Large eddy simulation of a complete harrier aircraft in ground effect

This paper aims to demonstrate the viability of using the large eddy simulation (LES) CFD methodology to model a representative, complete STOVL aircraft geometry at touch down. The flowfield beneath such a jet-borne vertical landing aircraft is inherently unsteady. Hence, it is argued in the present work that the LES technique is the most suitable tool to predict both the mean flow and unsteady fluctuations. and, with further development and validation testing, this approach could be a replacement, and certainly a complementary aid, to expensive rig programmes. The numerical method uses a compressible solver on a mixed element unstructured mesh. Examination of instantaneous flowfield predictions from these LES calculations indicate close similarity with many flow features identified from ground effect flow visualisations, which are well known to be difficult to model using RANS-based CFD. Whilst significant further work needs to be carried out, these calculations show that LES could be a practical tool to model, for example, Hot Gas Ingestion for the Joint Strike Fighter aircraft

Measurements in the annular shear layer of high subsonic and under-expanded round jets

An experimental study has been undertaken to document compressibility effects in the annular shear layers of axisymmetric jets. Comparison is made of the measured flow development with the well-documented influence of compressibility in planar mixing layers. High Reynolds number (~106) and high Mach number jets issuing from a convergent nozzle at nozzle pressure ratios (NPRs) from 1.28 to 3.0 were measured using laser Doppler anemometry instrumentation. Detailed radial profile data are reported, particularly within the potential core region, for mean velocity, turbulence rms, and turbulence shear stress. For supercritical NPRs the presence of the pressure waves in the inviscid shock cell region as the jet expanded back to ambient pressure was found to exert a noticeable effect on shear layer location, causing this to shift radially outwards at high supercritical NPR conditions. After a boundary layer to free shear layer transition zone, the turbulence development displayed a short region of similarity before adjustment to near-field merged jet behaviour. Peak turbulence rms reduction due to compressibility was similar to that observed in planar layers with radial rms suppression much stronger than axial. Comparison of the compressibility-modified annular shear layer growth rate with planar shear layer data on the basis of the convective Mach number (M C) showed notable differences; in the annular shear layer, compressibility effects began at lower M C and displayed a stronger reduction in growth. For high Mach number aerospace propulsion applications involving round jets, the current measurements represent a new data set for the calibration/validation of compressibility-affected turbulence models

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 has been carried out, enabling comparison of flow behavior for a clean nozzle and a nozzle with aft-deck. The data provided are proposed as a suitable benchmark validation test case for computational fluid dynamics studies of rectangular nozzle plumes with aft-deck interaction effects

Stereo-PIV measurements of spatio-temporal turbulence correlations in an axisymmetric jet

Stereoscopic three-component particle image velocimetry (3C-PIV) measurements have been made in a turbulent round jet to investigate the spatio-temporal correlations that are the origin of aerodynamic noise. Restricting attention to subsonic, isothermal jets, measurements were taken in a water flow experiment where, for the same Reynolds number and nozzle size, the shortest time scale of the dynamically important turbulent structures is more than an order of magnitude greater that in equivalent airflow experiments, greatly facilitating time-resolved PIV measurements. Results obtained (for a jet nozzle diameter and velocity of 40 mm and 1 m s1, giving Re D 4 104) show that, on the basis of both single-point statistics and two-point quantities (correlation functions, integral length scales) the present incompressible flow data are in excellent agreement with published compressible, subsonic airflow measurements. The 3C-PIV data are first compared to higher-spatial-resolution 2C-PIV data and observed to be in good agreement, although some deterioration in quality for higher-order correlations caused by high-frequency noise in the 3C-PIV data is noted. A filter method to correct for this is proposed, based on proper orthogonal decomposition (POD) of the 3C-PIV data. The corrected data are then used to construct correlation maps at the second- and fourth-order level for all velocity components. The present data are in accordance with existing hot-wire measurements, but provide significantly more detailed information on correlation components than has previously been available. The measured relative magnitudes of various components of the two-point fourth-order turbulence correlation coefficient (Rij;kl) – the fundamental building block for free shear flow aerodynamic noise sources – are presented and represent a valuable source of validation data for acoustic source modelling. The relationship between fourth-order and second-order velocity correlations is also examined, based on an assumption of a quasi-Gaussian nearly normal p.d.f. for the velocity fluctuations. The present results indicate that this approximation shows reasonable agreement for the measured relative magnitudes of several correlation components; however, areas of discrepancy are identified, indicating the need for work on alternative models such as the shell turbulence concept of Afsar (Eur. J. Mech. (B/Fluids), vol. 31, 2012, pp. 129–139)

Effect of scarfing on rectangular nozzle supersonic jet plume flow characteristics

An experimental and computational fluid dynamics study is reported of supersonic jets issuing from a high-aspect-ratio rectangular convergent–divergent nozzle with and without a scarfed exit. Schlieren visualization and laser Doppler anemometry measurements captured near-field aerodynamic development of an unheated jet at overexpanded, design, and underexpanded conditions. Reynolds-averaged Navier–Stokes computational fluid dynamics predictions using an eddy viscosity closure (Spalart–Allmaras model) for clean and scarfed geometries were compared with measurements to examine the ability to capture nozzle scarfing effects. The measured plume shape for a scarfed nozzle was strongly affected at overexpanded conditions (a distorted four-lobe shape was observed), whereas a rectangular shape was retained for underexpanded flow although plume bifurcation occurred. The development of the plume shape and the mixing rate was a consequence of the strong vortices that occur with rectangular nozzles, with extra vortices introduced by scarfing. The nozzle exit static pressure changed dramatically when scarfing was added, influencing plume secondary flows and near-field development. The main features of scarfed jet development were predicted qualitatively correctly; the four-lobe overexpanded shape was reproduced but the strength of pressure-driven secondary velocities was overpredicted. The experimental data provided represent a challenging validation test case for computational fluid dynamics studies of three-dimensional supersonic jet plumes with scarfed interaction effects

Evaluation of LES and RANS CFD modelling of multiple steady states in natural ventilation

This paper reports research carried out with the aim of evaluating and comparing the performance of Large Eddy Simulation (LES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) modelling for predicting the multiple steady states observed in experiments on a buoyancy-driven naturally ventilated enclosure. The sub-grid scales of the flow have been modelled using a Van Driest damped Smagorinsky sub-grid scale model in the case of LES and an RNG k-ε turbulence model has been used for URANS. A novel mesh design strategy was introduced to design the LES mesh to identify an optimum 'well-resolved' mesh, assuming that the flow investigated is free-shear dominated. It was found that the URANS solution eventually settled down into a permanent steady state, displaying no evidence of continuing instabilities or periodic unsteadiness. Both URANS and LES solutions captured the existence of three steady states as observed in experimental studies. However, LES was more accurate in predicting the temperatures inside the enclosure compared to URANS. In the URANS solutions, it was observed that for smaller lower opening areas the average indoor temperature had noticeable discrepancies when compared with experimental results. Unlike URANS, LES correctly predicted different steady state temperatures for different opening areas and the time to reach steady state agreed closely with theoretical predictions

LES of turbulent liquid jet primary breakup in turbulent coaxial air flow

A robust two-phase flow Large Eddy Simulation (LES) algorithm has been developed and applied to predict the primary breakup of an axisymmetric water jet injected into a surrounding coaxial air flow. The high liquid/gas density and viscosity ratios are known to represent a significant challenge in numerical modelling of the primary breakup process. In the current LES methodology, an extrapolated liquid velocity field was used to minimise discretisation errors, whilst maintaining sharp treatment of fluid properties across the interface. The proposed numerical approach showed excellent robustness and high accuracy in predicting coaxial liquid jet primary breakup. Since strong turbulence structures will develop inside the injector at high Reynolds numbers and affect the subsequent primary breakup, the Rescaling and Recycling Method (RM) was implemented to facilitate generation of appropriate unsteady LES inlet conditions for both phases. The influence of inflowing liquid and gas turbulent structures on the initial interface instability was investigated. It is shown that liquid turbulent eddies play the dominant role in the initial development of liquid jet surface disturbance and distortion for the flow conditions considered. When turbulent inflows were specified by the RM technique, the predicted core breakup lengths at different air/water velocities agreed closely with experimental data. © 2013 The Authors

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

Modelling buoyant thermal plumes in naturally ventilated buildings

The aim of the work reported in this paper was to evaluate the performance of Large Eddy Simulation (LES) for modelling natural ventilation driven by twin plumes. The flow is characterised by an interface height which separates the warm buoyant air above from the cooler air below, and a merging height for coalescence of the two plumes. Comparison between the LES predictions and theory for the interface height and volume flow rate in the merged plumes is good, giving confidence that LES has potential for modelling this important class of flows

An LES turbulent inflow generator using a recycling and rescaling method

The present paper describes a recycling and rescaling method for generating turbulent inflow conditions for Large Eddy Simulation. The method is first validated by simulating a turbulent boundary layer and a turbulent mixing layer. It is demonstrated that, with input specification of mean velocities and turbulence rms levels (normal stresses) only, it can produce realistic and self-consistent turbulence structures. Comparison of shear stress and integral length scale indicates the success of the method in generating turbulent 1-point and 2-point correlations not specified in the input data. With the turbulent inlet conditions generated by this method, the growth rate of the turbulent boundary/mixing layer is properly predicted. Furthermore, the method can be used for the more complex inlet boundary flow types commonly found in industrial applications, which is demonstrated by generating non-equilibrium turbulent inflow and spanwise inhomogeneous inflow. As a final illustration of the benefits brought by this approach, a droplet-laden mixing layer is simulated. The dispersion of droplets in the near-field immediately downstream of the splitter plate trailing edge where the turbulent mixing layer begins is accurately reproduced due to the realistic turbulent structures captured by the recycling/rescaling method