Jet Oscillation Research Articles

Experimental and theoretical study on the association between entrainment vortex evolution and pressure oscillation of steam jet condensation

Condensation pressure oscillation of steam jet has great influence on direct contact condensation (DCC) equipment. Therefore, it is essential to reveal the microscopic mechanism of pressure oscillation of steam jet condensation. In this study, the association between entrainment vortex evolution and pressure oscillation of steam jet condensation is investigated. Firstly, the periodic variation of characteristic parameters of jet oscillation caused by periodic vortex shedding is obtained based on the image processing techniques. Secondly, by synchronous comparison between the pressure oscillation cycles and the jet parameters oscillation cycles, it is found that the periodic evolution and shedding of two-phase entrainment vortexes formed in the upper and lower mixing layers is the microscopic mechanism which causes condensation pressure oscillation. Specifically, the high-frequency pressure oscillation near the steam jet results from the high-frequency transverse oscillation of the steam jet which is caused by the periodic vortex evolution in the mixing layer and the periodic shedding of the vortex blob at the tail of the jet; the low-frequency pressure oscillation near the steam jet results from the low frequency oscillation of the whole steam jet caused by periodic shedding of the paired vortex blobs; the pressure oscillation in the downstream of the steam jet is caused by the condensation oscillation of periodic shedding vortex blobs. Finally, the dominant frequency distribution of condensation pressure oscillation under various test conditions is analysed. Combined with the vortex shedding model, a prediction equation of the dimensionless characteristic frequency St of the shedding vortexes is established. The predicted values had good agreement with the experimental results.

Chain oscillations in liquid jets

Pour coffee into a mug, and you might notice that the water cascading from the jug resembles a chain: A series of ``links'' oriented at 90\ifmmode^\circ\else\textdegree\fi{} to one another. Such oscillating jets occur frequently in our daily lives, but their origin is still not fully understood. In this study, we performed experiments and direct numerical simulations to show that the wavelength and amplitude of the jet's oscillations and surfaces are directly linked to the flow rate and size of the opening through which the liquid flows.

Influence of wave height on round jet in regular waves: Direct numerical simulations

The influence of wave height on the dynamics of a turbulent round jet is analyzed using direct numerical simulation. The vortex dynamics associated with the interaction of the jet with waves of different heights is compared using the swirling strength criterion and the contours of vorticity. An analysis of the vortex roll-up frequency reveals that it increases with increasing wave height. With an increase in the wave height, the deflection of the jet elevates, and therefore, the span of the vortex rings in the near-field reduces in the direction of wave propagation. Further, an analysis of the braid region reveals that the number of lateral jets reduces to three with increasing wave height despite the number of counter-rotating vortex pairs remaining as four. The longer time evolution suggests that as the jet moves from the crest to the zero down-crossing, the fluid in the far-field gets detached from the main jet flow for the stronger waves. The evaluation of the preferred mode reveals that it remains helical for all wave heights. The proper orthogonal decomposition (POD) establishes that the highest energy is captured by the modes of the highest wave height. Also, as the wave height increases, a fewer number of modes capture 90% of the energy. Further, contours of POD modes represent the jet oscillations as well as the kinetic energy gain in the shear layer and the far-field. The time-averaged quantities quantitatively demonstrate an increase in mixing and entrainment in the jet as the wave height increases.

Liquid jet injection in supersonic crossflow: Self-induced jet oscillations and its effects

Liquid jet injection into supersonic crossflow is one of the conceptually simple ways of fuel atomization in Scramjet engines. In the present work, we experimentally investigate the flow and spray formed due to the injection of a liquid jet in supersonic crossflow with free stream Mach number of M∞ = 2.5. The main focus of the work is on the unsteady aspects of the flow and spray, which are found to be self-induced and occur naturally in the flow. High speed shadowgraphy shows large variations in the windward spray edge and penetration with time. Unsteady pressure measurements in the liquid injection line show that in the presence of a supersonic crossflow, there exist significant self-generated mass flow fluctuations that occur in the injection line. The frequency of these oscillations is seen to be close to a Strouhal number, St = fδ /U∞ ≈ 0.007, close to the typical low frequencies seen in Shock-Wave Boundary Layer Interaction studies (δ = boundary layer thickness). The variations in mass flow rate within the injection line leads to significant fluctuations of the spray penetration and shocks. In particular, the variations in spray penetration with time is observed to lead to significant variations in both the droplet numbers and size at a given location in the generated spray, which can have significant implications for applications where such configurations are used.

Rapid quasi-periodic oscillations in the relativistic jet of BL Lacertae.

Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales1-3. This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare4 in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet5, plasma instabilities6,7 or orbital motion in an accretion disc7,8. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref. 9). BL Lac, the prototype of a subclass of blazars10, is powered by a 1.7 × 108 MSun (ref. 11) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref. 12)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities6 near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.

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.

Oscillations of a particle-laden fountain.

Different regimes are usually observed for fluid migration through an immersed granular layer. In this work, we report a puzzling behavior when injecting water at a constant flow rate through a nozzle at the bottom of an immersed granular layer in a Hele-Shaw cell. In a given range of parameters (granular layer height and fluid flow rate) the granular bed is not only fluidized, but the particle-laden jet also exhibits periodic oscillations. The frequency and amplitude of the oscillations are quantified. The Strouhal number displays a power-law behavior as a function of a nondimensional parameter, J, defined as the ratio between the jet velocity at the initial granular bed height and the inertial particle velocity. Fluid-particle coupling is responsible for the jet oscillations. This mechanism could be at the origin of the cyclic behavior of pockmarks and mud volcanoes in sedimentary basins.

Cavitation bubble collapse in a vicinity of a rigid wall with a gas entrapping hole

This paper aims to reveal the influence of a rigid wall with a gas entrapping hole on the characteristics of the dynamic behavior of a laser-induced bubble collapse. A high-speed camera system was used to record the oscillation process of the laser-induced bubble on a rigid wall with a gas entrapping hole. When a bubble is generated by a laser above the wall with a gas entrapping hole, the entire bubble collapse stays away from the wall or splits into two bubbles because of a radial jet induced by bubble contraction. These two distinctive collapse modes are dependent on the distance between the wall and the bubble. The focus of this study is the quantitative analysis of the jet formation, bubble migration, and oscillation period, and compared with the behavior of the bubble near a rigid wall. The results show that unlike the generation of the bubble near a rigid wall, a rigid wall with a gas entrapping hole affects the morphology of the jet and changes the direction of migration of the bubble and decreases the oscillation period. Thus, the rigid wall with a gas entrapping hole could be effective for reducing cavitation erosion on the wall surface, which is supported by our experiment results.

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.

Global oscillations in low-density round jets with parabolic velocity profiles

Experiments and spatiotemporal stability analysis are carried out to study the global oscillations in laminar low-density round jets with parabolic velocity profiles. The experimental results of laminar low-density jets with parabolic velocity profiles exhibit global axisymmetric oscillations. The spatiotemporal stability results based on base profiles from numerical simulations are consistent with the present experimental results. These results differ from the prediction of stability study by Coenen et al. (Phys. Fluids, vol. 20, 2008, p. 074104). They reported that the low-density jets with near parabolic velocity profiles show global helical oscillations. It is observed that the method used by Coenen et al. is not able to predict the nature of global oscillations observed in experiments for low-density jets with near parabolic profiles. The present spatiotemporal stability results demonstrate that the base flows from Navier–Stokes equations are required to predict the critical conditions observed in experiments for low-density jets with near parabolic velocity profiles. The breakdown distance of globally unstable low-density jets scales with $(Re-Re_c)^{-1/2}$ ( $Re_c$ is the critical Reynolds number), consistent with the scaling law obtained from the nonlinear global mode of the Ginzburg–Landau equation.

Numerical study on satellite droplet formation in laminar jet breakup for a liquid droplet radiator

Liquid Droplet Radiator is promising for the waste heat removal of space nuclear power systems, and droplet properties have an impact on its performance. Breakup behaviors of laminar liquid jets with continuous droplets formation were numerically studied. Results verified that satellite droplets formation would destroy uniform droplet streams. The effects of We, Oh, Ca, and k on laminar jet breakup behaviors were investigated. Increasing Oh shifted jet breakup pattern from downstream pinching to upstream pinching, and it reversed as We increased. The jet breakup was more delayed with Oh increased at low We and jet oscillation amplitude became larger with We increased. At low We, breakup length and breakup time decreased firstly and then increased with k increased, but increased with Ca increased. However, the influence of Ca became slight at high We. Moreover, quantitative criteria were proposed for accurately identifying three satellite droplet formation regimes.

Analysis of Swirl Number Effects on Effusion Flow Behavior Using Time-Resolved Particle Image Velocimetry

Abstract The analysis of the interaction between the swirling and cooling flows, promoted by the liner film cooling system, is a fundamental task for the design of turbine combustion chambers since it influences different aspects such as emissions and cooling capability. In particular, high turbulence values, flow instabilities, and tangential velocity components induced by the swirling flow deeply affect the behavior of effusion cooling jets, demanding for dedicated time-resolved near-wall experimental analysis. The experimental setup of this work consists of a non-reactive single sector linear combustor test rig scaled up with respect to engine dimensions; the test section was equipped with an effusion plate with standard inclined cylindrical holes to simulate the liner cooling system. The rig was instrumented with a 2D time-resolved particle image velocimetry system, focused on different field of views. The degree of swirl for a swirling flow is usually characterized by the swirl number, Sn, defined as the ratio of the tangential momentum flux to axial momentum flux. To assess the impact of such parameter on the near-wall effusion behavior, a set of three different axial swirlers with swirl number equal to Sn = 0.6–0.8–1.0 were designed and tested in the experimental apparatus. An analysis of the main flow field by varying the Sn was first performed in terms of average velocity, root mean square, and Tu values, providing kinetic energy spectra and turbulence length scale information. In a second step, the analysis was focused on the near-wall regions: the strong effects of Sn on the coolant jets were quantified in terms of vorticity analysis and jet oscillation.

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.

Influence of the Solar Cycle on the North Atlantic Oscillation

AbstractWe examine the influence of the solar cycle on the North Atlantic Oscillation (NAO) on its pathway from the upper stratosphere to the surface by applying lagged regression analyses to recent observations, historical observations covering 194 years, and an Earth system model simulation covering 165 years. The propagation of the solar signal can well be explained by a top‐down mechanism, but one that was strongly affected by air‐sea coupling. The solar signal first appears in the subtropical upper stratosphere as a temperature signal. The associated zonal wind signal then propagates downward to the surface in response to stratospheric variability known as the Polar‐night Jet Oscillation. The NAO signal tends to appear in February during years of peak solar activity. The solar signal is further modulated such that positive NAO signals tend to appear earlier in winter with increasing years after peak solar activity associated with the appearance of the NAO‐related SST signal. There are also 50‐year scale fluctuations and amplitude modulations of the solar‐NAO signal, but the tendency of increasing NAO amplitude with increasing NAO‐related SST amplitude suggests a strong influence of decadal ocean dynamics on these modulations.

Flow Transition Analysis of Multiple Jets in Narrow Parallel Walls using POD and DMD

We experimentally and numerically study the cavity oscillation of a compound jet in narrow parallel plates. Particle Image Velocimetry (PIV) obtains the time series velocity distribution with four jets. To perform the mode decomposition of the flow field, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are applied to the PIV data. The POD analysis reveals that the cavity oscillation involves two hierarchical structures: one corresponds to the cavity-scale oscillating of the jet, and the other to the advecting multiple smaller vortices. The DMD analysis classifies the oscillation modes into three categories: (i) oscillation mode on the container scale, (ii) pulsating mode on the container scale, and (iii) oscillation modes of individual jets. The results suggest that the superposition of these oscillation modes at the high flow rate is observed as cavity oscillation.

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.

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.

Investigation of jet control using pulsed jets with three azimuthal modes

In this paper, a turbulent jet manipulated by pulsed control jets is investigated using large-eddy simulation. Four pulsed jets are arranged around the nozzle exit and work under three azimuthal modes; i.e., symmetric, flapping or first mixed and second mixed modes. Time-averaged statistics revealed that the shear layer exhibits different spreading rates along different azimuthal directions. In addition, the complexity of the generated coherent structures depends on the forcing method. Under the symmetric mode, the shear layer in the forced plane rolls up early, and the coherent structure is stretched along the downstream direction in the unforced plane, forming a distorted ring ultimately. Under the flapping mode, staggered vortex rings are formed when viewed from the forced plane. These structures introduce heavily oscillation of the primary jet in the near field, which eventually bifurcates. Under the second mixed mode, the single distorted vortex rings resemble those under the symmetric mode. However, the axes of neighboring rings are orthogonal to each other. Therefore, a strong mutual interaction occurs between neighboring rings. In particular, the front part of a ring is inhaled into its preceding counterpart. Meanwhile, the rear part of the preceding ring is pushed outward, resulting in a more distorted structure. The mode analysis suggests that the disturbance generated by the second mixed mode is sustained for longer distance in the flow field.

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.