Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C13: Convection and Buoyancy-driven Flows: Environmental Flows |
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Chair: Nigel Kaye, Clemson University Room: 304 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C13.00001: Flow structures and kinetic-potential energy exchange in forced rotating stratified turbulence Tianyi Li, Minping Wan, Jianchun Wang, Shiyi Chen We investigate the long-time evolution of flow structures and the kinetic-potential energy exchange e\textunderscore KtoP in rotating stratified turbulence. Numerical simulations of forced homogeneous rotating stratified turbulence with different Froude numbers are performed. In the presence of stratification, with two box scale structures with opposite signs of vertical vorticity formed at later times, there are numerous small vortices spreading in the flow. Cyclonic vortices, though being less numerous than anticyclonic vortices, grow faster, thus forming the box-scale structure earlier. Moreover, the intense area of e\textunderscore KtoP is associated with the cyclonic structures. We also find a relation between vertical vorticity and the distribution of the density in the vortices during the evolution of turbulence. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C13.00002: Prandtl Number Dependence of Stratified Turbulence Jesse Legaspi, Michael L. Waite Stratified turbulence is affected by buoyancy forces that suppress vertical motion, resulting in horizontal layers of quasi-two-dimensional vortices. The Prandtl number $Pr$ (or Schmidt number) quantifies the relative strengths of viscosity and buoyancy diffusivity which damp small-scale fluctuations at different microscales. Direct numerical simulations (DNS) must resolve the smallest features, requiring high resolution for large $Pr$ (e.g. $Pr=7$ in heat-stratified water and 700 in salt-stratified water). To reduce this computational demand $Pr=1$ is often assumed, possibly introducing discrepancies between DNS and real geophysical flows. In this work, DNS of homogeneous forced stratified turbulence with $Pr=$ 0.7, 1, 2, 4, and 8 are performed for varying stratification strength. Energy spectra, buoyancy flux spectra, spectral energy flux, and physical space fields are compared for scale-specific $Pr$-sensitivity. Intermediate and large scale $Pr$-dependence was found in addition to expected small-scale sensitivity. Based on our results, the $Pr=1$ assumption is not realistic for DNS of $Pr>1$ stratified turbulence: the effects at intermediate, and in some cases, large horizontal scales must be considered, though the computational demand can be prohibitive. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C13.00003: Examination of temperature spectra in stably stratified boundary layers measured using nano-scale probe Fiona Spencer, Tyler Van Buren, Alexander J Smits, Owen Williams Thermally stable turbulent boundary layers are prevalent in the polar regions and nocturnal atmospheric surface layer but measurements are challenged by changing conditions and small fluxes. Here, we examine the influence of increasing stratification on the spectrum of temperature fluctuations in a laboratory-scale boundary layer over a rough surface. A nanoscale cold-wire (T-NSTAP) is employed to significantly increase frequency response and resolution compared to conventional cold-wires. This method is used to examine boundary layer conditions from near-neutral, through to the collapse of turbulence. This novel dataset allows examination of changes to temperature spectra, their energy cascade and wall-normal locations of maximum spectral energy in both inner and semi-local coordinates. These results when compared to atmospheric data, provide insights into the separation of Reynolds and Richardson number influences. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C13.00004: Dynamics of subsiding shells in actively growing clouds with vertical updrafts Vishnu Nair The dynamics of a subsiding shell at the edges of actively growing shallow cumulus clouds with updrafts is analyzed using direct numerical simulations with grid sizes up to 3072 x 1536 x 1536. The actively growing clouds have a fixed in-cloud buoyancy and velocity. Turbulent mixing and evaporative cooling at the cloud edges generate a subsiding shell which grows with time. A self-similarity analysis reveal that contrary to classical self similar flows, the turbulent kinetic energy budget terms and the velocity moments scale according to the buoyancy and not with the mean velocity. The shell accelerates ballistically with a magnitude defined by the saturation value of the buoyancy of the cloud-environment mixture. In this regime, the shell is buoyancy driven and independent of the in-cloud velocity. The shell thickness and the velocity continue to grow indefinitely and could possibly be limited only by the lifetime of the cloud or thermal. The entrainment coefficient is observed to be a function only of the initial state of the cloud and the environment. This coefficient is linked to the fractional entrainment rate used in cumulus parameterization schemes for large scale models and is shown to be of the same order of magnitude. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C13.00005: Impact of ambient stratification on gravity currents propagating over a submerged canopy Jian Zhou, S. Karan Venayagamoorthy The dynamics of lock-exchange gravity currents propagating over a submerged canopy in a linearly stratified environment is investigated using highly resolved three-dimensional large-eddy simulations. The canopy is composed of a bottom-mounted array of square cylinders. It is highlighted that the structure and propagation of the gravity currents show remarkable variation across the parameter space of canopy density and ambient stratification. For sparse canopies, the gravity current propagates through the canopy interior, and the reduction of front speed is less sensitive to the canopy drag under stronger ambient stratification where there is less wake-induced buoyancy loss of the current head. For denser canopies, the transition to over-flow on the canopy top and the accompanied recovery of front speed are found to occur earlier in canopy density as the ambient stratification becomes stronger, which is the outcome of three mechanisms: (i) weaker canopy drag due to the altered propagation pathway; (ii) less convective mixing of the over-head with the underlying lighter fluid; and (iii) more regain of effective horizontal buoyancy gradient as the current is lifted up. The latter two mechanisms occur due to the background stratification. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C13.00006: Computational modeling of dense gas flushing from urban canyons Rasna Sharmin, Nigel Kaye Results are presented from a series of CFD simulations of the flushing of a dense pollutant from a model urban canyon. The simulations are run using both RANS and LES simulations for the flushing from a square canyon formed between two square prismatic buildings. Results are presented for the flushing rate for a range of Richardson numbers with higher Richardson number flows exhibiting lower flushing rates. The influence of the upstream flow conditions on the flushing rate are also explored. A series of simulations for different upstream surface roughness, formed by imposing a saw-tooth geometry of various aspect ratios on the flow floor, elucidate the role of the boundary layer structure on the flow over the canyon and resulting flushing rate. The computational results are compared to prior experimental results for the same geometry with the goal of further elucidating the flow structure under more controlled conditions. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C13.00007: Coaxial moist plumes in stationary and windy environments Shuo Li, Morris Flynn We report on the dynamics of coaxial plumes in both stationary and windy ambient environments. The coaxial plumes consist of hot, humid air as the inner, circular plume and warm, less humid air as the outer, annular plume. For a stationary ambient, a double plume model is proposed using a three-way entrainment formulation that involves three undetermined entrainment coefficients. Two body force formulations are discussed, which regard either the ambient or the outer plume as the reference fluid for the inner plume. Meanwhile, a planar laser-induced fluorescence (PLIF) technique is employed to visualize the flow and quantify the scalar concentration of either the inner or outer plumes. The PLIF experiment reveals different near-field entrainment behaviors for cases wherein the inner plume is more/less buoyant than the outer plume. Moreover, the optimal values for the entrainment coefficients are determined by a pixel-by-pixel comparison of the scalar concentration between theory and experiment. For a windy ambient, theoretical results show that wind tends to rapidly dilute the heat and moisture transferred from the inner to the outer plume. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C13.00008: Large-scale and Small-scale Turbulent Structures in a Stably Stratified Shear Layer Tomoaki Watanabe, James J. Riley, Koji Nagata, Keigo Matsuda, Ryo Onishi Title: Large-scale and Small-scale Turbulent Structures in a Stably Stratified Shear Layer Turbulent structures in a stably stratified shear layer are investigated with direct numerical simulations (DNS). Initial mean streamwise velocity and density are given by hyperbolic tangent functions and, therefore, shear and stratification are localized in a thin layer with an initial thickness of $h_0$. The DNS uses a very large computational domain in horizontal directions. The Reynolds number and Richardson number based on $h_{0}$ and the density and velocity differences across the shear layer are 2000 and 0.06, respectively. Turbulence develops from the initial conditions, grows, and then it rapidly decays with time. Flow visualization employing the second invariant of the velocity gradient tensor confirms that a large number of hairpin vortices appear near the edge of the shear layer at late time in the simulation. Very elongated flow structures are also found for the streamwise velocity visualized on a horizontal plane, where the length of these long structures is about 10 times larger than the shear layer thickness. It is also shown that the length scales associated with the hairpin vortices and the elongated flow structures make large contributions to turbulent kinetic energy. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C13.00009: On Correlations of Lagrangian and Eulerian Accelerations in Turbulent Stratified Shear Flows Frank Jacobitz, Kai Schneider The correlations of Lagrangian and Eulerian accelerations of homogeneous turbulence with uniform shear and stable stratification are investigated using data from direct numerical simulations. In order to vary the relative importance of shear and stratification, a Richardson number range from Ri = 0, corresponding to unstratified shear flow, to Ri = 10, corresponding to strongly stratified shear flow, is considered. The correlations between Lagrangian and Eulerian accelerations are observed to increase with increasing Richardson number. Using a wavelet-based scale decomposition of the accelerations, their correlations at different scales of motion are also investigated. It was found that the Lagrangian and Eulerian accelerations are strongly correlated at large, energy-containing scales of motion. However, the correlations decrease with decreasing scale of the turbulent motion and the accelerations are mainly decorrelated at small, dissipative scales of motion. In addition, the correlations of Lagrangian and Eulerian time-rates of change of fluctuating density are also considered. Again, stronger correlations are obtained at larger Richardson numbers and at larger scales of the turbulent motion. [Preview Abstract] |
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