Browsing by Subject "Large-eddy simulation"

Now showing 1 - 4 of 4
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    Euler-Lagrangian simulations of turbulent bubbly flow.
    (2011-03) Mattson, Michael David
    A novel one-way coupled Euler-Lagrangian approach, including bubble-bubble collisions, coalescence and variable bubble radius, was developed in the context of simulating large numbers of cavitating bubbles in complex geometries using direct numerical simulation (DNS) and large-eddy simulation (LES). This dissertation i) describes the development of the Euler-Lagrangian approach, ii) outlines the novel bubble coalescence model derived for this approach and iii) describes simulations performed of bubble migration in a turbulent boundary layer, bubble coalescence in a turbulent pipe ow and cavitation inception in turbulent flow over a cavity. The coalescence model uses a hard-sphere collision model is used and determines coalescence stochastically. The probability of coalescence is computed from a ratio of coalescence timescales, which are dynamically determined from the simulation. Coalescence in a bubbly, turbulent pipe ow (Re#28; = 1920) in microgravity was simulated with conditions similar to experiments by Colin et al. [1] and excellent agreement of bubble size distribution was obtained. With increasing downstream distance, the number density of bubbles decreases due to coalescence and the average probability of coalescence decreases due to an increase in overall bubble size. The Euler-Lagrangian approach was used to simulate bubble migration in a turbulent boundary layer (420 < Re#18; < 1800). Simulation parameters were chosen to match Sanders et al. [2], although the Reynolds number of the simulation is lower than the experiment. The simulations show that bubbles disperse away from the wall as observed experimentally. Mean bubble diffusion and profiles of bubble concentration are found to be similar to the passive scalar results, except very near the wall. The carrier-fluid acceleration was found to be the reason for moving the bubbles away from the wall. The one-way coupled Euler-Lagrangian approach was applied to simulate the experiment of cavitating turbulent ow over a cavity by Liu and Katz [3]. The classical Rayleigh-Plesset equation is integrated using adaptive time-stepping to accurately and efficiently solve for the change of the bubble radius over time. The one-way coupled Euler-Lagrangian model predicts cavitation inception at the trailing edge of the cavity and also in the vortices shed from the leading edge, in qualitative agreement with experiment.
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    LES of turbulent cavitating flows using the homogeneous mixture model
    (2020-09) Bhatt, Mrugank
    The objective of this dissertation is to develop LES (large-eddy simulation) capabilities to study cavitation in complex hydrodynamic geometries. A fully-compressible homogeneous mixture model with a finite rate mass transfer is used in the simulations. The ability of the homogeneous mixture approach to capture resolved small-scale vapor bubbles is evaluated by a vapor bubble collapse problem. The effects of physical length scale, surface tension, driving pressure, and dimensionality of the problem are assessed using the parametric study. The finite rate effects of the cavitation model are discussed using the non-dimensional parameters and compared to the flow advection time scales. The expression for the finite rate mixture speed of sound is derived. Partial cavitation over incipient, transitory, and periodic regimes in the experimental sharp wedge configuration of Ganesh et. al. (2016) is investigated. The vapor void fractions obtained from LES shows very good agreement with X-ray measurements in each of the regimes. Physical mechanisms of cavity transition, both re-entrant jet and bubbly shock waves are captured in the LES. Conditions favoring the formation of either the re-entrant jet or the bubbly shock waves are studied through a detailed analysis of streamline curvature, vapor production, and vorticity transport. Flow over a five-bladed marine propeller is studied at design conditions. The assessment of propeller shaft orientation, numerical dissipation, the pressure drop in vortex cores, free-stream nuclei, and grid resolution revealed that the propeller performance is sensitive to the free-stream nuclei content, lower values showing a better comparison to the experiments. A numerical approach based on the preconditioning and the DTS is proposed to address the acoustic stiffness; thereby, enabling the low free-stream nuclei calculations. The novelty of the method lies in the application of preconditioning to a fully-compressible cavitation solver; where the characteristic-based filtering is modified based on the all-speed Roe-type scheme in addition to the traditional time-derivative matrix. The results are demonstrated for the unsteady flow over a cylinder under wetted and cavitation inception conditions, and the LES of low over a propeller under wetted conditions.
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    A stochastic particle method for the investigation of turbulence/chemistry interactions in large-eddy simulations of turbulent reacting flows
    (2013-12) Ferrero, Pietro
    The main objective of this work is to investigate the effects of the coupling between the turbulent fluctuations and the highly non-linear chemical source terms in the context of large-eddy simulations of turbulent reacting flows. To this aim we implement the filtered mass density function (FMDF) methodology on an existing finite volume (FV) fluid dynamics solver. The FMDF provides additional statistical sub-grid scale (SGS) information about the thermochemical state of the flow - species mass fractions and enthalpy - which would not be available otherwise. The core of the methodology involves solving a transport equation for the FMDF by means of a stochastic, grid-free, Lagrangian particle procedure.Any moments of the distribution can be obtained by taking ensemble averages of the particles. The main advantage of this strategy is that the chemical source terms appear in closed form so that the effects of turbulent fluctuations on these terms are already accounted for and do not need to be modeled.We first validate and demonstrate the consistency of our implementation by comparing the results of the hybrid FV/FMDF procedure against model-free LES for temporally developing, non-reacting mixing layers. Consistency requires that, for non-reacting cases, the two solvers should yield identical solutions. We investigate the sensitivity of the FMDF solution on the most relevant numerical parameters, such as the number of particles per cell and the size of the ensemble domain. Next, we apply the FMDF modeling strategy to the simulation of chemically reacting, two- and three-dimensional temporally developing mixing layers and compare the results against both DNS and model-free LES. We clearly show that, when the turbulence/chemistry interaction is accounted for with the FMDF methodology, the results are in much better agreement to the DNS data. Finally, we perform two- and three-dimensional simulations of high Reynolds number, spatially developing, chemically reacting mixing layers, with the intent of reproducing a set of experimental results obtained at the California Institute of Technology. The mean temperature rise calculated by the hybrid FV/FMDF solver, which is associated with the amount of product formed, lies very close to the experimental profile. Conversely, when the effects of turbulence/chemistry coupling are ignored, the simulations clearly over predict the amount of product that is formed.