Indicators for the evaluation of wind tunnel test section flow quality and application to a numerical closed-circuit wind tunnel

The flow in a wind tunnel test section must meet high standards to obtain accurate and reliable measurement data. Good flow quality demands a certain degree of spatial uniformity and temporal steadiness of velocity and pressure. In this paper, a set of six new indices is developed and presented that relate spatial aspects of the mean velocity field to flow quality. One index quantifies the degree of uniformity of the velocity field and can be used directly as a flow quality indicator. The five other indices are related to different types of deviations from spatially uniform flow; skewed flow and angularity (up-flow and down-flow, swirl, cross-flow, diverging and converging flow). The indices can be used to evaluate the flow quality in existing tunnels, to assess the impact of design modifications, as well as to carry out CFD-based design of new wind tunnels. As an example, the indices are applied to assess the impact of guide vanes and screens on the test section flow quality in a numerical closed-circuit wind tunnel.