Supersonic Turbulent Flow Research Articles

Wall temperature effects in high-enthalpy supersonic turbulent channel flows considering air dissociation

Wall temperature effects in high-enthalpy supersonic turbulent channel flows considering air dissociation

Detached-Eddy Simulation of Supersonic Turbulent Flow over a Cylinder/Skewed Flare Configuration

The computational campaign reported in this article focuses on a series of experiments at Mach 2.85 carried out in the 1980s at NASA Ames Research Center on a set of cylinder/skewed flare configurations designed to produce highly three-dimensional shock-wave/boundary-layer interactions in the absence of end-wall effects. Computations carried out in that era were unable to match the experimental results using the numerical techniques, turbulence models, and grid resolution available at the time. In the present work, newer Reynolds-averaged Navier–Stokes and detached-eddy simulation methods have been applied to these flows, and relatively good agreement has been obtained with the experimental data. Difficulty in capturing the correct separation-bubble size was encountered with initial detached- eddy simulations, but the introduction of resolved turbulence via a boundary-layer trip produced much better results. The present paper reports on results obtained for four inclination angles (0, 5, 10, and 23 deg) of the skewed 30-deg flare. Detached-eddy simulation is seen to be an economical alternative to large-eddy simulation for capturing many features of large-scale separation unsteadiness over long time intervals at true Reynolds number.

Comparisons between the first- and second-order spectral stochastic estimations in investigating the multiphysics couplings for a supersonic turbulent channel flow

Comparisons between the first- and second-order spectral stochastic estimations in investigating the multiphysics couplings for a supersonic turbulent channel flow

A non-absorption-loss immune TDLAS sensor for online Mach number evaluation in supersonic flows

Tunable diode laser absorption spectroscopy (TDLAS) techniques are widely used in combustion monitoring by using direct absorption spectroscopy (DAS). However, the DAS method is sensitive to laser intensity fluctuations and fails to work in harsh application cases. A non-absorption-loss immune TDLAS sensor was proposed for robust online evaluation of Mach number from temperature and speed in supersonic flows and in situ validated on a scramjet. Spectral lines of H2O at 7185.58 and 7444.35 cm−1 were selected to perform evaluations of target molecules, i.e., of water vapor. In highly turbulent supersonic flows of Mach 2.8, the proposed TDLAS sensor has improved the signal-to-noise ratio (SNR) of the transmitted laser intensity by about 6.46 dB, compared to the conventional single-ended TDLAS sensor. When the ramjet was running stably, the average Mach number was 2.82, and standard deviations of the Mach number were less than 0.09. Relative errors of measured Mach numbers were less than 1.39 %. The proposed sensor monitors the entire ramjet engine process from ignition to propellant cut-off at a rate of 5 kHz and yields detailed insights for ramjet performance evaluation.

Direct numerical simulation of fully-developed supersonic turbulent channel flows with dense vapors

This work aims to investigate the impact of the molecule-complexity effect and the non-ideal effect on wall-bounded turbulent flows by applying direct numerical simulation (DNS) to fully-developed channel flows of two typical organic vapors: R1233zd(E) and octamethyltrisiloxane (MDM). For each vapor, three thermodynamic states are analyzed: one in the dilute-gas region, one near the saturation line, and one in the supercritical region. For mean flow fields, it is found that, due to smaller Prandtl and Eckert numbers, both the molecule-complexity effect and the non-ideal effect reduce the mean temperature rise from the cold wall to the channel center. Meanwhile, the molecule-complexity effect weakens the mean density drop, while the non-ideal effect strengthens the drop. Furthermore, once the density and viscosity variations are considered, the mean streamwise velocity profiles of dense vapors are practically the same as the ideal gas. For turbulent fluctuations, it is found that the correlations between T′, p′, and ρ′ in dense vapors are more complicated than the ideal gas: for the ideal gas, fluctuations are dominated by “vorticity mode”; hence, ρ′ and T′ are strongly related to u′ but independent of p′; however, for dense vapors, “acoustic mode” can also play an important role. A newly derived equation illustrates that, through the “acoustic mode,” the molecule-complexity effect obviously enhances the positive correlation between ρ′ and p′, while the non-ideal effect can enhance the positive correlation between T′ and p′. Further analysis of instantaneous flow fields shows that p′ is isotropic. The isotropic character affects fluctuation magnitudes but has limited effect on the specified wall-direction turbulent transport. Consequently, Walz's equation and Reynolds analogy in terms of enthalpy are still valid. Finally, a comparison between the DNS energy budget and k equation of Reynolds-averaged Navier–Stokes (RANS) model has been carried out. Results show that obvious deviation happens on the production term in spite of the careful selection of eddy viscosity model.

Role of very large-scale motions in shock wave/turbulent boundary layer interactions

The present study investigates the cause of low-frequency unsteadiness in shock wave/turbulent boundary layer (TBL) interactions. A supersonic turbulent flow over a compression ramp is studied using wall-resolved large eddy simulation (LES) with a freestream Mach number of 2.95 and a Reynolds number (based on δ0: the thickness of the incoming TBL) of 63 560. From the view of stability analysis, the effect of intrinsic instability on such low-frequency unsteadiness is excluded from the flow system by designing a ramp angle of 15°, and our attention is paid to the convective instability contributed by the incoming TBL. The LES results are analyzed by linear and nonlinear disambiguation optimization (LANDO), spectral proper orthogonal decomposition (SPOD), and resolvent analysis. The LANDO results reveal a streamwise scale-frequency relation of coherent structures in a very long (around 60δ0) TBL, which indicates that the dynamics of very large-scale motions (VLSMs) in the TBL are featured by a low frequency. The SPOD results reveal that the most energetic SPOD mode features a low frequency that is identical to the dominant low frequency of the wall-pressure spectrum. Additionally, coherent structures of the mode resemble the VLSMs in the incoming TBL. These consistencies imply that the dynamics of VLSMs contribute to the low-frequency unsteadiness of the present flow. A resolvent analysis then further suggests that the origins of low-frequency dynamics of the present flow are from the VLSMs, which can be optimally amplified by the forcing in the turbulent flow.

Large eddy simulation of a supersonic lifted hydrogen flame: Impacts of Lewis, turbulent Schmidt and Prandtl numbers

Parametric large eddy simulations (LES) of a supersonic lifted hydrogen flame are reported. The emphases are on two aspects: impacts of (1) Lewis number (Lei of the ith species) and (2) turbulent Schmidt and Prandtl numbers (Sct and Prt) on supersonic turbulent flame and flow structures. Five cases are considered: species-specific Lei, Sct=Prt=1.0 (C0); unity Lei, Sct=Prt=1.0 (C1); species-specific Lei, Sct=0.5, Prt=1.0 (C2); species-specific Lei, Sct=1.0, Prt=0.5 (C3); and species-specific Lei, Sct=Prt=0.5 (C4). Numerical results of instantaneous and/or time-averaged species mole fractions, mixture fraction, heat release rate, flame base location, and mixed modes of premixed and diffusion combustion are compared between cases C0 and C1. Differences in auto-ignition locations and strengths and flame structures and stabilization specify the impacts of Lewis number. They are triggered by different predictions of species mass and thermal diffusions at fuel-coflow and/or coflow-ambient air mixing layers. These differences are rationalized by a scale analysis of mass/thermal diffusion and convection for case C0, which suggests the relatively low but non-negligible former against the latter. Cases C0 and C2–C4 barely see differences in terms of instantaneous and/or time-averaged temperature, velocity, and mixed combustion modes except for further downstream areas where combustion occurs. Both Sct and Prt impose less significant influences than Lewis number, as sub-grid scale mass/thermal diffusion is subordinate to its resolved counterpart according to their scale analysis for case C4.

The Impact of Upstream Static Deformation on Flow Past a Cylinder/Flare

Reynolds-averaged Navier–Stokes simulations are performed for supersonic turbulent flow over a cylinder/flare with upstream surface distortion representative of structural deformation induced via fluid–structural and fluid–thermal–structural behavior. Broad parametric analysis is carried out through the generation of Kriging-response surfaces from a database of general simulations. A posteriori simulations are then carried out at parametric combinations that correspond to extrema in the Kriging response surfaces to gain deeper insights into the interaction between the surface distortion and flow responses. Upstream distortions tend to decrease, rather than increase, the peak pressure and heat flux loads on the flare compared to an undeformed cylinder. Furthermore, decreases in these quantities reach up to O(10%) compared to up to O(1%) for increases. Integrated quantities over the flare are relatively insensitive to upstream distortion. The corner separation length is the most sensitive quantity to upstream distortion, with protrusions tending to increase the separation length and recessions reducing the separation length. Modifications in the separation length of up to 40% are observed. Reductions in peak loads tend to correspond to increases in the corner separation length. The movement of the surface distortion relative to the corner indicates a negligible impact beyond 1.5 distortion lengths from the corner, and the largest impact on the corner separation length occurs when distortion is directly adjacent. These results are an important step toward understanding and quantifying the impact of surface deformations on downstream components.

Direct numerical simulations of compressible turbulent channel flows with asymmetric thermal walls

This paper extends the work of Tamano & Morinishi (J. Fluid Mech., vol. 548, 2006, pp. 361–373) by simulating supersonic turbulent channel flow with asymmetric thermal walls using a larger computational domain and a finer mesh. Direct numerical simulation is carried out for four cases with different thermal wall boundaries at the top wall at fixed $Ma=1.5$ , $Re=6000$ and $Pr=0.72$ , while the bottom wall is maintained at a constant temperature of $T_L$ equal to the reference temperature. These cases are referred to as the adiabatic case TAd, where the top wall is adiabatic; the pseudo-adiabatic case T32, where the top wall is isothermal with temperature $T_{w,t}=T_A$ ; the sub-adiabatic case T25, with $T_{w,t}=0.77T_A$ ; and the super-adiabatic case T40, with $T_{w,t}=1.24T_A$ . Here, $T_A=3.234$ is the mean temperature at the adiabatic wall in the TAd case. The objective of this study is to compare and contrast the TAd case with its corresponding T32 case, and to investigate the effect of the wall temperature difference between the two isothermal walls. Comparisons of the basic turbulent statistics, the heat transfer between the Favre-averaged mean-flow kinetic energy, the Favre-averaged turbulent kinetic energy and the Favre-averaged mean internal energy, as well as the wall heat transfer properties, indicate that the TAd case and its corresponding T32 case are generally equivalent. The only discernible difference is in the region very close to the top wall for the temperature-fluctuation-related quantities. The analysis reveals that the asymmetry of the thermal walls causes asymmetry in the flow and thermal fields. In addition, the transfer of the heat generated by the pressure dilatation and the viscous stress is facilitated by the turbulent heat flux term and the mean molecular heat flux term.

Direct numerical simulations of high-enthalpy supersonic turbulent channel flows including finite-rate reactions

Direct numerical simulations of temporally evolving high-enthalpy supersonic turbulent channel flows are conducted at a Mach number of 3.0 and Reynolds number of 4880 under isothermal wall conditions. Air is assumed to behave as a five-species mixture, and chemical non-equilibrium and equilibrium assumptions are adopted to investigate the influence of finite-rate reactions on the turbulent statistics and large-scale structures. The two wall temperatures of 1733.2 and 3500 K are such that the mixture components undergo strong dissociation and recombination reactions along the channel. Investigation shows that the turbulent intensity is weakened and the mean and fluctuating temperatures are smaller when finite-rate reactions are considered. The mean dissociation degree is a quadratic function of the normal position, and its curvature under the chemical non-equilibrium assumption is greater than that under the chemical equilibrium assumption. The fluctuating mass fractions of the generated species seem to decrease slightly in the near-wall region, and their distributions are obviously different from those of the fluctuating velocity and fluctuating temperature. Finite-rate reactions increase the proportion of turbulent kinetic energy production in the skin friction decomposition, especially when the wall temperature is 3500 K. The large-scale structures visualized by the cross correlation between temperature and species mass fraction become stronger in the normal direction. The turbulent Schmidt number and several velocity–temperature correlations, including the recovery enthalpy and strong Reynolds analogy, are insensitive to the chemical reaction rate and wall temperature.

FLEW: A DNS Solver for Compressible Flows in Generalized Curvilinear Coordinates

We present FLEW, an in-house high-fidelity solver for direct numerical simulation (DNS) of turbulent compressible flows over arbitrary shaped geometries. FLEW solves the Navier–Stokes equations written in a generalized curvilinear coordinate system, in which the surface coordinates are non-orthogonal, whereas the third axis is normal to the surface. Spatial discretization relies on high-order finite-difference schemes. The convective terms are discretized using an hybrid approach, combining the near-zero numerical dissipation provided by central approximations with the robustness of weighted essentially non-oscillatory (WENO) schemes, required to capture shock waves. Central schemes are stabilized using a skew-symmetric-like splitting of convective derivatives, endowing the solver with the energy-preserving property in the inviscid limit. The maximum order of accuracy is eighth for central schemes (also used for viscous terms discretization) and seventh for WENO. The code is oriented to modern high-performance computing (HPC) platforms thanks to message passing interface (MPI) parallelization and the ability to run on graphics processing unit (GPU) architectures. Reliability, accuracy and robustness of the code are assessed in the low-subsonic, transonic and supersonic regimes. We present the results of several benchmarks, namely the inviscid Taylor–Green vortex, turbulent curved channel flow, transonic laminar flow over a NACA 0012 airfoil and turbulent supersonic ramp flow. The results for all configurations proved to be in excellent agreement with previous studies.

Turbulent drag reduction in compressible flows using streamwise traveling waves

Skin-friction drag reduction (DR) in supersonic turbulent channel flows using streamwise traveling waves of spanwise velocity (STWSV) has been studied using direct numerical simulations. In the present study, simulations are carried out for different phase speeds of the upstream and downstream traveling waves for a bulk Mach number Mab=1.5 and bulk Reynolds number Reb = 3000. The efficacy of the control has been investigated for other bulk Reynolds numbers as well. The Stokes layer generated by the control affects the near-wall statistics primarily by modifying the coherent structures, thereby either reducing or aggravating the skin-friction drag. For Reb = 3000 and Mab=1.5, a maximum drag reduction (DRmax) of 49% is achieved, primarily due to suppression of sweeps and ejections on application of control. The efficacy of skin-friction drag reduction varies in a small range when Reb is varied from 3000 to 6000; however, the maximum drag reduction is observed for Reb = 4000, owing to a drastic decrease in sweep events. Quadrant analysis reveals that for drag reduction (DR) cases, STWSV reduce magnitude of streamwise u′ and wall-normal w′ velocity fluctuations magnitude and stacks them around zero. In contrast, w′ is increased for drag increase (DI) cases. Skin-friction decomposition using Fukagata, Iwamoto and Kasagi (FIK) identity suggests that the control alters the turbulent component significantly. The behavior of the control is also studied at Mab = 0.3 and 2.5, and it is observed that the maximum drag reduction obtained decreases for high bulk Mach numbers.

Effects of Viscosity Law on High-Temperature Supersonic Turbulent Channel Flow for Chemical Equilibrium

Direct numerical simulations of temporally evolving high-temperature supersonic turbulent channel flow for chemical equilibrium were conducted with a Mach number of 3.0, a Reynolds number of 4880, and a wall temperature of 1733.2 K to investigate the influence of the viscosity law. The mean and fluctuating viscosity for the mixture rule is higher than that for Sutherland’s law, whereas an opposite trend is observed in the mean temperature, mean pressure, and dissociation degree. The Trettel and Larsson transformed mean velocity, the Reynolds shear stress, the turbulent kinetic energy budget, and the turbulent Prandtl number are insensitive to the viscosity law. The semilocal scaling that take into account local variation of fluid characteristics better collapses the turbulent kinetic energy budget. The modified strong Reynolds analogies provide reasonably good results for the mixture rule, which are better than those for Sutherland’s law. The streamwise and spanwise coherencies for the mixture rule are stronger and weaker than those for Sutherland’s law, respectively. The relationship between viscosity and species components can help to identify the traveling wave packet.

Coexistence of different mechanisms underlying the dynamics of supersonic turbulent flow over a compression ramp

Supersonic turbulent flow over a compression ramp is studied using wall-resolved large eddy simulation with a freestream Mach number of 2.95 and a Reynolds number [based on δ0: the thickness of incoming turbulent boundary layer (TBL)] of 63 560. The unsteady dynamics of the present shock wave/turbulent boundary layer interaction (STBLI) flow are investigated by using dynamic mode decomposition techniques, linear and nonlinear disambiguation optimization, local stability analysis (LSA), and global stability analysis (GSA). By analyzing the dynamic system for the STBLI flow, three dynamically important modes with characteristic spanwise wavelengths of 2δ0, 3δ0, and 6δ0 are captured. The 2δ0 mode approximates the spanwise scale of the Görtler-like vortices and Görtler mode of LSA, suggesting the presence of Görtler instability, which is believed to be related to the unsteady motion of streaks downstream of reattachment in the flow. The features of the 3δ0 mode are also observed in large-scale motions of the incoming TBL, implying the existence of a convective mechanism that is excited and maintained by such motions. Additionally, the GSA results show the most unstable mode features a spanwise wavelength of around 6δ0, indicating the existence of global instability that is believed to be related to the oscillating motion of separation shock. The coexistence of these three mechanisms is confirmed. Discussions on the above findings provide an interpretation for low-frequency unsteadiness that the unsteadiness of surface streaks results from the combined effects of the Görtler instability near flow reattachment and the convection of large-scale motions in the incoming boundary layer, while the low-frequency shock motion may be related to a global mode driven by upstream disturbances.

Preface

The 2023 UIT (Italian Union of Thermo-Fluid Dynamics) International Conference, hereafter referred as 40th UIT 2023, was organized by the Department of Engineering at the University of Perugia (Italy), in collaboration with the UIT, on June 26-28 2023, at Palazzo Bernabei, Assisi.The annual UIT Conference was held in Assisi for the first time. The scope of the Conference covered a range of topics in computational fluid-dynamics and heat transfer; thermophysical properties; heat and mass transfer for sustainable energy systems; experimental techniques for heat and mass transfer; multiphase fluid-dynamics; natural, forced and mixed convection.The 40th UIT Heat Transfer Conference 2023 program scheduled three keynote lectures by international recognised scientists: Ibrahim Dincer from Ontario Tech. University (Oshawa, Ontario) about the role of thermodynamics in integrated energy systems; Gary Neil Coleman from NASA Langley Research Center (Hampton, USA) about numerical studies of turbulent supersonic plane-channel flows; and Sauro Filippeschi from University of Pisa (Italy) about wickless two phase heat transfer devices. A total of 97 papers were submitted to the 40th UIT Heat Transfer Conference 2023, 75 of which presented by the Authors in oral sessions and the remaining 22 in one poster session.About 150 researchers participated to the 40th UIT 2023. The Conference was a useful occasion to stimulate discussion, further understanding about heat transfer and related phenomena, present the state-of-the-art of some topics, discuss emerging trends, and promote collaborations.The Organizing Committee hopes that the event results constituted a significant contribution to the knowledge in the fields of thermos-fluid dynamics and heat transfer.A special thanks to UIT, all the people who contributed to the success of the event, the International Advisory Committee, the Local Organizing Committee, and to all the Conference attendees.With Kind RegardsFranco Cotana, Università di Perugia, ItalyFederico Rossi, Università di Perugia, ItalyCinzia Buratti, Università di Perugia, ItalyList of Committees are available in this pdf.

Numerical Simulation of Supersonic Turbulent Separated Flows Based on k–ω Turbulence Models with Different Compressibility Corrections

The accurate prediction of supersonic turbulent separated flows involved in aerospace vehicles is a great challenge for current numerical simulations. Based on the k–ω equations, several different compressibility corrections are incorporated in turbulence models to improve their prediction capabilities. Two benchmark test cases, namely the ramped cavity and the compression corner, are adopted for the numerical validation. Detailed comparisons between simulations and experiments are conducted to evaluate the effect of compressibility corrections on turbulence models. The computed results indicate that compressibility corrections have a significant impact on turbulence model performance. The compressibility correction, considering the effects of both dilatation dissipation and pressure dilatation, is suitable for the prediction of compressible free shear layers, but it may have a negative impact on the prediction of low-speed flows in the near-wall region due to the severe underprediction of the wall skin friction coefficient. In comparison, the compressibility correction only considering the effects of dilatation dissipation is conservative, with decreased predictability of free shear layers in supersonic flows, although it improves the predictions of the original models without corrections.

Accuracy evaluation of modern effective codes by comparing calculated and experimental data on the example of problem about supersonic viscous turbulent gas flow around the tandem of back-forcing and forward-forcing steps

Дано описание различных программных кодов для расчета полей течения в окрестности сложных конфигураций вязким потоком турбулентного сжимаемого газа. Проведена серия тестовых расчетов для тандема трехмерных клиньев разрежения и сжатия. Целью расчетов является анализ точности разрешения отрывных зон с использованием классических и вновь создаваемых методов. Для достижения цели описаны физические особенности течения и выполнено сравнение результатов расчетов, полученных разными кодами в рамках тестового случая «тандем клиньев сжатия и разрежения».

Wall heat flux in supersonic turbulent expansion flow with shock impingement

We perform direct numerical simulations to investigate the characteristics of wall heat flux (WHF) in the interaction of an oblique shock wave at an angle of 33.2° and free-stream Mach number M ∞ = 2.25 impinging on supersonic turbulent expansion corners with deflection angles of 0o (flat plate), 6o and 12o. The effect of the expansion on the WHF characteristics is analysed by comparing it to the interaction with the flat plate under the same flow conditions and a fixed shock impingement point. In the post-expansion region, the decreased mean WHF is found to collapse onto the flat plate case when scaled with the mean wall pressure. The statistical properties of the WHF fluctuations, including probability density function, frequency spectra, and space–time correlations, are comparatively analysed. The expansion causes an increase in the occurrence probability of negative extreme events, an enhancement of high-frequency energy, and an inhibition of intermediate-frequency energy. The increased expansion angle also results in a faster recovery of characteristic spanwise length scales and an increase in convection velocity. We use the mean WHF decomposition method in conjunction with bidimensional empirical mode decomposition to quantitatively analyse the impact of expansion on scale contributions. It is demonstrated that the presence of the expansion corner has no significant impact on the decomposed results, but it significantly reduces the contribution associated with outer large-scale structures.

Numerical Investigation of Confinement Effect on Supersonic Turbulent Flow Past Backward Facing Step With And Without Transverse Injection

Effect of confinement has been investigated numerically for supersonic turbulent flow past backward facing step in a nonreacting scramjet combustor. Flow structure downstream of the backward facing step has been studied by considering various configurations with different combustor heights as well as unconfined flow. Staged transverse sonic injectors with different combustor heights are also considered to find out the effect of confinement on the penetration, spreading and other flow features in the flow field. Three dimensional Navier Stokes equations along with k-ε turbulence model are solved using a commercial CFD software. The simulation captures all essential features of the flow. Good comparisons of various flow profiles have been obtained between experimental and computed values. Although confinement creates complicated shock reflections in the combustor, the length of the recirculation bubble behind the backward facing step remains almost constant. For the injection case, the recirculation region is extended upto the bow shock arising upstream of the injector and two recirculation bubble is seen between the backward facing step and the injector.

Numerical Study of Benchmark Experiments of Supersonic Compressor Cascades

Benchmark cases of turbulent supersonic flow through compressor cascades were computed using a modern upwind scheme (Roe with MUSCL pre-processing), with the k-ω turbulence model, and the classical MacCormack method. The test cases derive from experiments through ARL SL 19 blade cascades at Detroit Diesel Allison (DDA) at the design Mach number, and at an off-design condition from ONERA. A verification of the codes was obtained by computing an inviscid flow through a wedge cascade at similar conditions. Predictions are in very good agreement for the DDA test case for the blade surface pressure distributions. Cleaner solutions were obtained using the Roe scheme. The MacCormack solution exhibits oscillations corresponding to oscillations of the shock-induced separating boundary layer on the pressure surface near the trailing edge. Wake flow Mach number distributions were very well predicted, but not flow angle distributions. Predictions are less accurate for the ONERA test case which was at a slightly higher Mach number and which moves shock locations sufficiently to cause a more sensitive response from the suction surface boundary layer.