Vortical structures on three-dimensional shock control bumps

Three-dimensional shock control bumps have long been investigated for their promising wave drag reduction capability. However, a recently emerging application has been their deployment as “smart” vortex generators, which offset the parasitic drag of their vortices against their wave drag reduction. It is known that three-dimensional shock control bumps produce streamwise vortices under most operating conditions; however, there have been very few investigations that have aimed to specifically examine the relevant flow structures. In particular, the strength of the vortices produced as well as the factors influencing their production are not well known. This paper uses a joint experimental and computational approach to test three different shock control bump shapes, categorizing their flow structures. Four common key vortical structures are observed, predominantly shear flows, although all bumps also produce a streamwise vortex pair. Both cases with and without flow separation on the bump tails are scrutinized. Finally, correlations between the strength of the main wake vortices and pressure gradients at various locations on the bumps are assessed to investigate which parts of the flow control the vortex formation. Spanwise flows on the bump ramp are seen to be the most influential factor in vortex strength.The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Clean Sky Joint Technology Initiative as part of the NextWing program under grant agreement no. 271843.This is the author accepted manuscript. The final version is available from the American Institute of Aeronautics and Astronautics via http://dx.doi.org/10.2514/1.J05466

Vortical structures on three-dimensional shock control bumps

Three-dimensional shock control bumps have long been investigated for their promising wave drag reduction capability. However, a recently emerging application has been their deployment as “smart” vortex generators, which offset the parasitic drag of their vortices against their wave drag reduction. It is known that three-dimensional shock control bumps produce streamwise vortices under most operating conditions; however, there have been very few investigations that have aimed to specifically examine the relevant flow structures. In particular, the strength of the vortices produced as well as the factors influencing their production are not well known. This paper uses a joint experimental and computational approach to test three different shock control bump shapes, categorizing their flow structures. Four common key vortical structures are observed, predominantly shear flows, although all bumps also produce a streamwise vortex pair. Both cases with and without flow separation on the bump tails are scrutinized. Finally, correlations between the strength of the main wake vortices and pressure gradients at various locations on the bumps are assessed to investigate which parts of the flow control the vortex formation. Spanwise flows on the bump ramp are seen to be the most influential factor in vortex strength.The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Clean Sky Joint Technology Initiative as part of the NextWing program under grant agreement no. 271843.This is the author accepted manuscript. The final version is available from the American Institute of Aeronautics and Astronautics via http://dx.doi.org/10.2514/1.J05466

Three-dimensional shock control bumps: Effects of geometry

© 2015, American Institute of Aeronautics and Astronautics Inc. All rights reserved. Results from wind tunnel experiments with three different geometry shock control bumps (SCBs) are presented and discussed. The primary aims of this study are to explore how 3-D SCBs generate stream-wise vorticity and to compare the flow features produced by different geometry 3-D SCBs. Results are presented in two parts: In part I, the flow development over a simple geometry baseline bump is described in detail with a focus on describing the evolution of vortical flow structures. In part II, results from tests with two different bumps (with subtle geometric variations from the baseline design) are presented and compared with those from the baseline bump. It is observed that the mechanism of vortex production for all 3-D SCBs tested is broadly in agreement with that reported in literature: Specifically, vortical flow structures are only observed downstream of the crest, on the SCB tail. Differences between the three SCB geometries tested are relatively subtle; increasing the width of the SCB tail reduces the extent of shock-induced separation at the crest, giving an anticipated reduction in viscous drag; steepening the SCB sides also reduces shock-induced separation at the crest (although to a lesser extent) and modifies the pressure profile on the off the bump such that the flow experiences lower span-wise pressure gradients. These findings suggest both geometric modifications considered may yield a benefit in performance for a 3-D SCB mounted on a transonic wing

Vortical structures on three-dimensional shock control bumps

The vortical wake structure produced by a three-dimensional shock control bump (SCB) is thought to be useful for controlling transonic buffet on airfoils. However, at present the vorticity produced is relatively weak and the production mechanism is not well understood. Using a combined experimental and computational approach, a preliminary investigation on the wake vorticity for different bump geometries has been carried out. The structure of the wake for on and off-design conditions are considered, and the effects on the downstream boundary layer demonstrated. Three main vortical structures are observed: a primary vortex pair, weak inter-bump vortices and shear flow in the lambda-shock region. The effect of pressure gradients on vortex strength is examined and it is found that spanwise pressure gradients on the front section of the bump are the most significant parameter influencing vortex strength. © 2013 by S.P. Colliss et al

Joint experimental and numerical approach to three-dimensional shock control bump research

Previous studies of transonic shock control bumps have often been either numerical or experimental. Comparisons between the two have been hampered by the limitations of either approach. The present work aims to bridge the gap between computational fluid dynamics and experiment by planning a joint approach from the outset. This enables high-quality validation data to be produced and ensures that the conclusions of either aspect of the study are directly relevant to the application. Experiments conducted with bumps mounted on the floor of a blowdown tunnel were modified to include an additional postshock adverse pressure gradient through the use of a diffuser as well as introducing boundary-layer suction ahead of the test section to enable the in-flow boundary layer to be manipulated. This has the advantage of being an inexpensive and highly repeatable method. Computations were performed on a standard airfoil model, with the flight conditions as free parameters. The experimental and computational setups were then tuned to produce baseline conditions that agree well, enabling confidence that the experimental conclusions are relevant. The methods are then applied to two different shock control bumps: a smoothly contoured bump, representative of previous studies, and a novel extended geometry featuring a continuously widening tail, which spans the wind-tunnel width at the rear of the bump. Comparison between the computational and experimental results for the contour bump showed good agreement both with respect to the flow structures and quantitative analysis of the boundary-layer parameters. It was seen that combining the experimental and numerical data could provide valuable insight into the flow physics, which would not generally be possible for a one-sided approach. The experiments and computational fluid dynamics were also seen to agree well for the extended bump geometry, providing evidence that, even though thebumpinteracts directly with the wind-tunnel walls, it was still possible to observe the key flow physics. The joint approach is thus suitable even for wider bump geometries. Copyright © 2013 by S. P. Colliss, H. Babinsky, K. Nubler, and T. Lutz. Published by the American Institute of Aeronautics and Astronautics, Inc

Shock control bump robustness enhancement

Robustness enhancement for Shock Control Bumps (SCBs) on transonic wings is an ongoing topic because most designs provide drag savings only in a relatively small band of the airfoil polar. In this paper, different bump shapes are examined with CFD methods which are validated first by comparison with wind tunnel results. An evaluation method is introduced allowing the robustness assessment of a certain design with little computational effort. Shape optimizations are performed to trim SCB designs to maximum performance on the one hand and maximum robustness on the other hand. The results are analysed and different and parameters influencing the robustness are suggested. Copyright © 2012 by Klemens Nuebler

Numerical and experimental examination of shock control bump flow physics

A method allowing a detailed investigation of the flow physics of shock control bumps (SCBs) on an unswept airfoil has been developed by comparison of the results of experiments and computations. A simple wind tunnel set-up is proposed which is shown to generate representative baseline conditions, allowing fine details of the flow to be measured using an array of techniques. Computational data for the same bump configuration is then validated against the experimental results, allowing a more intimate analysis of the flow physics as well as relating wind tunnel results to the performance of the SCB on an unswept wing. © Springer-Verlag Berlin Heidelberg 2013