DNS / LES Studies of Turbulent Flows Based on the Cumulant Lattice Boltzmann Approach
IRMB/TU Braunschweig (Germany)
Local Project ID:
HPC Platform used:
Hermit of HLRS
In many industrial and environmental problems turbulent flows over porous surfaces are encountered which also penetrate the porous medium to different extents. Although there is a wealth of literature on macroscopic models of such phenomena which do not take the pore scale explicitly into account, these approaches typically require some additional transport coefficients to match experimentally obtained statistics for mass, momentum and energy transport across such interfaces.
In this project a team of scientists of the Technische Universität Braunschweig conducts Direct Navier-Stokes (DNS) and Large Eddy Simulation (LES) computations of turbulent flows which explicitly take into account specific pore scale geometries obtained from computer tomography imaging and do not use any explicit turbulence modeling.
In this first part of the project the researchers conduct validation studies for two canonical turbulent flows, i.e. flow around a plate and flow in a porous channel. Subsequently, they compare simulation results of turbulent flows over a porous sand and to experimental results and demonstrate the validity of this approach. Finally this approach is discussed to address evaporation processes on a pore scale which is based on a separation of time-scales. The newly developed cumulant Lattice Boltzmann scheme implemented as part of this research Code VirtualFluids shows a favorable behavior with respect to parallelization efficiency as well as to numerical stability and accuracy.
The numerical prediction of aeroacoustic properties of porous material inlays as part of wing profiles in combination with their aerodynamic performance facilitates the design of suitable porous media with specific porosity and permeability properties as well as the design of optimal coverage locations. As the pore scale of porous media under consideration is several orders of magnitude below the wing scale, the scientists are faced with a multi-scale problem which cannot be solved by a single-scale model bridging all relevant length and time scales. Thus CFD models addressing mean flow properties such as RANS (Reynolds-averaged Navier-Stokes) models are favorable for the wing scale aerodynamics and DNS simulations are appropriate to resolve the fluid dynamics inside the porous medium and its turbulent boundary layer.
For this scale, the researchers utilize a recently developed weakly compressible CFD (computational fluid dynamics) model based on the Cumulant Lattice Boltzmann approach which is described below. As a first validation study towards the real world problem, the scientists successfully conducted a massively parallel highly resolved fully turbulent flow simulation over a plate which has been partially covered with a porous inlay represented by a pore scale resolved geometric model based on realistic tomography scans which compare well to the resulting experimental boundary layer profiles. The numerical setup contains more than 4 billion degrees of freedom.
The quantification of soil evaporation and of soil water content dynamics near the soil surface are critical in the physics of land-surface processes on many scales and are dominated by multi-component and multi-phase mass and energy fluxes between the ground and the atmosphere. Although it is widely recognized that both liquid and gaseous water movement are fundamental factors in the quantification of soil heat flux and surface evaporation, their computation has only started to be taken into account using simplified macroscopic models.
As the flow field over the soil can be safely considered as turbulent, it would be natural to study the detailed transient flow dynamics by means of DNS / LES where the three-dimensional flow field is resolved down to the laminar sub-layer. Yet this requires very fine resolved meshes allowing a grid resolution of at least one order of magnitude below the typical grain diameter of the soil under consideration.
In order to gain reliable turbulence statistics, up to several hundred eddy turnover times corresponding of more than ten seconds have to be simulated. Yet, the time scale of the receding saturated water front dynamics in the soil is on the order of hours. Thus the researchers are faced with the task of solving a transient turbulent flow problem including the advection-diffusion of water vapor over the soil-atmospheric interface represented by a realistic tomographic reconstruction of a real porous medium taken from laboratory probes.
Research Team & Contact Information:
Prof. Dr.-Ing. habil. Manfred Krafczyk, Dr. rer. nat. Martin Geier, Kostyantyn Kucher, Ying Wang
Institut für rechnergestützte Modellierung im Bauingenieurwesen (IRMB) der Technischen Universität Braunschweig
Pockelstraße 3, D-38106 Braunschweig/Germany