Low-order stochastic modelling of low-frequency motions in reflected shock-wave/boundary-layer interactions

A combined numerical and analytical approach is used to study the low-frequencyshock motions observed in shock/turbulent-boundary-layer interactions in theparticular case of a shock-reflection configuration. Starting from an exact formof the momentum integral equation and guided by data from large-eddy simulations,a stochastic ordinary differential equation for the reflected-shock-foot low-frequencymotions is derived. During the derivation a similarity hypothesis is verified for thestreamwise evolution of boundary-layer thickness measures in the interaction zone. Inits simplest form, the derived governing equation is mathematically equivalent to thatpostulated without proof by Plotkin (AIAA J., vol. 13, 1975, p. 1036). In the presentcontribution, all the terms in the equation are modelled, leading to a closed form ofthe system, which is then applied to a wide range of input parameters. The resultingmap of the most energetic low-frequency motions is presented. It is found that whilethe mean boundary-layer properties are important in controlling the interaction size,they do not contribute significantly to the dynamics. Moreover, the frequency of themost energetic fluctuations is shown to be a robust feature, in agreement with earlierexperimental observations. The model is proved capable of reproducing available lowfrequencyexperimental and numerical wall-pressure spectra. The coupling betweenthe shock and the boundary layer is found to be mathematically equivalent to afirst-order low-pass filter. It is argued that the observed low-frequency unsteadinessin such interactions is not necessarily a property of the forcing, either from upstreamor downstream of the shock, but an intrinsic property of the coupled system, whoseresponse to white-noise forcing is in excellent agreement with actual spectra

Direct numerical simulations of a high-pressure turbine vane

In this paper, we establish a benchmark data set of a generic high-pressure (HP) turbine vane generated by direct numerical simulation (DNS) to resolve fully the flow. The test conditions for this case are a Reynolds number of 0.57 × 106 and an exit Mach number of 0.9, which is representative of a modern transonic HP turbine vane. In this study, we first compare the simulation results with previously published experimental data. We then investigate how turbulence affects the surface flow physics and heat transfer. An analysis of the development of loss through the vane passage is also performed. The results indicate that freestream turbulence tends to induce streaks within the near-wall flow, which augment the surface heat transfer. Turbulent breakdown is observed over the late suction surface, and this occurs via the growth of two-dimensional Kelvin–Helmholtz spanwise roll-ups, which then develop into lambda vortices creating large local peaks in the surface heat transfer. Turbulent dissipation is found to significantly increase losses within the trailing-edge region of the vane.Partnership for Advanced Computing in Europe (PRACE) and the UK Turbulence Consortium funded by the EPSRC under Grant No. EP/L000261/

Large-Scale Streamwise Vortices in Turbulent Channel Flow Induced by Active Wall Actuations

© 2017, Springer Science+Business Media B.V., part of Springer Nature. Direct numerical simulations of turbulent flow in a plane channel using spanwise alternatively distributed strips (SADS) are performed to investigate the characteristics of large-scale streamwise vortices (LSSVs) induced by small-scale active wall actuations, and their role in suppressing flow separation. SADS control is obtained by alternatively applying out-of-phase control (OPC) and in-phase control (IPC) to the wall-normal velocity component of the lower channel wall, in the spanwise direction. Besides the non-controlled channel flow simulated as a reference, four controlled cases with 1, 2, 3 and 4 pairs of OPC/IPC strips are studied at M = 0.2 and Re = 6,000, based on the bulk velocity and the channel half height. The case with 2 pairs of strips, whose width is Δz+ = 264 based on the friction velocity of the non-controlled case, is the most effective in terms of generating large-scale motions. It is also found that the OPC (resp. IPC) strips suppress (resp. enhance) the coherent structures and that leads to the creation of a vertical shear layer, which is responsible for the LSSVs presence. They are in a statistically steady state and their cores are located between two neighbouring OPC and IPC strips. These motions contribute significantly to the momentum transport in the wall-normal and spanwise directions showing potential for flow separation suppression

Surviving Sepsis Campaign: International guidelines for management of severe sepsis and septic shock: 2008

SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Compressible mixing layer growth rate and turbulence characteristics

Direct numerical simulation databases have been used to study the effect of compressibility on mixing layers. The simulations cover convective Mach numbers from 0.2 to 1.2 and all contain a fully resolved turbulent energy cascade to small spatial scales. Statistical information is extracted from the databases to determine reasons for the reduced growth rate that is observed as the convective Mach number is increased. It is found that the dilatational contribution to dissipation is negligible even when eddy shocklets are observed in the flow. Also pressure-dilatation is not found to be significant. Using an accurate relation between the momentum thickness growth rate and the production of turbulence kinetic energy together with integrated equations for the Reynolds stress tensor it is shown that reduced pressure fluctuations are responsible for the changes in growth rate via the pressure–strain term. A deterministic model for the required pressure fluctuations is given based on the structure of variable-density vortices and the assumption that the limiting eddies are sonic. Simple anisotropy considerations are used to close the averaged equations. Good agreement with turbulence statistics obtained from the simulations is found.\u3cbr/\u3

Direct numerical simulations of shocklet-containing turbulent channel counter-flows

Counter-flow or counter-current configurations can maintain high turbulence intensities and exhibit a significant level of mixing. We have previously introduced a wall-bounded counter-flow turbulent channel configuration (Physical Review Fluids, 6(9), p.094603.) as an efficient framework to study compressibility effects on turbulence. Here, we extend our previous direct numerical simulation study to a relatively higher Mach number (M = 0.7) to investigate strong compressibility effects (also by reducing the Prandtl number from Pr = 0.7 to 0.2), and the formation and evolution of unsteady shocklet structures. It is found that the configuration is able to produce highly turbulent flows with embedded shocklets and significant asymmetry in probability density functions of dilatation. A peak turbulent Mach number close to unity is obtained, for which the contribution of the dilatational dissipation to total dissipation is nevertheless found to be limited to 6%