Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A13: Free Surface Flows I: Wakes and Turbulence |
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Chair: Laura Pauley, Penn State University Room: 201 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A13.00001: A Reduced Order Model for Wake Surface-Wave Interaction Laura Pauley, Amir Mehdizadeh Surface waves can change the radar and optical signatures initially produced by the wake of a towed or self-propelled object on the sea surface. To date, most investigations considered the effects of the wake on the surface waves. Here our intention is to study how surface waves affect the wake. The wake of a moving object can persist thousands of diameters downstream. Due to the extensive domain, a reduced order method (2D+t) is often used to sweep downstream through the wake development. The 2D+t computation approximates the development of the wake at a fixed location as an object moves past but applies cyclical boundary conditions in the streamwise direction. A modified (Parabolized) Navier-Stokes (PNS) method has the same numerical efficiency as the classical 2D+t method but includes additional streamwise gradient terms derived in the transformation from a moving reference frame to a fixed reference frame. The present paper aims to assess the capability of the 2D+t and PNS methods for a laminar/turbulent wake interacting with a surface wave described by the Stokes drift velocity distribution. Results from 3D simulations will be used for validation to find criteria where 2D+t and PNS methods deliver results with an acceptable level of accuracy. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A13.00002: Modeling variable density turbulence in the wake of an air-entraining transom stern Kelli Hendrickson, Dick Yue This work presents \emph{a priori} testing of closure models for the incompressible highly-variable density turbulent (IHVDT) flows in the near wake region of a transom stern. This three-dimensional flow is comprised of convergent corner waves that originate from the body and collide on the ship center plane forming the ``rooster tail" that then widens to form the divergent wave train. These violent free-surface flows and breaking waves are characterized by significant turbulent mass flux (TMF) at Atwood number $At=(\rho_{2} - \rho_{1})/(\rho_{2} + \rho_{1})\approx 1$ for which there is little guidance in turbulence closure modeling for the momentum and scalar transport along the wake. To whit, this work utilizes high-resolution simulations of the near wake of a canonical three-dimensional transom stern using conservative Volume-of-Fluid (cVOF), implicit Large Eddy Simulation (iLES), and Boundary Data Immersion Method (BDIM) to capture the turbulence and large scale air entrainment. Analysis of the simulation results across and along the wake for the TMF budget and turbulent anisotropy provide the physical basis of the development of multiphase turbulence closure models. Performance of isotropic and anisotropic turbulent mass flux closure models will be presented. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A13.00003: A Study of Water Wave Wakes of Washington State Ferries Bradley Perfect, James Riley, Jim Thomson, Endicott Fay Washington State Ferries (WSF) operates a ferry route that travels through a 600m-wide channel called Rich Passage. Concerns of shoreline erosion in Rich Passage have prompted this study of the generation and propagation of surface wave wakes caused by WSF vessels. The problem was addressed in three ways: analytically, using an extension of the Kelvin wake model by Darmon et al.\ ({\em J.\ Fluid Mech.}, {\bf 738}, 2014); computationally, employing a RANS Navier-Stokes model in the CFD code OpenFOAM which uses the Volume of Fluid method to treat the free surface; and with field data taken in Sept-Nov, 2014, using a suite of surface wave measuring buoys. This study represents one of the first times that model predictions of ferry boat-generated wakes can be tested against measurements in open waters. The results of the models and the field data are evaluated using direct comparison of predicted and measured surface wave height as well as other metrics. Furthermore, the model predictions and field measurements suggest differences in wake amplitudes for different class vessels. Finally, the relative strengths and weaknesses of each prediction method as well as of the field measurements will be discussed. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A13.00004: Footprints of turbulence over a viscous liquid Marc Rabaud, Anna Paquier, Frederic Moisy We observe the dynamics of tiny deformations at the surface of a viscous liquid sheared by a turbulent airflow using Free-Surface Synthetic Schlieren, which allows for time-resolved measurements of the topography with a micrometric accuracy. We are interested here in the low-velocity regime, before the onset of quasi-monochromatic wind waves. In this regime,~the observed small and disorganized surface deformations directly result from the applied turbulent pressure field filtered by viscous and capillary effects. The amplitude of the footprints is found to increase linearly with air velocity, and the spatio-temporal dynamics is compatible with the known dynamics of the streaks of the turbulent boundary layer over a flat rigid wall. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A13.00005: The Turbulent Boundary Layer Near the Air-Water Interface on a Surface-Piercing Flat Plate Nathan Washuta, Naeem Masnadi, James H. Duncan Turbulent fluctuations in the vicinity of the water free surface along a flat, vertically oriented surface-piercing plate are studied experimentally using a laboratory-scale experiment. In this experiment, a meter-wide stainless steel belt travels horizontally in a loop around two rollers with vertically oriented axes, which are separated by 7.5 meters. 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 between rollers. The belt is launched from rest with a 3-$g$ acceleration in order to quickly reach steady state velocity. This creates a temporally evolving boundary layer analogous to the spatially evolving boundary layer created along a flat-sided ship moving at the same velocity, with a length equivalent to the length of belt that has passed the measurement region since the belt motion began. Cinematic Stereo PIV measurements are performed in planes parallel to the free surface by imaging the flow from underneath the tank in order to study the modification of the boundary layer flow field due to the effects of the water free surface. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A13.00006: DNS of air entrainment at the air-water interface of a temporally developing turbulent boundary layer Farshad Nasiri, Elias Balaras The entrainment of air at the free-surface of a turbulent boundary layer remains a poorly understood problem. This flow, typically found at the side of surface vessels, is characterized by highly turbulent bubbly region close to the surface and two-phase mixing. In this study, we will consider a turbulent boundary layer developing over an infinitely long moving plate. Our primary objective is to test the hypothesis that there is a critical combination of Fr and We numbers -based on the local momentum thickness- that plays a critical role to the onset of entrainment. In particular, we will report two-phase, DNS of a temporally developing turbulent boundary layer with waterside Reynolds number ranging from $Re_\theta=900$ to $1200$. The computational domain is large enough to accommodate the range of eddies found in such flow. We utilize a conservative solver, where the air-water interface is sharply defined using a level-set formulation. Turbulent statistics away from the surface are presented. Rate of entrainment and the range of scales of entrained droplets are considered. Conditional averages of the flow field (i.e. vorticity, curvature, etc.) are reported to identify the mechanisms that trigger air-entrainment. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A13.00007: Micro-swimmer dynamics in free-surface turbulence subject to wind stress Cristian Marchioli, Salvatore Lovecchio, Alfredo Soldati We examine the effect of wind-induced shear on the orientation and distribution of motile micro-swimmers in free-surface turbulence. Winds blowing above the air-water interface can influence the distribution and productivity of motile organisms via the shear generated just below the surface. Swimmer dynamics depend not only by the advection of the fluid but also by external stimuli like nutrient concentration, light, gravity. Here we focus on gyrotaxis, resulting from the gravitational torque generated by an asymmetric mass distribution within the organism. The combination of such torque with the viscous torque due to shear can reorient swimmers, reducing their vertical migration and causing entrapment in horizontal fluid layers. Through DNS-based Euler-Lagrangian simulations we investigate the effect of wind-induced shear on the motion of gyrotactic swimmers in turbulent open channel flow. We consider different wind directions and swimmers with different reorientation time (reflecting the ability to react to turbulent fluctuations). We show that only stable (high-gyrotaxis) swimmers may reach the surface and form densely concentrated filaments, the topology of which depends on the wind direction. Otherwise swimmers exhibit weaker vertical fluxes and segregation at the surface. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A13.00008: Understanding capillary wave turbulence using discrete quasi-resonant kinetic equation Yulin Pan, Dick Yue Weak turbulence power-law spectrum can be physically understood from the the kinetic equation (KE), which governs the evolution of wave spectrum due to nonlinear resonant interactions. For capillary waves, KE yields a stationary solution of a power-law spectrum, with energy flux from large to small scales due to triad resonant interactions. The condition of triad resonance, however, may not be satisfied if wavenumber can only take discrete values. This happens physically when the wavefield is finite, or numerically when discrete waveumber grid is used. Under this situation, energy flux is governed by quasi-resonant interactions; KE is not directly applicable and the underlying physics is not fully understood. We conduct a numerical study of KE on a discrete grid, where the frequency mismatch $\Delta \omega$ of a triad is restrained from being nonzero. The energy transfer within such triads is accounted for by a generalized delta function $\delta_g(\Delta \omega)$, which obtains its maximum at $\Delta \omega=0$ and rapidly decreases as $\Delta \omega$ increases. The width $\epsilon$ of $\delta_g(\Delta \omega)$ thus characterizes the nonlinear broadening. The simulation results elucidate the physics for different levels of nonlinear broadening relative to a given discrete grid. [Preview Abstract] |
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