Reacting Hypersonic Boundary Layer Stability with Blowing and Suction

Abstract

The stability of hypersonic boundary layers over flat plates and cones is analyzed using CFD simulations of the mean flows and linear stability analysis of the boundary layer profiles. Blowing and suction effects on disturbance amplification rates and predicted transition locations are investigated. In the case of blowing gas into the flow over a cone, it is found that the computational results match the trends observed in the experiments of increasing instability with increasing blowing rate. Next for the case of blowing, both reacting and non-reacting gases are considered, and the possible stabilizing effect of chemical reactions is investigated. Stability analyses are performed for small rates of CO2 injection into a low-enthalpy air flow, and the results show only slight disturbance amplification. Further analysis is required to see if CO2 injection into flows at higher enthalpies can have a more significant effect. I. Introduction Understanding the factors that affect boundary layer stability and transition to turbulence is very important for a number of hypersonic vehicle applications. A number of studies have been performed looking at the effects of blowing or suction on boundary layer stability. This can occur in the case of forced mass flow through a porous surface or in the case of an ablative surface material which releases gas into the boundary layer. Schneider 1 performed a review of published open literature experimental results relating to the effect of blowing on boundary layer stability, with a focus on experimental data which may be suitable for comparison with the results of calculations. The results were summarized in the conclusions that blowing generally moves transition upstream with higher mass flow rates, lighter injection gases, and injection occurring farther upstream having greater effects. Using computational fluid dynamics and linear stability analysis, we attempted to reproduce some of the results of the experiments which were reviewed. This effort was hindered in many cases by missing details of the model geometries or experimental conditions. One series of experiments by Pappas and Okuno 2 contained a sufficient amount of detail for us to attempt a computational analysis with a few assumptions, and the results of that analysis will be presented. We also have begun to investigate the effects of blowing or suction in high-enthalpy flows where, in the case of blowing, the injected gas will not only mix with the free-steam gas, but may also chemically react, and some preliminary results from that work will be presented. For all our simulations, we use the STABL 3 suite of tools. The laminar mean flow solutions for this analysis are generated using an optimized 2D/axisymmetric CFD solver based on the implicit Data-Parallel Line Relaxation (DPLR) method. 4 The DPLR CFD solver and the stability analysis code, PSE-Chem, have been extensively tested and validated for linear stability theory (LST) and parabolized stability equations (PSE) calculations in the analysis of many different problems. 3, 5, 6 The ability to calculate laminar mean flows and boundary layer stability properties for models with suction or gas injection is new to STABL and this capability will first be validated by comparison with other published results.