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Perret L.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment | Savory E.,University of Western Ontario
Boundary-Layer Meteorology | Year: 2013

An analysis of the dynamics of the flow over a street canyon immersed in an atmospheric boundary layer is presented, using particle image velocimetry measurements in a wind tunnel. Care was taken to generate a 1:200 model scale urban type boundary layer that is correctly scaled to the size of the canyon buildings. Using proper orthogonal decomposition (POD) of the velocity field and conditional averaging techniques, it is first shown that the flow above the opening of the canyon consists of a shear layer separating from the upstream obstacle, animated by a coherent flapping motion and generating large-scale vortical structures. These structures are alternately injected into the canyon or shed off the obstacle into the outer flow. It is shown that unsteady fluid exchanges between the canyon and the outer flow are mainly driven by the shear layer. Finally, using POD, the non-linear interaction between the large-scale structures of the oncoming atmospheric boundary layer and the flow over the canyon is demonstrated. © 2013 Springer Science+Business Media Dordrecht. Source

Gibbs J.A.,University of Oklahoma | Fedorovich E.,University of Oklahoma | Van Eijk A.M.J.,Applied Scientific Research | Van Eijk A.M.J.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment
Journal of Applied Meteorology and Climatology | Year: 2011

Weather Research and Forecasting (WRF) model predictions using different boundary layer schemes and horizontal grid spacings were compared with observational and numerical large-eddy simulation data for conditions corresponding to a dry atmospheric convective boundary layer (CBL) over the southern Great Plains (SGP). The first studied case exhibited a dryline passage during the simulation window, and the second studied case was used to examine the CBL in a post-cold-frontal environment. The model runs were conducted with three boundary layer parameterization schemes (Yonsei University, Mellor-Yamada-Janjić, and asymmetrical convective) commonly employed within the WRF model environment to represent effects of small-scale turbulent transport. A study domain was centered over the Atmospheric Radiation Measurement Program SGP site in Lamont, Oklahoma. Results show that near-surface flow and turbulence parameters are predicted reasonably well with all tested horizontal grid spacings (1, 2, and 4 km) and that value added through refining grid spacing was minimal at best for conditions considered in this study. In accord with this result, it was suggested that the 16-fold increase in computing overhead associated with changing from4- to 1-km grid spacing was not justified. Therefore, only differences among schemes at 4-km spacing were presented in detail. WRF model predictions generally overestimated the contribution to turbulence generation by mechanical forcing over buoyancy forcing in both studied CBL cases. Nonlocal parameterization schemes were found to match observational datamore closely than did the local scheme, although differences among the predictions with all three schemes were relatively small. © 2011 American Meteorological Society. Source

Guilmineau E.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment
SAE International Journal of Passenger Cars - Mechanical Systems | Year: 2010

This paper presents a finite-volume-based detached-eddy simulation for the prediction of flow around a passenger vehicle. The flow solver used is ISIS-CFD developed by the CFD Department of the Fluid Mechanics Laboratory of Ecole Centrale de Nantes. The validation is carried out by a crosswind simulation around the squareback Willy model. The model was designed in order that separations are limited to the region of the base for a moderate yaw angle. This model without sharp corners on the fore body and a square base is more convenient for the analysis of unsteady separations limited on its leeward side and base. The angle between the upstream velocity and the direction of the model varies between 0° and 30°. The results are compared to a previous numerical study obtained with a RANS simulation and experimental data. All comparisons (aerodynamic forces, wall pressures, and topology of total pressure) show that DES simulations give a better agreement with experimental data, particularly for the large yaw angles. The second model is the classical Ahmed body with the slant angle of 25°. With a RANS simulation, we have a massive separation in the wake while with DES simulation, the agreement is better with the experimental data. This paper shows that DES simulations give an improvement of the wake prediction for the automotive flow. © 2010 SAE International. Source

Maiboom A.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment | Tauzia X.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment
Fuel | Year: 2011

Automotive Diesel engines exhaust emissions must constantly be reduced to comply with more and more stringent regulations, all over the world. The introduction of water in the combustion chamber is already used on some large marine diesel engines to cut down NOx emission. In this paper an experimental study is conducted on a modern automotive 1.5 l HSDI Diesel engine while injecting a water-in-diesel emulsion (WDE) with a volumetric water-to-fuel ratio of 25.6%. Four injection strategies are considered with and without pilot injection, with two levels of injection pressure. First, the injection of WDE is compared to diesel-fuel in terms of combustion and NOx and PM emissions without using exhaust gas recirculation (EGR). Depending on the WDE fuelling rate and injection strategy (with or without a pilot injection before main injection), NOx emissions are most often reduced (of up to 50%), and PM emission are most often decreased as well (the maximum relative reduction being 94%). The combustion is largely affected by the injection of WDE as compared with pure diesel-fuel, the main observations being an increased of the ignition delay and an improved mixing-process between the fuel and the surrounding gases. After that, the use of WDE in parallel with EGR (with various EGR rates) is tested with the aim at improving the NOx-PM trade-off (reduction of NOx emission at a given PM emission level or reduction of PM emission at a given NOx emission level). The results show that this method is an effective way for NOx and PM emission reduction on an automotive Diesel engine. © 2011 Elsevier Ltd. All rights reserved. Source

Josset C.,CNRS Nantes Thermocinetique Lab | Babarit A.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment | Clement A.H.,CNRS Laboratory for Hydrodynamics, Energetics & Atmospheric Environment
Proceedings of the Institution of Mechanical Engineers Part M: Journal of Engineering for the Maritime Environment | Year: 2015

This paper describes a numerical wave-to-wire model of the second-generation wave energy converter called SEAREV. Governing equations are given in the time domain for the motion of the masses involved in the device and for the hydraulic power take-off (PTO) used to convert the motion into electricity. The hydrodynamic forces are derived using the standard linear potential theory. The memory term in the radiation force is replaced by additional states using the Prony method in order to change the equation of motion into the ordinary differential equation form. The PTO is composed of hydraulic rams, an accumulator, and a hydraulic generator, which delivers electricity when there is enough energy stored in the accumulator.Using the MATLAB Simulink tool, the equation of motion is solved to simulate the full device (including the power take-off) from the incident wave to the electricity delivered to the grid. Simulation results are presented in the paper and comparisons are made with a simpler PTO: a linear damper. They show that the torque applied to the hydraulic PTO must exceed a threshold to start absorbing energy, unlike the linear damping model. They also show that the power production can be very discontinuous, depending on the level of the incident wave power. This is due to the fact that the generator considered can transform the energy stored in the accumulator faster than the energy transmitted by the rams into the accumulator. It could therefore be interesting to use several generators to adapt the electrical energy production to the level of incident wave power, or a generator that could work efficiently at part load in order to achieve continuous energy production. © 2007 Institution of Mechanical Engineers. Source

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