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.

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.

Internal Geometry and External Wall Effects on Fluidic Oscillator Behavior

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.

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.

Enhanced transport and mixing of Arctic ozone during SSWs

<p><span>The extreme disruptions of the wintertime stratospheric circulation during sudden stratospheric warmings (SSW) have important effects on tracer concentrations through alterations in transport and mixing properties. In this presentation we will examine the dynamics that control changes of Arctic ozone during the life cycle of SSWs, providing a quantitative analysis of both advective transport and mixing of Arctic ozone. We use output from four ensemble members (60 years each) of the Whole Atmospheric Community Climate Model, and also use reanalysis and satellite data for validation purposes. The composite evolution of ozone displays positive mixing ratio anomalies up to 0.5 – 0.6 ppmv above 550 K (∼50 hPa) around the central warming date and negative anomalies below (-0.2 to -0.3 ppmv), consistently in observations, reanalysis and model.</span></p><p><span>Our analysis shows a clear temporal offset between ozone eddy transport and diffusive ozone fluxes. The initial changes in ozone are mainly driven by isentropic eddy fluxes linked to enhanced wave drag responsible for the SSW. The recovery of climatological values in the aftermath of SSWs is slower in the lower than in the upper stratosphere, and is driven by the competing effects of cross-isentropic motions (which work towards the recovery) and isentropic mixing (which delays the recovery). These features are enhanced in strength and duration during sufficiently deep SSWs, particularly those also labeled as Polar-night Jet Oscillation (PJO) events. It is found that SSW-induced ozone concentration anomalies below 600 K (∼40 hPa), as well as total column estimates, persist around one month longer in PJO than in non-PJO warmings.</span></p>

Jet Oscillation Frequency Characterization of a Sweeping Jet Actuator

The time-resolved flowfield 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 practical range of mass flow rates providing flow conditions ranging from incompressible to subsonic compressible flows. A linear relationship between the mass flow rate and the jet oscillation frequency is found. The results of the numerical model are compared with the experimental data in the literature, and good agreement is found. Additionally, it is observed that the SWJ actuator frequency response switches from one linear mode to another linear mode when the compressibility effects become important.

Analysis of the Forces Driving the Oscillations in 3D Fluidic Oscillators

One of the main advantages of fluidic oscillators is that they do not have moving parts, which brings high reliability whenever being used in real applications. To use these devices in real applications, it is necessary to evaluate their performance, since each application requires a particular injected fluid momentum and frequency. In this paper, the performance of a given fluidic oscillator is evaluated at different Reynolds numbers via a 3D-computational fluid dynamics (CFD) analysis. The net momentum applied to the incoming jet is compared with the dynamic maximum stagnation pressure in the mixing chamber, to the dynamic output mass flow, to the dynamic feedback channels mass flow, to the pressure acting to both feedback channels outlets, and to the mixing chamber inlet jet oscillation angle. A perfect correlation between these parameters is obtained, therefore indicating the oscillation is triggered by the pressure momentum term applied to the jet at the feedback channels outlets. The paper proves that the stagnation pressure fluctuations appearing at the mixing chamber inclined walls are responsible for the pressure momentum term acting at the feedback channels outlets. Until now it was thought that the oscillations were driven by the mass flow flowing along the feedback channels, however in this paper it is proved that the oscillations are pressure driven. The peak to peak stagnation pressure fluctuations increase with increasing Reynolds number, and so does the pressure momentum term acting onto the mixing chamber inlet incoming jet.

Enhanced Stratosphere/Troposphere Coupling During Extreme Warm Stratospheric Events with Strong Polar-Night Jet Oscillation

Extreme warm stratospheric events during polar winters from ERA-Interim reanalysis and CMIP5-ESM-LR runs were separated by duration and strength of the polar-night jet oscillation (PJO) using a high statistical confidence level of three standard deviations (strong-PJO events). With a composite analysis, we demonstrate that strong-PJO events show a significantly stronger downward propagating signal in both, northern annular mode (NAM) and zonal mean zonal wind anomaly in the stratosphere in comparison with non-PJO events. The lower stratospheric EP-flux-divergence difference in ERA-Interim was stronger in comparison to long-term CMIP5-ESM-LR runs (by a factor of four). This suggests that stratosphere–troposphere coupling is stronger in ERA-Interim than in CMIP5-ESM-LR. During the 60 days following the central date (CD), the Arctic oscillation signal was more intense during strong-PJO events than during non-PJO events in ERA-Interim data in comparison to CMIP5-ESM-LR runs. During the 15-day phase after CD, strong PJO events had a significant increase in stratospheric ozone, upper tropospheric zonally asymmetric impact, and a regional surface impact in ERA-Interim. Finally, we conclude that the applied high statistical threshold gives a clearer separation of extreme warm stratospheric events into strong-PJO events and non-PJO events including their different downward propagating NAM signal and tropospheric impacts.