High-fidelity simulations are performed to characterize the turbulence-induced wall pressure fluctuations on a sharp cone at a 5.5-degree angle-of-attack in a Mach 8 flow. Wall-resolved large-eddy simulation (LES) and wall-modeled large-eddy simulation (WMLES) results are compared to measurements at several locations on the cone body. Simulations are also compared to each other, and WMLES show good comparison in the autospectra, but modest comparison in the coherence.
During hypersonic flight, compressional and viscous heating of the air can form a plasma layer which encases the aircraft. If the boundary layer becomes turbulent, then the electron density fluctuations can effect a parasitic modulation in microwave signals transmitted through the plasma. We developed an approach for studying the interaction of microwave signals with a turbulent, hypersonic plasma layer. The approach affords a great deal of flexibility in both the plasma layer model and the antenna configuration. We then analyzed a situation in which microwaves, transmitted from a rectangular aperture antenna, propagate through a turbulent plasma layer to a distant receiver. We characterized the first- and second-order statistics of the computed parasitic modulation and quantified the depolarization of the signal. The amplitude fluctuations are lognormally distributed at low frequencies and Rice-distributed at high frequencies. Fluctuations in the copolarized phase and amplitude of the far-field signal are strongly anticorrelated. We used a multioutput Gaussian process (MOGP) to model these quantities. The efficacy of the MOGP model is demonstrated by recovering the time evolution of the copolarized phase given the copolarized amplitude and occasional measurements of the phase.
The size of a pressure transducer is known to affect the accuracy of measurements of wall-pressure fluctuations beneath a turbulent boundary layer because of spatial averaging over the sensing area of the transducer. In this paper, the effect of finite transducer size is investigated by applying spatial averaging or wavenumber filters to a database of hypersonic wall pressure generated from a direct numerical simulation (DNS) that simulates the turbulent portion of the boundary layer over a sharp 7° half-angle cone at nominally Mach 8. Here, a good comparison between the DNS and the experiment in the Sandia Hypersonic Wind Tunnel at Mach 8 is achieved after spatial averaging is applied to the DNS data over an area similar to the sensing area of the transducer. The study shows that a finite sensor size similar to that of the PCB132 transducer can cause significant attenuation in the root-mean-square and power spectral density (PSD) of wall-pressure fluctuations, and the attenuation effect is identical between cone and flat plate configurations at the same friction Reynolds number. The Corcos theory is found to successfully compensate for the attenuated high-frequency components of the wall-pressure PSD.
Direct numerical simulations (DNS) were conducted of a high-velocity flat plate boundary layer with time-periodic fluctuating inflow. The DNS fluctuation growth and evolution over the plate is then compared to the solution as computed using classical linear stability theory (LST) and the parabolized stability equations (PSE) of a second mode eigen function. The decay rate of the free stream perturbations is also compared to LST and the choice of shock-capturing method and the associated dissipation rate is characterized. The agreement observed between the eigen function from LST and the fundamental harmonic of the temporal Fourier transform (FT) of the DNS simulation demonstrates the ability of the solver to capture the initiation and linear growth of a hypersonic boundary layer instability. The work characterizes the shock-capturing numerical dissipation for slow and second mode growth as well as provides confidence in the numerical solver to study further development towards non-linear growth and eventual transition to turbulence.
A wind tunnel test from AEDC Tunnel 9 of a hypersonic turbulent boundary layer is analyzed using several fidelities of numerical simulation including Wall-Modeled Large Eddy Simulation (WMLES), Large Eddy Simulation (LES), and Direct Numerical Simulation (DNS). The DNS was forced to transition to turbulence using a broad spectrum of planar, slow acoustic waves based on the freestream spectrum measured in the tunnel. Results show the flow transitions in a reasonably natural process developing into turbulent flow. This is due to several 2nd mode wave packets advecting downstream and eventually breaking down into turbulence with modest friction Reynolds numbers. The surface shear stress and heat flux agree well with a transitional RANS simulation. Comparisons of DNS data to experimental data showreasonable agreement with regard to mean surface quantities aswell as amplitudes of boundary layer disturbances. The DNS does show early transition relative to the experimental data. Several interesting aspects of the DNS and other numerical simulations are discussed. The DNS data are also analyzed through several common methods such as cross-correlations and coherence of the fluctuating surface pressure.
Here we consider the shock stand-off distance for blunt forebodies using a simplified differential-based approach with extensions for high enthalpy dissociative chemistry effects. Following Rasmussen [4], self-similar differential equations valid for spherical and cylindrical geometries that are modified to focus on the shock curvature induced vorticity in the immediate region of the shock are solved to provide a calorically perfect estimate for shock standoff distance that yields good agreement with classical theory. While useful as a limiting case, strong shock (high enthalpy) calorically perfect results required modification to include the effects of dissociative thermo-chemistry. Using a dissociative ideal gas model for dissociative equilibrium behavior combined with shock Hugoniot constraints we solve to provide thermodynamic modifications to the shock density jump thereby sensitizing the simpler result for high enthalpy effects. The resulting estimates are then compared to high enthalpy stand-off data from literature, recent dedicated high speed shock tunnel measurements and multi-temperature partitioned implementation CFD data sets. Generally, the theoretical results derived here compared well with these data sources, suggesting that the current formulation provides an approximate but useful estimate for shock stand-off distance.
High-enthalpy hypersonic flight represents an application space of significant concern within the current national-security landscape. The hypersonic environment is characterized by high-speed compressible fluid mechanics and complex reacting flow physics, which may present both thermal and chemical nonequilibrium effects. We report on the results of a three-year LDRD effort, funded by the Engineering Sciences Research Foundation (ESRF) investment area, which has been focused on the development and deployment of new high-speed thermochemical diagnostics capabilities for measurements in the high-enthalpy hypersonic environment posed by Sandia's free-piston shock tunnel. The project has additionally sponsored model development efforts, which have added thermal nonequilibrium modeling capabilities to Sandia codes for subsequent design of many of our shock-tunnel experiments. We have cultivated high-speed, chemically specific, laser-diagnostic approaches that are uniquely co-located with Sandia's high-enthalpy hypersonic test facilities. These tools include picosecond and nanosecond coherent anti-Stokes Raman scattering at 100-kHz rates for time-resolved thermometry, including thermal nonequilibrium conditions, and 100-kHz planar laser-induced fluorescence of nitric oxide for chemically specific imaging and velocimetry. Key results from this LDRD project have been documented in a number of journal submissions and conference proceedings, which are cited here. The body of this report is, therefore, concise and summarizes the key results of the project. The reader is directed toward these reference materials and appendices for more detailed discussions of the project results and findings.
The Multi-Fidelity Toolkit (MFTK) is a simulation tool being developed at Sandia National Laboratories for aerodynamic predictions of compressible flows over a range of physics fidelities and computational speeds. These models include the Reynolds-Averaged-Navier-Stokes (RANS) equations, the Euler equations, and modified Newtonian aerodynamics (MNA) equations, and they can be invoked independently or coupled with hierarchical Kriging to interpolate between high-fidelity simulations using lower-fidelity data. However, as with any new simulation capability, verification and validation are necessary to gather credibility evidence. This work describes formal code- and solution-verification activities as well as model validation with uncertainty considerations. Code verification is performed on the MNA model by comparing with an analytical solution for flat-plate and inclined-plate geometries. Solution-verification activities include grid-refinement studies of HIFiRE-1 wind tunnel measurements, which are used for validation, for all model fidelities. A thorough treatment of the validation comparison with prediction error and validation uncertainty is also presented.
The Multi-Fidelity Toolkit (MFTK) is a simulation tool being developed at Sandia National Laboratories for aerodynamic predictions of compressible flows over a range of physics fidelities and computational speeds. These models include the Reynolds-Averaged Navier–Stokes (RANS) equations, the Euler equations, and modified Newtonian aerodynamics (MNA) equations, and they can be invoked independently or coupled with hierarchical Kriging to interpolate between high-fidelity simulations using lower-fidelity data. However, as with any new simulation capability, verification and validation are necessary to gather credibility evidence. This work describes formal code-and solution-verification activities. Code verification is performed on the MNA model by comparing with an analytical solution for flat-plate and inclined-plate geometries. Solution-verification activities include grid-refinement studies of HIFiRE-1 wind tunnel measurements, which are used for validation, for all model fidelities.
The Multi-Fidelity Toolkit (MFTK) is a simulation tool being developed at Sandia National Laboratories for aerodynamic predictions of compressible flows over a range of physics fidelities and computational speeds. These models include the Reynolds-Averaged Navier–Stokes (RANS) equations, the Euler equations, and modified Newtonian aerodynamics (MNA) equations, and they can be invoked independently or coupled with hierarchical Kriging to interpolate between high-fidelity simulations using lower-fidelity data. However, as with any new simulation capability, verification and validation are necessary to gather credibility evidence. This work describes formal model validation with uncertainty considerations that leverages experimental data from the HIFiRE-1 wind tunnel tests. The geometry is a multi-conic shape that produces complex flow phenomena under hypersonic conditions. A thorough treatment of the validation comparison with prediction error and validation uncertainty is also presented.
Four Direct Numerical Simulation (DNS) datasets covering effective freestream Mach numbers of 8 through 14 are used to investigate the behavior of turbulence-induced aero-optical distortions in hypersonic boundary layers. The datasets include two from simulations of flat plate boundary layers (Mach 8 and 14) and two from simulations of flow over a sharp cone (Mach 8 and 14). Instantaneous three-dimensional fields of density from each DNS are converted to refraction index and integrated to produce distributions of the Optical Path Differences (OPD) caused by turbulence. These values are then compared to experimental data from the literature and to an existing model for the root-mean-square of the OPD. Although the model was originally developed for flows with Mach ≤ 5, it provides a basis to which we compare the hypersonic data.
Quantifying gas-surface interactions for hypersonic reentry applications remains a challenging and complex problem where credible models are needed to design and analyze thermal protection systems. A flexible sensitivity analysis approach is demonstrated to analyze finite-rate ablation models to identify reaction parameters and mechanisms of influence on predicted quantities of interest. Simulations of hypersonic flow over a sphere-cone are presented using parameterized Park, Zhluktov and Abe (ZA), and MURI finite-rate models that describe the oxidation and sublimation of carbon. The results presented in this study emphasize the importance of characterizing model inputs that are shown to have a high impact on predicted quantities and build evidence to assess credibility of these models.