Vortex-Induced Vibration of Circular Cylinders Using Multi-Block Immersed Boundary-Lattice Boltzmann Method

Despite decades of research, vortex-induced vibration (VIV) of circular cylinders is still a topic of strong interest in fluid mechanics, as it is of great importance in many engineering disciplines, such as bridges, nuclear reactors and high-rise buildings. In order to provide an in-depth understanding of complex fluid-structure interaction during VIV, this thesis considers the following physical scenarios using an in-house code developed based on immersed boundary-lattice Boltzmann method (IB-LBM). First, a system with two fixed cylinders with an intermediate centre-to-centre spacing is considered. It is found that the frequency component of the force on each individual cylinder changes from a single value to multiple ones, then to a large number of discrete ones and eventually to a broadband continuous spectrum, as the alignment angle increases. Second, the vibration of a cylinder may occur due to fluid-structure interaction, and thus the free motion is investigated using the results from the corresponding forced oscillation. It is shown that when a cylinder is in periodic free motion, its motion will remain the same if the combined mass-damping parameter remains unchanged and the variations of body mass and stiffness follow a particular pattern. Here, the damping ratio is redefined using the motion frequency of the body instead of the commonly adopted natural frequency of the body. Third, large-eddy simulation as turbulence model is implemented in the computer code and multi grids are adopted in IB-LBM to improve computation efficiency and accuracy. Turbulent flow is then studied. The results show that the effect of the Reynolds number on the well-known three response branches at different reduced velocities, or initial, upper and lower branches, is significant. When Reynolds number is fixed, at its lower range calculated, there are only initial and upper branches, and at higher range, there are only upper and lower branches

A study of computational methods for wake structure and base pressure prediction of a generic SUV model with fixed and rotating wheels

This study is an evaluation of computational methods in reproducing experimental data for a generic SUV geometry and an assessment on the influence of fixed and rotating wheels for this geometry. Initially, comparisons are made in wake structure and base pressures between several CFD codes and experimental data. It was shown that steady-state RANS methods are unsuitable for this geometry due to a large scale unsteadiness in the wake caused by separation at the sharp trailing edge and rear wheel wake interactions. URANS offered no improvements in wake prediction despite a significant increase in computational cost. DES and Lattice Boltzmann methods showed the best agreement with experimental results in both wake structure and base pressure, with LBM running in approximately a fifth of the time for DES

Vehicle surface contamination, unsteady flow and aerodynamic drag

The rear surfaces of blunt-ended vehicles, such as SUVs, are vulnerable to the build-up of contaminants thrown up from wet road surfaces by their tyres. This can compromise drivers’ vision, vehicle visibility, sensor performance and aesthetics. Vision will be reduced if the rear screen and lenses of camera systems become obscured. Similarly, sensing methods such as Light Detection and Ranging [LIDAR], introduced to support higher-level Advanced Driver Assistance Systems [ADAS] and autonomous driving are also vulnerable to contaminant accumulation. In addition, vehicle users may find that dirt is transferred to their hands and clothes as they access the rear load space. Finally, rapid soiling of external surfaces can be perceived as degrading the aesthetics of premium vehicles. Such deposition is a manifestation of unsteady aerodynamics – particularly the interaction between tyre spray, wheel wakes and the vehicle rear wake. These wake structures also strongly influence aerodynamic drag which, in turn affects CO2 emissions for Internal Combustion Engine [ICE] powered cars and the range of Battery Electric Vehicles [BEV]. Hence, automotive manufacturers need a simulation approach that can be used to minimise these characteristics concurrently during vehicle development. This work met that need by developing and deploying an innovative simulation process which predicts both contaminant accumulation and drag at the same time, by numerically representing unsteady aerodynamics, tyre spray and surface water behaviour. It is now integrated into the vehicle development process at Jaguar Land Rover [J/LR] where it is being used to develop new cars. This has been achieved by using a series of novel simplified vehicle geometry and spray systems to incrementally develop and validate the simulation strategy. The work culminated with its application to a production vehicle and subsequent validation against full scale experiments, providing the first quantification of accuracy for simulations of rear surface contamination. This novel simulation approach is combined with original experiments to show that reduced vehicle ride heights can lead to increased rear surface contamination, by reducing underbody flow and moving the vehicle wake closer to the highly contaminated wheel wakes. This provides a challenge for vehicle developers as lower ride heights are used to reduce aerodynamic drag; an increasingly important objective for both ICE and BEV product development, to support lower CO2 emissions and enhanced range, respectively. Finally, the first evidence is presented to suggest that aerodynamically improved underfloors can increase rear surface contamination, or at least redistribute it towards the lower regions of the vehicle rear, such as the bumper. This raises a risk for future BEVs which combine aerodynamically advantageous smooth underfloors with vulnerable ADAS features, such as rear bumper mounted LIDAR

Lattice-Boltzmann simulation of two-dimensional flow over two vibrating side-by-side circular cylinders

Author name used in this publication: Yang LiuVersion of RecordPublishe

Extensive study of flow characters for two vertical rectangular polygons in a two-dimensional cross flow

Fluid dynamics problems have a significant impact on the growth of science and technologies all over the world. This study investigates viscous fluid’s behavior when interacting with two rectangular polygons positioned vertically and aligned in a staggered configuration. Two physical parameters, Reynolds Number and Gap spacings, are discussed using the Lattice Boltzmann Method for two-dimensional flow. Results are discussed in vortex snapshots, time trace histories of drag and lift coefficient, and power spectra analysis of lift coefficient. Nine distinct flow vortex streets are identified based on increasing gap spacings between the pair of two rectangular polygons. The vortex shedding mechanism is disturbed at small gap spacings and becomes optimal at large gap spacings. Different physical parameters of practical importance, like mean drag coefficient, root mean square values of drag coefficient, root mean square values of lift coefficient, and Strouhal number, approach the single rectangular polygon value at large gap spacings

Application of Lattice Boltzmann Method for Simulating Stably Stratified Flows past Cylinders

This research looks into the intriguing subject of ambient density-stratified flows, which have long captivated researchers due to their representation of real-world physical phenomena such as diapycnal mixing in oceans driven by environmental influences. The study specifically focuses on the flow past a cylindrical object within such stratified flows, which introduces complexities involving buoyancy and viscous effects. A major focus of this research is the examination of the lattice Boltzmann method as a novel approach to model stratified flows around circular cylinders by solving coupled Navier-Stokes and advection-diffusion equations. The study investigates the impact of stratification on wake characteristics and various flow parameters for a single cylinder at six Reynolds numbers ranging from 10 to 600 and Froude numbers from 2.19 to 7.51. Additionally, the investigation includes the case of two cylinders arranged in tandem at a Reynolds number of 100, with similar Froude numbers. This research demonstrates the suitability and robustness of the lattice Boltzmann method in modeling stratified flows past cylinders. The findings reveal that even moderate levels of stratification can significantly influence the wake pattern, potentially leading to changes in the flow regime. Moreover, the study demonstrates that the introduction of stratification is associated with a reduction in the drag coefficient and shedding frequency, leading to altered flow behaviors. Furthermore, in the case of flow past two cylinders, the presence of stratification increases the critical spacing between the cylinders

Aerodynamic Analysis of Grand Prix Cars Operating in Wake Flows

The effect of the upstream wake of a Formula 1 car on a following vehicle has been investigated using experimental and computational methods. Multiple vehicle studies in conventional length wind tunnels pose challenges in achieving a realistic vehicle separation and the use of a short axial length wake generator provides an advantage here. Aerodynamic downforce and drag were seen to reduce, with greater force reductions experienced at shorter axial spacings. With lateral offsets, downforce recovers at a greater rate than drag, returning to the level for a vehicle in isolation for offsets greater than half a car width.The effect of the wake was investigated in CFD using multiple vehicle simulations and non-uniform inlet boundary conditions to recreate the wake. Results closely matched those for a full two-vehicle simulation provided the inlet condition included unsteady components of the onset wake. Creating a nonuniform inlet condition allowed the wake parameters to be modified to test sensitivity to different wake features. Dynamic pressure deficit in the wake is shown to have the greatest impact on the following vehicle, reducing loading on the downforce producing surfaces. Wake up-wash and vortex flows are shown to have a smaller effect on downforce generated by the following car, but have an important role in diverting the dynamic pressure deficit upwards and over the following car.Future regulation changes, aimed at reducing the downforce loss experienced when following another car, should aim to reduce the velocity deficit onset to the following car; either by reducing wheel and underbody wakes, or by extracting the wake using up-wash from the rear wing

The Numerical Simulation of Fluid Flow

This book collects the accepted contributions to the Special Issue "The Numerical Simulation of Fluid Flow" in the Energies journal of MDPI. It is focused more on practical applications of numerical codes than in its development. It covers a wide variety of topics, from aeroacoustics to aerodynamics and flow-particles interaction

A Comparative Study of Simulated and Measured Gear-Flap Flow Interaction

The ability of two CFD solvers to accurately characterize the transient, complex, interacting flowfield asso-ciated with a realistic gear-flap configuration is assessed via comparison of simulated flow with experimental measurements. The simulated results, obtained with NASA's FUN3D and Exa's PowerFLOW for a high-fidelity, 18% scale semi-span model of a Gulfstream aircraft in landing configuration (39 deg flap deflection, main landing gear on and off) are compared to two-dimensional and stereo particle image velocimetry measurements taken within the gear-flap flow interaction region during wind tunnel tests of the model. As part of the bench-marking process, direct comparisons of the mean and fluctuating velocity fields are presented in the form of planar contour plots and extracted line profiles at measurement planes in various orientations stationed in the main gear wake. The measurement planes in the vicinity of the flap side edge and downstream of the flap trailing edge are used to highlight the effects of gear presence on tip vortex development and the ability of the computational tools to accurately capture such effects. The present study indicates that both computed datasets contain enough detail to construct a relatively accurate depiction of gear-flap flow interaction. Such a finding increases confidence in using the simulated volumetric flow solutions to examine the behavior of pertinent aer-odynamic mechanisms within the gear-flap interaction zone