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 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.
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 eigenfunction. The agreement observed between the eigenfunction 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 enables the study of the perturbation’s evolution in the boundary layer as well as provides confidence in the numerical solver to study further development towards non-linear growth and eventual transition to turbulence.
Direct numerical simulations (DNS) were conducted to characterize the pressure fluctuations under the turbulent portion of the boundary layer over a sharp 7◦ half-angle cone at a nominal freestream Mach number of 8 and a unit Reynolds number of Reunit = 13.4 × 106/m. The axisymmetric cone geometry and the flow conditions of the DNS matched those measured in the Sandia Hypersonic Wind Tunnel at Mach 8 (Sandia HWT-8). The DNS-predicted wall pressure statistics, including the root-mean-square (r.m.s.) fluctuations and the power spectral density (PSD), were compared with those measured in the Sandia HWT-8. A good comparison between the DNS and the experiment was shown for the r.m.s. and PSD of wall-pressure fluctuations after spatial averaging was applied to the DNS data over an area similar to the sensing area of the transducer. The finite size of the PCB132 transducer, with a finite sensing area of d+ ≈ 50, caused significant spectral attenuation at high frequencies in the experimentally measured PSD, and the loss in sensor resolution resulted in an approximately 27% reduction in r.m.s. pressure fluctuations. The attenuation due to finite sensor sizes has only a small influence on wall-pressure coherence, as indicated by the good comparisons between the DNS without spatial filtering and the experiment for transducers with either streamwise or spanwise separations. The characteristics of turbulent pressure fluctuations at the cone surface were also compared with those over a flat plate and at the wind-tunnel nozzle wall to assess the effect of flow configurations on the scaling relations of turbulent pressure fluctuations. The inner scale was found to successfully collapse wall-pressure PSD of the cone with those over a nozzle wall and on a flat plate at a similar freestream Mach number. For all the three flow configurations, the Corcos model was found to deliver good predictions of wall pressure coherence over intermediate and high frequencies, and the Corcos parameters for the streamwise and spanwise coherence at Mach 8 were found to be similar to those reported in the literature at lower supersonic Mach numbers.
[Abstract] To support reduced order modeling of heat transfer for reentry bodies we develop an approximate solution method is identified that provides good estimates for the local wall derivative (and thereby the skin friction and Nusselt numbers) for a wide range of self-similar laminar formulations. These formulations include: Blasius flow, axisymmetric and planar stagnation flows and the Faulkner-Skan flows. The approach utilized is simply an extension of the classical Weyl formulation for the Blasius equation. Using this solution form estimates that naturally represent combined flow behaviors are represented without post-solution interpolation. An important example, namely axisymmetric stagnation equally combined with laminar zero pressure gradient (flat plate) flow, shows a difference of 10% between the pre-solution combination developed here and s simple post-solution arithmetic average. Clearly, the nonlinearity inherent to these solutions prevails in terms of these simple solutions. Compressible extensions to the basic incompressible result are achieved by including a near wall Chapman-Rubesin term making these solutions suitable for adiabatic wall problems. Direct comparison of the wall gradient estimation procedure developed here demonstrates excellent agreement with empirically fit blunt body heat transfer models such as the asymptotically consistent model of Kemp et. al. which are deemed more appropriate than the classical stagnation point scaling approaches.
This report documents the initial testing of the Sandia Parallel Aerodynamics and Reentry Code (SPARC) to directly simulate hypersonic, turbulent boundary layer flow over a sharp 7- degree half-angle cone. This type of computation involves a tremendously large range of scales both in time and space, requiring a large number of grid cells and the efficient utilization of a large pool of resources. The goal of the simulation is to mimic and verify a wind tunnel experiment that seeks to measure the turbulent surface pressure fluctuations. These data are necessary for building a model to predict random vibration loading in the reentry flight environment. A low-dissipation flux scheme in SPARC is used on a 2.7 billion cell mesh to capture the turbulent fluctuations in the boundary layer flow. The grid is divided into 115200 partitions and simulated using the Knight's Landings (KNL) partition of the Trinity system. The parallel performance of SPARC is explored on the Trinity system, as well as some of the other new architectures. Extracting data from the simulation shows good agreement with the experiment as well as a colleague's simulation. The data provide a guide for which a new model can be built for better prediction of the reentry random vibration loads.
Wartemann, Viola; Wagner, Alexander; Wagnild, Ross M.; Pinna, Fabio; Miró, Fernando M.; Tanno, Hideyuki; Johnson, Heath
In the present study, three boundary-layer stability codes are compared based on hypersonic high-enthalpy boundary-layer flows around a blunted 7 deg half-angle cone. The code-to-code comparison is conducted between the following codes: the Nonlocal Transition analysis code of the DLR, German Aerospace Center (DLR); the Stability and Transition Analysis for hypersonic Boundary Layers code of VirtusAero LLC; and the VKI Extensible Stability and Transition Analysis code of the von Kármán Institute for Fluid Dynamics. The comparison focuses on the role of real-gas effects on the second-mode instability, in particular the disturbance frequency, and deals with the question on how far not accounting for real-gas effects compromises the stability analysis. The experimental test cases for the comparison are provided by the DLR High Enthalpy Shock Tunnel Göttingen and the Japan Aerospace Exploration Agency High Enthalpy Shock Tunnel. The focus of the comparison between the stability results and the measurements is, besides real-gas effects, the influence of uncertainties in the mean flow on the stability analysis.