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 A20: Geophysical Fluid Dynamics General I |
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Chair: Jerome Neufeld, Cambridge Room: 602 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A20.00001: Formulation and implementation of free-slip boundary condition on a deformed domain in the context of continuous Galerkin high-order element-based discretization. Theodoros Diamantopoulos, Peter Diamessis, Marek Stastna The free-slip condition is a convenient choice of boundary condition (BC) used in the simulation of stratified flows to circumvent the resolution of fine-scale no-slip-driven bottom boundary layers when numerically solving the incompressible Navier-Stokes equations (INS). The nodal spectral element method is a commonly used strategy in the discretization of the INS where the free-slip condition arises as a natural BC from the integration-by-parts of the viscous term in the INS. In the context of a deformed domain, the tangential component of the force associated with the free-slip condition, couples the Cartesian velocity components leading to a larger system of equations for the calculation of the velocity field. One approach to mitigate this complexity is to impose a pseudo-traction BC, where each velocity component can be solved for independently. Effectively, the velocity components can be treated as scalar quantities allowing the use of the same computational machinery as for the calculation of the density and the pressure. In a series of test problems, which include the propagation of an internal solitary wave, the effective numerical drag produced by the pseudo-traction BC will be quantified thereby enabling the assessment of the accuracy of the pseudo-traction approach. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A20.00002: Role of aeolian hysteresis and secondary turbulence in dust entrainment over arid landscapes Santosh Rana, William Anderson Saltation, the wind-blown hopping motion of sand particles, plays an effective role in dust emission from the sediment bed. Low and high momentum regions appear as long streaks in atmospheric turbulence. Positive and negative vertical velocity are associated with low and high momentum regions respectively. High momentum regions initiate saltation when the fluid velocity exceeds the fluid threshold value and saltation continues till the fluid velocity drops below the impact threshold value. This phenomenon is called hysteresis or the lag between the initiation and cessation of saltation. Saltation occurs during this hysteresis period. The sweeping motion and the negative vertical velocity of the high momentum regions do not allow entrainment of the dust released during saltation. However, the positive vertical velocity associated with the low momentum region picks up the dust. A conditional averaging method is employed to study this paradox as two modes of dust entrainment. In the primary mode, the dust released by the saltating particles of the high momentum region is entrained by the neighboring low momentum region. In the secondary mode, dust released due to saltation by a very recent high momentum region is entrained by a closely following low momentum region in the flow direction. Here, we explain how dust exposed to the low momentum region as a result of hysteresis gets entrained by two modes of transport. The methodology explained here is applicable to both earth and mars conditions. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A20.00003: Revisiting eddy diffusivity models in geophysical boundary layers Tomas Chor, James McWilliams, Marcelo Chamecki Eddy diffusivity models have proved invaluable when modeling turbulent fluxes in many situations. However, in atmospheric and oceanic boundary layers, large-scale motions (for example convective plumes and Langmuir circulations) do not conform to the usual assumptions necessary for their application. This has prompted the creation of several ad-hoc models, each designed to work under rather specific conditions, and each of them often failing to work well outside their intended operating area. In this work we present an alternative unifying solution that estimates the total eddy diffusivity without a priori assumptions about its shape or scaling. The approach is based on the fact that the eddy diffusivity should depend only on the flow, which we use as a basis for an optimization procedure that uses Large-Eddy simulation data. The result of our approach is that most of the fluxes are modelled with an eddy diffusivity, while the rest (which depends on specific sources of scalars and is attributed to large scale motions) is modelled as non-diffusive processes. We present an application of our proposed approach to the classic case of a convective boundary layer and show that we are able to predict the heat flux from quantities measured using passive tracers. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A20.00004: Semi-Empirical Models of Surface Pressure in the Planetary Boundary Layer in terms of Second-Order Structure of Atmospheric Turbulence Gregory Lyons, Carl Hart The fluctuations in surface pressure beneath turbulent boundary layers have long been of interest due to the aerodynamic noise and structural vibrations they induce. In the planetary boundary layer, analogous fluctuations at the ground surface contribute to instrument noise directly as static pressure, recorded by microbarometers and microphones, and indirectly as ground motion, detected by seismographs and geophones. It is hypothesized that the fast (i.e. linear) term of the pressure Poisson equation makes the principal contribution to surface fluctuations, so that only second-order structure of the inhomogeneous velocity field is necessary for modeling of second-order pressure statistics. The mirror flow model due to Kraichnan and rapid-distortion theory models proposed by Mann, which both derive inhomogeneous turbulence from initially homogeneous fields, are used to produce semi-empirical models of the surface pressure wavenumber spectrum. With an effective convection velocity, frequency pressure spectra are derived and contrasted. Using meteorological observations from a recent experiment, model parameters are estimated from velocity spectra, and the resultant pressure spectral models are compared with measurements from flush-mounted pressure sensors. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A20.00005: The poroelastic aquifer: river flux response to solid Earth tidal forcing Jerome Neufeld, Eric Lajeunesse, Olivier Devauchelle The water flux through rivers is chiefly determined by the intermittent charging of groundwater in laterally extensive shallow aquifers aquifers by rainfall and drawdown by seepage to the river bank. Here we show that the pattern of drawdown is modulated by the small-scale forcing of shallow poroelastic aquifers by the solid Earth tides. We use a shallow poroelastic model of flow in a deformable matrix to show that the modulation of the solid stress by solid Earth tides gives rise to the observed 2-4 cm height variation in the far-field aquifer depth. This oscillatory groundwater table also drives a modulation of the water flux into the river which is apparent in long-term records of stream flux. By understanding the poroelastic response of aquifers to solid Earth tides as observed in the river flux data, we are therefore able to infer properties of distributed aquifers from readily available records of the river flux. The fluid dynamical modelling, motivated by observations at the Quick creek catchment, Guadeloupe, also demonstrates a generic framework for understanding the interaction between periodic solid stresses and fluid flow in deformable porous media. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A20.00006: The Sphered Cube and other applications of sparse spectral methods in spheres Keaton Burns, Daniel Lecoanet, Geoffrey Vasil, Jeffrey Oishi, Benjamin Brown, Eric Hester Developing efficient spectral discretizations of hydrodynamical models in spherical coordinates is a challenging task. For instance, the analytical behavior of scalars, vectors, and tensors differ near the coordinate singularities at the poles and the origin, so care must be taken to maintain accuracy in spectral representations of such quantities. We have recently developed bases for arbitrary-order tensors in the ball that incorporate these different analytical behaviors and possess banded differential operators similar to sparse Chebyshev methods. These new bases enable the efficient and systematic discretization of broad ranges of tensorial PDEs in spherical coordinates. We will discuss these bases and their implementation in the open-source spectral framework Dedalus. We will then present a variety of uncommon simulations in spherical domains, including Rayleigh-Benard convection in a cube using immersed boundaries, and the evolving interface between an ice shell and an underlying ocean using the phase-field method. These examples illustrate the robustness of these spherical bases and their utility for complex simulations of stellar and planetary interiors. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A20.00007: Fluid dynamical models of lunar crustal formation Callum Watson, Jerome Neufeld, ChloĆ© Michaut Typical models of the thermal evolution of the Moon involve a solid crust growing via crystal flotation, with an underlying magma ocean then cooling by conduction through the crust. However, convection dominated by a temperature-dependent viscosity, producing a stagnant-lid, may be more relevant to the early lunar magma ocean and explain the long timescales for crust formation. This involves convection with in a fluid whose viscosity varies by several orders of magnitude as a function of temperature and crystal fraction. The temperature scale most relevant to the convection is that over which the viscosity varies. This scale is greatly reduced during rapid changes in viscosity, such as when jamming occurs at a critical melt fraction. We consider a one-dimensional model in which the lunar magma ocean is considered as one body of convecting silicate, with a viscosity that depends strongly on temperature, pressure and solid fraction. The resulting surface heat flux depends primarily on the temperature of the mixed interior and on the hydrostatic pressure at the base of the stagnant lid. This results in an initial unstable scenario with a lid that is mostly entrained back into the interior after approximately 200 Ma, which may explain the long persistence of the Lunar dynamo. [Preview Abstract] |
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