I am struggling to understand what characteristic scales mean in the context of thin shear layer equations. I am referring mostly to the following paragraph from the book "Turbulent Shear Layers in Supersonic Flow" by Smits and Dussauge (2nd edition):
Thin shear layers are flows where the characteristic scale in the cross-stream direction is much smaller than the characteristic scale in the streamwise direction. As a consequence, derivatives of mean quantities taken in the direction across the flow are always much larger than similar derivatives taken in the freestream direction. Typical examples include mixing layers, jets, wakes and boundary layers where the pressure gradients are not too large. For this class of flows, we can derive a set of approximate equations that are useful for the understanding of compressible turbulent shear layers.
Does characteristic scale here refer to a some kind of order of magnitude analysis? Thank you very much!
