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Publication Access status: Open Access , Lattice-Boltzmann-Verfahren hoher Ordnung zur Simulation kompressibler Strömungen(2023)In this thesis, a compressible semi-Lagrangian lattice Boltzmann method is newly developed and tested. The lattice Boltzmann method is a rapidly advancing numerical method for computational fluid dynamics. However, in its original form, the lattice Boltzmann method is limited to weakly compressible flows with low Mach number. Previous attempts to extend the lattice Boltzmann method to supersonic flows suffered either from poor stability, from impractically large velocity sets, or from small time step sizes. As an alternative to previous approaches, a semi-Lagrangian streaming step is used in this work. Semi-Lagrangian methods decouple the spatial, time, and velocity space discretization of the original lattice Boltzmann method by interpolation during the streaming step. Following the introduction, the second and third chapters of this thesis first detail the basics of the lattice Boltzmann method and list previous approaches to simulate compressible flows. Subsequently, the compressible semi-Lagrangian lattice Boltzmann method is developed and described. In the fourth chapter of the thesis, new cubature-based velocity sets are developed and tested, including a D3Q45 velocity set for the computation of compressible flows, which significantly reduces the computational cost compared to conventional velocity discretizations. In the fifth chapter of the thesis, simulations of one-dimensional shock tubes, two-dimensional Riemann problems, and shock-vortex interactions are performed for validation. Thereafter, simulations of compressible Taylor-Green vortices as well as wall-bounded problems demonstrate the advantages of the method for compressible flow simulations. The latter include the supersonic flow around a two-dimensional NACA-0012 profile and around a three-dimensional sphere as well as a supersonic channel flow. The simulation section is followed by an extensive discussion of the semi-Lagrangian lattice Boltzmann method in comparison to other methods. The advantages of the method include comparatively large time step sizes, compatibility with body-fitted meshes, and the intrinsic stability of the method even without artificial viscosity.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Strömungs- und ThermodynamikVolume number:93 11 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Pseudopotentialbasierte Lattice-Boltzmann-Verfahren für Mehrphasenströmungen(2019)Pseudopotential (PP)-based lattice Boltzmann methods are widely used to simulate multiphase flows. As they ground on a phenomenological approach, their usage involves a high modelling effort and, additionally, they suffer from so-called spurious velocities that compromise numerical stability and accuracy. Therefore, this work advances PP-models by three novel aspects. First, the PP-model is shown to develop spurious droplets when contact angles are modelled with common approaches in combination with enhanced forcing schemes. These droplets are eliminated by a novel approach that relies on additional boundary conditions for all interaction forces. This not only prevents the spurious droplets from emerging but also increases the stability in wallbound flows. Second, a novel procedure to reduce spurious velocities is introduced. Therefore, the discretization of the interaction forces is extended and the additional free coefficients are optimized numerically using static droplet simulations. The resulting scheme is shown to greatly reduce spurious velocities and is successfully validated in stationary and transient testcases. Third and final, the diffusion characteristics in multicomponent systems are investigated in detail, revealing a critical dependency between the macroscopic diffusion coefficients and the forcing scheme. This analysis forms the basis for the comparison and development of new potential functions (in multicomponent systems) and reduces the modeling effort.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Strömungs- und ThermodynamikVolume number:53 3 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Widerstandsreduktion kompressibler turbulenter Strömungen durch wandbasierte Beeinflussung(2025)An active flow control method is used with the aim of friction drag reduction. The method utilizes streamwise oscillation of spanwise velocity at the wall. Owing to reduced turbulence intensity, less energy is required to drive the flow against viscous resistance. The key question is how compressibility affects the drag reduction results. A huge dataset of different flow cases with carefully selected parameter combinations has been created to reliably extract the influence of important flow parameters on the flow behavior. Direct numerical simulations and large eddy simulations of subsonic and supersonic channel flow were run with Mach numbers based on the mean velocity of Ma = 0.3, 1.5 and 3.0 and Reynolds numbers Re based on the friction velocity in the range of 190 up to 2540. Mean property effects are predominant in supersonic channel flow through large variations of the mean density and temperature and wall cooling. Consequences are a higher Reynolds stress anisotropy, increased length scales in the viscous sublayer flow and enhanced streak stability. The pressure-strain correlation and spanwise dissipation terms undergo a strong attenuation compared to the incompressible counterparts, especially in the controlled flows. Higher drag reduction and a larger optimum control wavelength are observed compared to the corresponding incompressible flows. Increasing the Mach number enhances these effects due to stronger mean property variations. A mitigation of near-wall compressibility effects occurs with an increase of the bulk Reynolds number, though, which attenuates the drag reduction benefits in the controlled flow as well. In the second part of this work, cooling terms were introduced in the Navier-Stokes equations to mimic the wall-normal temperature profile of an external boundary layer flow and to minimize variable property effects within the complete channel, respectively. This approach allows for a better isolation of intrinsic compressibility effects and wall cooling effects. The cooling strategies highlight the importance of intrinsic compressibility effects, which contribute to an increased control efficiency, too. As a potential case of application for the flow control method, a jet flow emanating from a round pipe is considered in the third part of this work. The turbulent subsonic pipe flow is manipulated by the transverse wall velocity and the focus is on the impact on the jet development. No direct interference of the streanwise oscillation pattern of transverse velocity within the pipe and the jet region is detected. However, the lowered turbulence intensity at the pipe exit noticeably affects the jet flow development, independent of how the reduction is achieved. The shear layer of the controlled jet flow develops with higher turbulence intensities associated with intensified Kelvin-Helmholtz instabilities. The effects can be relativized if the maximum transverse velocity is located directly at the pipe exit. Consequently, a jet with similar characteristics to the uncontrolled one can be generated with the advantage of a substantially lowered energy expenditure.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Strömungs- und ThermodynamikVolume number:127 6 - Some of the metrics are blocked by yourconsent settings
Publication Access status: Open Access , Lattice-Boltzmann-Methoden zur Simulation inkompressibler Wirbelströmungen(2017)The present work develops novel methodological extensions to the lattice Boltzmann method (LBM). These extensions enable the method to efficiently simulate incompressible vortical flows. They cure two major drawbacks of the standard LBM: its instability in under-resolved turbulence and its restriction to regular computational grids. At first, a pseudo-entropic stabilizer (PES) is developed, which combines ideas from multiple-relaxation-time (MRT) models and entropic models. The new PES is local, explicit, and flexible. It modifies the collision step in a way that enables stable simulations and produces qualitatively matching results, even on strongly underresolved grids. To extend the LBM towards simulations on irregular grids, a recent discontinuous Galerkin lattice Boltzmann method is studied and enhanced by more stable time integrators. This study illustrates the severe shortcomings of existing off-lattice Boltzmann methods (OLBMs). Based on these findings, the present work succeeds in developing a semi-Lagrangian lattice Boltzmann method (SLLBM). This novel approach allows unstructured grids, large time steps, and high-order accurate representations of the solution to be used in a unique way. Applications to exemplary flows demonstrate how and why the new method outperforms other recent OLBMs in both efficiency and accuracy. Additionally, this work describes the development of a modular off-lattice Boltzmann code and shows that the method’s convergence order can be increased by implicit multistep methods.Source Type:Doctoral ThesisSeries:Schriftenreihe des Lehrstuhls für Strömungs- und ThermodynamikVolume number:34 8
