Nearly all high-enthalpy transition experiments take place in shock tunnels or expansion tubes. However, it is well known that transition occurs prematurely in these noisy facilities sometimes via mechanisms that are not relevant to flight. Extrapolation of noisy tunnel results to flight conditions is based on ad hoc tuning of the stability codes but this approach runs the risk of applying the incorrect transition mechanism assumptions to flight-vehicle transition prediction. Accurate knowledge of the appropriate mechanisms requires knowledge of the basic state and disturbance environment but these are difficult to quantify in short duration facilities. The goal of this research task is to provide a better scientific understanding of the instability and transition processes in high enthalpy facilities.
We will develop laser diagnostics for transition studies and then characterize the flow on conincal models in high-enthalpy impulse facilities. First, the molecular tagging velocimetry (MTV) technique under development at TAMU will be further advanced to facilitate characterization of velocity profiles and turbulence in high enthalpy impulse facilities. The technique is based on laser-induced fluorescence of NO. Thus, a useful byproduct of the measurement is the rotatational state of NO, which provides the mean temperature profile. The principles behind the method are described in Hsu et al (2009). More recently, we demonstrated single-shot two-component velocity maps (Fig. 7). Using custom algorithms, we can quantify the 2-D velocity and strain rates with a resolution of 100 μm. This technique will also be used to measure instantaneous velocities in the freestream which, through spatial averaging, will provide the freestream turbulence levels and spatial spectra. Second, we will apply coherent Anti-Stokes Raman spectroscopy to characterize the vibrational state of the major species within freestream.
Collectively, these methods provide the basic state, inflow and freestream boundary conditions. The instrumentation development verification will take place in the TAMU shock tunnel. This facility, operating in the tailored mode, provides stagnation enthalpies from about 0.5 to 5.0 MJ/kg. We also intend to utilize our laser diagnostics to characterize the flow on the same models in the Caltech T5 facility. This facility provides stagnation enthalpies in the range of 2.0 to 20.0 MJ/kg which produces significant non-equilibrium chemistry effects.
Finally, we will characterize stability and transition through DNS and stability calculations anchored to the experimental basic state and boundary conditions. We will perform high fidelity numerical simulations to characterize the growth modes and breakdown processes. The simulations will be anchored to the experiments through the basic state and freestream conditions. The simulation tools will then be used to isolate phenomena to provide improved understandings of the basic physical processes. The outcomes of these tasks will be an improved understanding of how to interpret the stability and transition processes in high-enthalpy facilities and advanced instrumentation for future studies in national and university facilities.