Session H09: Geophysical Fluid Dynamics: General (5:45pm - 6:30pm CST)

Turbulent flow structure associated with interacting barchan dunes

Barchan dunes are 3D, crescent-shaped bedforms, and while most commonly associated with aeolian environments, recent observations have shown them to form in subaqueous and extraterrestrial environments as well. As barchans migrate in the direction of the flow, they interact with their neighbors, typically by way of a collision. The morphodynamics of such collision processes are complex, where the role of the turbulent flow structure is strongly coupled to that of the sediment transport and morphological change. Here we study the flow structure in a decoupled manner through measurements of the turbulent flow over fixed-bed models of barchan dunes in various configurations involved in a barchan collision process. Particle image velocimetry is used to measure the flow in a refractive-index matched flume environment that enables uninhibited access to the whole flow field around these geometrically complex bedforms. Presented herein are results from temporally resolved stereo-PIV measurements showing the dynamics of turbulent flow structure in the cross-plane. Aspects of the flow turbulence with implications for sediment transport are analyzed in terms of the impulses of sweeps and ejections, which are further explained in terms of reconstructed vortex structure and wavelet analysis.   

Properties of long-running submerged turbidity currents

Turbidity currents are sediment-laden flows that travel along sloping surfaces, typically the submarine bottom. They are driven by the density difference between the current and the deep layer of quiescent ambient fluid above them. The interaction of the current with the bottom may result in the generation of sedimentary features on the seafloor called bedforms, and the interaction with the ambient fluid causes sediment-free fluid entrainment into the current. In this work we focus our attention on the flow dynamics of the body of turbidity currents moving on non-erodible beds and the turbulent interaction between the near-wall layer and interfacial layers. For this we use a combination of highly resolved direct numerical simulations (approx. 113 million grid points) and large eddy simulations (approx. 20 million grid points) of spatially evolving turbidity currents in a long domain, of length equal to 150 times the inlet height. The flows are simulated using the spectral element method with the open-source computational fluid dynamics solver Nek5000. We assess the effect of bed slope, settling velocity of the sediment (i.e., sediment size) and bottom boundary conditions for both sub and super-critical regimes.   

Intermittent cascading interfacial instabilities in gravity currents

Submarine gravity currents are flows that are submerged beneath a deep layer of quiescent fluid and that can travel over long distances along the oceanic floor. They are driven by the density difference between the current and the clear ambient fluid above. In this work we present results on highly resolved direct numerical simulations of gravity currents. We perform two simulations of currents in the well-established flow regimes: supercritical and subcritical regimes. We also report on the existence of an intermediate transcritical regime. While the current slowly evolves in the subcritical and supercritical regimes in a near self-similar manner, the transcritical current with its unique cyclical evolution exhibits a limit-cycle like behavior. We investigate how departure from self-sustaining equilibrium state is the mechanism responsible for this cyclic evolution of the transcritical regime.   

Early Lunar convection with pressure-dependent rheology

The Moon's crust is known to have formed over 4 Ga ago during the convective cooling of an early global magma ocean. Due to the strongly temperature dependent viscosity of magma, a thick boundary layer known as a {\em stagnant lid} is thought to have formed, the cooling of which resulted in the Lunar crust. Observations of the Lunar gravitational field and topography show that there is a significant hemispheric crustal dichotomy: the far side is far thicker than that the near side.\\ We consider a simple one-dimensional model with pressure- and temperature- dependent silicate viscosity, with a stagnant lid divided into two hemispheres overlying a well-mixed silicate interior and ferrous core. We find that when the viscosity increases sufficiently rapidly with pressure, an symmetric state is unstable to antisymmetric perturbations in stagnant-lid thickness. This instability saturates only when a significant dichotomy has developed in the stagnant lid.\\ The crustal thickness is reflective of the asymmetric stagnant lid thickness, by having a crust that forms from compaction of the stagnant lid. Our model suggests a mechanism for the observed crustal dichotomy, and additionally resolves issues with the timescale of formation.   

System Identification Approaches to Modeling of Tornado Acoustic Mechanisms

Recent studies have shown evidence of tornadic infrasound production prior to tornadogenesis though the life of the tornado. Oklahoma State University has recently developed experimental apparatuses to provide flexible measurements of local atmospheric conditions around severe storm events. Although the physical mechanisms of tornadic infrasound are not well known, system identification methods can be applied to this newly available data to develop and validate models from leading hypotheses. Required adaptations and measurement considerations to fit system identification frameworks with existing models are discussed. Preliminary examples of applying of system identification techniques to identification focused severe storm simulations show progress in quantifying the state information and system structure needed to concisely model tornados. When combined with experimental measurements, these identification frameworks will provide insight into scaling of models and future tornado tracking   

The effect of roughness on the onset of nonlinear flow in fractures

In fractures where surface fluctuations are large compared to their aperture (\textit{narrow fractures}) the flow is forced to move in tortuous paths that produce additional viscous friction and may affect the importance of inertia effects. We consider the relation between the magnitude of surface roughness and the onset of inertial effects in the pressure driving the flow through a single open fracture. We performed experiments systematically varying the average aperture of the open fracture and covering a wide range of Reynolds numbers. For each aperture, we analyze the data in terms of the Forchheimer equation and show that the critical Reynolds number, defined as the Reynolds number at which inertial effects contribute 10{\%} of the total pressure losses is highly correlated with the roughness of the surface. We show that inertial effects appear at lower Reynolds numbers as the relative magnitude of surface roughness increases. We present results showing that the magnitude of the deviations in the pressure field compared to a linear profile, taken at different points in the fracture along the flow direction, are also directly related to the relative magnitude of surface roughness in the fracture. Finally, we present computational results in simple sinusoidal fractures to explore the role of the relative wavelength of the surface fluctuations on the onset of inertial effects.   

Unmanned Aircraft for Mapping Atmospheric Boundary Layer Induced Geomorphological Changes

The process of acquiring lower atmospheric measurements onboard unmanned aircraft systems (UAS) is becoming a widely available solution. This study is a continuation of data collection that combines atmospheric observations using UAS with photogrammetrically mapped rapid geomorphology also using UAS to observe and relate the terrain induced effects on lower atmospheric phenomena and vice versa. Through a series of flights, lower atmospheric boundary conditions are characterized through wind velocity direction measurements onboard. During the date of atmospheric measurement collection, images are taken at the site and a photogrammetric technique called Structure from Motion (SfM) is used to reconstruct high fidelity models of the topography. The atmospheric data collection systems include 3D and 2D anemometers evaluated in both body and earth fixed configurations. Additional testing has been done to characterize the flow of air around the sensor while in flight ensuring the calibration and validation of the windspeed and direction data. The results of photogrammetry and wind observations both analyzed and related to compare the coupled effects. Wind induced terrain variations are compared through multiple terrain models taken over the past 2 years at the Little Sahara State Park.   

PIV measurements of intracrater flow dynamics utilizing a mound-bearing impact crater model in a refractive index matched environment

Impact craters are the most dominant large-scale topographic features on Mars and play a critical role in uncovering Mars' ancient history. One topography of interest concerns craters hosting a central mound. Knowledge of intracrater wind circulations is crucial in assessing the aerodynamic mechanisms that dictate the morphological evolution of impact craters. These vortical circulations are formed and modified due to an interplay between vortex shedding, ambient velocity gradients, and inhomogeneity of Reynolds stresses. Planar particle image velocimetry (PIV) measurements were conducted at multiple Reynolds numbers of flow over an idealized crater model in a refractive index matched (RIM) flume environment. The RIM technique acts to render the acrylic model transparent by equating its RI with that of the working fluid. This in turn affords optical access to the interior of the crater model and near-surface measurements. Mean flow statistics reveal large-scale symmetric counter-rotating intracrater recirculation regions coupled with a complex dynamic of detaching and reattaching flow. By collecting vector fields at two laterally offset wall-normal planes and vertically offset wall-parallel planes, we are able to infer the highly three-dimensional flow structure.   

Internal vs Forced Variability metrics for Geophysical Flows using Information theory

We propose a metric for measuring internal and forced variability in ensemble geophysical flow models using information theory: Shannon entropy and mutual information. Information entropy fundamentally determines variability by measuring the amount of variation in a distribution, as opposed to variance measuring the second moment. Shannon entropy and mutual information naturally take into account correlation coefficient, apply to any data, and are insensitive to outliers as well as a change of scale. We combine these two to quantify internal vs forced variability in (1) idealistic Gaussian vectors, (2) a realistic coastal ocean model and we show our metric's advantage over variance metrics. Our metric applies to any ensemble flow models where intrinsic and extrinsic factors compete to control variability.   

Oklahoma City Contaminant Dispersion: 3D Velocity and Concentration Data Analysis for a Scaled Puff Release Experiment

Magnetic resonance (MR) techniques were leveraged to experimentally obtain velocity and concentration measurements for a novel puff release contaminant dispersion study, with the motivation of providing a high fidelity, three-dimensional data set for comparison with JU2003-related studies. The study involved a 1:2206 scaled model of downtown Oklahoma City as it was in 2003, placed inside a water channel for boundary layer simulation. Flow through the channel was fully turbulent with a Reynolds number of 36,000 based on hydraulic diameter. An MRI system was used to take scans at 12 time-specific measurement phases throughout the puff release cycle, allowing for incremental visualization of the contaminant plume during its transport downstream. The resulting data set is analyzed using isosurfaces, streamtraces, and contour planes, while tracer flux analysis is leveraged to characterize flow into and out of an intersection and several street canyons. Comparison with wind tunnel data from an experiment involving similar geometry is also discussed in detail. The MR data set provides a means of comparison for other related studies and computational models, and can be used to identify dispersion characteristics relevant to emergency response efforts and city planning.   

Single-particle Lagrangian statistics from direct numerical simulations of rotating-stratified turbulence

To study the influence of rotation and stratification (RaS) on turbulent transport, we perform a DNS study of Lagrangian statistics in the case where the ratio between the frequencies associated with RaS, $f$ and $N$, is $N/f=5$. We vary the Froude number $Fr$ in the range $0.03 < Fr < 0.2$. We separate the motion in the horizontal and vertical directions. As the intensity of RaS increases, a sharp transition is observed from a regime dominated by eddies to a regime dominated by waves, when the product $N \tau_\eta$ becomes larger than 1, $\tau_\eta$, being the Kolmogorov time based on the mean kinetic energy dissipation. In the regime $N\tau_\eta <1$, acceleration statistics exhibit characteristics of isotropic turbulence in both directions. This includes probability density functions with wide tails and acceleration variance approximately scaling as per Kolmogorov theory, contrary to what happens when waves prevail ($N \tau_\eta > 1$). In the regime $N\tau_\eta <1$, rotation enhances the mean-square displacements in horizontal planes at short times but suppresses them at longer times, in the diffusive regime. In all cases, the displacements in the vertical direction are always reduced, and in the $N\tau_\eta >1$ regime, the motion is essentially trapped in horizontal plane.