The Plenum Method Versus Blockage Corrected Nozzle Method for Determining Climatic Wind Tunnel Air Speed 2004-01-0668

Recently, computational fluid dynamics (CFD) was applied to investigate blockage (or velocity) corrections using the nozzle method for a climatic wind tunnel (CWT) test environment (SAE 2003-01-0936). The study included two blockage corrections to the nozzle method reference velocity: vehicle frontal velocity and vehicle upper surface pressure trace. These methods resulted in well correlated predictions between the open road and CWT flow conditions.

These CFD predicted blockage corrections are experimentally verified in a climatic wind tunnel in this study. A non-intrusive method applying particle image velocimetry is applied to acquire the velocity field in front of the test vehicle. The experimental data verifies the blockage correction predictions derived from the previous CFD work. Furthermore, the verification experiment also confirms the blockage non-uniformity around the test vehicle, which emphasizes the importance of customizing blockage correction for different test objectives.

The same CFD solutions and an additional CFD simulation for the same climatic wind tunnel with the empty test section were applied to explore the plenum method in the simulated climatic wind tunnel. The same local velocity curve fit procedure used to define blockage correction factors (V_actual/V_ΔP) in the previous nozzle method study was applied using the pressure differentials from the tunnel stilling chamber to plenum locations to derive V_ΔP. The study showed that the climatic wind tunnel plenum method is able to achieve blockage correction values close to 1.0 for both frontal velocity and surface pressure simulation cases when vehicle blockage was 60% and below. When the optimum static pressure location was selected, the blockage correction values differed from 1.0 by -2.8% to +3.1%. For the range of plenum static pressure locations investigated, this range increased to -4.7% to 5.6%. No consistent correlation curve was found that allowed these blockage corrections to be applied in a universal manner as was found in the previous nozzle method study. In the absence of such a correlation, use of the nozzle method with the unified nozzle blockage correction correlations from SAE 2003-01-0936 are considered a more accurate method for obtaining the correct effective flow velocity for climatic wind tunnel testing than using the plenum method.

Frontal velocity blockage corrections for a sedan with a 76% blockage ratio positioned close to the nozzle exit did not fall into the pattern of the 60% and below blockage cases. This was also true for the previous nozzle method study³. The differences between frontal velocity blockage correction factor and surface pressure blockage correction factor for this case were large for both nozzle and plenum methods. These results suggest that a minimum test D_WB greater than 1.65m be developed with consistency with lower blockage results as a selection criterion.