Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session M16: Free-Surface Flows: Waves & General II |
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Chair: Kenneth Kiger, University of Maryland Room: North 130 |
Monday, November 22, 2021 1:10PM - 1:23PM |
M16.00001: Analyzing free-surface deformations for flow over submerged bedforms Saksham Gakhar, Jeffrey R Koseff, Nicholas Ouellette Submerged features in a turbulent free-surface flow may leave an imprint on the water surface, but these surface expressions are difficult to distinguish from typical turbulent fluctuations. To characterize them, we implemented Polarimetric Slope Sensing (PSS), which allows us to measure spatiotemporal reconstructions of the free-surface deformations. We will present the results of laboratory experiments conducted in a 6m circulating water flume and analyzed using PSS for flow over a variety of canonical bedforms modeled after common submerged features in littoral and fluvial settings. |
Monday, November 22, 2021 1:23PM - 1:36PM |
M16.00002: Inferring subsurface flow structures from surface manifestation in free-surface flows using neural network Anqing Xuan, Lian Shen For free-surface flows, subsurface turbulent structures can create distinguished and complex manifestations on the free surface. We have built a neural network based framework to infer the subsurface turbulent motions from the observations of the surface elevation and velocity fluctuations. Through a series of convolutional layers, the neural network extracts features from the surface measurements and then reconstructs the three-dimensional subsurface velocity field. As an example, we consider a turbulent open channel flow. We train the neural network using the data obtained from direct numerical simulations, which use a free-surface boundary-fitted grid to resolve the surface motions accurately. It is found that the neural network can infer certain turbulent coherent structures, such as the inclined vortices arising from the bottom boundary layer, and reveal their correlations with the surface manifestations. |
Monday, November 22, 2021 1:36PM - 1:49PM |
M16.00003: Dynamics of the rapid creation and collapse of dry spot regions int fluid layers Jacob D Bruney, Roberto Camassa, Sarah Davis, Russell Engle
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Monday, November 22, 2021 1:49PM - 2:02PM |
M16.00004: Numerical investigation on the principal stage of wind-wave generation Tianyi Li, Lian Shen We use a combined numerical and theoretical approach to study the dynamics of wind-generated water waves in the principal stage of the Phillips theory (Phillips, J. Fluid Mech., 1957). Using a wave-surface-fitted grid, we perform direct numerical simulation of turbulent wind over initially calm water to capture the multistage generation of water waves. Detailed analyses are conducted to evaluate the Phillips theory in both physical space and wavenumber space. We obtain numerical evidence on the existence of the principal stage when the surface elevation variance grows linearly with time. The wavenumber spectrum of surface wave elevations is further analyzed using a time-dependent norm to elucidate the role of the resonance mechanism in wave generation. In the physical space, we propose an improved model to obtain a quantitative prediction of the growth rate of surface elevation variance in the principal stage, which agrees with the direct numerical simulation results better than the Phillips theory. |
Monday, November 22, 2021 2:02PM - 2:15PM |
M16.00005: Spatio-temporal evolution of the surface wind-wave spectrum due to nonlinear internal solitary waves Liangyi Yue, Oliver B Fringer, Lian Shen Internal solitary waves (ISW) in the ocean produce surface currents that modify surface waves in the capillary-gravity wave regime, leaving a distinct signature on the ocean surface of alternating rough and smooth banded patterns that are observable in synthetic aperture radar satellite imagery. We study the evolution of the surface wave spectrum due to ISWs using a coupled surface-internal wave numerical model. The surface waves are computed with a higher-order spectral method which solves for the velocity potential and free-surface elevation at the surface boundary. This model takes as its input the surface currents due to a propagating ISW computed with the Dubreil-Jacotin-Long (DJL) equation which gives the density and velocity field associated with fully nonlinear inviscid internal gravity waves. The surface waves are initialized with the JONSWAP spectrum, and we use the coupled model to simulate the surface wave spectrum in response to an ISW using parameters that are typical of those found in the South China Sea. The model is validated by comparison to results of the two-layer model of Hao and Shen (JFM 2020), and we present a detailed analysis of the spatio-temporal evolution of the surface wave spectrum due to the ISW. |
Monday, November 22, 2021 2:15PM - 2:28PM |
M16.00006: An improved adjoint-based ocean wave reconstruction and prediction method Jie Wu, Xuanting Hao, Lian Shen We propose a method for the reconstruction and prediction of nonlinear wave field from coarse-resolution measurement data. We adopt the data assimilation framework using the adjoint equation to search for the optimal initial wave field to match the given measurement data. Compared with the conventional approach where the surface elevation and velocity potential are independent, our method features an additional constraint to dynamically connect these two control variables based on the dispersion relation of waves. The performance of our new method and the conventional method is assessed with the synthetic nonlinear wave data generated from phase-resolved nonlinear wave simulations using the high-order spectral method. We consider a variety of wave steepness and noise levels for the nonlinear irregular wave fields. It is found that the conventional method tends to overestimate the surface elevation in the high-frequency region and underestimate the velocity potential. In comparison, our new method shows significantly improved performance in the reconstruction and prediction of instantaneous surface elevation, surface velocity potential, and high-order wave statistics including the skewness and kurtosis. |
Monday, November 22, 2021 2:28PM - 2:41PM |
M16.00007: Transport of fibres in surface waves Nimish Pujara, Gabrielle A Every Fibres from the breakdown of clothing, fishing lines, and other debris are dispersed in lakes and oceans by surface waves. We examine fibre transport in waves via models that take into account inertial effects in successive approximations of slender body. We use these models to explore how fibre properties (i.e., length, diameter, desnity) and orientation influence fibre transport under different wave conditions. We find solutions to the fibre transport in terms of drift velocities that are analogues of the classical Stokes drift. We find that inertialess slender body theory predicts that neutrally buoyant fibres drift at the same rate as the Stokes drift irrespective of fibre properties, since wave-induced flow is dominated by a single length scale. However, this picture is altered as both fibre and fluid inertia become important. Our results show that, counter intuitively, fibre transport is enhanced by inertial effects. |
Monday, November 22, 2021 2:41PM - 2:54PM |
M16.00008: Experimental study on the effects of microplastics and surfactants on ocean surface roughness Yukun Sun, Christopher Ruf, Thomas Bakker, Yulin Pan High correlations between ocean surface roughness and the presence of floating microplastics are recently detected by the NASA CYGNSS satellites. In order to understand the mechanisms underlying the correlation, we conduct wave tank experiments to study the effects of surface particles and surfactants on surface roughness (as two hypothetical mechanisms). In a 35-meter wave tank, we compare the surface roughness (measured by mean square slope, MSS) of irregular waves generated by a mechanical wave maker with and without the presence of particles/surfactants. For particles, we find that MSS is increased compared to the clean-water situation with low concentration of particles (probably due to the diffracted waves). With the increasing concentration, the MSS gradually decreases and is eventually subject to a damping effect (relative to clean-water MSS). For surfactants, we observe significant damping of MSS, which is explained through the Marangoni damping effect. |
Monday, November 22, 2021 2:54PM - 3:07PM |
M16.00009: Spatial and temporal scales of free-surface turbulence Filippo Coletti, Roumaissa Hassaini, Yaxing Li, Kelken Chang, Baptiste Gousset, Claudio Mucignat We study the motion of tracer particles floating on the free surface of turbulent water. Experiments are carried out in a large open channel flow facility, over the fully developed region behind a square mesh grid. Millions of floating tracer trajectories are reconstructed by particle tracking velocimetry, allowing to explore single-point and two-point statistics in both the Eulerian and the Lagrangian frames. We focus on a regime in which turbulence is not strong enough to significantly deform the free surface against gravity and surface tension. Despite the tracer being confined to a quasi-flat material surface, their motion is consistent with the canons of Kolmogorov’s classic theory of homogeneous isotropic turbulence. This is revealed by their velocity fluctuations, accelerations, and velocity structure functions, and by Lagrangian dispersion displaying the expected ballistic-to-diffusive transition. The key parameters of the free-surface flow, including the turbulence dissipation rate and integral length scale, closely match those of the sub-surface turbulence as measured by particle image velocimetry. The floating tracers cluster due to the compressibility of the free surface, and do so over spatial and temporal scales even larger than the integral scales of the turbulence. |
Monday, November 22, 2021 3:07PM - 3:20PM |
M16.00010: Validated theoretical model of wind-wave evolution Meital Geva, Lev Shemer The current model temporally and spatially describes the entire wind-wave evolution, from initial ripples to steady state. This model has no adjustable parameters and it was validated against extensive experimental measurements performed in our lab. Two distinct theoretical approaches currently exist: The Miles’ approach that is linear, deterministic and is based on shear-flow instability. The Phillips’ approach which is nonlinear and stochastic, wave excitation is attributed to pressure fluctuations in the airflow. There is no model so far that attempts to describe the entire evolution process. Experiments in our lab on excitation of waves by impulsively applied wind show that the process consists of four distinct stages; the wave field is stochastic with multiple length scales. Our quasi-linear theory is based on the solution of the coupled Orr-Sommerfeld equations; each harmonic grows exponentially. Unlike other studies, we account for co-existence of all unstable harmonics resulting in the expected surface elevation. Breaking imposes limit on the maximum wave steepness; the fetch limits the maximum growth duration. |
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