Ashvinkumar Chaudhari

Large Eddy Simulation of Boundary-Layer Flows over Two-Dimensional Hills

Large Eddy Simulations (LES) are performed for turbulent boundary-layer flows over two-dimensional (2D) hills or ridges of two different slopes at Reynolds number equal to 3,120 based on the hill height and the free stream velocity. The surface of the hill is assumed to be aerodynamically smooth. The hill height is considerably smaller than the boundary-layer depth. The hill models used in this study are the same as those used in the RUSHIL wind tunnel experiment carried out by Khurshudyan et al. (United States Environmental Protection Agency Report, EPA-600/4-81-067, 1981) and LES results are compared with the wind tunnel measurements. This study focuses on the overall flow behaviour changes as a function of the hill slope. The results of the mean velocity, the flow separation, and the turbulence quantities are discussed in the paper. It is shown that LES produces overall satisfactory results on the turbulent flow over the 2D hills. Especially for less steep hill, the flow behaviour is well predicted by LES.

Numerical study of how stable stratification affects turbulence instabilities above a forest cover: application to wind energy

Forest areas are of increasing interest for the wind energy industry. However, they induce complex flows with strong shear and high turbulence levels. Stably stratified atmospheric conditions, typical during nighttime and especially in winter, add to the challenge of accurately estimating wind resources. Such conditions typically imply strong wind shear and cause larger structural fatigue loads to wind turbines. In this work, large-eddy simulations are performed in neutral and stable conditions over a forest to analyze the influence of the combined effect of forest and thermal stabilities on the unsteady characteristics of the wind flow. Taking advantage of the unsteady resolution provided by the simulations, turbulent characteristics of each thermal stability including the organization of turbulent structures are presented. The resulting comparison between the two cases is put into perspective for wind energy applications.

Full-Scale Experimental Validation of Large-Eddy Simulation of Wind Flows over Complex Terrain: The Bolund Hill

Numerical simulation of local atmospheric flows around a complex topography is important for many wind energy applications, as it can help in locating and optimizing wind farms. The aim of this paper is to simulate the atmospheric flows over the challenging and complex site of Bolund Hill (in Denmark) using Large-Eddy Simulations (LES) and to validate the simulation methodology against full-scale measurements. Wind prediction over a potential inland wind park is the main application of the validated LES methodology. For the first time, LES-based results from more than one wind direction of the Bolund case are reported and analyzed in detail herein. The LES results from each direction are compared with the Bolund field data in a quantitative way, for example, by using simulation error. According to the results comparison, it is found that the LES model provides by far the most accurate prediction for turbulent kinetic energy. The simulation error of this LES model for predicting turbulent kinetic energy is 19% smaller than that of all other models, whether experimental or numerical, employed previously over Bolund Hill.

Numerical Modelling of Wind Flow over Hills

The paper demonstrates when the Wind Atlas Analysis and Application Program (WAsP) is comparable to Computational Fluid Dynamics (CFD) in order to use the WAsP wind prediction later for time consuming CFD simulations. Three different numerical methods (WAsP, RANS, LES) for observation of wind flow over the hills are described and compared with the wind-tunnel experiment. The paper shows that WAsP provides reasonably realistic results for the flow over the commonly found in nature shallow hills.

Numerical Study of the Impact of Atmospheric Stratification on a Wind-Turbine Performance

This paper is aimed to obtain a better understanding of how a stably stratified atmospheric condition involving low-level jet, in comparison to neutral condition, affects to a wind-turbine performance. In this work, large-eddy simulation approach in combination with actuator-line model is used to simulate the wake flows in an infinitely long wind farm. Here, numerical simulations of the wind-turbine wakes are carried out under the influence of neutral and stable conditions. The numerical results are compared between the two atmospheric conditions. It has been seen that the wind-turbine performance is highly influenced under the stable conditions. For an infinitely long wind-farm scenario, the power produced by a turbine under the stable condition is smaller by 81% than that under the neutral condition.

Effects of the canopy created velocity inflection in the wake development in a large wind turbine array

Large Eddy Simulations (LES) are carried out using OpenFOAM to investigate the canopy created velocity inflection in the wake development of a large wind turbine array. Simulations are performed for two cases with and without forest separately. Results of the simulations are further compared to clearly show the changes in the wake and turbulence structure due to the forest. Moreover, the actual mechanical shaft power produced by a single turbine in the array is calculated for both cases. Aerodynamic efficiency and power losses due to the forest are discussed as well.

Numerical Simulation of Bubble Coalescence and Break-Up in Multinozzle Jet Ejector

Designing the jet ejector optimally is a challenging task and has a great impact on industrial applications. Three different sets of nozzles (namely, 1, 3, and 5) inside the jet ejector are compared in this study by using numerical simulations. More precisely, dynamics of bubble coalescence and breakup in the multinozzle jet ejectors are studied by means of Computational Fluid Dynamics (CFD). The population balance approach is used for the gas phase such that different bubble size groups are included in CFD and the number densities of each of them are predicted in CFD simulations. Here, commercial CFD software ANSYS Fluent 14.0 is used. The realizable - turbulence model is used in CFD code in three-dimensional computational domains. It is clear that Reynolds-Averaged Navier-Stokes (RANS) models have their limitations, but on the other hand, turbulence modeling is not the key issue in this study and we can assume that the RANS models can predict turbulence of the carrying phase accurately enough. In order to validate our numerical predictions, results of one, three, and five nozzles are compared to laboratory experiments data for Cl2-NaOH system. Predicted gas volume fractions, bubble size distributions, and resulting number densities of the different bubble size groups as well as the interfacial area concentrations are in good agreement with experimental results.