Fluid mechanics, especially theoretical studies of transitional and turbulent shear flows in open systems, numerical fluid mechanics, coherent structures in turbulent flows, nonlinear dynamical systems, wind turbine aerodynamics.
AFOSR - Characterization and Low-Dimensional Modeling of Urban Fluid Flow
This work is aimed at addressing the problem of operating micro-air vehicles (MAVs) in the geometrically and fluid mechanically complex environment of urban street canyons. Both operation and design of such flight vehicles are hampered by a lack of accurate data on the statistics of the unsteady turbulent wind flows in such an environment. Given the difficulty of this environment, in conjunction with the restrictions in the amount of propulsion energy available for MAVs, it becomes paramount to optimize both the precision and efficiency of maneuvering such flight vehicles. Moreover, in order to design MAVs that are well adapted for the flow conditions and resulting gust loads they are likely to encounter in an urban flow field, a better understanding of the spatio-temporal structure of these velocitiy fields is highly desirable.
In order to address these issues, we are developing a methodology that will allow us to accurately characterize the statistics of wind gusts that are relevant for the operation of micro-air vehicles in the urban environment. Based on low-dimensional descriptions via truncated sets of POD modes we can generate good approximations of the gust statistics as they are encountered by actual MAVs. Detailed descriptions of both spatial and temporal statistics and combinations thereof can be obtained either from field measurements, or from wind tunnel experiments.
As a result, we have access to both a fully three-dimensional, well-resolved spatial description (encoded in our numerically generated POD modes), as well as very long time series (from our wind tunnel measurements) that currently cannot be economically generated in computations. Together this information allows us to construct comprehensive models for the spatio-temporal structure of urban flow fields.
Aerodynamics of Vertical-Axis Wind Turbines
The aerodynamics of vertical-axis wind turbines (VAWT) is much less understood than the one of the conventional horizontal-axis machines (HAWT) we find in wind farms around the country. This is despite the fact that VAWTs can achieve efficiencies and performance that is equivalent to HAWT, while offering distinct advantages in certain situations, such as building-integrated turbines, and whenever the option to modify the aspect ratio of the turbine cross-section is advantageous.
The complexity of VAWT aerodynamics is associated with the inherent unsteadiness of the flow around their blades that is characterized by periodically changing angles of attack and velocity magnitude. In addition, many VAWTs operate in a regime were the flow around the turbine blades is stalled during part of the rotational period, leading to complex dynamic stall effects. In our group we are trying to get an improved fundamental understanding of these flow phenomena, through both numerical simulation and experiment.
Urban Wind Power Generation
The subject of this project area is the generation of electrical energy using wind power near or at the point of its consumption. Efficient wind turbines of appropriate type are integrated with the design of multi-use high-rise buildings or, in some cases, wide span structures. Comprehensive and additional power generation may be achieved by the addition of solar electric, solar thermal or concentrated solar installations.
Energy is consequently generated at its point of use for users of the building with surplus for export. Considerable power losses incurred from long distance power transmission are thus eliminated especially when point of generation is a long distance from point of consumption. Regional and smart power networks would benefit from this concept.
Research, development and testing work already done over several years at IIT indicate the essential feasibility of this concept. Optimization of the work and testing the final configurations is now required leading to larger scale trials and commercial launching. The impact of this project could lead to important economic and technical advances in energy production and distribution.
Optimal Placement of Wind Turbines in Wind Farms
Arrangement of the wind turbines is crucial to the efficiency of wind farms. The objective of this work is to develop a systematic methodology for the optimal placement of wind turbines in wind farms. The methodology we are developing for this project can be decomposed into the following three stages:
- Develop a wind power generation model that takes into account both the aerodynamics and the uncertainty of the wind conditions.
- Use the particle swarm optimization method to find the optimal locations of a given number of wind turbines maximizing the wind power generation.
- Construct the economic model combining both the profits from the optimal wind power generation and the costs of installation and maintenance of the wind turbines. Then choose the optimal number of wind turbines to be installed to maximize the economic model.
Through the three stages of modeling and optimization, the proposed method can find the best arrangement of the wind turbines for the wind farm based on its geophysical location and environmental factors including weather, terrain topography, natural landscaping, etc.
Heisenberg Grant of DFG (German National Science Foundation), 1996-2001
Hermann-Reissner-Award for Aerospace Engineering, 1992
Summa Cum Laude (mit Auszeichnung) for Dr.-Ing., University of Stuttgart, 1991
Fellowship, Ph.D.-program of Studienstiftung des deutschen Volkes, 1989-1991
Honors (mit Auszeichnung bestanden) for Dipl.-Ing., University of Stuttgart, 1988
Award of Bundeswettbewerb Mathematik (German National Mathematics Contest), 1978
Professional Society Memberships
ZHOU, T.; REMPFER, D. (2012) "Numerical Study of Detailed Flow Field and Performance of Savonius Wind Turbines." Renewable Energy, Vol. 51, pp. 373–381.
BOCKENFELD, D.; CHEN, H.; KAMINSKI, M. D.; ROSENGART, A. J.; REMPFER, D. (2010) "A Parametric Study of a Portable Magnetic Separator for Separation of Nanospheres from the Circulatory System." Separation Science and Technology, Vol. 45, No. 3, pp. 355–363.
MOKHASI, P.; REMPFER, D. (2009) "Nonlinear System Identification Using Radial Basis Functions." International Journal of Numerical Methods in Fluids.
MOKHASI, P.; REMPFER, D. (2009) "Sequential Estimation of Velocity Fields Using Episodic POD." Physica D, Vol. 237, No. 24, pp. 3197–3213.
MOKHASI, P.; REMPFER, D. (2008) "Predictive Flow-Field Estimation." Physica D.
SPASOV, M.; REMPFER, D.; MOKHASI, P. (2008) "Simulation of a Turbulent Channel Flow with an Entropic Lattice Boltzmann Method." International Journal of Numerical Methods in Fluids, Vol. 60, No. 11, 2009.
ASEN, P.-O.; KREISS, G.; REMPFER, D. (2008) "Direct Numerical Simulations of Localized Disturbances in Pipe Poiseuille Flow." Theoretical and Computational Fluid Dynamics.
REMPFER, D. (2008) "Two Remarks on a Paper by Sani et al." International Journal of Numerical Methods in Fluids, Vol. 56, No. 10, pp. 1961–1965.
CHEN, H.; BOCKENFELD, D.; REMPFER, D.; KAMINSKI, M. D.; ROSENGART, A. J. (2007) "Three-dimensional Modeling of a Portable Medical Device for Magnetic Separation of Particles from Biological Fluids". Physics in Medicine and Biology, Vol. 52, pp. 5205–5218.
CHEN, H.; BOCKENFELD, D.; REMPFER, D.; RITTER, J. A.; KAMINSKI, M. D.; LIUA, X.;ROSENGART, A. J. (2007) "A Comprehensive In-vitro Investigation of a Portable Magnetic Separator Device for Human Blood Detoxification." Physics in Medicine and Biology, Vol. 52, pp. 6053–6072.
CHEN, H.; BOCKENFELD, D.; REMPFER, D.; KAMINSKI, M. D.; LIUA, X.; ROSENGART, A. J. (2007) "Preliminary 3-D Analysis of a High Gradient Magnetic Separator for Biomedical Applications." Journal of Magnetism and Magnetic Materials , Vol. 320, Nos. 3–4, pp. 279–284.
JOSHI, V.; REMPFER, D. (2007) "Energy Analysis of Turbulent Channel Flow Using Bi-Orthogonal Wavelets." Phys. Fluids, Vol. 19, No. 8, pp. 085106–12.