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Both the friction drag and the pressure drag can be sig- nificant. The drag force is proportional to the square of the velocity, and the increase in velocity at higher Reynolds numbers usually more than offsets the decrease in the drag coefficient.

For blunt bodies such as a circular cylinder or sphere, however, an increase in the surface roughness may actually decrease the drag coefficient, as shown in Figure 7—19 for a sphere.

### Mecanica de Fluidos Fundamentos y Aplicaciones – Yunus Cengel by Yunus A. Cengel – PDF Drive

At this point, the drag is mostly about 95 percent due to pressure drag. For a given hit, this means a longer distance for the ball. This is done by tripping the flow into turbulence at a lower Reynolds number, and thus causing the fluid to close in behind the body, narrowing the wake and reducing pressure drag considerably.

Flow in the wake region is characterized by random vortex formation and pressures much lower than the descragar point pressure. This results in a sudden decrease in drag of a flying body and insta- bilities in flight.

The flow in the boundary layer is laminar in this range, but the flow in the separated region past the cylinder or sphere is highly turbulent with a wide turbulent wake. For a table traneferencia ball, however, the distances are very short, and the balls never reach the speeds in the turbulent range.

In the range of Reynolds numbers where the flow changes from laminar to turbulent, even the drag force FD decreases as the velocity and thus Reynolds number in- creases. The nature of the flow across a cylinder or sphere strongly affects the total drag coefficient CD. At higher velocities, the fluid still hugs the cylinder on the frontal side, but it is too fast to remain attached to the surface as it approaches the top of the cylinder.

## Mecanica de Fluidos Fundamentos y Aplicaciones – Yunus Cengel

This is especially the case for streamlined bodies. Therefore, the drag coefficient in this case is reduced by a fac- tor of 5 by transferrencia roughening the surface.

Thus, the fluid follows the curvature of the cylinder. A decrease in the drag coefficient does not necessarily indicate a decrease in drag. This behavior is characteristic of blunt bodies. Experienced golfers also give the ball a spin during the hit, which helps the rough ball develop a lift and thus travel higher and further.

This large reduction in CD is due to the flow in the boundary layer becoming turbulent, which moves the separation point further on the rear of the body, reducing the size of the wake and thus the magnitude of the pressure drag. The high pressure in the vicinity of the stagnation point and the low pressure on the opposite side in the wake produce a net force on the body in the direction of flow.

Determine the drag force ex- erted on the pipe by the river. Assumptions 1 The outer surface of the pipe is smooth so that Figure 7—17 can be used to determine the drag coefficient. The drag force that acts on the pipe is to be determined. This results in a much smaller drag coefficient and thus drag force for a rough- surfaced cylinder or sphere in a certain range of Reynolds number compared to a smooth one of identical size at the same velocity.

This is in contrast to streamlined bodies, which experience an increase in the drag coefficient mostly due to friction drag when the boundary layer becomes turbulent.

The d above shows that czlor the surface can be used to great advantage in reducing drag, but cengsl can also backfire on us if we are not careful—specifically, if we do not operate in the right range of Reynolds num- ber. Therefore, the surfaces of table tennis balls are made smooth.

Effect of Surface Roughness We mentioned earlier that surface roughness, in general, increases the drag coefficient in turbulent flow. Once the drag coefficient is available, the drag force acting on a body in cross flow can be determined from Eq. There is no flow separation in this regime.

## Cap 7 e 8 Cengel – Transferencia de calor e massa

The curves exhibit differ- ent behaviors in different ranges of Reynolds numbers: The average drag coefficients CD for cross flow over a smooth single circu- lar cylinder and a sphere are given in Figure 7— As a result, the boundary layer detaches from the surface, forming a separation region behind the cylinder. Enviado por Fernanda flag Denunciar. The occurrence of turbulent flow at this Reynolds number reduces the drag coefficient of a golf ball by half, as shown in Figure 7— The delay of separation in turbulent flow is caused by the rapid fluctuations of the fluid in the transverse direction, which enables the turbulent boundary layer to travel further along the surface before separation occurs, resulting in a narrower wake and a smaller pressure drag.

Obviously, roughening the sphere in this case will increase the drag by a factor of 4 Fig. It should be kept in mind that the free-stream turbulence and disturbances by other bodies in flow such as flow over tube bundles may affect the drag coefficients significantly.