Flow separation-pressure drag

Also in this equation the effects of drag flow and pressure flow can be separated the drag flow is proportional to the rotational speed of the screws, and the pressure flow is proportional to the ratio of pressure gradient and viscosity. 

This partial-slip condition, over a proportionately large surface area felt by the flow passing over the sharks skin, inhibits flow reversal which would otherwise lead to separation. Thus, the overall effect for the shark would be a reduction in pressure drag. 

If the boundary layer is laminar, fluid particles move parallel to the dashed line in Fig. 6.1. There is no opportunity to replace energy converted to heat within the boundary layer by energy transport from the free stream. However, when the boundary layer is turbulent, there are transverse velocity components along the dashed line of Fig. 6.1. This enables the energy going into heat to be replenished from the main stream which, in turn, delays flow separation. The pressure drag will, thus, be less for a turbulent boundary layer than for a laminar one. 

An example of the first paradox is evident in Fig. 6.5 at a Reynolds Number of about 2 x 10. At this particular Reynolds Number, the boundary layer becomes turbulent which, in turn, postpones flow separation, and greatly reduces the pressure drag with little change in viscous drag. 

For the flow of a viscous fluid past the cylinder, the pressure decreases from A to B and from A to C so that the boundary layer is thin and the flow is similar to that obtained with a non-viscous fluid. From B to D and from C to D the pressure is rising and therefore the boundary layer rapidly thickens with the result that it tends to separate from the surface. If separation occurs, eddies are formed in the wake of the cylinder and energy is thereby dissipated and an additional force, known as form drag, is set up. In this way, on the forward surface of the cylinder, the pressure distribution is similar to that obtained with the ideal fluid of zero viscosity, although on the rear surface, the boundary layer is thickening rapidly and pressure variations are very different in the two cases. 

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