Hello,

Does anyone know what is the difference between a Shear Layer and a Mixing Layer?

Thanks, MAZ

They're the same thing in general ( a mixing layer is also a shear layer) - although you could argue that the converse is not true; i.e. not all shear layers are mixing layers. This is however just semantics.

Thanks for your reply. Why the converse is not true?

Actually I have seen in couple of strong fluid dynamic papers that they distinguish between mixing layer and shear layer for example they say: "this flow behaves more like a shear layer than a mixing layer" but they do not explain the difference. I though may be to be more specific a mixing layer is created of the interaction of two different fluids that both have a non-zero velocity and probably different densities, while the shear layer is created when one is quasi-still like a the interaction of a flow past the quasi-still flow inside an open cavity. But in general I believe that both are pretty much the same.

In general a mixing layer is what is commonly called a "free shear layer". The mixing aspect arises because of the Kelvin-Helmholtz instability which causes it to wrap up into the so-called cats-eyes structures.

The reason the converse is not generally the case is that boundary layers (especially with separation) are layers with strong shear and can be called (and in the case with separation are) shear layers; i.e. there are significant boundary effects which are not present in the free shear layer case.

Got it. Thanks Tom!

[afroz_javed]

when two parallel streams of fluids meet at an interface with a velocity difference a shear layer is formed. By defination a shear layer is a layer of flow where a shear or velocity gradient exists. Going by this, boundary layer is also a form of shear layer. But in boundary layer the momentum transport is affected by a solid interface and viscous forces are dominant upto some distance. In case of a shear layer forming at a parallel fluid flow interface the viscous forces are not dominant and the flow is dominated by turbulent transport of momentum. This layer is sometimes called mixing layer also. But some authors call this fluid interface shear layer as mixing layer when the mixing of two species is involved, e.g., mixing of two parallel streams of two different gases at different velocities.

[Avr.Tomer]

I have what I find as quite a good heuristic explanation about the different free shear flow in:
https://cfdisraelblog.wordpress.com/...w-of-the-wall/