Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H11: Jets: Impingement on Surfaces and Crossflows |
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Chair: Constantine Megaridis, University of Illinois at Chicago Room: C120-121-122 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H11.00001: Liquid jet impinging orthogonally on a wettability-patterned surface Theodore Koukoravas, Aritra Ghosh, pallab Sinha Mahapatra, Ranjan Ganguly, Constantine Megaridis Jet impingement has many technological applications because of its numerous merits, especially those related to the ability of liquids to carry away heat very efficiently. The present study introduces a new configuration* employing a wettability-patterning approach to divert an orthogonally-impinging laminar water jet onto a predetermined portion of the target surface. Diverging wettable tracks on a superhydrophobic background provide the means to re-direct the impinging jet along paths determined by the shape of these tracks on the solid surface. In a heat transfer example of this method, an open-surface heat exchanger is constructed and its heat transfer performance is characterized. Since this approach facilitates prolonged liquid contact with the underlying heated surface through thin-film spreading, evaporative cooling is also promoted. We demonstrate flow cases extracting 100 W/cm$^{\mathrm{2}}$ at water flow rates of O(10 mL/min). By comparing with other jet-impingement cooling approaches, the present method provides roughly four times more efficient cooling by using less amount of coolant. The reduced coolant use, combined with the gravity-independent character of this technique, offer a new paradigm for compact heat transfer devices designed to operate in reduced- or zero-gravity environments. * T. P. Koukoravas, A. Ghosh, P. Sinha Mahapatra, R. Ganguly and C. M. Megaridis, Intl J. Heat Mass Transfer 95, 142-152, 2016. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H11.00002: Spreading Characteristics of Newtonian Free Surface Liquid Jets Impacting a Moving Substrate Hatef Rahmani, Yuchen Guo, Sheldon Green, Ali Vakil The impingement of high-speed liquid jets on a solid substrate is salient to a number of industrial processes, including surface coating in the railroad industry. The impingement of Newtonian liquid jets is studied both experimentally and through simulation. On impingement the liquid jet spreads laterally from the impingement location to form a lamella that is then convected downstream, producing an overall U-shaped liquid surface. A variety of jet and substrate velocities, liquid viscosities, and jet diameters were studied. It is found that the lamella dimensions (width (W), radius (R), thickness (h)) vary with the jet Reynolds number ($Re_{jet} )$ and vary inversely with the substrate Reynolds number ($Re_{sub} )$. Interestingly, the ratio W/R is almost constant, independent of the jet viscosity, diameter, and speed, and also independent of the substrate speed. Furthermore, the lamella radius and width scale as $Re_{jet} /\sqrt {Re_{sub} } $ and the lamella thickness scales as $1/\sqrt {Re_{sub} } $. The experimental results were in good agreement with volume-of-fluid (VOF) CFD simulations, which implies that the simulations may be used to probe the physics of impingement. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H11.00003: Numerical study of an impinging jet to a turbulent channel flow in a T-Junction configuration Michail Georgiou, Miltiadis Papalexandris In this talk we report on Large Eddy Simulations of an impinging planar jet to a turbulent channel flow in a T-Junction configuration. Due to its capacity for mixing and heat transfer enhancement, this type of flow is encountered in various industrial applications. In particular, our work is related to the emergency cooling systems of pressurized water reactors. As is well known, this type of flow is dominated by a large separation bubble downstream the jet impingement location. Secondary regions of flow separation are predicted both upstream and downstream the impinging jet. We describe how these separation regions interact with the shear layer that is formed by the injection of the jet to the crossflow, and how they affect the mixing process. In our talk we further examine the influence of the jet's velocity to characteristic quantities of the jet, such as penetration length and expansion angle, as well as to the first and second-order statistics of the flow. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H11.00004: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 11:32AM - 11:45AM |
H11.00005: Magnetic Resonance Velocimetry analysis of an angled impinging jet Alexandre Irhoud, Michael Benson, Claire VerHulst, Bret Van Poppel, Chris Elkins, David Helmer Impinging jets are used to achieve high heat transfer rates in applications ranging from gas turbine engines to electronics. Despite the importance and relative simplicity of the geometry, simulations historically fail to accurately predict the flow behavior in the vicinity of the flow impingement. In this work, we present results from a novel experimental technique, Magnetic Resonance Velocimetry (MRV), which measures three-dimensional time-averaged velocity without the need for optical access. The geometry considered in this study is a circular jet angled at 45 degrees and impinging on a flat plate, with a separation of approximately seven jet diameters between the jet exit and the impingement location. Two flow conditions are considered, with Reynolds numbers of roughly 800 and 14,000. Measurements from the MRV experiment are compared to predictions from Reynolds Averaged Navier Stokes (RANS) simulations, thus demonstrating the utility of MRV for validation of numerical analyses of impinging jet flow. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H11.00006: The Time-Resolved Flow Field of a Spatially Oscillating Jet in Crossflow F. Ostermann, R. Woszidlo, C.N. Nayeri, C.O. Paschereit Spatially oscillating jets in crossflow emitted by fluidic oscillators have been proven beneficial for flow control applications in recent studies. However, the driving mechanism behind the efficacy remains unknown. The presented study examines the fundamental, time-resolved flow field of a spatially oscillating jet in crossflow. The inclination angle between oscillation plane and crossflow is 90deg . The underlying experimental dataset is acquired plane-by-plane by a traversable stereoscopic particle image velocimetry system. Phase-averaging reduces stochastic noise, compensates low sampling rates, and allows combining the individual planes to a time-resolved three-dimensional flow field. The trajectory of the oscillating jet is much shallower than a steady jet. Two counter-rotating streamwise vortices are revealed. The sense of rotation is opposite to that of the counter-rotating vortex pair of steady jets in crossflow. This sense of rotation enables the vortices to prevail far downstream because they push each other toward the wall. The strength of the vortices is alternating. This vortex pair is a promising candidate to be the driving mechanism behind the high efficacy in separation control. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H11.00007: Structural and mixing characteristics in actively controlled transverse jets Takeshi Shoji, Andrea Besnard, Elijah Harris, Robert M'Closkey, Ann Karagozian, Luca Cortelezzi These experiments explore the effect of external excitation on gaseous transverse jet (TJ) structural and mixing characteristics, emphasizing axisymmetric jet forcing. Sinusoidal as well as single and multiple square wave pulses, the latter with variable amplitudes, are explored for a range of jet-to-crossflow momentum flux ratios $J$, spanning regimes\footnote{Megerian, et al, JFM \textbf{593}, pp. 93-129, 2007} of absolutely unstable upstream shear layers ($J < 10$) and convectively unstable shear layers ($J > 10$). The studies utilize acetone PLIF imaging of the jet, as done for unforced jets\footnote{Gevorkyan, et al, JFM \textbf{790}, pp. 237-274, 2016}. Axisymmetric forcing, irrespective of the waveform, can enhance cross-sectional symmetry of the TJ for convectively unstable conditions, but generally disrupts the usually symmetric counter-rotating vortex pair (CVP) observed for the absolutely unstable TJ. Conditions producing deeply penetrating, periodic vortical structures, such as square wave forcing at critical stroke ratios, increase jet spread, but do not always optimize molecular mixing. Creating multiple vortex structures of different strengths via multiple square pulses leads to enhanced interactions and accelerated vortex breakdown, potentially increasing mixing. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H11.00008: Analysis of the stability and sensitivity of jets in crossflow Marc Regan, Krishnan Mahesh Jets in crossflow (transverse jets) are a canonical fluid flow in which a jet of fluid is injected normal to a crossflow. A high-fidelity, unstructured, incompressible, DNS solver is shown (Iyer {\&} Mahesh 2016) to reproduce the complex shear layer instability seen in low-speed jets in crossflow experiments. Vertical velocity spectra taken along the shear layer show good agreement between simulation and experiment. An analogy to countercurrent mixing layers has been proposed to explain the transition from absolute to convective stability with increasing jet to crossflow ratios. Global linear stability and adjoint sensitivity techniques are developed within the unstructured DNS solver in an effort to further understand the stability and sensitivity of jets in crossflow. An Arnoldi iterative approach is used to solve for the most unstable eigenvalues and their associated eigenmodes for the direct and adjoint formulations. Frequencies from the direct and adjoint modal analyses show good agreement with simulation and experiment. Development, validation, and results for the transverse jet will be presented. Supported by AFOSR (Iyer, P. S. {\&} Mahesh, K. 2016 A numerical study of shear layer characteristics of low-speed transverse jets. J. Fluid Mech. 790, 275-307) [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H11.00009: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 12:37PM - 12:50PM |
H11.00010: Large bouncing jets Karl Cardin, Mark Weislogel We experimentally investigate the phenomena of large jet rebound (bounce), a mode of fluid transfer following oblique jet impacts on hydrophobic surfaces. We initially seek to describe the regimes of such jet bounce in tests conducted in the weightless environment of a drop tower. A parametric study reveals the dependence of the rebound mode on the relevant dimensionless groups such as Weber number W$e_{\perp}$ defined on the velocity component perpendicular to the surface. We show that significantly larger diameter jets behave similarly as much smaller jets demonstrated during previous terrestrial investigations when W$e_{\perp}\approx 1$. For W$e_{\perp}>1$, large jet impacts create fishbone-like structures. We also explore rebounds from nonplanar substrates. Improving our understanding of such jet rebound opens avenues for unique transport capabilities. [Preview Abstract] |
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