Emmanuel Guilmineau

Effect of Side Wind on a Simplified Car Model: Experimental and Numerical Analysis

A prior analysis of the effect of steady cross wind on full size cars or models must be conducted when dealing with transient cross wind gust effects on automobiles. The experimental and numerical tests presented in this paper are performed on the Willy square-back test model. This model is realistic compared with a van-type vehicle; its plane underbody surface is parallel to the ground, and separations are limited to the base for moderated yaw angles. Experiments were carried out in the semi-open test section at the Conservatoire National des Arts et Metiers, and computations were performed at the Ecole Centrale de Nantes (ECN). The ISIS-CFD flow solver, developed by the CFD Department of the Fluid Mechanics Laboratory of ECN, used the incompressible unsteady Reynolds-averaged Navier-Stokes equations. In this paper, the results of experiments obtained at a Reynolds number of 0.9×10 6 are compared with numerical data at the same Reynolds number for steady flows. In both the experiments and numerical results, the yaw angle varies from 0 deg to 30 deg. The comparison between experimental and numerical results obtained for aerodynamic forces, wall pressures, and total pressure maps shows that the unsteady ISIS-CFD solver correctly reflects the physics of steady three-dimensional separated flows around bluff bodies. This encouraging result allows us to move to a second step dealing with the analysis of unsteady separated flows around the Willy model.

Incompressible flow calculations of blunt leading edge separation for a 53 degree swept diamond wing (Invited)

One of the challenges in simulating aerodynamic flows is the accurate prediction of onset and progression of separation. In earlier STO research, it was found that various predictions for three-dimensional airfoils at angle of attack showed different pitch moments due to differences in the predicted location of separation areas. Therefore, a diamond shaped wing was developed within the NATO STO AVT-183 research group, to isolate the blunt leading edge separation and understand the underlying physical mechanisms. In the present paper, two viscous-flow solvers dedicated to hydrodynamic applications are used to predict the flow around the diamond wing. Both codes solve the incompressible flow using unstructured grids and finite volume discretisation. The results comprise different grid set-ups, with/without peniche and different turbulence models ranging from isotropic or anisotropic statistical turbulence closures to hybrid LES turbulence models. Moreover, the role played by the discretisation error is assessed by using anisotropic automatic grid refinement procedures based on flow-related criteria. A detailed discussion is made, highlighting the similarities and differences of the results, the respective influence of modeling and discretisation errors and showing the potential of CFD tools to predict the type of flow under consideration.

A comparison between full RANSE and coupled RANSE-BEM approaches in ship propulsion performance prediction

The paper compares the development of the coupling between a viscous Reynolds-averaged Navier-Stokes (RANSE) method and an inviscid Boundary Element method (BEM) with application to the prediction of the propulsive performance of a propelled ship. The BEM computational model is implemented into the PRO-INS code developed by CNR-INSEAN. It is based on a boundary integral formulation for marine propellers in arbitrary onset non-cavitating and cavitating flow conditions. The RANSE approach is based on the unstructured finite-volume flow solver ISIS-CFD. An essential feature for full RANSE simulations with the ISIS-CFD code developed by ECN-CNRS is in the use of a sliding grid technique to simulate the real propeller rotating behind a ship hull. The STREAMLINE tanker and propeller are proposed as validation test case. Full RANSE simulations are performed for design speed only, while hybrid RANSE/BEM self-propulsion computations are performed for a speed range. Both computations are compared with experimental data and show good agreement for ship resistance and for propeller thrust and torque.Copyright

Impact of Turbulence Closures and Numerical Errors for the Optimization of Flow Control Devices

The simulation of turbulent flows including active flow control devices, such as synthetic jets, is still a difficult task. Numerical parameters (grid size, time step, etc.) may have a significant influence on the result, while the choice of the turbulence closure is often critical. In this context, we investigate the use of a Kriging-based global optimization method to determine optimal control parameters. The objective of this study is twofold: firstly, we quantify the impact of some numerical and modeling parameters on the Kriging model constructed using a design of experiment approach. In a second phase, we conduct an optimization process and measure the impact of numerical and modeling errors on the optimal control parameters found. An approach to account for some numerical errors during the optimization is finally presented. The turbulent flow over a backward facing step, including a synthetic jet actuator, is considered as test-case. The time-averaged recirculation length is considered as control criterion, while jet frequency and amplitude are optimized.

Hybrid LES–RANS-Coupling for Complex Flows with Separation

The investigations carried out within the French-German DFG-CNRS Research Initiative on ‘LES of Complex Flows (FOR 507)’ aimed at the development and application of hybrid LES-RANS methods for turbulent flows with separation. The objectives were twofold.On the one hand, the performance of a well-established hybrid approach, namely the detached-eddy simulation, was studied based on really challenging test cases such as the flow over the 3D hill and around the Willy car model. On the other hand, a new hybrid LES-RANS methodologywas set up which aims at overcoming certain drawbacks associated with DES. It relies on the coupling of a near-wall RANS model with LES for the outer flow which allows an appropriate representation of large-scale flow phenomena. In order to take the anisotropies in the near-wall region rigorously into account, an explicit algebraic Reynolds stress model was chosen and its performance in comparison with linear eddy-viscosity models was analyzed for different test cases such as the flow over 2D hills and in a 3D diffuser. A further advantage of the new hybrid method is that no interface predefinition is required. Instead the interface is dynamically determined on-the-fly based on instantaneous physical quantities which guarantees that local changes in the flow are accounted for.

Numerical Study of the Turbulent Flow Around a Square Cylinder Near a Wall

Calculations are reported for the flow around a two-dimensional, square cylinder at Re = 22,000 (based on the prism side dimension, D, and the free-stream velocity) placed at various distances from an adjacent wall. The nominal boundary layer thickness is 1.5D. Experiments have indicated that unsteady vortex shedding is suppressed when the wall is relatively close to the cylinder. The turbulent fluctuations are simulated with three turbulence models: the one-equation model of Spalart & Allmaras (1992), the two-equations SST K–ω model (Menter, 1993) and a Reynolds stress Rij –ω closures (Deng & Visonneau, 1999). The paper consists in comparing simulation and experimental results for configurations S/D = 1 (periodic case) and S/D = 0.25 (stationary case). Predicted and measured distributions of the mean velocity, Reynolds stress tensor and surface pressures are compared. Although the agreement is very good in general, observed discrepancies are discussed.© 2002 ASME

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.

Numerical Simulation of Flow Around a Generic Pickup With ISIS-CFD

Computational Fluid Dynamics (CFD) is used to simulate the flow over a pickup truck. The flow solver used is ISIS-CFD developed by the CFD Department of the Fluid Mechanics Laboratory of Ecole Centrale de Nantes. CFD simulations are carried out with the Explicit Algebraic Reynolds Stress Model (EARSM) turbulence model and the Detached Eddy Simulation (DES). The focus of the simulation is to assess the capabilities of ISIS-CFD for vehicle aerodynamic development for pickup trucks. Detailed comparisons are made between the CFD simulations and the existing experiments for a generic pickup truck. The comparisons between the simulation results and the time-averaged measurements reveals that the CFD calculations are able to track the flow trends.Copyright