Jet Oscillation Frequency Research Articles

Flow Performance Enhancement of a Fluidic Oscillator Through the Integration of Rectangular Ribs on Coanda Surface

Abstract Fluidic oscillators utilize internal flow dynamics to produce oscillatory fluid jets. The Coanda surface in the mixing chamber of a fluidic oscillator plays a critical role by facilitating controlled fluid manipulation through flow attachment and redirection. The mixing chamber pressure drop, jet oscillating frequency, and deflection angles are hence dependent on the geometry of the Coanda surface. In this study, the Coanda surface is modified by using rectangular ribs of different aspect ratios. The effects of ribbed Coanda surface on oscillating jet characteristics are computed numerically through two-dimensional unsteady Favre-averaged Navier–Stokes equations. The aspect ratio (ARribs), the ratio of rib height to rib base, is varied from 0.64 to 1.56 and air is used as a working fluid. An increase in the ARribs increases the jet oscillation frequency. The highest aspect ratio achieves an oscillation frequency of 820 Hz, contrasting with 355 Hz for the smooth case. On the other hand, the jet deflection angles are decreased as the aspect ratio increases. Interestingly the introduction of the ribs on the Coanda surface decreased the pressure drop in the oscillator. A decrease in pressure drop of 22% for an aspect ratio of 1.56 was achieved as compared to the smooth case. These results are attributed to the influence of the ribs on the formation of a separation bubble formed in the mixing chamber. The jet performance parameter, frequency-deflection-pressure ratio, was found to be 43% higher for ARribs of 1.56 as compared to the smooth case.

Experimental and numerical study of a fluidic oscillator frequency domain characteristics

Fluidic oscillators are fluid devices that provide steady flow at the inlet while generating unsteady flow at the outlet. This paper aims to investigate the frequency domain characteristics of internal pressure fluctuations in a specific type of fluidic oscillator to gain a deeper understanding of its dynamic behavior. The following conclusions were drawn: (1) Within a certain range, as the inlet-to-outlet pressure ratio increases, the oscillation frequency of the fluidic oscillator continuously rises, and the oscillation amplitude strengthens. (2) As the jet nozzle velocity increases within a certain range, the jet oscillation frequency exhibits a linear upward trend. These research findings provide essential academic and engineering guidance for designing and applying fluidic oscillators, assisting engineers in better understanding and controlling the device’s performance to meet the requirements of various application domains.

Delayed Detached Eddy Simulation of Flow Separation Control Using Fluidic Oscillator

The investigation of flow separation control over a NACA 0015 airfoil model using a fluidic oscillator (FO) is conducted through delayed detached eddy simulation. First, the flowfield within and outside an FO operating in quiescent air is resolved simultaneously. The oscillation of the jet flow induced by the FO is attributed to the cyclic expansion and contraction of the recirculation bubbles located near the two Coanda surfaces in the mixing chamber. Significantly, the predicted jet oscillation frequency closely matches the experimental data, validating the accuracy of our findings. Next, the FO is integrated into an airfoil model to suppress the flow separation. The airfoil is under deep stall conditions, with angles of attack of 20 and 17° and a Reynolds number of Re=4.8×105. The key driving force for flow reattachment is the spanwise vortices induced by the oscillating jet, which substantially enhance the mixing between the separated flow and the external high-momentum flow. Consequently, the aerodynamic performance of the airfoil is notably improved. Additionally, both spectral and dynamic mode decomposition analyses indicate that the flow, under the influence of the FO forcing, becomes more orderly and well-organized and is effectively locked into the forcing frequency.

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.

CONTROL SYSTEM FOR WATERJET CUTTING PROCESS WITH SWIRLING WORKING FLUID JET

In mechanical engineering, the procurement process is one of the most important in the life cycle stage of manufacturing products from metal materials. In this regard, there are disadvantages in the manufacture of parts: low productivity, complexity and high cost of cutting tools, and the impossibility of cutting. The work proposes an approach to increase the productivity of using the cutting properties of abrasive grains and expanding the contact area of a waterjet jet with the workpiece surface being processed by twisting the jet and adjusting the values of the jet oscillation frequency to the thickness of the workpiece. As a result of research verification, the approach showed good prediction accuracy.

Influence of jet parameters of fluidic oscillator-type fuel injector on the mixing performance in a supersonic flow field

Numerical analyses are performed to clarify the influence of jet parameters, i.e., the sweeping jet angle and oscillation frequency, of a fluidic oscillator on the mixing performance. Specifically, hydrogen is injected into the supersonic freestream through a fluidic oscillator, and the corresponding mixing performance is evaluated. The mixing performance evaluated is penetration length, area of a flammable region, and mixing efficiency. In all cases, the fluidic oscillator has better mixing performance than a steady transverse jet. The results show that the influence of the sweeping jet angle on the mixing performance is more notable than that of the oscillating frequency. At 58 and 580 Hz frequency, 24° has the highest penetration length ratio and largest area of the flammable region. The tendency of the mixing efficiency ratio depending on the sweeping angle at low frequencies is unclear. At high frequencies, 24° has the biggest mixing efficiency and largest area of the flammable region, whereas 18° has the highest penetration length ratio. The results showed that the appropriate combination of the frequency and sweeping jet angle to maximize the mixing performance.

Additive manufacturing impact on a fluidic oscillator with respect to surface roughness

This experiment investigates the impact of areal surface roughness Sa on the performance of a fluidic oscillator produced by additive manufacturing (AM) versus the conventionally machined samples. The fluidic oscillator used is a classical curved (Type-II) Sweeping Jet (SWJ) actuator with two feedback channels. The interior surface finish of each sample is characterized using an optical 3D measurement system. The performance of the emanated jet is evaluated using hot-wire anemometry downstream of the outlet nozzle while the required supply pressure is recorded using a high-frequency pressure transducer at the actuator inlet. A bifurcated velocity profile of the external flow typical for the SWJ is observed in all cases, irrespective of the manufacturing process and surface roughness Sa< 20 μm. The jet velocity profiles are found to be satisfactorily self-similar, confirming that the surface roughness in the examined range does not qualitatively change the internal flow dynamics. However, increasing the surface roughness causes a reduction in the output jet spreading and a concomitant increase in the jet oscillation frequency. A linear model is presented to estimate the jet spreading versus the surface roughness.

Active Flow Control of Supersonic Jet Using Streamwise Pulsed Blowing

The present study investigates a novel control strategy to create a sweeping oscillatory jet using fluidic actuation principle. A supersonic underexpanded primary jet issued into a suddenly expanding duct is oscillated by giving cyclic pulsed secondary jet blowing in the top and bottom step regions of the suddenly expanding duct. The current study employs the bistable nature of jet expansion seen in a suddenly expanding duct for the jet oscillation. The cyclic perturbations (pulsed secondary jet blowing) given in the streamwise direction switch the direction of the asymmetric jet expansion of the underexpanded primary supersonic jet to the top and bottom wall alternatively, thereby creating an oscillatory supersonic jet. It is found that the switching of the direction of the primary jet to any one wall is caused by the Coanda effect produced by the secondary jet issued in the corresponding wall side. The results show that the primary jet quickly responds to the secondary jet perturbations and that the supersonic primary jet oscillation frequency nearly matches secondary jet perturbation frequency. A parametric study to investigate the jet oscillation characteristics with various secondary jet oscillation frequencies and mass flow rate ratios are also being carried out in the present study. It is found that the jet oscillation frequency increases with increase in the secondary jet pulsation frequency. The jet deflection is found to increase with increase in the secondary jet to primary jet mass flow ratio (blowing ratio) up to a critical limit, after which the increment in jet deflection ceases due to the restrictions imposed by the duct geometry.

Flow physics of planar bistable fluidic oscillator with backflow limbs.

Fluidic oscillators (FOs) are used in a variety of applications, including process control and process intensification. Despite the simple design and operation of FOs, the fluid dynamics of FOs exhibit rich complexities. The inherently unstable flow, jet oscillations, and resulting vortices influence mixing and other transport processes. In this work, we computationally investigated the fluid dynamics of a new design of a planar FO with backflow limbs. The design comprised of two symmetric backflow limbs leading to bistable flow. The unsteady flow dynamics, internal recirculation, jet oscillations, secondary flow vortices were computationally studied over a range of inlet Reynolds numbers (2400-12,000). The nature and frequency of the jet oscillations were quantified. The computed jet oscillation frequency was compared with the experimentally measured (using imaging techniques) jet oscillation frequency. The flow model was then used to quantitatively understand mixing, heat transfer, and residence time distribution. The approach and the results presented in this work will provide a basis for designing FO's with desired flow and transport characteristics for various engineering applications.

The Influence of Exit Nozzle Geometry on Sweeping Jet Actuator Performance

When pressurized with a fluid, the sweeping jet actuator (SWJA) emits a self-induced and self-sustained temporally continuous, but spatially oscillating bi-stable jet at the outlet. The SWJA adds up local momentum using the Coanda extension without any moving parts and, therefore, can be a promising tool for suppressing aerodynamic flow separation. However, the SWJA needs to be integrated into curved aerodynamic surfaces with an angle. The present study focuses on investigating the effects of various exit nozzle geometries on the flow field. The geometric parameters considered were the exit nozzle angle, diffuser arm length, and curvature. The working fluid was air, and the mass flow rate was 0.015 lb/s. A set of time-dependent flow fields was computed using a two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) simulation. The time history of pressure was recorded inside the upper and lower feedback channels. The jet oscillation frequency was obtained by employing the fast Fourier transform (FFT) for all datasets. The results were compared against the baseline case and data available in the literature. The results showed that external geometric variations at the nozzle exit had a negligible impact on the oscillation frequency. However, there were notable effects on the pressure and velocity distribution in the flow field, indicating that the actuator had sensitivity towards the geometric variation of the exit nozzle—the wider the exit nozzle, the lower the downstream velocity. Notably, we observed that the mean velocity at the exit nozzle downstream for the curvature case was 40.3% higher than the reference SWJA.

A comparative analysis of sweeping jet actuators

The characteristics of spatially oscillating jets emanating from various Sweeping Jet Actuators (SJAs) are investigated in the present study by performing numerical analysis. The four different SJA designs commonly referred in the literature for the flow control applications, namely, the feedback free type, single feedback type, sharp dual feedback type and curved dual feedback type are considered for this investigation. The various characteristics of the oscillating jets such as instantaneous velocity field, jet oscillation frequency, spread angle and the mean centerline velocity variation have been evaluated and compared with the aim of finding an effective design for application in flow control. The present study reveals that the oscillation frequency and jet penetration are more for a feedback free SJA when compared to the other designs for the same conditions whereas the spread angle is high for a curved dual feedback SJA. Based on these findings, it is inferred that the feedback free type and curved dual feedback type are more suitable for flow control applications.

A detailed analysis of flow and heat transfer characteristics under a turbulent intermittent jet impingement on a concave surface

A computational study is carried out of the 3-D flow field and heat transfer under a turbulent intermittent circular jet impingement on a concave surface. The control-volume procedure with the SIMPLE algorithm is employed to solve the unsteady RANS (use full form) equations. The RNG k-? model is implemented to simulate turbulence due to its success in predicting similar flows. The numerical results are validated by comparing them with the experimental data. The effects of jet Reynolds number and oscillation frequency on the flow and heat transfer are evaluated. The profiles of instantaneous and time-averaged Nusselt numbers exhibit different trends in axial, x-directions and circumferential, s-directions. It is found that increasing frequency from 50 to 200 Hz results in considerable time-averaged Nusselt number enhancement in both axial and curvature directions. The intermittent jet at a frequency of 200 Hz enhances the total average Nusselt number by 51.4%, 40%, and 33.7% compared to the steady jet values at jet Reynolds numbers of 10000, 23000, and 40000, respectively. In addition, a correlation for the average Nusselt number is proposed depending on the Reynolds number and the Strouhal number.

Experimental study of flow dynamics of a sweeping jet in a slot channel

In recent years, fluidic oscillators have been actively applied as devices for flow control in the field of aero and hydrodynamics. This study aims to investigate the structure of a flow of sweeping jet ejected from a fluidic oscillator into a confined area – slot channel. Dynamics of sweeping jet flow are investigated using the PIV method with high temporal resolution. The effect of the Re number on the sweeping jet oscillation frequency was studied in the range from 1 500 to 8 000. Linear frequency dependence on Re number was obtained. Bounding walls affect the dynamics of sweeping jet flow that leads to a change of average velocity field. For low Re numbers, obtained results are in good agreement with the results of other studies.

Numerical investigation of flow separation control over an airfoil using fluidic oscillator

Leading edge flow separation control over a stalled National Advisory Committee for Aeronautics 0015 airfoil using a fluidic oscillator (FO) is investigated by means of numerical simulation possibly for the first time to elucidate the flow control mechanism and evaluate control authority. The flow is assumed to be two-dimensional and fully turbulent and resolved using unsteady Reynolds-averaged Navier–Stokes calculations with the elaborate Reynolds stress turbulence model employed. Our simulation is proved to be able to successfully resolve the basic characteristics of a FO operating in quiescent air, which include both the qualitative prominent flow structures and quantitative jet oscillation frequency. It is seen that the driving force behind the self-induced and self-sustaining oscillation of jet flow inside the oscillator is Coanda effect induced alternating development of a recirculation bubble on either side of the mixing chamber walls. We provide a comprehensive analysis of the flow control procedure over an airfoil at Reynolds number of Re = 4.8 × 105 and an elucidation of the flow control mechanism. It is found that the most prominent flow feature resulting from the interplay between an oscillating jet and external crossflow over an airfoil is the production of spanwise vortices. The strong entrainment effect of the induced spanwise vortices is the dominant mechanism leading to the mitigation of flow separation. Periodic jet oscillation generates a series of downstream moving vortices over an airfoil surface and results in a greatly delayed flow separation. The recovery of a strong suction pressure peak near the leading edge and significant lift enhancement and drag reduction reflects the improvement of an aerodynamic performance of the airfoil under control. Also observed is the phenomenon of local flow frequency lock-in to forcing frequency near the leading edge region. Moreover, the mass supply rate at the inlet of the oscillator is found to have an appreciable effect on the flow control authority. A higher mass flow usually leads to a better flow control performance.

Numerical investigation of characteristics of fluidic oscillator operating in quiescent air and a crossflow

We present a detailed numerical study of characteristics of fluidic oscillator operating in originally quiescent air and in a crossflow. Our study is focused on the visualization of flow field evolution associated with fluidic oscillator in an attempt to shed some light into the elucidation of mechanism of jet oscillation and interaction of jet flow with crossflow and into the revelation of flow control mechanism of fluidic oscillator. Reynolds-averaged two-dimensional computation with elaborate Reynolds stress turbulence model allows us to reproduce the experimentally observed salient flow features like recirculation bubbles in actuator and oscillating jet. The excellent agreement between simulation and experiment in both qualitative flow structures and quantitative jet oscillation frequency gives credibility to a number of new observations made from simulation, which, however, are not apparent in experimental data. For example, it is found from our simulation that the scaling of a specific actuator does not significantly influence the characteristics of the scaled actuator and mechanism of associated flow oscillation. In addition, the interaction of oscillating jet with external crossflow leads to the generation of spanwise vortices, which, in the presence of flow separation, is believed to be able to entrain high momentum external flow towards body surface and play a dominant role in flow separation control. Moreover, when the intensity of crossflow increases, the frequency of jet oscillation decreases. Thus, our work presents an updated description of working principles and mechanisms of fluidic oscillator.

Effects of Fluidic Oscillator Nozzle Angle on the Flowfield and Impingement Heat Transfer

Experimental and numerical investigations were performed to evaluate the effects of the exit nozzle configuration of a fluidic oscillator on the flowfield and impingement heat transfer performance downstream on a flat plate. A conventional wall-attachment-type fluidic oscillator with a fan-shaped exit configuration was studied. Eight different exit fan angles () were considered, and the effects of each nozzle configuration on the jet oscillation frequency, average fluid fan angle , and discharge coefficient were studied. Results show that the exit fan angle has a significant effect on the average fluid fan angle. Initially, increases along with the fan angle up to (); beyond which, eventually plateaus with further increase in . The jet oscillation frequency appears to be independent of nozzle configurations. A case study was performed that includes sweeping jet impingement heat transfer in order to assess the practical implication of the exit geometry. Heat transfer experiments were performed for each nozzle configuration at a jet-to-wall spacing of and three coolant mass flow rates (, 1.48, and ). The average heat transfer tends to deteriorate with increasing fan angle. However, a pronounced improvement in cooling uniformity was observed at large-fan-angle configurations (). The time-resolved velocity field obtained from an unsteady Reynolds-averaged Navier–Stokes simulation revealed that separation bubble starts to appear at the diverging wall of the nozzle exit when that prevents the lateral motion of the jet, thus reduces the average fluid fan angle.

On Predicting the Frequency of a Flip-Flop Jet Nozzle

This research deals with the oscillation frequency of a classical and simple flip-flop jet nozzle with one feed-back loop based on the measurements of pressure and velocity. From these measurements, the traces of pressure difference between both ends of a connecting tube are modeled by a triangular wave, and the flow velocities in the connecting tube are calculated numerically. The resulting accumulated flow work, which consists of the time integral of mass flux into the re-circulation region on the low pressure side wall from the re-circulation region on the opposite high-pressure side wall through the connecting tube, is more adequate to determine jet-oscillation frequency than any other time integrals. This includes those of momentum flux and kinetic-energy flux. It is confirmed that jet-oscillation frequency predicted on the basis of this accumulated flow work agrees well with experiment.

Internal Geometry and External Wall Effects on Fluidic Oscillator Behavior

Abstract This study quantified the correlation of internal geometry (including Coanda effects) and external walls on oscillation frequency for a fluidic oscillator that was tested for a variety of mass flow rates using CO2 gas. The oscillator designs were modified by altering the aspect ratio (AR) with respect to the exit nozzle and changing the cross-sectional area ratio (MR) between the exit throat and power nozzle. The AR and cross-sectional MR were shown to be correlated with frequency. External walls parallel to each other and perpendicular to the oscillator exit throat were added at varying separation distances to observe how they affected the jet oscillation angle and frequency. By increasing the convexity of the exit throat, Coanda effects were about three times more effective in increasing the oscillation angle compared to wall effects. The internal geometry effects were combined by nondimensional analysis to find a function for predicting the frequency of an oscillator in terms of aspect and area ratios. The function showed that the oscillators converged to a single Strouhal number of 0.016.

Low Frequency Pulsating Jet Impingement Boiling and Single Phase Heat Transfer

Experiments are carried out to characterize the influence of jet pulsations on the single phase and boiling heat transfer under a confined and submerged impinging jet, using water and FC-72. A novel jet pulsation mechanism is developed for introducing the mean flow pulsations at different jet flow rates. The effects of jet Reynolds number, pulsation frequency and amplitude are studied by comparison of the time-averaged Nusselt numbers and boiling curves against steady state data. The root-mean-squared dimensionless temperature oscillations in the heater are correlated to the jet Strouhal number, mean Reynolds number and the diffusion Fourier number of the heater that is based on the frequency of jet oscillations. Under both boiling and single phase cooling conditions, temperature oscillations are found to be inversely related to the Strouhal number with an exponent in the range 0.63-0.75. During pulsating jet impingement boiling, a periodic renewal of the boiling process is observed, where the bubbles on the heater surface are cyclically flushed downstream of the stagnation point in-phase with the pulsating flow. For the cases considered, the root-mean-squared dimensionless temperature is found to be negligible during fully developed nucleate boiling, indicating that the heat transfer process is pseudo-stationary. Here, vigorous ebullition is the dominant mechanism of heat transfer and suppresses any temperature oscillations that can arise due to the flow oscillations. For heat fluxes in the partial nucleate boiling regime and close to critical heat flux however, temperature oscillations decrease with an increase in jet Strouhal number. No marked influence of low frequency jet pulsations is found on the time averaged single phase or boiling heat transfer characteristics (including critical heat flux) for the range of parameters studied.

Jet Oscillation Frequency Characterization of a Sweeping Jet Actuator

The time-resolved flow field of a spatially oscillating jet emitted by a sweeping jet (SWJ) actuator is investigated numerically using three-dimensional Reynolds-averaged Navier–Stokes (3D-URANS) equations. Numerical simulations are performed for a range of mass flow rates providing flow conditions varying from incompressible to subsonic compressible flows. After a detailed mesh study, the computational domain is represented using two million hexagonal control volumes. The jet oscillation frequency is predicted by analyzing velocity time histories at the actuator exit, and a linear relationship between the jet oscillation frequency and time-averaged exit nozzle Mach number is found ( f = 511.22 M + 46.618 , R² = 0.97). The results of our numerical model are compared with data from the literature, and a good agreement is found. In addition, we confirmed that the Strouhal number is almost constant with the Mach number for the subsonic oscillating jet and has an average value of St = 0.0131. The 3D-URANS model that we presented here provides a computationally inexpensive yet accurate alternative to the researchers to investigate jet oscillation characteristics.