LAMINAR-TURBULENT TRANSITION IN THE BOUNDARY LAYER

The boundary layer can be either laminar or turbulent. At the forward stagnation point, it will always be laminar. However, as energy is removed in the form of heat, the boundary layer thickens, which, in turn, gives rise to an increase in the mean Reynolds Number for the boundary layer. Upon reaching a critical value of Reynolds Number, the boundary layer becomes turbulent. 

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. 

While the viscous drag is greater with a turbulent boundary layer, the pressure drag will be less. At certain values of Reynolds Number, the decrease in pressure drag due to boundary layer turbulence far exceeds the increase in friction drag. In such cases, the total drag decreases with an increase in turbulence in the boundary layer. 

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