Abstract: We present synchronized time-resolved measurements of the wing kinematics and wake velocities for a medium sized bat, Cynopterus brachyotis, flying at low-medium speed in a closed-return wind tunnel. Measurements of the motion of the body and wing joints, as well as the resultant wake velocities in the Trefftz plane are recorded at 200 Hz (approximately 28-31 measurements per wing beat). Circulation profiles are found to be quite repeatable although variations in the flight profile are visible in the wake vortex structures. The circulation has almost constant strength over the middle half of the wing beat (defined according the vertical motion of the wrist, beginning with the downstroke). A strong streamwise vortex is observed to be shed from the wingtip, growing in strength during the downstroke, and persisting during much of the upstroke. At relatively low flight speeds (4.3 m/s), a closed vortex structure behind the bat is postulated.
Abstract: Droplet deposition onto a hydrophobic surface is studied experimentally and numerically. A wide range of droplet sizes can result from the same syringe, depending strongly on the needle retraction speed. Three regimes are identified according to the motion of the contact line. In region I, at slow retraction speeds, the contact line expands and large droplets can be achieved. In region II, at moderate needle speeds, a quasicylindrical liquid bridge forms resulting in drops approximately the size of the needle. Finally, at high speeds (region III), the contact line retracts and droplets much smaller than the syringe diameter are observed. Scaling arguments are presented identifying the dominant mechanisms in each regime. Results from nonlinear numerical simulations agree well with the experiments, although the accuracy of the predictions is limited by inadequate models for the behavior of the dynamic contact angle.
Abstract: Results are presented that demonstrate the successful use of live bacteria as mechanical actuators in microfabricated fluid systems. The flow deposition of bacteria is used to create a motile bacterial carpet that can generate local fluid motion inside a microfabricated system. By tracking the motion of tracer particles, we demonstrate that the bacterial cells that comprise the carpet self-organize, generating a collective fluid motion that can pump fluid autonomously through a microfabricated channel at speeds as high as 25 mu m s(-1). The pumping performance of the system can also be augmented by changing the chemical environment. The addition of glucose to the working buffer raises the metabolic activity of the bacterial carpet, resulting in increased pumping performance. The Performance of the bacterial pump is also shown to be strongly influenced by the global geometry of the pump, with narrower channels achieving a higher pumping velocity with a faster rise time.
Abstract: The deformation of an elastic rod rotating in a viscous fluid is considered, with applications related to flagellar motility. The rod is tilted relative to the rotation axis, and experiments and theory are used to study the shape transition when driven either at constant torque or at constant speed. At low applied torque, the rod bends gently and generates small propulsive force. At a critical torque, the rotation speed increases abruptly, and the rod forms a helical shape with increased propulsive force. We find good agreement between theory and experiment. A simple physical model is presented to capture and explain the essential behavior.
Abstract: Accurate interpretation of recruitment rate measurements of microscale particles, such as cells and microbeads, to biofunctional surfaces is difficult because factors such as uneven ligand distributions, particle collisions, variable particle fluxes, and molecular-scale surface separation distances obfuscate the ability to link the observed particle behavior with the governing nanoscale biophysics. We report the development of a hydrodynamically conditioned micropattern catch strip assay to measure microparticle recruitment kinetics. The assay exploited patterning within microfluidic channels and the mechanostability of selectin bonds to create reaction geometries that confined a microbead flux to within 200 nm of the surface under flow conditions. Systematic control of capillary action enabled the creation of homogeneous or gradient ligand distributions. The method enabled the measurement of particle recruitment rates (k(eff), s(-1)) that were primarily determined by the interaction of the biomolecular pair being investigated. The method is therefore well suited for relative measurements of delivery vehicle and cellular recruitment potential as governed by surface-bound molecules.
Abstract: We present results from a series of experiments demonstrating the use of single quantum dots (QDs) as simultaneous temperature and velocity probes at the micro-scale. The fluorescence intensity of QDs varies predictably with temperature due to changes in quantum efficiency. We use total internal reflection fluorescence microscopy to study the region within 200 nm of a fluid-solid interface. A two-color, time-averaged temperature sensing technique based on the ensemble intensity changes of single QDs as compared to a reference dye (rhodamine 110) is presented. Many single QD intensity measurements are used to build intensity distributions which can be mapped to fluid temperature. Simultaneously, we track the motion of individual QDs, building a distribution of particle displacements, where the mean displacement yields the local fluid velocity. We also show that the width of the displacement distribution (or the diffusion coefficient) captures the scaling of the temperature to viscosity ratio, which may allow for independent viscosity measurement.
Abstract: Body motions (kinematics) of animals can be dimensionally complex, especially when flexible parts of the body interact with a surrounding fluid. In these systems, tracking motion completely can be difficult, and result in a large number of correlated measurements, with unclear contributions of each parameter to performance. Workers typically get around this by deciding a priori which variables are important (wing camber, stroke amplitude, etc.), and focusing only on those variables, but this constrains the ability of a Study to uncover variables of influence. Here, we describe ail application of proper orthogonal decomposition (POD) for assigning importances to kinematic variables, using dimensional complexity as a metric. We apply this method to bat flight kinematics, addressing three questions: (I) Does dimensional complexity of motion change with speed? (2) What body markers are optimal for capturing dimensional complexity? (3) What variables should a simplified reconstruction of bat flight include in order to maximally reconstruct actual dimensional complexity? We measured the motions of 17 kinematic markers (20 joint angles) on a bat (Cynopterus brachyotis) flying in a wind tunnel at nine speeds. Dimensional complexity did not change with flight speed, despite changes in the kinematics themselves, suggesting that the relative efficacy of a given number of dimensions for reconstructing kinematics is conserved across speeds. By looking at subsets of the full 17-marker set, we found that using more markers improved resolution of kinematic dimensional complexity, but that the benefit of adding markers diminished as the total number of markers increased. Dimensional complexity was highest when the hindlimb and several points along digits III and IV were tracked. Also, we uncovered three groups of joints that move together during flight by using POD to quantify correlations of motion. These groups describe 14/20 joint angles, and provide a framework for models of bat flight for experimental and modeling purposes. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: The physical environment of the aerosphere is both complex and dynamic, and poses many challenges to the locomotor systems of the three extant evolutionary lineages of flying animals. Many features of the aerosphere, operating over spatial and temporal scales of many orders of magnitude, have the potential to be important influences on animal flight, and much as marine ecologists have studied the relationship between physical oceanography and swimming locomotion, a subfield of aeroecology can focus attention on the ways the biology of flight is influenced by these characteristics. Airflows are altered and modulated by motion over and around natural and human-engineered structures, and both vortical flow structures and turbulence are introduced to the aerial environment by technologies such as aircraft and wind farms. Diverse aspects of the biology of flight may be better understood with reference to an aeroecological approach, particularly the mechanics and energetics of flight, the sensing of aerial flows, and the motor control of flight. Moreover, not only does the abiotic world influence the aerospheric conditions in which animals fly, but flying animals also, in turn, change the flow environment in their immediate vicinity, which can include the air through which other animals fly, particularly when animals fly in groups. Flight biologists can offer considerable insight into the ecology of the aerial world, and an aeroecological approach holds great promise for stimulating and enriching the study of the biology of flight.
Abstract: We demonstrate the use of Escherichia coli and their chemotactic characteristics to enhance mixing in a microchannel in a controlled and bi-directional manner. The presence of a chemoattractant in one arm of a three-junction microchannel results in an asymmetric increase in the effective diffusion coefficient of extremely high molecular weight TMR-Dextran (MW 2 000 000), which rises linearly with the concentration of attractant from a baseline value of 8-42 mu m(2)/s at a concentration of 0.1 M. The response to a repellent is similar, with the opposite bias.
Abstract: Electrokinetic effects and electrostatic repulsion between tracer particles and glass surface have both been proposed as possible sources that would lead to false slip results obtained from velocimetry-based measurements. Using a three-dimensional total internal reflection velocimetry technique, we address such a concern by comparing the measured slip lengths between nonionic solutions and electrolyte solutions whose ionic concentrations have been predicted to reduce the electricity-induced slip effect to a submolecular level. It is observed that the presence of electrolytes has no effect on the measured slip lengths, suggesting that the observed slip velocities are most likely not due to electrostatic and electrokinetic effects, but are consequences of true boundary slip.(c) 2007 American Institute of Physics.
Abstract: We demonstrate that flagellated bacteria can be utilized in surface arrays (carpets) to achieve mixing in a low-Reynolds number fluidic environment. The mixing performance of the system is quantified by measuring the diffusion of small tracer particles. We show that the mixing performance responds to modifications to the chemical and thermal environment of the system, which affects the metabolic activity of the bacteria. Although the mixing performance can be increased by the addition of glucose (food) to the surrounding buffer or by raising the buffer temperature, the initial augmentation is also accompanied by a faster decay in mixing performance, due to falling pH and oxygen starvation, both induced by the higher metabolic activity of the bacterial system.
Abstract: By applying the three-dimensional total internal reflection velocimetry (3D-TIRV) technique to freely suspended micron-sized fluorescent particles, we are able to simultaneously observe the three-dimensional anisotropic hindered diffusion for values of the gap-size-to-radius ratio much less than one. We demonstrate that the 3D-TIRV can be used to accurately track freely suspended 1.5-mu m radius particles. The displacement measurements reveal that the hindered diffusion coefficients are in close agreement with the theoretical values predicted by the asymptotic solutions of Brenner [Chem. Eng. Sci. 16, 242 (1961)] and Goldman [Chem. Eng. Sci. 22, 637 (1967)] for gap-size-to-radius ratio much less than one, while hindered diffusion anisotropicity is simultaneously observed in all data sets.
Abstract: Synovial fluid is a semidilute hyaluronate (HA) polymer solution, the rheology of which depends on HA-protein interactions, and lubricin is a HA-binding protein found in synovial fluid and at cartilage surfaces, where it contributes to boundary lubrication under load. Individuals with genetic deficiency of lubricin develop precocious joint failure. The role of lubricin in synovial fluid rheology is not known. We used a multiple-particle-tracking microrheology technique to study the molecular interactions between lubricin and HA in synovial fluid. Particles (200 nm mean diameter) embedded in normal and lubricin-cleficient synovial fluid samples were tracked separately by using multiple-particle-tracking microrheology. The time-dependent ensemble-averaged mean-squared displacements of all of the particles were measured over a range of physiologically relevant frequencies. The mean-squared displacement correlation with time lag had slopes with values of unity for simple HA solutions and for synovial fluid from an individual who genetically lacked lubricin, in contrast to slopes with values less than unity (alpha approximate to 0.6) for normal synovial fluid. These data correlated with bulk rheology studies of the same samples. We found that the subdiffusive and elastic behavior of synovial fluid, at physiological shear rates, was absent in fluid from a patient who lacks lubricin. We conclude that lubricin provides synovial fluid with an ability to dissipate strain energy induced by mammalian locomotion, which is a chondroprotective feature that is distinct from boundary lubrication.
Abstract: Colloidal lithography was used to make a novel array (2-D) of micro-rings, dots, and interconnected-honeycomb structures. These geometries are controlled using the curing temperature-dependent rheological properties of the siloxane elastomer precursor. Serratia marcescens was patterned on the interconnected honeycomb microstructure demonstrating a potential application for microbioanalytical devices, microfluidics, and bio-micro-electromechanical systems.
Abstract: We present a statistical approach to particle tracking velocimetry developed to treat the issues associated with nanometer-sized tracer particles such as fluorescent molecules and quantum dots (QDs) along with theory and experimental results. Extremely small tracers pose problems to traditional tracking methods due to high levels of thermal motion, high levels of intensified camera noise, high drop-in/drop-out rates and, in the case of QDs, fluorescence intermittency (âblinkingâ). The algorithm presented here compensates for these problems in a statistical manner and determines the physical velocity distributions from measured particle displacement distributions by statistically removing randomly distributed, non-physical tracking events. The algorithm is verified with both numerically simulated particle trackings and experiments using 54 nm diameter fluorescent dextran molecules and 6 and 16 nm diameter QDs.
Abstract: The existence and magnitude of slip velocities between deionized water and a smooth glass surface is studied experimentally. Sub-micron fluorescent particles are suspended in water and intaged using total internal reflection velocimetry (TIRV). For water flowing over a hydrophilic surface, the measurements are in agreement with previous experiments and indicate that slip, if present, is minimal at low shear rates, but increases slightly as the shear rate increases. Surface hydrophobicity is observed to induce a small slip velocity, with the slip length reaching a maximum of 96 nm at a shear rate of 1800 s(-1). Issues associated with the experimental technique and the interpretation of results are also discussed.
Abstract: Control of compressor tip clearance flows is explored in a linear cascade using three types of fluidic actuators; normal synthetic jet (NSJ; unsteady jet normal to the mean flow with zero net mass flux), directed synthetic jet (DSJ; injection roughly aligned with the mean flow), and steady directed jet (SDJ), mounted on the casing wall. The effectiveness of each active control technique is determined in terms of its ability to achieve: (1) reduction of tip leakage flow rate, (2) mixing enhancement between tip leakage and core flow, and (3) increase in streamwise momentum of the flow in the endwall region. The measurements show that the NSJ provides mixing enhancement only, or both mixing enhancement and leakage flow reduction, depending on its pitchwise location. The DSJ and SDJ actuators provide streamwise momentum enhancement with a consequent reduction of clearance-related blockage. The blockage reduction associated with the use of NSJ is sensitive to actuator frequency, whereas that with the use of DSJ is not. For a given actuation amplitude, DSJ and SDJ are about twice as effective as NSJ in reducing clearance-related blockage. Further the DSJ and SDJ can eliminate clearance-related blockage with a time-averaged momentum flux roughly 16% of the momentum flux of the leakage flow. However achieving an overall gain in efficiency appears to be hard; the decrease in loss is only about 30% of the expended flow power from the present SDJ actuator Guidelines for improving the efficiency of the directed jet actuation are presented.
Abstract: A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This paper presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.
Abstract: Squeeze film damping and hydrodynamic lift for a micromechanical perforated proof mass are calculated and measured. This paper has resulted in closed-form expressions that can be used to design accelerometers, tuning-fork gyroscopes (TFGs), and other micromechanical devices. The fluid damping and lift are determined using finite-element analyses of the normalized and linearized governing equations where the boundary condition of the pressure relief holes is derived using pipe How analysis. The rarefaction of gas is incorporated in the governing equations based on slip How condition. As a further check, a one-dimensional (1-D) network model is developed to account for the boundary condition of the holes on a tilted proof mass. Both closed-form and numerical solutions are compared against experimental data over a range of pressures.
Abstract: Results concerning the design and fabrication of electromagnetic actuators, and their application to affect the wall shear stress in a fully turbulent channel flow are discussed. The actuators utilize a Lorentz force to induce fluid motion due to the interaction between a magnetic field and a current density. The actuators are comprised of spanwise-aligned rows of permanent magnets interlaced with surface-mounted electrodes, segmented to allow the Lorentz force to be propagated in the spanwise direction. Problems commonly associated with electromagnetic flow control-electrolysis, bubble formation, and electrode corrosion are substantially reduced, and in most cases eliminated by the use of a conductive polymer coating. The actuators generate velocity profiles with a penetration depth into the flow of approximately 1 mm (set by the electrode/magnet pitch) and maximum velocities of approximately 4 cm/s. The actuation velocities are found to scale linearly with forcing voltage and frequency. The electrical to mechanical efficiency is found to be very low (approximate to10(-4)), primarily due to the limitations on the magnetic field strength and the low conductivity of the working fluid (saltwater). The actuators are used in a fully turbulent low Reynolds number channel flow and their effect on the turbulent skin friction is measured using a direct measurement of drag. Maximum drag reductions of approximately 10% are measured when the flow is forced using a spanwise oscillating Lorentz force. A scaling argument for the optimal amplitude of the current density is developed. The efficiency of this method for drag reduction, and its application at higher Reynolds numbers is also discussed. (C) 2004 American Institute of Physics.
Abstract: A new velocimetry system has been developed for use in microdevices that incorporate silicon as their structural material. The system is designed to illuminate and measure particle and surface motions using infrared wavelengths, taking advantage of the fact that silicon is largely transmissive to light with wavelength above 1 gm. The system allows the observation of motion inside silicon-based microdevices, which are otherwise opaque to light at visible wavelengths. By analyzing these images using both time-of-flight and phase-locked techniques, quantitative measurements are demonstrated concerning the position and speed of internal surfaces and the motion of fluids inside complex microfabricated devices. The system as demonstrated has a resolution of approximately 360 nm, although higher resolution is possible with future improvements.
Abstract: The effect of bacterial motion on the diffusion of a molecule of high molecular weight is studied by observing the mixing of two streams of fluid in a microfluidic flow cell. We show that the presence of motile E. coli bacteria in one of the streams results in a marked increase in the effective diffusion coefficient of Dextran, which rises linearly with the concentration of bacteria from a baseline value of 0.2x10(-7) to 0.8x10(-7) (cm(2)/s) at a concentration of 2.1x10(9)/ml (approximately 0.5% by volume). Furthermore, we observe that the diffusion process is also observed to undergo a change from standard Fickian diffusion to a superdiffusive behavior in which the diffusion exponent rises from 0.5 to 0.55 as the concentration of bacteria rises from 0 to 2.1x10(9)/ml. (C) 2004 American Institute of Physics.
Abstract: Total internal reflection velocimetry (TIRV) is used to measure particle motion in the near-wall region of a microfluidic system. TIRV images are illuminated with the evanescent field of an incident laser pulse and contain only particles that are very close to the channel surface. Sub-micron-sized fluorescent particles suspended in water are used as seed particles and their images are analyzed with a particle tracking velocimetry (PTV) algorithm to extract information about apparent slip velocity. At relatively low shear rates (less than 2,500 s(-1)), a velocity proportional to the shear rate was observed. The statistical difference between velocities measured over hydrophilic and hydrophobic surfaces was found to be minimal. The results suggest that the slip length, if present, is less than 10 nm, but uncertainty regarding the exact character of the illumination field prevents a more accurate measurement at this time. Numerical simulations are presented to help understand the results and to provide insight into the mechanisms that result in the experimentally observed distributions. Issues associated with the accuracy of the experimental technique and the interpretations of the experimental results are also discussed.
Abstract: Escherichia coli ( E. coli) and other bacteria are propelled throughwater by several helical flagella, which are rotated by motors embedded at random points on the cell wall. Depending on the handedness and rotation sense, the motion of the flagella induces a flow field that causes them to wrap around each other and form a bundle. Our objective is to understand and model the mechanics of this process. Full-scale flagella are 10 mum in length, 20 nm in diameter, and turn at a rate of 100 Hz. To accurately simulate bundling at a more easily observable scale, we built a scale model in which 20-cm-long helices are rotated in 100,000 cp silicone oil (Poly-di-methyl-siloxane). The highly viscous oil ensures an appropriately low Reynolds number. We developed amacro-scale particle image velocimetry (PIV) system to measure the full-field velocity distribution for rotating rigid helices and rotating flexible helices. In the latter case, the helices were made from epoxy-filled plastic tubing to give approximately the same ratio of elastic to viscous stresses as in the full-scale flagella. Comparison between PIV measurements and slender-body calculations shows good agreement for the case of rigid helices. For the flexible helices, we find that the flow field generated by a bundle in the steady state is well approximated by the flow generated by a single rigid helix with twice the filament radius.
Abstract: The slip effects of water flow in hydrophilic and hydrophobic microchannels of 1 and 2 mum depth are examined experimentally. High-precision microchannels were treated chemically to enhance their hydrophilic and hydrophobic properties. The flow rates of pure water at various applied pressure differences for each surface condition were measured using a high-precision flow metering system and compared to a theoretical model that allows for a slip velocity at the solid surface. The slip length was found to vary approximately linearly with the shear rate with values of approximately 30 nm for the flow of water over hydrophobic surfaces at a shear rate of 10(5) s(-1). The existence of slip over the hydrophilic surface remains uncertain, due to the sensitivity of the current analysis to nanometer uncertainties in the channel height. (C) 2003 American Institute of Physics.
Abstract: An experimental system for liquid flow-rate measurements of pressure-driven flow through microchannels is described. The displacement of a meniscus is tracked in a precision borehole using a laser distance meter mounted on a feed-back controlled traversing stage. Successful measurements of flow rates as small as 30 pl/s are reported when using non-evaporative hexadecane, and evaporative fluids such as ethanol are measured at flow rates of 0.1 nl/s. The set-up can also be used for measurements in time-dependent flows. Finally, a discussion of the influence of dissolved nitrogen on the measured flow rates is given.
Abstract: An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer (Re-theta=1960) using a linear active control scheme. System identification in the boundary layer provides optimal transfer functions that predict the downstream characteristics of the streamwise velocity fluctuations. Enhanced detection techniques isolate the large-scale turbulent motion and improve the downstream correlations, resulting in greater controllability. The control is applied using a spanwise array of resonant synthetic jet actuators that introduce pairs of streamwise vortices into the flow. Control results show that a maximum reduction of 30% in the streamwise velocity fluctuations is achieved. This reduction is greatest at the point of optimization but spans a few hundred viscous lengths downstream of the actuator, about 50 viscous lengths in the wall-normal direction and 150 viscous lengths in the spanwise direction. The wall pressure fluctuation and the mean wall shear stress (measured approximately using mean velocity profiles near the wall) were reduced by 15% and 7% respectively. The bursting frequency, based on VITA event detection was also reduced by up to 23%.
Abstract: Escherichia coli and other bacteria use rotating helical filaments to swim. Each cell typically has about four filaments, which bundle or disperse depending on the sense of motor rotation. To study the bundling process, we built a macroscopic scale model consisting of stepper motor-driven polymer helices in a tank filled with a high-viscosity silicone oil. The Reynolds number, the ratio of viscous to elastic stresses, and the helix geometry of our experimental model approximately match the corresponding quantities of the full-scale E. coli cells. We analyze digital video images of the rotating helices to show that the initial rate of bundling is proportional to the motor frequency and is independent of the characteristic relaxation time of the filament. We also determine which combinations of helix handedness and sense of motor rotation lead to bundling.
Abstract: Oblique transition was experimentally investigated in a Blasius boundary layer formed on a flat plate. This transition mechanism was provoked by exciting a pair of oppositely oriented oblique Orr-Sommerfeld (O-S) modes given by (omega/omega(ts) +/-beta/beta(ts)) = (1, +/-1) in the frequency-wavenumber (spanwise) space. Surface waviness with height Deltah and a well-defined wavenumber spectrum that is synchronized with the neutral O-S wavenumber at Branch I, (alpha(w) +/-beta(w)) = (alpha(ts,I) +/-beta(ts,I)), was used to provide a steady velocity perturbation in the near-wall region. A planar downstream-travelling acoustic wave of amplitude e was created to temporally excite the flow near the resonance frequency, omega(ts)(= 2pif(o)), of an unstable eigenmode corresponding to k(ts) = k(w) (where k = +/-[alpha(2) + beta(2)](1/2)). Possible mechanisms leading to laminar-to-turbulent breakdown were examined for various forcing combinations, epsilonDeltah. For small values of epsilonDeltah, a peak-valley structure corresponding to a spanwise wavenumber of 2beta(w) was observed. As expected, the maximum r.m.s. narrow-band streamwise velocity fluctuations, u(t)(f(o)), occur at peak locations, which correspond to regions with mean streamwise velocity, U, deficits. For the largest value of epsilonDeltah, significant mean-flow distortion was observed in the spanwise profiles of U. Large spanwise velocity gradients, \textbackslashdU/dxi\textbackslash, exist between peaks and valleys and appear to generate an explosive growth in the velocity fluctuations. The maximum values of it, no longer occur at peak locations of the stationary structure but at locations of spanwise inflection points. The magnitude of u(t) scales with \textbackslashdU/dxi\textbackslash. A nonlinear interaction of two non-stationary modes was conjectured as a possible mechanism for the enhancement of the streak amplification rate.
Abstract: The effect of axially-varying clearance on microfabricated gas journal bearings is explored. This variation commonly arises from difficulties inherent to etching deep, narrow channels. Two types of clearance variation commonly observed in etched bearings are investigated: taper and bow. Both shapes are shown to have a detrimental effect on load capacity and bearing stability compared to a cylindrical bearing with the minimum clearance. For the same variation magnitude, taper is shown to have a more serious effect, including complete closure of the stability corridor at low speed for some cases. Methods are suggested for estimating variable-clearance bearing performance using cylindrical bearing data.
Abstract: A low-order model was created to analyze a small-scale gas bearing with a diameter of 4.1 mm, designed to spin at 2.4 million rpm. Due to microfabrication constraints, the bearing lies outside the standard operating space and stable operation is a challenge. The model is constructed by reference to Newtonâs second law for the rotor and employs stiffness and damping coefficients predicted by other models. Ar any operating point it is able to predict (1) whether the journal can sustain stable operation, and (2) the whirling frequency of the journal. Analysis shows that the best way to operate the bearing is in a hybrid mode where the bearing relies on hydrostatics at low speeds and hydrodynamics at high speeds. However, in transitioning from hydrostatic to hydrodynamic operation, the model shows that the bearing is prone to instability problems and great care must be taken in scheduling the bearing pressurization system in the course of accelerating through low and intermediate rotational speeds.
Abstract: An experimental investigation was conducted to examine acoustic receptivity and subsequent boundary-layer instability evolution for a Blasius boundary layer formed on a flat plate in the presence of two-dimensional and oblique (three-dimensional) surface waviness. The effect of the non-localized surface roughness geometry and acoustic wave amplitude on the receptivity process was explored. The surface roughness had a well-defined wavenumber spectrum with fundamental wavenumber k(w). A planar downstream-travelling acoustic wave was created to temporally excite the flow near the resonance frequency of an unstable eigenmode corresponding to k(ts) = k(w). The range of acoustic forcing levels, is an element of, and roughness heights, Deltah, examined resulted in a linear dependence of receptivity coefficients, however, the larger values of the forcing combination is an element of Deltah resulted in subsequent nonlinear development of the Tollmien-Schlichting (T-S) wave. This study provides the first experimental evidence of a marked increase in the receptivity coefficient with increasing obliqueness of the surface waviness in excellent agreement with theory. Detuning of the two-dimensional and oblique disturbances was investigated by varying the streamwise wall-roughness wavenumber alpha (w) and measuring the T-S response. For the configuration where laminar-to-turbulent breakdown occurred, the breakdown process was found to be dominated by energy at the fundamental and harmonic frequencies, indicative of K-type breakdown.
Abstract: The design and implementation of a novel dynamic calibration technique for shear-stress sensors is presented. This technique uses the oscillating wall shear stress generated by a traveling acoustic wave as a known input to the shear-stress sensor. A silicon-micromachined, floating-element shear-stress sensor has been dynamically calibrated up to 4 kHz using this method. These data represent the first broadband, experimental verification of the dynamic response of a shear-stress sensor.
Abstract: Efficient heating of a fluid is demonstrated using a novel heat exchanger in which bulk Silicon forms both the heater structure and the resistive heating elements. Current passed through the heater raises the temperature of the heater fins and this energy is transferred to a fluid flowing between adjacent fins. By exploiting the change in sign of the temperature coefficient of resistivity of the heavily doped silicon, the temperature of the system is stably maintained at the intrinsic point. A heat exchanger of this nature is integrated with a nozzle, resulting in a microthruster with elevated chamber temperature, which greatly improves the specific impulse, or thrust per unit weight flow of propellant. A numerical model is presented to optimize the heater design. Benchtop tests demonstrate the inherent stability of the intrinsic point heater design, while thrust tests demonstrate the improved fuel economy of the micropropulsion system. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: High-precision experimental results are reported showing the tangential momentum accommodation coefficient (TMAC) for several gases in contact with single-crystal silicon to be less than unity. A precise and robust experimental platform is demonstrated for measurement of mass flows through silicon micromachined channels due to an imposed pressure gradient. Analytic expressions for isothermal Maxwellian slip flows through long channels are used to determine the TMAC at a variety of Knudsen numbers. Results from experiments using nitrogen, argon and carbon dioxide are presented. For all three gases the TMAC is found to be lower than one, ranging from 0.75 to 0.85.
Abstract: We discuss deep reactive ion etching (DRIE) as a promising technology that can be readily applied in the micromanufacturing of low-thrust propulsion systems to be used on future generations of micro- and nanosatellites. This dry processing technique permits the fabrication of high-aspect-ratio silicon structures and intricate morphologies, both with tight tolerances, in a repetitive and controllable fashion that lightweight space vehicles will exploit with the introduction of smaller thrust components for precise; maneuvering and attitude control. The etching approach described herein is counted among the present state of the art techniques utilized in the current trend toward miniaturization of sensors and actuators. This trend is being propelled by the increased technological capability as the enabling factor for size reduction. Scaling laws, especially the cube-square law, can be successfully applied for obtaining macropower from microdevices manufactured with the silicon technology that has developed for microelectronics applications, including DRIE. The application of this plasma etching technique in the fabrication and testing of silicon supersonic micronozzles is also described.
Abstract: A numerical investigation is made into the use of linear predictive (Wiener) filters to predict flow quantities in the near-wall region of a turbulent boundary layer for use in active control algorithms. Optimal filters for the prediction of Reynolds stress and fluctuating streamwise velocity components using wall-shear stress information are developed and their dependence on the number and location of shear sensors is explored. It is found that a densely populated, wall-based, sensor system can predict the near-wall Reynolds stress with good accuracy, and that 76% of the optimal performance can be achieved with as few as four sensors whose locations coincide with the strongest weights of the Wiener filter derived using a very dense network of input sensors. (C) 2000 American Institute of Physics. [S1070- 6631(00)50112-3].
Abstract: A journal bearing simulation tool developed to aid the design of the MIT microturbo-machine bearings is described. This tool uses an orbit method with a pseudospectral technique for treating the Reynolds equation. Comparison is made to various published data. Two types of stability chart are presented and their application to turbomachine bearing design is discussed. Simulations of imbalance, noncircular geometry, and nonuniform pressures at the bearing ends are also demonstrated.
Abstract: We present a measurement technique designed to accurately measure small flow rates near atmospheric pressure, and demonstrate the ability to measure flows on the order of 10(-10) mol/sec. The technique is based upon a modification of a constant-volume mass accumulation scheme where the flow rate is measured by monitoring the change in pressure of a known volume of gas. We identify two phenomena, thermally-induced fluctuations and thermodynamically-induced fluctuations, which will affect the resolution and dynamic range of the instrumentation and discuss how the problems associated with them can be mitigated. We estimate the resolution to be approximately 2.0 X 10(-13) mol/s, and the maximum measurable flow rate to be to the order 2 X 10(-9) mol/s.
Abstract: An analytic and experimental investigation into gaseous flow with slight rarefaction through long microchannels is undertaken. A two dimensional (2-D) analysis of the Navier-Stokes equations with a first-order slip-velocity boundary condition demonstrates that both compressibility and rarefied effects are present in long microchannels. By undertaking a perturbation expansion in epsilon, the height-to-length ratio of the channel, and using the ideal gas equation of state, it is shown that the zeroth-order analytic solution for the streamwise mass flow corresponds well with the experimental results. Also, the effect of slip upon the pressure distribution is derived, and it is obtained that this slip velocity leads directly to a wad-normal migration of mass. The fabrication of wafer-bonded microchannels that possess well-controlled surface structure is described, and a means for accurately measuring the mass flow through the channels is presented. Experimental results obtained with this mass-how measurement technique for streamwise helium mass how through microchannels 52.25-mu m wide, 1.33-mu m deep, and 7500-mu m long for a pressure range of 1.6-4.2 atmospheres (outlet pressures at atmospheric) are presented and shown to compare favorably with the analysis. [147]
Abstract: The characteristics of a typical flow control actuator design are discussed, The device is based on a resonating structure that interacts with a closed volume of fluid to create a concentrated jet through an exit orifice. The resulting unsteady how through the orifice introduces viscous effects that are characterized by the Stokes parameter based on the orifice diameter. An optimum operating Stokes parameter is then computed by matching this viscous dominated solution to an ideal, inviscid result. The actuator is modeled with a system of coupled equations that describe its fluid-structural behavior. This model is compared to experimental results and is seen to predict time and frequency characteristics well. Experimental data also show that, away from the exit orifice, the jet is self-similar and its intensity is also governed by the Stokes parameter.
Abstract: The evolution of a wavepacket in a laminar boundary layer is studied experimentally, paying particular attention to the stage just prior to the formation of a turbulent spot. The initial stages of development are found to be in very good agreement with previous results and indicate a stage in which the disturbance grows according to linear theory followed by a weakly nonlinear stage in which the subharmonic grows, apparently through a parametric resonance mechanism. In a third stage, strong nonlinear interactions are observed in which the disturbance develops a streaky structure and the corresponding wavenumber-frequency spectra exhibit an organized cascade mechanism in which spectral peaks appear with increasing spanwise wavenumber and with frequencies which alternate between zero and the subharmonic frequency. Higher harmonics are also observed, although with lower amplitude than the low-frequency peaks. The final (breakdown) stage is characterized by the appearance of high-frequency oscillations with random phase, located at low-speed âspikeâ regions of the primary disturbance. Wavelet transforms are used to analyse the structure of both coherent and random small-scale structure of the disturbance, In particular, the breakdown oscillations are also observed to have a wavepacket character riding on the large-amplitude primary disturbance.
Abstract: An experimental investigation is made into the active control of the near-wall region of a turbulent boundary layer using a linear feed-forward control algorithm. A wall-based detection scheme is described which effectively detects coherent structures and predicts downstream flow behavior. A simple demonstration, using three wall-based sensors and a single actuator, achieves a maximum of 31% reduction in u(rms) and 17% reduction in p(rms). (C) 1997 American Institute of Physics.
Abstract: Experimental results are presented concerning the evolution of instabilities generated by the interaction between low-level broad-band acoustic waves and small two-dimensional roughness elements. Streamwise perturbation velocity spectra are measured and it is found that on the smooth plate, naturally-occurring Tollmien-Schlicting (T-S) waves grow in a manner consistent with a resonant subharmonic wave interaction. However, in the presence of small two-dimensional roughness elements, a strong primary T-S mode is forced by an interaction with the background acoustic field. This leads to a K-type of nonlinear interaction characterized by the generation of harmonics (up to six harmonics are observed) at the expense of the subharmonic. The scaling of the T-S waves and their harmonics with the number and amplitude of the wall roughness is also considered. (C) 1996 American Institute of Physics.
Abstract: A direct simulation Monte Carlo (DSMC) investigation of flows related to microelectromechanical systems (MEMS) is detailed This effort is intended to provide tools to facilitate the design and optimization of micro-devices as well as to probe the effects of rarefaction, especially in regimes not amenable to other means of analysis. The code written for this purpose employs an unstructured grid, a trajectory-tracing particle movement scheme, and an ââinfinite channelâ boundary formulation. Its results for slip-flow and transition regime micro-channels and a micro-nozzle are presented to demonstrate its capabilities.
Abstract: In order to determine the effect of surface irregularities on local convective heat transfer, the variation in heat transfer coefficients on small (2-6 mm diam) hemispherical roughness elements on a flat plate has been studied in a wind tunnel using IR techniques. Heat transfer enhancement was observed to vary over the roughness elements with the maximum heat transfer on the upstream face. This heat transfer enhancement increased strongly with roughness size and velocity when there was a laminar boundary layer on the plate. For a turbulent boundary layer, the heat transfer enhancement was relatively constant with velocity, but did increase with element size. When multiple roughness elements were studied, no influence of adjacent roughness elements on heat transfer was observed if the roughness separation was greater than approximately one roughness element radius. As roughness separation was reduced, less variation in heat transfer was observed on the downstream elements. Implications of the observed roughness enhanced heat transfer on ice accretion modeling are discussed.
Abstract: The calibration of high-bandwidth sensors is typically carried out in an steady environment or at least a well-controlled unsteady flow. A simple technique for calibration of sensors in flow fields with arbitrary unsteadiness (such as a turbulent field) is described. Although the method requires a DC reference measurement at each calibration point, the resulting calibration is accurate for both average and unsteady measurements up to the full bandwidth of the sensor. Applications and limitations of the technique are also discussed.
Abstract: The Navier-Stokes equations for circular pipe flow are integrated using direct numerical simulation for the case of transitional Reynolds number. Previous work on linear disturbances (reported in Part I) is exploited for the simulation of low to moderate amplitude disturbances where it is found that the transient growth mechanism persists in the nonlinear development with the evolution attributable to the linear mechanism remaining of considerable significance. A hypothesis of Trefethen et al. [Science 261, 578 (1993)] concerning the role of nonlinearity in the transition process and ultimately in turbulence is elucidated and given support. It is suggested that nonlinearity is essential in continually perturbing the eigenmodes of the flow in such a way that each mode is never permitted to relax to its least stable eigenstate (damped in the subcritical case). In this way, the linear growth mechanism can be regarded as an underpinning component of the general nonlinear feedback insofar as it is the only part which can extract energy from the mean flow and thus yield a net increase in disturbance energy. The physical aspects of the flow simulations are consistent with puff formation where, using a pair of helical waves as initial data, a sharp trailing front is formed naturally; axisymmetric ring vortices are generated and the general flow characteristics are in broad agreement with experiment.
Abstract: The evolution of localized three-dimensional disturbance in two- and three-dimensional laminar boundary layers is examined. The linearized Navier-Stokes equations for three-dimensional disturbances in a three-dimensional parallel shear flow are solved numerically using Fourier transform Chebyshev collocation techniques. Modal analysis shows that substantial short-term energy growth can be obtained even when all instability waves are damped. This transient growth can increase the initial disturbance energy by two or three orders of magnitude, at which stage nonlinear interactions might lead to a breakdown to turbulent flow, bypassing the traditional Tollmien-Schlichting instability mechanism. The dependence of the transient growth on wave number, Reynolds number, sweep angle and Hartree parameter is determined and a method for predicting the maximum transient growth is proposed and found to be reasonably accurate over a wide parameter range. Localized disturbances are also examined and it is found that the bypass growth mechanism can enhance the formation of cross-flow vortices in a three-dimensional flow. Some implications are discussed, particularly with respect to the observed effects of roughness on transition location.
Abstract: The behavior of very low-amplitude disturbances in a circular pipe is considered. Direct simulation of the Navier-Stokes equations is used to compute the evolution of two- and three-dimensional waves and the results are found to be in good agreement with solutions to the Orr-Sommerfeld equation for Hagen-Poiseuille flow. Transient growth mechanisms are also investigated computationally, in which case it is found that the growth of disturbances with large but finite streamwise wavelength exhibits a very rich structure of temporal evolution depending on the particular initial condition chosen. Comparison with recent results reported by Bergstrom on optimal disturbances is also given. In Part II of this study these findings will be extended to the nonlinear development of like disturbances.
Abstract: The development of localized disturbances in parallel shear flows is reviewed. The inviscid case is considered, first for a general velocity profile and then in the special case of plane Couette flow so as to bring out the key asymptotic results in an explicit form. In this context, the distinctive differences between the wave-packet associated with the asymptotic behavior of eigenmodes and the non-dispersive (inviscid) continuous spectrum is highlighted. The largest growth is found for three-dimensional disturbances and occurs in the normal vorticity component. It is due to an algebraic instability associated with the lift-up effect. Comparison is also made between the analytical results and some numerical calculations. Next the viscous case is treated, where the complete solution to the initial value problem is presented for bounded flows using eigenfunction expansions. The asymptotic, wave-packet type behaviour is analyzed using the method of steepest descent and kinematic wave theory. For short times, on the other hand, transient growth can be large, particularly for three-dimensional disturbances. This growth is associated with cancelation of non-orthogonal modes and is the viscous equivalent of the algebraic instability. The maximum transient growth possible to obtain from this mechanism is also presented, the so called optimal growth. Lastly the application of the dynamics of three dimensional disturbances in modeling of coherent structures in turbulent flows is discussed.
Abstract: The transition process of a small-amplitude wave packet, generated by a controlled short-duration air pulse, to the formation of a turbulent spot is traced experimentally in a laminar boundary layer. The vertical and spanwise structures of the flow field are mapped at several downstream locations. The measurements, which include all three velocity components, show three stages of transition. In the first stage, the wave packet can be treated as a superposition of two- and three-dimensional waves according to linear stability theory, and most of the energy is centred around a mode corresponding to the most amplified wave. In the second stage, most of the energy is transferred to oblique waves which are centred around a wave having half the frequency of the most amplified linear mode. During this stage, the amplitude of the wave packet increases from 0.5% to 5% of the free-stream velocity. In the final stage, a turbulent spot develops and the amplitude of the disturbance increases to 27% of the free-stream velocity. Theoretical aspects of the various stages are considered. The amplitude and phase distributions of various modes of all three velocity components are compared with the solutions provided by linear stability theory. The agreement between the theoretical and measured distributions is very good during the first two stages of transition. Based on linear stability theory, it is shown that the two-dimensional mode of the streamwise velocity component is not necessarily the most energetic wave. While linear stability theory fails to predict the generation of the oblique waves in the second stage of transition, it is demonstrated that this stage appears to be governed by Craik-type subharmonic resonances.
