User:Egm6936.f09/Turbulence seminars

= Charles Meneveau, UF, 19 Jan 2010 =

William F. Powers Lecture

Tuesday, January 19, 2010, 4:00 pm in Room 303 MAE-A

Fluid mechanics and turbulence in the wind-turbine array boundary layer

Charles Meneveau

Department of Mechanical Engineering and Center for Environmental and Applied Fluid Mechanics Johns Hopkins University

Abstract

When wind turbines are deployed in large arrays, their ability to extract kinetic energy from the flow decreases due to complex interactions among them and the atmospheric boundary layer. In order to improve our understanding of the vertical transport of momentum and kinetic energy across a boundary layer flow with wind turbines, Large Eddy Simulations and wind-tunnel experimental studies are undertaken. A suite of LES, in which wind turbines are modeled using the classical `drag disk' concept, are performed for various wind turbine arrangements, turbine loading factors, and surface roughness values. In the wind tunnel studies, the boundary layer flow includes a 3 by 3 array of lightly loaded model wind turbines. The results of both the simulations and experiments are used to shed light on the vertical turbulent transport of momentum and kinetic energy across the boundary layer. The results are also used to develop improved models for effective roughness length scales. This work is a collaboration with M. Calaf, J. Meyers, R. Cal, J. Lebron, H.S. Kang, and L. Castillo, and is supported by the National Science Foundation.

Biography

Charles Meneveau is the Louis M. Sardella Professor in the Department of Mechanical Engineering at Johns Hopkins University. He also has a joint appointment in the Geography and Environmental Engineering Department and serves as the director of the Center of Environmental and Applied Fluid Mechanics at Johns Hopkins. He received his B.S. degree in Mechanical Engineering from the Universidad Técnica Federico Santa María in Valparaíso, Chile, in 1985 and M.S, M.Phil. and Ph.D. degrees from Yale University in 1987, 1988 and 1989, respectively. During 1989/90 he was a postdoctoral fellow at the Stanford University/NASA Ames' Center for Turbulence Research.

Professor Meneveau has been on the Johns Hopkins faculty since 1990. His area of research is focused on understanding and modeling hydrodynamic turbulence, and complexity in fluid mechanics in general. He combines experimental, computational and theoretical tools for his research. Special emphasis is placed on the multiscale aspects of turbulence, using appropriate tools such as subgrid-scale modeling, downscaling techniques, fractal geometry, wavelet analysis, and applications to Large Eddy Simulation. The insights that have emerged from Professor Meneveau’s work have led to new numerical models for Computational Fluid Dynamics and applications in engineering and environmental flows. With his students and co-workers, he has authored over 110 peer-reviewed articles. In 2005 the ISI has recognized the article “Scale Invariance and Turbulence Models for LES" (2000, with J. Katz) as a "Highly Cited Article", placing it in the top 1% within its field.

Professor Meneveau is a foreign corresponding member of the Chilean Academy of Sciences, and a Fellow of the American Academy of Mechanics, the U.S. American Physical Society and the American Society of Mechanical Engineers. He has received the 2004 UCAR Outstanding Publication award (with students and other colleagues at JHU and NCAR), the Johns Hopkins University Alumni Association's Excellence in Teaching Award (2003), and the APS' François N. Frenkiel Award for Fluid Mechanics (2001). He is now the Joint Editor-in-Chief of the Journal of Turbulence, an Associate Editor for the Journal of Fluid Mechanics and is a member of the Editorial Committee of the Annual Reviews of Fluid Mechanics. Between 2001 and 2004 he was an Associate Editor for Physics of Fluids.

= Michael Reeks at UF on Tue, 27 Oct 2009 =

"HOW DOES IT RAIN? The influence of turbulent structures in clouds", by Michael Reeks, School of Mechanical & Systems Engineering, University of Newcastle, UK. Abstract When warm air rises it expands adiabatically, cooling until it becomes supersaturated with water vapor. The water vapor then nucleates by inhomogeneous nucleation and the droplets so formed grow in size by condensation. The process continues until the droplets are large enough to precipitate by gravitational settling. Unfortunately the timescales for precipitation by this process alone are much too long (~103 greater than what is observed). Furthermore the size distribution of the droplets is very broad contrary to simple models for condensation growth. So how does it rain? It is believed that a major enhancement of the growth of droplets and the very broad range of size distribution is due to the way droplets interact with both the large and small scales of the turbulence in a cloud. Furthermore, contrary to traditionally held views, droplets do not mix in a turbulent flow but segregate out into regions of high strain rate in the flow. My lecture is thus divided into 2 parts. In the first part I will describe how the small and large scales of the turbulence in a cloud influence the condensation rate and how that leads to a broad size distribution of droplet size consistent with what is observed. And in the second part, I will describe how the droplet segregation depends upon the persistence and morphology of the small scales of the turbulence and in turn how the segregation process leads to enhancement of the collision processes and finally to the reductions in  times scales for precipitation.

It is stated in his seminar that DNS is very expensive and can only be applicable to low Reynolds number flows, whereas particle flow in clouds has a wide range of Reynolds number. LES, which is less expensive than DNS, is therefore more suitable for particle-laden flow in clouds. Reeks introduced into the simulation a random excitation in terms of Fourier expansion (that looks like a series of plane waves), with coefficients concocted to preserve incompressibility. Question: Could one consider the carier flow in a cloud to be incompressible? Not clear, need to clarify.

Discussed in the seminar were some issues related to whether it was appropriate to use the two-way coupling the way Reeks did, or was it more equivalent to a one-way coupling ?? Need to clarify.