Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session L20: Free Surface Flows: Turbulence and MixingFree Surface Turbulence
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Chair: Tracy Mandel, Stanford University Room: 704 |
Monday, November 20, 2017 4:05PM - 4:18PM |
L20.00001: Wave turbulence in a two-layer fluid: Coupling between free surface and interface waves Eric Falcon, Bruno Issenmann, Claude Laroche We experimentally study gravity-capillary wave turbulence on the interface between two immiscible fluids of close density with free upper surface. We locally measure the wave height at the interface between both fluids by means of a highly sensitive laser Doppler vibrometer. We show that the inertial range of the capillary wave turbulence regime is significantly extended when the upper fluid depth is increased: The crossover frequency between the gravity and capillary wave turbulence regimes is found to decrease whereas the dissipative cut-off frequency of the spectrum is found to increase. We explain these observations by the progressive decoupling between waves propagating at the interface and the ones at the free surface, using the full dispersion relation of gravity-capillary waves in a two-layer fluid of finite depths. The cut-off evolution is due to the disappearance of parasitic capillaries responsible for the main wave dissipation for a single fluid. [Preview Abstract] |
Monday, November 20, 2017 4:18PM - 4:31PM |
L20.00002: Transition from weak wave turbulence regime to solitonic regime Roumaissa Hassani, Nicolas Mordant The Weak Turbulence Theory (WTT) is a statistical theory describing the interaction of a large ensemble of random waves characterized by very different length scales. For both weak non-linearity and weak dispersion a different regime is predicted where solitons propagate while keeping their shape unchanged. The question under investigation here is which regime between weak turbulence or soliton gas does the system choose ? We report an experimental investigation of wave turbulence at the surface of finite depth water in the gravity-capillary range. We tune the wave dispersion and the level of nonlinearity by modifying the depth of water and the forcing respectively. We use space-time resolved profilometry to reconstruct the deformed surface of water. When decreasing the water depth, we observe a drastic transition between weak turbulence at the weakest forcing and a solitonic regime at stronger forcing. We characterize the transition between both states by studying their Fourier Spectra. We also study the efficiency of energy transfer in the weak turbulence regime. We report a loss of efficiency of angular transfer as the dispersion of the wave is reduced until the system bifurcates into the solitonic regime. [Preview Abstract] |
Monday, November 20, 2017 4:31PM - 4:44PM |
L20.00003: Nonlinear waves from a localized vortex source in strongly correlated fluids Akanksha Gupta, Rajaraman Ganesh, Ashwin Joy Highly charged quasi two-dimensional grain medium (complex plasma) is a remarkable test-bed to study wave like phenomena [1,2]. Understanding of such wave propagation has many important applications in geophysics, petroleum engineering, and mining, earthquakes, and seismology [3]. In the present study, for the first time, the propagation of nonlinear wave which originates from localized coherent vortex source has been studied using molecular dynamics simulation taking Yukawa liquids as a prototype for strongly correlated fluid. In this work, the coupling of transverse and longitudinal mode, effect of azimuthal speed of vortex source on the linear and nonlinear properties of generated wave will be presented as a function of strong correlation[4]. \\ \\$[1]$Gregor E. Morfill and Alexei V. Ivlev. Rev. Mod. Phys., 81, 1353, (2009). \newline[2] A. Piel et. al Phys. Rev. Lett., 89, 085004, (2002). \newline [3]Jos\'{e} M Carcione et .al, Geophysical Journal International, 95(3), 597--611 (1988). \newline[4]Akanksha Gupta, PhD Thesis Submitted (June 2017); Manuscript Under Preparation (2017) [Preview Abstract] |
Monday, November 20, 2017 4:44PM - 4:57PM |
L20.00004: Identification of nonlinear coupling in wave turbulence at the surface of water Antoine Campagne, Roumaissa Hassaini, Ivan Redor, Quentin Aubourg, Joël Sommeria, Nicolas Mordant The Weak Turbulence Theory is a theory, in the limit of vanishing nonlinearity, that derive analytically statistical features of wave turbulence. The stationary spectrum for the surface elevation in the case of gravity waves, is predicted to $E(k)\propto k^{-5/2}$. This spectral exponent -5/2 remains elusive in all experiments. in which the measured exponent is systematically lower than the prediction. Furthermore in the experiments the weaker the nonlinearity the further the spectral exponent is from the prediction. In order to investigate the reason for this observation we developed an experiment in the CORIOLIS facility in Grenoble. It is a 13m-diameter circular pool filled with water with a 70 cm depth. We generate wave turbulence by using two wedge wavemakers. Surface elevation measurements are performed by a stereoscopic optical technique and by capacitive probes. The nonlinear coupling at work in this system are analyzed by computing 3- and 4-wave correlations of the Fourier wave amplitudes in frequency. Theory predicts that coupling should occur through 4-wave resonant interaction. In our data, strong 3-wave correlations are observed in addition to the 4-wave correlation. Most our observations are consistent with field observation in the Black Sea (Leckler et al 2015). [Preview Abstract] |
Monday, November 20, 2017 4:57PM - 5:10PM |
L20.00005: Identification of weak-turbulent wave-number region in stratified turbulence Naoto Yokoyama, Masanori Takaoka The anisotropic gravity-wave turbulence at small wave numbers and the isotropic Kolmogorov turbulence at large wave numbers coexist in stratified turbulence. The Ozmidov wave number, at which the linear time scale and the nonlinear time scale are comparable, has been considered as the wave number that separates the weak and strong turbulence. Here, the linear time scale and the nonlinear time scale are respectively the inverse of the Brunt-V\"ais\"al\"a frequency and the eddy-turnover time. However, the Ozmidov wave number does not reflect the effect of anisotropy in the weak turbulence. It was numerically found that the wave kinetic energy is dominant in the total kinetic energy over the vortical energy, the wave kinetic energy is close to the potential energy, and the polarization anisotropy part is dominant in the vertical kinetic energy at the wave numbers where the ratio of the linear time scale to the nonlinear time scale is smaller than $1/3$. Namely, the anisotropic gravity-wave turbulence is dominant at the wave numbers where the ratio is smaller than $1/3$. Such wave-turbulence region is encompassed by the Ozmidov wave number. These facts are consistent with the critical balance. [Preview Abstract] |
Monday, November 20, 2017 5:10PM - 5:23PM |
L20.00006: Glimpses of Kolmogorov’s spectral energy dynamics in nonlinear acoustic waves Prateek Gupta, Carlo Scalo Gupta, Lodato, and Scalo (AIAA 2017) have demonstrated the existence of an equilibrium spectral energy cascade in shock waves formed as a result of continued modal thermoacoustic amplification consistent with Kolmogorov’s theory for high-Reynolds-number hydrodynamic turbulence. In this talk we discuss the derivation of a perturbation energy density norm that guarantees energy conservation during the nonlinear wave steepening process, analogous to inertial subrange turbulent energy cascade dynamics. The energy cascade is investigated via a bi-spectral analysis limited to wave-numbers and frequencies lower than the ones associated with the shock, analogous to the viscous dissipation length scale in turbulence. The proposed norm is derived by recombining second-order nonlinear acoustic equations and is positive definite; moreover, it decays to zero in the presence of viscous dissipation and is hence classifiable as a Lyapunov function of acoustic perturbation variables. The cumulative energy spectrum wavenumber distribution demonstrates a -3/2 decay law in the inertial range. The governing equation for the thus-derived energy norm highlights terms responsible for energy cascade towards higher harmonics, analogous to vortex stretching terms in hydrodynamic turbulence. [Preview Abstract] |
Monday, November 20, 2017 5:23PM - 5:36PM |
L20.00007: Numerical Investigation of Surface Wave Effect on Turbulence Underneath Anqing Xuan, Bingqing Deng, Lian Shen We perform numerical simulations to study the fundamental mechanism of the distortion of turbulence by surface waves. In our model setup, statistically steady, isotropic turbulence is generated by random forcing in the bulk flow under a monochromatic progressive wave. The simulations are performed on a wave-surface-fitted grid with fully nonlinear free-surface kinematic and dynamic boundary conditions, so that we are able to resolve wave motions directly to include both the instantaneous distortion effects and the phase-averaged accumulative effects of the wave. The simulations capture the distortion of turbulence and reveal detailed information of the instantaneous turbulence field. We analyze the turbulence statistics and observe Reynolds shear stress generated under the wave. Reynolds stress is found to be dependent on wave phase. Further analysis shows that both the wave strain rate and free surface kinematics contribute to the distortion of turbulence underneath. [Preview Abstract] |
Monday, November 20, 2017 5:36PM - 5:49PM |
L20.00008: Multiphase three-dimensional direct numerical simulation of a rotating impeller with code Blue Lyes Kahouadji, Seungwon Shin, Jalel Chergui, Damir Juric, Richard V. Craster, Omar K. Matar The flow driven by a rotating impeller inside an open fixed cylindrical cavity is simulated using code {\it Blue}, a solver for massively-parallel simulations of fully three-dimensional multiphase flows. The impeller is composed of four blades at a 45$^{\circ}$ inclination all attached to a central hub and tube stem. In {\it Blue}, solid forms are constructed through the definition of immersed objects via a distance function that accounts for the object’s interaction with the flow for both single and two-phase flows. We use a moving frame technique for imposing translation and/or rotation. The variation of the Reynolds number, the clearance, and the tank aspect ratio are considered, and we highlight the importance of the confinement ratio (blade radius versus the tank radius) in the mixing process. {\it Blue} uses a domain decomposition strategy for parallelization with MPI. The fluid interface solver is based on a parallel implementation of a hybrid front-tracking/level-set method designed complex interfacial topological changes. Parallel GMRES and multigrid iterative solvers are applied to the linear systems arising from the implicit solution for the fluid velocities and pressure in the presence of strong density and viscosity discontinuities across fluid phases. [Preview Abstract] |
Monday, November 20, 2017 5:49PM - 6:02PM |
L20.00009: The evolution of the surface signature of a canopy-generated shear instability using free-surface synthetic Schlieren Tracy Mandel, Hayoon Chung, Jeffrey Koseff We present results from a laboratory imaging technique, free-surface synthetic Schlieren, to remotely measure surface turbulence based on the apparent distortion of submerged roughness features. The shear instability generated by a model vegetative canopy yields a clear signal in the surface slope field. We measure the propagation speed, frequency, and length scale of the Kelvin-Helmholtz vortices at the surface and connect these to properties of the subsurface flow and canopy geometry. We also observe that in more energetic flows, the vortices break up into strong surface-impacting boils before they reach the end of the canopy. These dynamics are related to changes in the Reynolds stress within the water column and show a transition from periodic coherent structures to chaotic turbulent motion, yielding important insights into mass and momentum transfer in vegetative canopies. More broadly, these results suggest that the surface signature generated by bottom roughness can be used to characterize the structure of the bed and the flow, in hopes that future field and laboratory studies can reduce the number and scope of measurements required to study the dynamics of interest. [Preview Abstract] |
Monday, November 20, 2017 6:02PM - 6:15PM |
L20.00010: Air Entrainment and Surface Ripples in a Turbulent Ship Hull Boundary Layer Naeem Masnadi, Martin Erinin, James H. Duncan The air entrainment and free-surface fluctuations caused by the interaction of a free surface and the turbulent boundary layer of a vertical surface-piercing plate is studied experimentally. In this experiment, a meter-wide stainless steel belt travels horizontally in a loop around two rollers with vertically oriented axes. This belt device is mounted inside a large water tank with the water level set just below the top edge of the belt. The belt, rollers, and supporting frame are contained within a sheet metal box to keep the device dry except for one 6-meter-long straight test section. The belt is accelerated suddenly from rest until reaching constant speed in order to create a temporally evolving boundary layer analogous to the spatially evolving boundary layer that would exist along a surface-piercing towed flat plate. Surface ripples are measured using a cinematic laser-induced fluorescence technique with the laser sheet oriented parallel or normal to the belt surface. Air entrainment events and bubble motions are recorded from underneath the water surface using a stereo imaging system. Measurements of small bubbles, that tend to stay submerged for a longer time, are planned via a high-speed digital in-line holographic system. [Preview Abstract] |
(Author Not Attending)
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L20.00011: Boundary layers at a dynamic interface: air-sea exchange of heat and mass Andrew Szeri Exchange of mass or heat across a turbulent liquid-gas interface is a problem of critical interest, especially in air-sea transfer of natural and man-made gases involved in climate change. The goal in this research area is to determine the gas flux from air to sea or vice versa. For sparingly soluble non-reactive gases, this is controlled by liquid phase turbulent velocity fluctuations that act on the thin species concentration boundary layer on the liquid side of the interface. If the fluctuations in surface-normal velocity and gas concentration differences are known, then it is possible to determine the turbulent contribution to the gas flux. However, there is no suitable fundamental direct approach in the general case where neither of these quantities can be easily measured. A new approach is presented to deduce key aspects about the near-surface turbulent motions from remote measurements, which allows one to determine the gas transfer velocity, or gas flux per unit area if overall concentration differences are known. The approach is illustrated with conceptual examples. [Preview Abstract] |
Monday, November 20, 2017 6:28PM - 6:41PM |
L20.00012: Motion of a rigid sphere through an elastic tube with a lubrication film Marie Tani, Thomas Cambau, Jose Bico, Etienne Reyssat The motion of large objects through narrow tubes is a common problem in physiology and more generally in the biological world. We built a model experiment where a rigid sphere is moved inside a narrower elastic tube coated with a lubricating fluid. The friction force is generally lower than in a non-lubricated situation. Interestingly, the force increases with the pulling velocity to the power 1/3, and also depends on the viscosity of the lubricant, the geometry and the mechanical properties of the tube. All our experimental data are well described by a scaling law combining lubrication and elasticity equations. We furthermore measured the thickness of the lubricant film. We present all these results. [Preview Abstract] |
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