Effect of vortex generators on corner flow separation caused by shock wave-boundary-layer interaction

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Wind tunnel experiments were conducted to investigate the effect of vortex generators on a corner flow separation caused by an interaction between a normal shock wave and the boundary layer in a Mach 1.4 flow. The shape of the vortex generators was rectangular. The vortex generators were mounted on a bottom wall of the test section. The parameters of the vortex generators were the rotation direction of the vortex, their size and their location. When the leading edge of the vortex generators turn towards the corner, the effect of the vortex generators on the corner flow separation decreased monotonically as the vortex generators size decreased. An independent separation appeared on the bottom wall in the case. When the leading edge of the vortex generators point in the opposite direction, the flow structure was changed by the size and the location of the vortex generators. We categorized the flow structures into three modes. The effect of the vortex generators and the three modes were successfully collapsed with the location parameter normalized by the second power of the scale parameter

Nozzle geometry effects on corner boundary layers in supersonic wind tunnels

Experiments on supersonic flows are typically conducted in wind tunnels with rectangular cross-sections, which use two-dimensional nozzles of two different types. A “full” setup consists of two contoured nozzle surfaces symmetric about the tunnel centre-height. The “half” configuration is also common, with a curved ceiling nozzle surface and a straight horizontal floor.Air Force Research Laboratory UK National Wind Tunnel Facilit

Force production mechanisms for a flat plate wing at low reynolds numbers (Invited)

© 2018, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. Micro Air Vehicles (MAVs) operate in highly unsteady and often separated flow conditions which are well outside the usual design space of conventional lifting surfaces. This paper aims to coalesce theory and simply explain the physical mechanisms behind force production for such unsteady and separated flows with regards to added mass and circulatory (vortex dynamics) effects. Particle Image Velocimetry (PIV) and flow visualisation are additionally used to show that the ’non-circulatory’ added mass forces are responsible for production of vorticity bound to the wing surface, thus can be confused with free vortices in the flow field. Changes to the ’added mass vorticity’ and thus forces derived from this are shown to be independent of flow topology and perhaps viscous effects in general with exception to edge conditions. A technique to simply quantify the distribution of the bound vorticity from PIV measurements is additionally described and utilised

Experiments to investigate lift production mechanisms on pitching flat plates

© 2016, The Author(s).Pitching flat plates are a useful simplification of flapping wings, and their study can provide useful insights into unsteady force generation. Non-circulatory and circulatory lift producing mechanisms for low Reynolds number pitching flat plates are investigated. A series of experiments are designed to measure forces and study the unsteady flowfield development. Two pitch axis positions are investigated, namely a leading edge and a mid-chord pitch axis. A novel PIV approach using twin laser lightsheets is shown to be effective at acquiring full field of view velocity data when an opaque wing model is used. Leading-edge vortex (LEV) circulations are extracted from velocity field data, using a Lamb–Oseen vortex fitting algorithm. LEV and trailing-edge vortex positions are also extracted. It is shown that the circulation of the LEV, as determined from PIV data, approximately matches the general trend of an unmodified Wagner function for a leading edge pitch axis and a modified Wagner function for a mid-chord pitch axis. Comparison of experimentally measured lift correlates well with the prediction of a reduced-order model for a LE pitch axis

Boundary layer vortex sheet evolution around an accelerating and rotating cylinder

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Effect of Vortex Generators on Corner Separation Caused by Shock Wave-Boundary-Layer Interaction

Wind tunnel experiments were conducted to investigate the effect of vortex generators on a transonic corner flow separation, resulting from the interaction of a normal shock wave with a turbulent boundary layer in a duct at M = 1.4. The shape of the vortex generators was rectangular. The vortex generators were mounted on the bottom wall of the test section. The investigation studied the effects of the rotation direction of the vortex, the VG size and position relative to the shock and the walls. When the leading edge of the vortex generators turn towards the corner, the effect of the vortex generators on the corner flow separation monotonically decreased as the vortex generators size reduced. In these cases, it was observed that independent separations appeared on the bottom wall. When the leading edge of the vortex generators points in the opposite direction away from the corner, the flow structure was found to depend on the size and the location of the vortex generators. We categorized the flow structures into three modes. The effect of the vortex generators and the three modes were successfully collapsed with a location parameter normalized by the second power of the non- dimensional size

Boundary-Layer development downstream of normal shock in transonic intakes at incidence

The flowfield around five transonic inlet lips at high incidence is investigated for a variety of flow conditions around a design point representative of high incidence manoeuvring. Changes to the operating point are simulated by varying the angle of incidence and the mass flow rate over the lip, intended to mimic the effect of an increase in engine flow. For these inflow conditions, the flow on the lip is characterised by a supersonic region, terminated by a near-normal shock wave. Of particular interest is the effect of lip geometry and operating point on the boundary layer at the equivalent fan location. The parametric investigation revealed a significant effect of lip shape on the position and severity of the shockwave-boundary layer interaction. From correlation studies, it appears that the extent of shock-induced separation is the main factor affecting the boundary layer state downstream of the normal shock wave-boundary layer interaction. Somewhat surprisingly, this was found to be independent of shock strength

Low reynolds number surge response of a flat plate wing at 90 degrees incidence

© 2017, American Institute of Aeronautics and Astronautics Inc. All rights reserved. This article presents an experimental investigation into the unsteady force response of a at plate wing when accelerated from rest at 90° incidence to the free stream direction. The work was performed to test a low order model developed by the NATO AVT-202 task group for pitching and surging wings at low Reynolds numbers. The model expresses the force response on a wing as the sum of circulatory components due to vortex dynamics, and non-circulatory components due to added mass. Force measurements, flow visualisation, and PIV measurements were taken for a fast and slow acceleration rate to relatively isolate the effect of added mass. Measurements suggest that the modified Wagner function can give a surprisingly reasonable prediction of vortex growth during the acceleration phase of a surging wing, however, incorrectly predicts vortex growth at steady state. It is proposed that added mass is a mechanism that generates vorticity, thus PIV measurements of circulation of the vortex pair can also contain the impulse due to added mass. When PIV measurements are compensated for added mass, good agreement between the drag predicted by the low order model and force balance measurement is shown

The Effect of Cross-Flow Vortex Trap Devices on the Aerodynamic Drag of Road Haulage Vehicles

The effect of Cross-Flow Vortex Trap Devices (CVTDs) on the local flowfield and vehicle drag at a range of yaw angles has been investigated in wind tunnel experiments. The CVTD is a flow control device proposed by Bauer and Wood that aims to reduce the sensitivity of articulated road haulage vehicles to crosswinds by managing the tractor- trailer gap cross-flow. A 1/10th scale model is used in a low speed wind tunnel at a Reynolds number of 900,000. The aerodynamic drag force is measured using a load cell connected to a rotating, raised ground plane. This research also uses tuft flow visualisation to examine the local flow- fields, and pressure taps to determine trailer pressure dis- tributions. It is found that a configuration of four 45% length CVTDs reduces the wind-averaged drag coefficient by 12%. The drag mechanisms that are responsible for the reduced drag include a lower average pressure on the trailer front face, a removal of the separation on the leeward side of the trailer due to a reduction in gap cross-flow, and an increase in pressure on the leeward side of the trailer be- hind the tractor-trailer gap. Furthermore, it is found that the drag reduction performance increases with CVTD length but does not vary with the number of CVTDs between one and four. These results suggest that using a single CVTD or flexible sheet of material at the centreline of the cab-gap is the most viable solution, as there is no further benefit to us- ing multiple devices. In addition, it allows for the greatest CVTD length without impeding articulation

Linking the unsteady force generation to vorticity for a translating and rotating cylinder

A boundary layer surface vortex sheet is utilized to investigate the development of the unsteady forces acting on a rotating and surging cylinder at a Reynolds number of 20 000. Planar particle image velocimetry and force balance measurements are employed to assess the flow field and force response. The boundary layer vortex sheet is categorized into non-circulatory and circulatory components. During acceleration the non-circulatory or added mass vortex sheet is successfully recovered from experimental data. This makes a decomposition of the forces, calculated only from particle image velocimetry measurements, possible. They compare well to force balance data. Additionally, the evolution of the boundary layer vortex sheet is assessed as the flow separates to form a vortical structure in the surrounding flow. When the influence of varying freestream velocity and rotation rate are removed, the surface vortex sheet at the location of separation varies little during vortex formation, even as the separation point moves along the surface of the cylinder