Supercomputing Institute Research Bulletin Summer 1997

Turbulent Flow and its Effect on Hypersonic Vehicles

Investigation of turbulent flow and its effect on hypersonic vehicles, such as future reusable launch vehicles, benefits from the use of supercomputers. As a hypersonic vehicle flies through the atmosphere, the thin layer of gas near the vehicle’s surface transitions from smooth, laminar flow to chaotic, turbulent flow. The point at which this transition occurs is known as the “transition location.” When this layer becomes turbulent, the aerodynamic drag increases–but more importantly, surface heating increases significantly. Thus, an accurate prediction of the point at which the flow transitions to turbulence is necessary for optimum design of the vehicle’s thermal protection system. In the past, experimental evidence was used to predict the transition location, but these measurements are usually made in wind tunnels that cannot reproduce actual flight conditions. Therefore, it is necessary to use very conservative transition estimates to design the vehicle, resulting in excessively heavy heat shields. With the advent of multiple-use vehicles, like single-stage-to-orbit vehicles and hypersonic cruise vehicles, weight limitations are increasingly important and require more accurate predictions of the transition location. Ultimately, the vehicle geometry should be optimized to delay or even eliminate transition entirely, thus significantly reducing vehicle weight. Aerospace Engineering and Mechanics Associate Professor Graham Candler and members of his research group have developed computational tools to determine the point at which a hypersonic flow transitions to turbulence. There are two aspects to this analysis. First, the mean flow is computed by numerically solving the partial differential equations that describe the conservation of mass, momentum, and energy within a flow field. Additionally, since the flow is chemically reacting, equations are solved that describe the rate of reaction of air. In this case, five chemical species are included (molecular nitrogen and oxygen, and the reaction products nitric oxide and atomic nitrogen and oxygen). The second part of the analysis uses linear stability theory to determine how small amplitude disturbances are amplified or damped as they pass through the computed flow field. The effects of the finite-rate chemical reactions are included in this portion of the analysis as well. These models provide an accurate description of the hypersonic flow field and can be used to predict the point at which the boundary layer becomes turbulent. The computational methods have been used to simulate some recent experiments in a high-energy shock tunnel at the California Institute of Technology. The tunnel is capable of simulating important aspects of flight conditions, including the chemical reactions that occur in the flow field. The experiments indicate that increasing flight speeds, and therefore increasing rates of chemical reactions, stabilize the boundary layer and delay transition. If this effect can be verified, it would have important implications in the design of new hypersonic vehicles because the thermal protection system weight could be reduced. The computations reproduce this effect, although the predicted transition locations are consistently larger than the experimental data. This is likely a result of the noisy flow environment in the wind tunnel. The figure illustrates the effect that the flow field chemical reactions have on the stability of the boundary layer. If the chemical reactions are not included in the stability analysis, i.e., if the flow is treated as nonreacting, the amplification rate is about 150/m. If the reactions absorb energy, i.e., if they are treated as endothermic, as they are in a realistic air boundary layer, the amplification rate is reduced significantly and the boundary layer is stabilized. Finally, if the reactions release energy, i.e., if they are exothermic, the amplification rate is increased. Therefore, endothermic reactions absorb the fluctuation energy and increase the boundary layer stability. Further work is under way to fully understand this interaction so that it can be used to reduce the thermal protection requirements of future hypersonic vehicles. Figure 1 Amplification rate as a function of disturbance frequency in a hypersonic boundary layer. Nonreacting indicates that the effects of chemical reactions are not included in the analysis; endothermic includes the realistic air reactions that absorb energy; exothermic uses chemical reactions that release energy. Increased amplification rate implies a more unstable boundary layer that transitions to turbulence more easily.

In This Issue: 1997 Summer Undergraduate Intern Program Relational Drug Design Workshop Rayleigh-Taylor Instability How Alumina Phases Impact the Ruby Scale > Turbulent Flow and Hypersonic Vehicles Origin 2000 Arrives • BORDER="0"> Seminar Synopses Research Reports