Luis P. Bernal

Streamwise vortex structure in plane mixing layers

The development of three-dimensional motions in a plane mixing layer was investigated experimentally. It is shown that superimposed on the primary, spanwise vortex structure there is a secondary, steamwise vortex structure. Three aspects of this secondary structure were studied. First, the spanwise vortex instability that generates the secondary structure was characterized by measurements of the critical Reynolds number and the spanwise wavelength at several flow conditions. While the critical Reynolds number was found to depend on the velocity ratio, density ratio and initial shear-layer-profile shape, the mean normalized wavelength is independent of these parameters. Secondly, flow visualization in water was used to obtain cross-sectional views of the secondary structure associated with the streamwise counter-rotating vortices. A model is proposed in which those vortices are part of a single vortex line winding back and forth between the high-speed side of a primary vortex and the low-speed side of the following one. Finally, the effect of the secondary structure on the spanwise concentration field was measured in a helium-nitrogen mixing layer. The spatial organization of the secondary structure produces a well-defined spanwise entrainment pattern in which fluid from each stream is preferentially entrained at different spanwise locations. These measurements show that the spanwise scale of the secondary structure increases with downstream distance.

An experimental study of deep water plunging breakers

Plunging breaking waves are generated mechanically on the surface of essentially deep water in a two‐dimensional wave tank by superposition of progressive waves with slowly decreasing frequency. The time evolution of the transient wave and the flow properties are measured using several experimental techniques, including nonintrusive surface elevation measurement, particle image velocimetry, and particle tracking velocimetry. The wave generation technique is such that the wave steepness is approximately constant across the amplitude spectrum. Major results include the appearance of a discontinuity in slope at the intersection of the lower surface of the plunging jet and the forward face of the wave that generates parasitic capillary waves; transverse irregularities occur along the upper surface of the falling, plunging jet while the leeward side of the wave remains very smooth and two dimensional; the velocity field is shown to decay rapidly with depth, even in this strongly nonlinear regime, and is simila...

Interaction of a Turbulent Round Jet with the Free Surface

Abstract : An experimental study of the interaction of an underwater turbulent round jet with the free surface was conducted. Flow visualization, surface curvature measurements and hot film velocity measurements were used to study this flow. It is shown that surface waves are generated by the large scale vortical structures in the jet flow as they approach the free surface. These waves propagate at an angle with respect to the flow direction. The propagation angle increases as the strength of the interaction is increased by increasing the momentum flux of the jet or reducing the distance of the jet to the free surface or both. Propagation of these waves in the flow direction is suppressed by the surface current produced by the jet. Far downstream the surface motions are caused by the large scale vortical structures interacting directly with the surface. The fundamental scaling parameters of the free-surface jet have been determined. The velocity scale is the velocity obtained from the combination of jet momentum, density and depth of the jet and the length scale is the distance of the et to the free surface. It is shown that the centerline velocity decay when scaled with these parameters collapses to a universal curve for different depths of the jet.

Vortex ring dynamics at a free surface

The results of a flow visualization study of the evolution of vortex rings near a free surface are presented. The vortex rings were formed underwater with their axis parallel to the free surface one ring diameter below the surface. It is shown that vortex lines in the ring open during the interaction with the surface. The resulting flow field consists of vortex lines, beginning and terminating at the free surface. Data are reported on several features of the vortex ring evolution, which show a large reduction of the propagation speed and an oscillation of the vortex eccentricity as it propagates downstream after the interaction.

A fully integrated high-efficiency peristaltic 18-stage gas micropump with active microvalves

We report the design, fabrication, and test results of a fully integrated peristaltic 18-stage gas micropump consisting of 18 serially-connected pumping chambers and 19 microvalves. The peristaltic micropump achieves (1) high-pressure differentials by accumulating small pressure differentials that are evenly distributed across the individual stages using a low-compression multi-stage design, (2) high flow rate by operating pumping membranes at fluidic resonance and high frequency (>10 kHz), and (3) gas flow regulation by actively controlling the open/close timing of microvalves for either high flow or high pressure. The 18-stage micropump includes several new innovations, such as checkerboard microvalves, dual drive electrodes, and dual pumping chambers to achieve efficient electrostatic pumping. The fabricated 18-statge pump has produced an air flow rate of ~4.0 seem and maximum pressure differential of-17.5 kPa with a total power of only ~57 mW. It has a total package volume of 25.1 x 19.1 x 1 mm3 and each individual membrane is only 2x2 mm2.

Experiments and Computations on Abstractions of Perching

The flight maneuver of perching is abstracted as a linear pitch ramp, with and without a deceleration in the free-stream direction. We consider, first, experimental-computational comparison for flowfield and aerodynamic force coefficients for an SD7003 airfoil pitching from α = 0o to 45o; and second, an experimental survey of reduced frequency and pivot point for a range of flat plate pitching cases from 0o to 90o. The computational approach is 3D Large Eddy Simulation, and the experimental approach is by three degree of freedom electric motion-rig in a water tunnel. Accurate flowfield resolution in deep-stall is seen to require a large spanwise extent of computational domain. Meanwhile, experiment can be plagued with blockage, and dynamic blockage was seen to behave differently than static blockage. Even very low reduced frequencies of motion give lift overshoot beyond static stall, but comparatively large frequencies are necessary before the lift curve slope changes, either due to rate effects or acceleration effects. Moving the pitch pivot point further aft tends to attenuate both lift and drag production, and pitching about the three-quarter chord point cancels the rate-effect, in agreement with quasi-steady linear airfoil theory. Surprisingly, the aerodynamic coefficient history differs little between cases with and without streamwise deceleration, except towards the very end of the motion. The implication is that perching-type of ground tests or computations can be adequately conducted in a steady free-stream.

A Micropump-Driven High-Speed MEMS Gas Chromatography System

We report (1) the integration of the first functioning MEMS gas chromatography system ( muGC) featuring a micropump, a micro-column, and a micro-chemiresistor sensor array; and (2) experimental demonstration of the state-of-the-art multi-vapor gas separation and detection. In particular, we report the best GC analysis data from the first micropump-driven muGC system to date: the separation and detection of 11 volatile organic compounds (VOC)s within only 78 seconds while consuming only 15.1 mW of power within a small volume of 0.5 cc. We also report the use of temperature programming (TP) of the separation column for fast analysis, which shortened the analysis time from 78 seconds to 24 seconds while maintaining gas analysis resolution.

Experimental Investigation of the Near Wake of a Pick-up Truck

The results of an experimental investigation of the flow over a pickup truck are presented. The main objectives of the study are to gain a better understanding of the flow structure in near wake region, and to obtain a detailed quantitative data set for validation of numerical simulations of this flow. Experiments were conducted at moderate Reynolds numbers (~3×10 5 ) in the open return tunnel at the University of Michigan. Measured quantities include: the mean pressure on the symmetry plane, unsteady pressure in the bed, and Particle Image Velocimetry (PIV) measurements of the flow in the near wake. The unsteady pressure results show that pressure fluctuations in the forward section of the bed are small and increase significantly at the edge of the tailgate. Pressure fluctuation spectra at the edge of the tailgate show a spectral peak at a Strouhal number of 0.07 and large energy content at very low frequency. The velocity field measurements in the symmetry plane show that shear layers form at the top of the cab and the underbody flow region. The cab shear layer evolves more slowly than the underbody flow shear layer and does not interact strongly with the tailgate for the present geometry. Behind the tailgate there is no recirculating flow region in the symmetry plane believed to be due to downwash from streamwise vorticity in the near wake. There are small recirculating regions on the sides of the tailgate symmetry plane extending approximately one tailgate height downstream.

Experimental Investigation of Pitching and Plunging Airfoils at Reynolds Number between 1x10^4 and 6x10^4

An experimental investigation was performed on a nominally two-dimensional pitching and plunging SD7003 and flat plate at Reynolds number 1 × 10, 3 × 10, and 6 × 10. The experiment was conducted at the University of Michigan water channel facility using phaseaveraged particle image velocimetry (PIV) technique to quantify the flow field. Two sets of airfoil kinematics were used in this study; a combined pitching and plunging motion, and a pure plunging motion. The flow topology and wall velocity profiles from the PIV measurements showed a Re dependence on a pitching and plunging SD7003 where the extent of flow separation is reduced at a relatively high Re. On the contrary, flat plate displayed a large leading edge separation flow characteristic that was independent of Re. For both airfoil cross-sections used in the experiment, turbulence statistics indicated laminar to turbulent transition phenomena at low Re. The study shows the leading edge shape effect on the flow transition and separation characteristics. A pure plunging motion of SD7003 and flat plate at Re = 60,000 showed the formation of the leading and trailing edge vortices. In addition, a quantitative analysis showed an apparent phase lag present on SD7003 relative to the flat plate. In order to validate the experimental data, a flow comparison between the University of Michigan and AFRL was performed.

An Integrated Combustor-Thermoelectric Micro Power Generator

This paper reports a micromachined thermoelectric (TE) power generator based on catalytic combustion in a micromachined combustion chamber. The 2mm×8mm×0.5mm combustion chamber is covered by a dielectric diaphragm supported on a silicon substrate that is bonded to a glass substrate. The generator integrates polysilicon-Pt thermopiles on the diaphragm to take advantage of high combustion temperature as well as excellent thermal isolation of the diaphragm. Combustion of hydrogen and air mixture is self-sustained inside the combustor after ignition and output power level of ∼1µW/thermocouple has been obtained. Temperature measurements indicate that power level of 10µW/thermocouple could be achieved by slight geometric modification to fully exploit the high temperature gradient of combustion on diaphragm.