Fluidic Research Articles

Manipulation of the flow induced by afterbody vortices using sweeping jets

In this study, the impact of oscillatory fluids on the flow induced by afterbody vortices, resembling those of a high-speed train, was investigated in wind tunnel experiments. The afterbody vortices were generated by a cylindrical model with a slanted base at Reynolds numbers ranging from 87 000 to 200 000. A fluidic oscillator was utilized to produce sweeping jets (SWJ) acting on the vortical flow, which is similar to those observed over the rear of a typical high-speed train. Our particle image velocimetry measurements on cross-stream planes reveal that the oscillatory flow produced by the sweeping jet significantly reduces the magnitude of induced flow, resulting in a 17.7% and 15.9% decrease in the total velocity and horizontal slipstream, respectively, at a velocity ratio of 6.4. This is attributed to the injection of the kinetic energy of sweeping jets into the core of the afterbody vortices, which leads to a higher dispersion of vorticity and incoherent energy distributions. The iso-surfaces of resultant velocities reveal that the fluid injection by the sweeping jet increased local velocities, resulting in a spanwise shrinkage of the induced velocity structure downstream. The study demonstrates the capability of sweeping jets in decreasing induced flow by manipulating afterbody vortices and their potential for reducing the slipstream produced by high-speed trains at the tracksides.

Flow mechanism study and geometrical parameters analysis of fluidic oscillators based on pressure-sensitive paint measurements and modal analysis

Flow mechanism study and geometrical parameters analysis of fluidic oscillators based on pressure-sensitive paint measurements and modal analysis

Enhanced Rectification Performance in Bipolar Janus Graphene Oxide Channels by Lateral Electric Fields.

Improving the ionic rectification in nanochannels enables versatile applications such as biosensors, energy harvesting, and fluidic diodes. While previous work mostly focused on the effect of channel geometry and surface charge, in this work via a series of molecular dynamics simulations, we find a striking phenomenon that the ionic current rectification (ICR) ratio in Janus graphene oxide (GO) channels can be tremendously promoted by lateral electric fields. First, under a given axial electric field, an additional lateral electric field can improve the ICR ratio by several times to an order, depending on the channel symmetry. The symmetric channel has an obviously greater ICR ratio because it maintains a more pronounced ion transport disparity at opposite axial fields. The underlying mechanism for the function of the lateral electric field is that it promotes the lateral migration of ions and thus amplifies the ion-residue electrostatic interaction at opposite axial fields, enlarging the ion dynamical difference. Furthermore, for different axial electric fields, the ICR ratio can always be improved by lateral electric fields (up to two orders), suggesting that the ICR improvement is universal. Our results demonstrate that applying a lateral electric field could be a new method to improve the rectification performance of nanochannels, providing valuable guidance for the design of efficient ionic diode devices.

Analysis of a Novel Fluidic Oscillator under Several Dimensional Modifications

To activate the boundary layer in Active Flow Control (AFC) applications, the use of pulsating flow has notable energy advantages over constant blowing/suction jet injections. For a given AFC application, five parameters, jet location and width, inclination angle, frequency of injection, and the momentum coefficient, need to be tuned. Presently, two main devices are capable of injecting pulsating flow with a momentum coefficient sufficient to delay the boundary layer separation: these are zero-net-mass-flow Actuators (ZNMFAs) and fluidic oscillators (FOs). In the present study, a novel FO configuration is analyzed for the first time at relatively high Reynolds numbers, and fluid is considered to be incompressible. After obtaining the typical linear correlation between the incoming Reynolds number and the outlet flow oscillating frequency, the effects of dimensional modifications on outlet width and mixing chamber wedge inclination angle are addressed. Modifications of the outlet width were observed to create large variations in FO performance. The origin of self-sustained oscillations is also analyzed in the present manuscript and greatly helps in clarifying the forces acting on the jet inside the mixing chamber. In fact, we can conclude by saying that the current FO configuration is pressure-driven, although the mass flow forces appear to be much more relevant than in previously studied FO configurations.

Performance Development of Fluidic Oscillator Nozzle for Cleaning Autonomous-Driving Sensors

Contaminated autonomous-driving sensors frequently malfunction, resulting in accidents; these sensors need regular cleaning. The autonomous-driving sensor-cleaning nozzle currently used is the windshield-washer nozzle; few studies have focused on the sensor-cleaning nozzle. We investigated the flow characteristics of the nozzle to improve its performance in cleaning the autonomous-driving sensor. The nozzle concept was based on the fluidic oscillator nozzle. Various performance parameters of the fluidic oscillator nozzle were selected and investigated. Transient fluid flow was simulated to determine the effect of the design parameters to maximize the oscillation flow phenomenon. Additionally, the spray angle and frequency were calculated. Analysis results showed that the change in flow speed affects the frequency, and the change in feedback-channel-inlet flow rate affects the angle change. To verify the simulation result, the three best models (R4+RC10, R6+RC11, R8+RC10) and the base model were manufactured and tested. The test results were consistent with the simulation results within a 6% error.

Numerical parametric study of a sweeping-vortex low-frequency fluidic oscillator

Fluidic oscillators are garnering increasing attention in various fields due to their self-induced oscillation characteristics. This study proposes a novel fluidic oscillator, the sweeping-vortex fluidic oscillator (SVFO), characterized by unique feedback properties. These properties exploit fluid feedback from the side opposite the attached wall and the spin delay of the captive vortex to yield low-frequency oscillating jets. The oscillation characteristics and the influence of main geometric parameters were investigated using computational fluid dynamics approach. Results show that the evolution of the flow field within the SVFO demonstrates an intricately dynamic interacting. The deflection switching of the main jet is primarily controlled by a pair of counter-rotating moving vortexes. Geometric parameters influence the stability of the dynamic evolution of both captive and moving vortexes, thereby altering the signal components of the output performance transmitted to the inlet. The overall width of the oscillator has a pronounced impact on the oscillation frequency, and with an increase in the overall width, a maximum frequency increase of 87 % can be observed. Furthermore, the height of the sweeping chamber and the diameter of the vortex chamber outlet have a noticeable effect on the pressure pulse amplitude. A pressure fluctuation coefficient greater than 1.1 was recorded. The results also suggest that the geometric parameters of the fluidic oscillator need to be designed within a reasonable range; otherwise, the jet oscillator will not function. A more refined optimization of geometric parameters could be obtained with the aim of achieving a stable main jet, feedback flow of appropriate intensity, and optimal attachment conditions. This work reveals the oscillation mechanism and the influence of geometric parameters on SVFOs, providing a theoretical basis for designing related oscillators.

Computational and experimental study of the synchronization strategies of two pulsing jet fluidic oscillators

Fluidic oscillators are interesting actuators for flow control purposes as they produce unsteady jets without any moving part. Flow separation control in a large scale, for instance on a wing, needs an array of such actuators, whose efficiency can be improved if the pulsed jets are synchronized. In this paper, two synchronization configurations based on interconnections of the feedback loops have been applied successfully to two bi-stable fluidic oscillators. The first configuration permits to obtain jets pulsating at a similar frequency as the jets produced by the oscillators working separately. The second configuration, which differs by the interconnection pattern, leads to a much lower frequency. Two different phase lags between the jets produced by the two oscillators have also been identified, depending on the interconnection pattern. These experimental results have been completed by a numerical study of the internal flow patterns of the two oscillators for an in-depth analysis of the physical mechanisms controlling the oscillation dynamics. In the first synchronization configuration, the oscillation is shown to be mainly controlled by the back and forth propagation of pressure waves in the oscillators' branches and feedback loops and its frequency can be estimated by the same simple relation as the one used for single oscillators. In the second synchronization configuration, the jet switching time is no more negligible compared to the pressure waves propagation time, leading to more complex oscillation dynamics.

Heat transfer enhancement of a channel via vortex-based fluidic oscillator: A numerical study

This paper numerically investigated the effects of incorporating three distinct vortex-based fluidic oscillators on heat transfer and pressure drop within a two-dimensional channel with constant heat flux at different Reynolds numbers ranging from 10,000 to 50,000. These oscillators consist of primary and secondary chambers connected by a throat and a return channel. The dimensions of the base vortex-based fluidic oscillators were halved to design the second oscillator while keeping the inlet and outlet throats unchanged to reduce pressure drop across the oscillator. Finally, a protrusion was incorporated into the wall profile of the second oscillator, intensifying the vortex in the secondary chamber and augmenting the transverse oscillation amplitude to boost heat transfer. The finite volume method with the k–ω shear-stress transport (SST) turbulence model was used to solve the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations and energy transport. Transient numerical results demonstrated intensified jet oscillations in the channel, enhancing heat transfer, particularly at the inlet zones of the channel owing to vigorous collisions with upper and lower channel walls. The time-averaged numerical results indicated that the first, second, and third oscillators respectively raised the average Nusselt number of the channel by 40%, 21%, and 24% at X/D = 5. Similarly, at X/D = 10, the oscillators led to increases of 22%, 9%, and 11 %, compared to the simple channel without oscillators. The small-size second and third oscillators also increased the performance evaluation criterion (PEC) compared to the base vortex-based fluidic oscillator. The maximum PEC values were 0.95 for the base oscillator, 1 for the second oscillator, and 0.98 for the third oscillator at Re = 10,000. These oscillators can be used as inserts for enhancing heat transfer in channels.

Mixing enhancement of transverse jets in supersonic crossflow using an actively controlled novel fluidic oscillator

This study investigates the fluid dynamics and mixing characteristics of an oscillating sonic jet injected into a supersonic cross flow of Mach 2.1 using experimental and computational techniques. The oscillating jet is produced by a novel fluidic oscillator, which consists of a primary rectangular duct that expands into an outer duct with sudden expansion. Control jets are injected in the lateral direction from the side walls of the sudden expansion in an out-of-phase manner to oscillate the injected jet in the spanwise direction of the crossflow. Experimental and numerical investigations based on wall static pressure and mass fraction fluctuations, respectively, revealed that the injected jet oscillation frequency matches the control jet frequency. The iso-surface of lambda-2 criterion showed the presence of various dominant vortex structures, such as counter-rotating vortex pairs, horseshoe vortex, sidewall vortices, and trailing vortices. Helicity contour plots showed that the streamwise vortices oscillate in the spanwise direction with the control strategy and promote the spread of the injected jet in the spanwise direction. The spatiotemporal reconstruction (z–t plot) of the density gradients at a particular streamwise location revealed that the bow shock produced by the interaction of the injected jet and the crossflow oscillates with the actuation of the control strategy. The power spectral density of the z–t plot revealed that the shock wave oscillation frequency matches the control jet frequency. The oscillating jet produced by the control strategy showed significant mixing enhancement in supersonic crossflow compared to a simple rectangular injection.

Spectral analysis of non-contrast fluoroscopy for evaluation of pulmonary perfusion: Feasibility and sensitivity testing with a phantom.

Oscillating x-ray attenuation in the lungs provides an opportunity to evaluate pulmonary perfusion without contrast. Recent intensity-based methods have been compared to pulmonary scintigraphy and CT angiography but lack rigorous phantom studies. A new method to quantify the periodic signal amplitude was employed using spectral analysis. Performance was characterized using a water phantom capable of creating an oscillating x-ray attenuation at physiologic amplitudes. Feasibility in detecting abnormal perfusion was performed on a volunteer with pulmonary vascular disease and compared to pulmonary angiography, the clinical gold standard. For each fluoroscopic acquisition, the normalized temporal signal from each pixel was decomposed into its frequency components using Fourier transformation, and the spectral amplitude, defined as the x-ray pulsatility index (XPI), was determined at the desired frequency using a band-pass filter. XPI was displayed as a pixel-wise parametric colormap. Based on XPI maps generated using two human volunteers, a water bath phantom was constructed with a fluctuating fluid height and a 1cm diameter pulsatility defect. Contrast-to-noise (CNR) of the defect was measured using fluoroscopy images acquired at variable fluid height fluctuation (0.1-1.9mm) and oscillation frequency (30-60bpm). Various sampling frame rates (3-30 fps) and acquisition durations (1.8-8s) using truncated datasets were reconstructed from full datasets. Fluoroscopic images were obtained in a patient just prior to pulmonary angiography in the same projection. XPI maps in human subjects showed high signal to background contrast with high central XPI measuring up to 0.5. Phantom experiments revealed CNR was linearly correlated to fluid height change (r2=0.998). CNR is proportional to increasing sampling frame rate and increasing acquisition duration as expected with Fourier analysis. XPI map displayed multifocal perfusion defects in good agreement with pulmonary angiography. Spectral analysis is an accurate and sensitive method to detect small changes in periodic x-ray attenuation using a short fluoroscopic acquisition. This method demonstrated good agreement to pulmonary angiography and shows promise for clinical imaging of pulmonary perfusion using standard fluoroscopic methods.

Experimental study on the effect of fluidic oscillator jet position on the performance of a high speed compressor cascade

In order to further improve the performance of the cascade, this paper focused on a high-load diffuser cascade and a pulsed-type fluidic oscillator for flow control was also applied. Experiments were conducted at design incidence of 0° and near stall incidence of +4°, investigating different jet positions of the fluidic oscillators. The results indicate that at Mach number of 0.67 and design incidence, the pulsed jet effect is optimal when the jet position of the fluidic oscillator is at 35% of chord, resulting in an 8.74% decrease in total pressure loss. At Mach number of 0.5 the total pressure loss decreases by 12.08%. At the Mach number of 0.67 near the stall incidence of +4°, the fluidic oscillator jet position at 7% of chord shows the most significant decrease in total pressure loss, which is 5.3%. When the pulsed jet is positioned at the separation starting point of the corner, the low-pressure region on the suction surface near the endwall decreases, while the high-pressure region on the pressure surface increases, enhancing the pressure expansion capability of the cascade. The pulsed jet can successfully decrease the vorticity in the passage, such as passage vortex, trailing shedding vortex and concentrated shedding vortex, and can resist the transverse secondary flow in the cascade passage, reducing the range of corner separation in the spanwise direction and total pressure loss.

The Effect of Curved Geometry on Exiting Flow of Fluidic Oscillators

Abstract Traditionally fluidic oscillators are designed to be planar. However, there are applications that may desire the exiting fluid to move in the third dimension. This could allow these oscillators to be more effective in applications such as fuel sprays, cooling flow, or flow control devices with its increase in effective spray area. This investigation designed a series of oscillators that curved the whole body and/or the exit nozzle to understand how to maximize out of plane motion. These configurations were compared to a baseline planar oscillator with no curved characteristics. Velocities were measured downstream of these oscillators within a data collection grid using a hot wire probe to determine the 3D shape of the exiting jet. Results show that configurations with only one of the two curved physical characteristics (i.e., only a curved body or a curved nozzle) produced the most curvature. Having both of the curved physical characteristics caused the nozzle width to decrease causing the axial spacing to decrease. Additionally, these curved exiting flows were only seen at mass flow rates below 40 standard liters per minute (SLPM). Higher mass flow rates caused the exiting flow to flatten, returning the flow to the baseline result of in-plane oscillations. This led to a decrease in jet spread.

A study on the flow behavior inside fluidic oscillation tools for friction-reduction in extended reach well

The drilling distance of extended-reach wells is limited by mechanical friction between the drill string and the borehole wall. The fluidic oscillator axial vibratory tools (AVTs) can effectively reduce friction and increase the weight on bit, thereby extending the reach of horizontal wells. However, few studies have delved into the evolutionary mechanism of the internal flow field of fluidic oscillator AVTs. In this paper, two types of fluidic oscillators with different flow path are presented. One is the reverse-feedback oscillator, and the other is sweeping-chamber oscillator. Through computational fluid dynamics method, the internal flow field of these two fluidic oscillator AVTs were analyzed, and the frequency and pressure drop were verified by experiments. The results show that the LES turbulence model provides more accurate simulations, showing an error range of less than 9%. The pressure drop and vibration frequency of the two tools are linearly related to the flow rate. The pressure drop of the reverse feedback oscillation tool is in the range of 0.1–1 MPa, and the vibration frequency is between 13 and 50 Hz. The sweeping-chamber oscillator can reduce vibration frequency to below 15 Hz. The results also show that it is crucial to consider the lag in pressure signals when designing and analyzing fluid oscillator systems, as it can affect the timing and behavior of the system. This research provides profound insights for understanding and optimizing fluidic oscillator systems.

Characterization of Out-of-Plane Curved Fluidic Oscillators

A novel fluidic oscillator design, in which the oscillator is curved along the primary flow direction, was evaluated. The effects of mass flow rate, hydraulic diameter, aspect ratio, curvature radius, surface roughness, inlet orientation, and nonconstant curvatures were experimentally investigated. Measurements of the oscillation Strouhal number, discharge coefficient, spreading angle, and sweeping angle were used to characterize the jet. The resulting oscillators were found to fall into one of three categories: oscillated at the same frequency and sweep angle as conventional flat oscillators; oscillated at a slightly higher frequency and lower sweep angle than flat oscillators; or no dominant oscillation frequency detected and with no sweeping action. An unsteady Reynolds-averaged Navier–Stokes computational fluid dynamics simulation revealed fundamental differences in the internal flow mechanisms between flat and curved oscillators that drive the sweeping jet. The curvature between 37.5 and 62.5% of the total length, or the region from the inlet nozzle to halfway through the main chamber, was a primary factor influencing the response type of a design. Due to the curvature of these oscillators, they have the ability to be used in geometrically constrained spaces, such as the leading edge of wings and turbine vanes.

Functionality integration in stereolithography 3D printed microfluidics using a "print-pause-print" strategy.

Stereolithography 3D printing, although an increasingly used fabrication method for microfluidic chips, has the main disadvantage of producing monolithic chips in a single material. We propose to incorporate during printing various objects using a "print-pause-print" strategy. Here, we demonstrate that this novel approach can be used to incorporate glass slides, hydrosoluble films, paper pads, steel balls, elastic or nanoporous membranes and silicon-based microdevices, in order to add microfluidic functionalities as diverse as valves, fluidic diodes, shallow chambers, imaging windows for bacteria tracking, storage of reagents, blue energy harvesting or filters for cell capture and culture.

Numerical study on role of foot of a flue organ pipe: relative phases in oscillations among pipe, foot and jet

Recent numerical studies showed that the foot of flue organ pipes closed at one end acts as a Helmholtz resonator and influences the acoustic oscillation in the pipe. Namely, the relative phase of oscillations between the pipe and foot changes depending on the resonance frequency of the foot. This fact can be explained by the theory of forced harmonic oscillators, in which the pipe works as a force driving the foot. However, in the damping-controlled regime, in which the resonance frequency of the foot, fH, is nearly equal to the frequency of acoustic oscillation, fa, the amplitude of the acoustic oscillation is smaller than those in the mass-controlled regime fH < fa and the stiffness-controlled regime fH > fa, even though the pressure oscillation in the foot lags behind that in the pipe by π/2. In this paper, to investigate this problem and to explore the role of the foot in detail, using a three-dimensional model of a closed flue organ pipe, we numerically investigate the relative phases among the pressure oscillation in the pipe, that in the foot and the jet velocity. We conclude that total balance among them is crucial for sustaining acoustic and fluid oscillations in the sound generation process. The oscillating jet drives the pipe with appropriate timing to sustain the acoustic oscillation for the stiffness-controlled and mass-controlled regimes. In contrast, the jet inefficiently drives the pipe with inappropriate timing for the damping-controlled regime.

Numerical analysis and experimental study on slurry diffusion characteristics of vortical oscillatory grouting technology considering soil interface

Based on the fluidic jet theory, this study proposes an effective vortical oscillatory grouting technology. The fluid switching mechanism and pressure pulse characteristics are investigated through numerical simulation and experiment. The slurry diffusion performance of oscillatory grouting is compared with that of steady-pressure grouting under various soil parameters and slurry parameters. The effects of soil interface on diffusion behavior are also studied. The results show that the growth and dissipation of the vortex within the tool generate periodic oscillatory fluid, which further produces pressure pulses. During oscillatory grouting, the pressure recovery process inside the fluidic oscillator reduces the pressure of the soil. With the increase in porosity and particle size, the pressure of the soil grouted by oscillatory pressure is noticeably reduced. Despite the lower pressure of the soil, oscillatory grouting demonstrates better diffusion performance, owing to the dynamic flow field and pressure fluctuation. The performance of oscillatory grouting is also influenced by the oscillatory frequency and pressure pulse amplitude. When the soil interfaces are taken into consideration, different fluid domains exhibit complex slurry diffusion patterns. It is observed that the interface that is perpendicular to the grouting direction has a greater influence on grouting behavior. The oscillatory grouting diffusion behavior is more sensitive to the soil interface. This study is expected to improve grouting efficiency and promote the development of new grouting technology.

Estimation of Internal Heat Flux on Pulsating Heat Pipes using Kalman Filter: Numerical and Experimental Results

Pulsating Heat Pipes (PHPs) are two-phase passive thermal devices characterized by the presence of significant fluid oscillations inside the tubes that permit to fast and efficiently transfer heat from a hot region to a cold one. They are present in many engineering applications, e.g., electronics, sustainable energies, aerospace. They are very attractive due to their high heat removal capability, flexibility, and low manufacturing cost. Although they have been widely studied their working principles are still not fully understood. To better comprehend their thermal behaviour recent works presented different approaches to assess the internal heat flux in PHPs. However, all the adopted approaches are based on whole-domain techniques that require a higher computational cost compared with sequential estimations. Therefore, to allow heat flux estimation with lower computational effort, in this work a procedure based on the Kalman filter has been adopted. The heat flux has been estimated by solving the inverse heat conduction problem in the PHP’s wall adopting as input data the temperature measurement on the external surface of the pipe acquired by an infrared camera. Firstly, the procedure has been validated adopting synthetic data. Then, experimental data referring to actual operating conditions have been employed to estimate the internal heat flux. The Kalman filter has been adopted as technique to solve the classical instability intrinsically present in inverse problems. It is relatively simple to implement and requires only previous time information, requiring low computational costs. Moreover, the Kalman filter allows the real time estimation of the heat flux: assessing the heat flux on PHPs during operation could be a useful instrument to provide information about on-time working conditions and thus avoid any possible malfunctioning.

Asymptotics of the Spectrum of Variational Problems Arising in the Theory of Fluid Oscillations

Asymptotics of the Spectrum of Variational Problems Arising in the Theory of Fluid Oscillations

A Review Study of Sweeping and Normal Impingement Jets

This review focuses on the experimental and numerical studies of sweeping impingement jets that serve in cooling of hot surfaces. It is known that the impinging jets produce high-localized heat transfer coefficient. The sweeping jet covers a wider area on a hot target to improve the heat transfer rate, they could be used to increase the cooling rate of the impingement surface by disturbing the boundary layer. To display a readable survey, the current review was partitioned to four groups based on engineering configurations. The review shows that the sweeping nozzle gives better efficiency in heat transfer, improved Nusselt number and uniform target surface temperature, compared with the conventional normal jets. The current review reveals that the sweeping-jet mechanism can be achieved either by fluidic oscillator or by exciting a flexible wall forming an oscillating jet. Most of the fluidic oscillator researches are conducted experimentally (27%), while the researches that use flexible wall are about 24%.