Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session H12: Turbulence: Shear Layers I |
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Chair: Stavros Tavoularis, University of Ottawa Room: 315 |
Monday, November 21, 2011 10:30AM - 10:43AM |
H12.00001: Statistics and scaling of turbulence in a spatially developing shear layer Antonio Attili, Fabrizio Bisetti A direct numerical simulation of a turbulent mixing layer has been performed. The flow achieves a Reynolds number based on Taylor's microscale equal to 250. The flow originates from a laminar, hyperbolic tangent inlet profile, which is perturbed with white noise. The turbulence statistics in the self-similar state agree with a number of previous results, where the flow evolves from turbulent inlets. Our data suggests that the final state of the layer could be universal. The degree of convergence to high Reynolds number asymptotic behavior has been evaluated by analyzing spectra and second order structure functions. The scaling exponents of high order structure functions are calculated using the Extended Self-Similarity. Over a wide range of scales, the exponents are found to match those in homogeneous isotropic turbulence. However, an additional scaling range is found at larger scales, where the effects of mean shear and coherent large structures are not negligible. The values of the exponents recovered in this second range agree well with those in flows with strong shear (boundary layers, channels and wakes). [Preview Abstract] |
Monday, November 21, 2011 10:43AM - 10:56AM |
H12.00002: Generation of a buoyant jet by a sphere moving vertically in a stratified fluid Hideshi Hanazaki, Keisuke Shimobata, Hiroyasu Yoshikawa We consider the flow past a sphere moving vertically at a constant speed in salt-stratified fluid. Experiments have shown that vertical jets are generated even if the vertical mean density gradient is not large. At least seven types of wake structures have been found, including a thin jet with a surrounding bell-shaped structure. In this numerical study, we investigate the unsteady generation mechanism of buoyant jets with the focus on the diffusion effects of the density/salt. When a sphere descends in a stratified fluid, the density is initially conserved along the movement of the fluid and the originally horizontal isopycnal surfaces are simply deformed vertically and pulled down by the sphere. As time proceeds, diffusive effects become significant in the density boudary layer on the sphere surface and in the thin jet, and the density is no longer conserved. This violation of conservation would be the origin of the jet which is composed of the fluids which moves up to return to thier original heights. [Preview Abstract] |
Monday, November 21, 2011 10:56AM - 11:09AM |
H12.00003: Turbulence and wave motion driven by horizontal shear layers below/above stably stratified regions Mohamed Moustaooui, Julian C.R. Hunt, Alex Mahalov Analytical studies and high resolution 3-dimensional numerical simulations of a jet show how eddies in turbulent shear flows moving with mean velocity $\Delta U$ relative to external upper or lower stably stratified regions with buoyancy frequency $N_{o}$ interact with these regions. Internal waves are not generated if the Richardson number is less than a critical value \textit{Ri}$^{\ast }$. If \textit{Ri$<$ Ri}$^{'}$ the stratification only perturbs the irrotational motions outside the shear layer. But if \textit{Ri}$^{'}$ \textit{$<$Ri$<$Ri}$^{\ast }$ intense interfacial shear layers forms at the top of the shear layer and then a second layer forms at a height of order $w^{+}/N_{o}$, that bounds a mixed layer of trapped turbulent eddies and weak mean shear. This double layer structure has similar scales as the natural layering process in turbulent stratified flows. When \textit{Ri$>$Ri}$^{\ast }$, first the upper and then the lower interfacial layers break up, the peak shear in the interface decreases and a narrow band spectrum of internal waves are generated which propagate upwards although they do not significantly interact with the turbulence in the shear layer. The Reynolds shear stress associated with the waves varies with \textit{Ri}, but over a wide range of \textit{Ri} is as much as about 1/5 of the peak shear in the shear layer; when normalised on the vertical variance at the interface it increases in proportion to$\sqrt {Ri-1} $. [Preview Abstract] |
Monday, November 21, 2011 11:09AM - 11:22AM |
H12.00004: The effects of weak stratification on turbulence within a mixing layer Roi Gurka, Eliezer Kit In mixing-layers between two parallel streams of different densities, shear and gravity effects interplay; buoyancy acts as a restoring force and the Kelvin Helmholtz mode is known to be stabilized by the stratification. The effect of stratification on streamwise and spanwise vortical structures in the stratified mixing layer has been investigated numerically (DNS) and experimentally (PIV). Two cases were compared: homogeneous flow (Ri=0) vs. weakly stratified (Ri=0.03). Mixing of the heated upper layer with unheated lower layer occurred in the wake of the splitter plate with attached oscillating Chevron type flappers. Controlling the attached to splitter plate flappers enabled to form different type of disturbances (2D and 3D) followed by various types of instabilities within the mixing layer. It was found that the energy of 3D modes at the initial stage increased faster in the case of stratified flow compared to the homogeneous case, indicating that some of the modes are convectively unstable and growth faster than they do in homogeneous flow. In addition, the lateral RMS distribution of the velocity components in stratified case was wider indicating faster spreading of the mixing layer. Those points out to convective instability within the primary roll caused by the density layers overturn occurring throughout the roll formation. [Preview Abstract] |
Monday, November 21, 2011 11:22AM - 11:35AM |
H12.00005: Viscous and inviscid velocity contributions near the turbulent/nonturbulent interface in a planar turbulent jet Filipe Soares Pereira, Carlos da Silva, Gerrit Elsinga, Jerry Westerweel Direct numerical simulations (DNS) of turbulent planar jets and shear free turbulence are used to assess the viscous and inviscid contributions to the entrainment velocity at the turbulent/non-turbulent (T/NT) interface, and their dependency on the Reynolds number and flow type. Whereas in shear free turbulence the viscous contribution dominates in the turbulent planar jet the inviscid contribution is more important. Furthermore, it is shown that as the Reynolds number increases the inviscid contribution becomes larger in both flows. Moreover it was observed that the for the planar turbulent jet configuration the indirect estimation of the T/NT interface thickness using the local velocity does not match the T/NT interface thickness as observed from the conditional vorticity profile. [Preview Abstract] |
Monday, November 21, 2011 11:35AM - 11:48AM |
H12.00006: Passive scalar dynamics near the turbulent/nonturbulent interface in a jet Rodrigo R. Taveira, Carlos da Silva The present work uses several direct numerical simulations (DNS) of turbulent planar jets at Reynolds number ranging from $Re_\lambda=120$ to $Re_\lambda=160$ and Schmidt numbers raging from $Sc=0.7$ to $7.0$ to analyze the nature and properties of the \textit{''scalar interface''} and to investigate the dynamics of turbulent mixing of a passive scalar. Specifically, we employ conditional statistics in relation to the distance from the T/NT interface in order to eliminate the intermittency that affects common turbulence statistics close to the jet edge. The physical mechanisms behind scalar mixing near the T/NT interfaces and their associated turbulent scales and topology are investigated. A sharp scalar interface exists separating the Turbulent and the irrotational flow regions. The thickness of this scalar interface $\delta_{\theta}$ is also of the order of the Taylor micro-scale, $\lambda$. However, the thickness of the scalar gradient variance $\left< \theta^2 \right>_I$ (where $G_j = \partial \theta/\partial x_j$) is much smaller. Very intense scalar gradient sheet structures along regions of intense strain, in particular at the T/NT interface. The scalar gradient transport equation is analyzed in order to further investigate the physical mechanism of scalar turbulent mixing at the jet edge. Almost all mixing takes place in a confined region close to the interface, beyond which they become reduced to an almost in perfect - balance between production and dissipation of scalar variance. [Preview Abstract] |
Monday, November 21, 2011 11:48AM - 12:01PM |
H12.00007: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 12:01PM - 12:14PM |
H12.00008: Passive Scalar Dispersion in Uniformly Sheared Turbulence Christina Vanderwel, Stavros Tavoularis The dispersion of a plume of dye from a continuous point source is investigated experimentally in uniformly sheared, nearly homogeneous, turbulent flow generated in a water tunnel. The flow has a turbulence Reynolds number of $Re_\lambda \approx 150$ and a shear rate parameter of $S^* \approx 12$. Neutrally buoyant fluorescent dye (fluorescein and/or rhodamine B/6G) is injected isokinetically from a streamlined injection tube into the developed turbulent flow. Instantaneous concentration and velocity fields of the plume are measured simultaneously using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV). Joint statistics of the concentration and velocity fields are measured and compared to predictions of theoretical models of both absolute and relative (two-particle) dispersion. The contribution to scalar transport by horseshoe vortices, shown earlier to be the dominant coherent structures of uniformly sheared turbulence, is determined by analysis of coincident instantaneous maps of concentration and velocity. [Preview Abstract] |
Monday, November 21, 2011 12:14PM - 12:27PM |
H12.00009: Similarity Theory for an Axisymmetric Turbulent Wake with Rotation Martin Wosnik Axisymmetric wakes are special cases of turbulent shear flows in the sense that the local Reynolds number based on velocity deficit and wake width decreases with downstream position. Recently, Johansson et al. (Physics of Fluids, \textbf{15}, no.3, 603-617, 2003) showed that two distinct similarity solutions for the non-swirling axisymmetric turbulent wake exist -- one for infinite and one for low local Reynolds number. Every axisymmetric wake, no matter how high the initial Reynolds number, will eventually transition to the low Reynolds number similarity state in the far wake. Here equilibrium similarity considerations are applied to axisymmetric turbulent wakes with rotation (swirl), as can be found downstream of wind or hydrokinetic turbines. By examining under which conditions the reduced momentum and Reynolds stress transport equations for swirling wakes as well as the momentum integrals admit to similarity solutions, asymptotic scaling relations for the decay of velocity deficit and swirl are found. Swirl is introduced as an initial condition, and additional constraints on the similarity solution are introduced from the turbine (wake generator) operating parameters, e.g., tip speed ratio, angular induction, etc. The consequences of having a non-point source of thrust (drag) and angular momentum are investigated. Implications of the findings on the operation of wind and hydrokinetic turbines and turbine arrays are discussed. [Preview Abstract] |
Monday, November 21, 2011 12:27PM - 12:40PM |
H12.00010: Pressure time correlation in turbulent shear flows Li Guo, Xin Zhang, Guowei He Pressure time correlations are important to turbulence-generated noise and flow-induced vibration. In this study, we numerically and theoretically investigate pressure time correlations in turbulent shear flows. We calculate the pressure time correlation from the direct numerical simulation (DNS) of turbulent channel flows. It is observed that the pressure time correlations decay faster than the streamwise velocity time correlations. This is different from isotropic homogenous turbulence, where the pressures have the same decorrelation time scales as velocities. A theoretical model is developed to confirm this observation. We further check Taylor frozen-flow model for pressure time correlation in turbulent channel flows. It is found that Taylor's model is not valid for strong shear rates. Based on this observation, we develop a non-frozen flow model, which takes the mean shear effects into account. This model is also verified using the DNS data of turbulent channel flows. [Preview Abstract] |
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