Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L39: Turbulence: Geophysical Fluid Dynamics |
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Chair: Dhiraj Kumar Singh, University of Utah Room: 355 E |
Monday, November 25, 2024 8:00AM - 8:13AM |
L39.00001: How Prandtl number affects the scale-dependent distribution of turbulent kinetic and potential energy in stably stratified turbulence Soumak Bhattacharjee, Stephen M de Bruyn Kops, Andrew D Bragg Recent direct numerical simulation (DNS) results have revealed striking effects of Prandtl number (Pr) on the dynamics of stably stratified turbulence (Riley, Couchman and de Bruyn Kops, Journal of Turbulence 2023), which are of great importance for understanding flows where Pr>>1. The theoretical study of Bragg and de Bruyn Kops (2024) revealed the mechanism that causes the turbulent kinetic energy (TKE) and turbulent potential energy (TPE) dissipation rates in such flows to be much more strongly dependent on Pr than in flows where the density field is passive. However, the mechanisms underlying the Pr-dependence of the TKE and TPE fields across the full range of scales in the flow are not well understood. To address this, we use an anisotropic filtering approach to analyze the mechanisms governing the TKE and TPE across scales. We use data from massive-scale DNS that explore the effect of Pr for flows with different buoyancy Reynolds numbers and Froude numbers. The analysis reveals how the mechanism presented in Bragg and de Bruyn Kops (2024) can be extended to different scales in the flow, providing insight into the mechanism responsible for the reversal of the buoyancy flux below a certain scale. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L39.00002: Resolvent analysis of viscosity stratified channel flows Pulkit Kumar Dubey, Anagha Madhusudanan, Greg P Chini, Rama Govindarajan Flows with temperature-dependent viscosity have been studied in the context of turbulent drag reduction and liquid cooling systems, where significant temperature differences between channel walls can be maintained. Direct numerical simulations of such flows are computationally challenging and expensive. Here, we employ resolvent analysis to gain insight into turbulent flow structures and energy spectra in viscosity-stratified channel flows at minimal computational cost. Specifically, we study the low-rank nature of the resolvent operator and identify the range of streamwise and spanwise wavenumbers that exhibit this low rank behavior as a function of the stratification. The effects of stratification on the energy spectra and the structure of the singular modes are also documented. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L39.00003: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 8:39AM - 8:52AM |
L39.00004: Turbulence budgets in shearless, inhomogeneous, and stably stratified turbulence Ryan Hass, Sanjiva K Lele Large scale simulations of the Earth’s oceans and atmosphere rely on crude parameterizations of turbulent processes using bulk quantities due to the prohibitive mesh resolution required to capture these processes directly. An interesting flow regime useful for evaluating existing models is one in which shearless turbulence, generated in a localized region of space, decays and interacts with background stratification, a scenario common in geophysical settings. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L39.00005: What Sets the Spatial Size of Coherent Objects Produced in a Reverse Cascade of Turbulent Energy? Sungkyu Kim, Philip S Marcus Reverse energy cascades occur in rapidly rotating and/or highly stratified turbulence. Earlier, we studied reverse cascades with the 2D quasigeostrophic (QG) equations used in geophysical fluid dynamics. Randomly forcing at high wave numbers, we created statistically steady, turbulent, coherent objects (in this case, we created an alternating band of β-plane jets that formed a “staircase” of potential vorticity and qualitatively looked like the multiple jet streams of Jupiter, Saturn, and Neptune). The width of the jets was the length where the reverse cascade of energy stopped. What determines that length? At each wave number k in a 2D QG fluid, the ratio of the kinetic energy (KE) to potential energy (PE) is (kLR)2, where LR is the Rossby deformation radius, so at large k, KE dominates; while at small k, PE dominates. Our 2D QG simulations suggested that the KE injected at small spatial scales, reverse cascades to smaller k until the PE equals the KE, so that the cascade stops at k = 1/LR. In a continuously-stratified 3D fluid, LR depends on the Coriolis parameter and the stratification. Here, we examine whether our earlier 2D QG findings about what sets the scale of the largest coherent objects are valid in real 3D stratified flows. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L39.00006: Experimental investigation of stable density-stratified confined mixing layers Maegan Vocke, Ralf Kapulla, Chris Morton The study of stratified flows provides important insight towards turbulent mixing in chemical and nuclear reactors, geophysical flows, and combustion processes. If the flow is stably stratified, buoyancy forces can impede the growth of Kelvin-Helmholtz (KH) instabilities, leading to a decay in turbulence production. In this work, stereo particle image velocimetry (PIV) measurements are performed in IDEFIX (HIgh gas Density differEnce Facility for mIXing) to characterize turbulent mixing mechanisms in shear-buoyancy driven flow. The facility consists of an open gas mixing loop that supplies two parallel gas flows with independent velocities (u1, u2) and independent densities (rho1, rho2) to a horizontal mixing chamber. The velocity and density ratios, R = (u1-u2)/(u1+u2) and A=(rho1-rho2)/(rho1+rho2), considered include R = 0.2, 0.4, 0.5, 0.6 and A = 0, 0.2, 0.4, respectively, where the half channel Reynolds number Re = 4400 is kept constant in the upper leg. The PIV measurements are used to characterize empirical growth laws across a wide parameter range. The density fields are estimated from continuity to provide an assessment of the local gradient Richardson number and identify different flow regimes based on the Miles-Howard criterion. Finally, proper orthogonal decomposition (POD) is applied to extract and compare the organization of coherent structures across different density ratios to further describe the different modes of turbulent mixing. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L39.00007: Amplitude modulation of Atmospheric Boundary Layer in presence of the monsoon low level jet over Indian subcontinent Shibani Bhatt, Abhishek Gupta, Harish Mangilal Choudhary, Pranav Sood, Neetesh Singh Raghuvanshi, Prajyot Sapkal, Thara Prabhakaran, Shivsai A Dixit The majority of rainfall in India is reported during June, July, August, and September, collectively known as the Monsoon season in the Indian Subcontinent. One of the primary indicators suggesting the onset of this season is the formation of a strong westerly jet at a height of approximately 1 to 2 km from the surface, known as the monsoon low-level jet (MLLJ). This study investigates scale interactions in the atmospheric boundary layer (ABL) during the presence of a strong MLLJ. Data from a weather flux tower, equipped with a sonic anemometer placed 4m above the ground, are analyzed, along with data from a wind profiling radar to identify the presence of the MLLJ. Scales equal to or larger than the ABL height (δ) are considered large scales in the flow. The amplitude modulation coefficient is calculated to quantify the effect of these large scales on the smaller scales within the ABL. Prior studies mention similarity between a laboratory wall jet and ABL with low level jet in terms of mean velocity profile and turbulence kinetic energy budget (Gupta et.al. JFM 2020). Heeding this, amplitude modulation coefficient (RAM) is also analyzed in a turbulent plane wall jet and compared with the one calculated from the sonic anemometer data near the ABL surface layer. The atmospheric surface layer appears to exhibit same order of magnitude of amplitude modulation as observed in the laboratory wall jet. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L39.00008: Spatial and Stability-driven variability of the TKE Dissipation Rate in the Marine Atmospheric Surface Layer: A LiDAR Experiment Giacomo Valerio Iungo, Sayahnya Roy, Mojtaba Shams Soulari The dissipation rate of turbulent kinetic energy, ε, is a critical parameter for turbulence characterization as it encompasses fundamental processes related to scalar, momentum, and energy transport. Dissipation rate can significantly affect accuracy in turbulence predictions for many applications, such as wildfire development, air traffic control, pollutant dispersion, and wind energy production. In the marine atmospheric boundary layer at coastal region, the shore-normal variability due to the presence of the coastline and air-sea interaction induces complex modulations in ε, which are difficult to predict with classical turbulence models. The main goal of this work is to characterize and model ε variability with the atmospheric stability, height, and sea/wave conditions. In this study, ε is estimated from streamwise velocity measurements collected with a scanning Doppler LiDAR deployed in coastal region. The turbulent dissipation rate is calculated using methods involving the second-order structure function or the power spectral density of the streamwise velocity. Special attention is paid to the identification of the inertial subrange from the LiDAR turbulence measurements. Findings indicate that colder offshore winds, which are typically associated with unstable atmospheric conditions, generate higher ε compared to milder onshore winds associated with stable atmospheric conditions. Clear trends of ε are also identified as a function of the wave age and the reference wind velocity. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L39.00009: Detecting Coherent Turbulence Structures in Planetary Boundary Layers via Koopman Mode Decomposition and Data-Driven Methods Milad Rezaie, Mostafa Momen The planetary boundary layer (PBL) exhibits highly nonlinear dynamics, which stems from its turbulent and chaotic nature. While many studies attempted to characterize coherent turbulence structures in PBLs, there is currently no overarching data-driven method for detecting such structures under different PBL regimes. This study aims to bridge this gap by using Koopman mode decomposition (KMD), unsupervised clustering, and large eddy simulations (LES). To this end, eight LESs of convective, neutral, and unsteady PBLs are conducted. The LES results show that increasing the buoyancy-to-shear ratio alters roll vortices to convective cells in PBLs. KMD was shown to detect non-trivial dynamical modes of such PBLs. Using timescale and quadrant analyses, we attributed these modes to pressure gradient, Coriolis, and buoyancy forces. It is found that only ~5% of the Koopman modes can reconstruct the primary PBL flow field compared to the actual LES data even under unsteady conditions. Furthermore, we combined convolutional neural networks with K-means clustering to efficiently classify Koopman modes according to their intrinsic dynamics. This study offers new insights into the PBL dynamics and presents a data-driven framework for characterizing complex spatiotemporal turbulence structures. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L39.00010: Momentum transport above forested gentle topography Gregory Q Torkelson, Marcelo Chamecki Much of Earth's surface is covered by forest, agricultural, and urban canopies. Turbulent transport in the atmospheric boundary layer (ABL) over the Amazon rainforest is a key area of study due to the wide array of gases emitted within the forest. The Amazon is characterized by a tall, dense canopy and rivers that run through the area, creating a series of gentle hills and valleys. In this study, a large-eddy simulation dataset is used to focus on the coupling between the two components that make up these topographical configurations, an upslope and a downslope. The streamwise length of the valley floor and hilltop are varied in our idealized configuration, altering the coupling between these two components. When the upslope and downslope are separated by a large enough distance, the flow has many similarities to turbulent flow over a forward and backward facing step. This includes local re-circulations and a growing shear layer across the canopy top. As these length scales shrink, the differences in the valley and hilltop cases become more apparent. An enhancement of turbulence across the shear layer is observed in the hilltop cases, with little change in the mean re-circulations. In the valley cases, mean re-circulations are found to merge and grow while turbulent mixing is reduced. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L39.00011: How does the rotation of particles affect precipitation settling velocity in the atmospheric surface layer Dhiraj K Singh, Timothy J Garrett, Eric R Pardyjak
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Monday, November 25, 2024 10:23AM - 10:36AM |
L39.00012: Uncovering the turbulence structure of Rayleigh-Taylor instabilities through direct numerical simulations on a small spatial domain Aaron Nelson, Guillaume Blanquart The buoyant turbulence generated by Rayleigh-Taylor (RT) instabilities plays a central role in the physics of wildfires, supernovae, and inertial confinement fusion. While the global evolution of RT instabilities has been extensively documented in experiment and simulation, the local turbulence structure is poorly understood; the presence of small-scale anisotropy seems to contradict Kolmogorov theory. To analyze this structure, a new simulation framework is developed to isolate the small scales of turbulence generation from the large spatial domain of an RT instability. This framework, referred to as homogeneous buoyant turbulence (HBT), is a direct numerical simulation (DNS) in a 3D periodic box. To ensure the periodic boundary conditions are applicable, a transformation is performed on the RT flow variables in the Navier-Stokes (NS) equations. This leads to new equations similar in form to the NS equations with additional terms that act to maintain the turbulence. These terms are closed by leveraging data from prior RT DNS. HBT is distinct from the homogeneous isotropic turbulence (HIT) framework, where the turbulence is maintained by a linear forcing term in the momentum equation. The results are first validated against existing RT DNS. The statistics are then compared to the corresponding HIT results to identify the key factors that separate buoyant turbulent structure from isotropic, Kolmogorov turbulence. |
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