Turbulent shear layer what is it and its aeroacoustic properties

[alchemist]

Hi,

i would just like to clarify some terms used in aerodynamics with you guys here and find out more about these terms at the same time.

Turbulent shear layer: Does this term refer to the first layer of air immediately adjacent to the wall surface over which the air flows? Turbulent eddies here would have a very small length scale as compared to the solid surface? How is this layer different from/related to turbulent boundary layer and turbulent wake?

From my understanding, turbulent wake is simply the turbulence downstream of the airfoil, generated by perhaps vortex shedding at the trailing edge.

Turbulent boundary layer is simply the layer of airflow over the surface where the turbulent shear layer is a part of.

Also, what are the different acoustic characteristics generated by each of these entities?

thank you!

 

[boneh3ad]

Technically speaking, a shear layer doesn't have to be a boundary layer against a wall. A shear layer can be any layer for which there is a shear stress which results in a velocity gradient. Oftentimes boundary layers are referred to as shear layers, so knowing the context of what you are reading would help.

A turbulent wake is as you have described it.

The turbulent boundary layer is a boundary layer whose fluid instabilities have grown to the point where the flow exhibits strong mixing and repeating vortical structures at progressively smaller scales. A turbulent boundary layer is not actually stochastic as it is sometimes treated, but is actually deterministic. There is some strong evidence that it is a both spatially and temporally chaotic system, though it hasn't been proven.

Again, how it differs from a turbulent shear layer depends on the context. A boundary layer is a type of shear layer, but your source may well have been referring to a free-shear layer as well, which is different and can also be turbulent and are typically jets erupting into a stationary (or slower) medium.

 

[NatSusan]

Hi there,

I'm not an expert in aerodynamics, but I'll try my best to answer your questions based on my understanding.

Firstly, yes, the turbulent shear layer refers to the first layer of air immediately adjacent to the wall surface over which the air flows. This layer is characterized by the presence of turbulent eddies, which have a very small length scale compared to the solid surface. This layer is different from the turbulent boundary layer because it is specifically referring to the layer of air closest to the wall, whereas the turbulent boundary layer encompasses a larger area of air flow over the surface.

The turbulent wake, on the other hand, refers to the turbulence that is created downstream of the airfoil, usually as a result of vortex shedding at the trailing edge. This is different from the turbulent shear layer because it is not directly adjacent to the surface, but rather further downstream.

In terms of acoustic characteristics, each of these entities can contribute to noise production in different ways. For example, the turbulent shear layer can create a type of noise known as boundary layer noise, which is caused by the turbulent eddies interacting with the solid surface. The turbulent wake can also contribute to noise, particularly at low frequencies. The turbulent boundary layer can also produce noise, but it is often at a higher frequency range compared to the other two entities.

I hope this helps clarify some of the terms and their relationships. Again, I'm not an expert in this field, so I would recommend doing some further research or consulting with an aerodynamics specialist for more in-depth information. Good luck!