Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session EV: Geophysical Flows: Mixing |
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Chair: Jean-Marc Chomaz, LadHyX CNRS-Ecole Polytechnique Room: Ballroom B |
Sunday, November 23, 2008 4:10PM - 4:23PM |
EV.00001: Transient growth of perturbations on stratified mixing layers Cristobal Arratia, Jean-Marc Chomaz, Sabine Ortiz We perform a study on the optimal perturbations developing on mixing layers. The basicly 2-dimensional Kelvin-Helmoltz instability that develops in this type of flow is known to become unstable leading to the development of streamwise vortices and eventually turbulence. This process is essential in many geophysical and industrial flows, where it greatly influences mixing and dissipation. We explore different types of 3-D optimal perturbations by means of numerical integration of the direct and adjoint Navier-Stokes equations in the Boussinesq approximation. Knowledge of these optimal perturbations reveals characteristics of the flow receptivity and helps to understand the different physical mechanisms present in its dynamics. These are key aspects in order to predict and control the flow evolution. [Preview Abstract] |
Sunday, November 23, 2008 4:23PM - 4:36PM |
EV.00002: LES of mixing in coastal areas Vincenzo Armenio, Federico Roman Large eddy simulations of near shore mixing are carried out using a new large-scale model, LES-COAST. The model integrates the Boussinesq form of the NS equations using a curvilinear formulation of the fractional step method. Complex geometry is reproduced with a combination of curvilinear mesh and immersed boundaries. Anisotropy of the problem ($\Delta x_{vert}/\Delta x_{hor} \sim 10^{-2}$) has required a two eddy-viscosities model. Specifically we use a mixed model, composed of an anisotropic scale-similar part and an eddy viscosity, Smagorinsky part. Two eddy-viscosities are used, $\nu_{T,v/h}=(C_{v/h} \Delta_{v/h})^2 |\overline{S}_ {v/h}|$ respectively in the two directions. The quantities $\Delta_{v/h}$ are chosen proportional to the local grid spacings in the two directions. Density stratification is also considered using a very simple SGS model, based on the assumption that $Pr_T = Sc_T =1$. The algorithm is being used for real applications. The following studies will be discussed: the intrusion of fresh water of the Tevere river in the Tirreno sea; mixing in the Muggia Bay (Region of Trieste) under the action of the breeze forcing. Results have clearly shown the reliability of new- generation LES large-scale models for applications in problems with horizontal and vertical scales respectively of the order of $10 km$ and $50 m$. [Preview Abstract] |
Sunday, November 23, 2008 4:36PM - 4:49PM |
EV.00003: Mixed surface layer deepening by Langmuir circulation and shear turbulence Jan-Bert Flor, Emil Hopfinger, Estelle Guyez We consider upper layer mixing by mixing processes such as by shear generated turbulence and by Langmuir circulation that are relevant to the surface mixed layer deepening in oceans and large lakes. In order to gain insight into the mixing process by continuously driven horizontal vortices, such as Langmuir circulation, with respect to mixing by shear generated turbulence, we consider recent experimental results on the mixing at a two-layer fluid interface by Taylor vortices. Relating the vortex induced mixing and the shear induced mixing to a surface friction-velocity $u^*$, we show that up to a Richardson number of $Ri_*= \frac{\Delta b h}{u_*^2} \leq 80$ layer deepening is dominated by shear turbulence, whereas it is taken over by Langmuir circulation for $Ri_* > 80$. The mixing efficiency of Langmuir circulation decreases gradually with increasing Richardson number, implying significant mixing also for higher Richardson numbers. As a consequence, there is no critical Froude-number criterion for the arrest of mixing by Langmuir cells as has been suggested previously. In agreement with in situ observations, the initial upper layer deepening is dominated by shear turbulence, and the subsequent principal layer deepening is due to Langmuir circulation. [Preview Abstract] |
Sunday, November 23, 2008 4:49PM - 5:02PM |
EV.00004: Large-eddy simulation of tidally driven mixing in estuaries Senthil Radhakrishnan, Ugo Piomelli, Ming Li, W. Rockwell Geyer The estuarine boundary layer is affected by the horizontal density gradient and exhibit mixing and restratification over a tidal cycle. Large Eddy Simulation(LES) is used to investigate the physics controlling the entrainment and growth of the boundary layer during the flood tide and the restratification during the ebb tide. While small-scale turbulent eddies generated by the bottom stress are the major flux-carriers in the well-mixed bottom boundary layer, relatively large spanwise vortices, dominate salt and momentum fluxes in the outer part of the bottom boundary layer between mid-flood and mid-ebb. Turbulent kinetic energy, momentum flux and bottom stress show a strong flood-ebb asymmetry. Strong bed stress during the flood tide causes the entrainment and growth of the bottom boundary layer whereas tidal straining during the ebb tide causes restratification in the water column. Additional LES runs conducted by switching off the baroclinic pressure gradient term in the momentum equation and the tidal straining term in the salinity equation show that the baroclinic pressure gradient is the main mechanism responsible for generating the flood-ebb asymmetry in turbulent mixing. [Preview Abstract] |
Sunday, November 23, 2008 5:02PM - 5:15PM |
EV.00005: Gas transfer through the air-water interface in LES of Langmuir circulation in shallow water Cigdem Akan, Andr\'es E. Tejada-Mart\'Inez Over the past century the study of gas exchange rates between the atmosphere and the ocean has received increased attention because of concern about the fate of slightly soluble, greenhouse gases such as CO$_{2}$ released into the atmosphere. Of recent interest is the oceanic uptake of CO$_{2}$ along US shallow water coastal regions (e.g. see http://www.nacarbon.org). We present surface gas transfer results from large-eddy simulation (LES) of wind-driven shallow water flow with and without wave effects. Wave effects, parameterized by the well-known Craik-Leibovich vortex force, lead to the generation of Langmuir circulation (LC), serving as a mechanism for surface renewal of low concentration fluid. Our computations are motivated by the infrared imagery of Marmorino et al. (2004) suggesting that LC can affect gas transfer across the surface through straining and stretching of the gas concentration boundary layer. Preliminary LES shows that shallow water LC can increase the surface gas transfer rate by about 30 percent. Here we will focus on the accuracy of surface renewal models in predicting gas transfer velocity, a measure of gas transfer efficiency, in the presence of LC. Gas transfer velocity predicted by the surface renewal models will be compared to the prediction obtained directly from the LES. [Preview Abstract] |
Sunday, November 23, 2008 5:15PM - 5:28PM |
EV.00006: Transition in energy spectrum of stably stratified turbulence Yoshi Kimura, Jackson Herring Energy spectra for forced stably stratified turbulence are investigated numerically using the Direct Numerical Simulations (DNS) with $1024^3$ grid points. The calculation is done by solving the 3D Navier-Stokes equations under the Boussinesq approximation pseudo-spectrally. Using toroidal-poloidal decomposition (Craya-Herring decomposition), the velocity field is divided into the vortex mode ($\phi_1$) and the wave mode ($\phi_2$). With the initial kinetic energy being zero, the $\phi_1$ spectra as a function of horizontal wave numbers, $k_{\perp}$, first develops a $k_{\perp}^{- 3}$ spectrum for the whole $k_{\perp}$ range, and then $k_{\perp}^{-5/3}$ part appears with rather a sharp transition wave number. Meanwhile the $\phi_2$ spectra shows $k_{\perp}^{-2}$ first, and then $k_{\perp}^{-5/3}$ part appears with the same transition wave number. Spectra for different values of the Brunt-- V\"ais\"al\"a frequency $N^2=1, 10, 50 {\rm and} 100$ are investigated, and we found that the $k_{\perp}^{-3}$ part at the large scale in the $phi_1$ spectra is characterized as 2d turbulence, and that the whole spectrum has the form of $E(k_{\perp})=a\eta_{\perp\phi_1}^{2/3}k_{\perp}^{-3}+C_K \varepsilon_{\perp\phi_1}^{2/3}k_{\perp}^{- 5/3}$ where $\eta_{\perp\phi_1}$ is the horizontal enstrophy dissipation based on the $\phi_1$ energy, and $\varepsilon_{\perp\phi_1}$ is the horizontal $\phi_1$ energy dissipation. Meanwhile we obtain $E(k_{\perp})=b\sqrt{N\varepsilon_{\perp\phi_2}} k_{\perp}^{-2} +C_K\varepsilon_{\perp\phi_2}^{2/3}k_{\perp}^{-5/3}$ for $\phi_2$ where $\varepsilon_{\perp\phi_2}$ is the horizontal $\phi_2$ energy dissipation. For both cases, $C_K\approx 1.2\sim2.0$ is obtained being close to the Kolmogorov constant. [Preview Abstract] |
Sunday, November 23, 2008 5:28PM - 5:41PM |
EV.00007: Turbulence Identification in Stably Stratified Flows David Hebert, Stephen de Bruyn Kops Flows subject to stable density stratification often occur in natural settings such as the ocean and atmosphere. Unlike turbulence in homogeneous flows, when turbulence occurs in density stratified flows it does so in localized, intermittent patches. It is in these turbulent patches that important features of the flow occur, e.g., mixing of species, dissipation of energy. As such, it would be beneficial to be able to identify where turbulent patches occur. Turbulence is a three dimensional phenomena associated with flow vorticity. In this talk, several published vortex identification methods are employed to identify turbulent patches from three dimensional high resolution direct numerical simulations (DNS) of density stratified flows. Vortex identification methods include those based on the second invariant as well as the eigenvalues of the velocity gradient tensor $\nabla v$. The use of DNS allows the results of the identification methods to be compared with actual flow dynamics, such as areas of high energy dissipation rate and enstrophy. Results suggest that while each vortex identification method is able to efficiently extract areas of turbulent activity, they depend on a subjective threshold criterion for meaningful results. [Preview Abstract] |
Sunday, November 23, 2008 5:41PM - 5:54PM |
EV.00008: Direct numerical simulations of stratified turbulence at higher Reynolds numbers Vishal Vasan, Olof Grundestam, James Riley, Pui-Kuen Yeung Stratified turbulence\footnote{D.K.\ Lilly, 1983, {\em J.Atmos.\ Sci.}, {\bf 40}, 749.} is a flow dominated by stable density stratification, and is characterized by low internal Froude numbers and high Reynolds numbers; such parameter regimes occur often in the atmosphere and oceans. We report on direct numerical simulations of stratified turbulence at very high resolution (up to 2048$^3$ grid points) enabling, in particular, fairly high Reynolds numbers to be achieved, well above the critical value at which instabilities and smaller-scale turbulence are expected to occur. A highly parallelized 3D Fourier pseudo-spectral code based on a ``pencils'' domain decomposition is used to solve the Navier-Stokes equations, and the Boussinesq approximation is made in accounting for stable density stratification. Both decaying and statistically stationary, forced flows are simulated. We examine various features of the flows, including the spectral energy transfer, horizontal fluid particle dispersion, mechanisms leading to instabilities and smaller-scale turbulence, and the parameterization of these flows for larger-scale models. [Preview Abstract] |
Sunday, November 23, 2008 5:54PM - 6:07PM |
EV.00009: Wind-driven turbulent oscillating channel flow under a stable stratification Werner Kramer, Herman Clercx, Vincenzo Armenio An LES investigation of the oscillating channel flow subjected to a constant wind stress revealed strong shear production of the turbulence in the wall and free-surface layers. The flow during most of the phases is well described by a combination of two log-law boundary layers. If the driving oscillating pressure gradient and wind stress are aligned turbulent streaks are observed in the entire domain. For a wind stress at a $45^{\circ}$ angle, the streaks in the free-surface and wall layer are not aligned. This results to more isotropic turbulence in the interior. We aim to investigate the effects if a stable stratification is added to this model problem for estuarine flows. The stratification is caused by a constant heat flux at the free-surface. Previous studies observed a strong stratification in the free-surface layer, which suppresses turbulent fluctuations. This might lead to a partially decoupling of the top and bottom layers. Moreover, internal waves could stir up the dynamics of the flow. [Preview Abstract] |
Sunday, November 23, 2008 6:07PM - 6:20PM |
EV.00010: Two-layer continuously stratified flow John McHugh Continuously stratified flow in two layers is considered. The interface between layers is defined by a jump in the Brunt-Vaisala frequency, N, and a sudden shift in the direction of the mean horizontal wind. This configuration is suggested by recent observations over isolated mountains showing the presence of very large amplitude internal waves. A coordinate system is chosen that makes this background state equivelent to the two-dimensional Kelvin-Helmholtz problem, now with the jump in N. Previous work has shown that resonant over-reflection, where the mean flow creates waves without any incident wave, may occur even with constant N throughout. Linear theory shows that the addition of the jump in N results in a larger interval of wavenumbers for resonant over-reflection. Linear and weakly nonlinear theory do not show any evidence of larger amplitude motion near the interface, and this feature of the experiments may be a strongly nonlinear phenomena. Numerical simulation with this background state shows that waves are not spontaneously created in the parameter region where resonant over-reflection should occur, but must be initiated with some type of disturbance. The results depend strongly on the form and strength of this disturbance. Internal waves and incoherent noise have been separately treated as disturbances. The results indicate that resonant over-reflection is one plausible explanation for the large amplitude waves that were found in the measurements. [Preview Abstract] |
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