The regions with shear stress and mean velocity gradient of opposite sign often existin complex turbulent shear flows.In these cases,the eddy viscosity hypothesis breaksdown.Hinze regards the,departure from eddy viscosity hypothesis as a result from transportationof mean momentum over distance by the large structures and arrives at a shearstress expression including the second order derivatives of the mean velocity.However,hisexpression greatly overestimates the shear stress.This implies that the flow particles areunlikely to have enough memory of the mean momentum over distance.By assuming thedeparture from eddy viscosity hypothesis as a result from transportation of the shear stresscontained in smaller eddies over distance by the large structures,the present author hasarrived at a new shear stress expression.The shear stress estimated so far is in goodagreement with the experiments.
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