Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D30: Geophysical Fluid Dynamics: Wakes and Boundaries in Stratified Flows |
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Sponsoring Units: DFD Chair: James Riley, University of Washington Room: 311 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D30.00001: Characteristics of turbulent/non-turbulent interfaces in wakes in stably-stratified fluidsO Tomoaki Watanabe, James Riley, Stephen de Bruyn Kops, Peter Diamessis, Qi Zhou The evolution of turbulent patches generated by the wake of a sphere in stably-stratified fluids is studied using direct numerical simulations. The DNS data analysis focuses on the investigation of the characteristics of turbulent/non-turbulence interfaces forming at the wake edge. Unlike the case for non-stratified fluids, because of the non-negligible enstrophy level in internal waves outside the stratified wake, enstrophy cannot be used as a marker for turbulent regions. We show that potential enstrophy is appropriate as a marker for turbulent regions in flows where both turbulence and internal waves exist. Therefore the interface is detected as an isosurface of constant potential enstrophy, and statistics can be calculated conditioned on the distance from the interface. Various quantities are examined from the wake interior to the region outside the wake, and show how the flow properties are adjusted between turbulent and non-turbulent regions near the interface. Based on the conditional analysis, we also report evidence for the strong influence of internal waves on turbulence inside the wake. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D30.00002: Dynamic mode decomposition identifies internal wave and vortical modes in stably stratified wakes Xinjiang Xiang, Kevin Chen, Trystan Madison, Geoffrey Spedding Though detailed information has been assembled to describe the late wakes behind various objects in stably stratified fluids, less is known about the dynamics at early stages, when the flow first interacts with the ambient density gradient, beginning the transition to the late wake regime. Detailed velocity fields (and derivatives) were reported by Xiang et al. (\textit{J. Fluid Mech.} 775, 149-177, 2015) for the near wake of a towed grid, with Re $\in \{2700, 11000\}$ and Fr $\in \{0.6,9.1\}$. Here using dynamic mode decomposition (DMD), the spatial and temporal evolution of the lee wave and shearing modes are extracted and examined for the same data set. Both dynamic modes show systematic dependence on Fr and Re, consistent with previous analysis. The results show the potential of DMD in analyzing the contribution of different modes in a complex, near wake evolution, including, but not limited to towed grids, and the wakes of more complicated towed geometries. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D30.00003: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D30.00004: Stratified Shear Flows In Pipe Geometries George Harabin, Roberto Camassa, Richard McLaughlin Exact and series solutions to the full Navier-Stokes equations coupled to the advection diffusion equation are investigated in tilted three-dimensional pipe geometries. Analytic techniques for studying the three-dimensional problem provide a means for tackling interesting questions such as the optimal domain for mass transport, and provide new avenues for experimental investigation of diffusion driven flows. Both static and time dependent solutions will be discussed. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D30.00005: Density overturns and local stability measures in early stratified wakes Trystan Madison, Prabu Sellappan, Xinjiang Xiang, Geoffrey Spedding Though the dynamics of decaying stratified turbulence are sensitive to certain details of the initial generating conditions, the late-time evolution has also general characteristics that depend only on local stratification parameters, often characterised by a buoyancy Reynolds number, $Re_b = Re.Fr^2$. Bluff-body wakes, for example, have been shown to have universal characteristics that do not depend on details of the generator. Recent experiments on the near wake of a towed grid (Xiang et al. \textit{J. Fluid Mech.} 775, 149-177, 2015) show that the trajectory of solutions entering the late stratified turbulence regime vary significantly with both $Re$ and $Fr$, reflecting different balance between wake-edge shear instabilities and local, grid turbulence-generated motions. Here we show density profiles taken through the grid wakes for $Re = 2700$ and $Fr = \{2,4,9\}$. The relative importance of stabilising density gradients {\it vs.} destabilising shear flows is customarily measured by a global and/or local Richardson number, $Ri$, and such measures will be compared and contrasted to form a more complete and quantitative picture of the early wake instabilities than has been available hitherto. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D30.00006: Dynamics of particle laden plume in linearly stratified environment Harish Mirajkar, Sridhar Balasubramanian Particle laden plumes, which are common in geophysical flows, were simulated experimentally and their flow dynamics was studied. Particles having mean size, $d_{p}=100 \mu m$, density, $\rho_{p}$=2500 kgm$^{-3}$, and volume fraction, $\phi_{v}$= 0-0.7$\%$, were injected along with lighter buoyant fluid into a linearly stratified medium (N=0.67 s$^{-1}$). It was observed that a particle-laden plume intruding at the neutral layer is characterized by four spreading regimes: (i) radial momentum flux balanced by the inertia force; (ii) inertia buoyancy regime; (iii) fluid-particle inertia regime, and (iv) viscous buoyancy regime. The maximum height, Z$_{m}$ for $\phi_{v}>0\%$ was observed to be consistently lower than the single-phase case. In the inertia-buoyancy regime, the radial spread, R$_{f}$, for the particle laden plume advanced in time as R$_{f} \approx t^{0:68}$ which is slower compared to the single-phase plume that propagates at R$_{f} \approx t^{0.74}$. It was observed that the jet cone angle is higher for the case of particle-laden plume owing to flaring of the plume. Due to the presence of particles, `particle fall out' effect occurs forming a parabolic cloud below the plume spreading height. With increasing $\phi_{v}$, secondary umbrella formation was also observed. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D30.00007: Interaction of two spheres settling in a linearly stratified fluid Matthieu Mercier, Clement Toupoint, Patricia Ern The settling dynamics of small objects in stratified fluids is important to understand the fate of the biomass in lakes or oceanic environments, for industrial applications such as waste-water disposal. More specifically, the interaction of two settling bodies is a fundamental problem recently studied numerically for spheres. Experimental results are needed for validation, especially at low and moderate values of the Reynolds number, for different values of the Froude number, the other parameter of interest. We present experimental results on the interaction of two spheres settling in a linearly stratified fluid. The settling dynamics is investigated by tracking their trajectories in three dimensions, using a pair of cameras imaging two perpendicular planes. Two typical cases are observed, the horizontal repulsion of particles initially aligned horizontally, and the Drafting-Kissing-Tumbling of spheres initially aligned vertically. The influence of the initial positions of the spheres, the Reynolds and Froude numbers, is investigated to quantify these effects and their robustness, in comparison to the dynamics in an homogeneous fluid. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D30.00008: Internal hydraulic jumps with large upstream shear Kelly Ogden, Karl Helfrich Internal hydraulic jumps in approximately two-layered flows with large upstream shear are investigated using numerical simulations. The simulations allow continuous density and velocity profiles, and a jump is forced to develop by downstream topography, similar to the experiments conducted by Wilkinson and Wood (1971). High shear jumps are found to exhibit significantly more entrainment than low shear jumps. Furthermore, the downstream structure of the flow has an important effect on the jump properties. Jumps with a slow upper (inactive) layer exhibit a velocity minimum downstream of the jump, resulting in a sub-critical downstream state, while flows with the same upstream vertical shear and a larger barotropic velocity remain super-critical downstream of the jump. A two-layer theory is modified to account for the vertical structure of the downstream density and velocity profiles and entrainment is allowed through a modification of the approach of Holland et al. (2002). The resulting theory can be matched reasonably well with the numerical simulations. However, the results are very sensitive to how the downstream vertical profiles of velocity and density are incorporated into the layered model, highlighting the difficulty of the two layer approximation when the shear is large. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D30.00009: Measurement of High Reynolds Number Near-Field Turbulent Sphere Wakes under Stratified Conditions Kenneth Kalumuck, Alan Brandt, Kirk Decker, Kara Shipley To characterize the near-field of a stratified wake at Reynolds numbers, Re $=$ 2 x 10$^{\mathrm{5}}$ - 10$^{\mathrm{6}}$, experiments were conducted with a large diameter (0.5 m) sphere towed through a thermally stratified fresh water lake. Stratification produced BV frequencies, N, up to 0.07/s (42 cph) resulting in Froude numbers F $=$ U/ND $\ge $ 15. The submerged sphere and associated instrumentation including two Acoustic Doppler Velocimeters (ADVs) and an array of fast response thermistors were affixed to a common frame towed over a range of speeds. Three components of the instantaneous wake velocities were obtained simultaneously at two cross-wake locations with the ADVs while density fluctuations were inferred from temperature measurements made by the thermistors. These measurements were used to determine the mean, rms, and spectra of all three components of the turbulent velocity field and density fluctuations at multiple locations. The turbulence power spectra follow the expected -5/3 slope with wavenumber. Existing stratified near-field wake data for spheres are for Re $=$10$^{\mathrm{4}}$ and less, and only a very limited set of data under unstratified conditions exists at these large values of Re. Those data are primarily measurements of the sphere drag, surface pressure distribution, and separation rather than in wake turbulence. Advances in CFD modeling have enabled simulations at these high Reynolds numbers without quantitative data available for validation. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D30.00010: Internal gravity waves in stratified turbulent flow past a towed sphere at $Re = 3700$ Anikesh Pal, Sutanu Sarkar Direct numerical simulation (DNS) has been performed to investigate internal gravity waves in flow past a sphere at $Re = 3700$ in a stratified fluid with a focus on quantifying the distinction between body and turbulence generation of waves.The simulations show an excellent match between simulation and prior theory regarding amplitude and frequency of the waves generated by the body. With a decrease in $Fr$, the body generation mechanism become stronger and waves exhibit upstream propagation. In the downstream direction, there is a very clear distinction in the temporal and spatial structure between waves generated by the body and those by the turbulent flow. Turbulence leads to waves with high frequency modes that propagate into the background and interact with the low frequency lee waves. Quantitative analysis of the potential energy (PE) distribution as a function of $Fr$ and downstream distance has also been carried out. For $Fr=1$, a significant amount of the energy produced by the displacement of the fluid by the body is converted into PE of the lee waves. In contrast, most of the input energy goes into the turbulent wake for $Fr=3$. The $Fr=1$ case manifests a substantial decrease in the recirculation length and an increase in the wave drag as compared to $Fr=3$. [Preview Abstract] |
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