A. J. Saddington

A review of jet mixing enhancement for aircraft propulsion applications

Abstract This article reviews techniques applicable to enhancing the mixing of jets, with particular emphasis on infrared (IR) signature reduction of high-speed jets. Following a brief introduction to the IR signature of jet plumes and the fundamentals of jet mixing, this paper discusses rapid mixing technologies under the categories of: geometric modifications (to the nozzle); high shear stress mixing; normal stress mixing; self-acoustic excitation; external acoustic excitation; mechanically oscillated; self-oscillated. It is shown that mixing enhancements of the order of 100 per cent are possible with some techniques and that by combining techniques this can be increased by at least as much again. Simple geometric calculations are presented which demonstrate that with rectangular nozzles such high levels of mixing enhancement may be necessary in order to reduce IR signature. Some apparent rapid mixing technologies, however, have been shown to increase jet spreading without increasing entrainment, whereas other techniques can reduce entrainment as easily as they can increase it.

An experimental and numerical investigation of under-expanded turbulent jets

The work described here concentrates on under-expanded, axisymmetric turbulent jets issuing into quiescent conditions. Under-expanded turbulent jets are applicable to most aircraft propulsion applications that use convergent nozzles. Experimental studies used laser doppler velocimetry (LDV) and pitot probe measurements along the jet centreline. These measurements were made for two nozzle pressure ratios (2.5 and 4.0) and at various streamwise positions up to 10 nozzle diameters downstream of the nozzle exit plane. A computational fluid dynamics (CFD) model was developed using the Fluent code and utilised the RNG K-e two-equation turbulence model. A mesh resolution of approximately one hundredth of nozzle exit diameter was found to be sufficient to establish a mesh independent solution. Comparison of the jet centreline axial velocity (LDV data) and pressure ratio (pitot probe data) showed good agreement with the CFD model. The correct number of shock cells had been predicted and the shock strength agreed well between the experiments and numerical model

Planar visualization of vortical flows

Abstract This article presents two over-looked post-processing techniques which provide the investigator with additional tools for data analysis and visualization. Both techniques exploit the trend for planar experimental data collection and are implemented in two-dimensions. Critically, both techniques are suitable for use on computational and experimental datasets, require no a-priori knowledge of the flow-field, and minimal user interaction during processing. Firstly, line integral convolution will be introduced as an alternative to streamline or in-plane velocity vector visualization. Secondly, a feature identification procedure will be outlined that can be used to reduce datasets for clearer visualization and provide quantitative information about topological flow features.

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

We present 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 inflows 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

Flow Visualization and Measurements in a Short Take-off, Vertical Landing Fountain Flow

The interaction of multiple jets with the ground is of great importance for the design and operation of short take-off, vertical landing aircraft. The fountain upwash flow, generated by the impingement of two axisymmetric, compressible, turbulent jets onto a ground plane, was studied using laser-based flow visualization and particle image velocimetry. Measurements were made with nozzle pressure ratios of between 1.05 and 4 and nozzle height-to-diameter ratios of between 2.4 and 8.4, whilst nozzle spacing remained fixed at seven diameters. Mean fountain vertical velocity magnitude was observed to decrease with increasing nozzle height. Mean fountain upwash velocity profiles were found to be self-similar for all test conditions. A distinct frequency of fountain oscillation was identified at a nozzle height of 4.4 diameters.

Unsteady Features of Twin-Jet STOVL Ground Effects

The flowfield resulting from the impingement of a two parallel axisymmetric jets on a perpendicular flat plate, simulating the operation of a short take-off and vertical-landing (STOVL) aircraft, has been studied experimentally using high-speed flow visualisation. Experiments were conducted using air as the working medium, with the plane connecting the nozzle centrelines illuminated by a sheet of light obtained by spreading a laser beam. Configurations with H/D of 5 and 7 revealed the presence of largescale structures, oscillatory recirculation zones and a high interaction of the main jets with the fountain, although no specific fountain pattern seems to dominate the flow.

Effects of Scaling on High Subsonic Cavity Flow Oscillations and Control

The effects of scaling on cavity oscillations and control have been studied by measuring the unsteady pressure on the floor of three cavities of different scales. The cavities have a length-to-depth ratio of 5 and a length-to-width ratio of 2, and the corresponding linear dimensions are in the ratio 0.5∶1∶2. The experiments were conducted with clean cavities and cavities fitted with leading-edge sawtooth spoilers so as to study the influence of scaling on clean cavities as well as the effectiveness of the passive control method on different sized cavities. The results showed significant variation of certain spectral characteristics of the clean cavities. The control effectiveness of the spoilers also showed variations with a change in scale of the model. It is recommended that, before implementing a passive control device for practical applications, the device should be tested in the possible range of cavity length-to-boundary-layer-thickness ratio (L/δ) that can be experienced in actual flight.

PIV measurements in a twin-jet STOVL fountain flow

Mean velocity and first order turbulence measurements were obtained from a three-dimensional upwash fountain flow generated by the impingement of two compressible axisymmetric turbulent jets onto a normal plane. The jet impingement area and fountain formation regions were examined with data obtained through the use of particle image velocimetry. Seven configurations with different nozzle pressure ratios were considered to ascertain the influence of jet compressibility on the fountain development. Results indicate that the mixing of the fountain is dependent on the nozzle pressure ratio, leading to an increase in the fountain spreading rate with increase in nozzle pressure ratio.

Large-scale instabilities in a STOVL upwash fountain

ABSTRACT The fountain flow created by two underexpanded axisymmetric, turbulent jets impinging on a ground plane was studied through the use of laser-based experimental techniques. Velocity and turbulence data were acquired in the jet and fountain flow regions using laser doppler velocimetry and particle image velocimetry. Profiles of mean and rms velocities along the jet centreline are presented for nozzle pressure ratios of two, three and four. The unsteady nature of the fountain flow was examined and the presence of large-scale coherent structures identified. A spectral analysis of the fountain flow data was performed using the Welch method. The results have relevance to ongoing studies of the fountain flow using large eddy simulation techniques.

MODELLING AND EXPERIMENTS ON UNDEREXPANDED TURBULENT JET MIXING

ABSTRACT This paper presents a study of mixing in underexpanded, turbulent air jets issuing from circular plain and castellated nozzles into quiescent air. Our experimental studies have used probe traverses of total and static pressure and temperature, schlieren flow visualisation and LDA measurements of turbulence properties. These measurements were taken for a range of nozzle pressure ratios (NPR = p 0 /p a ) from 2.0 to 6.5 and at various downstream locations (x) along the jet up to x/D = 18 (where nozzle diameter, D = 29mm or 12mm). CFD studies used the Fluent code (v. 5.5). Five different turbulence models were evaluated for a circular jet at NPR = 3: ‘standard’ k −e, ‘realisable’ k −e, RNG k −e, RSM and Spalart-Allmaras. The RNG k −e model showed the best agreement with our experiments. Using this model the effect of nozzle exit turbulence intensity (Ti) was investigated. For the three values of Ti specified (2.7%, 4.5% and 6.0%), increasing turbulence intensity was found to reduce the peak Mach number in the shock cells. The streamwise spacing of the shock cells was, however, largely unaffected by changes in Ti . The effect of varying NPR was also investigated. Increasing NPR increased the potential core length of the jet from approximately 9 D at NPR = 2.5 to 12 D at NPR = 4. This was due to an increase in the streamwise length of the shock cells; the number of shock cells remained constant at nine in each case. The experimental and computational studies confirmed the mechanism for mixing enhancement in castellated nozzles: differential expansion of the jet fluid in the tooth and gap regions leading to the creation of streamwise vortices. The effect does not seem to persist far downstream.