Mark V. Finnis

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.

Aerodynamic characteristics of a wing-and-flap configuration in ground effect and yaw

The influence of the yaw angle on a model representative of a monoposto racing car’s front wing and nose section operating in close proximity to the ground is discussed. The yawed condition is representative of a car operating in a crosswind or with side-slip while cornering. Because of the need for downforce in corners rather than on a straight, it is standard practice to test a racing car at various orientations of yaw, pitch and roll quasi-statically. Wind tunnel testing with a 50%-scale model at a unit Reynolds number of 1.69 × 106 was used to investigate the forces and the surface flow structures. The results were then used to validate simulations with the three-equation k–kL–ω transitional turbulence model to observe the surface pressures and the wake structures. It was found that a change in the surface pressure caused asymmetric loading of the wing, the strengthening or inhibiting of vortices depending on their rotational sense and an overall reduction in both the downforce and the drag of the wing; all these were amplified as the yaw angle was increased or the ground clearance reduced. The fundamental aerodynamic flow features of a racing car’s front wing operating at yaw are established.

Density measurements for rectangular free jets using background-oriented schlieren

An experimental study incorporating the use of the Background-Oriented Schlieren (BOS) technique was performed to measure the density field of a rectangular supersonic jet. This technique is easier to set up than conventional schlieren since the optical alignment involving the various mirrors, lenses and knife-edge is replaced by a background pattern and a single digital camera. The acquired images which contain information of density gradients in the flow are solved as a Poisson equation and further processed using deconvolution and tomographic algorithms to generate a 3D domain which contains information about the actual density. 2D slices can then be extracted to quantitatively visualise the density along any required planes. The results from supersonic axisymmetric jets are used for validation of the code; these show excellent agreement with pre-validated CFD data. The results for a rectangular supersonic jet are then obtained. These show good agreement with the CFD data, in terms of shock-cell spacing and overall structure of the jet. The technique has proved useful for investigating axis-switching, a phenomenon generally associated with non-axisymmetric jets.

Force and moment measurements for a generic car model in proximity to a side wall

Force and moment data are presented for an Ahmed reference model with backlight angles of 10°, 25° and 40° at various distances from a side wall. Tests were run at a freestream velocity of 25 m/s and a rolling road provided ground simulation. Six-component force data were recorded and compared with previous experimental data. It is found that the proximity of the side wall causes a general increase in both the lift coefficient and the pitching moment for the Ahmed model, with the increase in the pitching moment becoming more rapid with decreasing wall distance. Increasing the proximity to the wall is further found to increase the overall Ahmed model drag. Furthermore, there is evidence of the breakdown of longitudinal vortices on the near-wall side of the model as the wall-to-model distance decreases, and a large pressure drop on the near-wall model side. This pressure drop increases in magnitude as the wall-to-model distance decreases, before dissipating at wall distances where the boundary layer restricts the flow.

Combined Stereoscopic Particle Image Velocimetry and Line Integral Convolution Methods: Application to a Sphere Sedimenting Near a Wall in a Non-Newtonian Fluid

The flow fields for a sphere sedimenting through a Newtonian and two non-Newtonian liquids near a wall in a square tank are investigated using 3-D stereoscopic particle image velocimetry (PIV) and line integral convolution (LIC) methods. The PIV data were taken using an angular stereoscopic configuration with tilt and shift arrangements for the Scheimpflug condition and a pair of liquid correction prisms. Data were recorded from planes perpendicular and parallel to the wall for each fluid case over a range of distances from the wall. The PIV and LIC results highlight significant differences in the wake structure for all three cases. Out of plane flow was also found to persist up to two sphere diameters downstream in the wake for all cases.

Forcing Boundary-Layer Transition on a Single-Element Wing in Ground Effect

The transition from a laminar to turbulent boundary layer on a wing operating at low Reynolds numbers can have a large effect on its aerodynamic performance. For a wing operating in ground effect, where very low pressures and large pressure gradients are common, the effect is even greater. A study was conducted into the effect of forcing boundary-layer transition on the suction surface of an inverted GA(W)-1 section single-element wing in ground effect, which is representative of a racing-car front wing. Transition to a turbulent boundary layer was forced at varying chordwise locations and compared to the free-transition case using experimental and computational methods. Forcing transition caused the laminar separation bubble, which was the unforced transition mechanism, to be eliminated in all cases and trailing-edge separation to occur instead. The aerodynamic forces produced by the wing with trailing-edge separation were shown to be dependent on trip location. As the trip was moved upstream the separation point also moved upstream, this led to an increase in drag and reduction in downforce. In addition to significant changes to the pressure field around the wing, turbulent energy in the wake was considerably reduced by forcing transition. The differences between freeand forced-transition wings were shown to be significant, highlighting the importance of modelling transition for ground-effect wings. Additionally, it has been shown that whilst it is possible to reproduce the force coefficient of a higher Reynolds number case by forcing the boundary layer to a turbulent state, the flow features, both onsurface and off-surface, are not recreated. 1 Aeromechanical Systems Group, Centre for Defence Engineering Cranfield University Defence Academy of the United Kingdom Shrivenham, SN6 8LA, UK Journal of Fluids Engineering 2 FE-16-1336 Roberts

A low profile retrodirective frequency selective surface for radar earth observation

This paper presents the design of a low profile retrodirective surface composed of a frequency selective surface (FSS). The principle of operation of re-radiation and scattering from the array surface is outlined. A full-wave analysis of individual FSS unit cells and the assembled reflect array is performed to better understand and design the reflector. A commensurate array of unit cells with progressive phase shift is realised using non-identical dimension dual resonant circular loop FSSs. A prototype reflect array was manufactured and the monostatic backscattered field measured in horizontal and vertical polarisation. Comparative measurements on similar sized flat plate and dihedral reflector surfaces were made that demonstrate the reflect array performance.

The characterisation of a gust generator for aerodynamic testing

An experimental investigation was conducted to characterise the gust environment generated by a cascade of sinusoidally oscillating vanes in the Cranfield University 1.52 m by 1.14 m low-speed, open-jet, closed-circuit wind tunnel operating at 7.5 ms−1 and 14.5 ms−1. Measurements were made at four stream-wise and three vertical positions in the flow-field downstream of the vanes using a cross-wire anemometer. The oscillation angle of the vanes was set at ±8° and ±12° with frequencies of 0.5 Hz, 1.0 Hz and 2.0 Hz corresponding to a reduced frequency of between 0.01 and 0.07. The vertical components of velocity and turbulence intensity were shown to be consistent for the range of measurement points chosen. The peak vertical velocity component downstream of the vanes was shown to be proportional to the maximum vane angle of attack but largely independent of reduced frequency.

Forcing Boundary-Layer Transition on an Inverted Airfoil in Ground Effect

The influence of the laminar boundary-layer state on a wing operating in ground effect has been investigated using experiments with a model that provides two-dimensional flow. The effect of a boundary-layer trip placed at varying distances from the leading edge was observed at various incidences in terms of on-surface characteristics, including pressure measurements, flow visualization, and hot-film anemometry, and off-surface characteristics with velocity surveys below and behind the wing. The act of forcing transition led to downforce being reduced and drag increased, moreover, it altered almost all aspects of the wing’s aerodynamic characteristics, with the effect becoming greater as the trip was placed closer to the leading edge. These aspects include the replacement of a laminar separation bubble with trailing-edge separation, a thicker boundary layer, and a thicker wake with greater velocity deficit. The importance of considering laminar phenomena for wings operating in ground effect has been shown.

Aerodynamic characteristics of a monoposto racing car front wing operating in high turbulence conditions

Racing cars are developed in different conditions to those experienced when on track following another car. An experimental and computational study was conducted in order to understand the influence of a leading car on the performance of the front-wing of a trailing car. Force measurement, surface flow visualization and wake surveys allowed for a better understanding of the flow conditions and the decrease in performance the trailing wing experiences. Performance reduction due to a lower dynamic pressure, increased turbulence intensity and local flow incidence changes were noted, as well as, a reduction in tip vortex strength on the trailing wing.