Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session L15: Free-Surface Flows: Waves & Turbulence |
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Chair: Filippo Coletti, ETH Zurich Room: 142 |
Monday, November 21, 2022 8:00AM - 8:13AM |
L15.00001: A comparison of optical techniques for free-surface measurements along wave- and turbulence-driven flows Vivek A Bheeroo, Tracy Mandel The free surface at an air-water interface has been shown to relay information about inner-flow characteristics, including the manifestation of turbulence through rising coherent structures. We explore existing optical techniques that enable free-surface reconstruction in a cost-effective and efficient manner. These techniques measure the apparent distortions of a reflected or refracted reference pattern that vary linearly with the gradient of the surface elevation. We compare the performance of three imaging techniques (Free-Surface Synthetic Schlieren, Total Internal Reflection-Deflectometry, Moon-Glade Background Oriented Schlieren) to characterize the perturbations along a dynamic free-surface. Surface reconstruction is performed using an optical flow algorithm that provides an improvement over cross-correlation techniques. This is in part due to the higher spatial resolution of the resulting displacement field coupled with a superior performance in computing larger displacements with higher straining/dilatation of the background reference pattern. Surface motions are induced in two ways: 1) turbulent free-shear flow emanating from a submerged jet that impinges the underside of the free-surface and 2) gravity-capillary waves that arise from flow across an overspill weir. These two flow conditions simulate surface dynamics in an open-channel flow where the bottom roughness triggers coherent structures that impact the free surface. The physics of each technique is assessed, and the surface reconstruction results are compared to one another. We will discuss the advantages and drawbacks of each technique in terms of accuracy and practicality. Finally, we use the surface elevation results to untangle the gravity-capillary wave mechanics in terms of their dispersive properties as well as to investigate the manifestation of turbulence within the energetic and random surface roughness that is generated from the submerged jet. |
Monday, November 21, 2022 8:13AM - 8:26AM |
L15.00002: Surface wave spectrum in boxed free-surface turbulence Eirini I Florou, Charles Fort, Philippe M Bardet In the present study free surface turbulence (FST) is explored in a canonical FST facility based on a non-intrusive optical diagnostic for reconstruction of liquid-gas interfaces in the 3D space and time with one single laser plane and camera. |
Monday, November 21, 2022 8:26AM - 8:39AM |
L15.00003: Turbulent diffusive transport of floating discs Filippo Coletti, Henri Sanness Salmon Understanding floating particle transport is crucial for the global response to plastic pollution. One key issue is the rate at which floating particles of different shapes and sizes spread over the surface of turbulent waters. Here we study experimentally the diffusive transport of floating discs in free-surface grid turbulence using particle tracking velocimetry. We consider discs larger than the dissipative flow scales and investigate their inertial filtering, i.e., the ability to filter out small-scale fluctuations. This behaviour, long identified in studies concerning particles in three-dimensional turbulence, may result in longer time-correlated motion as compared to fluid elements, and thus increased diffusivity. However, the applicability of classic turbulence theory to free-surface turbulence is debated. To substantiate this notion, we consider the Lagrangian motion of the discs and compare them to that of floating tracers of the free-surface flow. Specifically, we determine the discs' diffusion coefficient by integrating the velocity auto-correlation function following the classical framework of Taylor diffusion. We verify their spreading rate by calculating the mean-square displacement due to turbulent fluctuations along the particle trajectories. |
Monday, November 21, 2022 8:39AM - 8:52AM |
L15.00004: Reconstruction of three-dimensional turbulent flow structures using surface measurements for free-surface flows based on convolutional neural network Anqing Xuan, Lian Shen We present a machine learning model to reconstruct three-dimensional turbulent flow structures using surface measurements for free-surface flows. The proposed model, which is based on the convolutional neural network (CNN) and is trained using the data obtained from direct numerical simulations of turbulent open-channel flows, can predict the velocity fluctuation field near the free surface and large-scale flow structures away from the surface with high accuracy. The CNN model outperforms a traditional linear stochastic estimation (LSE) model considerably. Further analyses of the CNN model and LSE model provide insights into how the two models are related to the flow physics of the subsurface turbulence. |
Monday, November 21, 2022 8:52AM - 9:05AM |
L15.00005: Wave-resolved direct numerical simulation and statistical analysis of Langmuir turbulence Di Yang, Meng Li Langmuir turbulence generated by the interaction between progressive surface waves and wind shear-induced turbulence is simulated using a wave-resolved direct numerical simulation (DNS) approach. The incipient generation and time evolution of the Langmuir turbulence caused by wave-turbulence interaction are analyzed statistically. The DNS results are also compared with the numerical modeling results obtained using the classical wave-averaged Craik–Leibovich (CL) model. A detailed analysis of the turbulent kinetic energy (TKE) budget shows that although the characteristics of the Langmuir turbulence obtained by the two models are similar, the detailed mechanisms for TKE production through wave-turbulence interaction are different in the flow fields obtained from the wave-resolved DNS and the wave-averaged CL model. |
Monday, November 21, 2022 9:05AM - 9:18AM |
L15.00006: Validation of CFD Predictions of Wave Propagation and Forces on a Cylinder in a Long Flume Max Beeman, Hanul Hwang, Catherine Gorle During hurricanes, coastal residential structures are subjected to high winds in combination with storm surge and subsequent waves. Elevating a building above the Base Flood Elevation will reduce flood damage, but the effects of wind-wave interaction in the areas surrounding and beneath the structure on the pressure distributions seen during extreme weather events are not well understood. Our long-term goal is to enable computational fluid dynamics (CFD) predictions of combined wind-wave loading. |
Monday, November 21, 2022 9:18AM - 9:31AM |
L15.00007: Effects of turbulence in high-speed winds on wind-wave coupled simulations Hanul Hwang, Catherine Gorle The impacts of climate-driven hazards in coastal regions escalate due to sea level rise and anthropogenic warming, causing severe direct losses and fatalities. To enhance the resilience of communities to extreme events, the NICHE team aims to design a full-scale testing infrastructure to model the impacts of wind and coastal hazards. The design will rely on physical experiments in existing wind-wave facilities, as well as numerical simulations. |
Monday, November 21, 2022 9:31AM - 9:44AM |
L15.00008: Statistics of extreme ship motions in broad-band wave field Xianliang Gong, Yulin Pan We consider the evaluation of extreme ship motion probability in a broad-band irregular wave field. For this problem, the previous techniques developed for the parameterization of the narrow-band wave field, i.e., methods based on the wave group structures, cannot be directly applied. The group-based statistics that are widely used in existing works also become ambiguous. In this work, we develop a new framework to directly resolve the temporal exceeding probability, defined as the fraction of time that ship motions exceed a given threshold. The parameterization of the wave field will be based on structures with elevations exceeding some threshold that are well suited to broad-band waves. The effectiveness of the proposed approach will be demonstrated in multiple broad-band wave fields with ship motion simulated by nonlinear roll equations. |
Monday, November 21, 2022 9:44AM - 9:57AM |
L15.00009: On the linearity of wave energy temporal growth in the initial stage of Phillips theory Tianyi Li, Lian Shen In 1957, Phillips proposed a prominent theory to elucidate the role of the resonance mechanism in the initial growth of surface waves from a flat air-water interface. The resonance between turbulent airflow and water deformation occurs when the convection velocity of air pressure fluctuations at the water surface matches the phase velocity of water waves. Near the resonance curve in the wavenumber space, the wave energy spectrum component grows quadratically with time. The overall temporal evolution of surface elevation variance exhibits a linear feature. Here, we present numerical evidence on the resonance mechanism in the initial stage of the Phillips theory based on direct numerical simulation data. We also provide a rigorous proof of the linear growth of surface elevation variance in the initial stage. We show that for gravity waves the leading order term of surface elevation variance is bounded by a linear function of time from an asymptotic analysis. |
Monday, November 21, 2022 9:57AM - 10:10AM |
L15.00010: Modeling and predicting oscillating surge wave energy converter behavior using dynamic mode decomposition Brittany Lydon, Brian L Polagye, Steven L Brunton Modeling wave energy converters (WECs) to accurately predict their hydrodynamic behavior has been a notoriously difficult challenge for the wave energy field, particularly in polychromatic sea states. A key challenge is that accurate, physics-based WEC modeling is too computationally expensive to be used for future-state prediction and optimal control, two areas of active research in the wave energy field. We propose the use of dynamic mode decomposition (DMD) to provide a purely data-driven technique to overcome these issues. In this study, we use DMD to develop a linear model of a grid-scale Oscillating Surge WEC (OSWEC) operating in polychromatic seas. Our goal is to model and predict the behavior of the OSWEC for use in optimal control schemes, such as model predictive control (MPC), without requiring an infeasible computational cost or sacrificing accuracy. We show that by using DMD, we can accurately model and predict OSWEC behavior in response to polychromatic sea states with appropriate model rank and training time. These findings provide insight on the use of DMD on systems with limited time-resolved data and present a framework for applying similar analysis to lab-scale experiments, high-fidelity simulations, or data from OSWECs operating in the field. |
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