Determining Blockage Corrections in Climatic Wind Tunnels Using CFD

Computational Fluid Dynamics (CFD) was applied to investigate blockage effects (or velocity correction) in a climatic wind tunnel (CWT) test environment. Different blockage effects in the CWT were modeled using four simplified vehicles that approximated a sedan, an SUV, a pickup truck, and a minivan. Blockage dependence on nozzle size and spacing between the nozzle exit plane (NEP) and the vehicle were also investigated. The study quantified the blockage effect using different correction methods based on vehicle frontal velocity profiles and upper surface pressure traces. The blockage-free solution was also simulated for each vehicle in an ‘open road’ or free air condition.

The CFD study revealed that all the test cases resulted in blockage correction factors, defined by V_actual/V_simulated greater than 1.0. This is a condition in which the uncorrected wind tunnel velocity was higher than the ‘open road’ condition. It was determined that vehicles with higher C_DA_F values induced higher blockage correction factors. The blockage correction factor increased exponentially as the vehicle was positioned closer to the NEP. Small differences in the blockage factor between the frontal velocity profile method and the upper vehicle surface pressure trace method were found. These results led to two unified CWT blockage correlations based on the vehicle frontal velocity profiles and the upper surface pressure traces. Each correlation is valid across a range of vehicle frontal areas, drag coefficients, and separation distances between the nozzle exit plane and the vehicle.