Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L22: Turbulence: Mixing III |
Hide Abstracts |
Chair: Arne Johansson, KTH Royal Institute of Technology Room: 317 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L22.00001: Large Eddy Simulation of Mixing within a Hypervelocity Scramjet Combustor David Petty, Vincent Wheatley, Carlos Pantano, Michael Smart The turbulent mixing of parallel hypervelocity (U $=$ 3230 m/sec, M $=$ 3.86) air-streams with a sonic stream of gaseous hydrogen is simulated using large eddy simulation. The resultant mixing layers are characterized by a convective Mach number of 1.20. This configuration represents parallel slot injection of hydrogen via an intrusive centerbody within a constant area rectangular combustor. A hybrid shock-capturing/zero numerical dissipation (WENO/TCD) switch method designed for simulations of compressible turbulent flows was utilized. Sub-grid scale turbulence was modeled using the stretched vortex model. Visualizations of the three dimensional turbulent structures generated behind the centerbody will be presented. It has been observed that a span-wise instability of the wake behind the centerbody is initially dominant. Further downstream, the shear-layers coalesce into a mixing wake and develop the expected large-scale coherent span-wise vortices. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L22.00002: On the effect of fractal generated turbulence on the heat transfer of circular impinging jets Tommaso Astarita, Gioacchino Cafiero, Stefano Discetti The intense local heat transfer achieved by circular impinging jets is exploited in countless industrial applications (cooling of turbine blades, paper drying, tempering of glass and metals, etc). The heat transfer rate depends mainly on the Reynolds number, the nozzle-to-plate distance and the upstream turbulence. It is possible to enhance the heat transfer by exciting/altering the large scale structures embedded within the jet. In this work turbulent energy is injected by using a fractal grid at the nozzle exit. Fractal grids can generate more intense turbulence with respect to regular grids with the same blockage ratio by enhancing the jet turbulence over different scales. Consequently, they are expected to improve the convective heat transfer. The results outline that a significant improvement is achieved (for small nozzle-to-plate distances up to 100{\%} at the stagnation point and more than 10{\%} on the integral heat transfer over a circular area of 3 nozzle diameters) under the same power input. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L22.00003: Quantification of Mixing of a Sonic Jet in Supersonic Crossflow due to Thick Turbulent Boundary Layer Interaction Tobias Rossmann, Adam Pizzaia The upstream injection surface boundary layer is shown to have a significant effect on the mixing characteristics of a sonic jet in supersonic cross flow. A circular, high-pressure, sonic jet is injected into a M$=$3.5 supersonic crossflow through different boundary layer thickness ($\delta $/D $=$ 7.5 and 1), with variable injection angles (-20 to $+$20 degrees), and variable momentum ratios (J $=$ 2, 5, and 10). Planar Laser Mie Scattering of condensed ethanol droplets is used to quantitatively image the injected fluid concentration in both the side and end views. Jet fluid concentrations PDFs are constructed to better understand the mixing dynamics. These PDFs are integrated to create mixed fluid fraction profiles that are then reduced to mixing efficiency. Mixing efficiency values are computed from different two-dimensional planes to determine if centerline mixing efficiencies are characteristic of the entire three-dimensional flow. Through these analyses, it is seen that thick boundary layers tend to marginally alter jet penetration and spread, but significantly worsen jet mixing capabilities, regardless of momentum ratio or injection angle. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L22.00004: Turbulent Mixing of an Angled Jet in Various Mainstream Conditions Kevin Ryan, Filippo Coletti, Christopher Elkins, John Eaton The angled jet in crossflow has been studied in detail with specific emphasis on the turbulent mixing of the jet fluid with the mainstream flow. The interaction of the upstream boundary layer with the jet shear layer results in complex vortex patterns that cause large mean distortion of the jet and rapid turbulent mixing. Most previous studies have been conducted in flat plate flows with little attention paid to the characteristics of the boundary layer. The present study examines the effect of mainstream geometric changes on the jet trajectory, counter-rotating vortex pair strength, and turbulent mixing. Seven cases were examined including flat plate boundary layers with three different thicknesses, adverse and favorable pressure gradient cases, and flows with concave and convex streamwise curvature. Full field, 3D mean velocity and scalar concentration fields were measured using magnetic resonance imaging (MRI) techniques in a water flow. The distortion of the streamtube initiated at the hole exit was examined for each of the seven cases. The degree of mixing was quantified by measuring the amount of mainstream fluid entrained into the jet as well as the turbulent diffusivity as a function of streamwise position. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L22.00005: Interaction of Inflow Jet and Flow Recirculation in Large Mixing Tanks Markus Vaas, Emma Thompson, Jochen Kriegseis Jet inflow into a confined box-shaped domain (tank) can induce a slow, stable recirculation flow. Once developed, these large scale structures compete with the spatial distribution of the inflow jet. The objective of the present work is to understand the underlying mechanisms which lead to this interaction/competition of the respective flow patterns. Particular emphasis is placed on the jet's self-similarity and its axial velocity decay, since both are of prime importance for the resulting entrainment along the jet borders and the overall mixing efficiency in the tank. Particle image velocimetry (PIV) measurements have been performed in a tank with turbulent inflow jets of varying jet Reynolds numbers. Coherent large-scale structures superimposed to the aforementioned mean flow fields were identified by means of proper orthogonal decomposition (POD). As such, both the meandering character of the jet core as well as fluctuating patterns of the recirculation zone have been identified. The impact of the confinement is demonstrated by a direct comparison of the POD results with the most salient free-jet patterns. Based on these insights, implications for entrainment and mixing stemming from the interaction of the jet and the recirculating flow are discussed. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L22.00006: Dynamics of spinodal decomposition in turbulent flows Federico Toschi, Roberto Benzi, Herman Clercx, David Nelson, Prasad Perlekar When a binary mixture is cooled below its critical temperature it undergoes a phase transition and the mixture separates into its individual components: this phenomenon is widely known as spinodal decomposition. The dynamics proceeds through different regimes all characterized by a coarsening of the domain size. We investigate numerically the dynamics of such a system when the mixture of immiscible fluids is stirred at the large scale and thus turbulent. Under turbulent conditions we find that the coarsening of the domains is arrested and a similarity with the physics of dilute turbulent emulsions is possible. In particular we show that the typical domain size can be estimated by means of the Kolmogorov-Hinze argument for the stability of droplets in turbulence. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L22.00007: Free shearless multi-material turbulent mixing in the presence and absence of gravity Pooya Movahed, Eric Johnsen Using a novel set-up, we perform direct numerical simulations of free shearless turbulent multi-material mixing starting from an unperturbed material interface between two fluids in an isotropic turbulent velocity field. The energy dissipation rate is matched in each fluid, such that anisotropy in the initial set-up solely comes from the density gradient. At large scales, the mixing region grows self-similarly after an initial transient period; a one-dimensional turbulence-diffusion model in conjunction with Prandtl's mixing length theory is applied to describe the growth of the mixing region. The observed growth exponent tends to 2/7, as expected for Batchelor turbulence based on energy budget arguments for large Reynolds numbers. At small scales, flow isotropy and intermittency are measured. Results suggest that a large density ratio between the two fluids is required to make the velocity field anisotropic at the Taylor microscope, while the flow remains isotropic at the Kolmogorov microscale. Results with gravity in a similar set-up that is Rayleigh-Taylor unstable will also be presented. This novel set-up allows us to investigate the role of gravity and density gradient on flow statistics separately, as opposed to traditional Rayleigh-Taylor studies in which these effects are coupled. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L22.00008: Turbulent Buoyant Flows: A Structural Approach Phares Carroll, Guillaume Blanquart In a wide variety of natural phenomena, buoyancy plays a significant role in the evolution of turbulent physics. However, there is no consensus in the literature in regards to the structural implications of the interplay between turbulence and buoyancy. In this study, two incompressible, miscible fluids with different densities are considered in the presence of gravity. In the implemented configuration, it is possible to vary independently the Reynolds and Richardson numbers, allowing for the methodical study of the interactions between buoyancy and turbulence. The simulation results are used to analyze the structural changes induced by buoyancy on the evolving turbulent field. Specifically, attention is paid to the spectral distribution of kinetic energy via calculation and analysis of spectra (energy production, dissipation, and transfer). This is done to determine where gravity (buoyancy) deposits its energy from a spectral perspective. Also, the evolution of each constituent term in the energy equation is examined and compared to results obtained from isotropic turbulent cases. Further, analysis is conducted on the alignment of the vorticity field with the direction of principle strains to identify how the turbulent structure is modified in the presence of such a body force. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L22.00009: Turbulent fountains as a model for mixing at a density interface Nigel Kaye, William Martin III Numerical results are presented for the change in flow rate in a round turbulent fountain as it rises and falls. The theoretical models of Bloomfield and Kerr (2000) are used to examine the entrainment across a density interface due to the impingement of an axisymmetric turbulent shear flow such as a plume or jet. The numerical results indicate that there are two mixing regimes, a low Froude number (weak fountain) regime in which the entrainment coefficient scales approximately on $\alpha \sim Fr^2$ and a high Froude number (forced fountain) regime in which the entrainment coefficient scales like $\alpha \sim Fr$. This is in contrast to the experimental parameterization of Kumagai (1984) which had the entrainment coefficient as $\alpha \sim Fr^3$ for low Froude numbers and $\alpha$ approaching a constant in the limit of high $Fr$. The numerical results are compared to a broad range of experimental measurements in the literature. The sensitivity of the calculated entrainment coefficients to the internal body force model and entrainment assumption is also examined. The results are discussed in relation to models for mixing efficiency in stratified flows. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L22.00010: Experimental variable-density mixing statistics Sergiy Gerashchenko, Katherine Prestridge Velocity and density statistics are studied experimentally for variable density mixing of a heavy fluid jet into air coflow at two Atwood numbers. The effect of buoyancy is found to be important for most turbulent quantities measured. The high At jet with larger Reynolds number shows reduced lateral spreading compared to the low At jet of smaller Reynolds number. Some universal features of variable density mixing are elucidated from PDFs of density and density gradients. The low Atwood number PDFs show fast and uniform mixing. High Atwood number PDFs of density have skewness toward the larger densities, indicating reduced rate of mixing of pure heavy fluid due to its inertia. This skewness is related to strong local compression events that can lead to enhanced molecular mixing. Turbulent kinetic energy decreases with distance from the jet for low Atwood number but increases for high Atwood number due to flow acceleration and generation of extra shear and turbulence. This is clearly a buoyancy-mediated effect. Statistical characteristics of mixing such as Favre-averaged Reynolds stress and its anisotropy, turbulent mass flux velocity, density-specific volume correlation, density power spectra are also examined in the near and far field from the jet. [Preview Abstract] |
Monday, November 25, 2013 5:45PM - 5:58PM |
L22.00011: Experiments on the fragmentation of a buoyant liquid volume in another liquid Maylis Landeau, Renaud Deguen, Peter Olson Buoyancy-driven fragmentation of one liquid in another immiscible liquid was a common process during the formation of the terrestrial planets. Another example of this phenomenon is the sudden release of petroleum into the ocean during the Deepwater Horizon disaster. In this study, we present experiments on the instability and fragmentation of volumes of heavier liquid released into lighter immiscible liquids. We characterize the different fragmentation regimes in parameter space. We find that, at low and intermediate Weber numbers (measuring the importance of inertia versus surface tension forces), the fragmentation regime mainly results from a competition between the growth of Rayleigh-Taylor instabilities and the roll-up of a vortex ring. At high Weber numbers, a turbulent fragmentation regime is found, and the large-scale flow behaves as a turbulent vortex ring or a turbulent thermal. An integral model based on the entrainment assumption, and adapted to buoyant vortex rings with initial momentum, is consistent with our experimental data. This indicates that the concept of turbulent entrainment is valid for non-dispersed immiscible fluids at large Weber and Reynolds numbers. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2022 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700