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Item type:Publication, Access status: with files , An all-resolution cumulant-based lattice Boltzmann approach for simulating high Reynolds number engineering shear flows(2024); The thesis addresses the development of a lattice Boltzmann fluid solver for the accurate and efficient simulation of turbulent flows at high Reynolds numbers. The central goal is a combined treatment of turbulence and collision models. Building on a comprehensive study of frequently used collision operators, which are fundamental for modeling the flow physics, the advancement of a particular model based on a cumulant approach and its validation is performed. To comply with the flexible resolution requirements associated with simulating technical flows, an optional wall model is supplemented to the final formulation.Publicationtype: Doctoral Thesis109 473 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Scrutinizing dynamic cumulant lattice Boltzmann large eddy simulations for turbulent channel flowsThis technical paper outlines the predictive performance of a recently published dynamic cumulant lattice Boltzmann method (C-LBM) to model turbulent shear flows at all resolutions. Emphasis is given to a simple strategy that avoids a frequently observed velocity overshoot phenomenon near rigid walls when combining the C-LBM with an all-resolution (universal) wall function. The examples included are confined to turbulent channel flow results for a variety of friction Reynolds numbers within 180 and 50,000, obtained on a sequence of isotropic, homogeneous grids that feature non-dimensional lattice spacings using inner coordinates from 4 to 2200. The results indicate that adjusting the near-wall distance of the first fluid node, i.e., the intersection of the wall with the first lattice edge, to the resolution provides a reasonably simple, robust, and accurate supplement to the all-resolution C-LBM approach. The investigated wall function/C-LBM combination displays a remarkable predictive performance for all investigated resolutions.Publicationtype: Journal Article77 203 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Scale-resolving turbulent channel flow simulations using a dynamic cumulant lattice Boltzmann methodThis paper reports the assessment of a modified cumulant lattice Boltzmann method in turbulent channel flows. The suggested approach is of interest for its resolution-spanning capabilities, which are scrutinized for grid resolutions between 4 and 430 wall units. With the emphasis on a recently published parameterized cumulant collision operator, we suggest a resolution-sensitive regularization supplemented by a dynamic subgrid-scale model based upon third-order cumulant expressions, which seamlessly vanishes for direct numerical simulations. The strategy is optionally supplemented by a wall function approach to adjust the wall treatment. The analysis involves the mean flow field, resolved and modeled second moments, two-point correlations, and spectral data and correlation lengths obtained for four friction Reynolds numbers (180, 550, 2000, and 5200). Results display a resolution-independent capability to simulate the attached turbulent shear flow.Publicationtype: Journal ArticleScopus© Citations 4 79 752 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Periodic hill flow simulations with a parameterized cumulant lattice Boltzmann methodThe article is concerned with the assessment of a cumulant lattice Boltzmann method in wall-bounded, separated turbulent shear flows. The approach is of interest for its resolution-spanning success in turbulent channel flows without using a specific turbulence treatment. The assessment focuses upon the flow over a periodic hill, which offers a rich basis of numerical and experimental data, for Reynolds numbers within (Formula presented.). The analysis involves the mean flow field, second moments and their invariants, as well as spectral data obtained for a wide range of resolutions with (Formula presented.). With the emphasis on a recently published parameterized cumulant collision operator, the universality of the value assigned to a stability preserving regularization parameter is assessed. Reported results guide resolution-dependent optimized values and indicate a required minimum resolution of (Formula presented.). Analog to the findings for attached turbulent shear flows, the approach appears to adequately resolve complex turbulent flows without the need for ad hoc modeling for a range of scale resolving resolutions.Publicationtype: Journal ArticleScopus© Citations 4 79 562 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical simulation of nonlinear interactions in a naturally transitional flat plate boundary layer(2020-05-15); ; ; This paper presents numerical simulations of the natural laminar-turbulent transition in a flat plate boundary layer. Natural transition occurs in low disturbance environments and is triggered by the growth of boundary layer instabilities or disturbances. This contribution specifically aims at investigating the interaction of multiple disturbances and their effect on the transition mechanism. The numerical simulations employ a lattice Boltzmann method (LBM) in conjunction with a highly accurate cumulant collision operator and a shared memory multi-GPU framework. The numerical approach is validated by comparing the growth of single Tollmien-Schlichting (T-S) waves with results of the linear stability theory (LST) prior to transition. Moreover, a comprehensive comparison with available direct numerical simulation results (DNS) for the post transition and the fully turbulent regime confirms the validity of the employed numerical approach. Subsequently, the influence of disturbances on the transition process are investigated in detail and for different disturbance frequencies and amplitudes. Two types of natural transition scenarios (H- & K-type transition) are analyzed and the LBM results are compared to predictions of the classical eN-method. Both approaches are found to reproduce a similar frequency influence. Higher frequency T-S waves trigger an earlier transition until a critical frequency is reached, above which the boundary layer is stable. In addition, it is observed that in case of multiple interacting disturbances, the laminar-turbulent transition occurs earlier than predicted by simplified methods that ignore the nonlinear interactions between disturbances. A spectral analysis of the velocity field confirms that secondary disturbances are crucial for the resonance transition and can significantly accelerate the transition process. The paper offers a comparison of linear and nonlinear results using a classical LST map and furthermore the laminar-turbulent natural transition is comprehensively investigated using the LBM.Publicationtype: Journal Article199 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Performance of Under-Resolved, Model-Free LBM Simulations in Turbulent Shear FlowsThe paper outlines the predictive capabilities of lattice Boltzmann methods (LBM) in turbulent shear flows. Attention is devoted to a specific collision operator which relaxes the distribution functions in cumulant space. The study highlights the benefits of a carefully defined discrete collision operator by scrutinizing the numerical stability and the predictive accuracy for a wide scope of resolutions—ranging from DNS to RANS—when no ad hoc turbulence closure is employed. Examples included are concerned with two frequently computed fundamental flows, i.e. Taylor-Green vortex and channel flows. Results reveal a fair accuracy and a remarkably small resolution dependence for the investigated cumulant collision operator, which is quite the contrary for other collision models.Publicationtype: Book Part165 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , CUDA-accelerated computational fluid dynamics (NVIDIA Webinar)This webinar discusses how NVIDIA GPUs and NVIDIA CUDA can enable high-fidelity Computational Fluid Dynamics for Higher Education and Research. Flow field computations for transient and turbulent flow problems are very compute-intensive and time-consuming. Popular existing numerical techniques often compromise on the underlying physics or require a massive amount of computational resources. Accurate high-fidelity CFD simulations on locally available hardware hence are highly appreciated by academia and industry. After a brief review of the theoretical basics and implementation details of a CUDA-accelerated CFD solver, results from several international research projects are presented. They all demonstrate that GPU computing can be a game changer for state-of-the art research projects in many relevant areas of Computational Fluid Dynamics.Publicationtype: Conference Presentation218 823 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Validation of a GPU-accelerated LBM based numerical ice tank for a Voith Schneider propulsion system(2019-05-13); ; ; This contribution outlines the capabilities of an enhanced GPU-accelerated Lattice Boltzmann method (LBM) for the simulation of ice-going Voith Schneider propelled (VSP) ships. Reported results cover a collaboration between an academic and an industrial partner, i.e. TUHH and Voith, within a joint research project to assess the ice-induced loads for cycloidal propellers. The goal is to establish a simulation framework which supports extensive parameter studies in a competitive computational time. Simulation objectives are an insight into the ice clearing capabilities of a hull, the ice’s influence on the propulsion efficiency as well as the additional ice-induced blade loads. Investigated influences refer to the ice thickness, the ice floe geometry, the ship speed and/or the VSP’s parameters. The experimental and numerical ice tanks are restricted to pre-broken ice floes and ice-breaking is not considered. The simulated dynamics of the multiple rigid bodies system, i.e. the. ice floes, the hull and the VSP, are obtained from a monolithic coupling of the flow solver and a contact-dynamics Physics Engine (Open Dynamics Engine; ODE). For the coupling of the explicit LBM and the motion solver, a bidirectional and explicit coupling approach is used. Computations were performed on moderate LBM grids featuring about 20 million nodes using local grid refinement. Prior to the assessment of ice loads, hydrodynamic validation in open-water conditions of the LBM against established FV-simulations will be reported. Subsequently, the comparison of simulated and measured ice loads experienced by a 5-bladed VSP attached to a generic hull geometry will be analyzed. A comparison of the ice floe dynamics displays reasonable agreement and force assessments confirm that the simulation is able to capture all important effects. Final applications are concerned with a tug geometry - propelled by 2 5-bladed VSPs - in pack ice conditions.Publicationtype: Conference Presentation198 195 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Scrutinizing lattice Boltzmann methods for direct numerical simulations of turbulent channel flowsThe paper reports on the predictive performance of Lattice Boltzmann methods in turbulent channel flows. Attention is confined to model-free (direct) numerical simulations at Reτ=180 using essentially different collision models, i.e. the Bhatnagar–Gross–Krook (BGK), the Multiple-Relaxation-Time (MRT) and the Cumulant model. The three approaches are assessed by a comparison of the predicted mean flow and turbulence statistics against benchmark Navier–Stokes solutions. Initial studies employ a fine isotropic grid which resolves all relevant scales. Subsequently, a sequence of four gradually decreasing resolutions is utilized to analyze the sensitivity of the collision models for an inadequate resolution. Moreover, the influence of the Mach number and the discretization on the predictive accuracy of the weakly compressible LBM framework is briefly addressed. Whilst the Mach number influence is negligible below Ma < 0.1, and the predictive agreement with reference data is generally satisfactory in conjunction with all employed discretizations for the fine grid, significant disparities between the collision models are observed when the mesh is coarsened. The BGK approach offers an improved predictive agreement with Navier–Stokes results for an adequate resolution, which comes at the expense of severe (formerly reported) stability issues that emanate from the interface between the buffer and the log-layer for an under-resolved flow. Both the MRT and the Cumulant model are more demanding with respect to grid convergence. As opposed to the BGK and the MRT model, the Cumulant model remains stable over a wide range of resolutions. The reason can be attributed to a rigid alignment between predicted streamwise two-point correlation lengths and the grid spacing, which augments the damping of sub-grid scales. The latter is deemed to be an inherent feature of the model.Publicationtype: Journal Article157 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , GPU-accelerated LBM-VOF two-phase flow simulations with grid refinement(2017-05); ; ; The contribution is devoted to a novel grid refinement technique for GPU-accelerated Lattice Boltzmann Method (LBM) dedicated to free surface flow simulations in marine applications. LBM implementations are mostly based on homogeneous isotropic Cartesian discretizations of the computational domain. Challenges occur when a refined spatial and temporal resolution is locally required. To efficiently address this demand, local grid refinement is often deployed to regions featuring small scale structures and effects to be resolved. Within the LBM the spatial and temporal discretizations are strongly coupled. Hence, LBM solvers featuring local grid refinement are suitable for the simulation of transient, turbulent and free surface flows. The present LBM involves a Volume of Fluid (VOF) strategy to model two-phase flows. Herein, the advection of a mixture volume fraction serves to advance the free-surface in time. The employed grid refinement is based on overlapping grids with different resolution. The bidirectional information exchange between the grids is managed via bi[tri]-linear interpolations for two[three]-dimensional simulations. For single-phase flows this synchronisation step comprises the exchange of LBM specific variables solely. When attention is directed to two-phase flows, the intergrid coupling of the mixture fraction poses a special algorithmic problem. The suggested coupling approach is based on a Piecewise Linear Interface Construction (PLIC) method. Accordingly, the information about the surfaces orientation and position is employed to compute the fill level more accurate within the overlapping regimes. Examples included refer to two- and three-dimensional test cases and reveal that the proposed method is able to reproduce accurate results at fairly moderate computational effort.Publicationtype: Conference Presentation387 445