Jacek Rokicki

Adjoint Lattice Boltzmann for topology optimization on multi-GPU architecture

In this paper we present a topology optimization technique applicable to a broad range of flow design problems. We propose also a discrete adjoint formulation effective for a wide class of Lattice Boltzmann Methods (LBM). This adjoint formulation is used to calculate sensitivity of the LBM solution to several type of parameters, both global and local. The numerical scheme for solving the adjoint problem has many properties of the original system, including locality and explicit time-stepping. Thus it is possible to integrate it with the standard LBM solver, allowing for straightforward and efficient parallelization (overcoming limitations typical for discrete adjoint solvers). This approach is successfully used for the channel flow to design a free-topology mixer and a heat exchanger. Both resulting geometries being very complex maximize their objective functions, while keeping viscous losses at acceptable level.

Investigation of blending-function-based overlapping-grid technique for compressible flows

The paper presents the development and investigation of a blending-function-based overlapping-grid technique for compressible Euler flows. In this technique, local solutions, obtained on overlapping subdomains, are combined into a global function using blending functions as weights. This global function is subsequently used to impose new boundary conditions on interfaces. The local solutions are obtained using the Riemann solver and an implicit-unfactored method, for spatial and temporal discretization, respectively. Several numerical experiments are carried out for flows around circular-arc and two-element airfoils, and demonstrate that the blending function approach improves the numerical convergence for grid configurations containing multiple overlaps.

Dynamic contact of droplet with superhydrophobic surface in conditions favour icing

Flight like droplet impact with superhydrophobic substrate in conditions favour icing is discussed in this work. Test stand with fast camera and equipment eligible to obtain temperatures and humidity at different ranges, lead to results which can prove, that superhydrophobic surface might be good ice repellent substrate. The influence of air humidity on droplet freezing was confirmed.

Hybrid RANS/LES of Flow in a Rib-Roughened Rotating Channel

The aim of the present study is to verify the ability of a hybrid RANS/LES model, based on the newest version of the RANS k-ω model of Wilcox (2008), to reproduce the flow in a rib-roughened rotating channel. The numerical results are compared to experimental data by Coletti and Arts (2011), and Coletti et al. (2012). Results obtained with the basic RANS k-ω model are analysed as well, with and without the correction term for frame rotation by Hellsten (1998). It is demonstrated that the hybrid RANS/LES k-ω model produces accurate results for the rotating ribbed duct flow. However, the RANS model with the correction term for frame rotation produces quite good results as well for mean velocity profiles, although not as good as with the hybrid model. Further, the RANS model has lower quality in the representation of the small-scale fluctuations.

Anisotropic Mesh Adaptation in the Presence of Complex Boundaries

Grid adaptation is a very powerful tool for optimizing CFD calculations. However typical isotropic adaptation used for 3D flows still may result in excessive number of elements. This is especially the case when lower+dimensional features of the flow are dominant (boundary layers, shockwaves). The present paper investigates anisotropic adaptation for flows with complex boundaries. Of particular interest is the automatic adaptation for laminar / turbulent boundary layer for which case new error indicator is proposed.

Parallel performance of overlapping mesh technique for compressible flows

Abstract The paper presents parallelization of a blended-function-based overlapping mesh technique and its implementation in the case of inviscid compressible flows around complex geometries. The overlapping grid technique is based on the partition of the domain into overlapping subdomain. The solution in each subdomain is obtained by an implicit algorithm and a characteristics-based method, while the blended functions are implemented in order to allow the accurate interpolation of the boundary conditions in the case of multiple overlaps. The above are applied to flows around multi-element airfoil geometries and results from parallel computations on CRAY Server CS6400 and CRAY T3E computing platforms are presented. The load-balancing and parallel performance issues are investigated for several grid systems. A simple model has also been developed for describing the parallel efficiency of the simulations and it is shown that the model results agree well with the parallel experiments.

Anisotropic Adaptation for Simulation of High-Reynolds Number Flows Past Complex 3D Geometries

The paper presents the anisotropic adaptation algorithm applied to simulations of high-Reynolds turbulent compressible flows past complex 3D geometries. The adaptive algorithm relies on anisotropic grid-cell spacing definition provided by the error estimator in a form of a metric field. The error estimator is based on the Hessian of the solution with additional terms used to improve the grid spacing in the regions of high viscous shear forces. The adaptive algorithm is used for the two testcases the Onera M6 wing and the High Lift Prediction Workshop 1 trap wing.

Investigation of parallel efficiency of an adaptive flow solver

Abstract Parallel efficiency, in a domain decomposition based approach, strongly depends on a partitioning quality. For an adaptive simulation partitioning quality is lost due to the dynamic modification of the computational mesh. Maintaining high efficiency of parallelization requires rebalancing of the numerical load. This paper presents numerical experiment with adaptive and dynamically load balanced flow application. It is shown that through a relatively inexpensive process of repartitioning high parallel efficiency is maintained.

Parallel Performance of Chimera Overlapping Mesh Technique

In the paper we present the Chimera overlapping mesh technique applied to the solution of compressible flow problems. This technique is used to facilitate the grid generation in complex geometries allowing at the same time for natural parallelisation of the problem. The presented algorithm is particularly suitable for cases with large number of meshes overlapping in almost arbitrary manner (including multiple overlaps). The parallel implementation is based on the PVM approach.

Predictions of round impinging jet heat transfer with two k‐ω hybrid RANS/LES models

Purpose – Applicability of two k‐ω hybrid RANS/LES and a k‐ω RANS models is studied for simulation of round impinging jets at nozzle‐plate distance H/D=2 with Reynolds number 70000, H/D=2 with Reynolds number 5000 and H/D=10 with Reynolds number 5000 (D is the nozzle exit diameter). The aim is to verify two concepts of unified hybrid RANS/LES formulations, one of DES (Detached Eddy Simulation) type and one of LNS (Limited Number Scales) type in analysis of impinging jet flow and heat transfer. The grid resolution requirements are also discussed.Design/methodology/approach – The simulations are performed with two k‐ω based hybrid RANS/LES models of very different nature, one of DES type and one of LNS type, and the RANS k‐ω model. For the lower Reynolds number (5000), also dynamic Smagorinsky LES is done. Both hybrid model formulations converge to the same RANS k‐ω model in the near‐wall region and have the same Smagorinsky limit on fine isotropic grids in the LES mode of the hybrid models.Findings – With ...