Search

Noise control of large diesel and natural gas generators is achieved through industrial mufflers. Design of such mufflers relies heavily on general guidelines. However, these guidelines are not suitable for complex mufflers; instead, computer-based optimization provides an effective means of design. Optimization of a plug flow muffler is conducted in this work with a multi-objective (transmission loss and pressure drop) finite element simulation-based optimization using the efficient global optimization (EGO) algorithm. The EGO algorithm is shown to be well suited for computationally expensive muffler optimization, performing vastly better than genetic algorithms, such as the commonly used NSGA-II algorithm. © 2020 Institute of Noise Control Engineering

Collection efficiency transfer functions that compensate for wind-induced collection loss are presented and evaluated for unshielded precipitation gauges. Three novel transfer functions with wind speed and precipitation fall velocity dependence are developed, including a function from computational fluid dynamics modelling (CFD), an experimental fall velocity threshold function (HE1), and an experimental linear fall velocity dependence function (HE2). These functions are evaluated alongside universal (KUniversal) and climate-specific (KCARE) transfer functions with wind speed and temperature dependence. Transfer function performance is assessed using 30 min precipitation event accumulations reported by unshielded and shielded Geonor T-200B3 precipitation gauges over two winter seasons. The latter gauge was installed in a Double Fence Automated Reference (DFAR) configuration. Estimates of fall velocity were provided by the Precipitation Occurrence Sensor System (POSS). The CFD function reduced the RMSE (0.08 mm) relative to KUniversal (0.20 mm), KCARE (0.13 mm), and the unadjusted measurements (0.24 mm), with a bias error of 0.011 mm. The HE1 function provided a RMSE of 0.09 mm and bias error of 0.006 mm, capturing the collection efficiency trends for rain and snow well. The HE2 function better captured the overall collection efficiency, including mixed precipitation, resulting in a RMSE of 0.07 mm and bias error of 0.006 mm. These functions are assessed across solid and liquid hydrometeor types and for temperatures between −22 and 19 ∘C. The results demonstrate that transfer functions incorporating hydrometeor fall velocity can dramatically reduce the uncertainty of adjusted precipitation measurements relative to functions based on temperature.

The effect of the excitation frequency of synthetic jet actuators on the mean jet velocity of synthetic jets issuing from an array of circular orifices is investigated experimentally. Herein, the focus is placed on an array of circular orifices, rather than a single orifice, as it brings the advantage of covering long-span airfoils. The array consists of 16 circular orifices, each having a diameter of 3.42 mm, distributed over a span of 300 mm. The jets are generated by the excitation of a single cavity via 16 piezoelectric elements. Localized velocity measurements at the exit of the orifices show that the mean jet velocity varies with the excitation frequency. Several distinct resonant peaks were observed in the frequency response. Acoustic simulations of the cavity showed that these peaks correspond to acoustic mode shapes of the cavity. Due to the high-aspect ratio of the cavity, several acoustic mode shapes exist in the excitation frequency range aside from the Helmholtz resonance frequency. Moreover, the mean jet velocity emanating from the array shows a variation from orifice to orifice, depending on the excited acoustic mode.

The response of a separated boundary layer to synthetic jet flow control at the global wake instability (F+ ≈ 𝒪(1)) and the shear-layer instability (F+ ≈ 𝒪(10)) measured by particle image velocimetry are presented. The visualization shows that in each of the control cases, coherent vorticity develops and breaks down into a turbulent wake. When the jets are actuated by burst-modulation at the wake instability frequency, they induce regular formation and detachment of large-scale vorticity to form a wide turbulent wake. Excitation at the shear-layer instability frequency, on the other hand, produces a train of alternating velocity fluctuations in the boundary layer which dissipate to a narrower wake. Proper orthogonal decomposition of the velocity fields show that the physical extent of the jet-induced coherent structures is decreased with increasing addition of momentum for both excitation frequencies.

To reduce the environmental impact and cost, energy and water consumption of multi-resident buildings should be improved while ensuring resident comfort. Inefficient mixing of hot and cold-water streams and a non-optimal domestic hot-water (DHW) distribution system design can cause higher energy consumption, component failures, and dissatisfied residents. An OpenModelica (OM) system-wide model of a 14-story building consisting of a controlled-loop injection (CLI) device and a DHW distribution system is presented. The OM results are validated against field measurements at discreet locations within a single-zone closed-loop circuit to ensure the validity of time-varying temperature and flowrate. The study demonstrates that OM is a useful engineering tool to model single and multi-zone high-rise buildings that allows advanced analysis, including system-wide optimization, advanced on-demand controls, and energy and water-usage efficiencies.

A new method for assessing collection efficiency using wind speed and hydrometeor fall velocity is presented for the unshielded Geonor T-200B3 precipitation gauge based on computational fluid dynamics results. Time-averaged Navier–Stokes simulations with a k–e turbulence model were used to determine the airflow around the gauge for 0 to 10 m s−1 wind speeds. Hydrometeor trajectories and collection efficiencies were determined using Lagrangian analysis for spherical 10 hydrometeor fall velocities between 0.25 to 10 m s−1 for rain (0.01–3.9 mm diameter), wet snow (0.2–21 mm diameter), dry snow (0.2–7.1 mm diameter), and ice pellets (1.5–4.3 mm diameter). The model results demonstrate that gauge collection efficiency strongly depends on both wind speed and hydrometeor fall velocity. Collection efficiency differences for identical hydrometeor fall velocities are within 0.05 for wind speeds less than 4 m s−1, despite differences in hydrometeor type, diameter, density, and mass. An empirical expression for collection efficiency with dependence on wind speed and fall velocity is 15 presented based on the numerical results, giving a RMSE of 0.03 for dry snow, wet snow, and rain, for wind speeds between 0 and 10 m s−1. The use of fall velocity captures differences in collection efficiency due to different hydrometeor types and sizes, and can be broadly applied even where the precipitation type may be unknown or uncertain. Results are compared to previous models and good model agreement with experimental results is demonstrated in Part II.

This paper presents an analysis of double emulsion formation through a cross junctional flow-focusing microchannel which uses an improved color gradient lattice Boltzmann method. The model with the...

A comparison of numerical and experimental analysis of cavity shape influence on flow properties of a synthetic jet is presented. The goal is to highlight areas that are challenging to measure and to better understand the in-cavity flow and exit characteristics. In the examination of three cavities it is found that, in accordance with experiments, the cavity with the sharpest nozzle-to-cavity transition transmits the most momentum at the exit. The sharp transition within the cavity results in a stronger vortex, higher self-induced inward velocity, and ultimately more room for flow to exit the cavity during the expulsion phase. It is shown that the shape of the internal cavity plays an important role in the flow behavior at the nozzle exit. From a computational perspective, the results suggest that the flow field within the cavity must be computed to obtain accurate exit conditions for the synthetic jet.

The response of precipitation to turbulent fluctuations near gauges is studied using time-averaged ( RANS ) and unsteady ( LES ) turbulence modeling. Updrafting effects on catch performance are analyzed for unshielded and shielded gauges. The effective precipitation catchment area of the gauge for both wind-induced effects and snowflake characteristics is found to reduce significantly for small particles in high winds but can be partly recovered by shielding. The variation in the amount of precipitation caught is quantified for different free-stream wind speeds using LES and RANS . The fluctuations, captured with LES are analyzed to determine the local structure of eddies near the orifice plane. Wind-induced drag on precipitates are modeled for a wide range of particle Reynolds numbers from low speed Stokes flow condition to high speed flows with inertial effects. Results show noticeable effect of drag–force model on catch performance calculation of precipitation gauges with uncertainties of up to 40% in high winds and large snowflake sizes. Finally, particle–wall collision on the catch performance is studied for different restitution conditions. These simulations have differences of up to 5% in catch performance for large particle sizes in high winds, dependent on whether the particles undergo elastic or plastic collisions. Comparing RANS and LES results, turbulence fluctuations show a considerable influence on shielding performance degeneration at high winds. Double shielding the gauge can improve efficiency by maintaining a lower fluctuation-to-mean catch ratio as wind speed increases.

The sensitivity of the Orr–Sommerfeld equation due to base flow deviations is investigated by means of a Monte Carlo-type perturbation strategy and a Chebyshev collocation method. In particular, the sensitivities of the peak growth rate and frequency, both of which are of importance in airfoil applications, are investigated in the low Reynolds number regime. A separated boundary layer with a nominal shape factor of H = 5.9 was perturbed for both velocity and wall-normal position deviations. Wide bands of eigenvalue spectra are obtained due to both types of perturbations. The standard deviation of the peak growth rate and frequency due to both perturbations does not exhibit a pronounced Reynolds number dependence. To broaden the results, six boundary layers were investigated with shape factors ranging from H = 5.9 − 22 . Perturbations resulting in a standard deviation of 1% of the nominal shape factor were applied. It is found that sensitivities of both the peak growth rate and frequency are more pronounced at lower shape factors, with a decrease in sensitivity with increasing shape factor. This result suggests that, at low Reynolds numbers, boundary layers with larger separated regions are less sensitive to base flow perturbations.

To better quantify measurement uncertainty associated with solid precipitation gauges, time averaged and transient turbulent flow over gauges is studied using a Computational Fluid Dynamic (CFD) approach. By numerically solving the Reynolds-Averaged Navier-Stokes ( RANS ) equations, vortex formation in the bulk structure of the flow is examined. To study transient effects, Large-Eddy Simulation ( LES ) quantified the turbulence and eddy dynamics. The simulated gauge is a Geonor T200B gauge shielded with Alter slats for a wind velocity ranging from 1 to 10 m/s. A multi-layer mesh generation technique allows the capture of a wide range of turbulent length scales. Gauge exposure effects on the wind characteristics are investigated for unshielded and shielded cases. Updrafting results in an effective collection area of only 30% of the orifice for capturing precipitation. The RANS results show wind momentum reduction above the gauge orifice from shielding with vertical slats assists catch efficiency of the gauge. However, tilting the slats in high winds degenerates shielding benefits. LES results show considerable production of turbulence kinetic energy near the inlet orifice area. Transient analysis with LES demonstrates that the shields intensify the turbulence near the gauge. With the addition of wind shields, the flow momentum above the gauge is reduced.

In this work, a large eddy simulation (LES) of a typical subsonic diffuser provides data used to analyze coherent structure in a separated flow with dynamic mode decomposition (DMD). From this, a low–dimensional approximation, which retains the main dynamic characteristics of the original flow fields, is obtained. In particular, specific dynamic structures associated with a unique frequency are isolated. The spatial structures of the real and imaginary parts of the DMD mode are similar but with a phase difference. The contribution of the conjugate modes to the evolution of the DMD modes over time is discussed. The dominant frequency is found to be related to the wake mode. The scale of wake will saturate, and the shear layer will become weaker and merges into the wake structure as it develops downstream. This allows direction for effective flow control strategies using this information.

A compact, lightweight, low-power piezoelectric micro-blower was characterized using particle image velocimetry to determine its flow control potential. The micro-blower has been operated in continuous mode as well as in burst mode using two different actuation frequencies. The maximum mean velocity measured with the micro-blower operating in continuous mode was approximately Ūmax = 13 m/s which occurred at the centerline at an approximate stream-wise location of x/d = 4. The velocity profiles in the developed region resemble those of turbulent jets. The momentum-flux from the micro-blower in continuous mode was significantly greater than a typical synthetic jet actuator which was successfully used for flow control, indicating that the micro-blower can impart the necessary momentum to be effective for flow control. With burst mode, the results show that the micro-blower could impart an even greater momentum.

Particle image velocimetry (PIV) data processing time can constrain data set size and limit the types of statistical analyses performed. General purpose graphics processing unit (GPGPU) computing can accelerate PIV data processing allowing for larger datasets and accompanying higher order statistical analyses. However, this has not been widespread likely due to limited accessibility to the GPU-PIV hardware and software. Most GPU-PIV software is platform dependent and proprietary, which restricts the computing systems that can be used and makes the details of the algorithm unknown. This work highlights the development of an open-source, cross-platform, GPU-accelerated, PIV algorithm. Validation of the algorithm is done using both synthetic and experimental images. The algorithm was found to accurately resolve the time-averaged flow, instantaneous velocity fluctuations, and vortices. All data processing was done on a GPU supercomputing cluster and notably outperformed the central processing unit version of the software by a factor of 175. The algorithm is freely available and included in the OpenPIV distribution.

Miniaturized ion mobility spectrometry (IMS) presents several advantages as portable chemical detection tools. They are fast, simple and inexpensive. However, the implementation of ion gates in miniaturized designs is difficult. Studies also showed that ion gates leads to electrostatic effects that negatively impacts the performance of IMS. This paper presents an alternative which eliminates ion gates by using the pulsating regimes of electrospray to generate small ion packets and a current-based measurement method to monitor drift times. A feedback loop was designed to control the number of pulsations generated by the electrospray. The results demonstrate that pulses of the axial spray mode I generate ion emissions of about 200 μs in ambient gas, which is comparable to conventional gating methods. The measured mobility is roughly 2.5 times higher than with conventional gating methods, so a new library of ion mobilities for this new method must be created for reference. The method presented is inexpensive and can be easily implemented in microfabricated IMS and similar devices.

Building codes require domestic hot water supplied to high-rise residential buildings to be stored at 60∘C or above to avoid bacterial contamination (e.g., Legionella), while the supplied water delivered to residential units should not exceed 49∘C to avoid scalding. Inefficient mixing of hot and cold water to lower the water temperature can cause temperature and pressure fluctuations of supplied water resulting in resident discomfort, higher energy consumption and risk to plumbing fixture. This work compares the efficiency of two mixing devices (a) an electronic mixing valve (EMV) and (b) a thermostatic mixing valve (TMV) experimentally and numerically. The energy savings, pressure drop, and temperature fluctuation caused by each device in the same building are presented. Each 24–hour period is divided into 4 time periods based on hot water demand. The results show that the EMV can provide steadier temperature during the transition between low demand period overnight and peak consumption in the early morning, and lower pressure drop in the hot water system. In addition, depending on the circulation flowrate in the building, the EMV can potentially reduce energy consumption by more than 40% particularly during low demand periods.

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

Impedance spectroscopy has the potential for label-free integrated electrochemical detection in microfluidic lab-on-a-chip applications. Its capability to identify and discern between surface and bulk processes in solid–liquid systems finds particular use for the detection of biorecognition events or conductivity measurements. The electrochemical transducer can be in the form of interdigitated electrode structures to increase sensitivity. Experimental work was performed to characterize two different transducer designs. Applications included the monitoring of protein films on contact-less interdigitated electrode structures and conductivity detection of droplets on insulated two-electrode structures. The use of electrode passivation eliminated electrode degradation. Experimental results were compared to theoretical analytical models, and were found to closely correlate with one another. The analytical models were used to design the transducer for optimal conductivity detection. The results inform the current research efforts for the development of in-line impedance spectroscopy in digital microfluidics and confirm the use of simple analytical models for the first-order estimation of the frequency response of interdigitated electrode structures.

Interdigitated electrode structures have applications in a myriad of fields and have become attractive for in-line electrochemical detection in lab-on-a-chip microsystems. Analytical models can replace complex and expensive numerical simulations to determine the capacitance of interdigitated electrode structures in cases where simple geometries can be assumed. Closed-form analytical expressions derived from Schwarz–Christoffel conformal mappings were used to determine the capacitance of multi-layer interdigitated electrode structures with an additional parallel continuous electrode. The partial capacitance approach was reformulated to find the capacitance of multi-layer structures with non-monotonically changing permittivities. The analytical model was found to be in close agreement with finite-element simulations. The model is used for optimization of an insulated transducer for sensitive detection of surface processes.

Wind-tunnel experiments are used to study the effect of momentum coefficient and excitation frequency on flow separation using synthetic jet actuation. Experiments are conducted on a NACA 0025 airfoil at a chord-based Reynolds number of 100,000 and angle of attack of 10 deg. The actuator is located near the leading edge, downstream of the mean separation location. High-frequency excitation is able to reattach the flow and eliminate the large-scale vortex shedding in the wake, leading to a decrease in drag of approximately 45%. Low-frequency excitation is employed to target the instabilities associated with the separated shear layer and vortex shedding in the wake. Excitation of the wake instability also causes the flow to reattach; however, it leads to organization of the large-scale vortex shedding. By forcing the boundary layer at the frequency of the shear-layer instability, the threshold momentum required to reattach the flow is an order of magnitude smaller as compared with high-frequency excitation,...

A synthetic jet is an unsteady jet that transfers linear momentum to the surroundings by alternately ingesting and expelling fluid from a cavity containing an oscillating diaphragm. This work presents the first experimental effort to validate the limited number of numerical investigations that have postulated synthetic jets are insensitive to cavity shape. Three axisymmetric synthetic jets with different cavity shapes were used to examine jet performance while keeping other parameters constant such as cavity volume, nozzle length and orifice diameter. Cylindrical, conical and contraction shaped cavities were considered. The cavity pressure and velocity at the orifice exit plane were measured using a microphone and hot-wire, respectively. The results demonstrated that for several operating conditions near Helmholtz resonance of the cavity, noticeable differences were observed in the radial velocity profiles between the three geometries. The Reynolds number decreased sequentially from the cylindrical to conical to contraction cavity. The momentum flux, which is relevant in flow control applications, followed the same trend. In general, the experimental results showed that synthetic jet performance is, to some degree, dependant on cavity shape.

This work was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC). In addition, the authors acknowledge the support and guidance of Afshin Rahimi during this study.

The reattachment of post-stall separated flow on a NACA 0025 airfoil at a Reynolds number of 100,000 and angle-of-attack of 12 $$^\circ$$ using synthetic jet control was studied experimentally. The goal of this work was to study the effect of control parameters including blowing ratio, frequency, and in particular, slot location, on aerodynamic performance. Slot locations both upstream and downstream of the mean separation point were tested. The results showed that for both drag reduction and lift increase, once a particular blowing ratio was met, the benefits of control saturated. Positioning the slot at the most upstream location was found to be the most effective, both in terms of having the lowest threshold blowing ratio and producing the largest lift-to-drag ratios. A monotonic increase in threshold blowing ratio (defined as being the blowing ratio required for fully attached flow) and decrease in lift-to-drag was observed as the slot location moved downstream. It was also found that while forcing at a frequency corresponding to the wake instability led to maximum lift increase, forcing in the range of the separated shear layer instability (frequencies an order of magnitude larger) led to maximum drag reduction.

This paper reports on a hybrid electrowetting-thermal passive microvalve for a flow control mechanism to interface droplet flow with continuous flow. When heated, a thermo-responsive polymer, in the form of a dilute aqueous droplet, forms a gel, and upon cooling, it liquefies. The gel blocks and its subsequent liquefaction unblocks the pressurized flow. Electrowetting on dielectric (EWOD) actuation was used to control and position a polymer droplet at a desired valving location. The valve interfaced a pressurized flow to a planar EWOD-based droplet flow in a single device. It was fabricated using a standard planar microfluidic glass platform. It has a three-layer architecture bonded by oxygen plasma bonding. The valve demonstrated zero leakage and no cross-mixing across the interface.

The influence of periodic excitation from synthetic jet actuators on boundary-layer separation and reattachment over a NACA 0025 airfoil at a low Reynolds number is studied. Flow-visualization results showed a vertical jet pulse accompanied by two counter-rotating vortices being produced at the exit of the simulated slot, with the vortices shed at the excitation frequency. Hot-wire measurements determined the maximum jet velocity for a range of excitation frequencies and voltages, and were used to characterize the excitation amplitude in terms of the momentum coefficient Cμ. With the synthetic jet actuator installed in the airfoil, flow-visualization results showed that excitation produces boundary-layer reattachment, with the associated significant reduction in wake width. Wake-velocity measurements were performed to characterize the effect of flow-control excitation amplitude and frequency on airfoil drag and wake topology. The results demonstrate that Cμ is the primary governing flow-control parameter....

Large-eddy simulation and linear stability analysis were performed on a NACA 0025 airfoil at a chord Reynolds number of 105 and four angles of attack. The computations showed that the initial vortex roll-up quickly breaks down to three-dimensional turbulence. Flow separation was observed at all angles, whereas only the lowest angle of attack formed a laminar separation bubble due to flow transition occuring close to the airfoil surface. A Chebyshev collocation method was employed to solve the viscous and inviscid stability equations. Linear stability analysis demonstrated that high-frequency disturbances occur in the laminar separation bubble case, whereas lower frequencies are present for the fully separated angles of attack. The maximum disturbance growth rates were dampened with the addition of viscosity but negligible change in peak frequency was noted.

The ability of active flow control through synthetic jet actuator to reattach the stalled flow on a NACA 0025 at low Reynolds number was investigated experimentally. Wind tunnel tests were performed where boundary layer velocity profiles were measured by hot-wire anemometry. The effects of excitation frequency and momentum input, quantified by the blowing ratio were the focus of the measurements. Using both low- and high-frequency actuation, the boundary layer was reattached in a time-averaged sense, however the mechanism of control was not the same. Low-frequency excitation lead to oscillations in the boundary layer at the control frequency due to the passage of a vortex transporting high-momentum to the inner boundary layer. Conversely, high-frequency control caused a steady reattachment of the boundary layer.

Real-time measurement of electrical properties are used to perform the first real-time detection of particle concentration and chemical reactions in electrowetting on dielectric devices without the need for optical access. For particle laden droplets, the change in both resistance and capacitance was found to be linear from zero to five-hundred particles with a resolution of approximately six particles for both measurements. Electrical properties were also measured for mixtures of alkaline phosphatase and p-Nitrophenyl Phosphate; reagents commonly used in immunoassays as the resultant chemical reaction produces a yellow precipitate. Experiments were performed with mixtures created off-chip and in droplet that were statically mixed on chip. The difference between the measured and expected capacitance was found to increase with the concentration of alkaline phosphatase and chemical reactions could be positively identified in mixtures made both on and off the chip. Real-time measurements of reagents mixed in a four electrode electrowetting on dielectric mixer were also taken. In these experiments, chemical equilibrium was reached after approximately 20 cycles and the difference between the measured and expected capacitance was over 8.5 times greater than the experimental uncertainty.

A method for achieving electrospray ionization directly from a droplet is reported. The electrospray is formed at the tip of a capillary wick bonded to a glass substrate with patterned electrodes. The droplet of sample solution is delivered to the base of the wick, forming a reservoir. Capillary action draws the solution from the reservoir to the tip of the wick, continuously replenishing the sample as it is emitted via electrospray. No external pressure source is required to drive the flow. The glass surface of the substrate is made hydrophobic to ensure the integrity of the reservoir. An 8 μL droplet of 0.4% (w/v) bovine serum albumin solution maintains a steady spray for 1–2 min operating in cone-jet mode. The electrospray emitter is compatible with digital microfluidic schemes that employ single-plate droplet actuation.

Integrating electrospray into microelectromechanical systems (MEMS) and microfluidic systems supports applications in diverse fields from biotechnology to aerospace. Electrospray also functions as a production tool, allowing for novel methods of MEMS fabrication. This review covers the three most significant applications of electrospray in MEMS and microfluidic systems technology: 1) as an integral part of a microfluidic device, most notably electrospray emitters for coupling a microfluidic chip to a mass spectrometer; 2) as a method for fabricating and manufacturing MEMS; and 3) for micropropulsion in aerospace applications using MEMS-based emitters. Perspectives on future research directions and opportunities are provided.

A passive mechanical method for the filtration of particles in electrowetting on dielectric (EWOD) devices is presented. Analytical and experimental results show that droplets actuated by EWOD cannot pass physical obstructions unaided at the scales considered here. However, it was possible to pull droplets past the same obstructions using a second droplet. The two droplets approach the obstruction from opposite sides and merge within the pore of the obstruction. The interface on the enabling side of the amalgamated droplet is then actuated to pull fluid through the obstruction. This technique was successful for pore sizes between half and two orders of magnitude below the confined droplet height. This wide range of viable pore sizes will allow for the filtration of particles by size in EWOD devices. It can also be used to filter large particles traditionally used in microfluidic immunoassays or allow for the use of smaller particles to increase sensitivity. Success at pore sizes as small as 2 μm also suggests that filtration of animal cells in EWOD devices is possible. The proposed process is performed without the use of surfactants, which may make it more attractive for applications using biological material.

This paper analyzes control strategies for a small scale wind turbine in intermittent winds using a computer model based on wind data collected on-site and computer simulation data. Standard generator control with a lowered gain was found to produce the greatest power capture by biasing the rotational speed towards high energy wind gusts. Optimal gain was able to be determined by calculating an estimated wind speed using physical and measured quantities. An augmentation to the standard control provides a method for capturing power safely beyond the rated wind speed.

A microfluidic dispensing device that is capable of generating droplets with volumes varying between 1 nL and 50 pL at an ejection frequency of up to 6 kHz is presented. In this device, a piezoactuator pushes onto an elastic membrane via piston tips; the mechanical bending of the membrane generates a pressure pulse pushing droplets out. An analytical model was developed solving bending characteristics of a plate-actuated fluidic dispensing system and used to calculate the displaced volume. The model was extended to perform stress analysis to find the optimum piston tip radius by minimizing design stresses. The optimum piston tip radius was found to be 67% of the chamber radius. The actuation force estimated using the analytical model was then used as input to a finite element model of the dispenser. A detailed numerical analysis was then performed to model the fluid flow and droplet ejection process and to find critical geometric and operating parameters. Results from both models were used together to find the best design parameters. The device contains three layers, a silicon layer sandwiched between two polydimethylsiloxane (PDMS) polymer layers. Silicon dry etching, together with PDMS soft lithography, was used to fabricate the chip. PDMS oxygen plasma bonding is used to bond the layers. Prototypes developed were successfully tested to dispense same-sized droplets repeatedly without unwanted droplets. The design allows easy expansion and simultaneous dispensing of fluids.

B LADES of small vertical-axis wind turbines operate at low Reynolds numbers and undergo continuous dynamic stall [1]. To improve the design of these blades, flow features such as separation and turbulent transition need to be captured accurately, which is still a challenge for most of the computational fluid dynamics codes. In a first attempt to simulate these flows, a code is developed and tested on a static airfoil at a low Reynolds number. When the chord Reynolds number Rec decreased below 500,000 the airfoil performance deteriorated by losing lift and increasing drag [2]. A free shear layer forms as the laminar boundary layer separates in an adverse pressure gradient forming a laminar separation bubble. This separated laminar boundary-layer transition to turbulence over an airfoil is a complex problem that is not completely understood. In particular, the information obtained so far is inadequate for clarifying the entire transition process, large-scale structures, and the instability mechanisms. Recently, an extensive experimental study of boundary-layer development for a NACA0025 airfoil at low Reynolds number was experimentally conducted [3]. Flow velocity datawere obtainedwith hot-wire anemometry.Wind-tunnel experiments were carried out for a range of Reynolds numbers from 10 to 10 and three angles of attack (5, 10, and 15 ). Two boundary-layer regimes were identified: 1) boundary-layer separation without reattachment and 2) separation bubble formation. Their results showed that transition to turbulence, which occurs because of the amplification of disturbances in the separated shear layer, plays a key role in boundary-layer reattachment. Their results also suggest that coherent structures form in the separated flow region and the formation of the roll-up vortices in the separated shear layer is linked to inviscid spatial growth of disturbances and is attributed to the Kelvin–Hemholtz instability. The final stage of transition is associated with the growth of a subharmonic component in the velocity spectrum, which can be attributed to the merging of the roll-up vortices. Given that the airfoil boundary layer at low Reynolds number is inherently unsteady and features important large-scale motions, this flow is not well predicted by Reynolds-averaged Navier–Stokes models. Instead a time-dependent simulation, such as a direct numerical simulation or large-eddy simulation (LES), is required for more accurate results. Large-eddy simulation is particularly suitable to investigate the generation and evolution of coherent structures in turbulent flows. It is ideal for low Reynolds number flows and explicit computation of coherent structures and has only limited sensitivity to modeling assumptions so that results are feasible for this practical problem. Flow past an experimentally studies NACA0025 airfoil is a challenging case for LES because of the different flow regimes around the airfoil, including the laminar boundary layer, laminar separation bubble, transition region, turbulent boundary layer, separation point, and separation region as well as wake region. In LES, it is important to resolve all important (large-scale) flow structures and model the remaining small-scale turbulence. Ideally, the goal would be to resolve the following flow regions: the laminar boundary layer with a sufficient amount of nodes (in the streamwise andwall-normal directions), the recirculation region (at the transition region and at the trailing edge), and the turbulent boundary layer. For blade design, it is also important to solve the flow in a short computational time. The code developed herein continues previous efforts to make LES applicable to complex flow simulations. The previous developed flow simulation methodology [4,5] also aims at obtaining accurate results on fewer grid nodes than standard LES. Although LES is promising, the drawback of LES is the large memory required if all the above mentioned regions are resolved. To control the size of the problem, an instantaneous wall function [6] is used, allowing the first nodes near the airfoil surface to be placed in the logarithm layer instead of in the sublayer. To further reduce computational cost, a pressure and velocity decoupled system is used. The consistent splitting method [7] evaluates the pressure by testing the momentum equation against gradients. The resulting Poisson equation for the pressure correction term avoids artificial boundary conditions. This scheme is free of splitting errors and the pressure error is smooth. Nonconforming finite elements with a lower requirement for degrees of freedom are chosen when compared to other conforming finite elements (e.g., the Taylor Hood element). This is a significant savings in memory and was suggested by [8] and implemented in FEATFLOW. Highly nonuniform meshes satisfy the resolution requirements of LES in the boundary layer. A traditional solver cannot damp the high frequencies generated by nonuniform meshes effectively, and a multigrid method is often needed. But the filtering operations on all the multigrid scales have to be separated from the traditional multigrid procedure to prevent errors between the different filtering scales. To get good LES results, a filtering operation is developed in the present work and discussed in Sec. II. In LES, the assumption of constant viscosity can not be made because of the eddy viscosity model used. This leads to a full coupling of the velocity components if an implicit treatment is applied for the viscous term. Two approaches have been traditionally used. One is the full coupled iterative method that can be solved iteratively but is expensive. The other is to treat the viscous term explicitly at the expense of reductions in the admissible time-step size due to stability restrictions. A third variable-viscosity formulation [9] can be used to discretize the variable-viscosity term.However, stability is not ensured because the variable-viscosity term is included on the right-hand side; in addition, this approach is computationally expensive. To simplify the above approach, a new scheme is adopted, where the variable viscosity is assumed piecewise constant in each element. Presented as Paper 4199 at the 18th AIAAComputational Fluid Dynamics Conference, Miami, FL, 25–28 June 2007; received 9 March 2009; accepted for publication 24 September 2009. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/10 and $10.00 in correspondence with the CCC. ∗Department of Mechanical & Industrial Engineering; taoxu@mecheng1. uwaterloo.ca. Department of Mechanical & Industrial Engineering; sullivan@mie. utoronto.ca. Department of Mechanical & Industrial Engineering; paraschi@me. concordia.ca. JOURNAL OF AIRCRAFT Vol. 47, No. 1, January–February 2010