Ground Vortex Research Articles

Oblique impingement of a circular jet in a cross flow

The distributions of temperature and dye concentration produced by a cross flow jet impinging on a flat surface are examined experimentally. Measurements show that for sufficiently high jet injection speeds, a ground vortex region upstream of the jet develops. The dimensionless gross geometrical features of this recirculating region are related by a simple linear law and are independent of the jet — surface spacing or the injection speed ratio.

Impinging jets in cross-flow

AbstractThe flow-fields associated with single and twin jets impinging in cross-flows have been studied. Parameters which affect the position of the ground vortex have been investigated: cross-flow-to-nozzle velocity ratio, cross-flow boundary layer thickness, nozzle height, nozzle pressure ratio, vector angle and nozzle splay with both fixed and moving ground planes. Results show that the ground vortex moves away from the nozzle centre-line as the ratio of cross-flow velocity to nozzle exit velocity is decreased; the rate of change of position, however, depends on other parameters (including the precise definition of nozzle exit velocity). Increasing nozzle height above the ground appears to cause little consistent variation in vortex position for a single jet but a marked forward movement of ground sheet separation point in the case of twin jets. For all cases there is an increase in penetration with increasing nozzle pressure ratio up to choking, with the subsequent behaviour dependent on the definition of nozzle equivalent velocity and cross-flow velocity ratio. The effect of the moving ground plane is to reduce vortex penetration significantly; this suggests that a moving ground plane simulation (or moving model) is essential when testing design configurations in ground effect in wind-tunnels. It is also shown that rig design can produce a blockage effect which moves the ground vortex significantly and can change other apparent parametric effects. Self-similarity laws are proposed for the ground vortex and the wall jet.

Computation of normal impinging jets in cross‐flow and comparison with experiment

AbstractThe PHOENICS code has been used to model the flow field surrounding subsonic and underexpanded jets impinging on a ground plane in the presence of a cross‐flow, for cases with both a fixed ground plane and a ‘rolling road’. The standard k‐ε turbulence model is used, without correction factors. It is confirmed that this overpredicts the free jet entrainment rate; the wall jet spreading rate is slightly underpredicted but the initial thickness is too high. Agreement with experiment is, nevertheless, much better than for previous calculations, showing the importance of the extent of the grid used. The ground vortex formed in cross‐flow is shown to move with varying effective velocity ratio and with rolling road operation in the same manner as experimentally observed. Ground vortex self‐similarity is also accurately predicted with the numerical modelling.

Recent Research into the Aerodynamics of ASTOVL Aircraft in Ground Environment

A brief survey is presented of the aerodynamic phenomena associated with jet-lift aircraft in ground effect. The particular problems of modelling the ground vortex formed by impinging jets in cross-flows are then described, in the light of the authors' own experimental studies. Finally, areas of these flow fields exhibiting stochastic properties are highlighted.

Experiments on the unsteadiness associated with a ground vortex

The ground vortex formed by a jet impinging on the ground in the presence of a crossflow has been studied experimentally. High speed motion pictures and spectral measurements were obtained to study the unsteady features of this flowfield. A very low-frequency pulsation or 'puffing' instability was observed. Since this unsteadiness could not be correlated with any other oscillations in the flowfield, the low-frequency oscillations must come from the gross features of the ground vortex itself. Namely, jet fluid accumulates in the ground vortex until the vortex is so large that the flowfield breaks up, the ground vortex is swept away, a new smaller vortex forms, and the process repeats itself. Measurements of the frequency of these oscillations are presented for the first time, and data on the vertical extent (height) of the ground vortex are also shown.

Simulation of a delta wing with two jets in ground effect

Numerical simulations of the flow about a 60° delta planform wing, equipped with two thrust reverser jets, in ground effect are presented. These simulations are based on the solution of the three-dimensional thin-layer Navier-Stokes equations on a body-conforming, overset, multiple grid topology. An implicit, two-factor, numerical scheme with flux-splitting in the streamwise direction is used. The first set of simulations involves the validation of the numerical scheme by studying the impact of numerical parameters (e.g. grid refinement and dissipation levels) on the solution. The second set of simulations is performed to understand the effect of flow-field parameters (such as the height of the delta wing above the ground plane, the jet size, and the jet exit temperature) on the solution. Flow features such as jet deformation and ground vortex formation observed in experimental flow visualizations are captured. Comparisons with experimental data confirm the ability to simulate the loss of wing-borne lift, commonly referred to as “suck-down,” as the delta planform flies at slow speeds in close proximity to the ground. Detailed analysis of the numerical results has given insight into the structure of the ground vortex and the mechanisms of lift loss.

Experimental investigation of a jet impinging on a ground plane in crossflow

An experimental investigation has been conducted in a wind tunnel to model the impingement of high-velocity jet exhaust flow on the ground, as encountered by V/STOL aircraft. A constant jet velocity was maintained while varying the wind tunnel crossflow velocity, upstream boundary-layer thickness, and height from the ground to the jet exit plane. The radial wall jet, when interacting with the crossflow, forms an oscillating horseshoe-shaped separation bubble, commonly referred to in the literature as a ground vortex. The streamwise distance of the separation point from the jet impingement point is documented here as a function of the flow parameters and geometry. Flow visualization of the flowfield and two-component laser Doppler velocimeter measurements taken through the separation bubble indicate that the separation bubble is highly unsteady and nonsymmetric. This unsteadiness may be related to shear-layer vortices shed from the lip of the jet. Thickening of the upstream boundary layer on the ground plane caused the wall jet to penetrate further upstream. The addition of a large plate flush-mounted to the jet exit caused the ground vortex to move downstream and also decreased the size of the ground vortex.

Rotor Aerodynamics in Ground Effect at Low Advance Ratios

Abstract : The results of an experimental study of the aerodynamic characteristics of a helicopter rotor operating in ground effect at low advance ratios are presented. Flow visualization studies were conducted along with measurement of the forces and moments acting on the rotor as a function of advance ratio, height above ground and collective pitch. Steady state experiments as well as non-steady experiments involving translational acceleration were conducted. Three distinct flow regimes were noted from the flow visualization studies. At the low end of the advance ratio range a recirculating flow was present, at intermediate advance ratios a horseshoe shaped vorted formed under the rotor, and at the high end of the advance ratio range studied, the rotor wake flows entirely downstream. At test conditions were the recirculating flow or ground vortex is present there are marked departures from classical ground effect theory. Translational acceleration was found to have a significant effect on the rotor forces and moments indicating that some appreciable time is required for the establishment of the ground effect flow field when a recirculating flow or ground vortex exists in the steady state. The results indicate that magnitude of the ground effects experienced by a helicopter are sensitive to the flight path of the helicopter. (Author)

The static pressure in a vortex core

Vortices are of interest in many situations in fluid mechanics. One type of vortex with which the gas turbine designer is concerned is the line vortex springing like a miniature tornado from a point on the ground (or from a nearby aircraft surface) and entering the engine intake. It is possible for these vortices to suck stones up into the engine (Refs. 1, 2) and also to reduce the compressor surge margin (Refs. 3, 4). They are unsteady, in that their point of origin wanders about and so does their point of entry into the intake. This unsteadiness makes it very difficult to measure them, and so far no one known to the author has reported measurements of ground vortex strength, in the course of several investigations of the phenomenon (Refs. 1-3, 5-8).

Behavior of a Subsonic Turbulent Slot Jet in Crossflow

References 1 Heyson, H.H., Linearized Theory of Wind Tunnel JetBoundary Corrections and the Ground Effect for VTOL-STOL Aircraft, NASATRR-124, 1962. 2 Sullivan, M.C., An Investigation of a Ground Vortex Formed by the Interaction of a Rotor Wake With Free Air in the Presence of a Groundplane, M.S. Thesis, Dept. of Aero, and Astro., University of Washington, Seattle, Wash., 1972. South, P., Measurements of Flow Breakdown in Rectangular Wind Tunnel Working Sections, National Research Council of Canada, Ottawa, Canada, LR513, 1968. Turner, T., A Moving-Belt Ground Plane for Wind Tunnel Ground Simulation and Results for 2 Jet-Flap Configurations, NASATND-4228, 1967.

Fan Inlet Disturbances and Their Effect on Static Acoustic Data

There is evidence that measurements of fan-rotor inlet noise taken during static test situations are at variance with aircraft engine flight data. In particular, static tests generally yield a significantly higher tone at blade passing frequency than that measured during flight. An experimental program was carried out to investigate this discrepancy. Inlet ground vortices and large-scale inlet turbulence were generated intentionally in an anechoic test chamber. Far-field acoustic measurements and inlet flow field hot film mappings of a fan rotor were then carried out to study the influence of such inlet disturbances at a static test facility. Experimental results indicate that the acoustic effect of such disturbances appears to be less severe for supersonic than for subsonic tip speeds. Further, a reverse flow that occurs on the exterior cowl in static test facilities appears to be an additional prime candidate for creating inlet disturbances and causing variance between flight and static acoustic data.

Ground Vortex–Limit to Engine/Reverser Operation

It has been commonly accepted that engine stalls during reverser operation are caused by the temperature distortion created by reingested exhaust gasses. Recent studies have shown that compressors respond to instantaneous inlet distortions of a time duration on the order of one engine revolution. This paper describes the results of a test of a high bypass ratio engine during reverse thrust operation which revealed, through the use of high response temperature and pressure measurement, that instantaneous distortions caused by an induced ground vortex were the cause of engine stalls.

Closure to “Discussion of ‘Ground Vortex–Limit to Engine/Reverser Operation’” (1976, ASME J. Eng. Power, 98, pp. 263–264)

Closure to “Discussion of ‘Ground Vortex–Limit to Engine/Reverser Operation’” (1976, ASME J. Eng. Power, 98, pp. 263–264)

Effects of nonuniform inflow on fan noise

A prominant source of turbomachinery noise is due to high-speed unsteady flows interacting with blades and vanes. In particular, atmospheric turbulence, cross-wind effects, and the “ground vortex” apparent during most static test conditions can interact with the fan on turbofan engines to produce significant levels of noise. An analysis is presented that models this noise-generating mechanism and is capable of predicting propagating noise, accepting as input various inlet turbulence structures, temperature fluctuations, and flow distortions. Because of the random uature of most inflow disturbances, a statistical approach was used to develop the model which is a modification and extension of a previous analysis presented by Mani [“Noise Due to Interaction of Inlet Turbulence with Isolated Stators and Rotors,” J.S.V. 19 (1971)]. The analysis shows that the level of fan blade passing tone and harmonics and their relative forward and rearward radiated intensities are critically dependent on both the inlet turbulence intensity and correlation length scales in the axial, circumferential, and spanwise directions. This analysis is used in conjunction with measured fan noise data to infer the turbulence properties that would be required to account for levels of observed blade passage tone noise.

Numerical Investigation of Parameters Affecting the Thermal and Hydrodynamic Characteristics of Impinging Jets in Cross Flow

In this investigation the hydrodynamic and the thermal fields due to a single impinging jet in cross-flow have been investigated numerically, using a 2-D axisymmetric model in order to predict the ground vortex characteristics. The parameters investigated include the effective velocity ratio, the nozzle height, the nozzle pressure ratio, the intake location, the intake mass flow rate and the jet temperature ratio. It is interesting to note that even with the 2-D modeling limitations it was possible to capture most of the thermal and fluid field characteristics of the ground vortex. It was found that the temperature distribution in the flow field is greatly affected by the effective velocity, and the maximum penetration point of the ground vortex is equal to the hot gas penetration. The ground vortex strength increases slightly with increasing the intake mass flow rate but has a minor effect on the ground vortex geometry and on the penetration of the hot gases. The intake location has a significant effect on the ground vortex strength when it is located upstream of the ground vortex core. Key Words: Numerical investigation, Turbulence Models, Impinging jets, Cross-flow.

Ground, Symmetric and Central Vortex States in Rotating Bose-Einstein Condensates

We study ground, symmetric and central vortex states, as well as their energy and chemical potential diagrams, in rotating Bose-Einstein condensates (BEC) analytically and numer- ically. We start from the three-dimensional (3D) Gross-Pitaevskii equation (GPE) with an angular momentum rotation term, scale it to obtain a four-parameter model, reduce it to a 2D GPE in the limiting regime of strong anisotropic conflnement and present its semiclassical scaling and geometrical optics. We discuss the existence/nonexistence problem for ground states (depending on the angular velocity) and flnd that symmetric and central vortex states are independent of the angular rotational momentum. We perform numerical experiments computing these states using a continuous normal- ized gradient ∞ow (CNGF) method with a backward Euler flnite difierence (BEFD) discretization. Ground, symmetric and central vortex states, as well as their energy conflgurations, are reported in 2D and 3D for a rotating BEC. Through our numerical study, we flnd various conflgurations with several vortices in both 2D and 3D structures, energy asymptotics in some limiting regimes and ratios between energies of difierent states in a strong replusive interaction regime. Finally we report the critical angular velocity at which the ground state loses symmetry, numerical veriflcation of dimension reduction from 3D to 2D, errors for the Thomas-Fermi approximation, and spourous numerical ground states when the rotation speed is larger than the minimal trapping frequency in the xy plane.