CERES :: Browsing by Author "Finnis, Mark V."

Browsing by Author "Finnis, Mark V."

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Open Access
Aerodynamic characteristics of a wing-and-flap configuration in ground effect and yaw
(IMechE, 2015-08-11) Roberts, L.; Correia, J.; Finnis, Mark V.; Knowles, Kevin
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.
Open Access
Aerodynamics of a convex bump on a ground-effect diffuser
(Journal of Fluids Engineering, 2018-04-19) Ehirim, Obinna; Knowles, Kevin; Saddington, A. J.; Finnis, Mark V.
A ground-effect diffuser is an upward-sloping section of the underbody of a racing car that enhances aerodynamic performance by increasing the downforce, thus improving tire grip. The downforce generated by a diffuser can be increased by geometric modifications that facilitate passive flow control. Here we modified a bluff body equipped with a 17° diffuser ramp surface (the baseline/plane diffuser) to introduce a convex bump near the end of the ramp surface. The flow features, force and surface pressure measurements determined in wind-tunnel experiments agreed with previous studies but the bump favorably altered the overall diffuser pressure recovery curve by increasing the flow velocity near the diffuser exit. This resulted in a static pressure drop near the diffuser exit followed by an increase to freestream static pressure, thus increasing the downforce across most of the ride heights we tested. We observed a maximum 4.9% increase in downforce when the modified diffuser was compared to the plane diffuser. The downforce increment declined as the ride height was gradually reduced to the low-downforce diffuser flow regime.
Open Access
Analysis and exploitation of complex SAR phenomena produced from vibrating targets
(Institute of Acoustics, 2018-12-31) Corbett, Brandon; Andre, Daniel; Muff, D.; Finnis, Mark V.; Blacknell, D.
Open Access
Bistatic 3D SAR for wall parameter extraction in cluttered environments
(IET, 2021-07-28) Elgy, James; Andre, Daniel; Finnis, Mark V.
Through-wall radar is an emergent technology rooted in urban surveillance, a key component being synthetic aperture radar (SAR). Accurate through-wall SAR relies on knowledge of the refractive index and thickness of any obscuring walls. Such information is rarely known beforehand and is subject to change on a sample-by-sample basis. It is therefore necessary to obtain the material properties in conjunction with any SAR measurement. In this letter, a remote data-driven asymmetric bistatic SAR approach is taken by means of matching the range to the direct back face reflection with an explicit geometry-based model. The proposed method relies on an accurate knowledge of the bistatic measurement geometry. Using the bright reflection from the front face of the wall, a method for refining an estimate of the bistatic measurement geometry is proposed. This approach is extended to three-dimensions to improve usability in heavily cluttered environments. This method is empirically validated using three-dimensional SAR measurements of both a wall-only, and a heavily cluttered scene. The method is shown to accurately extract both the refractive index and thickness of a concrete wall, with both measurements in agreement with each other and an independent validation measurement.
Open Access
Boundary-layer transition on wings in ground effect
(2017-12) Roberts, L. S.; Finnis, Mark V.; Knowles, Kevin
The competitiveness of a high-performance racing car is extremely reliant on aerodynamics. Due to the current economic climate, track testing is often forsaken and the majority of aerodynamic development carried out using sub-scale wind tunnel testing and computational simulations. It is important, therefore, that experimental and computational approaches represent real-world conditions as closely as possible. Although racing cars travel at much higher speeds than typical passenger cars, in comparison to aircrafts they still operate at relatively low Reynolds numbers and, consequently, laminar and transitional phenomena are evident. Despite this, the bulk of relevant literature available for racing-car aerodynamics is undertaken with little regard to the influence of Reynolds number, and in the case of computational studies, the omission of laminar and transitional phenomena all together. The present work has demonstrated, using a super-scale two- dimensional wind-tunnel model, that laminar and transition flow phenomenon are important at Reynolds numbers equivalent to a full-scale racing car. Moreover, the influence of these aspects increased as the wing’s ground clearance reduced; meaning that in ground effect they are even more important. Further experiments with three-dimensional models of varying complexity, from a simple single-element wing to a highly complex F1-specification wing, showed that laminar phenomena are important for F1 applications as well as for lower-downforce capability racing cars. A transition-sensitive eddy-viscosity turbulence model, k-kL-w, was used to simulate inverted wings operating in ground effect. It was shown that that laminar and transitional flow states could be simulated easily inside a commercial solver, and that the model offered a substantial improvement over the classical fully-turbulent k-w SST in terms of both force coefficient prediction and surface-flow structures. This experiments and computational simulations described in this thesis show the Reynolds number sensitivity of, and importance of laminar phenomenon on, wings operating in ground effect. It has been shown that laminar boundary layers are an important aspect of the flow characteristics of wings in ground effect, at both full-scale and model-scale Reynolds numbers. As such, it is recommended that future studies incorporate laminar and transitional phenomena.
Open Access
Characteristics of boundary-layer transition and Reynolds-number sensitivity of three-dimensional wings of varying complexity operating in ground effect
(American Society of Mechanical Engineers, 2016-06-03) Roberts, Luke S.; Finnis, Mark V.; Knowles, Kevin
The influence of Reynolds number on the aerodynamic characteristics of various wing geometries was investigated through wind-tunnel experimentation. The test models represented racing car front wings of varying complexity: from a simple single-element wing to a highly complex 2009-specification formula-one wing. The aim was to investigate the influence of boundary-layer transition and Reynolds-number dependency of each wing configuration. The single-element wing showed significant Reynolds-number dependency, with up to 320% and 35% difference in downforce and drag, respectively, for a chordwise Reynolds number difference of 0.81 × 105. Across the same test range, the multi-element configuration of the same wing and the F1 wing displayed less than 6% difference in downforce and drag. Surface-flow visualization conducted at various Reynolds numbers and ground clearances showed that the separation bubble that forms on the suction surface of the wing changes in both size and location. As Reynolds number decreased, the bubble moved upstream and increased in size, while reducing ground clearance caused the bubble to move upstream and decrease in size. The fundamental characteristics of boundary layer transition on the front wing of a monoposto racing car have been established.
Open Access
Combined Stereoscopic Particle Image Velocimetry and Line Integral Convolution Methods: Application to a Sphere Sedimenting Near a Wall in a Non-Newtonian Fluid.
(The Visualization Society of Japan, 2005) Lawson, Nicholas J.; Finnis, Mark V.; Tatum, J. A.; Harrison, G. M.
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.
Open Access
Density Measurements for Rectangular Free Jets Using Background Oriented Schlieren
(Royal Aeronautical Society, 2013-08-30T00:00:00Z) Tipnis, T. J.; Finnis, Mark V.; Knowles, Kevin; Bray, Derek
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 3-D domain which contains information about the actual density. 2-D 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.
Open Access
Discrimination of buried objects using time-frequency analysis and waveform norms
(IEEE, 2017-01-09) Morrow, Ivor L.; Wirth, Sebastian G.; Finnis, Mark V.
Ground Penetrating Radar (GPR) are widely used to probe the sub-surface. Recently, various time-frequency analyses has been proposed to discriminate buried land mines from other clutter objects and thus reduce GPR false alarm rates. This paper examines the possibility for discrimination and assesses it experimentally. The approach uses the Choi-Williams time-frequency transform to analyse ultra-wideband signal returns from a range of shallow buried objects. Single Value Decomposition is performed on isolated object time-frequency signatures. The signatures are evaluated using a set of waveform norms that discriminate in time, frequency and energy content. The results indicate that this approach could improve land mine detection rates and reduce false alarms.
Open Access
Forcing boundary-layer transition on a single-element wing in ground effect
(ASME, 2017-06-14) Roberts, Luke S.; Finnis, Mark V.; Knowles, Kevin
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 free- and 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 on-surface and off-surface, are not recreated.
Open Access
Forcing boundary-layer transition on an inverted airfoil in ground effect
(AIAA, 2017-07-21) Roberts, L. S.; Finnis, Mark V.; Knowles, Kevin; Lawson, Nicholas J.
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.
Open Access
Forcing boundary-layer transition on an inverted airfoil in ground effect and at varying incidence
(American Institute of Aeronautics and Astronautics Inc, AIAA, 2016-06-17) Roberts, L.; Finnis, Mark V.; Knowles, Kevin; Lawson, Nicholas J.
The influence of the laminar boundary-layer state on a wing operating in ground effect at Re = 6 × 10 has been investigated using experiments with a model that provides two-dimensional flow and computations with a panel-method code. 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 visualisation and hot-film anemometry, and off-surface characteristics with LDA 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
Open Access
Imaging SAR phenomenology of concealed vibrating targets
(ADE (IEEE), 2018-08-16) Corbett, Brandon; Andre, Daniel; Muff, Darren; Morrow, Ivor L.; Finnis, Mark V.
This paper describes the novel imaging of SAR phenomena produced from vibrating targets with multipath effects. It has been established, through numerical SAR experiments, that different physical mechanisms interact to produce new artefacts. The computations demonstrated that the edges of a dielectric medium can act as a source for multipath effects to emanate from, leading to the hypothesis that SAR artefacts can arise from through-wall SAR imagery. This deduction and mechanism of origin were validated through several experimental measurements, undertaken at Cranfield University’s Antennas and Ground-based SAR laboratory, yielding results that closely match those predicted.
Open Access
The influence of external disturbances on the aerodynamic performance of a wing in ground effect
(2016-10-03) Correia, J.; Knowles, Kevin; Finnis, Mark V.
Aerodynamic development of race cars is mostly performed in wind tunnels, where consistent and repeatable conditions can be found. This leads to race cars being designed to be very efficient in a uniform, low-turbulence flow. However, while running on the track the airflow passing over a car is frequently altered by the presence of other cars, ambient wind, or track conditions, which may lead to a change in aerodynamic performance. An experimental and computational study was set up in an attempt to understand the effects of real on-track performance-limiting factors such as roll, yaw and immersion in the wake of a leading car. The study was broken down into independent phases that allowed the collection of reference data, from a wing operating with and without roll and yaw in an undisturbed flow, wake data from a leading wake-generator representative of a monoposto car, and data from the wing operating in the wake of the leading vehicle. Results from the wake survey led to the identification of three main flow characteristics: low dynamic pressure in the wake, counter-rotating vortices that lead to local velocity components and high turbulence intensities in the wake. The negative influence of these flow characteristics was confirmed once the wing was immersed in the wake, as the wing suffered from a significant reduction in downforce throughout the tested ride height range. The results from the roll and yaw experiments showed that when operating in either condition an inverted wing in ground effect will experience the same downforce enhancement mechanisms as those present on a wing operating in an unrolled and unyawed condition. Although either roll or yaw showed reductions in the downforce generated by the wing, the roll results showed a small ride height range where it is possible to increase the generated downforce under certain flap and roll settings.
Open Access
Localising vibrating scatterer phenomena in synthetic aperture radar imagery
(IET, 2020-01-21) Corbett, Brandon; Andre, Daniel; Finnis, Mark V.
Artefact phenomena resulting from synthetic aperture radar (SAR) image formation can pose a challenge for image interpretation. One such artefact is produced when a vibrating target is imaged. Suppression of these artefacts has previously been described, however little has been developed in the area of modelling the location and shape of such artefacts. The authors present an experimentally validated model that provides accurate location and shape of vibrating target paired echoes in both SAR near-field and SAR far-field imagery.
Open Access
Low-frequency 3D synthetic aperture radar for the remote intelligence of building interiors
(2017-06-12) Andre, Daniel; Faulkner, B.; Finnis, Mark V.
Low-frequency (LF) synthetic aperture radar (SAR) images offer a viable approach to determining the architecture and contents of buildings and underground bunkers via remote sensing. Often however, standard 2D SAR images can be difficult to interpret due to component signatures from different heights being projected into the scene leading to confused results. In this research, measurement results have shown that the full Nyquist 2D aperture scan approach to 3D through-wall LF SAR provides focussed 3D resolution of a wall and contents behind it in a number of frequency bands. Full-scale radar system upgrades are ongoing in order to investigate numerous other scenarios, however in the meantime, sparse 2D aperture scanning investigations have been undertaken with a prototype radar scanner. Whilst this kind of collection cannot achieve the low sidelobe levels of full Nyquist 2D aperture collections, these prototype scanner measurements are much faster to collect, and have shown encouraging results of sufficient image quality to determine the 3D configuration of prominent features in the target scene, albeit with higher sidelobe or image artefact levels.
Open Access
Modelling boundary-layer transition on wings operating in ground effect at low Reynolds numbers
(Sage, 2018-10-25) Roberts, Luke S.; Finnis, Mark V.; Knowles, Kevin
The transition-sensitive, three-equation k-kL-ω eddy-viscosity closure model was used for simulations of three-dimensional, single-element and multi-element wing configurations operating in close proximity to the ground. The aim of the study was to understand whether the model correctly simulated the transitional phenomena that occurred in the low Reynolds number operating conditions and whether it offered an improvement over the classical fully turbulent k-ω shear stress transport model. This was accomplished by comparing the simulation results to experiments conducted in a 2.7 m × 1.7 m closed-return, three-quarter-open-jet wind tunnel. The model was capable of capturing the presence of a laminar separation bubble on the wing and predicted sectional forces and surface-flow structures generated by the wings in wind tunnel testing to within 2.5% in downforce and 4.1% in drag for a multi-element wing. It was found, however, that the model produced insufficient turbulent kinetic energy during shear-layer reattachment, predicted turbulent trailing-edge separation prematurely in areas of large adverse pressure gradients, and was found to be very sensitive to inlet turbulence quantities. Despite these deficiencies, the model gave results that were much closer to wind-tunnel tests than those given by the fully turbulent k-ω shear stress transport model, which tended to underestimate downforce. Significant differences between the transitional and fully turbulent models in terms of pressure field, wake thickness and turbulent kinetic energy production were found and highlighted the importance of using transitional models for wings operating at low Reynolds numbers in ground effect. The k-kL-ω model has been shown to be appropriate for the simulation of separation-induced transition on a three-dimensional wing operating in ground effect at low Reynolds number.
Open Access
Multistatic dual-polarimetric through-wall 3D-SAR
(IEEE, 2021-07-02) Andre, Daniel; Watson, Francis; Finnis, Mark V.
Through-wall (TW) imaging is of great interest for both military and civilian applications. At sufficiently low radar frequencies, Synthetic Aperture Radar (SAR) provides a TW sensing solution, however making sense of cluttered and overlaid signatures can prove difficult. This work investigates the suitability of three-dimensional dual-polarimetric multistatic TW-SAR, for the determination of building architectures and the detection of targets within buildings. These results are relevant to radar sensing Unmanned Air Systems under current development
Open Access
On the near-wake of a ground-effect diffuser with passive flow control
(Springer Verlag, 2019-02-08) Ehirim, Obinna; Knowles, Kevin; Saddington, A. J.; Finnis, Mark V.; Lawson, Nicholas J.
A ground-effect diffuser is an upwardly-inclined section of an automobile’s underbody which increases aerodynamic performance by generating downforce. To understand the diffuser flow physics (force behaviour, surface and off-surface flow features), we established the near-wake (within one vehicle width of the base) velocity profiles and flow structures of an automotive ground-effect diffuser using a bluff body with a 17 degree slanted section forming the plane diffuser ramp surface (baseline geometry), and endplates extending along both sides of the ramp. Wind tunnel experiments were conducted at a Reynolds number of 1.8 million based on the bluff body length, and laser Doppler velocimetry was used to measure two-dimensional velocity components on three planes of the diffuser near-wake. We also measured the velocity field in the near-wake of diffusers with modified geometry (with an inverted wing or a convex bump) as passive flow control devices. The near-wake velocity profiles indicated that the passive flow control methods increased the diffuser flow velocity and that the longitudinal vortices along the diffuser determined the shape of the flow structures in the near-wake of the diffuser bluff body
Open Access
Passive flow control on a ground-effect diffuser using an inverted wing
(SAE, 2018-08-13) Ehirim, Obinna; Knowles, Kevin; Saddington, A. J.; Finnis, Mark V.
In this experimental and computational study a novel application of aerodynamic principles in altering the pressure recovery behavior of an automotive-type ground-effect diffuser was investigated as a means of enhancing downforce. The proposed way of augmenting diffuser downforce production is to induce in its pressure recovery action a second pressure drop and an accompanying pressure rise region close to the diffuser exit. To investigate this concept with a diffuser-equipped bluff body, an inverted wing was situated within the diffuser flow channel, close to the diffuser exit. The wing’s suction surface acts as a passive flow control device by increasing streamwise flow velocity and reducing static pressure near the diffuser exit. Therefore, a second-stage pressure recovery develops along the diffuser’s overall pressure recovery curve as the flow travels from the diffuser’s low pressure, high velocity inlet to its high pressure, low velocity exit. Consequently, downforce production is increased with the use of the wing. Across the range of ride heights investigated, computational fluid dynamics simulations, validated against wind tunnel measurements, show an increase in downforce, with the increase reaching a high of about 12% relative to the baseline (without the wing). However, the increment in downforce occurred at relatively high ride heights but not once the diffuser started stalling at relatively low ride heights.