| 1. |
- Fistler, Marco, 1989, et al.
(författare)
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A new LES subgrid-scale approach for turbulence modulation by droplets
- 2020
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Ingår i: ICLASS 2018 - 14th International Conference on Liquid Atomization and Spray Systems. ; 14
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Konferensbidrag (refereegranskat)abstract
- We present a new modelling approach for turbulence modulation by droplets on the subgrid-scale (SGS) level of Large-Eddy-Simulations (LES). Many SGS models exist for the effect of gas phase SGS on the droplet phase, but very few for the mechanisms vice versa on the turbulent intensity of the gas phase. The reasons are a lack of physical understanding and limited computational resources for extensive DNS studies. To address both problems a dimension-reduced and consequently less costly model, namely One-Dimensional- Turbulence (ODT), is used to gather information about this specific flow phenomena. ODT is a stochastic tool simulating turbulent flows along a notional 1D line of sights. For modeling the turbulent advection instantaneous maps are applied to the line which represent the effect of individual eddies on property fields and the dispersed phase. After validating the general test case of a droplet-laden shear flow against DNS data, a concept is presented on how to gather turbulence modulation for several parameter ranges in a data base and how to make them accessible on the flight for LES. The three most significant parameters, the unladen flow Reynolds number, the droplet loading and the particle momentum number, are chosen to construct an efficient data base.
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| 2. |
- Fistler, Marco, 1989
(författare)
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Turbulence modulation effects caused by small droplets using one-dimensional-turbulence
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents a stochastic model to study turbulence modulation effects on gas phases caused by small droplets or more generally speaking particles to achieve a better understanding about the physics and with the aim to provide data for a subgrid-scale (SGS) model for Large-eddy-simulations. The one-dimensional-turbulence (ODT) model addresses one of the major issues for multiphase flow simulations, namely computational costs. It is a dimension-reduced model resolving all turbulent time and length scales and reaching parameter ranges, which are inaccessible for Direct-Numerical-Simulations (DNS). ODT is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps which represent the effect of individual turbulent eddies on property fields. For an efficient investigation of turbulence modulation effects, ODT has been extended in this thesis in many ways. First, the Lagrangian particle tracking method developed by Schmidt et al. was modified for spatial, cylindrical flow simulations. Therefore, the two-way coupling mechanism was extended as well. This case serves to study the overall influence of particles on a jet configuration, which conforms with the droplet-laden flow in the dilute region of a spray. Here, the most significant effects are expected. Secondly, a concept for developing a SGS model is presented. Based on this concept, ODT was modified to capture two canonical test cases of stationary, forced isotropic turbulence (HIT) and homogeneous shear turbulence (HST). For this purpose, a forcing scheme that maintains statistical stationarity and a new energy redistribution mechanism during the eddy events are introduced. The latter enables ODT to predict anisotropic turbulent structures. ODT is validated against several data sets of DNS studies and showed its capability to access parameter ranges beyond previous limits. It turned out to have a lot of potential to contribute to a SGS closure of LES for turbulence modulation caused by small droplets.
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| 3. |
- Fistler, Marco, 1989, et al.
(författare)
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Turbulence modulation in particle-laden stationary homogeneous isotropic turbulence using one-dimensional turbulence
- 2020
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Ingår i: Physical Review Fluids. - 2469-990X. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- Turbulence modulation in particle-laden stationary homogeneous isotropic turbulence is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, ODT is extended in two ways. First, a forcing scheme that maintains statistical stationarity is introduced. Unlike direct numerical simulation (DNS) of forced turbulence, the ODT framework accommodates forcing that is not directly coupled to the momentum equation. For given forcing the ODT energy dissipation rate is therefore the same in particle-laden cases as in the corresponding single-phase reference case. Second, previously implemented one-way-coupled particle phenomenology is extended to two-way coupling using the general ODT methodology for flow modulation through interaction with any specified energy and momentum sources and sinks. As in a DNS comparison case for Re-lambda = 70, turbulence modulation is diagnosed primarily on the basis of the fluid-phase kinetic-energy spectrum. Because ODT involves subprocesses with straightforward physical interpretations, the ODT mechanisms of particle-induced turbulence modulation are clearly identified and they are plausibly relevant to particleladen Navier-Stokes turbulence. ODT results for the ratio of particle-phase and fluid-phase kinetic energies as a function of particle Stokes number and mass loading are reported for the purpose of testing these predictions in the future when these quantities are evaluated experimentally or using DNS.
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| 4. |
- Fistler, Marco, 1989, et al.
(författare)
-
Turbulence modulation in particle-laden stationary homogeneous shear turbulence using one-dimensional turbulence
- 2020
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Ingår i: Physical Review Fluids. - 2469-990X. ; 5:12
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Tidskriftsartikel (refereegranskat)abstract
- Turbulence modulation in particle-laden stationary homogeneous shear turbulence (HST) is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, an ODT formulation previously used to study turbulence modulation in forced homogeneous isotropic turbulence (HIT) is extended, so that the model emulates the anisotropic character of HST and, potentially, anisotropic turbulence in general. This is done by limiting the kinetic-energy redistribution during an eddy event to an exchange involving two velocity components, where the three possible choices of the omitted component define three eddy types whose relative likelihoods control the anisotropy. Comparisons of ODT and direct-numerical-simulation results with reference to signatures of turbulence modulation are the basis of a broader ODT parameter study that is reported. Owing to the reduced dimensionality of ODT, it is found that the fidelity of the model for single-phase HST does not extend to particle effects on flow anisotropy, but for quantities averaged over components, parametric trends are captured. The consistent approach to case comparisons that was introduced in the HIT study to evaluate sensitivities to particle-phase parameters in a given flow configuration is extended here to a cross-comparison of HST and HIT model results, and its efficacy is again confirmed. The results provide an overall characterization of the potential for ODT to support the incorporation of particle-induced turbulence modulation into subgrid-scale closures of large-eddy simulations.
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