Orthogonality in the Boundary Layer

To investigate the influence of the inclination of the grid in the boundary layer the reference grid (y0 = 1e – 6, y+ = 0.08, ratio = 1.1) were inclined to achieve angle variations of 0deg to 70deg in 10deg steps. Angles against the flow direction were tested only between – 10deg and -30deg. Here the results show the same influence as the angles in the flow direction. Hence the independence of the angle direction is assumed.

The variation of the inclination in the boundary layer shows with the one – equation models (SAO and SAE) a massive influence in the results (fig. 2 & 3). Compared with the results of the reference grid the friction coefficients differ by more than 100% at an inclination of 60deg (fig. 5). The same behavior is shown in the drag coefficient cD. This influence is however the result of inaccuracies arising from the computation of the wall-normal distance d (fig. 4), which is reflected most strongly in the results of the models with the strongest dependency on d. A lower er­ror occurs with the models SST and Hellsten EARSM and the error is negliable for the remaining models. With increasing inclination the calculated wall distance dif­fers from the correct values. This leads to the demonstrated errors with all turbulence models that depend on the wall distance. For this simple geometry, the analytically correct value of d could easily be determined. Replacing the computed d with the correct value gave results with negliable sensitvity to the grid inclination.