Jeroen Wackers

Multigrid solution method for the steady RANS equations

A novel multigrid method for the solution of the steady Reynolds-averaged Navier-Stokes equations is presented, that gives convergence speeds similar to laminar flow multigrid solvers. The method is applied to Menters one-equation turbulence model. New aspects of the method are the combination of nonlinear Gauss-Seidel smoothing on the finest grid with linear coarse-grid corrections, and local damping in the initial stages of the computation, to keep the solution stable; the damping needed is estimated with the nonlinear smoother. Efficiency on the finest grid is increased with full multigrid, second-order accuracy is obtained with defect correction. Tests on boundary layers and airfoil flows show the efficiency of the method.

A Simple and Efficient Space-Time Adaptive Grid Technique for Unsteady Compressible Flows

textabstractA space-time adaptive gridding technique for unsteady flows is presented. The technique is applied to the 2D unsteady Euler equations. The method is relatively simple, computationally very efficient and it can be easily adapted to other types of fluid flow. It consists of four parts: (i) a time-stepping algorithm that adapts the grid to the solution several times per coarse time step, (ii) a second-order accurate discretisation of the flow equations that combines a limited upwind discretisation of the fluxes with a two-step discretisation of the time derivatives and is well-suited for adapted grids, (iii) a simple data structure to store the solution and the grid geometry, and (iv) a refinement criterion. Two of these are tested, one based on the first and one on the second spatial derivative of the density. Results for two test problems, the classical forward-facing step problem and the shedding of vorticity from a flat plate, show that the method is much more efficient than comparable methods without adaptive gridding.

Can adaptive grid refinement produce grid-independent solutions for incompressible flows?

Abstract This paper studies if adaptive grid refinement combined with finite-volume simulation of the incompressible RANS equations can be used to obtain grid-independent solutions of realistic flow problems. It is shown that grid adaptation based on metric tensors can generate series of meshes for grid convergence studies in a straightforward way. For a two-dimensional airfoil and the flow around a tanker ship, the grid convergence of the observed forces is sufficiently smooth for numerical uncertainty estimation. Grid refinement captures the details of the local flow in the wake, which is shown to be grid converged on reasonably-sized meshes. Thus, grid convergence studies using automatic refinement are suitable for high-Reynolds incompressible flows.

Creating Free-Surface Flow Grids with Automatic Grid Refinement

The objective of this work is to create grids for free-surface water flow simulation entirely with automatic grid refinement. It is shown why it is necessary to refine the mesh iteratively as the solution converges and why refinement and derefinement of hexahedral cells must be treated anisotropically.The proposed refinement criterion is a combination of the pressure Hessian with refinement at the free surface, in order to capture the flow which drives the surface motion and the position of the surface itself. Smoothing is needed in the computation of the Hessian in order to remove oscillations in the pressure, the pressure Hessian is extrapolated through the free surface to remove its discontinuity there.Two test cases confirm that effective fine meshes for wave computation can be created with the proposed automatic refinement procedure.

Sliding Grids and Adaptive Grid Refinement for RANS Simulation of Ship-Propeller Interaction

Abstract The paper describes a computational approach for a numerical propulsion test. Key techniques concern the computation of free-surface viscous flows around propellers using the sliding grid technique and accurate wave capturing around the hull. An advanced numerical technique resolves very small-scale flow motion such as tip vortex. Efficiency and accuracy are balanced using an adaptive mesh refinement technique. This approach is validated against the KCS test case proposed for the Tokyo 2005 and Gothenburg 2010 CFD validation workshops.

Automatic grid adaptation for unstructured finite volumes

An automatic grid refinement method is presented for the simulation of ship flows. It provides directional refinement of unstructured grids and derefinement of refined grids for unsteady simulation, it is fully parallel and includes automatic dynamic load balancing. Different refinement criteria are implemented. Results are presented that confirm the increased accuracy of solutions obtained on refined grids. Refinement around the water surface proves to be very effective for the simulation of strong breaking waves. A pressure gradient criterion is shown to detect the main features of a ship flow and to be able to generate effective fine grids in their entirety.

Wake Prediction of a Marine Propeller: The Role of the Turbulence Closures

The marine propeller is the principal ship component to provide the necessary propulsion and when working at the aft of a ship, the propeller operates in a heterogeneous field because of the wake, created by the hull, which can have an effect on performance since the interaction is directly related to vibrations, noise and propulsion performances. So, the physical mechanisms that characterize the interactions between the propeller and the hull is very complex.

Wave and Current Generation in Wave Flumes Using Axial-Flow Pumps

We investigate a new concept for wave and current generation. It consists of axial-flow pumps driven such as to generate an oscillatory flow through an orifice located at one end of the flume. Oscillations of the flow lead to the generation of water waves at the free surface. If the average of the flow is different from zero, a current is generated that superposes on the waves. In this study, we explored the technical capabilities of this concept and the influence of geometric parameters on wave and current generation. We used numerical and experimental modelling. Most noticeably, the numerical results indicate that this concept is well suited for the generation of long and high waves. An experimental setup has been designed and built. We used it to make an experimental proof of concept for the wave and current generation, including waves propagating against the current.

Assessment of Turbulence Models for Flow Around Three-Dimensional Geometries

This paper presents a computational study of flow around three-dimensional geometries as the Ahmed body, which is a classical test case for automotive flow, but also as the JBC (Japan Bulk Carrier)which was first investigated in the framework of the Tokyo 2015 Workshop on Numerical Ship Hydrodynamics. For both test cases, an investigation of RANS (\(k-\omega \) SST and EARSM) and hybrid RANS-LES models (DES and IDDES) is conducted. All simulations have been performed with the ISIS-CFD flow solver, which is developed by Ecole Centrale de Nantes and CNRS. For both geometries, the hybrid RANS-LES models predict a high level of turbulent kinetic energy which is in better agreement with the experiments than the quantity predicted with a RANS turbulence model.

Assessment of statistical and hybrid LES turbulence closures for surface combatant DTMB5415 at 20° static drift condition.

This paper provides detailed validation of the complex free-surface flow around the surface combatant DTMB5415 at 20° static drift conditions. Particular emphasis is being placed on the onset and progression of the various vortical structures created at the sonar dome, at the free-surface and around bilge keels in conditions where free-surface breaking may occur. A detailed analysis of this complex free-surface flow is conducted on grids which are dynamically refined to capture the vortical structures. Local comparisons with recent experiments performed at the Iowa Institute of Hydraulic Research (IIHR) during a joint NATO-AVT (Advanced Vehicle Technologies) collaboration are used to perform detailed flow analysis and draw some conclusions about the actual potentialities of advanced Computation Fluid Dynamics (CFD) method in terms of physical and numerical modeling.Copyright