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A far-field solver based on the acoustic analogy of Ffowcs Williams and Hawkings for acoustic problems with convection and vortical waves was implemented in the open source package Overture. The numerical calculations of CAA test cases showed good agreement with analytical solutions or results from measurements and simulation data from the literature. This paper presents the implemented solver and its verification. In futher work, the developed framework will be used to investigate jet-flap interaction and the emitted sound field, numerically.

A new class of one-dimensional solar wind models is developed within the general polytropic, single-fluid hydrodynamic framework. The particular case of quasi-adiabatic radial expansion with a localized heating source is considered. We consider analytical solutions with continuous Mach number over the entire radial domain while allowing for jumps in the flow velocity, density, and temperature, provided that there exists an external source of energy in the vicinity of the critical point which supports such jumps in physical quantities. This is substantially distinct from both the standard Parker solar wind model and the original nozzle solutions, where such discontinuous solutions are not permissible. We obtain novel sample analytic solutions of the governing equations corresponding to both slow and fast wind., 13 pages, 3 figures, MNRAS, Accepted for publication

Pseudopotential-based lattice Boltzmann models provide an efficient means to simulate multiphase flows. Although thoroughly analysed and extended, they still suffer from various numerical deficiencies, such as stability issues at large density and viscosity ratios as well as spurious velocities in interfacial regions. In this work, we show how to optimise the model numerically with regard to the latter. This optimised scheme is analysed in static as well as oscillating droplet simulations and is compared to different higher-order isotropic schemes. We found that the optimised scheme substantially lowers spurious velocities and competes well with comparable schemes in both test cases.

Coaxial jets originating from a nozzle interacting with a two-element wing configuration consisting of a main wing and a flap are computed using large-eddy simulations (LES) to investigate in the long term the effect of the interaction on the sound field of this jet-wing-flap configuration. The secondary jet Mach number is \(M_s=0.37\) and the Reynolds number based on the secondary velocity and diameter is \(Re_{s}=1.32 \times 10^6\). The nozzle inlet boundary layers are modeled by Blasius velocity profiles with a boundary layer thicknesses of 20% of the nozzle channel half-width. The acoustic far-field is predicted by solving the linearized Euler equations (LEE) in a region attached to the LES domain. The jet streams interacts with the two-element wing configuration and lead to a significantly changed mean flow field in the wing-flap gap area and pressure at the wing surface. Additionally, a strong influence of the flap on the jet development is observed, especially in the case of a non-zero free stream velocity.

We propose and analyse a new strategy of shear flow turbulence control that can be realized by the following steps: (i) imposing specially designed seed velocity perturbations, which are non-symmetric in the spanwise direction, at the walls of a flow; (ii) the configuration of the latter ensures a gain of shear flow energy and the breaking of turbulence spanwise reflection symmetry: this leads to the generation of spanwise mean flow; (iii) that changes the self-sustained dynamics of turbulence and results in a considerable reduction of the turbulence level and the production of turbulent kinetic energy. In fact, by this strategy the shear flow transient growth mechanism is activated and the formed spanwise mean flow is an intrinsic, nonlinear composition of the controlled turbulence and not directly introduced in the system. In the present paper, a weak near-wall volume forcing is designed to impose the velocity perturbations with required characteristics in the flow. The efficiency of the proposed scheme has been demonstrated by direct numerical simulation using plane Couette flow as a representative example. A promising result was obtained: after a careful parameter selection, the forcing reduces the turbulence kinetic energy and its production by up to one-third. The strategy can be naturally applied to other wall-bounded flows, e.g. channel and boundary-layer flows. Of course, the considered volume force is theoretical and hypothetical. Nevertheless, it helps to gain knowledge concerning the design of the seed velocity field that is necessary to be imposed in the flow to achieve a significant reduction of the turbulent kinetic energy. This is convincing with regard to a new control strategy, which could be based on specially constructed blowing/suction or riblets, by employing the insight gained by the comprehension of the results obtained using the investigated methodology in this paper.

To understand the self-sustenance of subcritical turbulence in spectrally stable shear flows, we performed direct numerical simulations of homogeneous shear turbulence for different aspect ratios of the flow domain and analyzed the dynamical processes in Fourier space. There are no exponentially growing modes in such flows and the turbulence is energetically supported only by the linear growth of perturbation harmonics due to the shear flow non-normality. This non-normality-induced, or nonmodal growth is anisotropic in spectral space, which, in turn, leads to anisotropy of nonlinear processes in this space. As a result, a transverse (angular) redistribution of harmonics in Fourier space appears to be the main nonlinear process in these flows, rather than direct or inverse cascades. We refer to this type of nonlinear redistribution as the nonlinear transverse cascade. It is demonstrated that the turbulence is sustained by a subtle interplay between the linear nonmodal growth and the nonlinear transverse cascade that exemplifies a well-known bypass scenario of subcritical turbulence. These two basic processes mainly operate at large length scales, comparable to the domain size. Therefore, this central, small wave number area of Fourier space is crucial in the self-sustenance; we defined its size and labeled it as the vital area of turbulence. Outside the vital area, the nonmodal growth and the transverse cascade are of secondary importance. Although the cascades and the self-sustaining process of turbulence are qualitatively the same at different aspect ratios, the number of harmonics actively participating in this process varies, but always remains quite large. This implies that the self-sustenance of subcritical turbulence cannot be described by low-order models., 22 pages, 24 figures

Direct numerical simulation is used to investigate effects of heat release and compressibility on mixing-layer turbulence during a period of self-similarity. Temporally evolving mixing layers are analysed at convective Mach numbers between 0.15 and 1.1 and in a Reynolds number range of 15000 to 35000 based on vorticity thickness. The turbulence inhibiting effects of heat release are traced back to mean density variations using an analysis of the fluctuating pressure field based on a Green's function.

We performed direct numerical simulations of homogeneous shear turbulence to study the mechanism of the self-sustenance of subcritical turbulence in spectrally stable (constant) shear flows. For this purpose, we analyzed the turbulence dynamics in Fourier/wavenumber/spectral space based on the simulation data for the domain aspect ratio 1 : 1 : 1. Specifically, we examined the interplay of linear transient growth of Fourier harmonics and nonlinear processes. The transient growth of harmonics is strongly anisotropic in spectral space. This, in turn, leads to anisotropy of nonlinear processes in spectral space and, as a result, the main nonlinear process appears to be not a direct/inverse, but rather a transverse/angular redistribution of harmonics in Fourier space referred to as the nonlinear transverse cascade. It is demonstrated that the turbulence is sustained by the interplay of the linear transient, or nonmodal growth and the transverse cascade. This course of events reliably exemplifies the wellknown bypass scenario of subcritical turbulence in spectrally stable shear flows. These processes mainly operate at large length scales, comparable to the box size. Consequently, the central, small wavenumber area of Fourier space (the size of which is determined below) is crucial in the self-sustenance and is labeled the vital area. Outside the vital area, the transient growth and the transverse cascade are of secondary importance - Fourier harmonics are transferred to dissipative scales by the nonlinear direct cascade. The number of harmonics actively participating in the self-sustaining process (i.e., the harmonics whose energies grow more than 10% of the maximum spectral energy at least once during evolution) is quite large - it is equal to 36 for the considered box aspect ratio - and obviously cannot be described by low-order models.

Direct numerical (DNS) and large-eddy simulations (LES) of a strongly heated globally unstable round jet are juxtaposed with respect to aerodynamical mean characteristics and the sound being generated. The sound field is computed by a hybrid approach using the acoustic perturbation equations (APE). All used codes have been adopted to massive-parallel supercomputers. This way results can be obtained in a reasonable time frame. When compared to the DNS results, the LES is capable to capture the major characteristics of the emitted sound field in the forward direction. The sideline and backward direction that are dominated by small scale noise reveal larger discrepancies that are due to the inherent restrictions of large eddy simulations.

Direct numerical simulation (DNS) is used to explore similarities and differences between fully developed supersonic turbulent plane channel and axisymmetric non-swirling pipe flow bounded by isothermal walls. The comparison is based on equal friction Mach number, friction Reynolds number, Prandtl number, ratio of specific heats and viscosity exponent. The channel half-width and pipe radius are chosen to define the Reynolds numbers. To what extent and why mean flow quantities, second-order turbulence statistics and terms in the Reynolds stress equations coincide or diverge in both flows are investigated. The role of the fluctuating pressure in causing characteristic differences among correlations involving pressure fluctuations is identified via a Green-function-based analysis of the pressure field.

Direct numerical simulations of compressible turbulent channel flow including passive scalar transport have been performed at five Mach numbers, M, ranging from 0.3 to 3.5 and Reynolds numbers, Re, ranging from 181 to 1030. The Prandtl and Schmidt numbers are 0.7 and 1.0, respectively, in all cases. The passive scalar is added to the flow through one channel wall and removed through the other, leading to an S-shaped mean scalar profile with non-zero gradient in the channel centre. The paper describes the set of compressible flow equations, which is integrated using high-order numerical schemes in space and time. Statistical equations are presented for fully developed flow, including budgets for the Reynolds stresses, the turbulent scalar fluxes and the scalar variance. Results are presented for second order moments and the terms in the mentioned balance equations. Outer scalings are found suitable to collapse incompressible and compressible data. The reduction in the near-wall pressure-strain and pressure-scalar gradient correlations due to compressibility is explained using a Green-function-based analysis of the fluctuating pressure field.

Direct numerical simulations (DNS) of compressible supersonic channel flow of air at Reynolds numbers ranging from Re τ = 180 to Re τ = 560 and Mach numbers ranging from M = 0.3 to M = 3.0 have been performed. A Navier-Stokes solver of high order accuracy has been vectorized and parallelized to run efficiently on the Hitachi SR8000-F1. Budgets of the Reynolds stresses and the passive scalar fluxes are presented, as well as explanations concerning the reduction of the pressure-correlation terms, using a Green's function approach.

Direct numerical simulations (DNS) of turbulent supersonic channel flow of air at Reynolds numbers ranging from Re τ = 180 to 560 and Mach numbers ranging from M = 0.3 to 3.0 have been performed. The DNS data are used to explain the reduction of the pressure-correlation terms due to compressibility, using a Green’s function approach.

Direct numerical simulations of compressible supersonic turbulent channel flow with passive scalar transport are performed at the Reynolds numbers Re T of 455 and 544, at a Mach number of 1.5 and Prandtl and Schmidt numbers of 0.7 and 1.0, respectively. A mean scalar gradient is imposed across the mean flow. Budgets of the scalar variance and the scalar flux components are presented for both Reynolds numbers. A scaling of the Van-Driest transformed scalar similar to u VD + in the viscous is observed. Furthermore scalings of the Reynolds stress components and the pressure-rate-of-strain tensor that collapse both compressible and incompressible data in the outer layer are derived and investigated in more detail.

Direct numerical simulations of isothermal channel flow, with Mach numbers ranging from M = 0.3 to M = 3 and Reynolds numbers ranging from Re τ = 180 to Re τ = 560 have been performed. Their statistical analysis shows that changes in the turbulent stress peak values are linked to the changes in the corresponding pressure strain components. The strong attenuation of the pressure strain correlations relative to their incompressible values, is shown to be due to decreasing velocity derivative fluctuations for x 2 * < 35, as well as to the decreased mean density with respect to its wall value.

Turbulence in supersonic channel flow is studied using direct numerical simulation. The ability of outer and inner scalings to collapse profiles of turbulent stresses onto their incompressible counterparts is investigated. Such collapse is adequate with outer scaling when sufficiently far from the wall, but not with inner scaling. Compressibility effects on the turbulent stresses, their anisotropy, and their balance equations are identified. A reduction in the near-wall pressure-strain, found responsible for the changed Reynolds-stress profiles, is explained using a Green's-function-based analysis of the pressure field

A semi-implicit large-eddy simulation technique is used to predict transport and infinitely fast reaction processes of an / jet injected through a narrow spanwise slot into a subsonic turbulent air flow between isothermal channel walls. The large-eddy simulation (LES) technique is based on approximate deconvolution and explicit modelling of the filtered heat release term. Spatial derivatives are computed using sixth-order accurate central compact schemes. An explicit fourth-order Runge–Kutta algorithm serves for time-integration. Turbulent inflow conditions are generated by a separate LES of fully developed channel flow and are introduced well upstream of the injection station using characteristic boundary conditions. The complex transport processes in the vicinity of the injection region are highlighted by instantaneous and statistically averaged flow quantities by Reynolds stress and total enthalpy balances.

This article employs LES to simulate temporal mixing layers with Mach numbers ranging from M c = 0.3 to M c = 1.2. A form of approximate deconvolution together with a dynamic Smagorinsky subgrid model are employed as subgrid models. A large computational domain is used along with relatively good resolution. The LES results regarding growth rate, turbulence levels, turbulence anisotropy, and pressure–strain correlation show excellent agreement with those available from previous experimental and DNS results of the same flow configuration, underlining the effectiveness and accuracy of properly conducted LES. Coherent structures during the transitional stage change from spanwise aligned rollers to streamwise-aligned thinner vortices at high Mach number. In the quasi-self-similar turbulent stage, the resolved-scale vorticity is more isotropic at higher M c , and its vertical correlation length scale is smaller. The ratio of the vertical integral length scale of streamwise velocity fluctuation to a characteristic isotropic estimate is found to decrease with increasing M c . Thus, compressibility leads to increased spatial decorrelation of turbulence which is one reason for the reduction in pressure–strain correlation with increasing M c . The balance of the resolved-scale fluctuating vorticity is examined, and it is observed that the linear production by mean shear becomes less important compared to nonlinear vortex stretching at high M c . A spectral decomposition of the pressure fluctuations into low- and intermediate-to-high-wave numbers is performed. The low-wave number part of the pressure field is found not to correlate with the strain field, although it does have a significant contribution to the r.m.s of the fluctuating pressure. As a consequence, the pressure–strain correlation can be analyzed using a simplified Green’s function for the Poisson equation as is demonstrated here using the LES data.