Wake Related Wind Tunnel Corrections for Closed Wall Test Sections 2006-01-0567

In closed wall test sections the total correction to the measured drag usually consists of several parts: solid blockage corrections related to the displacement of the model, horizontal buoyancy corrections due to empty tunnel gradients and the wake blockage corrections, which are necessary to handle effects created by the displacement effect of the wake. The latter will be investigated in more detail in the paper.

The wake blockage correction usually consists of two parts: a correction to the measured dynamic pressure (q-correction) and a gradient correction, the so-called wake induced drag increment. Both corrections are directly dependent on the source strength which is equivalent to the displacement effect of the wake. Therefore the displacement of the wake is analyzed in more detail. It will be shown by using analytical as well as CFD results that the usual assumption of a displacement thickness equal to the momentum thickness introduces a significant error which results in severe undercorrection in high blockage conditions. This error affects both q-correction and wake induced drag increment, if the wake displacement is not determined from measured wall signatures (pressure, velocity).

It will be shown that the displacement effect of the wake is typically not constant but is dependent on the stream-wise distance from the model. The increase in displacement will be calculated for a number of cases, including streamlined and bluff bodies as well as laminar and turbulent flow in the wake. In order to keep an easily usable approximation for the wake displacement a proposal for an enhancement factor to the momentum thickness (which in the classical approach is used as approximation to the displacement) will be made.

Finally the total correction is tested using CFD results. Using the detailed flow information available from the numerical solutions the total correction is split up into the individual contributions of solid blockage, wake blockage and wake induced drag increment. It is found that the solid blockage correction and the wake induced drag increment are approximately correct, whereas the wake blockage contribution is still significantly too small. Possible reasons for this are discussed.