Effects of flow width in nominally two-dimensional turbulent separated flows

Pulsed-wire measurements of mean and fluctuating wall shear stress have been measured beneath a nominally two-dimensional separated and reattaching flow, where the flow width has been varied by means of end plates. End effects are much larger near the surface than they are in the outer flow. Residual effects of the presence of the end walls on the mean wall shear stress are seen for a flow width as large as seven bubble lengths. It is inferred that the effects of the end-wall boundary layers extend to a substantially smaller distance. The influence of the end plates on the rms of the fluctuations is markedly less than that on the mean stress.     

Review on time-resolved two-dimensional gas concentration mapping in turbulent flows using molecule-sensitive light scattering techniques

The applicability of laser-induced molecular fluorescence, Rayleigh and Raman scattering for making quantitative, time-resolved two-dimensional gas concentration measurements in turbulent flows has been reviewed. Preliminary experimental results obtained as well in cold and chemically non-reacting as in hot and reactive gas flows by several working groups indicate, that all the three techniques can be used to monitor the concentration distribution in selected sections of turbulent flows using an optical multichannel analyzer for signal acquisition. The resulting data allow to quantitatively study the turbulent mixing mechanism. Time-resolved information about large-scale structures is obtained.

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Übersicht über die zeitaufgelöste zweidimensionale Erfassung der Konzentrationsfelder in turbulenten Gasströmungen mittels molekül- sensitiver Streulichttechniken

Zusammenfassung In einem Übersichtsbeitrag wird die Anwend-barkeit von laser-induzierter Molekülfluoreszenz, Rayleigh und Raman Streuung zur quantitativen, zeitaufgelösten, zweidimensionalen Aufnahme der Gaskonzentration in turbulenten Strömungen dargestellt. Erste experimentelle Ergebnisse, von mehreren Arbeitsgruppen sowohl in kalten und chemisch nichtreagierenden als auch in heißen und reagierenden Gasströmungen erzielt, zeigen auf, daß alle drei Techniken genutzt werden können, um die Konzentrationsverteilung in ausgewählten Teilen von turbulenten Strömungen mit Hilfe eines zweidimensionalen optischen Vielkanal-Analysators darzustellen. Die gewonnenen Daten ermöglichen so eine quantitative Erforschung der turbulenten Mischungsmechanismen, und eine zeitaufgelöste Information über die sogenannten Large-scale-Strukturen ist erhältlich.

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Dedicated to Professor E. R. G. Eckert on the occasion of his 80th birthday     

Nonlinear galerkin method and subgrid-scale model for two-dimensional turbulent flows

The nonlinear Galerkin method, derived by Marion and Temam from results in dynamical systems theory, is investigated in the framework of numerical simulation of two-dimensional incompressible turbulent flows. We use the nonlinear Galerkin method together with a hyperviscosity subgrid-scale parametrization. We state the theoretical background, define the scheme, and derive some technical improvements for the method. We also report numerical experiments conducted for freely decaying as well as forced turbulence. Our main conclusion is that the performance of the nonlinear Galerkin method is important only if the dissipation range is large.This work was supported by Contract D.R.E.T. 90/1551/A000: Développement d'algorithmes de simulation numérique des équations de Navier-Stokes.     

Assessment of the SSG pressure-strain model in two-dimensional turbulent separated flows

Data from a number of benchmark two-dimensional turbulent separated flows are used to assess the performance of the Speziale, Sarkar and Gatski model [28] for the fluctuating pressure-strain correlations and, in particular, to determine whether the model can reproduce the effects of rigid boundaries without recourse to ad-hoc corrections. Comparative predictions are also obtained with a standard Reynolds-stress closure in which such correlations are necessary and these serve to distinguish the models' true performance from spurious computational artifacts. The models were implemented in a finite-volume method which solves the Navier-Stokes equations on non-orthogonal grids with colocated storage arrangement. Details of the numerical treatments adopted to obtain stable and fast-converging solutions are described and the outcome of rigorous grid-independence checks are reported for each test case. It is found that both models yield essentially similar results for the mean flow and turbulence fields. The implications of this result to the choice of Reynolds-stress closure for complex engineering simulations are discussed.     

Effects of irregular two-dimensional and three-dimensional surface roughness in turbulent channel flows

Wall-resolved Large Eddy Simulation of fully developed turbulent channel flows over two different rough surfaces is performed to investigate on the effects of irregular 2D and 3D roughness on the turbulence. The two geometries are obtained through the superimposition of sinusoidal functions having random amplitudes and different wave lengths. In the 2D configuration the irregular shape in the longitudinal direction is replicated in the transverse one, while in the 3D case the sinusoidal functions are generated both in streamwise and spanwise directions. Both channel walls are roughened in such a way as to obtain surfaces with statistically equivalent roughness height, but different shapes. In order to compare the turbulence properties over the two rough walls and to analyse the differences with a smooth wall, the simulations are performed at the same Reynolds number Re_τ = 395. The same mean roughness height h = 0.05δ (δ the half channel height) is used for the rough walls.The roughness function obtained with the 3D roughness is larger than in the 2D case, although the two walls share the same mean height. Thus, the considered irregular 3D roughness is more effective in reducing the flow velocity with respect to the 2D roughness, coherently with the literature results that identified a clear dependence of the roughness function on the effective slope (see Napoli et al. (2008)), higher in the generated 3D rough wall. The analysis of higher-order statistics shows that the effects of the roughness, independently on its two- or three-dimensional shape, are mainly confined in the inner region, supporting the Townsend’s wall similarity hypothesis. The tendency towards the isotropization is investigated through the ratio between the resolved Reynolds stress components, putting in light that the 3D irregular rough wall induces an higher reduction of the anisotropy, with respect to the 2D case.     

Improved subgrid scale model for dense turbulent solid-liquid two-phase flows

The dense solid-phase governing equations for two-phase flows are obtained by using the kinetic theory of gas molecules. Assuming that the solid-phase velocity distributions obey the Maxwell equations, the collision term for particles under dense two-phase flow conditions is also derived. In comparison with the governing equations of a dilute two-phase flow, the solid-particle‘s governing equations are developed for a dense turbulent solid-liquid flow by adopting some relevant terms from the dilute two-phase governing equations. Based on Cauchy-Helmholtz theorem and Smagorinsky model, a second-order dynamic sub-grid-scale (SGS) model, in which the sub-grid-scale stress is a function of both the strain-rate tensor and the rotation-rate tensor, is proposed to model the two-phase governing equations by applying dimension analyses. Applying the SIMPLEC algorithm and staggering grid system to the two-phase discretized governing equations and employing the slip boundary conditions on the walls, the velocity and pressure fields, and the volumetric concentration are calculated. The simulation results are in a fairly good agreement with experimental data in two operating cases in a conduit with a rectangular cross-section and these comparisons imply that these models are practical.     

Air entrainment in two-dimensional turbulent shear flows with partially developed inflow conditions

In plunging jet flows and at hydraulic jumps, large quantities of air are entrained at the intersection of the impinging flow and the receiving body of water. The air bubbles are entrained into a turbulent shear layer and strong interactions take place between the air bubble advection/diffusion process and the momentum shear region. New air-water flow experiments were conducted with two free shear layer flows: a vertical supported jet and a horizontal hydraulic jump. The inflows were partially developed boundary layers, characterized by the presence of a velocity potential core next to the entrapment point. In both cases, the distributions of air concentration exhibit a Gaussian distribution profile with an exponential longitudinal decay of the maximum air content. Interestingly, the location of the maximum air content and the half-value band width are identical for both flow situations, i.e. independent of buoyancy effects.     

Dissipation estimates in turbulent flows using the zero-wire-length technique

Azad and Kassab (1989) presented a new technique for estimating dissipation in turbulent flows and they referred to the method as the zero-wire-length technique. The validity of the approach has been here checked experimentally for the flow in the far wake of a circular cylinder for which Browne et al. (1987) had obtained reasonable estimates of the dissipation. It has been found that the zero-wire-length technique provides no more than an estimate of the isotropic dissipation: the actual dissipation values cannot be estimated by this technique.     

Pressure field,vorticity field,and coherent structures in two-dimensional incompressible turbulent flows

Geometrical arguments lead to the definition of two education criteria for coherent structures in two-dimensional incompressible turbulent flows. These criteria involve the pressure or the vorticity field and are compared.     

Spatial correlation measurement in turbulent flows using pulsed wire anemometry

Details of a new technique in pulsed wire anemometry, developed to allow measurement of two-point spatial velocity correlations in highly turbulent flows, are described. An outline of the interface devices necessary for linking two anemometers to a microcomputer is given and examples of the use of the technique are presented. Firstly, measurements of spatial correlations with and without time delay in the near wake of a nominally two-dimensional cylinder normal to a uniform stream confirmed the viability of the technique. Secondly, measurements in the highly turbulent, separated region behind a normal flat plate fitted with a downstream splitter plate are presented as a demonstration of the effectiveness of the technique in such difficult flows. We believe that these are the first direct measurement of spatial correlation functions within a separated flow.     

Quantitative visualization of compressible turbulent shear flows using condensate-enhanced Rayleigh scattering

This paper describes several flow visualization experiments carried out in Mach 3 and Mach 8 turbulent shear flows. The experimental technique was based on laser scattering from particles of H₂O or CO₂ condensate that form in the wind tunnel nozzle expansion process. The condensate particles vaporize extremely rapidly on entering the relatively hot fluid within a turbulent structure, so that a sharp vaporization interface marks the outer edge of the rotational shear layer fluid. Calculations indicate that the observed thin interface corresponds to a particle size of 10 nm or less, which is consistent with optical measurements, and that particles of this size track the fluid motions well. Further, calculations and experiments show that the freestream concentration of condensate required for flow visualization has only a small effect on the wind tunnel pressure distribution. Statistics based on the image data were compared to corresponding results from probe measurements and agreement was obtained in statistical measures of speed, scale, and orientation of the large-scale structures in the shear layer turbulence. The condensate-enhanced Rayleigh scattering technique is judged to be a useful tool for quantitative studies of shear layer structure, particularly for identifying the instantaneous boundary layer edge and for extracting comparative information on the large-scale structures represented there.     

Parallel computation of turbulent flows using moment base lattice Boltzmann method

This paper describes parallel computing approach for simulating turbulent flows using a moment base lattice Boltzmann method. The distribution functions of the lattice Boltzmann method are expressed by corresponding moments. Choosing proper relaxation times for higher order moments, a minimum numerical dissipation is implicitly added to stabilise the method at high Reynolds numbers. Validation of the method is made by computing free decaying periodic turbulent flows and fully developed turbulent channel flows on a GPU platform. Though the present method requires additional work to calculate the higher order moments, it is shown that additional computational cost is negligible in the GPU computing. The numerical results stably obtained for the turbulent flows are in good agreement with those of a pseudo-spectral method and corresponding DNS database.     

Spatio-temporal correlation analysis of turbulent flows using global and single-point measurements

A method to extract whole-field spatio-temporal correlations by combining global and single-point measurement techniques of different time resolutions is proposed. For fluid mechanics applications, the emphasis is on the combination of low repetition rate particle image velocimetry (PIV) results with experimental data obtained at largely higher sampling frequencies. The experimental feasibility of the procedure is established from results obtained in the wake of a cylinder, using PIV and constant temperature hot wire anemometry (CTA). The method is then applied to examine the shear layer in the core of a round subsonic jet using PIV and laser Doppler velocimetry (LDV). The accuracy of the cross-correlation functions is compared to the auto- and cross-correlation functions obtained from series of LDV and CTA measurements.     

Modelling turbulent and dispersion heat fluxes in turbulent porous medium flows using the resolved LES data

Resolved large eddy simulations (LESs) of turbulent conjugate heat transfer in porous media are performed by the lattice Boltzmann method (LBM) for modelling turbulent and dispersion heat flux terms of the double-averaged energy equation. The considered porous structures are square rod arrays, staggered cube arrays and body centred cubic foam. In the LBM, the double-distribution function method which solves the distribution functions for the velocity and the internal energy is used. For the velocity and thermal fields, the D3Q27 multiple-relaxation-time method and the regularized D3Q19 single-relaxation-time method are applied, respectively. A priori tests using the LES data suggest that the trends of the sum of the dispersion and volume-averaged turbulent heat fluxes can be well captured by the second order gradient diffusion model.     

Heterogeneous drag reduction in turbulent pipe flows using various injection techniques

The results of an experimental study of the injection of concentrated polymer solutions into the near-wall region of a turbulent pipe flow are reported. The injection experiments described here show drag reduction that was significantly larger than that obtained for homogeneous polymer solutions of the same average concentration. Local drag reduction and friction behavior was obtained by measuring pressure differences over a test section of 13 m in length. Furthermore the flow behaviour of the injected polymer solution was investigated by flow visualization experiments. Velocity profile measurements elucidate in case of near-wall injection that the turbulent structure could be altered in the near-wall and also in the core region of the pipe flow, indicating that the polymer lumps and threads created by the near-wall injection are able to influence a much wider spectrum of turbulent eddies in comparison to centreline injection or, all the more, to homogeneous drag reduction.     

Numerical study of turbulent trailing-edge flows with base cavity effects using URANS

Turbulent flows over lifting surfaces exhibiting trailing-edge vortex shedding often cause adverse and complex phenomena, such as self-induced vibration and noise. In this paper, a numerical study on flow past a blunt-edged two-dimensional NACA 0015 section and the same section with various base cavity shapes and sizes at high Reynolds numbers has been performed using the unsteady Reynolds-averaged Navier–Stokes (URANS) approach with the realisable κ–ε turbulence model. The equations are solved using the control volume method of second-order accuracy in both spatial and time domains. The assessment of the application of URANS for periodic trailing-edge flow has shown that reasonable agreement is achieved for both the time-averaged and fluctuating parameters of interest, although some differences exist in the prediction of the near-wake streamwise velocity fluctuation magnitudes. The predicted Strouhal numbers of flows past the squared-off blunt configuration with varying degrees of bluntness agree well with published experimental measurements. It is found that the intensity of the vortex strengths at the trailing-edge is amplified when the degree of bluntness is increased, leading to an increase in the mean square pressure fluctuations. The numerical prediction shows that the presence of the base cavity at the trailing-edge does not change the inherent Strouhal number of the 2D section examined. However, it does have an apparent effect on the wake structure, local pressure fluctuations and the lift force fluctuations. It is observed that the size of the cavity has more influence on the periodic trailing-edge flow than its shape does.