A turbulent flow can effectively relaminarize. R. Narasimha [76] distinguishes three principle types of relaminarization or reverse transition:

— Reynolds number relaminarization, due to a drop of the local (boundary – layer edge based) Reynolds number.

— Richardson relaminarization, if the flow has to work against buoyancy or curvature forces.

— Acceleration relaminarization, if the boundary-flow is strongly accelerated.

For acceleration relaminarization in two-dimensional flow a criterion is, see [77]

Kcrit = —~ 2 • 10~6. (8.29)

ue dx

The phenomenon of relaminarization can play a role also in the flow past hypersonic flight vehicles. Consider, for instance, the flow around the leading edge towards the leeward side of the Blunt Delta Wing, Section 3.3. The flow accelerates away from the two primary attachment lines towards the leading edges, Fig. 3.16, and is expanding around the latter towards the leeward side of the configuration, Fig. 3.17. This is accompanied by a drop of the unit Reynolds number, see the discussion at the end of Sub-Section 3.3.3. Whether the two effects, single or combined, would be strong enough in this case to actually relaminarize a turbulent flow coming from the windward side of a re-entry vehicle is not known. Also it is not known whether other flow
situations exist in high-speed flight, where relaminarization can play a role, including the phenomenon of relaminarization with subsequent re-transition, see for instance [78].

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