An electrical probe of the phonon mean-free path spectrum

Most studies of the mean-free path accumulation function (MFPAF) rely on optical techniques to probe heat transfer at length scales on the order of the phonon mean-free path. In this paper, we propose and implement a purely electrical probe of the MFPAF that relies on photo-lithographically defined heater-thermometer separation to set the length scale. An important advantage of the proposed technique is its insensitivity to the thermal interfacial impedance and its compatibility with a large array of temperature-controlled chambers that lack optical ports. Detailed analysis of the experimental data based on the enhanced Fourier law (EFL) demonstrates that heat-carrying phonons in gallium arsenide have a much wider mean-free path spectrum than originally thought

Flow optimization study of a batch microfluidics PET tracer synthesizing device.

We present numerical modeling and experimental studies of flow optimization inside a batch microfluidic micro-reactor used for synthesis of human-scale doses of Positron Emission Tomography (PET) tracers. Novel techniques are used for mixing within, and eluting liquid out of, the coin-shaped reaction chamber. Numerical solutions of the general incompressible Navier Stokes equations along with time-dependent elution scalar field equation for the three dimensional coin-shaped geometry were obtained and validated using fluorescence imaging analysis techniques. Utilizing the approach presented in this work, we were able to identify optimized geometrical and operational conditions for the micro-reactor in the absence of radioactive material commonly used in PET related tracer production platforms as well as evaluate the designed and fabricated micro-reactor using numerical and experimental validations

A numerical analysis of finite Debye-length effects in induced-charge electro-osmosis

For a microchamber filled with a binary electrolyte and containing a flat un-biased center electrode at one wall, we employ three numerical models to study the strength of the resulting induced-charge electro-osmotic (ICEO) flow rolls: (i) a full nonlinear continuum model resolving the double layer, (ii) a linear slip-velocity model not resolving the double layer and without tangential charge transport inside this layer, and (iii) a nonlinear slip-velocity model extending the linear model by including the tangential charge transport inside the double layer. We show that compared to the full model, the slip-velocity models significantly overestimate the ICEO flow. This provides a partial explanation of the quantitative discrepancy between observed and calculated ICEO velocities reported in the literature. The discrepancy increases significantly for increasing Debye length relative to the electrode size, i.e. for nanofluidic systems. However, even for electrode dimensions in the micrometer range, the discrepancies in velocity due to the finite Debye length can be more than 10% for an electrode of zero height and more than 100% for electrode heights comparable to the Debye length.Comment: 11 pages, Revtex, 7 eps figure

Investigation of turbulent boundary layer structure using particle image velocimetry

Particle-image velocimetry (PIV) is used to measure instantaneous velocity fields in the streamwise wall-normal plane of a zero-pressure-gradient turbulent boundary layer. In total, 340 PIV realizations of the boundary layer were obtained at Reynolds numbers based on momentum thickness of Ree = 930, 2370 and 6845. Each realization contains approximately 10000 two-component velocity vectors which are accurate to within 0.4 to 1.0% of the free-stream velocity.The PIV results are used to examine coherent structures throughout the boundary layer and determine their Reynolds-number dependencies. Coherent structures have been an important part of turbulent boundary-layer research for more than thirty years. By understanding coherent structures, their Reynolds-number dependencies and their dynamical characteristics, engineers and scientists will be better able to control and predict turbulent flows.A 20 m long low-turbulence boundary-layer research facility was developed for the measurement of turbulent boundary layers using particle-image velocimetry. Hot-film anemometer measurements show that the turbulence intensities at the test section inlet are about 0.16%, at a free-stream velocity of 6 m/s.A PIV interrogation and vector-validation system was developed to conduct the PIV measurements efficiently. The interrogation system utilizes eight i860 array processors in parallel to achieve a peak aggregate performance of 640 Mflops. Using 128 x 128 pixel cross correlations, it processes 100 vectors per second.Two-dimensional maps of in-plane velocity vectors, spanwise vorticity, Reynolds stress, and streamwise and wall-normal velocity contours show the instantaneous structures in the overlap and outer regions of the boundary layer. Details of large-scale motions protruding into the free stream are clearly shown in the outer region.The overlap region consists of shear layers oriented at 45\sp\circ from the wall and are associated with transverse vortical elements, which are located above and slightly downstream of the shear layers. Comparisons between measured coherent structures and theories of Theodorsen, Townsend and Perry et al. are given.Two-dimensional spatial correlation functions of streamwise velocity, wall-normal velocity, spanwise vorticity and Reynolds stress are used to examine the statistically relevant structure of the boundary layer and its Reynolds-number dependency. Stochastic estimation is used to estimate conditional eddies throughout the boundary layer and reveal their Reynolds-number dependencies.U of I OnlyETDs are only available to UIUC Users without author permissio

Structure of turbulent boundary layer using stereoscopic large format video-piv

Development of a stereoscopic particle image velocimeter for the measurement of three-dimensional vectors on a planar domain is described. The camera is based on two large format (2k x 2k) video cameras. Experiments in a turbulent boundary layer at Reθ= 2525 demonstrate its ability to measure threedimensional turbulent flow. In addition to the quantitative value of the out-ofplane component, it is found that having the complete three-dimensional vector also significantly improves the qualitative visualization of the flow.Air Force Office of Scientific Research 97/01; Office of Naval Researchearch 97/01; TSI Inc., St. Paul, Minnesota 97/0

Examination of the Slip Boundary Condition by μ-PIV and Lattice Boltzmann Simulations

This work examines the slip boundary condition by Lattice Boltzmann simulations, addresses the validity of the Navier's hypothesis that the slip velocity is proportional to the shear rate and compares the Lattice Boltzmann simulations to the experimental results of Tretheway and Meinhart (Phys. of Fluids, 14, L9-L12). The numerical simulation models the boundary condition as the probability, P, of a particle to bounce-back relative to the probability of specular reflection, 1- P. For channel flow, the numerically calculated velocity profiles are consistent with the experimental profiles for both the no-slip and slip cases. No-slip is obtained for a probability of 100% bounce-back, while a probability of 0.03 is required to generate a slip length and slip velocity consistent with the experimental results of Tretheway and Meinhart for a hydrophobic surface. The simulations indicate that for microchannel flow the slip length is nearly constant along the channel walls, while the slip velocity varies with wall position as a results of variations in shear rate. Thus, the resulting velocity profile in a channel flow is more complex than a simple combination of the no-slip solution and slip velocity as is the case for flow between two infinite parallel plates