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Title: Designing shock control bumps for transonic commercial aircraft
Author: Jones, Natasha Ruth
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2017
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Shock control bumps (SCBs) are considered promising flow control devices for transonic commercial aircraft. By generating a λ-shock structure, 2D SCBs offer large drag savings, but perform poorly when that structure breaks down off-design. Milder-performing 3D devices produce weak vortices, that may offer some boundary layer control, and SCBs also affect buffet via direct impact on shock motions and separation. To date however, design studies have largely ignored complications from the swept, spanwise-varying flows, so this thesis tackles the question of whether SCB arrays can offer useful benefits to the performance of transonic commercial aircraft. Using a numerical infinite-wing model, a simple rotation adaptation is shown to redress deficient on-design drag performance of 3D SCBs under swept flows. With the correct rotation (dependent on height, planform and spacing) bumps follow performance-design trends similar to those in unswept flow. With this knowledge, an array design method is developed to tailor 2D and 3D devices to local flow conditions on an aircraft model, aiming to maximise on-design drag performance. Careful infinite-wing setup means the influence of rotation and array height on performance is replicated on the aircraft. Predicted array designs achieve 74-87% of their estimated local drag savings. However, with wave drag being a smaller percentage of the total, the influence of arrays on lift is more significant and makes the optimal designs shorter than predicted. Strategies for improving off-design drag performance are then evaluated. Stagger, an alternating chordwise translation applied to 3D arrays, broadens operating range and lowers drag penalties by better accommodating off-design shock movements, but offers a less favourable trade-off against on-design drag than simply reducing the array height. However, a 2D array can always outperform a 3D on drag objectives. Lastly, buffet performance is inferred using steady indicators based on trailing edge pressure and shock location. These disagree regarding the impact on buffet onset, unresolvably due to a lack of validation data, but agree that arrays could alleviate flow development post-onset. Optimal array designs depend on prioritised objectives: considering buffet severity and on-design drag, tall 2D (or 3D) arrays; for buffet and minimum off-design drag penalties (similar to the motivation behind vortex generator application), 3D arrays of varying height and stagger. A simple flight fuel consumption model utilising the computed drag data shows that many arrays are neutral or offer small savings (up to 0.3%) across a range of mission profiles. While likely too small to merit application for solely drag purposes, this implies buffet benefits without cost to efficiency. Unsteady tests and proper assessment of buffet onset are needed to confirm this.
Supervisor: Jarrett, Jerome Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Shock Control Bumps ; Transonic ; Aircraft ; Design ; Flow Control ; CFD ; Shock Control