Pulsed Jets In Crossflow Research Articles

Effects of additional transmission chamber on flow dynamics of a pulsed jet in crossflow

This study examines the effects of an additional transmission chamber on a pulsed jet in crossflow using experimental and Large-eddy simulation methods, with a focus on the structural and mixing characteristics of the flow. Three pulse frequencies are selected (f = 20 Hz, 50 Hz, and 100 Hz), and the results of the model with an additional transmission chamber are compared with the round pipe model. The results show that the additional transmission chamber enhances jet penetration, diffusion, and mixing uniformity, particularly at higher pulse frequencies. The additional transmission chamber also alters the exit velocity profile, increasing peak velocity and promoting greater deflection under crossflow. These findings highlight the potential of transmission chambers to optimize pulsed jet performance in applications requiring effective mixing and penetration.

Penetration of vertical pulsed jets in crossflow at low velocity ratio

Using the visualization method, the initial rise and penetration of a circular turbulent pulsed jet into a transverse air flow are studied at the ratio of jet velocities to the transverse flow r = u j /u f = 0.67–2.33. A comparative assessment of the penetration of a pulsating jet into a transverse flow for frequencies from 0 to 20 Hz is carried out. The cases of both stationary and oscillating jet flows are analyzed. The penetration of a pulsating jet into a transverse flow is shown to be more significant than for a stationary one and depends on an increase in the ratio of velocities and frequency: it increases linearly at a fixed frequency and passes through a minimum at a fixed ratio of velocities.

Effects of backward inclination on a pulsed jet in crossflow

The effects of the backward inclination angle on the flow and jet dispersion characteristics of a pulsed jet in crossflow were studied experimentally in an open-loop wind tunnel. The pulsations were created by a loudspeaker which installed at the bottom of a nozzle assembly. The instantaneous and time-averaged flow patterns were obtained using the laser-light sheet assisted smoke flow visualization method. The instantaneous and average velocities, turbulence intensities, integral turbulence time scales were probed using a hot-wire anemometer. The tracer-gas detection technique was applied to examine the fluid dispersion capability of the jet. With excitation, the axial jet pulsations were transferred to transverse oscillations after the jet was deflected by the crossflow. A small jet backward inclination angle could lead to an increase of jet-fluid dispersion and transverse jet spreading width. There existed a critical jet backward inclination angle of 20° at which the jet-fluid dispersion index and transverse jet spreading width attained the maximum values. Arranging the jet backward inclination angle at values smaller than 20° could induce an increase in dispersion of the pulsed jet in crossflow and benefit the industrial applications which required fast and efficient jet-fluid dispersion and mixing. • Jet backward inclination angle presented significant effects on axial mean velocities, turbulence intensities, and dispersion properties. • There existed a critical jet backward inclination angle of 20° at which the jet spreading width and dispersion attain maximum values. • At jet backward inclination angle smaller than critical value, increasing inclination angle could increase jet-fluid dispersion. • At jet backward inclination angle larger than critical value, increasing inclination angle would significantly decrease jet-fluid dispersion. • Arranging jet backward inclination angle at critical value could benefit industrial applications.

High Frequency Pulsed Injection into a Supersonic Duct Flow

A study is presented of the effect of pulsation (100% modulation) of a sonic jet of helium into a supersonic (ducted) crossflow on its mixing. An injector was developed to provide a high-speed high-frequency (up to 13 kHz) pulsed jet. The injector nozzle is formed between fixed internal passages and a three- or four-sided wheel embedded within the device that rotates due to the flow in the nozzle. For a given geometry the pulsation frequency is repeatable, constant above a certain pressure ratio, and scales with the speed of sound of the gas. The pulsed jet in the crossflow was visualized in a side view by schlieren photography. The plume of injected helium was visualized in a cross section, 69 effective jet diameters downstream of injection, by seeding the helium with a small amount of ethanol, which condensed as tiny particles and illuminated with a laser light sheet. Results indicate a modest reduction in mean plume cross-sectional size (and, therefore, reduction of mixing) with pulsation and an increase in mean helium penetration. The visualization results are consistent with a transitional state between the turbulent puffs and vortex rings previously observed in low-speed experiments on subsonic pulsed jets in crossflow.

Large eddy simulation of a pulsed jet in cross-flow

AbstractThis study quantifies the mixing that results from a pulsed jet in cross-flow in the near jet region. By large eddy simulation computations, it also helps to understand the physical phenomena involved in the formation of the pulsed jet in cross-flow. The boundary conditions of the jet inlet are implemented via a Navier–Stokes characteristic boundary condition coupled with a Fourier series development. The signals used to pulse the jet inlet are a square or a sine wave. A new way of characterizing the mixing is introduced with the goal of easily interpreting and quantifying the complicated mixing process involved in a pulsed jet in cross-flow flow fields. Different flow configurations, pulsed and non-pulsed, are computed and compared, keeping the root mean square value of the signal constant. This comparison not only allows the characterization of the mixing but also illustrates some of the properties of the mixing characterization.

Modeling Wall Film Formation and Breakup Using an Integrated Interface-Tracking/Discrete-Phase Approach

We propose a computationally tractable model for film formation and breakup based on data from experiments and direct numerical simulations. This work is a natural continuation of previous studies where primary atomization was modeled based on local flow information from a relatively low-resolution tracking of the liquid interface [Arienti and Soteriou, 2007, “Dynamics of Pulsed Jet in Crossflow,” ASME Paper No. GT2007-27816]. The submodels for film formation proposed here are supported by direct numerical simulations obtained with the refined level set grid method [Herrmann, 2008, “A Balanced Force Refined Level Set Grid Method for Two-Phase Flows on Unstructured Flow Solver Grids,” J. Comput. Phys., 227, pp. 2674–2706]. The overall approach is validated by a carefully designed experiment [Shedd et. al., 2009, “Liquid Jet Breakup by an Impinging Air Jet,” Forty-Seventh AIAA Aerospace Sciences Meeting. Paper No. AIAA-2009-0998], where the liquid jet is crossflow-atomized in a rectangular channel so that a film forms on the wall opposite to the injection orifice. The film eventually breaks up at the downstream exit of the channel. Comparisons with phase Doppler particle analyzer data and with nonintrusive film thickness point measurements complete this study.

Optimization of pulsed jets in crossflow

We use direct numerical simulation to study the mixing behaviour of pulsed jets in crossflow. The pulse is a square wave and the simulations consider several jet velocity ratios and pulse conditions. Our objective is to study the effects of pulsing and to explain the wide range of optimal pulsing conditions found in experimental studies of the problem. The central theme is that pulsing generates vortex rings; the effect of pulsing on transverse jets can therefore be explained by the behaviour of vortex rings in crossflow. Sau & Mahesh (J. Fluid Mech., vol. 604, 2008, pp. 389–409) show that vortex rings in crossflow exhibit three distinct flow regimes depending on stroke and ring velocity ratios. The simulations of pulsed transverse jets in this paper show that at high velocity ratios, optimal pulse conditions correspond to the transition of the vortex rings produced by pulsing between the different regimes. At low velocity ratios, optimal pulsing conditions are related to the natural time scale on which hairpin vortices form. An optimal curve in the space of stroke and velocity ratios is presented. Data from various experiments are interpreted in terms of the properties of the equivalent vortex rings and shown to collapse on the optimal curve. The proposed regime map allows the effects of experimental parameters such as pulse frequency, duty cycle, modulation and pulse energy all to be predicted by determining their effect on the equivalent stroke and velocity ratios.

Analysis and reconstruction of a pulsed jet in crossflow by multi-plane snapshot POD

In this work, snapshot proper orthogonal decomposition (POD) is used to study a pulsed jet in crossflow where the velocity fields are extracted from stereoscopic particle image velocimetry (SPIV) results. The studied pulsed jet is characterized by a frequency f = 1 Hz, a Reynolds number Re j = 500 (based on the mean jet velocity $${\overline{U}_{j}}$$ = 1.67 cm/s and a mean velocity ratio of R = 1). Pulsed jet and continuous jet are compared via mean velocity field trajectory and Q criterion. POD results of instantaneous, phase-averaged and fluctuating velocity fields are presented and compared in this paper. Snapshot POD applied on one plane allows us to distinguish an organization of the first spatial eigenmodes. A distinction between “natural modes” and “pulsed modes” is achieved with the results obtained by the pulsed and unforced jet. Secondly, the correlation tensor is established with four parallel planes (multi-plane snapshot POD) for the evaluation of volume spatial modes. These resulting modes are interpolated and the volume velocity field is reconstructed with a minimal number of modes for all the times of the pulsation period. These reconstructions are compared to orthogonal measurements to the transverse jet in order to validate the obtained three-dimensional velocity fields. Finally, this POD approach for the 3D flow field reconstruction from experimental data issued from planes parallel to the flow seems capable to extract relevant information from a complex three-dimensional flow and can be an alternative to tomo-PIV for large volume of measurement.

Scaling of Fully Pulsed Jets in Crossflow

There is recent interest in the dynamics of pulsed jets in uniform crossflow. A classification scheme for fully pulsed Jets in crossflow is proposed based on the stroke ratio of the jet pulse and the duty cycle of the pulse train. Scaling relations for penetration and dilution of fully pulsed jets'in crossflow are derived from the self-similar scaling of turbulent vortex rings and puffs in quiescent media. Individual turbulent structures are assumed to drift freely with the crossflow in the proposed scaling. The penetration of fully pulsed jets scales with the fourth root of the velocity ratio, stroke ratio, and axial distance. The decay of mean concentration scales with the velocity ratio and axial distance to the -3/4th power. These scaling relations apply to distinct turbulent structures before any interaction between successive structures. The criteria for interaction among flow structures near the nozzle and in the far field are also presented.

PIV measurements of a zero-net-mass-flux jet in cross flow

Particle image velocimetry (PIV) is used to study a circular zero-net-mass-flux (ZNMF) jet in cross flow (JICF). ZNMF jets are formed by using the working fluid of the environment into which the jet is issuing without net transfer of fluid mass into the environment from the ZNMF jet system during one period of oscillation. Two ZNMF-JICF, which are characterized by Rej=1,240, R=4.6, St=0.016 and Rej=2,960, R=7, St=0.014, are investigated in this study. Phase-averaged in-plane velocity and out-of- plane vorticity vector fields in the jet near field are pre- sented and the time-dependent volume flow is calculated to study the structure of the jets. Ensemble-averaged in- plane velocity data in the jet far field were determined to study the mean jet structure. The ensemble-averaged vector fields are used to determine jet trajectory and the velocity variation along the trajectory. These results are discussed and compared to continuous and pulsed jets in cross flow. List of symbols a maximum diaphragm displacement (peak-to-peak), mm A, B parameters of jet trajectory equation d orifice diameter, mm D diaphragm diameter, mm f oscillation frequency, Hz IW cross-correlation digital PIV (CCDPIV) interro- gation window size, px

Structure, penetration, and mixing of pulsed jets in crossflow

Structure, penetration, and mixing of pulsed jets in crossflow