Forcing boundary-layer transition on an inverted airfoil in ground effect and at varying incidence

Presented at 34th AIAA Applied Aerodynamics ConferenceThe 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 show

Through-wall detection and imaging of a vibrating target using synthetic aperture radar

This paper explains the development of a through-wall synthetic aperture radar (SAR) simulator, which is being used to investigate the SAR artefacts originating from vibrating targets, known as paired echoes. The simulation and experimental results both show that paired echoes can be detected and imaged through a wall, with a noticeable reduction in intensity, resulting in the number of visible echoes to be reduced in brightness and appear shifted in location in a through-wall SAR image

Volumetric SAR near-field upsampling and basebanding

Highly sampled imagery offers many benefits to the radar practitioner, ranging from easier image coregistration to simple visual appeal. However, it is often overlooked due to the computational burden forming such an image imposes. Fast image formation typically imposes restrictions on the imaging scenario, for example synthetic aperture radar (SAR) far-field, and exploits parallelism through use of modern multi- core architecture. Imposing a SAR near-field requirement on the image formation limits the applicability of several of the faster algorithms, thus there is a need to create a general process to achieve highly sampled imagery, regardless of the imaging regime. In this letter, a method for accurately upsampling near-field (SAR) imagery is presented. This is applicable to both SAR near-field and SAR far-field scenarios. The methodology is discussed, and an example is provided in the form of a SAR near-field volumetric image of a miniature tank. The limitations to the approach are discussed and prospects for future work given

Localising vibrating scatterer phenomena in synthetic aperture radar imagery

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

Modelling boundary-layer transition on wings operating in ground effect at low Reynolds numbers

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

Low-frequency 3D synthetic aperture radar for the remote intelligence of building interiors

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

Discrimination of buried objects using time-frequency analysis and waveform norms

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

Passive flow control on a ground-effect diffuser using an inverted wing

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

Characteristics of boundary-layer transition and Reynolds-number sensitivity of three-dimensional wings of varying complexity operating in ground effect

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

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 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