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EEWR Brown Bag Seminar with Graduate Students Hossein Hezzaveh and Qi Li

Speaker: Hossein Hezzaveh and Qi Li, Graduate Students
Series: EEWR Brown Bag Seminars
Location: Engineering Quad E225
Date/Time: Friday, April 12, 2013, 12:00 p.m. - 1:00 p.m.

Abstract:

Speaker: Seyed Hossein Hezaveh
Title: Simulating Vertical Axis Wind Turbines In the ABL Bests

Due to the negative climatological effects of fossil fuels and the abundance of renewable energy resources, i.e. wind and sun, there is a growing interest in optimizing the efficiency of technologies that harvest these energies. Currently the majority of investigation and development in wind energy are directed towards Horizontal Axis Wind Turbines (HAWTs). However, HAWTs require specific wind and terrain conditions in order to be in their optimal working conditions. Vertical Axis Wind Turbines, (VAWTs), have several important advantages compared to HAWTs; mainly, they can be built with larger scales and their performance is not sensitive to wind direction. In this study, we developed a numerical model for simulating VAWTs in Atmospheric Boundary Layer (ABL) flows using Large Eddy Simulation (LES). In order to simulate the effect of turbine blades on the ABL flow, models of the blade forces are implemented into the LES, while tracking each blade position. We present results of interactions of VAWTs and Atmospheric Boundary Layer (ABL).

Speaker: Qi Li
Title: Momentum Roughness Length of Water Surfaces: New Perspectives on an Old Problem

Based on field experimental data collected over a lake, new approaches are proposed for the computation of the roughness lengths for momentum (z0m) of water surfaces. The new approach for computing and parameterizing z0m accounts for the surface wave state, which is shown to be relevant, and can thus capture and represent both types of waves. The results indicate that the classic Charnock's relation with constant coefficient parameterizes z0m sufficiently well only under wind-wave conditions. However, especially under swell conditions, we found that the surface waves’ phase velocity and directionality could affect the form drag at the surface, inducing a misalignment between the surface stress and wind velocity vectors. Under such conditions, the appropriate velocity parameter to use in similarity relations such as the one proposed by Monin and Obukhov is the wind velocity component parallel to the wave phase velocity, taken relative to that wave phase velocity. With this framework, instead of the conventional z0m, the critical height z0mr becomes the representative ‘momentum transfer roughness length’ in the new formulation of the Monin-Obukhov similarity theory. A new parameterization for z0mr is proposed based on wavelength and wave age, which gives consistent results under both wind-wave and swell conditions.