Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session EK: Free Surface Flows III |
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Chair: Sigurdur Thoroddsen, National University of Singapore Room: 102B |
Sunday, November 23, 2008 4:10PM - 4:23PM |
EK.00001: How do hydraulic jumps form? Anders Andersen, Tomas Bohr We present an experimental and theoretical study of the formation of stationary hydraulic jumps in a narrow channel. We start each measurement with an empty channel and change the flow-rate abruptly from zero to a constant positive value. This leads to the formation of a stationary hydraulic jump in a two stage process: first the channel fills by the advancing fluid front, which undergoes a transition from supercritical to subcritical at some position in the channel, and later the influence of the downstream boundary conditions makes the jump move upstream to its final position with exponentially decreasing speed. We compare our experimental findings with theoretical predictions based on Rayleigh type shock theory. [Preview Abstract] |
Sunday, November 23, 2008 4:23PM - 4:36PM |
EK.00002: Substrate hydrophobicity and meandering Peter Vorobieff, Bjorn Birnir, Keith Mertens, Vakhtang Putkaradze We present a study of the effect of surface properties on the meandering of a rivulet flowing down a non-eroding inclined plane. In this plane, we consider the behavior of meandering amplitude of the rivulet $h(x,t)$ for a variety of substrates, from partially wetting to hydrophobic, and present our results in terms of Fourier spectra of $h$ and in terms of the dimensionless growth rate of averaged absolute values of $h$ vs. downstream distance $x$. While the spectra have certain similarities in their scaling behavior for all the surfaces we studied, the dimensionless amplitude growth rate appears to depend rather strongly on the static contact angle characterizing the substrate. [Preview Abstract] |
Sunday, November 23, 2008 4:36PM - 4:49PM |
EK.00003: Scaling laws for meandering streams Keith Mertens, Vakhtang Putkaradze, Peter Vorobieff We report on the scaling laws associated with meandering of a rivulet flowing down a non-erodible, partially wetting incline. The meandering streams in this experiment are triggered by flow rate fluctuations and sustained by external noise forcing. In our experiments, the former is provided by an electronically controlled valve, and the latter is due to fluid droplets left on the surface by previous meanderings. Over the entire range of scales we observe, the averaged spectrum of the deviations of the stream from its centerline demonstrates a power-law scaling, thus precluding the possibility of a preferred wavelength in ongoing meandering. We derive a simple theoretical model of rivulet meandering from first principles, incorporating stream dynamics and external noise forcing. The model provides an accurate statistical description of the stream deviation from a non-meandering path. [Preview Abstract] |
Sunday, November 23, 2008 4:49PM - 5:02PM |
EK.00004: The effect of surface conditions on the statistics of the surface temperature field during mixed convection J. Kou, J.R. Saylor The statistics of the surface temperature field of an air/water interface are presented for the case of a clean water surface and a water surface covered with the surfactant monolayer oleyl alcohol. Experiments were conducted in a wind/water tunnel where the wind speed ranged from 1 - 4 m/s and the water was warmer than the air. The surface temperature field was acquired using an infrared camera. The root-mean-square and the skewness of the surface temperature field were computed and related to the heat flux and the wind speed for both the clean and surfactant-covered cases. Probability density functions of surface temperature were also computed and are presented to further reveal the effect of surfactant on the relationship between heat flux and wind speed and the surface temperature field. Some discussion of the mean temperature field is also presented. [Preview Abstract] |
Sunday, November 23, 2008 5:02PM - 5:15PM |
EK.00005: Meandering instability of a rivulet confined between two plates Laurent Limat, Adrian Daerr, Samuel Paillat, Jens Eggers We have investigated experimentally and theoretically the meandering instability of a rivulet flowing vertically between two plates. In contrast with previous works, we considered pure fluids with no surfactant effects. Experiments on silicon oils of low viscosities, reveal that there is a spontaneous instability leading to traveling meandering patterns, often disordered, but with a well defined wavelength. Strongly ordered patterns can be selected by forcing the entry with a well defined frequency. In both cases, the obtained wavelength is centimetrical and with a week dependence upon flow rate. Theoretically, this instability can be interpreted in terms of centrifugal effects competing with the friction of contact lines on the two plates. Starting from hydrodynamic equations, we have obtained a reasonably simple dispersion relationship that allows us to recover the selected wavelength and the pattern phase velocity. We suggest that this theory should also hold for rivulet on inclined plates, provided that the hysteresis and the noise introduced by the substrate are not too high. [Preview Abstract] |
Sunday, November 23, 2008 5:15PM - 5:28PM |
EK.00006: Detection of a liquid-metal surface using the DLP technique Martin Hillenbrand, Robert Stieglitz, Andreas Class, Paul Neitzel Numerous new reactor concepts (e.g., Accelerator-Driven Systems) utilize the impact of a high-energy particle beam onto a liquid-metal surface as a vital part of the reactor design. Consequently, there is a need to detect the liquid-metal surface with high spatial and temporal resolution. The detection of liquid-metal surfaces has inherent difficulties that limit the use of common measurement techniques, e.g., high reflectivity in vacuum coupled with high flow velocity and rapid surface motion. In order to develop a method capable of meeting the needs of such systems, a projection method has been modified by adding a second (transparent) screen. The resulting Double-Layer-Projection (DLP) technique was applied to obtain spatially and temporally resolved measurements of a circular hydraulic jump using the eutectic liquid GaInSn as the test fluid. The jump position and height were measured with the required accuracy of $\pm $0.3mm. The velocity of the dominant waves and the superimposed high-frequency disturbances of the liquid metal surface were also detected. [Preview Abstract] |
Sunday, November 23, 2008 5:28PM - 5:41PM |
EK.00007: Law of spreading of the crest of a breaking wave Patrice Le Gal, Timoth\'ee Nicolas, Timoth\'ee Jamin, Michael Le Bars, Yves Pomeau In a wide range of conditions ocean waves break. This can be seen as the manifestation of a singularity in the dynamics of the fluid surface, moving under the effect of the fluid motion of the underlying fluid. We show that, for shallow water waves at the onset of breaking, the wave crest expands in the span-wise direction as the square root of time. This is first derived from a theoretical analysis and then compared with experimental findings. The agreement is excellent. We then explore another configuration where the waves are generated by a parabolic wave maker. The focusing of the initially parabolic fronts induces interferences and also breaking of the waves if their amplitude is large enough. In this case a widening of the breaking proportional to the power $3/2$ of time is expected as it follows the Huygens cusp shape. [Preview Abstract] |
Sunday, November 23, 2008 5:41PM - 5:54PM |
EK.00008: Stagnation, folding, coiling, and breakup of viscous jets: a synthesis Neil Ribe, Mehdi Habibi, Yaser Rahmani, Daniel Bonn Using laboratory experiments and theoretical modeling, we have studied the dynamics of a thin jet of viscous fluid falling from a substantial height (tens of cm) onto a solid surface. Our experiments reveal surprisingly complex behavior as the viscosity decreases for a fixed flow rate. For the highest viscosities used, the jet coils at all heights for which it remains intact. As the viscosity decreases, one observes (1) chaotic alternation between coiling and planar folding; (2) alternation between coiling, folding, and axisymmetric stagnation-point flow; and (3) stagnation-point flow alone. In all cases, the jet breaks up via the Rayleigh instability if the fall height becomes sufficiently large. To understand these results theoretically, we have developed a mathematical model of a thin viscous jet that includes an exact representation of surface tension forces. The linearized forms of these equations that describe the stability of stagnation-point flow comprise three uncoupled subsets, corresponding respectively to planar folding, helical coiling, and the Rayleigh instability. We solve these equations numerically to determine phase diagrams for the different types of instability, and compare the results with our experimental observations. [Preview Abstract] |
Sunday, November 23, 2008 5:54PM - 6:07PM |
EK.00009: Apex Jets from Impacting Drops Jeremy Marston, Sigurdur Thoroddsen A new jetting phenomenon has been observed experimentally when a viscous drop, such as glycerin, impacts onto a low-viscosity, low-surface tension liquid pool, such as methanol. This jet is produced by the ejecta sheet which emerges from the free surface of the pool, moves up along and wraps around the surface of the drop. The convergence and closure of this sheet at the top apex of the drop produces a thin vertical jet along the axis of symmetry at velocities of more than 10 times that of the drop. These jets are only observed for a narrow range of impact conditions. The drop impact velocity must be high enough that the ejecta sheet has sufficient inertia to reach the apex, but not so high that it detaches. Thus we identify critical Reynolds and Weber numbers. Jetting has been observed both for drops which are miscible and immiscible with the pool liquid, under a different range of impact conditions but never for pools of water, as the surface tension is then significantly larger than that of the drop. Marangoni stresses may act in this case to promote separation and prevent the jetting. [Preview Abstract] |
Sunday, November 23, 2008 6:07PM - 6:20PM |
EK.00010: Thin-films flows over microtextured surfaces: Polygonal water sheets Adrian P. Delancy, Emilie Dressaire, Laurent Courbin, Howard A. Stone We study water sheets and bells resulting from the impact of a water jet onto circular targets of comparable diameter. Depending on the physical properties of the target surface, which may or may not be covered with a roughness at the micron-scale, i.e. arrays of cylindrical micron-size posts arranged on regular lattices, we obtain a variety of stable shapes including circles and polygons such as hexagons, eight corner stars. We vary the topographic features (height of the posts, lattice distance and geometry) and the jet properties (size of the nozzle, flow rate) and we measure the size and shape of the liquid sheet. We rationalize our results by taking into account the additional friction induced by the lattice providing a fluid velocity that depends on the orientation of the lattice. [Preview Abstract] |
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