On Blockage Effects in Wind Tunnels – A CFD Study 2001-01-0705
Validation studies often contains uncertainties due to boundary condition differences between computational fluid dynamics, CFD-, and experimental fluid dynamics, EFD-, in the setup for the cases involved.
The present study is in two parts where the first part is a 2D parametric study of two geometrical parameters and their variation with blockage. The parameters were radius in the transition region from the windscreen to the roof and the angle of attack for the windscreen. A mesh blending technology has been used where an interpolation between two meshes is conducted on a vertices level for the creation of a mix between the two grids. Many CFD simulations have been done of which some are presented.
The windscreen angle was more sensitive to blockage in the sense that trends in drag from two configurations may be erroneously predicted if large blockage is encountered. The front radius seems to be not so much sensitive in that respect and trends in drag and especially absolute drag levels changes only at very large blockage ratios.
A separate study of blockage correction factors in the Volvo wind tunnel, hereafter-denoted PVT after its Swedish name, is ongoing, and for this study CFD simulations will be used for investigation of blockage effects for a full-scale car in this wind tunnel. The initial stage of this work which is more or less being able to simulate a slotted wall wind tunnel are presented. Comparisons are done with a car in a free stream, e.g. a very large solid wall wind tunnel.