Jet Oscillation Research Articles

Fluidic Oscillators, Feedback Channel Effect under Compressible Flow Conditions.

Fluidic oscillators are often used to modify the forces fluid generates on any given bluff body; they can also be used as flow, pressure or acoustic sensors, with each application requiring a particular oscillator configuration. Regarding the fluidic oscillators’ main performance, a problem which is not yet clarified is the understanding of the feedback channel effect on the oscillator outlet mass flow frequency and amplitude, especially under compressible flow conditions. In order to bring light to this point, a set of three-dimensional Direct Numerical Simulations under compressible flow conditions are introduced in the present paper; four different feedback channel lengths and two inlet Reynolds numbers = 12,410 and = 18,617 are considered. From the results obtained, it is observed that as the inlet velocity increases, the fluidic oscillator outlet mass flow frequency and amplitude increase. An increase of the feedback channel length decreases the outlet mass flow oscillating frequency. At large feedback channel lengths, the former main oscillation tends to disappear, the jet inside the mixing chamber simply fluctuates at high frequencies. Once the Feedback Channel (FC) length exceeds a certain threshold, the oscillation stops. Under all conditions studied, pressure waves are observed to be traveling along the feedback channels, their origin and interaction with the jet entering the mixing chamber are thoroughly evaluated. The paper proves that jet oscillations are pressure-driven.

Asymmetric oscillation of pressurized dense gas-solid jet in a two-dimensional nozzle

The instability of pressurized dense gas-solid jets in a two-dimensional nozzle is experimentally studied using a high-speed camera, and the effects of the hopper pressure and the particle diameter on instability are considered. As particle velocity rises with hopper pressure, dense gas-solid jets become unstable and display an asymmetric oscillation, which is quasi-periodic and its frequency and evolution are further characterized. For particle diameters of d = 185 μm and 410 μm, the time-average frequencies increase linearly with the exit particle velocities. The asymmetric structures grow along the nozzle, and even undergo deformation and merging. The macroshot of the flow transition region between hopper and nozzle indicates that gas-solid separation generates fluctuations which will propagate both upstream and downstream. In the nozzle, fluctuations are amplified by the enhanced gas-solid interaction, evolving to the asymmetric structures.

Effects of Bent Outlet on Characteristics of a Fluidic Oscillator with and without External Flow

A fluidic oscillator with a bent outlet nozzle was investigated to find the effects of the bending angle on the characteristics of the oscillator with and without external flow. Unsteady aerodynamic analyses were performed on the internal flow of the oscillator with two feedback channels and the interaction between oscillator jets and external flow on a NACA0015 airfoil. The analyses were performed using three-dimensional unsteady Reynolds-averaged Navier-Stokes equations with a shear stress transport turbulence model. The bending angle was tested in a range of 0–40°. The results suggest that the jet frequency increases with the bending angle for high mass flow rates, but at a bending angle of 40°, the oscillation of the jet disappears. The pressure drop through the oscillator increases with the bending angle for positive bending angles. The external flow generally suppresses the jet oscillation, and the effect of external flow on the frequency increases as the bending angle increases. The effect of external flow on the peak velocity ratio at the exit is dominant in the cases where the jet oscillation disappears.

Experimental study on flow oscillating mechanism of non-condensable gas jet through one- or multi-hole sparger in quiescent water

Submerged gas jet plays a very important role in many industries occasion. It often causes load of pool structure induced by flow oscillation. Exploring the mechanism and key influencing factors of submerged non-condensable gas jet oscillation characteristics is of great significance to ensure the safe operation of the equipment. In the present work, the phenomenon of flow oscillation for non-condensable gas jet through a horizontal one- or multi-hole sparger from sonic to supersonic zone is studied experimentally. The steady and unsteady characteristic of flow process and flow structure are visualized by using high-speed camera, and pressure oscillation is obtained by simultaneous pressure measurement. Experimental results show that gas Froude number and sparger structure have great influence on the gas flow pattern. With Froude number increasing, gas flow pattern is obviously reinforced. Multi-hole sparger significantly weakens the gas jet behavior at horizontal direction away from holes comparing with one-hole sparger under the same Froude number. The spectrogram approaches of pressure oscillation indicates the dynamic pressure data contains two principal frequency components. First dominant frequency is identical with that of transient interfacial fluctuation, which is related to gas jet periodic expansion feedback and independent on Froude number and position. And second dominant frequency is not changed with the position, but decreases significantly with the increase of Froude number. Intensity of pressure oscillation increases with Froude number increasing and the distance from sparger decreasing. In addition, the multi-hole sparger can significantly reduce intensity of pressure oscillation under the same Froude number. Dimensionless relation is developed to evaluate the Froude number and sparger structure on the second dominant frequency and intensity of pressure oscillation. This study can be applied to well understanding of flow oscillation mechanism and provide technical support for looking for mitigation measures to reduce oscillation in the future.

Erosion wear research of wall-attachment jet component based on solid–liquid two-phase flow

ABSTRACT Erosion resistance is the evaluation indicator of the performance parameter of wall-attachment jet component, which is the key component of jet oscillation tool. For the erosion wear problem, the erosion wear research of jet component based on solid–liquid two-phase flow is conducted. According to the fluid dynamics and erosion theory, the Euler-Lagrangian method and the discrete phase model (DPM) are used to establish the internal erosion model of wall-attachment jet component to discuss the effect on particle parameters. The interaction response of main influence factors is analyzed by the response surface method (RSM). The results show that the erosion wear degree of the internal flow channel of jet component is positively correlated to the particle volume fraction, as well inversely proportional to the particle diameter. As the inlet velocity increases, the kinetic energy of particle increases, and the average erosion rate increases about 28.5%. When the drilling fluid viscosity increases, maximum erosion rate fluctuates in 0.5 ~ 1 mg/m−2.s−1 and average erosion rate changes from 0.2 mg/m−2.s−1 to 0.5 mg/m−2.s−1. As the drilling fluid viscosity increases, maximum and average erosion rate are 0.92 mg/m−2.s−1 and 0.55 mg/m−2.s−1, respectively. The experimental data indicate the erosion rate of jet component reduces about 0.015% and 0.025% when the time is at 50 h and 120 h, respectively. The research fingdings can be applied to the structure optimization of jet component and also provide reference for the field service of jet oscillation tool.

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.

Annual and Seasonal Precipitation and Their Extremes over the Tibetan Plateau and Its Surroundings in 1963–2015

Based on daily precipitation data from 115 climate stations, seasonal and annual precipitation and their extremes over the Tibetan Plateau and its surroundings (TPS) in 1963–2015 are investigated. There exists a clear southeast-northwest gradient in precipitation and extreme daily precipitation but an opposite pattern for the consecutive dry days (CDDs). The wet southeast is trending dry while the dry center and northwest are trending wet in 1963–2015. Correspondingly, there is a drying tendency over the wet basins in the southeast and a wetting tendency over the dry and semi-dry basins in the center and northwest in summer, which will affect the water resources in the corresponding areas. The increase (decrease) in precipitation tends to correspond to the increase (decrease) in maximum daily precipitation but the decrease (increase) in CDDs. Extreme precipitation events with 20-year, 50-year, 100-year, and 200-year recurrence occurred frequently in the past decades especially in the 1980s. The greatest extreme precipitation events tend to occur after the late 1990s and in the southeastern TPS. The ERA5 reanalysis and climate system indices reveal that (1) decreased moisture transports to the southeast in summer due to the weakening of the summer monsoons and the East Asian westerly jet; (2) increased moisture transports to the center in winter due to the strengthening of the winter westerly jet and north Atlantic oscillation; and (3) decreased instability over the southeast thus suppressing precipitation and increased instability over the northwest thus promoting precipitation. All these are conducive to the drying trends in the southeast and the wetting trends in the center.

Winter Persistent Extreme Cold Events in Xinjiang Region and Their Associations with the Quasi-Biweekly Oscillation of the Polar Front Jet

Winter persistent extreme cold events (WPECEs) often cause great damage to the development of economies and people’s lives. The sub-seasonal variation of the atmospheric circulation is regarded as one of important causes of extreme weather, and is key to propel the extended period prediction. In this paper, we mainly analyze the WPECEs in Xinjiang region and their relationship with the sub-seasonal variation of the East Asian polar front jet (PFJ). The results suggest the persistent extreme cold event (equal or greater than 7 days) occurs most frequently in Xinhe County of Xinjiang region, with obvious inter-annual and inter-decadal variations. Further analysis shows that the variation of the mean temperature in the key area has characteristics of intra-seasonal variation when the WPECE occurs. The result of composite analysis shows that this intra-seasonal variation is related to the sub-seasonal variation of atmospheric circulation, especially the PFJ anomalous activity near Lake Balkhash. By using the power spectrum analysis method, note that the PFJ activity has the characteristics of quasi-biweekly oscillation (QBWO) in WPECEs. On quasi-biweekly scale (10–20-day filtered), the weakening of PFJ, the intensification of the zonal easterly wind in the upper troposphere, the accumulation of the strong cold air, and the intensification of the meridional northerly wind in the lower troposphere enhance the occurrence of WPECEs in Xinjiang. Further investigation indicates that the quasi-biweekly PFJ mainly propagates eastward and southward before the WPECE occurs in Xinjiang, China.

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.

Study on the Spray Characteristics and Oscillation Mechanism of a Feedback-Free Internal Impinging Nozzle

Feedback-free internal impinging nozzle, which can generate fan-shape oscillating spray with constant pendulum angle, is well-suitable for flow control, as it has a wide range of operation frequency and does not require additional energy for moving parts. In this study, the spray characteristics of an internal impinging nozzle were investigated in an open atomization test bench by utilizing high-speed Schlieren technology and Malvern particle size analyzer. To better understand the jet oscillation mechanism and to evaluate its impact on the spray sweeping behaviors, a 3D CFD simulation was carried out, which takes the nozzle internal flow dynamics and the external spray morphology into account. The results indicate that the sweeping behavior of the emitted jet is caused by the impingement between two opposite internal jets inside the nozzle. A bi-model spray spatial flow distribution of the nozzle is observed with varying oscillating frequencies and constant spray pendulum angle at different injection pressures. The oscillation process can be divided into two stages depending on jet breakup processes, and the spray waveforms are influenced by the ratio of the durations of these two stages.

A feedback model of the edge tone, using the adjoint Orr–Sommerfeld equation

Abstract

Jet sweeping angle control by fluidic oscillators with master-slave designs

The fluidic oscillator is an instrument that can continuously generate a spatially sweeping jet entirely based on its internal geometry without any moving parts. However, the traditional fluidic oscillator has an inherent limitation, that is, the spreading angle cannot be controlled independently, rather by the jet volume flow rate and internal geometry. Accordingly, two types of fluidic oscillators based on the master-slave design are developed in current study to decouple this correlation. In both designs, the master layer inherits the similar oscillation mechanisms of a sweeping jet, and the slave layer resembles a steady jet channel. The difference between the two designs is that Design A has a short diverging exit in the slave layer, but Design B adds a long interaction chamber in the exit channel to intensify flow instability. The external flow fields and governing oscillation properties of these two designs are experimentally explored with time-resolved Particle Image Velocimetry (PIV), while the internal flow dynamics and driving oscillation mechanisms are numerically investigated. By fixing the total volume flow rate, the jet spreading angle of Design A can be increased smoothly from 0° to above 100° by increasing the proportion of master layer's flow rate from 0 to 100%. For Design B, the control authority of the master layer is significantly enhanced by adding the interaction chamber in the slave layer. In addition, the added chamber causes notable jet oscillation even when the master layer has none input.

FEATURES OF SOUND GENERATION BY THE JET-DRIVEN HELMHOLTZ OSCILLATOR

Experimental studies the model of a jet-driven Helmholtz oscillator has been carried out. The oscillator is a cylindrical chamber, closed on the sides by two covers with inlet and outlet holes. The oscillator was excited by a stream of airflow. The influence of the geometric dimensions of the chamber and the openings in the covers on the amplitude of pressure fluctuations was studied and the optimal channel configuration was determined. Particular attention is paid to the study of the influence on the amplitude of pressure fluctuations of the shape and size of the nozzle - the hole in the front cover. There were measured the following processes: the pressure drop across the nozzle, to calculate the speed of the jet at the chamber entrance, and the parameters of pressure fluctuations in the chamber, to plot the amplitude-frequency spectra. There were made certain observations of a jet tone appearance and acoustic modes excitation at the natural frequency of the chamber-resonator with a smooth increase in the jet velocity. The relationship between the jet tone and the resonant in the chamber is noted. Recommendations for designing a Helmholtz jet oscillator are presented.

Flow behaviours and velocity fields of non-oscillating and transversely oscillating jets

The near-field flow characteristics and jet fluid dispersion properties of non-oscillating and transversely oscillating jets issued from a fluidic oscillator were studied experimentally. A V-shaped fluidic oscillator was designed to induce transverse jet oscillations by merging two bifurcated alternatively issuing side jets. The characteristic flow patterns, evolution processes, velocities, vorticities, turbulence intensities and jet fluid dispersion capabilities in the median plane of the jets were examined. The characteristic flow patterns were obtained using laser-light-sheet-assisted smoke flow visualization method. The flow velocities were measured using particle image velocimetry. The jet fluid dispersion properties were analysed using tracer gas detection method. The non-oscillating jet was axial momentum-dominated—the axial momentum of the jet was limited within a narrow region around the central axis and was significantly larger than the transverse momentum. Therefore, the transverse velocities and vorticities were small and the dispersion of the jet fluid was relatively low. The transversely oscillating jets meandered in the near-field so the flow patterns presented a violent breakup of vortical flow structures and large transverse widths. The streamlines expanded transversely outwards and caused a part of the axial momentum to transfer towards the transverse direction. The vorticities and transverse velocities were increased and the axial velocities were decreased, so the dispersion of jet fluid was augmented in the transversely oscillating jets.

Effects of nanoparticles on the instability of liquid jets in a gaseous crossflow

Nanofluids have attracted interest for their high performance in enhancing heat transfer and improving combustion. However, few studies have investigated the influences of nanoparticles on spray processes. In this study, the surface waves, liquid column oscillation, and droplet size distribution of a nanofluid transverse jet were investigated at low gas Weber number. A new statistical image-processing method was proposed to identify the surface wavelength and jet oscillation amplitude. A new power index parameter was proposed to account for the effect of nanoparticles, and then introduced to correlate with the upwind wavelength and Sauter mean diameter (SMD) of nanofluid droplets in the far field. The results show that in bag breakup and multimode breakup, the wavelengths decrease along the jet flow direction while the amplitudes gradually increase in the initial stage. The characteristic wavelength and SMD diminish with the increase of nanoparticle concentration, and the oscillation angle of liquid column increases correspondingly. The nanoparticle additive at low concentration obviously enhances the flow instability and cavitation inside the jet. Therefore, the fineness and uniformity of nanofluid atomization in the far field are significantly improved.

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.

Machines for Mechanical Mining of Hardly Workable and Abrasive Rocks

Abstract Mechanical mining is a widely used method for separating material from mined rock in the mining branch, tunnelling, road and construction industries. Depending on the mechanical properties of rocks, most often defined by uniaxial compressive strength, various machines and tools are applied. Apart from rock strength, the efficiency of the mining process is determined by abrasiveness, which affects the rate of tool abrasive wear. Currently, disc mining by static pressure or rear undercutting are the most developed methods. Undercutting can be supported by high pressure water jet or disk oscillation. Mining with drills also utilizes static pressure. In addition to discs, conical picks are applied; they are mounted on cutterheads. In underground mining and tunnelling, there is an increasing need to cut abrasive rocks with a strength of more than 120 MPa in places where explosive materials cannot be used. In the article, the recently applied and developed methods of cutting abrasive rocks characterized by high strength, such as copper, gold, tungsten or platinum ores and diamond deposits, have been presented. Next, the latest machines and machine systems used for their mining are reviewed. Leaders in the mining machinery industry, such as Joy (Komatsu), Epiroc (Atlas Copco), Sandvik and Aker Wirth are developing proprietary designs of cutting machines based on both well-known and completely new ideas.

On the use of oscillating jet flames in a coflow to develop soot models for practical applications

Oscillating jet flames in a coflow mimic certain features of turbulent flows in a simplified and controllable way. Our study aims to identify the potential of the HMOM soot model, which is validated in steady laboratory flames to be applied in practical, mostly turbulent flames. The model is found to accurately predict all the measurement parameters in a steady laminar sooting jet flame of ethylene/nitrogen mixture. When applied to an oscillating flame with the same fuel mixture, it was found that, while velocity, temperature, and OH fields are well predicted, the peak soot volume fraction is over predicted and occurs away from the experimentally measured location. The potential for soot model improvement is analysed by correlating the transient gas-phase species, polycyclic aromatic hydrocarbon, and soot formation or destruction behaviour. A need for improved soot nucleation, condensation, and oxidation formulations is identified. It is further found that the soot number density distribution in mixture fraction space is comparable to the transitional turbulent flame regime dominated by Kelvin-Helmholtz rollers.

Effects of Reynolds number on flow and mixing characteristics of a self-sustained swinging jet

The effects of Reynolds number on flow and mixing characteristics of a merged single swinging jet induced by a V-shaped fluidic oscillator were studied experimentally. The jet Reynolds number was varied from 80 to 3000. A high-speed digital camera was used to capture the instantaneous flow evolution processes using the laser light-sheet-assisted flow visualisation method. Long-exposure images together with the binary edge detection technique were used to identify the jet spread width. The velocities, turbulence intensities, as well as turbulence macro time and length scales in the central axis were measured using a one-component hot-wire anemometer. Measurement results using the tracer-gas concentration detection method provided information on the dispersion characteristics of the merged swinging and non-swinging jets. Four characteristic flow modes (non-swinging, subcritical swinging, critical swinging, and supercritical swinging jets) were identified within different ranges of jet Reynolds number. At Reynolds numbers <150 ± 10, no jet swinging happened. At Reynolds numbers between 150 ± 10 and 1200 ± 23, the instantaneous velocities of the merged jet presented periodic oscillations of small amplitudes and was denoted as the subcritical swinging jet. At Reynolds numbers between 1200 ± 23 and 2350 ± 50, the merged jet proceeded irregular swinging motion and was called the critical jet. At Reynolds numbers higher than 2350 ± 50, the time-evolving velocities of the merged jet presented periodic oscillations of large amplitudes imposed by large turbulent fluctuations and was termed the supercritical swinging jet. The measured results of the axial velocities and turbulence intensities in the central axis revealed that in the regime of the supercritical swinging jet, the momentum conversion from the axial to transverse direction was rapidly performed in the near field at the axial distance <2.4 times of the jet exit width due to the induced transverse jet oscillation. The transverse oscillations of the swinging jets induced large turbulence intensities and small turbulence macro time and length scales, and led to large jet mass dispersion. Therefore, the mixing characteristics of the flow was significantly improved due to large jet mass dispersion.