Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T23: Free Surface Flows: Turbulence |
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Chair: Filippo Coletti, ETH Zurich Room: 149AB |
Monday, November 20, 2023 4:25PM - 4:38PM |
T23.00001: Surface-attached vortex imprints as a proxy for surface divergence Omer Babiker, Ivar Bjerkebæk, Anqing Xuan, Lian Shen, Simen Å Ellingsen Mass transfer between ocean and atmosphere is primarily governed by the turbulence in the topmost centimetres beneath the free surface. It has been frequently observed that areas of strong positive divergence of the surface-tangential velocity field correspond to efficient surface renewal and consequently increased transfer of mass. Patches of strong positive surface divergence occur in the form of intermittent upwelling events visible as ``boils'' on the surface. It has been qualitatively observed that surface-attached ``bathtub'' vortices tend to appear at the edges of upwelling boils, hence a correlation between the density of these long-lived vortices and average surface divergence might be expected. Surface-attached vortices leave imprints on the surface which are particularly simple to detect: they live for a long time, and their imprints are in the form of nearly circular dimples. |
Monday, November 20, 2023 4:38PM - 4:51PM |
T23.00002: Free liquid-air interface reconstruction in 3D space using polarimetry slope sensing Corentin Le Houedec, Philippe Bardet, Eirini I Florou The estimation of liquid-gas interfaces 3D shape is necessary for a variety of industrial and scientific applications. Here, an affordable, accurate, nonintrusive and easy to implement diagnostics is presented for profilometry in 3D. It relies on polarimetry slope sensing coupled with planar laser induced fluorescence. The measurement of the slope is performed via a polarized camera, and we consider a specular reflection model. A method to resolve the reflected angle ambiguity is also introduced. The elevation is obtained by integrating the slope measurement. To obtain accurate and absolute elevation data, one needs at least one boundary condition, which PLIF provides. In addition, the topography reconstruction algorithm is presented in detail, and single water drop impact events are measured to estimate its performance. Finally, free surface turbulence is studied at high-Reynolds number. |
Monday, November 20, 2023 4:51PM - 5:04PM |
T23.00003: A computational model to study the interaction of turbulence with a free-surface and its impact on air-entrainment at high Reynolds numbers Andre Calado, Elias Balaras Direct numerical simulations (DNS) of forced homogeneous turbulence interacting with a free-surface can be used as a building-block configuration to reproduce most mechanisms responsible for air-entrainement and bubble generation in ship wakes. Today, however, most studies are limited to very low Reynolds numbers (i.e. Reynolds number based on the Taylor microscale of order 100), due to cost considerations. The latter comes primarily from the cost of solving the pressure Poisson equation, which has variable coefficients and cannot be solved with fast poisson solvers (FPS) based on FFTs as in the single-phase flow counterpart. In this work we propose a fractional step formulation for this class of problems which utilizes FPS. As a result the cost is significantly reduced and higher Reynolds numbers can be considered. One important aspect of the approach is the use of a conservative level-set method, which provides an efficient and accurate representation of the free-surface. The level-set technique enables the tracking of the interface between the gas and liquid phases, allowing for the study of entrainment and mixing phenomena. The solver has been validated for different 2D and 3D test cases, and the computational approach demonstrates its capability to capture the complex dynamics of free-surface turbulent flows and gas-liquid entrainment. Ensemble and spatial averages for statistical quantities such as Reynolds stresses, vorticity and volume fraction are computed and presented. Entrained bubble size distributions are compared against literature correlations. |
Monday, November 20, 2023 5:04PM - 5:17PM |
T23.00004: Relative Dispersion in Free Surface Turbulence Filippo Coletti, Yaxing Li, Yifan Wang Transport along the free surface of turbulent flows is crucial in nature and industrial settings, but our understanding of this process is complicated by the fact that the surface velocity field is compressible. Here we report on an experimental study in which Lagrangian tracking is applied to millions of microscopic particles floating on the free surface of turbulent water. The experiments are carried out in a 2 cubic meter tank with randomly actuated jet arrays, leaving the free surface marginally deformed by the turbulence. We find that the surface divergence is prevalently positive at small (near-dissipative) scales, indicating fast separation rates of nearby particles. Based on this observation, we present a generalization of pair dispersion by classifying particle pairs based on their initial separation rate. When this does not exceed the relative velocity prescribed by Kolmogorov theory, the expected scaling of the Eulerian velocity structure functions is retrieved. Moreover, the separation of such particles in the inertial sub-range is compared with Richardson's theory. These findings highlight the degree of applicability of classic results derived for incompressible turbulence to free-surface transport, while revealing specific features of this type of flows. |
Monday, November 20, 2023 5:17PM - 5:30PM |
T23.00005: Turbulence statistics in open-box free surface turbulence Eirini I Florou, Philippe Bardet Free surface dynamics are of critical importance in many engineering applications and fundamental physical mechanisms. Surface deformation and gas entrainment can be triggered by turbulence (e.g., breaking waves or submersed jets). However, free surface topography and turbulence characteristics underneath are coupled through a balance mechanism between the disrupting turbulent kinetic energy and the stabilizing surface tension and gravity forces. Despite the great progress in Computational Fluid Dynamics, the effect of this balancing mechanism on the turbulence characteristics is not fully understood yet, mainly due to the lack of experimental evidence for the processes at play. This work introduces a new free surface turbulence facility to generate homogeneous and isotropic turbulence and investigate fundamental energy redistribution and air entrainment in high Reynolds number regimes. Facility design relies on an open-box symmetric configuration inspired by the forced equivalent turbulence boxes. Flow characteristics are investigated in a large region in the vicinity of the free surface by means of particle image velocimetry. Results reveal three main flow zones: a jet merging region, a homogeneous and isotropic core and a free surface influenced zone. We will discuss facility design principals and focus on turbulence statistics in both the free surface influenced, and homogeneous and isotropic regions. |
Monday, November 20, 2023 5:30PM - 5:43PM |
T23.00006: Connecting surface thermal features with subsurface buoyant plume dynamics Zeeshan Saeed, Tracy L Mandel Surface thermal signatures of a buoyancy-induced flow can be instrumental in investigating the evolution of subsurface flow structures, potentially allowing for remote sensing to replace in situ measurements. To study this, experiments were conducted using a vertically oriented plume in a tank (measuring 0.63m to a side) with a vertically mounted infrared camera imaging the free-surface. To this end, correlation length scale analysis at the free surface were conducted for a range of flow conditions to see how it varies with the Reynolds number defined by the source outlet variables. Furthermore, we analyze these trends by computing the power spectral density of the temperature fluctuations at the free surface. During this analysis, we also compare our results with the available literature, including similar experiments for a narrower range of Reynolds number by Judd et al. (2016, J. Visualization). To elucidate the role that the turbulent state of flow plays in the downstream evolution of the flow structures, and therefore the surface features, experiments are also conducted using turbulence-inhibiting polymers. Results are then analyzed and compared by including the Weissenberg number to the parameter space to draw upon the evolution of these structures, and thus the mechanics of the entrainment process. The need for appropriately defining the parameters quantifying the flow is also stressed in terms of the scale-separation concept underlying turbulent flows. |
Monday, November 20, 2023 5:43PM - 5:56PM |
T23.00007: Data-driven technique for decomposing the relative effects of waves and turbulence Julio E Chavez-Dorado, Isabel Scherl, Michelle H DiBenedetto The flow near the ocean surface modulates key climate processes ranging from mixed-layer dynamics to air-sea fluxes of heat, gas, and momentum. These quantities are commonly parameterized by the Reynolds stress or the friction velocity and can be inferred from point velocity measurements. However, these observations are frequently contaminated by surface waves which precludes bandpass filtering from accurately isolating the turbulence- and wave-induced velocity components. Decomposing wave and turbulence signals from ocean flow data is an ongoing challenge. Most previously developed methods are limited to ideal wave conditions and assume that waves and turbulence do not interact. We demonstrate a data-driven approach using dynamic mode decomposition (DMD) to recover and filter out dominant wave dynamics from turbulent flow data without any prior system knowledge or assumptions about the wave-turbulence interaction. We consider flow scenarios with different ratios of wave and shear strain to quantify the wave motion influence on turbulence. DMD shows promising results when applied to single-point measurements from field and laboratory data. |
Monday, November 20, 2023 5:56PM - 6:09PM |
T23.00008: A practical methodology to estimate bed shear stress in smooth wall-bounded turbulent flows using DNS data Harshit Mishra, Subhas Karan Venayagamoorthy The measurement of bed shear stress is incredibly challenging in the field as well as in the laboratory. Here, a practical method is presented to estimate the bed shear stress by appealing to the inner-outer layer coupling in conjunction with data obtained from high-Reynolds number Direct Numerical Simulations (DNS) of wall-bounded turbulent flows. The inner-outer layer coupling suggests that there exists a unique relationship between inner and outer length and time scales. The data from the DNS studies have been used to derive a relationship between bed shear stress and outer layer velocity scale. This relationship is then tested using experimental data of open channel flow in a laboratory setting and excellent agreement has been found. The new methodology simplifies the estimation of bed shear stress as the need of near boundary measurements has been obviated. This study also demonstrates the utility of DNS of turbulent flows for practical applications. |
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