Effect of side wind on a simplified car model: Experimental and numerical analysis

International audienceA prior analysis of the effect of steady cross wind on full size cars or models must be conducted when dealing with transient cross wind gusts 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 to a van-type vehicle; its plane under-body 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 Métiers (CNAM) 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 0.9 106 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° to 30°. 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

CFD Simulation of PMM Motion in Shallow Water for the DTC Container Ship

International audienceThis paper is devoted to the validation exercises with the ISIS-CFD code conducted for the test cases proposed for the MASHCON conference. CFD simulations have been performed for the 4 different pure yaw and pure sway test cases under shallow water condition. Predicted results are compared with the measurement data provided by FHR

Assessment of turbulence closures for detached flows control

International audienceIn this study, di fferent turbulence closures are compared for the prediction of detached flows over a backward facing step, including a flow control device based on oscillatory suction and blowing. An automated optimization of the control parameters (frequency and amplitude) is carried out for the di fferent closures, to assess the ability of Reynolds-Averaged Navier-Stokes (RANS) models to predict similar optimal parameters

Numerical and Modeling Issues for Optimization of Flow Control Devices

International audienceActive flow control has been a growing research area for the last decades, since this approach demonstrated its ability to improve aerodynamic performance, for a large range of applications. It is especially appealing in case of separated flows, for which natural instability phenomena can be efficiently exploited to manipulate flow characteristics using periodic flow excitation.In this context, a major difficulty is related to the choice of actuation parameters, such as excitation frequency, amplitude, location, to obtain the expected flow response. In cases implying a single isolated actuator, it is quite easy to carry out an experimental or numerical study to determine efficient control parameters. However, in the perspective of industrial applications based on hundreds of actuators, this task is far from being straightforward and the use of an automated optimization strategy is thus proposed, in the spirit of previous works.While optimization algorithms are now commonly employed in aerodynamics for shape optimization purpose, their use in the context of control devices yields new issues, which are detailed in the present study. In particular, we aim at quantifying the impact of the numerical errors (discretization, convergence) and modeling uncertainties (turbulence closure) on the optimization procedure. The choice of the optimization parameters is also explored. Different test-cases and flow solvers are considered in this study, e.g. NACA airfoil, backward facing step, 25 degrees ramp

Comparison of turbulence closures for optimized active control

International audienceActive flow control strategies, such as oscillatory blowing / suction, have proved their efficiency to modify flow characteristics for various purposes (e.g. skin friction reduction, separation delay, etc) in case of rather simple configurations. To extend this approach to industrial cases, the simulation of a large number of devices at real scale and the optimization of parameters are required. In this perspective, numerical simulations based on Reynolds-Averaged Navier-Stokes (RANS) equations seem to be the most appropriate framework, despite the well known limitations of turbulence closures in the context of unsteady separated flows. Thus, the objective of this work is to evaluate the ability of RANS models for the optimization of control devices and compare the results obtained using different turbulence closures. In this perspective, an incompressible RANS solver for unstructured grids is coupled with a surrogate-based global optimizer. The resulting tool is applied to derive an optimal actuation, based on an oscillatory blowing / suction device, for a set of turbulence closures including two-equation eddy-viscosity models and an explicit algebraic Reynolds stress model. As test-case, the reduction of the separation length for a backward facing step is targeted. Results are finally compared and analyzed, in terms of flow characteristics and optimal actuation parameters found

Towards automated computation with uncertainty estimation for industrial simulation of ship flow

Adaptive grid refinement is tested for routine, automated simulations of ship resistance in calm water. A simulation protocol for these computations is fine-tuned on one test case and then applied unchanged to three different cases. The solutions are numerically accurate and compare well with experiments. Effective numerical uncertainty estimation increases the trustworthiness of the solutions

Numerical study of VIV over a flexible riser

This study is based on the simulation of the fluid-structure interaction on risers. We aim to quantify the structural response of these long flexible pipes, used for the extraction of offshore petroleum when they are subjected to marine currents. Toe occurring phenomenon is known as VIV (Vortex induced VIbration). These problems are a relevant challenge for several offshore companies, which are associated with K-Epsilon in a Citeph project. Toe project's goal is to use the FSI simulation tool developed by K-Epsilon and initially used for the simulation offlexible membranes such as sails, to model these VIV phenomena. Toe problem of VIV in the case of a riser is a strongly coupled problem, meaning that the added mass is not negligible compared to the mass of the structure. This can be challenging for most fluid-structure interaction software. A strongly coupled algorithm is presented [6]. First, numerical results of fluid around cylinders are presented and compared to experimental results ([l], [10]) with several turbulence models, and time step sizes. Toen, Chaplin's benchmark is presented with experimental / numerical comparison [3]

Effet de vent latéral sur un modèle simplifié de voiture par une méthode DES

Cette étude traite des effets de vent latéral sur un modèle simplifié de voiture, appelée le modèle de Willy. Ce modèle sans arête sur la partie avant et avec un culot droit est plus pratique pour l'analyse des séparations instationnaires qui sont limitées au coté sous le vent et au culot. Les simulations sont réalisées par une approche DES et les résultats sont comparées à des données expérimentales ainsi qu'à des résultats obtenus par une simulation RANSE

CFD, potential flow and system-based simulations of fully appended free running 5415m in calm water and waves

5415M, course-keeping, waves, CFD, validation, NATO AVT-161 Abstract. The seakeeping ability of ships is one of the aspects that needs to be assessed during the design phase of ships. Traditionally, potential flow calculations and model tests are employed to investigate whether the ship performs according to specified criteria. With the increase of computational power nowadays, advanced computational tools such as Computational Fluid Dynamics (CFD) become within reach of application during the assessment of ship designs. In the present paper, a detailed validation study of several computational methods for ship dynamics is presented. These methods range from low-fidelity system-based methods, to potential flow methods, to high-fidelity CFD tools. The ability of the methods to predict motions in calm water as well as in waves is investigated. In calm water, the roll decay behavior of a fully appended self-propelled free running 5415M model is investigated first. Subsequently, forced roll motions simulated by oscillating the rudders or stabilizer fins are studied. Lastly, the paper discusses comparisons between experiments and simulations in waves with varying levels of complexity, i.e. regular head waves, regular beam waves and bi-chromatic waves. The predictions for all methods are validated with an extensive experimental data set for ship motions and loads on appendages such as rudders, fins and bilge keels. Comparisons between the different methods and with the experiments are made for the relevant motions and the high fidelity CFD results are used to explain some of the complex physics. The course keeping and seakeeping of the model, the reduction rate of the roll motion, the effectiveness of the fin stabilizers as roll reduction device and the interaction of the roll motion with other motions are investigated as well. The paper shows that only high-fidelity CFD is able to accurately predict all the relevant physics during roll decay, forced oscillation and sailing in waves