Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session E11: Turbulence: Mixing I |
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Chair: Gianluca Iaccarino, Stanford University Room: 314 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E11.00001: Direct numerical simulation of scalar transport in turbulent flows over progressive water waves Di Yang, Lian Shen The transport of passive scalars in turbulent flows over progressive water waves is studied using direct numerical simulation. A hybrid pseudo-spectral and finite-difference scheme on a wave-surface-fitted grid is used to resolve the flow and scalar fields near the wave surface. Various Schmidt numbers, wave phase speeds, and wave slopes are considered, and their effects on the instantaneous and statistics of scalar concentration are investigated. The scalar field shows apparent wave-phase dependence induced by the wave surface curvature and motions. For slow waves, the scalar field is mainly affected by the wave surface geometry with similarities with the case of a stationary wavy boundary. Differently, when the wave phase speed is large, the wave motion has strong influence on the scalar field. Budget for the intensity and flux of scalar concentration fluctuations are also studied. The analysis provides useful insight into the modeling of scalar transport over progressive waves. [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E11.00002: Turbulent mixing in a river confluence controls the fate of contaminated sediments Cristian Escauriaza, Christian Gonzalez, Paula Guerra, Pablo Pasten, Gonzalo Pizarro A river confluence in a 40-degree angle and a high momentum ratio (M=12.8) mixes water with different chemical compositions favoring the formation of arsenic-rich particles. The dynamics of turbulent coherent structures at the mixing interface between both streams controls the transport and fate of these particles. Motivated by these field measurements, we integrate an experimental investigation in the laboratory with numerical computations based on the detached-eddy simulation (DES) approach. Streamwise helical vortices and shear layers have a significant effect on the size and deposition of the particles and their interaction with bed sediment. Through this study we provide new insights on the dynamics of coherent structures in confluences and the influence of turbulent mixing on chemical processes and contaminant fate in aquatic environments. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E11.00003: Cumulative compressibility effects on population dynamics in turbulent flows Prasad Perlekar, David R. Nelson, Roberto Benzi, Federico Toschi Bacteria and plankton populations living in oceans and lakes reproduce and die under the influence of turbulent currents. Turbulent transport can interact in a complex way with the dynamics of populations because the typical reproduction time of microorganism is well within the inertial range of turbulence time scales. We quantitatively investigate the effect of flow compressibility on the dynamics of populations. While a small compressibility can be induced by several physical mechanisms, like density mismatch or the finite size of microorganisms with respect to the fluid turbulence, its effect on the the carrying capacity (average population concentration) of the ecosystem can be dramatic. We show that a tiny compressibility can produce a finite effect on the carrying capacity, this is due to an integrated effect made possible by the long replication times of the organisms with respect to turbulence time scales. We also present a full statistical quantification of the fluctuations of the population concentration that leads to a data collapse over a broad range in parameter space. [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E11.00004: Effect of LES models on the entrainment of a passive scalar in a turbulent planar jet Diogo Chambel Lopes, Carlos da Silva, Ricardo Reis, Venkat Raman Direct and large-eddy simulations (DNS/LES) of turbulent planar jets are used to study the role of subgrid-scale models in the integral characteristics of the passive scalar mixing in a jet. Specifically the effect of subgrid-scale models in the jet spreading rate and centreline passive scalar decay rates are assessed and compared. The modelling of the subgrid-scale fluxes is particularly challenging in the turbulent/nonturbulent (T/NT) region that divides the two regions in the jet flow: the outer region where the flow is irrotational and the inner region where the flow is turbulent. It has been shown that important Reynolds stresses exist near the T/NT interface and that these stresses determine in part the mixing and combustion rates in jets. The subgrid scales of motion near the T/NT interface are far from equilibrium and contain an important fraction of the total kinetic energy. Model constants used in several subgrid-scale models such as the Smagorinsky and the gradient models need to be corrected near the jet edge. The procedure used to obtain the dynamic Smagorinsky constant is not able to cope with the intermittent nature of this region. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E11.00005: An advection-diffusion model for the dispersion in quasi two-dimensional steady turbulent jets Julien R. Landel, C.P. Caulfield, Andrew W. Woods The study of turbulent jets in relatively enclosed geometries is relevant to many chemical engineering processes. Predicting the concentration of chemical reactants in time and space requires a good understanding of the jet dynamics. We have considered experimentally and theoretically the behaviour of liquid jets in a quasi-Hele-Shaw cell, where the jets are constrained in a narrow gap whose width is two orders of magnitude smaller than the length-scales of the other two flow dimensions. In this configuration, the dynamics shown by the jets is very rich. Detailed examinations of instantaneous structures of the flow reveal a high-speed sinuous core at the centre of the jet and large vortical structures on each side, which we analyse quantitatively using a variety of techniques (particle image velocimetry and dye experiments). These structures have a large impact on the mixing and dispersion properties of the jet. We propose a one-dimensional advection-diffusion model to account for the vertical dispersion in the jet. The diffusion coefficient assumed in the model is based on our understanding of the large-scale structures of these jets. The model is solved analytically using a similarity form in the case of a finite-volume release of tracers in the jet. The theoretical predictions and the experimental measurements show very good agreement. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E11.00006: Effect of background turbulence on an axisymmetric turbulent jet Laurent Mydlarski, Babak Khorsandi, Susan Gaskin An experimental investigation into the effect of different levels of background turbulence on the dynamics of a momentum-driven, axisymmetric, turbulent jet was conducted. An approximately homogenous isotropic turbulent background with zero mean flow was generated using a random jet array. The velocities were measured by acoustic Doppler velocimetry and flying hot-film anemometry. The results show that the mean axial velocities decay faster in the presence of background turbulence, while the mean radial velocities increase, especially close to the edges of the jet. The RMS velocity of the jet issuing into the turbulent background also increased compared to that of a jet emitted into a quiescent background. In addition, the width of the jet increased in the presence of the background turbulence. The mass flow rate of the jet decreased in the presence of the background turbulence, from which it can be inferred that the entrainment into the jet is reduced. It is hypothesized that, in the presence of background turbulence, large-scale engulfment (and not small-scale nibbling) is expected to be the main entrainment mechanism. [Preview Abstract] |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E11.00007: Uncertainty of RANS mixing model prediction for an underexpanded jet in a supersonic cross flow Catherine Gorle, Gianluca Iaccarino The complex phenomena involved in scramjet propulsion can be investigated using RANS simulations, but the simulations do not fully represent all the physics involved. The mixing of fuel and air inside the supersonic combustion chamber is one of the critical processes that requires modeling. The goal of the present work is the development of an uncertainty model for the RANS modeling of mixing that can be used to characterize the safety and operability limits of scramjet engines. To this end, RANS simulations of an underexpanded jet in a supersonic cross flow are considered. Experimental data and an LES database are available from literature. The uncertainties in the RANS simulations are related to the models for the Reynolds stresses and the mixture fraction fluxes. The sensitivity of the model outcome to all sources of uncertainty in these models is characterized and quantified by introducing perturbations in the Reynolds stresses and the mixture fraction fluxes. Through comparison of the RANS and LES results it is attempted to quantify the required range of perturbations to correctly represent the model uncertainties. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E11.00008: Redesigning a Film-Cooled Airfoil Trailing Edge using MRI Techniques Michael Benson, Christopher Elkins, John Eaton Trailing edges of modern gas turbine blades are film cooled through cutback slots on the airfoil pressure surface. The slots are spanwise divided, forming rectangular wall jets separated by tapered lands. The 3D wall jets mix rapidly with the mainstream flow reducing the cooling effectiveness. Experiments were conducted to document the 3D mean velocity and coolant concentration fields on a baseline configuration using Magnetic Resonance Imaging (MRI) in a water flow with Re = 110,000 based on airfoil chord length. Critical flow features causing rapid mixing were identified: a separation bubble behind the slot lip, and a pair of strong longitudinal vortices formed just downstream of the slot breakout. The geometry was modified to improve film cooling surface effectiveness obtained from the concentration field. The first redesign modified the slot lip and land shapes to minimize the slot lip separation bubble size and reduce 3D effects. The other redesigns modified the land shape to reduce the strength of the longitudinal vortices. These latter two designs produced a substantial reduction in the mixing rate of the coolant jet with the mainstream flow, improving the cooling system performance. The highly detailed concentration and velocity fields available with MRI-based experiments can be used to understand the flow physics and derive significant system improvements. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E11.00009: Three Dimensional Transient Turbulent Simulations of Scramjet Fuel Injection and Combustion Marwane Bahbaz Scramjet is a propulsion system that is more effective for hypersonic flights (M$>$5). The main objective of the simulation is to understand both the mixing and combustion process of air flow using hydrogen fuel in high speed environment s. The understanding of this phenomenon is used to determine the number of fuel injectors required to increase combustion efficiency and energy transfer. Due to the complexity of this simulation, multiple software tools are used to achieve this objective. First, Solid works is used to draw a scramjet combustor with accurate measurements. Second software tool used is Gambit; It is used to make several types of meshes for the scramjet combustor. Finally, Open Foam and CFD++ are software used to process and post process the scramjet combustor. At this stage, the simulation is divided into two categories. The cold flow category is a series of simulations that include subsonic and supersonic turbulent air flow across the combustor channel with fuel interaction from one or more injectors'. The second category is the combustion simulations which involve fluid flow and fuel mixing with ignition. The simulation and modeling of scramjet combustor will assist to investigate and understand the combustion process and energy transfer in hypersonic environment. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E11.00010: Stochastic modeling of statistically unsteady turbulent mixing A.T. Qamar, M. Cadjan, S.I. Abarzhi Rayleigh-Taylor and Richtmyer-Meshkov turbulent mixing are statistically unsteady processes. Their dynamics combines coherence and randomness, and their mean values and fluctuations are both time-dependent. These turbulent processes have a number of symmetries and are characterized by a set of invariant measures [EPL 91, 12867]. Employing these invariant measures, we developed a stochastic model for Rayleigh-Taylor and Richtmyer-Meshkov turbulent mixing in the case of sustained, time-dependent and impulsive acceleration. For the flow quantities, the effect of fluctuations on the mean values is studied and their statistical properties are analyzed. Requirements for statistical quality of experimental and numerical data are outlined. Mechanisms of mitigation and control of turbulent mixing processes are proposed. Their implementation in experiments and simulations is discussed. [Preview Abstract] |
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