„In fact, the assumption is far from trivial. For instance, when turbulence is generated in flow past an obstacle (e.g. one bar in a grid), one major effect of the flow is to create a surface of tangential discontinuity which starts from the obstacle and floats down stream. This surface is intrinsically unstable – most of the turbulence results from its expansion and contortion. It is not sure that the state obtained downstream (even if it is made approximately homogeneous and isotropic by suitable means) will be microscopically similar to what we have for instance in a convection cell, where the fluid is made turbulent by heating from below.“ (p. 11) #Gennes #flow #turbulence