Timothy Wei

Bio: Timothy Wei is an academic researcher from University of Nebraska–Lincoln. The author has contributed to research in topics: Vortex & Turbulence. The author has an hindex of 18, co-authored 55 publications receiving 1719 citations. Previous affiliations of Timothy Wei include Rutgers University & University of Medicine and Dentistry of New Jersey.

Modifying turbulent structure with drag-reducing polymer additives in turbulent channel flows

Abstract: New power spectra computed from LDA measurements of the fluctuating u- and v-velocity signals in a turbulent channel flow with and without drag-reducing polymer (polyethylene oxide) injection are presented. LDA data rates were sufficiently high to reconstruct the simultaneous time-dependent u- and v-velocity signals along with the time-dependent Reynolds stress signal. Time-averaged statistics of the turbulent flow are presented in conjunction with the power spectral measurements which show a dramatic reduction in both the v-velocity fluctuations and Reynolds stress fluctuations throughout the channel over all frequencies. There is also a redistribution of energy in the u-velocity fluctuations from high frequencies to low frequencies throughout the channel. Different injection conditions were examined: different polymer concentrations were injected at different flow rates such that the total amount of polymer in the channel remained constant. For certain polymer concentrations, ‘large’ negative Reynolds stress, -〈uv〉/uτ2 ≈ − 0.2, was measured in the near-wall region. In addition, there is a marked difference in the u-velocity spectra and the Reynolds stress spectra close to the wall for the different injection conditions.

Measurement of hydrodynamic force generation by swimming dolphins using bubble DPIV.

Abstract: Attempts to measure the propulsive forces produced by swimming dolphins have been limited. Previous uses of computational hydrodynamic models and gliding experiments have provided estimates of thrust production by dolphins, but these were indirect tests that relied on various assumptions. The thrust produced by two actively swimming bottlenose dolphins (Tursiops truncatus) was directly measured using digital particle image velocimetry (DPIV). For dolphins swimming in a large outdoor pool, the DPIV method used illuminated microbubbles that were generated in a narrow sheet from a finely porous hose and a compressed air source. The movement of the bubbles was tracked with a high-speed video camera. Dolphins swam at speeds of 0.7 to 3.4 m s −1 within the bubble sheet oriented along the midsagittal plane of the animal. The wake of the dolphin was visualized as the microbubbles were displaced because of the action of the propulsive flukes and jet flow. The oscillations of the dolphin flukes were shown to generate strong vortices in the wake. Thrust production was measured from the vortex strength through the Kutta–Joukowski theorem of aerodynamics. The dolphins generated up to 700 N during small amplitude swimming and up to 1468 N during large amplitude starts. The results of this study demonstrated that bubble DPIV can be used effectively to measure the thrust produced by large-bodied dolphins.

Laser anemometer measurements of Reynolds stress in a turbulent channel flow with drag reducing polymer additives

Abstract: Turbulence measurements with a high resolution laser Doppler anemometer in a channel flow with injection of a concentrated polyethylene oxide solution are reported. With polymer injection sufficient to yield a well mixed concentration of 10 ppm the total drag is reduced by 38%, but the sum of the Reynolds stress and the conventional molecular shear stress is only (2)/(3) of the force produced by the streamwise pressure gradient. Thus, the injected polymer solution has a significant effect on the turbulence and causes an additional retarding force equal to approximately (1)/(3) of the total drag.

Modeling Fluid Structure Interaction

Abstract: : The principal goal of this program is on integrating experiments with analytical modeling to develop physics-based reduced-order analytical models of nonlinear fluid-structure interactions in articulated naval platforms The critical research path for this effort is defined in the context of transitioning these models into engineering tools for design and analysis of existing and future marine systems The symbiosis of analysis and experiments provides unique opportunities for advance the state-of-the-art in analytical modeling by directly addressing nonlinear coupling effects, and linking individual terms in the analysis to physical parameters measured in the laboratory This research is also an excellent vehicle for training a new generation of workers who are adept at understanding fluid-structure interaction problems from both analytical dynamics and experimental fluid dynamics perspectives

Vortex-induced vibrations

Abstract: This review summarizes fundamental results and discoveries concerning vortex-induced vibration (VIV), that have been made over the last two decades, many of which are related to the push to explore very low mass and damping, and to new computational and experimental techniques that were hitherto not available. We bring together new concepts and phenomena generic to VIV systems, and pay special attention to the vortex dynamics and energy transfer that give rise to modes of vibration, the importance of mass and damping, the concept of a critical mass, the relationship between force and vorticity, and the concept of "effective elasticity," among other points. We present new vortex wake modes, generally in the framework of a map of vortex modes compiled from forced vibration studies, some of which cause free vibration. Some discussion focuses on topics of current debate, such as the decomposition of force, the relevance of the paradigm flow of an elastically mounted cylinder to more complex systems, and the relationship between forced and free vibration.

A Survey of Projection-Based Model Reduction Methods for Parametric Dynamical Systems

Abstract: Numerical simulation of large-scale dynamical systems plays a fundamental role in studying a wide range of complex physical phenomena; however, the inherent large-scale nature of the models often leads to unmanageable demands on computational resources. Model reduction aims to reduce this computational burden by generating reduced models that are faster and cheaper to simulate, yet accurately represent the original large-scale system behavior. Model reduction of linear, nonparametric dynamical systems has reached a considerable level of maturity, as reflected by several survey papers and books. However, parametric model reduction has emerged only more recently as an important and vibrant research area, with several recent advances making a survey paper timely. Thus, this paper aims to provide a resource that draws together recent contributions in different communities to survey the state of the art in parametric model reduction methods. Parametric model reduction targets the broad class of problems for wh...

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Abstract: To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Balanced Model Reduction via the Proper Orthogonal Decomposition

Abstract: A new method for performing a balanced reduction of a high-order linear system is presented. The technique combines the proper orthogonal decomposition and concepts from balanced realization theory. The method of snapshotsisused to obtainlow-rank,reduced-rangeapproximationsto thesystemcontrollability and observability grammiansineitherthetimeorfrequencydomain.Theapproximationsarethenusedtoobtainabalancedreducedorder model. The method is particularly effective when a small number of outputs is of interest. It is demonstrated for a linearized high-order system that models unsteady motion of a two-dimensional airfoil. Computation of the exact grammians would be impractical for such a large system. For this problem, very accurate reducedorder models are obtained that capture the required dynamics with just three states. The new models exhibit far superiorperformancethanthosederived using a conventionalproperorthogonal decomposition. Although further development is necessary, the concept also extends to nonlinear systems.