Reduction In Core Length Research Articles

Subsonic Jet Mixing via Active Control Using Steady and Pulsed Control Jets

A comparison of the performance of steady and pulsed control jets in enhancing the mixing rate of a high Reynolds number (10 6 ) high subsonic Mach number (0.9) jet is reported. The key parameters affecting control jet performance are identified as control jet pressure ratio, Strouhal number (St), and mode number (m), and an existing nozzle test facility is modified to allow detailed investigation of these parameters. Optimization of the control jet design lead to increased mixing, such that a primary jet potential core length reduction of 50% was achieved. The increase in jet/ ambient entrainment interfacial area was seen as an important factor. Pulsation at St = 0.22 in an antisymmetric mode (m = 1) gave the best results. The data provided, based on extensive laser Doppler anemometry mean velocity and turbulence measurements, allowed an improved understanding of the physical processes that are crucial for enhanced mixing and contribute a useful new data set for computational fluid dynamics validation of rapid mixing jet flows.

Corrugated Tabs for Supersonic Jet Control

The efficiency of corrugated tabs in promoting the mixing of Mach 1.8 axisymmetric jet has been investigated experimentally. Two rectangular tabs of 4.2% blockage, with corrugations at the edges, located diametrically opposite at the exit of a Mach 1.8 convergent-divergent nozzle were found to be better mixing promoters than identical rectangular tabs without corrugations, at overexpanded, correctly expanded, and underexpanded states of the jet. Furthermore, the corrugated tabs were found to be more efficient in weakening the shocks in jet core compared with the plain tabs. As high as 78 % of reduction in core length was achieved with corrugated tabs for the jet operated at nozzle pressure ratio of 7, the corresponding reduction with the plain tabs is only 54%. The mixing effectiveness of corrugated tabs increases progressively with increase of nozzle pressure ratio whereas, the maximum mixing effectiveness of the plain tabs is found to be at the correctly expanded state. The shadowgraph pictures for the uncontrolled and controlled jets clearly demonstrate the effectiveness of corrugated tabs in weakening the waves in the jet core. The speculation of smaller vortices generated by the corrugated tab is supported by a preliminary visualization with water flow channel.

Flow Control by Dielectric Barrier Discharge Actuators: Jet Mixing Enhancement

This study is focused on a new approach to control the mixing at the exhaust of a jet nozzle. Two dielectric barrier discharges are used to enhance the turbulent quantities in the jet wake. An axisymmetric air jet is equipped with a 22-degree-angle diffuser that houses two dielectric barrier discharge actuators, symmetrically placed along the lips of the diffuser. Streamwise, cross-stream planes, and turbulent spectra for free air jet velocities from 10 to 40 m/s are investigated by stereoscopic particle image velocimetry and laser Doppler velocimetry up to 8 jet diameters. The airflow is separated naturally along the diffuser wall and the results demonstrate that a full (at 10 m/s) or partial (at 20 m/s) flow reattachment can be performed, this process being associated with the shedding of coherent structures. The turbulent spectra suggest that the frequency of the vortex shedding is dictated by the electrical frequency applied to the actuators and that the flow structures can be manipulated up to 40 m/s. The time-averaged data confirm that a significant enhancement of the jet spreading, a jet core length reduction, and a turbulent kinetic energy increase are performed up to 30 m/s for a quasi-steady actuation. Unsteady actuations are also investigated and the results highlight that a Strouhal number from 0.25 to 0.32 is particularly effective to enhance the turbulent kinetic energy in the overall flowfield.

Effect of Cross-Wire Location on the Mixing of Underexpanded Sonic Jets

This paper presents the results of an experimental investigation carried out to study the effectiveness of a passive control in the form of a cross-wire located at the nozzle exit and downstream of the nozzle exit for promoting jet mixing. The advantage of locating the cross-wire at downstream of the nozzle exit is to keep the flow at the nozzle exit undisturbed, to avoid thrust loss. Sonic jets at nozzle pressure ratios (NPR) of 3, 5, and 7 were investigated with cross-wire at the nozzle exit, at 1De and 2De from the nozzle exit. The cross-wire influenced the supersonic core, causing the significant reduction of core length and mixing enhancement. The cross-wire also results in diffusion of shocks for all the cross-wire locations and at all tested NPRs.

Active control of high-speed and high-Reynolds-number jets using plasma actuators

Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm exit diameter and a Reynolds number based on the nozzle exit diameter of about 1.1×106. Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle exit, are used to force various azimuthal modes over a large frequency range (StDF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 (f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D/θ ∼ 250) also responded to forcing with various azimuthal modes (m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.

Subsonic and Transonic Jet Control with Cross-Wire

Cross-wire eectiv eness for subsonic and sonic jet control is reported in this paper. Mach 0.4, 0.6, 0.8 and 1.0 axisymmetric jets from a convergent nozzle with cross-wire along a diameter at the exit were studied. The cross-wire was found to be eectiv e in promoting jet mixing right from the nozzle exit, at all Mach numbers. For the underexpanded sonic jet at NPR 3, 5 and 7, the cross-wire inuenced the core and the shock cells, causing signican t reduction of core length and weakening of the shocks at all levels of underexpansion. The jets from the nozzle with cross-wire spread faster in the direction normal to the cross-wire.

Development and use of localized arc filament plasma actuators for high-speed flow control

The paper discusses recent results on the development of localized arc filament plasma actuators and their use in controlling high-speed and high Reynolds number jet flows. Multiple plasma actuators (up to 8) are controlled using a custom-built 8-channel high-voltage pulsed plasma generator. The plasma generator independently controls pulse repetition rate (0–200 kHz), duty cycle and phase for each individual actuator. Current and voltage measurements demonstrated the power consumption of each actuator to be quite low (20 W at 20% duty cycle). Emission spectroscopy temperature measurements in the pulsed arc filament showed rapid temperature increase over the first 10–20 µs of arc operation, from below 1000 °C to up to about 2000 °C. At longer discharge pulse durations, 20–100 µs, the plasma temperature levels off at approximately 2000 °C.Modelling calculations using an unsteady, quasi-one-dimensional arc filament model showed that rapid localized heating in the arc filament on a microsecond time scale generates strong compression waves. The results of the calculations also suggest that flow forcing is most efficient at low actuator duty cycles, with short heating periods and sufficiently long delays between the pulses to allow for convective cooling of high-temperature filaments. The model predictions are consistent with laser sheet scattering flow visualization results and particle imaging velocimetry measurements. These measurements show large-scale coherent structure formation and considerable mixing enhancement in an ideally expanded Mach 1.3 jet forced by eight repetitively pulsed plasma actuators. The effects of forcing are most significant near the jet preferred mode frequency (ν = 5 kHz). The results also show a substantial reduction in the jet potential core length and a significant increase in the jet Mach number decay rate beyond the end of potential core, especially at low actuator duty cycles.

Infra-red signature reduction study on a small-scale jet engine

AbstractThis paper presents infra-red signature data for a small-scale, low pressure ratio turbojet engine typical of that used in unmanned air vehicle applications. The aim of the study was to test a number of different convergent nozzle designs concentrating on those with trailing edge modifications. The engine used in the tests has a single stage centrifugal compressor and radial inflow turbine and is designed to produce approximately 150N of thrust at 103,500rpm using liquid propane fuel. The test rig consisted of a calibrated thrust stand whilst the engine was controlled through an electronic engine control unit and laptop PC. The jet plume was visualised using an infra-red spectroradiometer which yielded qualitative data across the infra-red spectrum. Simultaneous measurements were also made of the engine thrust. A Pitot probe was used to take pressure readings across different sections of the exhaust flow. Analysis of the infrared signature of the engine exhaust plume and any thrust penalty yielded a performance comparison for each of the nozzles tested. Correlation of engine thrust with engine rpm showed that, within the accuracy of the measurements, there was no significant thrust penalty associated with the notched nozzles. Infra-red imagery of the plain and 60° notched nozzles indicated that the latter reduced the length of the hottest part of the exhaust plume by approximately 33%. The spectroradiometer data shows a significant reduction in spectral radiance for the CO2 wavelength of approximately 4·3µm when the notched nozzles are used. The 60° notched nozzle appeared to perform best in reducing the spectral radiance at this wavelength. Centreline total pressure measurements in the exhaust plume correlated well with the infra-red imagery in that a potential core length reduction of up to 30% could be achieved using the 60° notched nozzle. Total pressure contours recorded 20mm (0·43D) downstream of the nozzle exit plane suggest that the notched nozzles are promoting increased mixing through radial spreading of the jet possibly associated with increased streamwise vorticity (although the latter could not be confirmed). There were also signs that the jet plumes being investigated were swirling.