Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D28: Free-Surface Flows II |
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Chair: Javier Rodriguez-Rodriguez, Carlos III University of Madrid Room: 32A |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D28.00001: Numerical simulations of transient air entrainment by rough and smooth plunging jets Ken Kiger, Nabil Kharoua, Lyes Khezzar Plunging jets are intimately linked to the process of air or gas entrainment into liquid pools, and can play either a beneficial or detrimental role in many environmental and industrial flows. The purpose of the present work is to assess the capability of combined LES/VOF algorithms to simulate water/air plunging jet flows, starting with the transient impact of the free jet, initial cavity formation, pinch off, and evolution towards a continuous entrainment phase. We focus on what happens in the transient impact phase for weakly and highly disturbed jets, operating with impact conditions of $Re =UD/\nu=10,500$, $We = \rho U^2D/\sigma=300$ and $Fr=U^2/gD=83$. In particular, the study investigates the ability of the simulations to capture liquid surface instabilities and the influence of the exiting jet turbulence content on the entrainment behavior. The results indicate that the qualitative behavior of the entrainment process follows very closely what is observed in experiments, with the rough jet exhibiting surface instabilities at impact that are not present in the smooth jet. These have an effect on the development of the initial air cavity and interfacial area, leading to a doubling of the interfacial area for a nominally similar entrained volume of air. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D28.00002: Lagrangian observations of acceleration and bubble dynamics in plunging breakers Miguel Canals, Andre Amador Understanding the three-dimensional structure of plunging waves is one of the most difficult problems in fundamental fluid dynamics. In this presentation we provide an analysis of field data collected in breaking waves using novel Lagrangian drifters with a diameter of 5-10 cm and equipped with miniature HD cameras and inertial measurement units. These drifters were deployed, using a personal watercraft, into the breaking region of waves ranging from 1-5 meters in height. We analyze in detail the time series of particle acceleration and rotation and how these quantities relate to the imagery captured by the camera aboard the drifters. This data represents the first dedicated study of the three-dimensional particle dynamics of plunging breakers. Going beyond the basic statistical analysis of the acceleration data, we make an attempt at characterizing the intensity of the wave breaking process using the bubble size and characteristics obtained from the HD video images. We also attempt to relate the spectral statistics of acceleration and particle rotation to existing Lagrangian turbulence models in the hopes of obtaining estimates of the kinetic energy dissipation in breaking waves, while taking into account the unsteady and heterogeneous nature of the turbulent flow. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D28.00003: An Experimental Study of Droplets Produced by Plunging Breakers D. Wang, D. Dai, X. Liu, J.H. Duncan The production of droplets by breaking water waves greatly affects the heat, mass and momentum transfer between the atmosphere and the sea surface. In this study, the production of droplets by mechanically generated breaking water waves was explored in a wave tank. The breakers were generated from dispersively focused wave packets (average frequency 1.15~Hz) using a programmable wave maker. Two overall wave maker amplitudes were used to create a strong spilling and a strong plunging breaker. The profile histories of the breaking wave crests along the center plane of the tank were measured with a cinematic laser-induced fluorescence technique, while the droplet diameter distributions and motions were measured at different locations along a horizontal line, which is 1 $cm$ above the maximum height of the wave crest, using a double-pulsed cinematic shadowgraph technique. It is found that droplets are primarily generated when the plunging jet of the wave generates strong turbulence during impact with the wave's front face and when large air bubbles, entrapped during the plunging process, rise to the free surface and pop. The differences between the generation mechanisms in spilling and plunging breakers is highlighted. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D28.00004: Quantification of air entrainment in a turbulent breaker from Bubble Image Velocimetry (BIV) data Javier Rodriguez-Rodriguez, Juan Lasheras One of the most difficult to quantify features of free surface turbulent flows is the amount of air that they engulf. Among other reasons, the large void fractions found in these flows preclude the application of many diagnostic techniques commonly used in fluid mechanics. In this work, we investigate the possibility of taking advantage of the large void fraction found in a turbulent breaker to simultaneously characterize the velocity field and the flux of air entrained across the mean free surface. To that end, we use a technique known as Bubble Image Velocimetry (BIV) that consists of applying standard PIV algorithms to a sequence of images of a bubble laden flow. Under certain conditions, that will be explained in this talk, the bubbles follow the motion of the large turbulent structures that are the main responsible for the entrainment of air bubbles. Thus tracking the motion of the bubbles, and measuring their number and volume distribution, it is possible to evaluate the flux of air across the mean free surface. The results obtained with this method are compared with those found in the literature that were acquired with fiber-optic void fraction probes. Supported by the ONR under contract N00014-05-1-0121 and by the Spanish Government through grant: DPI2011-28356-C03-02. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D28.00005: Laminar jet impingement and hydraulic jump behavior on a superhydrophobic surface with isotropic slip Julie Vanderhoff, Joseph Prince, Daniel Maynes We present an analytical model describing laminar jet impingement and the resulting hydraulic jump on a flat horizontal superhydrophobic surface with uniform surface slip in all directions. Due to the relatively thin film dynamics associated with the growth of the laminar jet after impingement, the influence of slip on the fluid physics is significant. An analysis based on momentum considerations is presented that allows prediction of the relevant thin film parameters as a function of radial position from the impingement point, jet Reynolds number, and constant relative slip length of the surface. The hydraulic jump can be located as a function of the laminar jet characteristics and imposed downstream liquid depth. The results reveal that at a given radial location, for increasing slip, the boundary layer growth and thin film thickness decrease while the surface velocity of the thin film increases. Increasing slip length also leads to the formation of a hydraulic jump at increasing radial location. A prediction equation is formulated to estimate the location of the hydraulic jump as a function of the magnitude of the slip and all other influencing variables. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D28.00006: Computational Study of Air Entrainment by Plunging Jets -- Influence of Jet Inclination Suraj Deshpande, Mario Trujillo The process of air entrainment by a continuous liquid jet plunging into a quiescent liquid pool is studied computationally. Our earlier study [APS2011] focused on shallow impacts and the discernible periodicity of air cavity formation. Here, we consider the effect of jet angle. For steep impacts, we see a chaotic formation of small cavities, in agreement with the literature. To explain the difference, we track evolution of the flow from initial impact to quasi-stationary state, for different jet inclinations. The initial impact always yields a large air cavity, regardless of jet angle. Difference emerges in the quasi-stationary state where shallow jets demonstrate the periodicity but the steep jets do not. We show that this is a manifestation of the air entrainment being a function of flow disturbance. For shallow jets, the disturbance originates from strong wavelike motion of the cavity which results in a total disruption of the jet. Thus, the resulting cavities are large and occur periodically. For the steep jets, entrainment happens by collapse of a thin gas film uniformly enshrouding the submerged jet. Such a thin film is very sensitive to the local flow disturbances. Thus, its collapse occurs stochastically all around the jet causing chaotic entrainment of small air pocket. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D28.00007: Laminar Jet Impingement and Hydraulic Jump Behavior on a Horizontal Surface with Anisotropic Slip Joseph Prince, Michael Johnson, Julie Vanderhoff, Daniel Maynes We present an analytical model that describes the influence anisotropic slip exerts on the thin film and hydraulic jump dynamics of laminar jet impingement on horizontal surfaces. Superhydrophobic surfaces with alternating microscale ribs and cavities exhibit anisotropic slip in the azimuthal direction and thus are described by this model. The thin film dynamics are predicted by an integral momentum based analysis as a function of the jet Reynolds number and for a specified slip length that varies azimuthally. In the analysis the thickness of the thin film at a given radius is assumed to be independent of the azimuthal coordinate. The model shows that the boundary layer grows more slowly parallel to the ribs compared to other directions. A second momentum balance was performed that predicts the radial location of the hydraulic jump as a function of imposed downstream depth. Deviation from the classical no slip case and from the scenario of isotropic slip was determined over a range of possible slip lengths. The results show that the hydraulic jump radius in the direction parallel to the ribs is larger than in the transverse direction and the shape of the hydraulic jump is nearly elliptical. Comparisons between the model results and experimental measurements are also provided. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D28.00008: Flow visualization of the water impact problem Hans Mayer, Rouslan Krechetnikov When a flat plate impacts the surface of an incompressible viscous liquid, the liquid directly beneath the plate is set into motion and an ejecta -- a high speed jet -- forms at the plate edge giving rise to the familiar ``splashing'' behavior. We present the results of our experimental investigation of the water impact problem using a particle image velocimetry (PIV) system to quantify the flow field beneath the plate immediately after impact with the speeds of the order of $1 \, \mathrm{m/s}$. The early-time formation of the ejecta for this flat plate geometry, including the influences of liquid viscosity ($1 < \mu < 10 \, \mathrm{mPa \cdot s}$) and surface tension ($20 < \sigma < 70 \, \mathrm{mN/m}$), are also studied with the PIV and high speed photography. Quantitative results for the flow field in the region beneath the plate and the growth of the ejecta are compared to existing and newly-developed theories. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D28.00009: Origin of ejecta in the water impact problem Rouslan Krechetnikov In this work we present a new analysis of the early time evolution of ejecta -- jet forming during the impact of a flat plate on the surface of an incompressible viscous liquid. The key goals here are to clarify the effects of viscosity and surface tension. In the course of construction of the solution, first the standard assumptions behind the existence of the inviscid approximation are revisited. Second, scalings of the structure of the solution near the plate edge are determined, with which the viscous solution in the Stokes approximation near the edge is constructed analytically. Third, the structure of a uniformly valid solution matching the Stokes solution to the inviscid one is revealed here. Finally, the analysis of both viscous and inviscid solutions allows us to uncover the scalings for the early time-evolution of the ejecta. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D28.00010: Three-dimensional oblique water-entry problems at small deadrise angles Matthew Moore, Sam Howison, John Ockendon, James Oliver We extend two-dimensional Wagner theory for the ideal, incompressible normal impact of rigid bodies that are nearly parallel to the surface of a liquid half-space. In particular, the impactors we consider are three-dimensional and have an oblique impact velocity that is an order of magnitude larger than the normal impact velocity of the body. A reformulation of the leading-order problem in terms of the displacement potential reveals the relationship between the oblique and corresponding normal impact solutions. We exploit this relationship to find new solutions to three-dimensional oblique impact problems. In the particular example of axisymmetric impactors, we consider several oblique-impact geometries in which singularities can develop on the boundary of the wetted region. We present the corresponding pressure profiles and models for the splash sheets and discuss the consequences of this breakdown of the theory. [Preview Abstract] |
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