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 G13: Convection and Buoyancy-driven Flows: Stratified Flows |
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Chair: Alberto Scotti, U. of North Carolina Room: 304 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G13.00001: Buoyant displacement flows of miscible fluids in axially rotating pipes Seyed Mohammad Taghavi, Shan Lyu This work experimentally considers the effects of a pipe axial rotating motion on buoyant displacement flows of miscible fluids in an inclined pipe. A heavy Newtonian fluid displaces a light Newtonian or yield stress fluid. The experiments are performed in a long transparent pipe so that the displacement flow patterns can be visualized using high speed imaging techniques. The analysis of our experimental results reveals that the displacement process can be affected by variations of the experimental parameters, such as the pipe inclination angle, the density difference, and most importantly the pipe rotation speed. For both Newtonian and yield stress displacements, increasing the pipe rotation speed enhances the transverse mixing between the fluids and, above a critical transition, leads to a complete removal of the displaced fluid. The results are quantitatively analyzed using the flow dimensionless groups, such as the Rossby number, the Froude number, and the Bingham number. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G13.00002: Nonnormal transient growth of triadic resonant internal gravity waves Kevin Ha, Jean Marc Chomaz, Sabine Ortiz Triadic resonant instability is a key component in the understanding of the dissipation process of inertia gravity waves for geophysical applications like oceanic circulation, which is still incompletely understood. A key process in the oceanic circulation is the vertical mixing, which makes it possible for dense deep water to reach the surface. Global warming regulation by the ocean then depends on mechanisms controlling the vertical mixing of deepwater masses. It was recently proposed by Garrett \& Kunze (2007) that the mixing results from the instabilities of internal gravity waves generated by interaction between the barotropic tide and bottom topography (continental shelf, underwater mount). The present work focuses on the energy approach of the triadic resonant instability and demonstrates that due to the nonnormality of the evolution operator, stable triadic resonant interactions result in a transient amplification of perturbation energy. Computations show that they can lead to a longer and more intense transient growth than unstable triads. Instead of being related to the differential growth of a stable and an unstable modes like for unstable triads, the transient growth of stable triads originates from the differential rotation (i.e. phase shift) of two stable eigenmodes. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G13.00003: Mechanisms of drag enhancement on bodies settling in a linearly stratified fluid Jacques Magnaudet, Jie Zhang, Matthieu Mercier Using DNS, we compute the flow past a sphere settling with constant speed in a linearly stratified fluid over a wide range of Reynolds, Prandtl and Froude numbers. Thanks to a rigorous mathematical decomposition procedure, we identify the various contributions to the stratification-induced drag, especially that due to the dragging of light fluid around the body and within its wake, and that induced by the distortion of the vortex lines. Combining DNS results and scaling laws provided by the dominant balances in the density and momentum equations, we show how these two contributions vary with the control parameters. It turns out that for large Prandtl numbers and Reynolds numbers up to ${\mathcal{O}}(10^3)$, the drag enhancement is essentially due to the specific structure of the vorticity field set in by buoyancy effects, while effect of the dragging of light fluid near the body surface takes over for larger Reynolds numbers. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G13.00004: Decay of convective boundary layer and effect of decreasing rate of surface heat flux Mingxiang Zhao, Lian Shen Decay of convective boundary layer (CBL) is a model of the late-afternoon transition of the unstably stratified atmospheric boundary layer. In this study, simulations were performed to investigate the decay process under different decreasing rates of the surface heat flux. We decomposed the turbulence fluctuations into different scales to investigate their responses to the decreasing surface heat flux. It is shown that the decreasing rate of the large-scale vertical velocity fluctuations is much greater than that of the small-scale fluctuations regardless how fast the decreasing rate of the surface heat flux is. Moreover, the large-scale fluctuations experience several stages of temporary enhancements due to the local upward heat flux, while the small-scale fluctuations decreases monotonously. For the decay process of temperature fluctuations, the present study showed essential differences from that of vertical velocity fluctuations. For instance, the temperature fluctuations respond to the decreasing surface heat flux more quickly and the small-scale fluctuations decay faster than the large-scale fluctuations. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G13.00005: Influence of shear on fingering dynamics in double diffusive convection Pejman Hadi Sichani, Cristian Marchioli, Francesco Zonta, Alfredo Soldati We examine the effect of shear on double-diffusive convection (DDC) in a confined fluid layer under the Oberbeck-Boussinesq condition. DDC is characterized by the competing action of a stably-stratified, rapidly-diffusing scalar (temperature) and an unstably-stratified, slowly-diffusing scalar (solute concentration). This problem has five governing parameters: The thermal Prandtl number, $Pr_T$ (momentum to thermal diffusivity ratio); the thermal Rayleigh number, $Ra_T$ (measure of the fluid instability due to temperature and density differences); the Lewis number, $Le$ (thermal to solute diffusivity ratio); the density stability ratio, $R_{rho}$ (measure of the effective flow stratification), and the shear rate, $\Gamma$. We investigate double-diffusive fingering subject to uniform shear in a wall-bounded domain by performing a campaign of highly-resolved numerical simulations in the ($Pr_T$, $Ra_t$, $Le$, $R_{rho}$, $\Gamma$) parameter space. Preliminary results show that shear tends to dampen the growth of fingering instability, leading to highly anisotropic DDC dynamics associated with the formation of regular sheets of the slowly-diffusing scalar. The resulting modifications of vertical heat transport and solute concentration are investigated at varying shear rates. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G13.00006: Estimation of mixing in a lock-exchange flow using molecular tagging velocimetry and thermometry Tanmay Agrawal, Jimmy Philip, Joe Klewicki Gravity currents produced by a lock-exchange experiment are studied using the single-component version of molecular tagging velocimetry (MTV) in conjunction with its thermal counterpart, molecular tagging thermometry (MTT). The experiments are conducted in a Perspex tank of 2.0 m x 0.2 m x 0.2 m where the lock is located mid-way. Therefore, the current is studied only during the slumping phase and there are no transitions associated with the end-wall reflection. For these experiments, the initial density difference is created by introducing a thermal inhomogeneity on either side of the lock as compared to the general experimental practice of dissolving a salt on one side. The flow is first visualized by mixing a dye on the heavier side to establish the experimental parameters. Subsequently, MTV/MTT images are acquired that contain approximately 1000 data points distributed across the interface of hot and cold fluid. This high-resolution velocity and temperature data is then used to quantify the mixing being taken place at the interface. Specifically, background potential energy of the flow is evaluated over time to estimate the extent of irreversible mixing while an equivalent Thorpe scale is calculated to estimate the size of overturning eddies. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G13.00007: Wall-bounded stably-stratified turbulence at high Reynolds number Francesco Zonta, Pejman Hadi Sichani, Alfredo Soldati Wall-bounded stably stratified turbulence is encountered in many industrial and natural processes. Examples include fluid motion in heat transfer devices or transport/mixing of organic species in terrestrial water bodies. In this work, we focus on stably stratified turbulent channel flow at high shear Reynolds number $Re_{\tau}$. We perform a campaign of pseudo-spectral direct numerical simulations (DNS) of the governing equations, written under the Boussinesq approximation, in the shear Richardson number space $Ri_{\tau}=Gr/Re^2_{\tau}$ (which measures the relative importance of buoyancy compared to inertia, with $Gr$ the Grashof number). In particular, we fix the Reynolds number $Re_{\tau}=1000$ and we change $Gr$ so to cover a broad range of $Ri_{\tau}$ values. For increasing $Ri_{\tau}$, turbulence is sustained only near the boundaries, whereas non-turbulent wavy structures (Internal Gravity Waves, IGW), also flavored by the presence of intermittent bursts, are observed at the core of the channel. Naturally, the presence of IGW alters the overall transfer rates of momentum and heat, as well as the mixing efficiency of the flow. We believe that the present results may give important contributions to future turbulence parametrization and modeling in this field. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G13.00008: Intermittency and modal dynamics of stratified mixing at oceanographic scales Alberto Scotti, Pierre-Yves Passaggia Stratified turbulent shear flows are known to generate strongly intermittent dynamics where the flow alternates irregularly between the growth of stratified shear instabilities, regions of decaying turbulence, and quasi laminar states. Quantifying intermittency at oceanographic scales from field measurements or numerical simulations may become intractable because of sampling issues. The former may offer too few spatial measurements while the latter may prove too computationally expensive. Laboratory experiments provide an interesting alternative and allow for circumventing both difficulties. This talk therefore explores this issue with the aid of an experimental dataset acquired in the UNC Joint Fluid Lab. In a large tank, we generate shear-driven turbulent mixing that spans a significant range of the parameter space encountered in the ocean and for several thousands of eddy turnover times. Hot wire and conductivity measurements are paired with PIV and LIF measurements in a plane to extract the modal features of the flow and their intermittency. These measurements are finally compared with measurements of the the bulk Richardson number which is found to be key driver of these dynamics. We finally discuss the implication for the interpretation of field data. [Preview Abstract] |
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