Hi, In mixing length model eddy viscosity is propotional to mean fluctuating velocity, but mean fluctuating velocity is always zero. So every time eddy viscosity is zero but it is not the real case.I am not right but what is correct plz tell me thanks

This seems to be a case of confusion caused by imprecise use of jargon. The mean fluctuating velocity is zero, but what goes into the mixing length model is some measure of the magnitude of the fluctuations, such as the root-mean-square average.

what do you mean by "caused by imprecise use of jargon" can you tell me what is the mixing length, characteristic length and characteristic velocity