsible for the development of the velocity boundary layer? What is the effect of the velocity on the thickness of the boundary layer?

> Define the frontal area of a body subjected to external flow. When is it appropriate to use the frontal area in drag and lift calculations?

> What does the friction coefficient represent in flow over a flat plate? How is it related to the drag force acting on the plate?

> During major windstorms, high vehicles such as RVs and semis may be thrown off the road and boxcars off their tracks, especially when they are empty and in open areas. Consider a 6000-kg semi that is 10 m long, 2.5 m high, and 2 m wide. The distance betw

> A fan is to accelerate quiescent air to a velocity of 8 m/s at a rate of 9 m3/s. Determine the minimum power that must be supplied to the fan. Take the density of air to be 1.18 kg/m3.

> The value of the gravitational acceleration g decreases with elevation from 9.807 m/s2 at sea level to 9.767 m/s2 at an altitude of 13,000 m, where large passenger planes cruise. Determine the percent reduction in the weight of an airplane cruising at 13

> A 7-m-diameter hot air balloon that has a total mass of 350 kg is standing still in air on a windless day. The balloon is suddenly subjected to 40 km/h winds. Determine the initial acceleration of the balloon in the horizontal direction.

> An 8-mm-diameter plastic sphere whose density is 1150 kg/m3 is dropped into water at 20°C. Determine the terminal velocity of the sphere in water.

> A 0.80-m-diameter, 1.2-m-high garbage can is found in the morning tipped over due to high winds during the night. Assuming the average density of the garbage inside to be 150 kg/m3 and taking the air density to be 1.25 kg/m3, estimate the wind velocity d

> Bill gets a job delivering pizzas. The pizza company makes him mount a sign on the roof of his car. The frontal area of the sign is A = 0.612 ft2, and he estimates the drag coefficient to be CD = 0.94 at nearly all air speeds. Estimate how much additiona

> Reconsider Prob. 15–32. Using appropriate software, investigate the effect of car speed on the required power to overcome (a) rolling resistance, (b) the aerodynamic drag, and (c) their combined effect. Let the car speed vary from 0 to 150 km/h in increm

> During steady motion of a vehicle on a level road, the power delivered to the wheels is used to overcome aerodynamic drag and rolling resistance (the product of the rolling resistance coefficient and the weight of the vehicle), assuming the friction at t

> Reconsider Prob. 15–30. Using appropriate software, investigate the effect of wind speed on the torque applied on the pivot. Let the wind speed vary from 0 to 50 m/s in increments of 5 m/s. Tabulate and plot the results. Data from Prob

> Which bicyclist is more likely to go faster: one who keeps his head and his body in the most upright position or one who leans down and brings his body closer to his knees? Why?

> A 70-kg bicyclist is riding her 15-kg bicycle downhill on a road with a slope of 8° without pedaling or braking. The bicyclist has a frontal area of 0.45 m2 and a drag coefficient of 1.1 in the upright position, and a frontal area of 0.4 m2 and a drag co

> An adiabatic closed system is accelerated from 0 m/s to 30 m/s. Determine the specific energy change of this system, in kJ/kg.

> A submarine can be treated as an ellipsoid with a diameter of 5 m and a length of 25 m. Determine the power required for this submarine to cruise horizontally and steadily at 40 km/h in seawater whose density is 1025 kg/m3. Also determine the power requi

> At highway speeds, about half of the power generated by the car’s engine is used to overcome aerodynamic drag, and thus the fuel consumption is nearly proportional to the drag force on a level road. Determine the percentage increase in fuel consumption o

> Advertisement signs are commonly carried by taxicabs for additional income, but they also increase the fuel cost. Consider a sign that consists of a 0.30-m-high, 0.9-m-wide, and 0.9-m-long rectangular block mounted on top of a taxicab such that the sign

> Suzy likes to drive with a silly sun ball on her car antenna. The frontal area of the ball is A = 2.08 × 10−3 m2. As gas prices rise, her husband is concerned that she is wasting fuel because of the additional drag on the ball. He runs a quick test in th

> A circular sign has a diameter of 50 cm and is subjected to normal winds up to 150 km/h at 10°C and 100 kPa. Determine the drag force acting on the sign. Also determine the bending moment at the bottom of its pole whose height from the ground

> Reconsider Prob. 15–22E. Using appropriate software, investigate the effect of frontal area on the annual fuel consumption of the car. Let the frontal area vary from 10 to 30 ft2 in increments of 2 ft2. Tabulate and plot the results. Data from Prob. 15-

> To reduce the drag coefficient and thus to improve the fuel efficiency, the frontal area of a car is to be reduced. Determine the amount of fuel and money saved per year as a result of reducing the frontal area from 20 to 13 ft2. Assume the car is driven

> The drag coefficient of a car at the design conditions of 1 atm, 25°C, and 90 km/h is to be determined experimentally in a large wind tunnel in a full-scale test. The height and width of the car are 1.25 m and 1.65 m, respectively. If the horizontal forc

> During a high Reynolds number experiment, the total drag force acting on a spherical body of diameter D = 12 cm subjected to airflow at 1 atm and 5°C is measured to be 5.2 N. The pressure drag acting on the body is calculated by integrating the pressur

> What is lift? What causes it? Does wall shear contribute to the lift?

> On a hot summer day, a student turns his fan on when he leaves his room in the morning. When he returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept closed.

> The resultant of the pressure and wall shear forces acting on a body is measured to be 430 N, making 30° with the direction of flow. Determine the drag and the lift forces acting on the body.

> A car is moving at a constant velocity of 110 km/h. Determine the upstream velocity to be used in fluid flow analysis if (a) the air is calm, (b) wind is blowing against the direction of motion of the car at 30 km/h, and (c) wind is blowing in the same d

> In general, how does the drag coefficient vary with the Reynolds number at (a) low and moderate Reynolds numbers and (b) at high Reynolds numbers (Re > 104)?

> Consider laminar flow over a flat plate. How does the local friction coefficient change with position? Answer: We are to discuss how the local skin friction coefficient changes with position along a flat plate in laminar flow. Analysis The local fric

> What is drafting? How does it affect the drag coefficient of the drafted body?

> What is flow separation? What causes it? What is the effect of flow separation on the drag coefficient?

> What is the effect of streamlining on (a) friction drag and (b) pressure drag? Does the total drag acting on a body necessarily decrease as a result of streamlining? Explain.

> What is the effect of surface roughness on the friction drag coefficient in laminar and turbulent flows?

> What is the difference between skin friction drag and pressure drag? Which is usually more significant for slender bodies such as airfoils?

> The cylindrical chimney of a factory has an external diameter of 1.1 m and is 20 m high. Determine the bending moment at the base of the chimney when winds at 110 km/h are blowing across it. Take the atmospheric conditions to be 20°C and 1 atm.

> For a cycle, is the net work necessarily zero? For what kinds of systems will this be the case?

> What is terminal velocity? How is it determined?

> What is drag? What causes it? Why do we usually try to minimize it?

> Water is to be withdrawn from a 7-m-high water reservoir by drilling a well-rounded 5-cm diameter hole with negligible loss near the bottom and attaching a horizontal 90° bend of negligible length. Taking the kinetic energy correction factor t

> Repeat Prob. 14–97 for hot-water flow of a district heating system at 100°C. Data from Prob. 14-97: A pipeline that transports oil at 40°C at a rate of 3 m3/s branches out into two parallel pipes made of commercia

> A pipeline that transports oil at 40°C at a rate of 3 m3/s branches out into two parallel pipes made of commercial steel that reconnect downstream. Pipe A is 500 m long and has a diameter of 30 cm, while pipe B is 800 m long and has a diameter

> Reconsider Prob. 14–95. Using appropriate software, investigate the effect of the second pipe diameter on the required pumping head to maintain the indicated flow rate. Let the diameter vary from 1 to 10 cm in increments of 1 cm. Tabula

> Water at 15°C is to be discharged from a reservoir at a rate of 18 L/s using two horizontal cast iron pipes connected in series and a pump between them. The first pipe is 20 m long and has a 6 cm diameter, while the second pipe is 35 m long an

> Shell-and-tube heat exchangers with hundreds of tubes housed in a shell are commonly used in practice for heat transfer between two fluids. Such a heat exchanger used in an active solar hot water system transfers heat from a water-antifreeze solution flo

> Repeat Prob. 14–92 except let the length of pipe A be three times that of pipe B. Compare this result to that of Prob. 14–92. Does the difference agree with your intuition? Explain. Data from Prob. 14-92: Two pipes of identical diameter and material ar

> Two pipes of identical diameter and material are connected in parallel. The length of pipe A is five times the length of pipe B. Assuming the flow is fully turbulent in both pipes and thus the friction factor is independent of the Reynolds number and dis

> What are the different mechanisms for transferring energy to or from a control volume?

> The velocity profile in fully developed laminar flow in a circular pipe, in m/s, is given by u(r) = 6(1 − 100r2), where r is the radial distance from the centerline of the pipe in m. Determine (a) the radius of the pipe, (b) the average velocity through

> A house built on a riverside is to be cooled in summer by utilizing the cool water of the river. A 20-m-long section of a circular stainless-steel duct of 20 cm diameter passes through the water. Air flows through the underwater section of the duct at 4

> How does surface roughness affect the pressure drop in a pipe if the flow is turbulent? What would your response be if the flow were laminar?

> The compressed air requirements of a manufacturing facility are met by a 120-hp compressor that draws in air from the outside through a 9-m-long, 22-cm-diameter duct made of thin galvanized iron sheets. The compressor takes in air at a rate of 0.27 m3/s

> In a geothermal district heating system, 10,000 kg/s of hot water must be delivered a distance of 10 km in a horizontal pipe. The minor losses are negligible, and the only significant energy loss arises from pipe friction. The friction factor is taken to

> In a laminar flow through a circular tube of radius of R, the velocity and temperature profiles at a cross section are given by u = u0(1 − r2/R2) and T(r) = A + Br2 − Cr4 where A, B, and C are positive constants. Obtain a relation for the bulk fluid temp

> Two pipes of identical length and material are connected in parallel. The diameter of pipe A is twice the diameter of pipe B. Assuming the friction factor to be the same in both cases and disregarding minor losses, determine the ratio of the flow rates i

> Water to a residential area is transported at a rate of 1.5 m3/s via 70-cm-internal-diameter concrete pipes with a surface roughness of 3 mm and a total length of 1500 m. In order to reduce pumping power requirements, it is proposed to line the interior

> Water is transported by gravity through a 10-cm diameter 550-m-long plastic pipe with an elevation gradient of 0.01 (i.e., an elevation drop of 1 m per 100 m of pipe length). Taking ρ = 1000 kg/m3 and ν = 1 × 10−6 m2/s for water, determine the flow rate

> Repeat Prob. 14–82 for cast iron pipes of the same diameter. Data from Prob. 14-82: A geothermal district heating system involves the transport of geothermal water at 110°C from a geothermal well to a city at about the same elevation for a distance of 1

> A steel rod of 0.5 cm diameter and 10 m length is stretched 3 cm. Young’s modulus for this steel is 21 kN/cm2. How much work, in kJ, is required to stretch this rod?

> A geothermal district heating system involves the transport of geothermal water at 110°C from a geothermal well to a city at about the same elevation for a distance of 12 km at a rate of 1.5 m3/s in 60-cm-diameter stainless-steel pipes. The fluid pressur

> Repeat Prob. 14–80 assuming pipe A has a halfway closed gate valve (KL = 2.1) while pipe B has a fully open globe valve (KL = 10), and the other minor losses are negligible. Data from Prob. 14-80: A certain part of cast iron piping of

> A certain part of cast iron piping of a water distribution system involves a parallel section. Both parallel pipes have a diameter of 30 cm, and the flow is fully turbulent. One of the branches (pipe A) is 1500 m long while the other branch (pipe B) is 2

> Consider laminar flow in a circular pipe. Is the wall shear stress τw higher near the inlet of the pipe or near the exit? Why? What would your response be if the flow were turbulent?