Hypersonic Turbulent Flow Research Articles

Heat flux measurement at hypersonic turbulent separated flow field induced by the blunt fins

Abstract Hypersonic separated flow induced by the protrusions is one of the most concerning research studies in aerodynamics due to the complex flow structure and the significance of engineering. An experimental study of three-dimensional hyper-sonic separated flow induced by the blunt fins is investigated. The tests are performed with inflow Mach number 6. The main flow properties of the shock systems are described by analyzing Schlieren photographs. The heat flux data are obtained by using high precision platinum film sensors. The effects of the fin sweep angle are also presented. Results show that the maximum heat flux decreases, and the range of the separation zone quickly reduces as the swept angle increases. Low-frequency oscillations of separated flow fields are analyzed through instantaneous heat flux signals at typical measuring points. It is shown that characteristic frequencies of shock wave oscillation with no rear sweep angle is about 1 k Hz.

Modeling of turbulence, chemistry, and heat transfer of a rocket nozzle

Calculating the composition of the combustion products serves as the starting point for the high-area ratio (HAR) rocket nozzle. The HAR rocket nozzle finds application in next-generation hypersonic vehicles and missiles. The gas dynamics, gas radiation, and turbulence-chemistry interactions (TCI) play a crucial role in simulations of hypersonic turbulent reactive flows through a rocket nozzle. The interaction of turbulence and chemistry leads to random species mixing and enhanced heat transfer. The k-omega shear stress transport (SST) model is employed to calculate the average flow fields. The eddy dissipation concept (EDC) with reduced chemistry is applied to capture the TCI in the nozzle. The spectral properties of H2O are calculated using the full spectrum k-distribution (FSK) model in conjunction with the first-order spherical harmonics method. The mass fractions of combustion products obtained from Particle Swarm Optimization (PSO) and Monte Carlo (MC) methods agree well with the Chemical Equilibrium with Applications (CEA) code. The flow fields obtained with the current solver are validated with the published experimental data. A significant rise and fall of 7–8% have been observed in the centerline flow properties when not including the TCI models in the flow solver. The centerline flow fields obtained with different discretization schemes are in good agreement. The radiative heat flux from the H2O species is more dominant than the convective.

A novel attention enhanced deep neural network for hypersonic spatiotemporal turbulence prediction

High Reynolds number turbulent flow of hypersonic vehicles exhibits multi-scale flow structures and non-equilibrium high-frequency characteristics, presenting a significant challenge for accurate prediction. A deep neural network integrated with attention mechanism as a reduced order model for hypersonic turbulent flow is proposed, which is capable of capturing spatiotemporal characteristics from high-dimensional numerical turbulent data directly. The network model leverages encoder–decoder architecture where the encoder captures high-level semantic information of input flow field, Convolutional Long Short-Term Memory network learns low-dimensional characteristic evolution, and the decoder generates pixel-level multi-channel flow field information. Additionally, skip connection structure is introduced at the decoding stage to enhance feature fusion while incorporating Dual-Attention-Block that automatically adjusts weights to capture spatial imbalances in turbulence distribution. Through evaluating the time generalization ability, the neural network effectively learns the evolution of multi-scale high-frequency turbulence characteristics. It enables rapid prediction of high Reynolds number turbulence evolution over time with reasonable accuracy while maintaining excellent computational efficiency.

Sub-filter-scale shear stress analysis in hypersonic turbulent Couette flow

Direct numerical simulations of hypersonic turbulent Couette flows are performed for top-wall Mach numbers of 6, 7 and 8, inspired by non-reactive high-enthalpy wind tunnel free-stream conditions, with the goal of analysing the physical processes driving the sub-filter-scale (SFS) stresses to inform development of large-eddy simulation techniques for hypersonic wall-bounded flows. Semi-local scaling laws collapse mean profiles and second-order turbulent statistics well, in spite of the strong wall-normal gradients of temperature and density. On the other hand, the SFS shear stresses exhibit an unexpected profile characterized by a region of pronounced shear stress deficit, which becomes more pronounced for higher Mach numbers. Instantaneous visualizations suggest that the SFS shear stress deficit is induced by the counter-gradient resolved momentum transport driven by residual velocity motions at the interface between high-density low-speed streaks being ejected away from the wall, and low-density high-speed ones replacing the displaced fluid, qualifying this as a compressibility effect. It is shown that the SFS shear stresses are primarily driven by second-order interactions between residual velocities, in spite of their triply nonlinear nature. This, in turn, motivated a statistical quadrant analysis revealing the presence of a SFS shear stress deficit, that is, SFS processes driving momentum transport of the resolved field towards the top wall. Additionally, higher-Reynolds-number simulations reveal that there is an upper limit of spatial filter widths to resolve large-scale structures, and such deficit is observable at any Reynolds number when a reasonable spatial filter is applied.

Stagnation enthalpy effects on hypersonic turbulent compression corner flow at moderate Reynolds numbers

Stagnation enthalpy effects on hypersonic turbulent compression corner flow at moderate Reynolds numbers

Effect of bulk viscosity on the hypersonic compressible turbulent boundary layer

The impact of bulk viscosity is unclear with considering the increased dilatational dissipation and compressibility effects in hypersonic turbulence flows. In this study, we employ direct numerical simulations to conduct comprehensive analysis of the effect of bulk viscosity on hypersonic turbulent boundary layer flow over a flat plate. The results demonstrate that the scaling relations remain valid even when accounting for large bulk viscosity. However, the wall-normal velocity fluctuations $v_{rms}^{\prime \prime }$ decrease significantly in the viscous sublayer due to the enhanced bulk dilatational dissipation. The intensity of travelling-wave-like alternating positive and negative structures of instantaneous pressure fluctuations $p_{rms}^{\prime }$ in the near-wall region decreases distinctly after considering the bulk viscosity, which is attributed mainly to the reduction of compressible pressure fluctuations $p_{c,rms}^{+}$ . Furthermore, the velocity divergence $\partial u_{i} / \partial x_{i}$ undergoes a significant decrease by bulk viscosity. In short, our results indicate that bulk viscosity can weaken the compressibility of the hypersonic turbulent boundary layer and becomes more significant as the Mach number increases and the wall temperature decreases. Notably, when the bulk-to-shear viscosity ratio of the gas reaches a few hundred levels ( $\mu _b/\mu =O(10^2)$ ), and mechanical behaviour of the near-wall region ( $\kern 0.06em y^+\le 30$ ) is of greater interest, the impact of bulk viscosity on the hypersonic cold-wall turbulent boundary layer may not be negligible.

Numerical Investigation of Wall-Cooling Effect on Aero-Optical Distortions for Hypersonic Boundary Layer

Compressible wall-modeled large-eddy simulations of Mach 8 turbulent boundary-layer flows over a flat plate were carried out for the conditions of the hypersonic wind tunnel at Sandia National Laboratories. The simulations provide new insight into the effect of wall cooling on the aero-optical path distortions for hypersonic turbulent boundary-layer flows. Four different wall-to-recovery temperature ratios, 0.3, 0.48, 0.71, and 0.89, are considered. Despite the much lower grid resolution, the mean velocity, temperature, and resolved Reynolds stress profiles from the simulation for a temperature ratio of 0.48 are in good agreement with those from a reference direct numerical simulation. The normalized root-mean-square optical path difference obtained from the present simulations is compared with that from reference direct numerical simulations, Sandia experiments, as well as predictions obtained with a semi-analytical model by Notre Dame University. The present analysis focuses on the effect of wall cooling on the wall-normal density correlations, on key underlying assumptions of the aforementioned model such as the strong Reynolds analogy, and on the elevation angle effect on the optical path difference. Wall cooling is found to increase the velocity fluctuations and decrease the density fluctuations, resulting in an overall reduction of the normalized optical path distortion. Compared to the simulations, the basic strong Reynolds analogy overpredicts the temperature fluctuations for cooled walls. Also different from the strong Reynolds analogy, the velocity and temperature fluctuations are not perfectly anticorrelated. Finally, as the wall temperature is raised, the density correlation length, away from the wall but inside the boundary layer, increases significantly for beam paths tilted in the downstream direction.

Transpiration-Cooling Heat Transfer Experiments in Laminar and Turbulent Hypersonic Flows

The design of a transpiration-cooled system requires detailed local heat transfer information on and in the vicinity of the porous injector; however, limited spatially resolved experimental studies exist, particularly in hypersonic flows. In this work, experiments were conducted in the University of Oxford’s high-density tunnel at Mach 6.1 in both laminar and turbulent regimes. Spatially resolved two-dimensional surface heat transfer measurements were acquired by imaging directly on and downstream of two microporous transpiration-cooled injectors (METAPOR® CE170 and zirconia) using high-speed infrared thermography. Whereas injection in the laminar regime results in a steady, monotonic reduction in heat transfer from the start of the injector, a flatter profile is present for the turbulent cases where turbulent mixing inhibits surface heat transfer reduction. It was found that a modification to existing relations from film theory successfully correlates the streamwise heat transfer distribution on the injector for different blowing rates of nitrogen and helium. A key result is that helium performs much better than reported in previous experiments. Finally, the downstream thermal effectiveness is characterized for turbulent flows. A collapse of the thermal effectiveness is achieved and a modified analytical correlation proposed.

Calibrating hypersonic turbulence flow models with the HIFiRE-1 experiment using data-driven machine-learned models

In this paper we study the efficacy of combining machine-learning methods with projection-based model reduction techniques for creating data-driven surrogate models of computationally expensive, high-fidelity physics models. Such surrogate models are essential for many-query applications e.g., engineering design optimization and parameter estimation, where it is necessary to invoke the high-fidelity model sequentially, many times. Surrogate models are usually constructed for individual scalar quantities. However there are scenarios where a spatially varying field needs to be modeled as a function of the model’s input parameters. We develop a method to do so, using projections to represent spatial variability while a machine-learned model captures the dependence of the model’s response on the inputs. The method is demonstrated on modeling the heat flux and pressure on the surface of the HIFiRE-1 geometry in a Mach 7.16 turbulent flow. The surrogate model is then used to perform Bayesian estimation of freestream conditions and parameters of the SST (Shear Stress Transport) turbulence model embedded in the high-fidelity (Reynolds-Averaged Navier–Stokes) flow simulator, using shock-tunnel data. The paper provides the first-ever Bayesian calibration of a turbulence model for complex hypersonic turbulent flows. We find that the primary issues in estimating the SST model parameters are the limited information content of the heat flux and pressure measurements and the large model-form error encountered in a certain part of the flow.

Hypersonic Turbulent Flow Reynolds-Averaged Navier–Stokes Simulations with Roughness and Blowing Effects

Thermal protection systems experience severe thermal load during atmospheric reentry of hypersonic vehicles. Inherent to the ablation process, roughness and blowing effects may appear, affecting the performance of the heat shield. The modeling and the simulation of these effects in the hypersonic regime are challenging and rarely compared to experimental data. In this paper, Reynolds-averaged Navier–Stokes simulations of hypersonic turbulent boundary layers with roughness and blowing wall conditions are performed on Holden’s experimental configurations (“Studies of Surface Roughness and Blowing Effects on Hypersonic Turbulent Boundary Layers over Slender Cones,” 27th Aerospace Sciences Meeting, AIAA Paper 1989-0458, 1989). Using the shear stress transport model. Four experimental configurations, characterized by different levels of turbulence compressibility, are considered. A detailed discussion is proposed about the behavior of equivalent sand grain corrections and blowing corrections in combination with turbulence compressibility corrections. The predictions of skin-friction coefficients and Stanton numbers are in good agreement with Holden’s experimental data. These results show that the combination of the Zeman compressibility correction and roughness/blowing wall corrections is promising for the simulation of hypersonic boundary layers over rough walls with blowing.

Mechanism analysis of magnetohydrodynamic control in hypersonic turbulent flow

Under the assumption of the low magnetic Reynolds number, the coupled model is established for the turbulent flow field and the externally applied magnetic field. The AUSMPW+ scheme and LUSGS method are used to solve turbulent magnetohydrodynamics (MHD) flow equations, in which the Spalart-Allmaras one-equation turbulence model is used. A series of numerical simulations over various geometry configurations, namely, a flat plate and a compression corner, is conducted by using an external electromagnetic field. Results show that the performance of MHD boundary layer flow control is determined mainly by the Lorentz force in the streamwise direction. With an external magnetic field used, the low velocity fluid in the boundary layer can decelerate and increase the static temperature locally. Moreover, the counter-flow Lorentz force always brings a negative effect on the turbulent skin friction coefficient, and the location for the MHD zone has a great influence on the control efficiency of the ramp-induced separation. A reasonable magnetic field layout scheme should be configured in practical engineering application.

Assessment of a high-order shock-capturing central-difference scheme for hypersonic turbulent flow simulations

High-speed turbulent flows are encountered in most space-related applications (including exploration, tourism and defense fields) and represent a subject of growing interest in the last decades. A major challenge in performing high-fidelity simulations of such flows resides in the stringent requirements for the numerical schemes to be used. These must be robust enough to handle strong, unsteady discontinuities, while ensuring low amounts of intrinsic dissipation in smooth flow regions. Furthermore, the wide range of temporal and spatial active scales leads to concurrent needs for numerical stabilization and accurate representation of the smallest resolved flow scales in cases of under-resolved configurations. In this paper, we present a finite-difference high-order shock-capturing technique based on Jameson’s artificial diffusivity methodology. The resulting scheme is ninth-order-accurate far from discontinuities and relies on the addition of artificial dissipation close to large gradient flow regions. The shock detector is slightly revised to enhance its selectivity and avoid spurious activations of the shock-capturing term. A suite of test cases ranging from 1D to 3D configurations (namely, perfect-gas and chemically reacting shock tubes, Shu–Osher problem, isentropic vortex advection, under-expanded jet, compressible Taylor–Green Vortex, supersonic and hypersonic turbulent boundary layers) is analyzed in order to test the capability of the proposed numerical strategy to handle a large variety of problems, ranging from calorically-perfect air to multi-species reactive flows. Results obtained on under-resolved grids are also considered to test the applicability of the proposed strategy in the context of implicit Large-Eddy Simulations.

Trapped waves in supersonic and hypersonic turbulent channel flow over porous walls – ERRATUM

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Compressibility effects on hypersonic turbulent channel flow with cold walls

Compressibility effects on velocity and temperature fluctuations in hypersonic turbulence over cold walls are investigated by exploiting a direct numerical simulation database. We found that the compressibility effects are enhanced by the decrease in wall temperature, which is directly reflected by the rapid increase in velocity divergence and turbulent Mach number. Helmholtz decomposition is adopted to evaluate the genuine compressibility effects by splitting the velocity fluctuation into a solenoidal component and a dilatational component. As the wall temperature decreases, the vertical motion is gradually dominated by the dilatational component, while the wall-parallel motions still by the solenoidal component. The dilatational components tend to decrease the skin friction by around 4%–6%. The instantaneous and conditionally averaged field around strong compressive motions further suggests that the dilatational structures behave as traveling-wave packets surrounded by vortex clusters. To improve the strong analogy between temperature and velocity fluctuations, their correlation in spectral space is studied. The results show that they are strongly correlated at the scales of energy-containing motions, but they are mutually independent at small scales.

Trapped waves in supersonic and hypersonic turbulent channel flow over porous walls

Abstract

Gas-Kinetic Scheme Coupled with Turbulent Kinetic Energy Equation for Computing Hypersonic Turbulent and Transitional Flows

ABSTRACT For simulation of hypersonic turbulent and transitional flows at high Reynolds numbers, a gas-kinetic scheme (GKS) strongly coupled with the turbulent kinetic energy equation in shear stress transport (SST) k−ω turbulence model is developed. To extend the current method to transitional flows, Langtry-Menter transition model is added together with SST k−ω turbulence model. Comparisons among the numerical solutions from current GKS method coupled with turbulent kinetic energy equation, GKS method uncoupled with turbulent kinetic energy equation, Reynolds-Averaged Navier-Stokes (RANS) solutions with turbulence/transition models, and experimental data are conducted. It is concluded that the current method can significantly improve the accuracy of numerical results.

HTR-1.2 solver: Hypersonic Task-based Research solver version 1.2

We present an updated version of the open-source Hypersonics Task-based Research (HTR) solver for hypersonic aerothermodynamics. The solver, whose first version was presented in Di Renzo et al. (2020), is designed for direct numerical simulation (DNS) of canonical hypersonic flows at high Reynolds numbers in which thermo-chemical effects induced by high temperatures are relevant. The solver relies on high-order spatial discretization on structured meshes and efficient time integrators for stiff systems within the Regent/Legion software stack, which makes the code highly portable and scalable in CPU and GPU-based supercomputers. The new version herein presented includes several optimizations and new tools for data analysis, along with novel user option for hybrid skew-symmetric/targeted essentially non-oscillatory numerics, to offer higher computational efficiency and lower numerical dissipation at moderate Mach numbers, inclusion of a new combustion mechanism for methane and oxygen, and new recycling–rescaling inlet boundary conditions targeted to the simulation of fully developed turbulent boundary layers. New version program summaryProgram Title: Hypersonics Task-based Research solverCPC Library link to program files:https://doi.org/10.17632/9zsxjtzfr7.2Developer’s repository link:https://github.com/stanfordhpccenter/HTR-solver.gitLicensing provisions: BSD 2-clauseProgramming language: Regent, C++, and CUDAJournal Reference of previous version: Di Renzo, M., Fu, L., & Urzay, J. (2020). HTR solver: An open-source exascale-oriented task-based multi-GPU high-order code for hypersonic aerothermodynamics. Computer Physics Communications, 107262.Does the new version supersede the previous version?: YesReasons for the new version: Release of new featuresSummary of revisions:•New optional sixth-order hybrid scheme has been implemented (activated by the flag “hybridScheme” in the input file). The scheme combines the energy-preserving properties of a sixth-order skew-symmetric central difference scheme [1] in smooth flow regions with the shock-capturing properties of a sixth-order targeted essentially non-oscillatory (TENO) scheme at points where shocks are involved. The switch between the two schemes is controlled by a TENO sensor whose cutoff value is adapted based on the maximum value of a modified Ducros sensor [2] across the reconstruction stencil. If the flag “hybridScheme” is set to false, the numerical scheme will revert to the TENO6-A scheme released in the previous version of the solver [3];•New recycling–rescaling inflow boundary conditions [4,5] for the simulation of turbulent compressible boundary layers are now available to the user;•Support for Legion tracing, which significantly improves the strong scalability of the solver, has been implemented;•A diagnostic tool to monitor the time evolution of the flow variables in a subvolume of the computational domain is now available;•A single-step chemistry mechanism for methane/oxygen combustion has been added to the mixtures handled by the HTR solver;•Sample scripts for strong scaling have been added to the “testcases” directory;•Unit test and regression test suites have been added to the repository;•The input file scheme has been modified in order to reduce verbosity and increase flexibility in specifying the boundary conditions and type of gas mixture;•Hyperbolic sine stretching functions have been made available to users during the grid generation process;•Computationally intensive tasks have been ported to C++ and CUDA in order to achieve higher efficiency on all hardware;•Several optimizations of the tasks body and mapper have been implemented in order to increase the computational efficiency and reduce the memory footprint.Nature of problem: This code solves the Navier–Stokes equations at hypersonic Mach numbers including finite-rate chemistry for dissociating air and multicomponent transport. The solver is designed for direct numerical simulations (DNS) of transitional and turbulent hypersonic turbulent flows under high-enthalpy conditions, and it accounts for thermochemical effects (vibrational excitation and chemical dissociation).Solution method: This code uses a low-dissipation sixth-order schemes for the spatial discretization of the conservation equations on Cartesian stretched meshes. Time advancement is carried out by either an explicit method if chemistry is slow, hence not introducing additional stiffness, or by an operator-splitting algorithm whereby chemical production rates are handled implicitly.Additional comments including restrictions and unusual features: The HTR solver builds on the runtime Legion [6,7] and is written in the programming language Regent [8,9] developed at Stanford University. Instructions for installation of the components are provided in the README file enclosed with the HTR solver and in the Legion repository [6].

The formulation of the RANS equations for supersonic and hypersonic turbulent flows

AbstractAccurate prediction of supersonic and hypersonic turbulent flows is essential to the design of high-speed aerospace vehicles. Such flows are mainly predicted using the Reynolds-Averaged Navier–Stokes (RANS) approach in general, and in particular turbulence models using the effective viscosity approximation. Several terms involving the turbulent kinetic energy (k) appear explicitly in the RANS equations through the modelling of the Reynolds stresses in such approach, and similar terms appear in the mean total energy equation. Some of these terms are often ignored in low, or even supersonic, speed simulations with zero-equation models, as well as some one- or two-equation models. The omission of these terms may not be appropriate under hypersonic conditions. Nevertheless, this is a widespread practice, even for very high-speed turbulent flow simulations, because many software packages still make such approximations. To quantify the impact of ignoring these terms in the RANS equations, two linear two-equation models and one non-linear two-equation model are applied to the computation of five supersonic and hypersonic benchmark cases, one 2D zero-pressure gradient hypersonic flat plate case and four shock wave boundary layer interaction (SWBLI) cases. The surface friction coefficients and velocity profiles predicted with different combinations of the turbulent kinetic energy terms present in the transport equations show little sensitivity to the presence of these terms in the zero-pressure gradient case. In the SWBLI cases, however, comparisons show that inclusion of k in the mean flow equations can have a strong effect on the prediction of flow separation. Therefore, it is highly recommended to include all the turbulent kinetic energy terms in the mean flow equations when dealing with simulations of supersonic and hypersonic turbulent flows, especially for flows with SWBLIs. As a further consequence, since k may not be obtained explicitly in zero-equation, or certain one-equation, models, it is debatable whether these models are suitable for simulations of supersonic and hypersonic turbulent flows with SWBLIs.

100 kHz PLEET velocimetry in a Mach-6 Ludwieg tube.

Picosecond laser electronic-excitation tagging (PLEET) was demonstrated in a Mach-6 Ludwieg tube at a repetition rate of 100 kHz using a 1064 nm, 100 ps burst-mode laser. The system performance of high-speed velocimetry in unseeded air and nitrogen Mach-6 flows at a static pressure in the range of 5-20 torr were evaluated. Based on time-resolved freestream flow measurements and computational fluid dynamics (CFD) calculations, we concluded that the measurement uncertainty of 100 kHz PLEET measurement for Mach 6 freestream flow condition is ∼1%. The measured velocity profiles with a cone-model agreed well with the CFD computations upstream and downstream of the shockwave; downstream of the shockwave the discrepancy between the CFD and experimental measurement could be attributed to a slight nonzero angle of attack (AoA) or flow unsteadiness. Our results show the potential of utilizing 100 kHz PLEET velocimetry for understanding real-time dynamics of turbulent hypersonic flows and provide the capability of collecting sufficient data across fewer tests in large hypersonic ground test facilities.

HTR solver: An open-source exascale-oriented task-based multi-GPU high-order code for hypersonic aerothermodynamics

In this study, the open-source Hypersonics Task-based Research (HTR) solver for hypersonic aerothermodynamics is described. The physical formulation of the code includes thermochemical effects induced by high temperatures (vibrational excitation and chemical dissociation). The HTR solver uses high-order TENO-based spatial discretization on structured grids and efficient time integrators for stiff systems, is highly scalable in GPU-based supercomputers as a result of its implementation in the Regent/Legion stack, and is designed for direct numerical simulations of canonical hypersonic flows at high Reynolds numbers. The performance of the HTR solver is tested with benchmark cases including inviscid vortex advection, low- and high-speed laminar boundary layers, inviscid one-dimensional compressible flows in shock tubes, supersonic turbulent channel flows, and hypersonic transitional boundary layers of both calorically perfect gases and dissociating air. Program summaryProgram Title: Hypersonics Task-based Research solverProgram Files doi:http://dx.doi.org/10.17632/9zsxjtzfr7.1Licensing provisions: BSD 2-clauseProgramming language: RegentNature of problem: This code solves the Navier–Stokes equations at hypersonic Mach numbers including finite-rate chemistry for air dissociation along with multicomponent transport. The solver is designed for direct numerical simulations (DNS) of transitional and turbulent hypersonic turbulent flows at high enthalpies, and accounts for thermochemical effects such as vibrational excitation and chemical dissociation.Solution method: This code uses a low-dissipation sixth-order targeted essentially non-oscillatory (TENO) scheme for the spatial discretization of the conservation equations on Cartesian stretched grids. The time advancement is performed either with an explicit method, when the chemistry is slow and therefore does not introduce additional stiffness in the integration, or with an operator-splitting method that integrates the chemical production rates with an implicit discretization.Additional comments: The HTR solver builds on the runtime Legion [1] and is written in the programming language Regent [2] developed at Stanford University. Instructions for the installation of the components are provided in the README file enclosed with the HTR solver and in the Legion repository [1].