High enthalpy double-wedge experiments

Abstract

A series of experiments studying nitrogen flow over double-wedge geometries has been conducted in the T5 shock tunnel at Caltech. These experiments were designed with computational fluid dynamics to test the nonequilibrium chemistry models used in computational fluid dynamics codes. Surface pressure and heat transfer rate measurements have been made. In addition, holographic interferometry was used to visualize the flow. Analysis of the data shows CFD cannot reproduce of the experimental results. The computed separation zones are smaller to those seen experimentally. The computed pressure peaks on the second wedge are smaller than the measured values. The computed heat transfer values match the experimental data in the separation zone. On the second wedge the computed heat transfer distribution matches the shape and heights of the experimental distribution but is shifted due to the difference in the size of the separation zones. The failure of the CFD to match the experiments is not believed to be due to grid resolution effects, modeling of the viscous terms, turbulence, or flow unsteadiness. While inadequate models for real gas and vibrational non-equilibrium eifects may be responsible for the failure of the CFD, no definite conclusions can be drawn yet. Further work is being done to explain these discrepancies. Introduction One of the biggest uncertainties in simulations of hypersonic flows is how to model reaction rates when the gas is in thermo-chemical nonequilibrium. In particular, the coupling between dissociation and vibrational relaxation is poorly understood in high speed, low density flows typical of re-entry conditions. Many models'' exist for this coupling, but none of them have been adequately validated with experimental data. Copyright ©1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. In an effort to provide this validation data, a set of experiments in the T5 Hypervelocity Shock Tunnel at Caltech has been performed. The T5 shock tunnel provides high enthalpy flows at the appropriate densities for vibrational nonequilibrium effects. The experiments were designed using computational fluid dynamics to provide data sensitive to the choice of vibration-dissociation coupling model used in the computation (For details see Ref. 4). A double-wedge geometry at four angles of attack was tested in a nitrogen freestream. This geometry was chosen because at appropriate enthalpies and values of the binary scaling parameter, pD, the computed flow varies depending on the vibration-dissociation coupling model used. Differences in the computed shock shapes, surface pressure, and surface heat transfer were large enough that the experimental data would be able to distinguish between the models. This sensitivity to vibration-dissociation coupling is explained by considering the shock interaction that occurs near the corner of the wedges. Figure 1 shows a schematic of this flow. The shape of the bow shock depends on the nonequilibrium chemistry occurring behind it. Different vibration-dissociation coupling models produce slightly different bow shock shapes, resulting in different impingement points of the transmitted shock on the second wedge. Small changes in the impingement point and in this shock angle can produce large differences in the size of the separation zone because of the different amounts of mass that are reversed into the separation zone. At relatively low wedge angles where the separation zone size is small, there is no difference between the predictions of the vibration-dissociation coupling models. As the second wedge angle increases, the differences between the predictions of various coupling models become greater.