Turbulent Shear Layer Research Articles

Multiple time-scale turbulence model for wall and homogeneous shear flows based on direct numerical simulations

The k-ϵ model is widely used and has proved effective in predicting wall shear layer turbulence. These models, however, perform poorly in prediction of homogeneous shear flows. Such inconsistency is not caused by crudeness of the eddy-viscosity approximation, but by inappropriate estimation of the relevant time-scale of turbulence. Thus, to settle this problem, we propose a low-Reynolds number-type “multiple time-scale” turbulence model on the basis of recent direct numerical simulations (DNS) of turbulence. Making a proposed model that is applicable to prediction of near-wall turbulent flows without the controversial wall functions and on reproducing the wall-limiting behavior of turbulence quantities is the emphasis. The proposed model has been tested in wall shear flows with and without a pressure gradient. We also tested the present model in homogeneous flows with and without mean strain. Assessments based on the latest DNS data and measurements indicate that the present model works well, regardless of the flow regimes.

Role of large coherent structures in turbulent compressible mixing

Turbulent compressible shear layers of round supersonic jets were excited by placing an open cavity near the nozzle exit, which produced pressure oscillations due to flow-induced cavity resonance. As a result of the excitation, large coherent structures were created in the highly compressible shear layers, making it possible to study the effect of such structures on turbulent compressible mixing. For this study, pressure-matched Mach 2 jets were used under both nonreacting and afterburning conditions. By varying the cavity dimensions, we could apply excitation over a wide range of frequencies and manipulate the coherent structure dynamics in the near field of the jets. Mie-scattering flow visualization images revealed that large coherent structures were generated when high-amplitude excitation occurred at frequencies close to the jet preferred mode, which was near the Strouhal number of about one-half. Growth rate of nonreacting shear layers was quantified as a function of excitation frequency; and, for afterburning jets, the change in global flame luminosity due to excitation was measured. It was observed that the growth rate was drastically increased in the near field when the coherent structures were organized. The afterburning characteristics were also modified by coherent structures and their dynamics. Under these conditions, afterburning intensity increased when the excitation frequency was higher than the preferred mode frequency and decreased at lower frequencies. The results suggest that the initial size of the coherent structures not only determined the rate of large-scale entrainment, but also modified molecular-level mixing by affecting the timing of large-structure breakdown.

Impact of a surfactant on a turbulent shear layer under the air‐sea interface

The effects of surface active material (surfactant) on a turbulent shear layer near the air‐sea interface are studied by direct numerical simulation of the flow. The study is motivated by the observation of the reduction of the scalar exchange rate across the air‐sea interface by a surfactant film. Though the model, based on the concept of surface renewal by turbulent eddies, explains the observation well, there has been little experimental or numerical validation of the detailed hydrodynamic mechanism. The present work aims to identify the differences in the underlying turbulence structures between clean and contaminated interfaces and to provide a basis for parameterizing the scalar transfer rate across a contaminated interface. The simulation results show that the inviscid blocking effect of a clean interface, the attenuation of vertical fluctuating velocities and the increase in horizontal turbulence intensities, is significantly reduced by surfactant contamination. The viscous sublayer adjacent to the interface is thickened with the presence of a surfactant. Within this viscous sublayer, both horizontal and vertical fluctuating turbulent intensities are attenuated. The upwelling (splatting) events, which are responsible for renewing the fluids in contact with the interface, are reduced drastically. In addition, the vortex connection onto the interface, which frequently occurs on a clean interface, is also diminished. The impact of a surfactant on the sheared turbulence is explained by the interaction between the transport of the surfactant and the underlying coherent horseshoe vortical flow. For turbulence beneath a clean interface, the impingement of the horseshoe vortex causes upwelling and, subsequently, a vortex connection onto the interface. If the surface is covered with a surfactant, surfactant concentration gradients are formed by the impinging turbulent eddies. Surface shear stresses are induced, which retard the approaching of the horseshoe vortices toward the interface, diminish the renewal of near‐surface fluids, and, consequently, reduce the scalar transfer rate.

Effects of surfactant on free-surface turbulent shear flow

We study the effects of surfactant (surface active material) on the turbulent shear layer near the free surface by analyzing the data from direct numerical simulation of the flow. It is found that the blocking effect of a clean surface (attenuation of the vertical fluctuating velocities and increase in the horizontal turbulence intensity) is significantly changed by the surfactant contamination. Within a viscous sublayer next to the contaminated surface, both horizontal and vertical turbulent intensities are attenuated approaching the surface. In addition, coherent vortex-connection signatures, which are found on a clean surface, are diminished with the presence of surfactant. The differences between the effects of clean and contaminated surfaces on the surface features and the underlying turbulence are explained by the coupling interaction between the surfactant transport and the underlying coherent horseshoe vortices.

On innovation in aerodynamics

AbstractA discussion is presented of innovative change in aerodynamics, in which the transformation of practical transonic aerodynamics by computational fluid dynamics is taken as a case study for the lessons to be learnt. The study focuses on the historic breakthroughs of Emmons and Murman and Cole in the calculation of transonic flows, and on the exploitation of these at the Royal Aircraft Establishment to provide industry with a new, and highly successful, tool for practical transonic wing design. It reaffirms the view that a first prerequisite for innovation is the active presence of individuals with clear vision and a spirit of adventure. Further, innovative work will flourish only if the climate is favourable, no matter how exceptional the individuals may be. The discussion concludes with remarks on opportunities for innovation in the treatment of Shockwaves, turbulent shear layers and complex practical configurations.

The effect of vortex pairing on particle dispersion and kinetic energy transfer in a two-phase turbulent shear layer

The effect of vortex pairing on particle dispersion and kinetic energy transfer in a two-phase turbulent shear layer

The effect of vortex pairing on particle dispersion and kinetic energy transfer in a two-phase turbulent shear layer

The effect of vortex pairing on particle dispersion and kinetic energy transfer in a two-phase turbulent shear layer

A semi-implicit method for internal boundary layers in compressible flows

A semi-implicit boundary-layer method is developed for application to spatially evolving internal boundary layers in compressible fluids. The method circumvents convergence difficulties that may arise with fully implicit marching schemes whenever the mean streamwise velocity profile is inflectional. The method is demonstrated for the special case of the compressible, laminar axisymmetric jet. However, the properties of the scheme make it well suited also to laminar or turbulent free shear layers, wakes and planar jets.

Large-scale structure evolution in supersonic interacting shear layers

An experimental investigation is made of the evolution of the large-scale structures in a flow consisting of two planar turbulent shear layers formed by a Mach-3 planar jet bounded by a Mach-5 freestream. The flow is characterized by two planar shear layers at a convective Mach number of 0.28 that are independent in the near field but that interact farther downstream. Measurements were made using fast-response hot-wire probes and planar laser scattering from a condensed alcohol fog. The hot-wire data were used to calculate power spectra and cross correlations from which large-scale-structure length scales and orientation were inferred. The images reveal roller-like structures, but they do not appear as dominant or as coherent compared with low-Mach-number shear layers at a similar Reynolds number. The hot-wire data confirm the relatively unorganized and three-dimensional nature of the independent shear layers. In the far field, the visualizations reveal that the shear layers interact to form a more organized structure, similar to vortex shedding in incompressible turhulent wakes. These organized structures result in a distinct peak in the power spectrum and larger spanwise coherence lengths than for the independent shear layer.

PREDICTION OF SUPERSONIC JET NOISE FROM A STATISTICAL ACOUSTIC MODEL AND A COMPRESSIBLE TURBULENCE CLOSURE

Acoustic radiation of shock free supersonic jets is modified in comparison with subsonic jet noise because of Mach wave emission. Intense noise is radiated when turbulent structures are convected supersonically relative to the ambient sound speed. Using the framework of Lighthill's equation, Ffowcs Williams and Maidanik developed an approximation of Lighthill's term for a supersonically converted acoustic source in a turbulent shear layer. From this result, a model can be deduced for axisymmetric jets. One then shows that the local knowledge of the mean flow and a characteristic time of turbulence in the source volume may be used to calculate the spectral directivity of Mach wave noise. As a consequence of this local formulation of the acoustic source term, the noise model contains a single unknown multiplicative constant. It also requires a calculation of the mean flow which may be carried out with a k–ϵ compressible turbulence model. Two cold jet cases at M= 1·7 and M= 2·0 and a hot jet at M= 2·0 and T j/ T o= 2·5 are analyzed. Comparisons with available experimental data and Tam's calculations based on alternative description in terms of instability waves travelling at the convection speed show a good agreement between calculations and measurements.

Arbitrary Blade Section Design Based on Viscous Considerations. Blade Optimization

A blade design and optimization procedure is presented in this work, which is based on viscous flow considerations. This procedure concerns the design of optimum rotating arbitrary compressible high subsonic compressor and turbine blade shapes. It takes into account the effects of wall curvature and Coriolis force on turbulence, while it allows the variation of stream surface radius, along which the blade shape is placed, as well as streamtube width, with meridional distance. In order to establish the inverse part of the viscous optimization procedure, aspects such as laminar stability, transition, optimum deceleration and, more generally, the behaviour of compressible attached and separated shear layers are discussed. A plane on which all the general properties of the compressible laminar and turbulent shear layers appear, is constructed and the generation of optimum shear layers for the critical side of the blade shape is established. The complete optimization (design) procedure is then described and discussed, while various designs realized by the present procedure are presented at the end of this paper.

Compressibility effects and mixing enhancement in turbulent free shear flows

Computational simulations have been performed to study compressible, spatially developing turbulent free shear layers with various velocity regimes—subsonic/subsonic, supersonic/subsonic, and supersonic/supersonic— for convective Mach numbers in the range of 0.14-1.28. The numerical code used the finite volume technique and a modified Godunov's scheme. The computed results for the supersonic/subsonic case are first compared with experimental axial mean-velocity profiles, vorticity thickness, and turbulence parameters. Mixing layers with various velocity regimes are then calculated to investigate compressibility effects on the evolution of large-scale structures through the flow visualization of the vorticity field, growth rate of the vorticity thickness, and vorticity dynamics analysis. Various forcing frequencies are applied at the inflow boundary to examine mixing enhancement for free shear layers with higher convective Mach numbers. It is found for the first time that the growth rate of supersonic/supersonic free shear layers increases markedly when the forced layers move up and down with time instead of forming vortex roll-up and pairing.

The effect of vortex pairing on particle dispersion and kinetic energy transfer in a two-phase turbulent shear layer

The transport of heavy, polydispersed particles and the inter-phase transfer of kinetic energy due to the viscous drag forces is measured experimentally in a turbulent shear layer. To study the effect of the large-scale vortex pairing event, the shear layer is forced simultaneously with a fundarmental and subharmonic perturbation. It is shown that vortex pairing plays a homogenizing role on the particulate field, but hte amount of homogenization is strongly dependent upon the particle's viscous relaxtion time, the eddy turnover time, as well as the time the particles interact with each scale prior to a pairing event. Thus, even though the smaller size particles become well-mixed across the large eddies, the larger sizes are still dispersed in an inhormogeneous fashion. It is also found that the kinetic energy transfer between the phases occurs inhomogeneously with energy being exchanged predominantly in a sublayer just outside the region of maximum turbulence intensity. The kinetic energy transfer is shown to exhibit notable positive and negative peaks located beneath the cores and stagnation points of the large-scale eddy field, and these peaks are shown to result from the irrotational velocity perturbations created by the vortices. This energy exchange mechanism remains a prominent process as long as the Stokes number of the particles relative to the vortices is of order unity.

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.

The damping of sound by wall turbulent shear layers

A frequency-dependent model is proposed for the eddy viscosity controlling the momentum and thermal wall boundary layers formed when an acoustic wave interacts with a low Mach number turbulent boundary layer whose thickness is much smaller than the acoustic wavelength. This is used to determine the effective acoustic admittance of the boundary layer, and to predict the attenuation of sound reflecting from a plane wall boundary layer, and low-frequency sound propagating in fully developed turbulent pipe flow. The predictions are shown to compare well with a recent experimental study of acoustic attenuation in pipe flows.

The effect of solid surfaces on turbulent jet noise

Abstract Lighthill’s acoustic analogy is applied to the problem of calculating the sound radiated by unit volume of jet-type shear-layer turbulence near a solid surface. The ensemble-averaged basic directivity patterns of the sound generated by randomly orientated longitudinal and lateral quadrupoles near a rigid plane are calculated, and are found to be very different for the two types of quadrupole. A method of modelling the acoustic sources, due to Ribner, is extended to the present case, enabling the basic directivity pattern to be calculated. The effect of the solid surface is found to depend strongly on the source frequency, and a method for estimating the source frequency in an actual turbulent shear flow is developed. The theory is applied to a plane two-dimensional wall jet, and predictions of some basic aero-acoustic characteristics are obtained.

Secondary instability of large scale structure in free turbulent shear layer

The secondary instability theory is used to study the behavior of spatially growing disturbance in free turbulem shear layer. The numerical results indicate that secondary instability of subharmonic mode shows a strong choice of spanwise wavenumber and the maximum growth rate occurs in two dimensional case. In contrast to that secondary instabilities of the fundamental mode occur in a wide scope of spanwise wavenumber. We have found so called translative instability at β=0 and bifurcation phenomenon for an amplitude of the KH wave larger than 0.06.

Control of Round Impinging Jet by Coaxial Annular Sub Jet. Effects of Velocity Ratio on Flow Characteristics.

This paper presents experimental and numerical analyses for the flow characteristics of double coaxial pipe jets, which consist of a central round main air jet and a coaxial annular sub air jet, impinging normally onto a flat plate. As the results, it was found that the mean flow characteristics, namely each velocity profile, decay of maximum velocity, spread of jet, and pressure distribution, and the turbulence characteristics, namely each turbulence component and Reynolds shear stress, of a round impinging jet can be controlled by an annular coaxial sub jet. In paticular, turbulent kinetic energy near the flat plate is augmented considerably by the existence of a turbulent shear layer between main and sub jets. The increase in a forced convective heat transfer on the plate is also expected. Numerical analysis was carried out using the k-ε turbulent model.

The application of classical POD and snapshot POD in a turbulent shear layer with periodic structures

The application of classical POD and snapshot POD in a turbulent shear layer with periodic structures

Applications of exact solutions to the Navier–Stokes equations: free shear layers

A family of exact solutions to the Navier—Stokes equations is used to analyse unsteady three-dimensional viscometric flows that occur in the vicinity of a plane boundary that translates and rotates with time-varying velocities. Such flows are important in the study of flows that are produced by rotating machinery. They are also useful in describing local behaviour in more complex global flows, such as that produced in a shear layer by the passage of a disturbance in the mainstream. An example is the flow produced in a turbulent shear layer by the passage of the core of a Rankine vortex. When the effect of viscosity is unimportant, the use of Lagrangian coordinates reduces the mathematical problem to that of solving a set of linear ordinary differential equations.