Using the generalized interpolation material point method for fluid-solid interactions induced by surface tension

This thesis is devoted to the development of new, Generalized Interpolation Material Point Method (GIMP)-based algorithms for handling surface tension and contact (wetting) in fluid-solid interaction (FSI) problems at small scales. In these problems, surface tension becomes so dominant that its influence on both fluids and solids must be considered. Since analytical solutions for most engineering problems are usually unavailable, numerical methods are needed to describe and predict complicated time-dependent states in the solid and fluid involved due to surface tension effects. Traditional computational methods for handling fluid-solid interactions may not be effective due to their weakness in solving large-deformation problems and the complicated coupling of two different types of computational frameworks: one for solid, and the other for fluid. On the contrary, GIMP, a mesh-free algorithm for solid mechanics problems, is numerically effective in handling problems involving large deformations and fracture. Here we extend the capability of GIMP to handle fluid dynamics problems with surface tension, and to develop a new contact algorithm to deal with the wetting boundary conditions that include the modeling of contact angle and slip near the triple points where the three phases -- fluid, solid, and vapor -- meet. The error of the new GIMP algorithm for FSI problems at small scales, as verified by various benchmark problems, generally falls within the 5% range. In this thesis, we have successfully extended the capability of GIMP for handling FSI problems under surface tension in a one-solver numerical framework, a unique and innovative approach.Chapter 1. Introduction -- Chapter 2. Using the generalized interpolation material point method for fluid dynamics at low reynolds numbers -- Chapter 3. On the modeling of surface tension and its applications by the generalized interpolation material point method -- Chapter 4. Using the generalized interpolation material point method for fluid-solid interactions induced by surface tension -- Chapter 5. Conclusions

Application of Dynamic Mesh Method in CFD to Engineering Designs of Needle-Free Liquid Jet Injector and Diaphragm-less Shock Tube

Many engineering devices have dynamic components and hence, their computational models are no longer fixed in space and time. In these cases, dynamic mesh method is often applied to analyze their motion or unsteady fluid dynamics around/inside them. This study deals with the engineering application of CFD particularly using dynamic mesh methods to simulate firstly the compressible transient flow in a needle-less liquid jet injector for biomedical application and secondly, the performance of a diaphragm-less shock tube design for investigation of high-speed compressible gas dynamics. The CFD software OpenFOAM® is used as the main research tool to carry out this study. For the first application, the dynamic behavior of the liquid jet is approximated using multi-phase compressible immiscible fluids LES solver together with the Volume-of-Fluid (VOF) method for the interface capturing. The liquid retained in the injector chamber is impacted by the moving grid boundary to mimic the injector piston driven by the driver air pressure; and the high speed liquid jet is emitted to atmosphere region though a nozzle. Numerical results are validated and discussed by comparing with experimental measurements. Performance plots as a function of various injector parameters are constructed and explained. The second application concerns with the diaphragm-less shock tube design which consists of an outer tube contained with high pressure and an inner one with low pressure. A particular design of diaphragm-less shock tube utilizes a rapid opening sleeve to mimic the rupture of a diaphragm which is traditionally used to separate the two pressure region. Applying CFD with dynamic mesh to the sleeve motion contributes to the analysis of the process of shock wave generation in this device and the shock tube parameters such as opening time of the sleeve for reliable performance. It is proven in this work that the numerical CFD models with dynamic mesh can accurately predict the performance of both engineering devices and provide a useful tool to analyze which parameters most significantly impact the performances

Topology optimization in OpenFOAM using a continuous adjoint framework

En aquest treball informem sobre l'addició de funcions a un solucionador d'optimització de topologia OpenFOAM existent. Comencem amb un resum detallat de les bases teòriques de l'optimització de la topologia en el cas de les equacions de Navier-Stokes en estat estacionari incompressibles sota un marc adjunt. A continuació, descriurem les bases teòriques per a la implementació del nostre solucionador estès. És a dir, hem implementat dues funcions noves: una funció objectiu de dissipació de potència i una restricció de volum. La validesa de la restricció de volum es prova amb èxit mitjançant un problema de joguina. Després de la validació, les sortides del solucionador recentment modificat es comparen amb la literatura per als casos que inclouen un sistema de conductes amb una entrada i dues sortides, una configuració alternativa del mateix sistema de conductes i un problema de doble coll d'ampolla de dues sortides. També incloem una revisió de la literatura actual sobre mètodes d'optimització de topologia i la implementació de restriccions de volum en mètodes basats en la densitat. See original in english in case of inaccurate translation.En este trabajo informamos sobre la adición de funciones a un solucionador de optimización de topología OpenFOAM existente. Comenzamos con un resumen detallado de la base teórica de la optimización topológica en el caso de las ecuaciones de Navier-Stokes de estado estacionario incompresible bajo un marco adjunto. Luego describimos la base teórica para la implementación de nuestro solucionador extendido. A saber, hemos implementado dos nuevas funciones: una función objetivo de disipación de potencia y una restricción de volumen. La validez de la restricción de volumen se prueba con éxito a través de un problema de juguete. Después de la validación, las salidas del solucionador recién modificado se comparan con la literatura para casos que incluyen un sistema de conductos con una entrada y dos salidas, una configuración alternativa del mismo sistema de conductos y un problema de cuello de botella doble y dos salidas. También incluimos una revisión de la literatura actual sobre métodos de optimización de topología y la implementación de restricciones de volumen en métodos basados ​​en densidad. See original in english in case of inaccurate translation.In this work we report on the addition of features to an existing OpenFOAM topology optimization solver. We begin with a detailed summary of the theoretical basis of topology optimization in the case of the incompressible steady-state Navier-Stokes equations under an adjoint framework. We then describe the theoretical basis for the implementation of our extended solver. Namely, we have implemented two new features: a power dissipation objective function and a volume constraint. The validity of the volume constraint is successfully tested via a toy problem. Following validation, outputs of the newly modified solver are benchmarked against literature for cases that include a duct system with one inlet and two outlets, an alternate configuration of the same duct system, and a dual bottleneck two-outlet problem. We also include a review of the current literature on topology optimization methods and the implementation of volume constraints in density-based methods

Numerical Simulation of the Lux Vertical Axis Wind Turbine

Wind energy can be characterized as a cheap, clean, and renewable energy source that is absolutely sustainable. With increasing demand for wind energy, it is productive to investigate the structural and operational factors that undermine the proficiency and the characteristic performance of the wind turbine. Of paramount importance to efficient wind energy generation is the aerodynamics of the wind turbine blades. The aerodynamic factors, such as drag, airfoil pro files, and wake interactions that often reduce the performance of the wind turbines, can be investigated through computational mathematics using computational fluid dynamics (CFD). CFD offers basic techniques and tools for simulating physical processes and proffers important insights into the ow data, which are demanding and costly to measure experimentally. In this thesis, we develop a simulation model in an open-source software package called OpenFOAM to investigate the performance characteristics of the Lux Vertical Axis Wind Turbine (VAWT). The Lux VAWT has a simpler design than its horizontal counterparts; however, its performance is affected by the unsteady aerodynamic due to a complex flow field. The turbulent flow field is governed by the incompressible Navier- Stokes equations. Simulations are carried out with an unsteady incompressible and dynamic flow solver, PimpleDyMFoam, on an unstructured mesh surface of the Lux VAWT geometry. The computational domain includes both the stationary and rotating mesh domains to accommodate the rotating motion of the turbine blades and the free-stream zone. The arbitrary mesh interface is applied as a boundary condition for the patches between the two domains to enable computation across disconnected but adjacent mesh domains. Meshing was done using two separate meshing tools, snappyHexMesh and ANSYS Mesher. The snappyHexMesh tool offered the most flexible and effective control over the mesh generation and quality. In order to derive the maximal power output from the Lux VAWT simulations, the Unsteady Reynolds Averaged Navier--Stokes (URANS) equations are solved with different time-stepping methods; the objective is to reduce the computational costs. While attempting to reduce the numerical diffusion from the non-transient terms of URANS, a stabilized trapezoidal rule with a second-order backward differentiation formula (TR--BDF2) time-stepping method was implemented in OpenFOAM. As a result, the transient aerodynamic forces of the blades, the torque, and power output are evaluated. The findings demonstrate that most of the transient aerodynamic force is generated along the axis of rotation of the rotor during one complete revolution. Similarly, the computations indicate that the BDF2 method results in the least computational cost and predicts a turbine power that is somewhat comparable to the experimental results. The difference between the simulation results and the experimental data is attributed partly to the pressure fluctuations on the turbine blades due to the mesh topology

Optimization of blade profiles for the Wells turbine

A Wells turbine, when coupled with an oscillating water column, allows the generation of power from the energy in waves on the surface of the ocean. In the present work, a tabu search is used to control the process of optimising the blade profile in the Wells turbine for greater performance, by maximising the torque coefficient. A free form deformation method is used as an efficient means of manipulating the blade profile and computational fluid dynamics in OpenFOAM are used to assess each profile in both two and three dimensions. Investigations into both the flow coefficient at which the optimisation is performed and the number of control variables in the free form deformation tool are performed before optimisations are done on a two-dimensional blade at the hub and tip solidities. This results in increases to the torque coefficient of 34% and 32% at the tip and hub solidities, respectively. These results are then applied to the three-dimensional turbine, giving a 14% increase in the torque coefficient. The results are assessed and an improved method of optimising the blade in two dimensions is proposed.Regione Autonoma Sardegna (grant funding co-authors from University of Cagliari

Flow control for road vehicle drag reduction

This thesis covers topics that span bluff-body aerodynamics, hybrid RANS-LES CFD methods, flow control and model-order reduction. These topics arise from investigating the flow past three geometries: the bullet shaped D-body, the canonical squareback Ahmed body and the commerical Nissan NDP. The study on the D-body was aimed at transitioning the research group from the restrictive block-structured formulated StreamLES solver to the more flexible OpenFOAM code that can use unstructured meshes. Linear feedback control for base pressure increase was applied as was done in the work by Dalla Longa et al. (2017). Identification of the plant, G(s), that represents the wake's response to forcing was completed and correlated well with the results from Dalla Longa et al. (2017). The same can also be said of the sensitivity based designed feedback control law, K(s). When applied in simulation, an attenuation of the base pressure fluctuations was, as desired, achieved, although the base pressure increased by 24.5% as opposed to the 38% achieved by Dalla Longa et al. (2017). In the study on the squareback Ahmed body, wall-resolving (WRLES) and wall-modelled (WMLES) large eddy simulation were successfully applied. First, a simulation setup that is both able to resolve wake bimodality, while remaining reasonable in computational resource use, was created. Subsequently, variants of this setup were used to identify a flow feature that plays a critical role in forcing wake bimodality events. More specifically, a heavily under-resolved WMLES simulation in which both the near-wall and part of the outer-region of the turbulent boundary layer are Reynolds-averaged did not capture the front recirculation bubble near the Ahmed body nose; neither did it resolve a bimodal wake switching event. Meanwhile, the simulations with a more refined near-wall mesh did capture the front separation bubble as well as bimodal switching events of the wake. This front separation bubble sends out powerful hairpin vortices that interact with the rear wake. Specifically, these vortices go on to produce significant amounts of TKE, which, upon convection to the rear of the Ahmed body, ultimately help trigger a bimodal event. The Ahmed body study also involved the application of linear feedback control for drag reduction as was done in the D-body study. In the short term, mean blowing did lead to a base pressure increase, but as the zero-net-mass-flux (ZNMF) jet settled, it oscillated around zero making its effects indiscernible. The final geometry analyzed was the Nissan NDP. This was done by performing benchmark wall-resolving LES (WRLES). First, the benefit of appending a rear cavity to an otherwise "squareback" geometry was assessed. It was concluded that the cavity allows the wake to move more freely about the rear base. Specifically, the wake is freed from its more restricted motion that is present with the "squareback" Nissan NDP. In doing so, the drag reduction achieved with the cavity appendage is about 13.6%. Work on the Nissan NDP also involved an assessment of a moving ground in the simulation. It was concluded that, in the stationary ground simulation, flow detachment at the ground where the flow exits from the underbody has an adverse drag effect. In other words, although moving ground simulations better replicate the real-world conditions, the stationary ground variant is in this case more conservative, as it returns slightly higher drag values.Open Acces