Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M05: Free-surface Flows: Near-surface Wakes |
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Chair: Tadd T. Truscott, Utah State University Room: Georgia World Congress Center B207 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M05.00001: The water entry of everything … a unifying approach Nathan B Speirs, Mohammad Mansoor, Jesse L. Belden, Tadd T Truscott The water entry of spheres has been shown to cause cavity formation above a critical impact velocity. Yet, previous foundational studies conflict over whether this finding applies at high contact angles. Through experimentation, we look closer at this conflict and show that cavity formation can occur well below the critical impact velocity as well. We also elucidate how changing the static contact angle alters the cavity shapes or regimes (e.g., quasi-static, shallow, deep and surface). Our findings indicate an alternate scaling for the Bond and Weber numbers that unifies the cavity regimes for many impacting body types (e.g., spheres, multi-droplet streams and jets). Our findings show that solid-liquid impact is quite similar to liquid-liquid impact and that both impact types should be considered as one in the same. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M05.00002: Surviving a cliff jump: go second Rafsan Rabbi, Nathan B Speirs, Jesse L. Belden, Tadd T Truscott Children and adults often ask us the question, “If I jump from a bridge into water can I throw an object at the water before I land and reduce the impact?” We tested this theory by dropping two spheres, axially aligned but distance separated, into a quiescent pool of water. The first sphere creates a cavity through which the second sphere enters. We show that the high peak force at the first few moments of the impact can be dramatically reduced. The diameter of the first sphere was varied while the diameter of the second sphere was kept constant. Through high-speed imaging and onboard accelerometers, we were able to divide the consecutive sphere entry into different regimes based on the cavity shape at the moment of the second sphere entry, revealing that the impact forces experienced in each regime are different and smaller than the single sphere case. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M05.00003: Study on translating and oscillating body near a Free surface S. Vengadesan, Prashant Khandelwal In this numerical study of fluid flow past elliptic cylinder, the effect of the presence of a free surface is analyzed. The fluid-fluid interface is tracked by the Level set method proposed by Osher and Sethian and the rigid elliptical cylinder is approximated by immersed boundary projection method, both of these methods are incorporated in an in-house code. The effect of the interface is observed for the forces experienced by the ellipse and the wake of the ellipse. The shape of the interface is also compared for different configurations and kinematics. The parameters investigated are Froude number and kinematics for a fixed Reynolds number. The considered motions are oscillation parallel to the interface with pure pitching motion near the interface. The interface vicinity changes the wake dynamics significantly for stationary bodies. While in some cases where flow becomes stabilized. Similarly, the forces on the body also vary as the interface proximity is changed. This study also compares the interfacial topology for various cases. Motion of the body causes instability in the shear layers present in the flow that leads to different vortex shedding mechanisms. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M05.00004: Concurrent vortex-shedding and surface-wave phenomena around a horizontal circular cylinder close to a free surface Dong Min Shin, Hyoungsoo Kim, Hyung Jin Sung, Yeunwoo Cho A fully submerged horizontal circular cylinder close to a free surface moves horizontally below the free surface in a still fluid. Concurrent vortex-shedding and surface-wave phenomena are observed according to the relevant Froude number (based on the cylinder diameter) and the gap ratio (distance between the cylinder and the free surface non-dimensionalized by the cylinder diameter). Both constrained and unconstrained cases are considered in terms of the vertical motion of the cylinder due to the vortex shedding. Overall, associated with the vortex shedding, five different surface-wave phenomena are observed; no waves, small-amplitude waves, surging breaking waves, collapsing breaking waves (observed only in unconstrained cases), straight-streamline jet-like flows over the upper surface of the cylinder. In particular, for a constant gap ratio, the jet-like flow becomes steeper as the Froude number increases and, for a constant Froude number, the jet-like flow becomes steeper as the gap ratio decreases. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M05.00005: Flow and air-entrainment around partially submerged vertical cylinders Valentin Ageorges, Jorge Peixinho, Gaële Perret This work presents experimental and numerical results of the flow around cylinders. Experimentally, the cylinders are partially immersed with a height h. The motion induces turbulent wake and free-surface deformation. Eleven cylinder diameters from D=1.4 to 16 cm were translated at varying speed for runs at constant h and h/D. The Reynolds and Froude numbers, based on the cylinder diameter, govern the flow. The explored range of parameters are in the regime of air-entrainment behind the cylinder with experiments carried out for 4 500 < Re < 240 000 and 0.2 < Fr < 2.4. The focus here is on drag force measurements and strong free-surface deformation up to rupture and air-entrainment. New data on air-entrainment mechanisms are provided. In addition, numerical simulations, based on finite volume method of high order, and level-set method are used to reproduce the flow behind the cylinder and free-surface deformation. Numerical and experimental results are compared with each other and with previous works without air-entrainment. The present work highlights free-surface effects on drag forces. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M05.00006: Enhancing Air-Water Gas Transfer Using Capillary-Gravity Wakes Katherine Adler, Edwin A. Cowen Air-water gas transfer rate limits some industrial processes, such as algae biofuel production. This limitation can be overcome through methods such as bubbling, waterfalls, or jets. However, these solutions are energy-intensive and disruptive. Therefore, this research explores more energy-efficient hydrodynamic means to enhance the transfer rate of low-soluble gases. Previous studies found that wind- or vibration-driven short-wavelength waves contribute to interfacial mass transport by straining the concentration boundary layer and increasing surface area. This presentation describes laboratory experiments conducted in a straight, open flume to determine the impact of capillary-gravity wakes on air-water gas transfer. An array of 1/8” dowels was partially submerged to disturb the water surface and form rippling wakes. The Bond number (BOD=D/λc) varied between 0.5 and 0.8, and the Froude number (Fr=U/√(gD)) varied between 1.8 and 2.0. During these experiments, the gas transfer velocity was 20-24% higher than the control case. Energy loss is estimated based on mean streamwise slope of the free surface. Further studies will modify dowel density and configuration and relate gas transfer enhancement to BOD and Fr. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M05.00007: The surface signature of a canopy-generated shear instability Tracy Mandel, Saksham Gakhar, Hayoon Chung, Itay Rosenzweig, Jeffrey R Koseff We present results from a laboratory study on the free-surface signal generated over an array of submerged circular cylinders, representative of submerged vegetation. We aim to understand whether aquatic ecosystems generate a surface signature that is indicative of both what is beneath the water surface as well as how it is altering the flow. A shear layer forms over the canopy, generating coherent vortex structures which eventually manifest in the free-surface slope field. Experimental measurements at the surface suggest a Strouhal number that is twice the most amplified mode predicted by linear stability theory. This implies that vortices may evolve between generation at the canopy height and their manifestation at the water surface. Additionally, it appears the flow never becomes fully-developed, likely due to interaction of flow structures with the free surface, as well as due to gradual changes in water depth from the driving barotropic pressure gradient. We connect the vortex properties measured at the surface with measurements of the bulk flow, and show that correlations between these quantities are adequate to create a parameterized model in which the interior velocity profile can be predicted solely from measurements taken at the free surface. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M05.00008: Effects of Surface Wave on the Turbulence underneath in Langmuir Circulation Anqing Xuan, Bing-qing Deng, Lian Shen The turbulence statistics in upper oceans and lakes can be significantly influenced by the forcing of surface waves and wind-driven shear, leading to Langmuir circulation. In the present work, the role of surface waves in the distortion of turbulence is studied using wave-phase-resolved simulations, where the Navier-Stokes equations are solved on a boundary-fitted curvilinear grid that evolves with the wave surface. In the simulation setup, we consider turbulent flows driven by a steady monochromatic wave and constant shear stress. The characteristics of Langmuir circulation, such as the elongated, counter-rotating vortices and the enhanced vertical fluctuations, are captured. Meanwhile, the phase-resolved simulations enable us to analyze the distortion of turbulence by the instantaneous wave orbital motions. The alternating normal and shear straining associated with the wave orbital motions are found to result in phase-variations of turbulence statistics, including the strength of vortices and the Reynolds stress. Analyses of vorticity dynamics and Reynolds stress budget in the Lagrangian framework also reveal how the phase-varying turbulent fluctuations contribute to the cumulative distortion of turbulence and the energy transfer between the wave and turbulence. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M05.00009: Surface Fluctuations and Air Entrainment in a Turbulent Free-Surface Boundary Layer Martin A. Erinin, Naeem Masnadi, Nathan Washuta, Farshad Nasiri, Elias Balaras, Nicholas Lawson, James H Duncan Air entrainment due to turbulence in a free surface boundary layer flow created by a horizontally moving vertical surface-piercing wall is studied through experiments and Direct Numerical Simulations (DNS). In the experiments, the moving wall is created by a stainless steel belt loop that is driven by two vertical rollers; one length of the belt between the rollers acts as the moving wall. The belt is accelerated suddenly from rest until reaching constant speed, thus creating a temporally-evolving boundary layer. Experimental observations and measurements of air-water interface deformation patterns and bubbles were performed with time-resolved photographic techniques. In the DNS, a temporally evolving boundary layer problem was studied with the computational domain consisting of a streamwise section of the belt flow field with periodic boundary conditions. The simulation uses a single-fluid, two-phase (air and water) formulation with an exact projection method on a staggered grid to resolve the air-water interface. From the DNS results, the physics and prevalence of several classes of air entrainment events is assessed. |
(Author Not Attending)
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M05.00010: Self-turbulizing cascade of wrinkles in a viscous thread Baptiste N\'eel, Emmanuel Villermaux A thread of viscous mineral oil is deposited at the surface of a clean water bath. Because of the lower surface tension of the oil, the thread first spreads, pulled by Marangoni forces acting on its sides in the plane of the bath. This motion however soon destabilizes and the thread core presents longitudinal sinuous wrinkles which amplify because the surface tension gradient is steepened at the extrema of the thread corrugations. We document and analyze the first steps of this instability, and show that the subsequent steps occur, in a catastrophic manner, on ever smaller timescales. The phenomenon indeed proceeds self-similarly down to ever smaller corrugations scales, in an accelerated cascade finally interrupted by the diffusive smearing of the oil thread core in the bath.
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