A numerical method for fluid-structure interactions of slender rods in turbulent flow

This thesis presents a numerical method for the simulation of fluid-structure interaction (FSI) problems on high-performance computers. The proposed method is specifically tailored to interactions between Newtonian fluids and a large number of slender viscoelastic structures, the latter being modeled as Cosserat rods. From a numerical point of view, such kind of FSI requires special techniques to reach numerical stability. When using a partitioned fluid-structure coupling approach this is usually achieved by an iterative procedure, which drastically increases the computational effort. In the present work, an alternative coupling approach is developed based on an immersed boundary method (IBM). It is unconditionally stable and exempt from any global iteration between the fluid part and the structure part. The proposed FSI solver is employed to simulate the flow over a dense layer of vegetation elements, usually designated as canopy flow. The abstracted canopy model used in the simulation consists of 800 strip-shaped blades, which is the largest canopy-resolving simulation of this type done so far. To gain a deeper understanding of the physics of aquatic canopy flows the simulation data obtained are analyzed, e.g., concerning the existence and shape of coherent structures

A coupled lattice boltzmann - immersed boundary method to model the behaviour of thin flexible structures in fluids

This article presents a computational model to simulate the fluid interaction with moving flexible thin structures. The model is based on a combination of three numerical approaches, (i) a Lattice-Boltzmann solver for the flow equations, (ii) a finite difference method to solve the solid equation, and (iii) an Immersed Boundary Method (IBM) to model the coupling between the fluid and the solid. The present IBM, based on a direct-forcing approach, preserves the no-slip boundary condition at the interface fluid-solid, and allows using Cartesian uniform lattice encompassing both fluid and solid domains. The flexible solid is modelled as an elastic structure. The resulting governing equation involves thus tension and bending forces as internal forces, the inertial and gravity forces and finally the action of the fluid represented by the IBM forcing. The method is first validated with reference to an academic test case dealing with numerical simulations of a flapping flag in a free stream. The model shows results in good agreement with the published academic test case. The validation extends to the free motion of rag in a water tunnel and a qualitative comparison is available against experimental data performed with Digital Image Correlation (DIC). Finally, the present method is used to simulate the behaviour of flexible rags in the presence of highly rotating flows at high Reynolds number, as it is the case in stirred tanks.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

A Lattice Boltzmann-Immersed Boundary method to simulate the fluid interaction with moving and slender flexible objects

A numerical approach based on the Lattice Boltzmann and Immersed Boundary methods is proposed to tackle the problem of the interaction of moving and/or deformable slender solids with an incompressible fluid flow. The method makes use of a Cartesian uniform lattice that encompasses both the fluid and the solid domains. The deforming/moving elements are tracked through a series of Lagrangian markers that are embedded in the computational domain. Differently from classical projection methods applied to advance in time the incompressible Navier–Stokes equations, the baseline Lattice Boltzmann fluid solver is free from pressure corrector step, which is known to affect the accuracy of the boundary conditions. Also, in contrast to other immersed boundary methods proposed in the literature, the proposed algorithm does not require the introduction of any empirical parameter. In the case of rigid bodies, the position of the markers delimiting the surface of an object is updated by tracking both the position of the centre of mass of the object and its rotation using Newtonʼs laws and the conservation of angular momentum. The dynamics of a flexible slender structure is determined as a function of the forces exerted by the fluid, its flexural rigidity and the tension necessary to enforce the filament inextensibility. For both rigid and deformable bodies, the instantaneous no-slip and impermeability conditions on the solid boundary are imposed via external and localised body forces which are consistently introduced into the Lattice Boltzmann equation. The validation test-cases for rigid bodies include the case of an impulsively started plate and the sedimentation of particles under gravity in a fluid initially at rest. For the case of deformable slender structures we consider the beating of both a single filament and a pair filaments induced by the interaction with an incoming uniformly streaming flow

Large eddy simulation for automotive vortical flows in ground effect

Large Eddy Simulation (LES) is carried out using the Rolls-Royce Hydra CFD code in order to investigate and give further insight into highly turbulent, unsteady flow structures for automotive applications. LES resolves time dependent eddies that are modelled in the steady-state by Reynolds-Averaged Navier-Stokes (RANS) turbulence models. A standard Smagorinsky subgrid scale model is used to model the energy transfer between large and subgrid scales. Since Hydra is an unstructured algorithm, a variety of unstructured hexahedral, tetrahedral and hybrid grids are used for the different cases investigated. Due to the computational requirements of LES, the cases in this study replicate and analyse generic flow problems through simplified geometry, rather than modelling accurate race car geometry which would lead to infeasible calculations. The first case investigates the flow around a diffuser-equipped bluff body at an experimental Reynolds number of 1.01 times 10 to the power 6 based on model height and inlet velocity. LES is carried out on unstructured hexahedral grids of 10 million and 20 million nodes, with the latter showing improved surface pressure when compared to the experiments. Comparisons of velocity and vorticity between the LES and experiments at the diffuser exit plane show a good level of agreement. Flow visualisation of the vortices in the diffuser region and behind the model from the mean and instantaneous flow attempts to explain the relation or otherwise between the two. The main weakness of the simulation was the late laminar to turbulent transition in the underbody region. The size of the domain and high experimental Reynolds number make this case very challenging. After the challenges faced by the diffuser-equipped bluff body, the underbody region is isolated so that increased grid refinement can be achieved in this region and the calculation is run at a Reynolds number of 220, 000, reducing the computational requirement from the previous case. A vortex generator mounted onto a flat underbody at an onset angle to the flow is modelled to generate vortices that extend along the length of the underbody and its interaction with the ground is analysed. Since the vortex generator resembles a slender wing with an incidence to the flow, a delta wing study is presented as a preliminary step since literature on automotive vortex generators in ground effect is scarce. Results from the delta wing study which is run at an experimental Reynolds number of 1.56 times 10 to the power 6 are in very good agreement with previous experiments and Detached Eddy Simulation (DES) studies, giving improved detail and understanding. Axial velocity and vorticity contours at several chordwise stations show that the leading edge vortices are predicted very well by a 20 million node tetrahedral grid. Sub-structures that originate from the leading edge of the wing and form around the core of the leading edge vortex are also captured. Large Eddy Simulation for the flow around an underbody vortex generator over a smooth ground and a rough ground is presented. A hexahedral grid of 40 million nodes is used for the smooth ground case, whilst a 48 million node hybrid grid was generated for the rough ground case so that the detailed geometry near the ground could be captured by tetrahedral cells. The geometry for the rough surface is modelled by scanning a tarmac surface to capture the cavities and protrusions in the ground. This is the first time that a rough surface representing a tarmac road has been computed in a CFD simulation, so that its effect on vortex decay can be studied. Flow visualisation of the instantaneous flow has shown strong interaction with the ground and the results from this study have given an initial understanding in this area

LARGE EDDY SIMULATION OF FLOW AROUND A FINITE SQUARE CYLINDER

The main objective of this research is to develop, document and study numerically the flow around finite-height square cylinders mounted on a ground plane, particularly in the near-wake region, under various geometrical conditions. Both the time-averaged and instantaneous flow fields are studied. This thesis consists of three main parts: a comprehensive study of flow over an aspect ratio AR = 5 square cylinder, the effect of sub-grid scale (SGS) models on the numerical simulation and the effect of aspect ratio on the flow structure. The first part of the thesis presents the time-averaged and instantaneous flow fields for flow over a wall-mounted finite-height square cylinder of aspect ratio of AR = 5 at a Reynolds number of Re = 500. The time-averaged flow field results are shown to be in good agreement with experiments. Comparison of the time-averaged results with the velocity field for a square cylinder immersed in a thicker boundary layer, suggests that the boundary layer thickness especially affects the upwash flow (Wang et al., 2009). The instantaneous velocity fields provide an in-depth view of the unsteady nature of the flow field. For the flow over a square cylinder of AR = 5, the instantaneous velocity fields are symmetric near the free end. However, antisymmetric patterns observed downstream may be an indication of the presence of periodic von-Karman type vortices. Since the wake regions are characterized by large-scale unsteady motions, turbulent flow over bluff bodies is well suited to large eddy simulation in which the large energy-containing scales of motion, which are responsible for most of the momentum transport, are resolved whereas the small-scale turbulent fluctuations are modeled. In the second part of the thesis, the performance of the three SGS models, the Smagorinsky model (SM), dynamic Smagorinsky model (DSM) and dynamic non-linear model (DNM) are studied for two grid sets of lower and higher resolution. The results indicated that in case of the DSM insufficient grid resolution leads to erroneous predictions, whereas the DNM is a major improvement as the predictions are similar on both the coarse and fine grids. In the third and final part of the thesis, the effect of aspect ratio on the flow over a wall-mounted finite-height square cylinder is numerically investigated. The wake of a finite square cylinder is studied for three aspect ratios of AR = 3, 5 and 7. The time-averaged vorticity was shown to vary with aspect ratio, e.g. as the aspect ratio increases, the vortex structures in a horizontal plane at mid-height became shorter and rounder in shape. The flow field of the finite cylinder is known to be strongly affected by the aspect ratio (Adaramola et al., 2006). For cylinders with relatively small aspect ratios, the two ends affect the flow patterns and significantly alter the flow structure

A modified Finite Element formulation for the imposition of the slip boundary condition over embedded volumeless geometries

This work describes a novel formulation for the simulation of Navier–Stokes problems including embedded objects. The new proposal is based on the use of a modified finite element space, which replaces the standard one within the elements intersected by the immersed geometry. The modified space is able to exactly reproduce the jumps happening at the embedded boundary while preserving the conformity across the faces intersected by the embedded object. The paper focuses particularly on the imposition of a slip boundary condition on the surface of the embedded geometry, proposing a new technique for the application of such constraint. The new proposal is carefully benchmarked using the results of a body fitted technique and of an alternative embedded approach. Potential applications of interest are also presented.Peer ReviewedPostprint (author's final draft

A FLUID STRUCTURE INTERACTION STRATEGY WITH APPLICATION TO LOW REYNOLDS NUMBER FLAPPING FLIGHT

In this work a structured adaptive mesh refinement (S-AMR) strategy for fluid-structure interaction (FSI) problems in laminar and turbulent incompressible flows is developed. The Eulerian computational grid consists of nested grid blocks at different refinement levels. The grid topology and data-structure is managed by using the Paramesh© toolkit. The filtered Navier-Stokes equations are evolved in time by means of an explicit second-order projection scheme, where spatial derivatives are approximated with second order central differences on a staggered grid. The level of accuracy of the required variable interpolation operators is studied, and a novel divergence-preserving prolongation scheme for velocities is evolved. A novel direct-forcing embedded-boundary method is developed to enforce boundary conditions on a complex moving body not aligned with the grid lines. In this method, the imposition of no-slip conditions on immersed bodies is done on the Lagrangian markers that represent their wet surfaces, and the resulting force is transferred to the surrounding Eulerian grid points by a moving least squares formulation. Extensive testing and validation of the resulting strategy is done on a numerous set of problems. For transitional and turbulent flow regimes the large-eddy simulation (LES) approach is used. The grid discontinuities introduced in AMR methods lead to numerical errors in LES, especially if non-dissipative, centered schemes are used. A simple strategy is developed to vary the filter size for filtered variables around grid discontinuities. A strategy based on explicit filtering of the advective term is chosen to effectively reduce the numerical errors across refinement jumps. For all the FSI problems reported, the complete set of equations governing the dynamics of the flow and the structure are simultaneously advanced in time by using a predictor-corrector strategy. Dynamic fluid grid adaptation is implemented to reduce the number of grid points and computation costs. Applications to flapping flight comprise the study of flexibility effects on the aerodynamic performance of a hovering airfoil, and simulation of the flow around an insect model under prescribed kinematics and free longitudinal flight. In the airfoil simulations, it is found that peak performance is located in structural flexibility-inertia regions where non-linear resonances are present