Detached eddy simulation of turbulent flow around square and circular cylinders on Cartesian cut cells

© 2016 Elsevier Ltd. All rights reserved. Square and circular cylinders in three-dimensional turbulent flows are studied numerically using the LES and DES turbulence models. One aim of the present study is to implement the LES and DES turbulence models in a cell-centered finite volume method (FVM) developed for solving the Navier-Stokes equations on Cartesian cut cells. The Cartesian cut cell approach is known to be robust for problems in geometrically complex domains with fixed or moving boundaries. For the purpose of validating the present numerical model, the current flow past fixed square and circular cylinders at moderate Reynolds numbers is tested first. Comparison of the computed results with experimental data reveals that the DES models are superior to the conventional LES and RANS models. The second aim of the present study is to assess the performance of different RANS based DES turbulence models. By means of the comparison of results obtained with the 0-equation mixing-length, 1-equation S-A and 2-equation k-ω based DES models for the flow over the same circular cylinder, some recommendations are proposed. According to the present study, in terms of accuracy the 1-equation S-A based DES model is very promising. Beside this, if the computational cost is the main concern, the 0-equation mixing-length based DES model might be an ideal option, achieving a good balance between accuracy and efficiency

Numerical simulation of water impact of solid bodies with vertical and oblique entries

The flow problem of hydrodynamic impact during water entry of solid objects of various shapes and configurations is simulated by a two-fluid free surface code based on the solution of the Navier-Stokes equations (NSE) on a fixed Cartesian grid. In the numerical model the free surface is captured by the level set function, and the partial cell method combined with a local relative velocity approach is applied to the simulation of moving bodies. The code is firstly validated using experimental data and other numerical results in terms of the impact forces and surface pressure distributions for the vertical entry of a semi-circular cylinder and a symmetric wedge. Then configurations of oblique water entry of a wedge are simulated and the predicted free surface profiles during impact are compared with experimental results showing a good agreement. Finally, a series of tests involving vertical and oblique water entry of wedges with different heel angles are simulated and the results compared with published numerical results. It is found that the surface pressure distributions and forces predicted by the present model generally agree very well with other numerical results based on the potential flow theory. However, as the current model is based on the solution of the NSE, it is more robust and can therefore predict, for example, the formation and separation of the thin flow jets (spray) from surface of the wedge and associated ventilation phenomena for the cases of oblique water entry when the horizontal velocity is dominant. It is also noted that the potential flow theory can result in over-estimated negative pressures at the tip of the wedge due to its inherent restriction to nonseparated flows. © 2013 Elsevier Ltd. All rights reserved

Modelling wave interaction with deformable structures based on a multi-region approach within OpenFOAM

© 2017 by the International Society of Offshore and Polar Engineers (ISOPE). This paper presents the development of a multi-region computational fluid-structure dynamics (CFSD) method which is integrated in our virtual wave structure interaction solver wsiFoam, based on the open-source OpenFOAM library, in order to account for the hydro-elastic effects produced by violent wave impacts against deformable bodies. This strategy relies entirely on the finite volume method (FVM) and does not require any third-party solvers, which renders it suitable for efficient parallel computing. We validate this novel approach against previous experimental and numerical results corresponding to a dam break of water impacting on a highly deformable plate as well as a flexible wedge entering water at a constant speed. In general, our preliminary results agree qualitatively well with previous data whilst the performance of parallel implementation evidences the potential of this method to be used in future high performing computing (HPC) applications

A GPU based compressible multiphase hydrocode for modelling violent hydrodynamic impact problems

This paper presents a GPU based compressible multiphase hydrocode for modelling violent hydrodynamic impacts under harsh conditions such as slamming and underwater explosion. An effort is made to extend a one-dimensional five-equation reduced model (Kapila et al., 2001) to compute three-dimensional hydrodynamic impact problems on modern graphics hardware. In order to deal with free-surface problems such as water waves, gravitational terms, which are initially absent from the original model, are now considered and included in the governing equations. A third-order finite volume based MUSCL scheme is applied to discretise the integral form of the governing equations. The numerical flux across a mesh cell face is estimated by means of the HLLC approximate Riemann solver. The serial CPU program is firstly parallelised on multi-core CPUs with the OpenMP programming model and then further accelerated on many-core graphics processing units (GPUs) using the CUDA C programming language. To balance memory usage, computing efficiency and accuracy on multi- and many-core processors, a mixture of single and double precision floating-point operations is implemented. The most important data like conservative flow variables are handled with double-precision dynamic arrays, whilst all the other variables/arrays like fluxes, residual and source terms are treated in single precision. Several benchmark test cases including water-air shock tubes, one-dimensional liquid cavitation tube, dam break, 2D cylindrical underwater explosion near a planar rigid wall, 3D spherical explosion in a rigid cylindrical container and water entry of a 3D rigid flat plate have been calculated using the present approach. The obtained results agree well with experiments, exact solutions and other independent numerical computations. This demonstrates the capability of the present approach to deal with not only violent free-surface impact problems but also hull cavitation associated with underwater explosions. Performance analysis reveals that the running time cost of numerical simulations is dramatically reduced by use of GPUs with much less consumption of electrical energy than on the CPU

Pure and aerated water entry of a flat plate

This paper presents an experimental and numerical investigation of the entry of a rigid square flat plate into pure and aerated water. Attention is focused on the measurement and calculation of the slamming loads on the plate. The experimental study was carried out in the ocean basin at Plymouth University’s COAST laboratory. The present numerical approach extends a two-dimensional hydro-code to compute three-dimensional hydrodynamic impact problems. The impact loads on the structure computed by the numerical model compare well with laboratory measurements. It is revealed that the impact loading consists of distinctive features including (1) shock loading with a high pressure peak, (2) fluid expansion loading associated with very low sub-atmospheric pressure close to the saturated vapour pressure, and (3) less severe secondary reloading with super-atmospheric pressure. It is also disclosed that aeration introduced into water can effectively reduce local pressures and total forces on the flat plate. The peak impact loading on the plate can be reduced by half or even more with 1.6% aeration in water. At the same time, the lifespan of shock loading is prolonged by aeration, and the variation of impulse is less sensitive to the change of aeration than the peak loading

Brand champion behaviour: Its role in corporate branding

yesBrand champions are responsible for encouraging employee commitment to the corporate brand strategy. They strongly believe in and identify with the brand concept—the company’s selected brand meaning, which underpins corporate brand strategy implementation. We conducted research to explore why and how brand champion behaviour operates within companies implementing a new corporate brand strategy. Against a backdrop of growing interest in brand champion behaviour in corporate branding research, we grounded our study in social identity theory and rhetorical theory from change management literature. Our findings show that articulating a compelling brand vision, taking responsibility, and getting the right people involved are the most widely used strategies by brand champions. We uncover how rhetorical strategies within brand champion behaviour generate employee commitment to a new corporate brand strategy. The dimension of brand champion behaviour that is effective depends on the type of brand evolution, involving shifts in the brand concept. We make suggestions for further studies underpinned by social identity theory and rhetorical theory to investigate brand champion behaviour processes within companies introducing a new corporate brand strategy

Numerical simulation of wave energy converters using Eulerian and Lagrangian CFD methods

During the last years many concepts of wave energy converters (WEC) have been proposed. All are designed to generate energy at competitive economic rates in average sea states and also to survive extreme wave conditions. Due to the complexity of most offshore wave energy devices and their motion response in different sea states, physical tank tests are common practice for WEC design. Full scale tests are also necessary, but are expensive and only considered once the design has been optimised. Computational Fluid Dynamics (CFD) is now recognised as an important complement to traditional physical testing techniques in offshore engineering. Once properly calibrated and validated to the problem, CFD offers a high density of test data and results in a reasonable timescale to assist with design changes and improvements to the device. Within the EPSRC funded research project "Extreme Wave Loading on Offshore Wave Energy Devices: a Hierarchical Team Approach" the two WECs Pelamis and the Manchester Bobber are investigated using different Eulerian and Lagrangian CFD techniques. Both devices float on the water surface and generate the electricity from the motion of the waves. Pelamis' overall movement is limited due to the mooring system, but the individual segments are allowed to move in 6 degrees of freedom and interact with the waves and the adjacent segments. The dynamics of the Manchester Bobber comprise the nominally vertical motion of the floats, which are arranged in an array, and the highly complex interactions between the floats and the waves. Two test cases leading towards simulation of the full dynamics of Pelamis and the Manchester Bobber have been modelled using different CFD techniques. The problems involve the interaction between regular waves and fixed horizontal cylinders of different levels of submergence. Results are compared with experimental data to calibrate the CFD codes. Furthermore, results for the fluid-structure interaction of an oscillating cone on the water surface are presented. The complexity of this problem is rather high, as it involves rigid body motion of an axisymmetric body. The motion is not linear, but is generated as a Gaussian focused wave packet. Complex jet-effects occur at the intersection of water and body surface. These and the forces on the structure are discussed. Four different CFD codes are applied to simulate the test cases: Smoothed Particle Hydrodynamics, a Cartesian Cut Cell method based on an artificial compressibility method with shock capturing for the interface, and two pressure-based Navier-Stokes codes, one using a Finite Volume and the other a control volume based Finite Element approach. © 2010 by The International Society of Offshore and Polar Engineers (ISOPE)

Numerical and Experimental Study of a Surging Point Absorber Wave Energy Converter

<p>Published in EWTEC 2009</p> <p> </p