What role will wind tunnels play in accomplishing the Apex objective of validating the CFD code?
Several organizations may have the capability to perform appropriate wind tunnel experiments to match the Mach and Reynolds numbers expected in flight. However, a basic problem exists with using wind tunnels to predict low Reynolds number, high Mach number airfoil performance. The problem stems from the level of freestream turbulence inherent in wind tunnels and the strong effect of freestream turbulence on boundary-layer dominated flows. Determining the relationship between freestream turbulence and airfoil performance would require the measurement of freestream turbulence in flight. Quantifying this relationship is not an objective of the Apex project. Since this measurement is extremely difficult and not required to satisfy the Apex objectives, no such measurement is currently planned. Therefore, there is no plan to use wind tunnel data to validate the CFD code as a part of the Apex project.
