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 M30: Geophysical Fluid Dynamics: Gravity Currents |
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Chair: Claudia Cenedese, Woods Hole Oceanographic Institution Room: 311 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M30.00001: Lock-release gravity currents over a sparse and dense rough bottom Claudia Cenedese, Roger Nokes, Jason Hyatt Dense oceanic overflows mix with surrounding waters along the descent down the continental slope. The amount of entrainment and dilution dictates the final properties of these overflows, and thus is of fundamental importance to the understanding of the formation of deep water masses. We will discuss laboratory experiments investigating the mechanisms by which bottom roughness enhances or inhibits entrainment and dilution in a lock-release dense gravity current. The bottom roughness has been idealized by an array of cylinders. Both spacing (sparse vs. dense configuration) and height of the roughness elements compared with the height of the current have been varied. Both density and velocity fields have been obtained. Experimental results suggest that enhancement of the entrainment/dilution of the current can occur due to two different mechanisms. For a sparse configuration the dense current propagates between the cylinders and the entrainment is enhanced by the vortices generated in the wake of the cylindrical obstacles. For a dense configuration the dense current rides on top of the cylinders and the dilution is enhanced by the onset of convective instability between the dense current above the cylinders and the ambient lighter water between the cylinders. For large values of the ratio of the lock height to the cylinder height, H/h$_{\mathrm{c}}$, the dense current behavior approaches that of a current over a smooth bottom, while the largest deviations from the smooth bottom case are observed for small values of H/h$_{\mathrm{c}}$. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M30.00002: Dynamics of double-diffusive lock-exchange gravity currents Nathan Konopliv, Eckart Meiburg The dynamics of double-diffusive gravity currents exhibiting the fingering instability were examined using 2D simulations of a lock exchange initial configuration. Both the initial stability ratio and the diffusivity ratio were varied. It was found that although the spreading of the currents was governed by a balance of buoyancy and turbulent drag forces, currents with more intense fingering spread faster than those with less intense or no fingering. This was due to an increase in the buoyancy of the currents with stronger fingering, which had a stronger effect than the increased drag. The fingering also affected the thickness of the currents, with more fingering corresponding to thinner currents. The mechanism that caused the thinner currents was also responsible for the creation of secondary and tertiary currents after a long time in a simulation that had intense fingering. If no secondary or tertiary currents formed, the density of the current was governed by a balance of double-diffusive and diffusive fluxes. An energy budget analysis revealed that double diffusive currents released more potential energy, had more dissipation and converted a significant amount of internal energy into potential energy via the diffusion of heat and salinity. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M30.00003: Mixing Induced by Colliding Gravity Currents Qiang Zhong, Christopher Hocut, Fazle Hussain, Harindra Fernando Colliding gravity current is a widespread phenomenon in complex-terrain meteorology. Nevertheless, only a few detailed studies have been conducted on the mixing and turbulent transport processes during collision, and no parameterization exists to incorporate the mixing effects of collision in mesoscale models. To this end, controlled laboratory experiments were conducted in a double lock-exchange configuration. Velocity and density measurements were made simultaneously using a PIV/PLIF system. Phase aligned ensemble-averaging was employed to elicit mean and turbulent quantities. Collisions cause localized instabilities both along the density interface and in the interior of gravity currents. The turbulence near the density interface induces strong mixing and, along with ambient fluid entrainment, produces strong fluctuations of buoyancy flux. A time scale for the evolution of ensuing turbulence as well as scaling for the entrainment velocity was delineated for the high Reynolds number cases. The flow was replete with turbulent vortices generated by the collisions, and they decayed exponentially with time. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M30.00004: Circulation-based modeling of gravity currents propagating into ambients with arbitrary shear and density stratification Mohamad Nasr-Azadani, Eckart Meiburg We develop a vorticity-based approach for modeling quasisteady gravity currents propagating into arbitrary density and velocity stratification. The model enforces the conservation of mass, horizontal and vertical momentum, and in contrast to previous approaches it does not rely on empirical, energy-based closure assumptions. Instead, the effective energy loss of the flow can be calculated \textit{a posteriori}. The present model results in the formulation of a second order, nonlinear ODE that can be solved in a straightforward fashion to determine the gravity current velocity, along with the downstream ambient velocity and density profiles. Comparisons between model predictions and DNS simulations show excellent agreement. They furthermore indicate that for high Reynolds numbers the gravity current height adjusts itself so as to maximize the loss of energy. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M30.00005: Entrainment dynamics in self-adjusting gravity currents using simultaneous velocity-density measurements Sridhar Balasubramanian, Qiang Zhong, Harindra Fernando Gravity currents can modify their flow characteristic by entraining and mixing with the ambient fluid. The entrainment in such systems may depend on a variety of intrinsic parameters such as, initial density difference, $\Delta\rho$, total height of the fluid, $H$, and slope of the terrain, $\alpha$. Thus, it is imperative to study the entrainment dynamics of a gravity current in order to have a clear understanding of the mixing transitions that govern the flow physics such as the shear layer thickness, $\delta_{u}$, and the mixing layer thickness, $\delta_{\rho}$. Experiments were conducted in a lock-exchange type facility, where a self-adjusting gravity current is formed, for which the only governing parameter is the Reynolds number, Re=$\frac{u_{f}H}{\nu}$, where $u_{f}$=0.4$\sqrt{g^{'}H}$ is the frontal velocity. Simultaneous PIV-PLIF technique is employed to get the velocity and density statistics. A control volume based flux method is used to calculate the flux entrainment coefficient, E$_{f}$, for a Reynolds number range of Re=400-12000 used in our experiments. The results show transition at Re~4x10$^{3}$, where the mixing occurs due to Kelvin-Helmholtz billows that promote small scale local mixing, and cause a spike in the flux entrainment velocity. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M30.00006: Gravity currents down a slope in the acceleration phase Yu-lin Huang, Albert Dai Gravity currents generated from an instantaneous buoyancy source propagating down a slope in the range of $0^\circ \le \theta < 90^\circ$ have been investigated. Front velocity history shows that, after the heavy fluid is released from rest, the flow goes through the acceleration phase, reaching a maximum front velocity $U_{f,max}$, and followed by the deceleration phase. The existence of a maximum of $U_{f,max}$ is found near $\theta=40^\circ$, which is supported by the theory. It is identified that the time of acceleration decreases as the slope angle increases, when the slope angle is approximately greater than $10^\circ$, and the time of acceleration increases as the slope angle increases for gravity currents on lower slope angles. A fundamental difference in flow patterns, which helps explain the distinct characteristics of gravity currents on high and low slope angles using scaling arguments, is revealed. Energy budgets further show that, as the slope angle increases, the ambient fluid is more easily engaged in the gravitational convection and the potential energy loss is more efficiently converted into the kinetic energy associated with ambient fluid. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M30.00007: Vorticity models for gravity currents propagating into two-layer stratified ambients Mohammad Amin Khodkar, Mohamad Nasr-Azadani, Eckart Meiburg We investigate the propagation of Boussinesq gravity currents into two-layer stratified ambients by means of vorticity models and two-dimensional Navier-Stokes simulations. The control volume-based vorticity model enforces the conservation of vertical momentum by balancing the in- and outflow of vorticity with the baroclinic vorticity generation inside the control volume. In this way, it avoids the need for energy-based closure assumptions, such as those invoked in earlier modeling efforts. We find that for flow fields both with and without upstream propagating bores, the model predictions regarding the gravity current and bore velocities are in good agreement with the simulation results. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M30.00008: On the propagation of a gravity current into a fluid with horizontal and vertical density gradient Hieu Pham, Sutanu Sarkar Large-eddy simulations are used to investigate the dynamics of a rotating gravity current propagating in the ocean surface mixed layer on top of a pycnocline. Two simulations with different conditions in the surface mixed layer are performed: one with a homogenous mixed layer and one with a horizontal density gradient. In the latter case, the density in the mixed layer decreases with propagating distance. In both cases, a nonlinear bore forms at the front of the gravity current with Kelvin-Helmholtz billows that develop below and in the region behind the bore. In the case with a homogeneous mixed layer, the bore propagates at a constant speed which is proportional to $\sqrt{g'H}$ where $g'$ is the reduced gravity and $H$ is the mixed layer depth. In the case with the horizontal gradient, the speed decreases in time. It is found that the horizontal density gradient influences the propagation of the bore in the following ways: (1) It reduces the buoyancy difference which drives the bore; (2) It generates a horizontal pressure gradient which drives a counter gravity current opposing the bore. The counter current creates a flow-converging zone ahead of the bore. The speed of the bore is found be dependent of the horizontal density gradient and the traveling distance of the bore. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M30.00009: Intrusive gravity currents interacting with obstacles in a continuously stratified environment Jian Zhou, Subhas Venayagamoorthy The flow dynamics of intrusive gravity currents past a surface-mounted obstacle was investigated using large eddy simulations. The propagation dynamics of a classical intrusive gravity current in the absence of an obstacle was first simulated to validate the numerical simulations. The numerical results showed good agreement with experimental measurements. An obstacle with a dimensionless height of $\tilde{D}=D/H$ ($H$ the total fluid depth) was then introduced and acted as a controlling factor of the downstream flow pattern. It is found that for short obstacles, the intrusion re-established itself downstream in a form similar to the classical intrusion (in the absence of an obstacle). However, for tall obstacles, the downstream flow was found to be a joint effect of horizontal advection, overshoot-springback phenomenon, and the Kelvin-Helmholtz instability. Three regimes of downstream obstacle-affected propagation speed were identified depending on values of $\tilde{D}$, i.e. a retarding regime ($\tilde{D} \approx 0 \sim 0.3$), an impounding regime ($\tilde{D}\approx 0.3\sim 0.6$), and a choking regime ($\tilde{D}\approx 0.6\sim 1.0$). [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M30.00010: Front conditions for gravity currents in channels of general cross-section: some general conclusions Marius Ungarish We consider the propagation of a high-Reynolds-number gravity current in a horizontal channel with general cross-section of width $f(z), 0 \le z\le H$; the gravity acceleration $g$ acts in $-z$ direction. (The rectangular case is $f(z) =$ const.) We assume a two-layer system of fluids of densities $\rho_c$ (current, of height $h$) and $\rho_a$ (ambient, filling the remaining part of the channel). We revisit the derivation of the nose Froude-number condition $Fr = U/(g' h)^{1/2}$; $U$ is the speed of propagation of the current and $g' = (\rho_c/\rho_a -1) g$. We present compact insightful expressions of $Fr$ and energy dissipation as a functions of $\varphi$ ($=$ area fraction occupied by the current in the cross-section), and show that a degree of freedom is present. We demonstrate that the extension of the closure suggested by Benjamin for the rectangular cross-section, namely that the bottom is a perfect stagnation line, produces $Fr$ solutions which are optimal with respect to several useful criteria. However, the energy conserving closure yields problematic $Fr$ results, as manifest in particular by invalidity for deep currents (small $h/H$). Connection with realistic time-dependent gravity currents is discussed. [Preview Abstract] |
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