Boundary layers separation

The boundary-layer separation and Reynolds flux analyses were suggested by Kutateladze and Leont ev (1964, 1966), Tong (1968b), and Wallis (1969, 1970). 

Boundary-layer separation and Reynolds flux. Kutateladze and Leont ev (1964, 1966) suggested that the flow boiling crisis can be analyzed using the concept of boundary-layer separation (blowoff) from a permeable flat plate with gas injection (without condensation), as shown in Figure 5.14. Kutateladze and Leont ev (1966) also give the critical condition of boundary layer separation from a flat plate with isothermal injection of the same fluid as... 

Figure 5.14 Kutateladze s concept of boundary-layer separation over a flat plate.

When velocity gradients are small, for example, near the boundary layer separation point and at the rear of a cylinder in separated flow, Eq. (33) is inaccurate. The separation point was determined with an accuracy of 1 degree by using twin strip electrodes of 125 /im length, separated by... 

Figure 8.5 shows a Venturi meter. The theory is the same as for the orifice meter but a much higher proportion of the pressure drop is recoverable than is the case with orifice meters. The gradual approach to and the gradual exit from the orifice substantially eliminates boundary layer separation. Thus, form drag and eddy formation are reduced to a minimum. 

As mentioned, a laminar boundary layer separates a greater distance from the surface of a curved body than a turbulent one. The laminar boundary layer in the upper photograph of Figure 5 is shown separating from the crest... 

Figure 5 Boundary layer separation Turbulent vs. laminar boundary flow close to an airfoil. Source From Ref. 89.

Fig. 5.11 Position of boundary layer separation and laminar/turbulent transition in the critical region and beyond. Experimental results of Achenbach (A3) and Raithby and Eckert (R3).

Equations (5-30) and (5-31) are plotted in Fig. 5.29. In agreement with numerical predictions (BIO, HI, H6), no boundary layer separation is predicted when there are no gradients of surface tension at the surface (H8). 

Figure 1. Boundary layer separation in inlet section of the vertical elutriator...

It will be seen that when the velocity is increasing along the duct the heat transfer rate is higher than when the velocity is constant. Simultaneously, when the velocity is decreasing along the duct the heat transfer rate is lower than when the velocity is constant. In the latter case, i.e., where the velocity is decreasing along the duct, boundary layer separation occurs at approximately X = 0.73 and the calculation is stopped just before separation occurs. 

We must consider the laminar and turbulent portions of the boundary layer separately because the recovery factors, and hence the adiabatic wall temperatures, used to establish the heat flow will be different for each flow regime. It turns out that the difference is rather small in this problem, but we shall follow a procedure which would be used if the difference were appreciable, so that the general method of solution may be indicated. The free-stream acoustic velocity is calculated from... 

Peridier, V.J., Smith, F.T. and Walker, J.D.A. (1991). Vortex-induced boundary-layer separation. Part 1. The unsteady limit problem. Re —> 00. J. Flmd Mech., 232, 99-131. 

The prediction of turbulent boundary-layer separation by MVF methods has not been very successful. Indeed, it may be appropriate to identify turbulent separation in terms of the turbulence near the wall, and this will require use of a more sophisticated model (i ITE or MRS), quite possibly in their full (rather than boundary-layer) form. 

FIGURE 716 Laminar boundary layer separation witli a turbulent wake flow over a circular cylinder at Re = 7.000. 

Flow visualization of flow over (fl) a smooth sphere at Re = 15,000, and (b) a sphere al Re = 30,000 with a trip wire. The delay of boundary layer separation is clearly seen by comparing Ihe two photographs. 

The range of liquid flow rates used to generate the date in Figure 23.10 is similar to the blood flow rates used in clinical practice. Figure 23.10 shows that for Reynolds numbers between 5 and 10, the slope of the friction factor versus Reynolds number curve changes suggesting the onset of boundary layer separation. Boundary layer separation will lead to mixing of the blood and a decrease in the blood side mass-transfer resistance. 

The boundary layer theory has been widely accepted and used to describe the transport phenomena in CVD processes. High-performance CVD systems require designers to focus on the geometrical parameters of the reaction chamber, the orientation and arrangement of the preforms in the chamber, as well as some other important components, such as pipes, distributor, exit and so forth. Due to drag effects around the boundary layer of preforms, it is very important to design the preforms and the reaction chamber and aim to avoid the boundary layer separation such that they experience a minimum drag force. The details of these effects are discussed in Chapter 6. 

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