49 research outputs found
Entropic Multi-Relaxation Models for Simulation of Fluid Turbulence
A recently introduced family of lattice Boltzmann (LB) models (Karlin,
B\"osch, Chikatamarla, Phys. Rev. E, 2014) is studied in detail for
incompressible two-dimensional flows. A framework for developing LB models
based on entropy considerations is laid out extensively. Second order rate of
convergence is numerically confirmed and it is demonstrated that these entropy
based models recover the Navier-Stokes solution in the hydrodynamic limit.
Comparison with the standard Bhatnagar-Gross-Krook (LBGK) and the entropic
lattice Boltzmann method (ELBM) demonstrates the superior stability and
accuracy for several benchmark flows and a range of grid resolutions and
Reynolds numbers. High Reynolds number regimes are investigated through the
simulation of two-dimensional turbulence, particularly for under-resolved
cases. Compared to resolved LBGK simulations, the presented class of LB models
demonstrate excellent performance and capture the turbulence statistics with
good accuracy.Comment: To be published in Proceedings of Discrete Simulation of Fluid
Dynamics DSFD 201
Drops bouncing off macro-textured superhydrophobic surfaces
Recent experiments with droplets impacting a macro-textured superhydrophobic
surfaces revealed new regimes of bouncing with a remarkable reduction of the
contact time. We present here a comprehensive numerical study that reveals the
physics behind these new bouncing regimes and quantify the role played by
various external and internal forces that effect the dynamics of a drop
impacting a complex surface. For the first time, three-dimensional simulations
involving macro-textured surfaces are performed. Aside from demonstrating that
simulations reproduce experiments in a quantitative manner, the study is
focused on analyzing the flow situations beyond current experiments. We show
that the experimentally observed reduction of contact time extends to higher
Weber numbers, and analyze the role played by the texture density. Moreover, we
report a non-linear behavior of the contact time with the increase of the Weber
number for application relevant imperfectly coated textures, and also study the
impact on tilted surfaces in a wide range of Weber numbers. Finally, we present
novel energy analysis techniques that elaborate and quantify the interplay
between the kinetic and surface energy, and the role played by the dissipation
for various Weber numbers
Fluid-Structure Interaction with the Entropic Lattice Boltzmann Method
We propose a novel fluid-structure interaction (FSI) scheme using the
entropic multi-relaxation time lattice Boltzmann (KBC) model for the fluid
domain in combination with a nonlinear finite element solver for the structural
part. We show validity of the proposed scheme for various challenging set-ups
by comparison to literature data. Beyond validation, we extend the KBC model to
multiphase flows and couple it with FEM solver. Robustness and viability of the
entropic multi-relaxation time model for complex FSI applications is shown by
simulations of droplet impact on elastic superhydrophobic surfaces
High speed flows with Particles on Demand: Boundary Conditions
The particles on demand (PonD) method is a new kinetic theory model that
allows for simulation of high speed compressible flows. While standard
Lattice-Boltzmann is limited by a fixed reference frame, significantly reducing
the range of applicable of Mach numbers, PonD takes advantage of adaptive
reference frames to get rid of the restrictions of standard LB and is able to
simulate flows at high speeds and with large temperature gradients. Previously,
PonD has been shown to be a viable alternative for simulation of flows with
strong discontinuities and for detonation modelling. However, treatment of
flows with complex boundaries has been lacking. Here, we present PonD augmented
with a non-equilibrium extrapolation based boundary condition. We present
several compressible test cases such as shock-vortex interaction in the
Schardin's Problem and supersonic flow over a two-dimensional cylinder at Mach
numbers up to 5. We observe that the results agree well with literature, paving
the way for a kinetic theory based approach for simulating compressible flows
in realistic scenarios
Simplification of reactive systems by the Relaxation Redistribution Method (RRM)
We review the novel Relaxation Redistribution
Method (RRM) for the construction of accurate discrete
approximations of slow invariant manifolds. Both formula-
tions (global and local) are discussed. A fully adaptive local
formulation, with a simple implementation in any dimension,
is worked out and illustrated with an example of autoignition
of the hydrogen-air mixture
Exploring shock-capturing schemes for Particles on Demand simulation of compressible flows
In this exploratory study, we apply shock-capturing schemes within the
framework of the Particles on Demand kinetic model to simulate compressible
flows with mild and strong shock waves and discontinuities. The model is based
on the semi-Lagrangian method where the information propagates along the
characteristics while a set of shock-capturing concepts such as the total
variation diminishing and weighted essentially non-oscillatory schemes are
employed to capture the discontinuities and the shock-waves. The results show
that the reconstruction schemes are able to remove the oscillations at the
location of the shock waves and together with the Galilean invariance nature of
the Particles on Demand model, stable simulations of mild to extreme
compressible benchmarks can be carried out. Moreover, the essential numerical
properties of the reconstruction schemes such as their spectral analysis and
order of accuracy are discussed
Quasiequilibrium lattice Boltzmann models with tunable bulk viscosity for enhancing stability
Taking advantage of a closed-form generalized Maxwell distribution function [ P. Asinari and I. V. Karlin Phys. Rev. E 79 036703 (2009)] and splitting the relaxation to the equilibrium in two steps, an entropic quasiequilibrium (EQE) kinetic model is proposed for the simulation of low Mach number flows, which enjoys both the H theorem and a free-tunable parameter for controlling the bulk viscosity in such a way as to enhance numerical stability in the incompressible flow limit. Moreover, the proposed model admits a simplification based on a proper expansion in the low Mach number limit (LQE model). The lattice Boltzmann implementation of both the EQE and LQE is as simple as that of the standard lattice Bhatnagar-Gross-Krook (LBGK) method, and practical details are reported. Extensive numerical testing with the lid driven cavity flow in two dimensions is presented in order to verify the enhancement of the stability region. The proposed models achieve the same accuracy as the LBGK method with much rougher meshes, leading to an effective computational speed-up of almost three times for EQE and of more than four times for the LQE. Three-dimensional extension of EQE and LQE is also discussed