Radial pumps and compressors are used in various engineering applications, including rocket turbopumps, automotive turbochargers, and refrigeration systems. Several physical effects, including viscous losses, flow separation, compressibility, and rotational dynamics, dominate radial impeller flow, making flow field prediction very difficult and requiring computationally expensive computational fluid dynamics (CFD). However, designers typically only require information at specific positions, resulting in most simulation data being unused. Accurately predicting the impeller exit flow field is often key to impeller design. Recent advances in reduced-order modeling and machine learning show promise for a priori flow field prediction. In this study, nine different reduced-order models (ROMs) were created to predict the dimensionless exit flow field of radial flow impellers in real-time. The ROMs consist of various linear and nonlinear dimensionality techniques paired with different regressors. Inputs include parameterized, dimensionless impeller geometry based on Bezier control points, number of blades, and dimensionless operating conditions. The ROMs were trained using over 1800 flow fields from high-fidelity CFD simulations representing a large radial flow compressor design space. ROMs were evaluated on relative error, training time, and evaluation time. Principal component analysis coupled with Gaussian process regression (PCA-GPR) emerged as the preferred ROM, training within 2 s, evaluating hundreds of cases in real-time, and matching the accuracy of computationally demanding nonlinear alternatives. PCA-GPR predictions show pressure, density, and velocity profiles within 5% average of CFD results. The ROM was validated through four test cases probing robustness across different operating conditions and geometries.

Water jet impingement is an effective method of rapidly cooling a surface, but heat transfer from the surface is highly dependent on the surface condition and properties. Here, the impact of a superhydrophobic (SH) surface on heat transfer to an impinging, axisymmetric, room-temperature water jet with Re = 6 x 10 3 to 18 x 10 3 is explored. SH surfaces are created by etching thin silicon wafers to form different micropatterns (posts or holes). Surfaces are heated to between 200 and 320 degrees C, and the local surface temperature is measured with a thermal camera. The time resolved heat transfer from the surface and speed at which the thin film front spreads are measured. Local surface heat flux from the surface to the jet is calculated using an instantaneous energy balance. Heat transfer is shown to be highly dependent on jet Re and initial surface temperature. Results also show that varying microstructure by feature shape, width or diameter, and pitch (distance between features) individually did not reveal a systematic effect. However, when a surface roughness parameter is computed, the data followed a systematic variation. An increase in roughness resulted in a corresponding increase in time for the thin film to advance and a decrease in heat transfer rate. Interestingly, microstructure height alone did yield an impact, where a decrease in post height from 25 to 5 mu m led to an increase in local heat flux of up to 90% for low Re cases.

This paper describes an experimental investigation of the interactions of side-by-side rotors at varying rotor spacings. The tests were performed in hover (J=0) using stereoscopic particle image velocimetry (SPIV). The rotors used were DJI 9443 rotors rotating at 4860 RPM (81 Hz). Rotor phase-locking was employed to synchronize the rotation rate with the captured PIV images. An isolated rotor case, along with four tip-to-tip separation distances of s/D=0.5, 0.2, 0.1, and 0.05, were explored. The shed tip vortices, rotor wake velocity profiles, and overall momentum flow rate measurements were analyzed at each separation distance case. Decreasing the tip-to-tip separation resulted in a 28% increase in wake boundary contraction ratio and a faster initial decay in tip vortex magnitude, which is three times faster for the smallest separation distance than for the isolated rotor. Interactions from the adjacent rotor also yielded a 47% decrease in vorticity of initial tip vortices and a 35% decrease in circulation at the smallest tip-to-tip spacing. The position of the rotor wakes was altered and shifted toward the adjacent rotor by up to 0.1D as separation distance decreased. The dual-rotor cases had an increase in momentum flux when compared to the isolated rotor case, with a maximum increase of 5.1% for the smallest tip-to-tip spacing case.

Accurate models for predicting drop dynamics, such as maximum drop departure sizes, are crucial for estimating heat transfer rates during condensation on superhydrophobic (SH) surfaces. Previous studies have focused on examining the heat transfer rates for SH surfaces under the influence of gravity or vapor flowing over the surface. This study investigates the impact of surface solid fraction and texture scale on drop mobility in a condensing environment with a humid air flow. Experiments recorded condensation with varying surface feature sizes from micro- to nano scale under different flow rates. Video analysis detected the drop-size distribution and maximum drop departure sizes. Particle image velocimetry (PIV) provided accurate shear force representations. Results showed that the maximum drop departure sizes decreased with lower surface solid fractions and higher flow rates. A force balance analysis revealed that coalescence-induced drop jumping aids drop departure. Different drop behaviors due to coalescence were linked to surface characteristics. The study quantified drop jump distances under shear force during condensation on SH surfaces, and found that jump distances increased when coalescing drop sizes were similar. Based on these findings, a method for tuning SH surfaces to control drop size at coalescence and departure was suggested. Drop mobility was measured in terms of Bond and Capillary numbers, showing a dependence on surface solid fraction and pitch size. By combining these into surface slip length, it was shown that drop mobility increases with increasing slip length.

Thermal atomization during droplet impingement on two distinctly different microstructured superhydrophobic (SH) surfaces is considered. Square posts and square holes with the same structure spacing and solid fraction are used to explore the relative importance of vapor escape and wetting dynamics during droplet impingement as a function of Weber number (We) and temperature. Experiments are accomplished for each configuration with structure heights of 4 mu m, 8 mu m, and 12 mu m. Surface temperature varies between 120 degrees C and 250 degrees C with impingement Weber numbers of 40, 85, 150, and 240. For nearly every scenario the maximum atomization intensity observed is greater on hole-structured SH surfaces than post-structured SH surfaces (which allow vapor escape between the structures). We find that the 4 mu m height structures behave as might be expected, where increasing We results in increased spreading and atomization. For taller structured surfaces the trends with We are not as clear. On the post SH surface, wetting increases as We increases (enhancing atomization intensity) until about We = 85-120 after which further increases in We result in and faster dewetting leading to decreasing atomization intensity with We. The Leidenfrost point is nominally the same for all surfaces except taller posts, for which it decreases with post height, presumably due to intermittent wetting. It is clear microstructure type and height as well as surface temperature and Weber number all significantly affect atomization intensity and the Leidenfrost point due to their competing impacts on the liquid and vapor dynamics associated with each microstructure configuration.

This paper reports on an experimental investigation of the interactions of two side-by-side propellers in hover ([Formula: see text]) using stereoscopic particle image velocimetry (SPIV). The propellers employed were DJI 9443 propellers rotating at 4860 RPM with a spacing of 5% of the propeller diameter. The SPIV measurement images were phase-locked with the synchronized propeller rotation. Three experimental scenarios were considered: a) a single propeller; b) dual, counter-rotating propellers rotating exactly in phase; and c) dual, counter-rotating propellers rotating 90 deg out of phase. The propeller tip vortices, full-field velocity measurements, and overall momentum flux measurements were analyzed for the three scenarios. The interactions of adjacent propellers caused a 55% faster decay in the peak vorticity of the tip vortices and significantly altered the tip vortex trajectories when compared to a single-propeller scenario. There was negligible change in the magnitude of the peak vorticity in tip vortex cores, the dissipation of the tip vortices, and axial velocity flowfields downstream of the propellers for the two differing phase-offset dual-propeller configurations. For both dual-propeller scenarios, the average momentum flow rate was 2.2% higher than for the single-propeller case, while the peak value was 5.2% higher.

Drops are retained or held on surfaces due to a retention force exerted on the drop by the surface. This retention force is a function of the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When external or body forces exceed the retention force, the drop begins to move. This work explores the conditions for which drop departure occurs on structured superhydrophobic surfaces in the presence of an applied shear flow. Drop departure is explored for five microstructured superhydrophobic surfaces, one nanostructured carbon nanotube surface and one smooth hydrophobic surface. Surface solid fractions range from 0.05 to 1.00, and measured static contact angles range from 121° to 161°. Droplet volumes of 5, 10, 20, 30, 40, and 50 μL are tested on each surface. For each experiment, increasing air velocity is applied to a droplet placed on a surface until the droplet departs. High-speed imaging is used to track droplet base length, height, cross-section area (as viewed from the side) and advancing/receding contact angles. Measurements of drop advancing and receding contact angles are reported at the point of departure, with increasing contact angle hysteresis observed prior to departure. Contact angle hysteresis is observed to be a good indicator of droplet mobility. Measurements of the average air velocity over the height of the droplet are determined at the point of departure for all conditions. The measured air velocity shows strong dependence on the surface solid fraction, and the required shear flow velocity decreases as the surface solid fraction decreases. This is most pronounced at very low solid fractions. A coefficient of drag for the departing drops in shear flow is calculated and is shown to decrease with increasing Reynolds number.

This paper presents an experimental exploration of thermal transport due to a water jet impinging on post patterned superhydrophobic surfaces and provides the first experimental data for this problem. The surface is heated to temperatures much lower than the saturation value. Results are obtained for a smooth hydrophobic surface and superhydrophobic surfaces with varying microfeature pitch (w = 8, 16, and 24 mu m) and cavity fraction (Fc = 0.56 and 0.85) and are compared to results derived from a previously published analytical model. The jet Reynolds number varied from 1.1 x 104 to 1.7 x 104 and the nominal surface heat flux varied from 2.5 x 104 to 4.9 x 104 W/m2. Results obtained for all superhydrophobic surfaces show a significant decrease in the local and average Nusselt numbers (up to 30 % reduction) compared to impingement on a smooth surface. Further, the reduction is a strong function of the surface post and cavity geometric parameters. The effective temperature jump length is determined for all scenarios considered, representing the first experimental measurements of temperature jump length for heat transfer at superhydrophobic surfaces. Functional relationships showing how the average Nusselt number and the non -dimensional temperature jump length depend on the superhydrophobic surface parameters are also presented.

Microfluidic-based techniques have been shown to address limitations of reconfigurable radio frequency (RF) antennas and filters in efficiency, power handling capability, cost, and frequency tuning. However, the current devices suffer from significant integration challenges associated with packaging, actuation, and control. Recent advances in reconfigurable microfluidics that utilize the motion of a selectively metalized plate (SMP) for RF tuning have demonstrated promising RF capabilities but have exposed a need for an accurate fluid actuation model. This research presents a model for the mechanical motion of a moving plate in a channel to relate the SMP size, microfluidic channel size, velocity, and inlet pressure. This model facilitates understanding of the actuation response of an RF tuning system based on a moving plate independent of the actuation method. This model is validated using a millimeter-scale plate driven by a gravitational pressure head as a quasi-static pressure source. Measurements of the prototyped device show excellent agreement with the analytical model; thus, the designer can utilize the presented model for designing and optimizing a microfluidic-based reconfigurable RF device and selecting actuation methods to meet desired outcomes. To examine model accuracy at device scale, recent papers in the microfluidics reconfigurable RF area have been studied, and excellent agreement between our proposed model and the literature data is observed.

An electrowetting on dielectric (EWOD) pump is a microfluidic pump that uses electrowetting to manipulate liquid droplets in a channel, offering an alternative approach to traditional mechanical pumps. EWOD-based pumps have significant potential for various microfluidic applications. For instance, by integrating microfluidic pumps with a radio frequency (RF) device, it is possible to create a microfluidic reconfigurable RF device. However, the current microfluidic-based devices have been primarily designed based on mechanical micropumps which require expensive clean-room fabrication methods. Here, we present an analytical model for an EWOD semi-continuous pump that can offer a promising alternative actuation for microfluidic actuation. However, a literature survey of recent advances in EWOD pumping has highlighted a gap in the modeling of the relationship between the fluid mechanics and the actuation dynamics. This paper presents an analytical model of an EWOD pump that determines the flow rate and pressure generated by taking into account the competing electrowetting force, friction force, and fluid inertia force in a one-degree-of-freedom channel. The analytical model is valuable for designing and optimizing EWOD-based pumps, as it provides insights into the dominant physical processes and could enable better control. The model is validated with an EWOD experiment and the data demonstrates less than a 6 % error between the measured and predicted maximum droplet velocity and a maximum 7.4 % error in the EWOD pump static pressure.

Recent advancements in Electrowetting on Dielectric (EWOD) systems, such as simplified fabrication, low-voltage actuation, and the development of more reliable materials, are expanding the potential applications of electrowetting actuators. One application of EWOD actuators is in RF devices to enable dynamic reconfiguration and allow real-time adjustments to frequency and bandwidth. In this paper, a method is introduced to actuate a panel using EWOD forces. In the EWOD system, the velocity of the plate increases by maximizing the actuation force, minimizing the moving mass (droplets and metalized plate), and reducing resistance (contact line drag, fluid drag). However, some of these are competing factors. For instance, the actuation force can be increased by increasing the number of droplets, but this also increases the inertia and the drag force. An analytical model of EWOD actuation is presented to understand system performance tradeoffs. The model is validated with an EWOD experiment, and the data demonstrate less than a 7.8% error between the measured and predicted maximum plate velocities for different voltage inputs. In addition, this study presents a 3D numerical FEM model to analyze the velocity profile and viscous force in the thin droplets, focusing on variations along the droplet’s height, which cannot be captured experimentally. The main advantage of the proposed system over previous works is the simple 2D manufacturing process, which allows embedding metalized plates and RF circuit boards, in addition to being compact, portable, and low-cost. In addition, the proposed method does not have any mechanical components, which can increase the system’s reliability in a harsh environment.

The thermal transport to drops that roll or slide down heated superhydrophobic surfaces is explored. High-speed infrared imaging is performed to provide time-resolved measurement of the heat transfer to the drop. Data are obtained for drops moving along smooth hydrophobic and structured superhydrophobic surfaces. Both post and rib style structures with surface solid fractions ranging from 0.06 to 1.0 are considered. The inclination angle of the surfaces was varied from 10 deg to 25 deg, and the drop volume was varied from 12 to 40 mu L. The measurements reveal that the drop speed is a strong function of both the inclination angle and the surface solid fraction. Further, the heat transfer is strongly affected by the surface solid fraction and the drop speed. Surfaces with low solid fraction result in a decrease in the initial heat transfer compared to the behavior observed for drops on a smooth surface. At the smallest solid fractions explored the reduction in heat transfer is nearly 80%. For rib structured surfaces, drop motion both along and perpendicular to the rib direction was considered and the heat transfer is larger for drops moving in the parallel rib configuration. This variation is likely caused by the increased rolling speed that prevails for the parallel rib case. Over 130 unique conditions were explored, and the results from all cases were used to develop correlations that enable prediction of the heat transfer to drops rolling or sliding down smooth hydrophobic and superhydrophobic surfaces.

This paper investigates the deviation in frictional characteristics from Stokes flow theoretical expectations for liquid flow through fused silica microtubes. Pressure drop data is used to characterize the friction factor for tube diameters ranging from 20−150 µm and over a Reynolds number range of 20−2000. Distilled water, hexane, and isopropanol were used in this study based on their distinct polarity and viscosity properties. Deviation from Stokes flow theory is observed in tubes with diameters less than 75 µm. The deviation appears to be independent of Re, represents a decrease in the friction factor and is similar for all three liquids at a fixed diameter. The measured deviation is 0% for 100−150 µm range and increases as the diameter of the tube decreases, from 5% at 75 µm to 30% at 20 µm. Possible sources for the observed deviation include shear heating, property variation with pressure, and surface phenomena. Of these effects, shear heating and property variation with pressure cannot explain the observed variations.

This paper presents an experimental study of drop impingement and thermal atomization on hydrophobic and superhydrophobic (SH) surfaces. Superhydrophobic surfaces having both microscale and nanoscale geometry are considered. Microscale SH surfaces are coated with a hydrophobic coating and exhibit micropillars and cavities which are classified using the surface solid fraction and center to center pitch. The solid fraction and pitch values explored in this study range from 0.05-1.0 and 8-60 μm respectively. Nanoscale textured surfaces are created by applying a blanket layer of carbon nanotubes. Both types of surfaces are further classified by a temperature jump length (λT). All experiments were conducted at We = 85. Results of atomization as a function of time for the impingement event are provided for several surfaces of varying surface geometry, surface temperature, and temperature jump length. Nanoscale SH surfaces are shown to completely suppress atomization at all conditions explored. Results of the maximum atomization that occurred on a given surface are also shown as a function of the surface temperature. The surface temperature at which the maximum atomization occurs varies with surface geometry. Further, the time after impact when the maximum atomization occurs is also a function of the SH surface parameters. In general, the maximum atomization magnitude and the surface temperature at which maximum atomization occurs each decrease with increasing λT. Further, the time when maximum atomization occurs increases with increasing λT.

At cruising speed, one of the most significant contributing factors to train fuel consumption is aerodynamic drag, and the leading locomotive experiences much more drag than any other car in the train. This work reports on the drag reduction that can be realized by the use of add-on nose fairings that are deployed on leading locomotives in a train set. Two types of fairing shapes were considered and all fairing walls are flat. It is anticipated that the fairing shapes would result from the deployment of easily stowed panels in an origami inspired manner. One of the fairing shapes has the appearance of a wedge and the other fairing is also wedge shaped, but with flow directing side wall features. For each general fairing shape, the important dimensions were parametrically varied in a systematic manner to identify the dimensions that yield maximum drag reduction. For the first shape, 45 different scenarios were considered; for the second shape, 15 were considered. A steady commercial computational fluid dynamic solver was employed to solve the flow field and locomotive drag for each of the scenarios. The best performing wedge-shaped fairing reduced the leading locomotive drag by nominally 14% and the best performing fairing shape with the side walls reduced the drag by 17%.

A stability control device (SCD) is a passive inlet cover treatment that can be applied to high suction performance inducers. Significant improvements in stability have been observed when an inducer operates with an SCD, including suppression of backflow at the leading edge of the inducer at low off-design flow coefficients. This is possible because of a local increase in mass flow at the leading edge of the pump, which allows an inducer to operate with an incidence near the design point value over a wide range of flow coefficients. In this paper, the suction performance of several inducers was explored with several different SCD geometries and at different flow coefficients. Specifically, five different SCD geometries were considered to explore the influence that SCD bleed slot width and resistance to flow through the SCD channel have on inducer performance. Further, removal of tangential velocity of the energized fluid transporting through the SCD channel was considered for some scenarios to highlight the impact of swirl on inducer performance. The results reveal that for all inducers and SCD combinations considered, the most important factor affecting the mass flow through the SCD, and subsequent mass flow gain, is the inlet diffusion of the inducer. This holds for both single-phase and multiphase scenarios. Further, the cavitation number where cavitation first starts to develop in the blade throat passage of the inducer is primarily dependent on the inlet blade angle and not the SCD geometry. Consequently, the shape of the cavitation breakdown curve is largely determined by the inducer blade angle.

Accurate models of retention forces between drops and superhydrophobic (SH) surfaces are required to predict drop dynamics on the surface. This retention force is, in turn, useful in modeling heat transfer rates for dropwise condensation on a SH surface. Drop contact angle distribution and base area on SH surfaces are essential factors for predicting retention forces. The present work measures the contact angle distribution and base area shapes of various drop sizes over a wide range of solid fraction for inclined microstructured SH surfaces at the point of drop departure. Base area shape was found to be well approximated using two ellipses with different aspect ratios, and the contact angle distribution was found to be best fit by a sigmoid function. At an incline near the roll-off angle, drop base area for surfaces with solid fraction close to 1 and close to 0 were found to be nearly circular, whereas the base area of drops on surfaces with an intermediate solid fraction deviated from circular behavior. In this work, maximum advancing and minimum receding contact angles were found as a function of solid fraction and used to calculate retention forces. Contact angle distribution and base area shapes are then used to calculate retention forces between drops and SH surfaces. These calculations are compared with the component of measured drop weight acting parallel to the plane on a tilted surface for validation. Previous retention force studies that investigate base area shape and contact angle distribution for smooth surfaces are not applicable for microstructured SH surfaces. The work shows that using a sigmoid contact angle distribution and modified base area shape yields retention forces that are on average 50% better than previously reported methods. Retention forces for smooth and SH surfaces calculated in this study were used to suggest retention force factor values for varying solid fraction surfaces.

Superhydrophobic (SH) surfaces possess desirable anti-fouling properties due to low wettability, but have also been shown to reduce heat transfer to subcooled water in impinging jet scenarios. In this work, superheated silicon substrates with varying wettability (hydrophilic or HPi, hydrophobic or HPo, SH) are quenched by an impinging water jet, where the substrate temperature is above the saturation temperature. Silicon wafers are either oxidized to create HPi surfaces, coated with Teflon to make the surface HPo, or plasma-etched and coated to create the necessary micro-texture for SH conditions. All wafers are integrated with an electric resistance heater and then heated to temperatures of 200-320 degrees C before impingement with an axisymmetric room temperature water jet of varying specified flow rates yielding jet Reynolds numbers between 60 00 and 18,000. High-speed visual data is collected, showing how the lamellar liquid contact region, limited by thermal breakup due to boiling, grows radially as the surface cools to temperatures below saturation. This data is correlated to temperature data recorded on the back side of the wafer using a thermal camera. Results of this study confirm previous conjecture that surface wettability can alter maximum heat flux, which is quantified here for the described scenario by up to 40%, and can also affect jet thin film spreading by up to 50%. Increasing initial surface temperature decreases thin film spreading rate on all surfaces, and increases heat transfer on all but the SH surfaces. Increasing Reynolds number yields an increase in heat flux, and affects both the thin film spreading rate as well as the maximum radius of the thin film region. (C) 2021 Elsevier Ltd. All rights reserved.