DNS of the interaction between a shock wave and a turbulent shear flow: some effects of anisotropy

Direct numerical simulation is used to study the interaction of a Mach 1.5 shock wave and various types of anisotropic turbulent flows. We compare the interaction of isotropic, axisymmetric and sheared turbulences (sometimes combined), with a specific interest for the sheared situation. The sign and magnitude of the correlation between the velocity and temperature fluctuations are found to have a crucial influence on the kinetic energy amplification across the shock. A decrease in magnitude is observed during the interaction for the velocity cross-correlation. The balance equation of this quantity is investigated and the terms responsible for this behaviour are identified. The shear stress effect upon fluctuating vorticity and the dissipation length scale is also presented. Thermodynamic fluctuations are finally analyzed, showing the departure from the isentropic state in the sheared situation compared to the isotropic one

A study of sheared turbulence/shock interaction: velocity fluctuations and enstrophy behaviour

Direct Numerical Simulations of the idealized interaction of a normal shock wave with several turbulent shear flows are conducted. We analyse the behaviours of velocity and vorticity fluctuations and compare them to what happens in the isotropic situation. Investigation of the budgets of these quantities allows to isolate the mechanisms underlying the physics of the interaction, and reveals the importance of enthalpic production and baroclinic torque in such flows

DNS of the interaction between a shock wave and a turbulent shear flow

Direct numerical simulation is used to study the interaction of a Mach 1.5 shock wave and various types of anisotropic turbulent flows. We compare the interaction of isotropic, axisymmetric and sheared turbulences (sometimes combined), with a specific interest for the sheared situation

Study of the turbulent mixing zone induced by the Richtmyer-Meshkov instability using Laser Doppler Velocimetry and Schlieren visualizations

An experimental study of the compressible mixing generated by the Richtmyer-Meshkov instability (RMI) is carried out in a vertical shock tube by means of two-components Laser Doppler Velocimetry (2C-LDV) measurements and Time-resolved Schlieren visualizations. An attempt is made to quantify the RMI-induced air/sulphurhexafluoride (SF6) mixing by measuring turbulence levels inside the mixing zone at a given stage of its development and by extracting the growth rate of the mixing zone from the Schlieren images

Investigation of the interface stretching within a reshocked mixing zone produced by the Richtmyer Meshkov Instability

The spatio-temporal evolution of a bi-dimensional (2D) and three-dimensional (3D) air/SF6 mixing layer issued from the development of a Richtmyer-Meshkov instability (RMI) under reshock is investigated using direct numerical simulations (DNS) at moderate Mach number (M=1.2) and high Atwood number (A=0.67). This study discusses the relevance of an original criterion based on the measurement of the gaseous interface stretching in the analysis of the mixing process. The first part of the work provides an estimation of the validity of a 2D approach in time for the retained simulation cases. To this avail, a 2D simulation for one typical parameter set is compared to its 3D counterpart. As a means of comparing the development of the mixing layer in both simulations, the classical criterion relying on the evaluation of the mixing layer thickness has been chosen. This criterion is commonly used to characterize baroclinic instability as it is intuitive and easy to compute and to analyze. However, this criterion only provides the mixing zone frontiers but does not provide information about the length scale content and its evolution on the interface. In order to tackle this issue, it is proposed to adapt a still documented criterion for the determination of the interface stretching, based on the computation of the temporal evolution of the mixing interface length for the study of various cases involving different initial interface perturbations, with reshock consideration

Experimental determination of the growth rate of Richtmyer-Meshkov induced turbulent mixing after reshock

The time evolution of the width of the turbulent mixing zone arising from the late development of Richtmyer-Meshkov instability is investigated in this work. This is achieved by means of the analysis of time-resolved Schlieren images obtained with a given set of shock-tube experiments. The post-reshock growth rate of the mixing zone width is found to be nearly insensitive to the development state of the mixing at the time of reshock

LES of shock wave/turbulent boundary layer interaction affected by microramp vortex generators

At large Mach numbers, the interaction of an oblique shock wave with a turbulent boundary layer (SWTBLI) developing over a flat plate gives rise to a separation bubble known to exhibit low-frequency streamwise oscillations around StL = 0.03 (a Strouhal number based on the separated region length). Because these oscillations yield wall pressure or load fluctuations, efforts are made to reduce their amplitude. We perform large eddy simulations to reproduce the experiments by Wang etal (2012) where a rake of microramp vortex generators (MVGs) were inserted upstream the SWTBLI with consequences yet to be fully understood. There is no consensus on the flow structure downstream MVGs and this is first clarified in the case of MVGs protruding by 0.47δ in a TBL at Mach number M = 2.7 and Reynolds number Reθ = 3600. Large-scale vortices intermittently shed downstream the MVGs are characterized by a streamwise period close to twice the TBL thickness and a frequency f ≈ 0.5Ue/δ, two orders of magnitude higher than the one of the uncontrolled SWTBLI. We then characterize the interaction between the unsteady wake of the MVGs with the SWTBLI resulting in the reduction of the interaction length and the high-frequency modulation of the shock feet motions

Towards the characterization of micro vortex generators effects on shock wave / turbulent boundary layer interaction using LES

We perform Large Eddy Simulations (LES) of the experimental configuration by Wang et al. (2012): a rake of microramp vortex generators (MVGs) were inserted upstream the SWTBLI. The configuration features MVGs protruding by 0.47 delta in a TBL at M = 2.7 and Re_theta = 3600. We first validate the flow solver and LES strategy on a baseline configuration without control and we retrieve the characteristic length scales and the low frequency motion (StL = 0.03) of the reflected shock foot. The configuration with MVGs exhibits successive regions alternating between either momentum deficit or momentum excess downstream the MVGs, with good agreement with the experiments. Classical wake recovery laws were retrieved as well as the frequency of St = 0.53 characteristic of the shedding of intermittent structures downstream the MVGs and we observe a significant 20% decrease of the separation bubble length

Simulations of shock wave/turbulent boundary layer interaction with upstream micro vortex generators

The streamwise breathing motion of the separation bubble, triggered by the shock wave/boundary layer interaction (SBLI) at large Mach number, is known to yield wall pressure and aerodynamic load fluctuations. Following the experiments by Wang et al. (2012), we aim to evaluate and understand how the introduction of microramp vortex generators (mVGs) upstream the interaction may reduce the amplitude of these fluctuations. We first perform a reference large-eddy simulation (LES) of the canonical situation when the interaction occurs between the turbulent boundary layer (TBL) over a flat plate at Mach number M=2.7 and Reynolds number Reθ=3600 and an incident oblique shock wave produced on an opposite wall. A high-resolution simulation is then performed including a rake of microramps protruding by 0.47δ in the TBL. The long time integration of the simulations allows to capture 52 and 32 low-frequency oscillations for the natural case and controlled SBLI, respectively. In the natural case, we retrieve the pressure fluctuations associated with the reflected shock foot motions at low-frequency characterized by StL=0.02−0.06. The controlled case reveals a complex interaction between the otherwise two-dimensional separation bubble and the array of hairpin vortices shed at a much higher frequency StL=2.4 by the mVGs rake. The effect on the map of averaged wall shear stress and on the pressure load fluctuations in the interaction zone is described, with a 20% and 9% reduction of the mean separated area and pressure load fluctuations, respectively. Furthermore, the controlled SBLI exhibits a new oscillating motion of the reflected shock foot, varying in the spanwise direction with a characteristic low-frequency of StL=0.1 in the wake of the mVGs and StL=0.05 in between

Direct numerical simulation of the interaction between a shock wave and various types of isotropic turbulence

Direct Numerical Simulation (DNS) is used to study the interaction between normal shock waves of moderate strength (M1=1.2 and M1=1.5) and isotropic turbulence. A complete description of the turbulence behaviour across the shock is provided and the influence of the nature of the incoming turbulence on the interaction is investigated. The presence of upstream entropy fluctuations satisfying the Strong Reynolds Analogy enhances the amplification of the turbulent kinetic energy and transverse vorticity variances across the shock compared to the solenoidal (pure vorticity) case. Budgets for the fluctuating-vorticity variances are computed, showing that the baroclinic torque is responsible for this additional production of transverse vorticity. More reduction of the transverse Taylor microscale and integral scale is also observed in the vorticity-entropy case while no influence can be seen on the longitudinal Taylor microscale. When the upstream turbulence is dominated by acoustic and vortical fluctuations, less amplification of the kinetic energy (for Mach numbers between 1.25 and 1.8), less reduction of the transverse microscale and more amplification of the transverse vorticity variance are observed through the shock compared to the solenoidal case. In all cases, the classic estimation of Batchelor relating the dissipation rate and the integral scale of the flow proves to be invalid. These results are obtained with the same numerical tool and similar flow parameters, and they are in good agreement with Linear Interaction Analysis (LIA)