My problem is a turbulent flow over flat plate. i have u-momentum equation and continuity equation. v-momentum equation is neglected by using order of magnitudes. no pressure difference in x-direction because of potential flow. i have defined wall boundary and inflow boundary in x-dir and top flow boundary in y-dir. outflow boundary in x-dir is not defined. i am using finite volume method. at the wall u velocity is calculated at yplus=30 by using loglaw. for log law shear stress at wall is calculated by meot*du/dy where meot is turb viscosity which is calculated by mixing length model. mixing length model is also used for modelling terms in u momentum eqn. The problem is after many iterations the distance from wall calculated from yplus crosses the first grid. please help me.

If I may seek some clarification:

How many equations are you solving: 1 (u) or 2 (u plus something else - y perhaps?)?

Are you fixing the mass flow and iterating for wall stress, vice-versa or something else?

I am not sure I fully understand the last sentence (the problem). Are you fixing the grid line at the wall to y+ = 30 and the next grid line away from the wall has a y+ < 30?