Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session EK: Free Surface Flows II |
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Chair: Andrew Bernoff, Harvey Mudd College Room: Salt Palace Convention Center 250 E |
Sunday, November 18, 2007 4:10PM - 4:23PM |
EK.00001: Coiling and Folding of Viscoelastic Jets Trushant Majmudar, Matthieu Varagnat, Gareth McKinley The study of fluid jets impacting on a flat surface has industrial applications in many areas, including processing of foods and consumer goods, bottle filling, and polymer melt processing. Previous studies have focused primarily on purely viscous, Newtonian fluids, which exhibit a number of different dynamical regimes including dripping, steady jetting, folding, and steady coiling. Here we add another dimension to the problem by focusing on mobile (low viscosity) viscoelastic fluids, with the study of two wormlike-micellar fluids, a cetylpyridinum-salicylic acid salt (CPyCl/NaSal) solution, and an industrially relevant shampoo base. We investigate the effects of viscosity and elasticity on the dynamics of axi-symmetric jets. The viscoelasticity of the fluids is systematically controlled by varying the concentration of salt counterions. Experimental methods include shear and extensional rheology measurements to characterize the fluids, and high-speed digital video imaging. In addition to the regimes observed in purely viscous systems, we also find a novel regime in which the elastic jet buckles and folds on itself, and alternates between coiling and folding behavior. We suggest phase diagrams and scaling laws for the coiling and folding frequencies through a systematic exploration of the experimental parameter space (height of fall, imposed flow rate, elasticity of the solution). [Preview Abstract] |
Sunday, November 18, 2007 4:23PM - 4:36PM |
EK.00002: Structure of a horizontal rectangular surface jet Eric Savory, Iftekhar Naqavi, Robert Martinuzzi, Roi Gurka A jet issuing close to or on a free surface is called a surface jet. Here a direct numerical simulation of a rectangular jet (hydraulic diameter $D)$ issuing at the free surface at Re=4420 and Fr=0.49 is studied. The mean velocity vectors on a spanwise-wall normal plane at a downstream location of $x$=4$D$ (within the transition zone) shows a counter rotating vortex pair (CVP). Further downstream in the fully developed region at $x$=16$D$ the CVP disappears and entraining fluid from below comes to the surface and spreads out. The surface normal vorticity on the $x-y$ plane at $z$=-0.25$D$ shows that the shear layers remain stable in the lateral direction up to \textit{x$\approx $4-5D}. However, the lateral vorticity on the $x-z$ plane at $y$=0 suggests that the shear layers in the vertical planes become unstable earlier. This earlier instability is responsible for CVP development in the transition region in the mean flow field. Once the shear layers become unstable in the lateral direction the CVP vanishes and the jet entrains fluid from below, mixes it with the jet fluid and expands along the surface. [Preview Abstract] |
Sunday, November 18, 2007 4:36PM - 4:49PM |
EK.00003: High-speed jet formation after solid object impact Stephan Gekle, Jos\'e Manuel Gordillo, Devaraj van der Meer, Detlef Lohse A circular disc impacting on a water surface creates a remarkably vigorous jet. Here we study this phenomenon in a controlled way by pulling the disc through the free surface at constant speed. An axisymmetric air-filled cavity is formed which eventually pinches off in a single point. Immediately after cavity closure the pinch-off location turns into a stagnation point and the flow pattern changes from radial sink flow into hyperbolic flow. The stagnation point deflects the incoming liquid leading to the formation of two fast sharp-pointed jets shooting up- and downwards from the closure point. We study the jet characteristics as a function of both impact velocity and disc radius. [Preview Abstract] |
Sunday, November 18, 2007 4:49PM - 5:02PM |
EK.00004: Three flow regimes of a viscous jet falling onto a moving surface under gravity Andriy Hlod, Annemarie Aarts, Fons van de Ven, Mark Peletier A thin jet of a viscous Newtonian fluid falling from a nozzle onto a moving surface can fall in three different regimes. In the first one the shape of the jet is convex and the jet makes zero angle with the surface. Here, the viscous force dominates inertia everywhere in the jet. In the second regime the fluid flows down vertically. Here, viscosity dominates near the nozzle, while inertia dominates near the surface. In the last regime, which occurs when the flow velocity at the nozzle is large, the jet's shape is concave, comparable to a ballistic trajectory. Then the fluid flow at the nozzle is aligned with the nozzle orientation, and inertia dominates everywhere in the jet. The model incorporates inertia, viscosity and gravity. Correct boundary conditions for the stationary jet as well as the regions of parameters for the three flow regimes are obtained by studying the characteristics of the equation of motion for a dynamic jet. The theoretical predictions of the three flow regimes correspond with experiments. For each flow regime we prove existence and investigate uniqueness. The solutions might not be unique if the nozzle does not point down vertically. In this case multiple stationary solutions are possible, which leads to instabilities, also observed in the experiments. [Preview Abstract] |
Sunday, November 18, 2007 5:02PM - 5:15PM |
EK.00005: On the breakup of non-circular liquid jets T.V. Kasyap, D. Sivakumar, B.N. Raghunandan It is well known that liquid jets discharging from non-circular orifices show interesting geometrical transformations on their free surface due to the competition between inertia and surface tension forces. A typical example is the axis-switching phenomenon observed on elliptical liquid jets. In this work, we present some hitherto unknown results obtained on the breakup of liquid jets issuing from elliptical, triangular and circular orifices of various geometric parameters but of same cross sectional area. Photographic techniques were employed to record the images of jets and relevant quantities like breakup length were measured from the photographs. The results show that elliptical and triangular jets behave exactly like the circular jet at very low and very high jet velocities. However within an intermediate range of flow conditions, liquid jets from these non-circular orifices show an enhanced destabilization which shortens their breakup lengths in comparison with the circular jet. We suggest that this is caused by the geometrical transformations on their free surface due to which there is more surface energy, making them less stable. [Preview Abstract] |
Sunday, November 18, 2007 5:15PM - 5:28PM |
EK.00006: Symmetry breaking: Swimming beneath free surfaces, Part 1 Sungyon Lee, Ophir Samson, Eric Lauga, A.E. Hosoi, Darren Crowdy The Scallop Theorem states that time-reversible motion cannot produce net propulsion in Stokes flow. One method for a swimmer to get around this theorem and propel itself is by using deformations of a free surface to break symmetry. We present here a simplified 2D swimmer, modeled as a stresslet point singularity plus a dipole. We obtain exact analytic solutions using conformal mapping techniques to describe the interplay between the swimmer and the free surface. [Preview Abstract] |
Sunday, November 18, 2007 5:28PM - 5:41PM |
EK.00007: Symmetry breaking: Swimming beneath free surfaces, Part 2 Ophir Samson, Sunyon Lee, Eric Lauga, Anette Hosoi, Darren Crowdy A swimmer modelled as a point stresslet and dipole in a Stokes flow has been shown to propel itself by deforming the shape of the interface and thus breaking the symmetry of the flow. The theory of this 2D point swimmer is now generalized to one having a deformable body. In this case, the swimmer can control its motion by ``squirming'' in any desired way. By using conformal mapping theory of doubly connected domains we can simultaneously capture both the deformation of the swimmer's shape and motion of the interface. [Preview Abstract] |
Sunday, November 18, 2007 5:41PM - 5:54PM |
EK.00008: An experimental study of the air/water interfacial surface temperature field during mixed convection J. Kou, K.P. Judd, J.R. Saylor The statistics of the surface temperature field of an air/water interface undergoing mixed convection were investigated experimentally. A body of water was exposed to a flow of air having a speed ranging from 1.0 to 4.0 m/s. The water body was warmer than the air and heat was transferred from the water to the air via mixed convection. The temperature field of the water surface was measured using an infrared camera. The statistics of the surface temperature field were computed, specifically the root mean square (rms) and the skewness. The rms was found to increase linearly with the heat flux emanating from the water surface, and was found not to be a function of the wind speed. The skewness was a weak function of the heat flux, and depended strongly on the wind speed. Although obtained under laboratory conditions, these results suggest that remotely sensed statistics of the surface temperature field can be used to measure the heat flux and the wind speed over water bodies. The temporally averaged surface temperature varied significantly across the water surface, requiring care in choosing a method for subtracting a mean prior to computing rms and skewness. The pdfs of the surface temperature field are also presented. [Preview Abstract] |
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