Parviz Behrouzi

Effect of tab parameters on near-field jet plume development

A full understanding of jet mixing behavior is essential in the aircraft design process, particularly when propulsion system integration issues are considered (for example, jet afterbody interactions, jet plume characterization, and jet noise reduction). The present work is motivated by an interest in unconventional jet exhaust nozzle design, specifically tabbed nozzles. The near-field mixing performance arising from a simple axisymetric jet shear layer and three-dimensional perturbed jet shear layers created via a range of solid tab designs introduced at the nozzle exit plane has been studied under subsonic and supersonic operating conditions. The effects of velocity ratio, tab shape, tab number, and tab orientation angle are investigated. Flow visualization of the tab effects is accomplished via laser-induced fluorescence in low-speed flow and schlieren imaging under supersonic conditions. The mean and rms axial velocity as well as pitot pressure and total temperature profiles have been measured along the jet centerline and on orthogonal cruciform radial traverse lines downstream of the nozzle exit. The performance of the solid tab in causing bifurcation of the jet was found to follow the same trend under both subsonic and supersonic conditions, indicating that the dominant features of the streamwise vorticity introduced by the tabs are essentially independent of the Mach number. The experimental results revealed that the decay of the jet core velocity was only weakly dependent on velocity ratio (over the range studied here), tab orientation angle, and tab shape. The mixing of the jet was, however, a strong function of the tab projected area, tab width, and tab number. The optimum tab number was found to be 2.

Jet Mixing Enhancement Using Fluid Tabs

Techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been studied in the present work both experimentally and computationally. Near-field jet development from a “clean” axisymmetric nozzle is used as a datum against which to judge mixing effectiveness. A combination of water tunnel and high-speed airflow facilities are used to assess the near-field jet behaviour experimentally. Classical solid tabs and “fluid” tabs (i.e. discrete radially discharging jets at nozzle exit) are examined. Both measurements and CFD calculations demonstrate that fluid tabs reproduce the same streamwise vortex formation process as solid tabs. For a tab jet flow rate of around 1% of the core nozzle flow, similar potential core length reductions and increased centre-line velocity decay rates are observed in the near field. Since fluid tabs can be switched off when not needed, hence avoiding associated drag/thrust loss penalties, this technology is a promising technique worthy of further development.

Laser Doppler velocimetry measurements of twin-jet impingement flow for validation of computational models

Abstract Jets discharged traversely into a cross-flow appear as a flow element in many propulsion related systems such as short take-off and vertical landing (STOVL) aircraft. The purpose of this paper is to report on a recent experimental study of a closely spaced pair of jets which may be of the same or different jet velocities and which interact with each other, with a cross-flow and with an opposite solid wall. In this paper an experimental facility, used to gather validation data suitable for testing computational fluids dynamics (CFD) model predictions of multi-jet ground impingement flows, is described. Water is used as the working fluid and laser Doppler velocimetry (LDV) measurements of twin impinging jets are reported, both with and without a cross-flowing stream. Emphasis is placed on the presentation of the mean and r.m.s. velocity contours in the fountain formation region between the jets. The effect of jet imbalance and velocity ratio were studied. Preliminary CFD predictions of the flow using a k – e turbulence model are presented. An overview is provided to guide users through the data and highlight several important flow features which have already emerged from the data and which are considered of importance to CFD model validation studies.

Active flow control of jet mixing using steady and pulsed fluid tabs

Flow control techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been investigated. A combination of water tunnel and high-speed airflow facilities is used to assess the near-field jet behaviour. Solid tabs, steady fluid tabs (i.e. discrete radially discharged control jets located close to the core jet exit), and pulsed fluid tabs are compared. The effect of fluid tab velocity amplitude, pulse rate, and pulse phase are studied using open-loop control. The measurements indicate that fluid tabs generate a similar streamwise vortex formation process (and hence display increased mixing) as previously observed in solid-tabbed nozzle flows. In incompressible testing the mixing effectiveness with a pair of pulsed fluid tabs 180° out-of-phase was as good as a twin solid tab nozzle for a control jet flowrate of only 0.5 per cent of the primary (core) jet flow. In preliminary high-speed testing similar benefits of fluid tabs over solid tabs were observed. Further study of pulsed fluid tabs is recommended; they have the attractive performance benefit that they can be easily switched off when not needed and offer increased flexibility as the basis of an optimized active control jet mixing device.

Underexpanded Jet Development from a Rectangular Nozzle with Aft-Deck

An experimental study is reported of underexpanded supersonic jet plumes issuing from a high-aspect-ratio convergent rectangular nozzle. Schlieren visualization, Pitot probe, and Laser Doppler Anemometry measurements are carried out to capture the plume development in the near field, and in particular the effect on the plume flow of a finite-length extended shelf or aft-deck attached to the lower nozzle wall. This creates asymmetry in the inviscid shock cell pattern and the entrainment characteristics, both of which influence shear-layer growth and plume trajectory. A net pressure force is induced on the aft-deck wall, which leads to transverse deflection of the jet plume once it leaves the aft-deck, both upward and downward, depending on aft-deck length and nozzle pressure ratio. For sufficiently high nozzle pressure ratio and a sufficiently long aft-deck, separation and reattachment of the plume from the aft-deck is observed. Detailed mapping of both mean velocity and turbulence in the plume near field ...

Effect of Tabs on Rectangular Jet Plume Development

The near-field mixing performance of a rectangular jet with and without solid tabs introduced at the nozzle lip was studied under supersonic (underexpanded) operating conditions. The effects of tab shape, tab location, and tab number were investigated. Flow visualization of the jet plume was accomplished via Schlieren imaging, and the velocity and turbulence fields quantified via laser Doppler anemometry measurements. Laser Doppler anemometry data were gathered along the jet centerline and on cruciform transverse and spanwise traverse lines downstream of the nozzle exit. Tab shape has only a minor effect, however tab location and number are sensitive parameters for jet plume development. Tabs located on the nozzle wide edge can lead to jet bifurcation, whereas tabs located on the narrow edge cause increased mixing primarily in the major axis direction. A nozzle with four tabs located on its wide edges was the most effective for overall enhancement of jet mixing.

Flow Control of Jet Mixing Using a Pulsed Fluid Tab Nozzle

Flow control techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been investigated experimentally. The near-field jet development of a clean axisymmetric nozzle is used as a datum to judge increased mixing effectiveness. A combination of water tunnel and high-speed airflow facilities are used to assess the nearfield jet behaviour. Classical solid tab, fluid tab (i.e. discrete radially discharging control jets located close to the core jet nozzle exit), and pulsed fluid tab nozzles are compared. The effect of the fluid tab flow rate, pulse rate and pulse phase are studied. The measurements indicate that nozzles equipped with fluid tabs and pulsed fluid tabs generate a similar streamwise vortex formation process (and hence display increased mixing) as occurs in a solidtabbed nozzle. The performance of pulsed fluid tabs is strongly influenced by the tab flow rate, pulse frequency and phase. In low speed tests the mixing effectiveness with a pair of pulsed fluid tabs working 180 o out of phase was as good as a twin solid tab nozzle for a total control jet flow rate of only 0.5% of the core jet flow. In high speed testing similar benefits of fluid tab control jets over solid tab designs were observed, although evidence was obtained that the interaction mechanisms between the streamwise vortices which produce the increased mixing across the core jet shear layer may be subtly different under low speed and compressible conditions. Further study of pulsed fluid tabs is recommended, since they have attractive performance benefits over other rapid jet mixing control mechanisms (can be easily switched off when not needed), and offer attractive flexibility as the basis for an optimum active control mixing device.

Computational fluid dynamics prediction of intake ingestion relevant to short take-off and vertical landing aircraft:

Abstract Intake ingestion can cause several major problems (e.g. compressor surge and stall) for short take-off and vertical landing (STOVL) aircraft operating in ground effect. Numerical predictions of the flowfield associated with a generic twin-jet plus intake model operating under ingestion flow conditions are reported using computational fluid dynamics (CFD) techniques. The results have been compared with laser Doppler anemometry (LDA) validation measurements taken in a specially designed test case configuration. The k-ε turbulence model and both first-order and second-order (QUICK) convection discretization schemes were employed. Fine meshes and second-order accurate discretization were found essential to produce solutions close to grid independence. A reasonable prediction of the general flow pattern has been achieved. Several features of the mean velocity field were close to the experimental results; however, the k-ε model was shown to produce significant errors in the prediction of the forward penetration distance of the ground sheet flow and in the shape of velocity profiles and turbulence levels near to the intake.

Numerical studies of twin‐jet impingement for STOVL flow application

Abstract The behaviour of turbulent jet impingement is of considerable importance to the performance of STOVL (Short Take‐Off and Vertical Landing) aircraft. Theoretical analyses of the flow field produced by jet impingement on a ground surface are formulated in terms of solving the full Reynolds‐Averaged Navier‐Stokes (RANS) equations. This paper presents predictions of a twin‐jet impingement on ground plane flow using the standard two‐equation k‐ϵ turbulence model. Predictions were performed for two impingement heights and two jet spacings. An examination of the effect of switching from first‐order (Hybrid) to second‐order (Quick) convection discretization was also studied. The results presented are sensibly independent of numerical influence and represent a true assessment of the performance of the turbulence model. The predictions were compared with LDV (Laser Doppler Velocimetry) experimental results. The main features of the flow field were correctly predicted. The fountain formation region was qualitatively predicted but quantitative under‐prediction of fountain development characteristics was observed to be around 50%. This is probably due to fountain unsteadiness, which is not included in the steady state CFD predictions presented here.

Effect of an Extended Shelf on Near Field Development of Supersonic Jet Plumes from Rectangular Nozzles

An experimental study is reported of underexpanded supersonic jet plumes issuing from a high aspect ratio rectangular nozzle. Schlieren visualisation, Pitot probe, and LDA measurements are made to capture the plume development in the near-field, and in particular the effect on the plume flow of the presence of a finite-length extended shelf (or aft-deck) attached to the lower wall of the nozzle. This creates asymmetry in inviscid shock cell patterns, entrainment, and shear layer growth, and induces a net pressure force on the aft-deck wall, which leads to transverse deflection of the jet plume. For sufficiently high Nozzle Pressure Ratios, and a sufficiently long shelf, separation (and possible reattachment) of the plume from the extended shelf wall is observed. Detailed mapping of both mean velocity and turbulence fields is carried out, enabling comparison of clean nozzle and nozzle with aft-deck near-field flow development to be made. The data provided are proposed as a suitable benchmark validation test case for CFD studies of rectangular nozzle plumes with aft-deck interaction effects.