Hypersonic Flight Conditions Research Articles

Real-Time Aerodynamic Load Estimation for Hypersonics via Strain-Based Inverse Maps

This work develops an efficient real-time inverse formulation for inferring the aerodynamic surface pressures on a hypersonic vehicle from sparse measurements of the structural strain. The approach aims to provide real-time estimates of the aerodynamic loads acting on the vehicle for ground and flight testing, as well as guidance, navigation, and control applications. Specifically, the approach targets hypersonic flight conditions where direct measurement of the surface pressures is challenging due to the harsh aerothermal environment. For problems employing a linear elastic structural model, the inference problem can be posed as a least-squares problem with a linear constraint arising from a finite element discretization of the governing elasticity partial differential equation. Due to the linearity of the problem, an explicit solution is given by the normal equations. Precomputation of the resulting inverse map enables rapid evaluation of the surface pressure and corresponding integrated quantities, such as the force and moment coefficients. The inverse approach additionally allows for uncertainty quantification, providing insights for theoretical recoverability and robustness to sensor noise. Numerical studies demonstrate the estimator performance for reconstructing the surface pressure field, as well as the force and moment coefficients, for the Initial Concept 3.X (IC3X) conceptual hypersonic vehicle.

Design and thermal–optical evaluation of an optical window module for a near-space remote sensor under ultra-high temperature conditions

Under hypersonic flight conditions, the optical window external surface of a near-space remote sensing system generates a large thermal load due to aerodynamic friction, resulting in serious additional contact thermal stresses and deformation of the flat window. Consequently, the imaging performance of the optical remote sensing system (ORSS) is reduced. To overcome this issue, a flexible support combination structure (FSCS) for an optical window suitable for ultra-high temperature conditions was developed in this study, which ensures the optical imaging quality requirements of the system. Based on the external thermal environment characteristics of a hypersonic aircraft, an aerothermal simulation model of the aircraft with a window component was established. More importantly, the FSCS was optimized and a thermal–structural model of the window component was established, and the opto-mechanical-thermal analysis method was proposed for the ORSS. Finally, the thermal-optical properties of the deformed window were evaluated. The results show that the optimized window component satisfies the optical imaging design requirements, which guides engineers in ultra-high temperature thermal analysis and optical imaging.

Multi-domain analysis and prediction of inductively coupled plasma jet dynamics via high-speed imaging of visible light emission

Inductively coupled plasma wind tunnels are crucial for replicating hypersonic flight conditions in ground testing. Achieving the desired conditions (e.g., stagnation-point heat fluxes and enthalpies during atmospheric reentry) requires a careful selection of operating inputs, such as mass flow, gas composition, nozzle geometry, torch power, chamber pressure, and probing location along the plasma jet. The study presented herein focuses on the influence of the torch power and chamber pressure on the plasma jet dynamics within the 350 kW Plasmatron X ICP facility at the University of Illinois at Urbana-Champaign. A multi-domain analysis of the jet behavior under selected power-pressure conditions is presented in terms of emitted light measurements collected using high-speed imaging. We then use Gaussian Process Regression to develop a data-informed learning framework for predicting Plasmatron X jet profiles at unseen pressure and power test conditions. Understanding the physics behind the dynamics of high-enthalpy flows, particularly plasma jets, is the key to properly design material testing, perform diagnostics, and develop accurate simulation models.

Analysis of the aerodynamic performance of a hypersonic gliding missile with a deflected warhead

In this study, we propose a scheme to control the deflection of the warhead based on the configuration of the hypersonic glide body (HGB) to solve the problems posed by its large control surface load and severe aerodynamic heat under hypersonic flight conditions. We conducted numerical simulations on the configurations of deflection of the warhead of an HGB analog under different flight modes as well as varying angles and directions of deflection. The results showed that once the warhead had been deflected, the overall configuration of the HGB analog still exhibited static longitudinal stability. An increase in the angle of deflection significantly reduced the lift-to-drag ratio of the configuration at large angles of attack. When the warhead was deflected upward, the configuration of the HGB analog exhibited static lateral instability, while it exhibited a high static lateral stability when the warhead was deflected downward.

Characterization of Particle-Surface Impacts on a Sphere-Cone at Hypersonic Flight Conditions

This study utilizes in-situ data of stratospheric particle size distributions to understand particle-surface impacts at hypersonic flight conditions. Particle diameters range from 0.2 to 10 μm. Simulations are performed to track Lagrangian particles through the flowfield and statistically gather the characteristics of particles that impact the surface. Probability density functions in terms of surface-impact locations, particle sizes, velocities, incident angles, and kinetic energies are presented. The results indicate that sub-micrometer-diameter particles most frequently impact the nosetip of the vehicle at a range of velocities that tend toward the freestream velocity at higher altitudes. However, it does not appear that sub-micrometer particles significantly contribute to the net surface-impact kinetic energy. It is shown that larger particles between 6 and 8 μm are responsible for the majority of net surface-impact kinetic energy, despite these surface-impact events being infrequent.

Effects of thermal plasma on the microstructure and mechanical behavior of alumina-mullite based oxide/oxide ceramic matrix composites

Oxide/oxide ceramic matrix composites have potential applications in thermal protection systems due to their ability to withstand very high temperatures while maintaining their load bearing capability and oxidation resistance. Under hypersonic flight conditions vehicle components may be exposed to temperatures above the melting points of alumina and mullite, as well as to chemically active ionized gases. Therefore, it is necessary to understand the response of these CMCs to not just thermal loads, but also the added challenges of plasma environments. In this study, alumina-mullite based oxide/oxide CMCs were exposed to an inductively generated plasma jet for 30 min at a heat flux of 24 W/cm2 in near vacuum conditions. Resulting microstructures were characterized with XRD and SEM/EDS. Silicon rich, sub-micron globules were observed on the surface of the impinged region. Also, the first 5 μm below the surface were enriched with silicon. The back surface showed no silicon enrichment but did have a granulated surface texture in comparison to as manufactured samples. Short beam strength tests were performed as a measure of structural performance. Differences in the overall response and short beam strength were noted between the as manufactured and plasma exposed cases.

Study on high-speed water entry process of non-rotating vehicles

The non-rotating trans-media vehicle can better adapt to the hypersonic flight conditions, but new problems in the hydrodynamic characteristics and motion stability arise during the water entry process. With the VOF multiphase model, Realizable k-ε turbulence model, and Schnerr-Sauer cavitation model, this paper establishes a simulation method of coupling the movement of non-roating body into water with multiphase flow fields based on overset grid technology, and the accuracy of the established model is verified. The simulation calculation of the oblique water entry process of the vehicle with different ratio of short axis to long axis in cross section is carried out. And the results show that although the ratio of short axis to long axis in cross section does not change the cavitation morphology, it affects the hydrodynamic and kinematic characteristics of the vehicle by influencing the relative position of the cavitation. As the ratio of short axis to long axis decreases, the motion stability of the vehicle increases gradually. However, if the ratio of short axis to long axis is too small, the drag reduction effect will be weakened, and the structure will suffer a bigger load.

IDEA: Artificial neural network models for 11-species air properties at thermochemical equilibrium

Accurate prediction of high-temperature air properties is essential in many aerodynamic applications under hypersonic flight conditions. Various curve-fit models using piecewise polynomial fittings have been commonly adopted to approximate equilibrium air properties at high temperatures. Several shortcomings including low accuracy, lack of diversity, and discontinuity at curve-fit boundary still remain, causing numerical troubles in computational procedures. To address the issues, IDEA, an open-source C++ library that enables fast and accurate computations of the equilibrium air properties and their first and second derivatives, is newly developed based on the artificial neural network (ANN).IDEA, which stands for the Infinitely Differentiable Equilibrium Air, predicts thermodynamic and transport properties of 11-species (N2, O2, N, O, NO, NO+, N+, O+, N++, O++, and e−) thermochemical equilibrium air at the temperature range up to 25,000 K and density range from 10−7 to 103 amagats. The training data is constructed from the kinetic molecular theory using the equilibrium constant method with the rigid-rotor, harmonic-oscillator model. As the name suggests, IDEA's models are infinitely differentiable in the application range; thus, they have enhanced convergence in computational fluid dynamics (CFD) when using gradient-based methods. Using a newly developed training process based on the Levenberg–Marquardt algorithm with weighted mean squared error loss, IDEA provides more accurate and diverse property models with much fewer parameters than previous piecewise polynomial fitting models. In addition, the proposed training method offers easy extensions to various property models with different species data. IDEA provides C interfaces that can be used for programs in various computer languages, such as C/C++, Fortran, Python, and MATLAB. IDEA's modeling routines are thread-safe, so they can be safely used for parallel programs without performance loss. The accuracy and enhanced convergence of IDEA is demonstrated via several high-speed flow computations Program summaryProgram title: IDEACPC library link to program files:https://doi.org/10.17632/84rhtfz9n2.1Developers' repository link:https://github.com/HojunYouKr/IDEALicensing provisions: BSD-3-ClauseProgramming language: C++11Nature of problem: Accurate prediction of equilibrium air properties is essential in many aerodynamic applications under hypersonic flight conditions. Direct calculation of air properties by the kinetic molecular theory requires time-consuming iterative methods to solve nonlinear equations of species concentrations. Various piecewise polynomial models have been developed to avoid such iterations. However, the regression error of the polynomial models requires improvement, and the lack of diversity of the polynomial models makes iterative computations inevitable, causing computational inefficiency. Furthermore, these polynomial models are not continuously differentiable, which deteriorates the convergence characteristics of computational fluid dynamics (CFD) solvers.Solution method: ANN is chosen to model the equilibrium properties of air owing to its excellent capability as a universal function approximator and better adaptability to a large dataset than kernel methods. The hyperbolic tangent activation function makes the ANN model infinitely differentiable, which enables to use a superlinear training process and thus improves convergence characteristics. The newly proposed superlinear training process based on the Levenberg–Marquardt algorithm with weighted mean squared error loss provides more accurate and diverse property models with much fewer parameters than the existing polynomial models. As a result, the proposed ANN model is completely free from iterative computations.

Informing air-carbon ablation modeling with theoretical calculations of atomic oxygen and nitrogen interacting with carbon surfaces.

To understand the gas-surface chemistry above the thermal protection system of a hypersonic vehicle, it is necessary to map out the kinetics of key elementary reaction steps. In this work, extensive periodic density functional theory (DFT) calculations are performed to elucidate the interaction of atomic oxygen and nitrogen with both the basal plane and edge sites of highly oriented pyrolytic graphite (HOPG). Reaction energies and barriers are determined for adsorption, desorption, diffusion, recombination, and several reactions. These DFT results are compared with the most recent finite-rate model for air-carbon ablation. Our DFT results corroborated some of the parameters used in the model but suggest that further refinement may be necessary for others. The calculations reported here will help to establish a predictive kinetic model for the complex reaction network present under hypersonic flight conditions.

Mathematical modeling and simulation of porosity on thermomechanical properties of UHTCs under hypersonic conditions

AbstractUltrahigh temperature ceramics (UHTCs) were analyzed for their suitability in hypersonic flight conditions using balanced heat equations, transport equation, and finite element modeling technique. Mathematical model was derived on the assumption that the induced porosity follows linear and parabolic solutions of Laplace equation and applied external load mimicking hypersonic conditions with critical heat flux ranging between 7 and 44 MW/m2. Simulations were carried out with four different UHTCs combinations and the results outlined a temperature rise exceeding 4700°C with deformation observed on the fixed area and where the heat flux was generated. The influence of porosity had a greater impact on the performance of the material as it led to a reduction in deformation compared to dense samples. Porous UHTCs exhibited a good thermal shock resistance owing to the release of thermal stresses through pores, which also enhanced the thermal insulation of the structure.

Stability analysis of streamwise vortices over a blunt inclined cone under a hypersonic flight condition

We studied the stability of leeward streamwise vortices over a hypersonic inclined blunt cone under a flight condition by solving the two-dimensional spatial eigenvalue problem (BiGlobal) and plane-marching parabolized stability equations (PSE3D). The stability analyses were performed based on the laminar flow obtained by direct numerical simulation. Due to the azimuthal pressure gradient and large bluntness, a pair of large-scale inward vortices dominate in the vicinity of the leeward ray. Such vortical structures are different from previous studies, where outward vortices are most prominent. Two types of unstable modes are identified, namely, “inner mode” with low phase velocities and “outer mode” with high phase velocities. The inner modes are unstable in a wide frequency range compared to the outer modes, differing from previous studies where inner modes are deemed to lie in a much lower frequency range compared to the outer modes. Moreover, the inner modes, with supersonic phase velocities, are found to be able to radiate weak acoustics outside the vortices. Mack mode, belonging to the outer-mode instability, is most amplified that it is likely responsible for the breakdown of the vortices. BiGlobal and PSE3D results show good agreement regarding the downstream developments of the Mack mode and inner modes and yet exhibit remarkable discrepancies in tracing the evolution of the outer modes, highlighting the necessity of considering the non-parallel effects when studying the stability of streamwise vortices.

A Novel Direct Optimization Framework for Hypersonic Waverider Inverse Design Methods

Waverider is a hypersonic vehicle that improves the lift-to-drag ratio using the shockwave attached to the leading edge of the lifting surface. Owing to its superior aerodynamic performance, it exhibits a viable external configuration in hypersonic flight conditions. Most of the existing studies on waverider employ the inverse design method to generate vehicle configuration. However, the waverider inverse design method exhibits two limitations; inaccurate definition of design space and unfeasible performance estimation during the design process. To address these issues, a novel framework to directly optimize the waverider is proposed in this paper. The osculating cone theory is adopted as a waverider inverse design method. A general methodology to define the design space is suggested by analyzing the design curves of the osculating cone theory. The performance of the waverider is estimated accurately and rapidly via combining a high-fidelity computational fluid dynamics solver and a surrogate model. A comparison study shows that the proposed direct optimization framework enables a more accurate design space and efficient performance estimation. The framework is applied to the multi-objective optimization problem, which maximizes internal volume and minimizes aerodynamic drag. Finally, general characteristics for waverider are presented by analyzing the optimized results with data mining methods such as K-means.

Fluid-thermo-structural response of actively cooled scramjet combustor in hypersonic accelerating-cruise flight

This paper presents a three-dimensional transient fluid-thermo-structural study of an actively cooled sandwich panel under hypersonic accelerating-cruise flight conditions. The thermo-structural loads are estimated using a high-speed gas-dynamic flow model combined with Eckert's reference temperature method. The thermal safeguard capacity with endothermic fuel-based active cooling shows design optimization scope compared to the passive system. A passive system has to survive excessively high temperatures with a less severe thermal gradient over the panel thickness and a high heat leakage into the structural interior or back wall. The actively cooled system with fuel develops significant panel bending due to a through-thickness thermal gradient in the cooling channels. For a Mach 7 flight, an actively cooled system reduces the bending deformation by 90% and improves the thermal safeguard capacity by 70% compared to a passive approach. Active cooling can effectively control the excessive thermo-structural deformation and fuel heating below its cracking temperature to improve combustion. Our parametric design study indicates the influence of fuel as a coolant for increasing combustion efficiency and the associated complexity and trade-off in the heat transfer and thermo-structural deformation behavior. This transient response under hypersonic accelerating-cruise flight conditions requires a careful review of the scramjet engine thermo-structural design philosophy. The design approach has applications in bodies encountering highly transient thermo-structural loads with differently purposed fuels/coolants.

Evaluation of Fay and Riddell formula under hypersonic flight conditions

PurposeThe Fay and Riddell (F–R) formula is an empirical equation for estimating the stagnation-point heat flux on noncatalytic and fully catalytic surfaces, based on an assumption of equilibrium. Because of its simplicity, the F–R has been used extensively for reentry flight design as well as ground test facility applications. This study aims to investigate the uncertainties of the F-R formula by considering velocity gradient, chemical species at the boundary layer edge, and the thermochemical nonequilibrium (NEQ) behind the shock layer under various hypersonic NEQ flow environments.Design/methodology/approachThe stagnation-point heat flux calculated with the F–R formula was evaluated by comparison with thermochemical NEQ calculations and existing flight experimental values.FindingsThe comparisons showed that the F–R underestimated the noncatalytic heat flux, because of the chemical composition at the surface. However, for fully catalytic heat flux, the F–R results were similar to values of surface heat flux from thermochemical NEQ calculations, because the F–R formula overestimates the diffusive heat flux. When compared with the surface heat flux results obtained from flight experimental data, the F–R overestimated the fully catalytic heat flux. The error was 50% at most.Originality/valueThe results provided guidelines for the F–R calculations under hypersonic flight conditions and for determining the approximate error range for noncatalytic and fully catalytic surfaces.

Design and analysis of the air-breathing aircraft with the full-body wave-ride performance

Waverider is expected to break the lift-to-drag ratio constraint in the hypersonic flight condition and it is commonly been studied as the forebody of the cruise vehicle. In this paper, we developed an approach to make waverider as the airframe for the air-breathing cruise vehicle. The fuselage that designed based on the cone-derived waverider theory with a unique upper boundary curve generates the same conical shock wave as that generated by the conical nose of the vehicle. Therefore, the vehicle is expected to have a full-body wave-ride performance and suitable for cruising in the hypersonic condition. Also, a gap between the conical nose and the fuselage is designed to be the inlet of the air-breathing engine. In comparison with the traditional cone-derived waverider, this kind of configuration has additional volume so that it can afford enough space for installing the engine system in the airframe. The performance of this kind of configuration is validated by the numerical method in the design Mach number. With the engineering estimation approach used to calculate the aerodynamic performance of the vehicle in hypersonic conditions, we studied the influence of design parameters on the aerodynamic performance. Results show that the increase of the design Mach number does negative impact on the lift-to-drag ratio but positive impact on the volumetric efficiency while the increase of the design direction angle has an opposite influence. However, in comparison with the response of the lift-to-drag ratio, the volumetric efficiency is less affected by the dihedral angle, which means that the dihedral angle can control the aerodynamic performance effectively within a limited volumetric efficiency range.

Investigating the mechanical behavior of multiscale porous ultra-high temperature ceramics using a quasi-static material point method

Ultra-high temperature ceramics (UHTCs) are a class of materials that maintain their structural integrity at high temperatures, e.g. 2000 °C. They have been limited in their aerospace applications because of their relatively high density and the difficulty involved in forming them into complex shapes, like leading edges and inlets. Recent advanced processing techniques have made significant headway in addressing these challenges, where the introduction of multiscale porosity has resulted in lightweight UHTCs and a more tailored thermal conductivity response. The effect of porosity on the properties of UHTCs must be characterized to enable design, but doing so experimentally can be costly, especially when attempting to mimic hypersonic flight conditions. As such, this paper seeks to computationally characterize and understand the effective mechanical properties of porous UHTCs, specifically titanium diboride, and validate those results against experimental results so as to build confidence in the model. An implicit quasi-static variant of the Material Point Method (MPM) is developed, whose capabilities include intrinsic treatment of large deformations and contact which are needed to capture the post-elastic behavior of simulated porous UHTC microstructures. It is found that the MPM can approximately obtain the elastic properties of porous UHTCs without calibration, and that the post-elastic results are found to be qualitatively consistent with experimental results. With calibrated input properties, significantly improved agreement is obtained.

Identification of traveling crossflow waves under real hypersonic flight conditions

Understanding crossflow instabilities in three-dimensional boundary layers triggered by either traveling crossflow waves or stationary crossflow vortices is of great importance for modeling, predicting, and controlling hypersonic laminar-turbulent transition. However, due to very limited available flight experiment data, the crossflow instability under real flight conditions is still far from fully understood. To gain further insight, the raw data of a recent model flight experiment conducted by China Aerodynamics Research and Development Center have been thoroughly analyzed in the present study. The instrumented model is an inclined blunt cone mounted with several pressure sensors. Distinct low-frequency signals detected by these pressure sensors are peaked at about 10 kHz, which are in good agreement with the traveling-crossflow-wave frequencies with the largest N factors predicted by the eN method. Moreover, propagation velocities and wave angles of these signals obtained from correlation analysis also agree with the results from linear stability theory. The present study confirms that the detected low-frequency signals are traveling crossflow waves and provides the first evidence of traveling crossflow waves under real flight conditions.

Effect of the reaction mechanism on the numerical prediction of the performance of a scramjet combustor at cruise flight 8 Mach number

A numerical investigation of the effect of the hydrogen-air reaction mechanism on the prediction of supersonic combustion was conducted. Different reaction mechanisms with different number of species and reactions were investigated at a Scramjet hypersonic flight trajectory. A generic three-dimensional supersonic combustor was investigated at flight 8 Mach number condition and at an altitude of 30 km. Numerical simulations were performed to determine the influence of the size of the reaction mechanism on the prediction of supersonic lifted flames, in a Scramjet flight trajectory were the combustor inflow conditions are close to the hydrogen-air self-ignition limit. Results showed significant influence of the number of species and reactions involved in the mechanism on the prediction of the ignition length. Shorter ignition delays were obtained with a reduced mechanism. Similarly, combustion efficiency was affected by the reduced mechanism dropping nearly 15%, compared to the reference scheme. On the other hand, no significant influence of the reaction mechanism was observed on the combustor pressure drop. Pressure losses nearly converge to the same values by the exit of the combustor. The present data suggest that the main observed differences are on the different prediction of the location where self-ignition occurred. We show how the ignition delay is affected by the omission of secondary species and steps reactions leading to shorter ignition delays or even attached flames which are not expected to occur at the investigated hypersonic flight condition.

Fast Doppler shift acquisition method for hypersonic vehicle communications

Large and rapidly changing Doppler shift poses a huge challenge on fast carrier acquisition for hypersonic vehicle communications. In this study, a novel Doppler shift acquisition method is proposed to increase the acquisition probability and reduce the acquisition time simultaneously. Firstly, the Doppler rate is estimated with a proposed fast blind estimation algorithm. The order of Doppler shift is reduced though twice instantaneous delay autocorrelation operation on the received signal and the modulated information is cancelled, and the Doppler rate can be obtained by the corresponding spectrum analysis. Secondly, the received signal is compensated by the estimated Doppler rate. Finally, the maximum value of the average power spectrum of the compensated result over multiple symbols is detected, and the Doppler shift and Doppler rate can be jointly determined. Furthermore, to improve the acquisition speed, a corrected narrower range, which is determined by the flight conditions of the hypersonic vehicles, is proposed to constrain the search range of the power spectrum peak. Simulation results indicate that, under typical hypersonic flight conditions, the acquisition probability of the Doppler rate increases from 0.52 to 0.86 under − 20 dB. The processing speed can be improved by 75.47% when the length of the combined symbols is 1000.

Influence of vibrational non-equilibrium on the polarizability and refraction index in air: computational study

Knowledge of optical effects accompanying hypersonic flight conditions is of critical importance to ensure reliable operation of on-board optical instrumentation. This paper presents results of calculations of scalar polarizability dependence on vibrational non-equilibrium in molecular nitrogen and oxygen. The non-equilibrium effects are evaluated on the basis of a semi-classical model for the molecular polarizability. It is shown that, depending on the vibrational and translational populations, changes in the scalar polarizability in excess of normal density and temperature scaling can be around 1%–5% at moderate gas temperatures and around 10% at high temperatures. The model was applied to predict the polarizability for non-equilibrium states in a nitrogen plasma formed by a single nanosecond pulse with different rise times.