Numerical investigation of transonic buffet control using a two-dimensional bump
Creators: name: YANG, Zheng
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401201
subject: Aerodynamics (excl. hypersonic aerodynamics)
classificationCode: 401204
subject: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)

description: The aerodynamic performance of a transonic aircraft wing is strongly influenced by the interaction between the shock wave and the boundary-layer. The shock wave caused by locally supersonic flow and the viscosity existing in a boundary-layer can lead to several undesirable phenomena such as flow instability, drag rise, and buffet. These phenomena can crucially limit the flight envelop and hence operation. Flow instability is caused by the flow unsteadiness that exists in shock-induced separation, thus creating turbulence around a streamlined body such as aerofoils and transonic gas turbine blades. Drag rise, also known as drag-divergence phenomenon, is related to a concept called sound barrier, which implies that the aerodynamic drag exerted on a body can be extremely high as the flow speed approaches to the speed of sound. Buffet is rather complex, as it can be recognised as a coexistence phenomenon of which both flow instability and drag-divergence that are likely to occur simultaneously. The implication of this statement can be interpreted as follows: buffet can be triggered at a critical flow condition when the separated airflow creates unsteady pressure waves that can interact with a shock wave caused by the formation of locally supersonic flow, and the flow speed enters a regime where compressibility effect is dominant. The mechanism of transonic buffet can be characterised in the forms of self-sustained shock oscillation, unsteady pressure fluctuation and the generation of wave drag. Self-sustained shock oscillation is a periodic shock movement taking place on 2D transonic aerofoils and Diverterless Supersonic Intakes (DSI) with a frequency at about 70Hz to 80Hz, and this frequency of oscillation depends on the flow Mach number and the angle of attack. Pressure fluctuation acts as a time response of pressure variation associated with shock wave/boundary-layer interaction. The level of unsteady pressure fluctuation becomes significant at the shock location and the trailing edge, which can be felt as a structural vibration on the tail unit of an airframe. Wave drag is an independent drag component, which is created by the loss of stagnation pressure across a shock wave. It has been demonstrated experimentally that a 3D surface bump configuration can fulfil two objectives in shock wave/boundary-layer control, drag reduction and buffet delay. However, a rather few studies that investigate the aerodynamic performance associated with an unsteady transonic flow in the presence of a surface bump, which still withholds a couple of unknown aspects that need to be found out, such as the control mechanisms, impacts, benefits, and drawbacks.

The current research is undertaken by performing Computational Fluid Dynamics (CFD) simulations for a 2D aerofoil fitted with a shock control bump at a transonic flow condition. Reynolds-Averaged Navier-Stokes equation coupled with turbulence models is adopted to solve the flowfield. The bump has a conventional geometric feature which consists of a ramp, a crest and a tail, and it has eight variants for which the designed parameters are reasonably specified according to the compressible aerodynamics theory. The bumps are placed at a forward and backward position on the suction side of the aerofoil, on which a shock wave is present. The forward position is 30% of the aerofoil chord and the backward position is 40% of the aerofoil chord. The chosen mathematical model is considered to be reliable as it has been well-validated by comparing the numerical results with the experimental data, in terms of the surface pressure distribution, unsteady pressure fluctuation and streamwise velocity profiles. Qualitative analysis is conducted through scrutinising the uncontrolled and controlled flowfields. The Mach number contours show that the forward bumps could generate a large oblique shock structure followed by an additional flow re-expansion due to the locally re-accelerated supersonic flow, while the backward bumps do not incur significant secondary expansion waves, and the oblique shock structure becomes relatively weaker and smaller on the backward bumps. Then, the control effectiveness of shock control bumps is analysed quantitatively. The control performance for each type of bump configuration is evaluated by computing the transient responses of the lift and drag coefficients, both in time and frequency domain. The results show that a shock control bump can mitigate the periodic lift oscillation that associates with a transonic buffet flow by attenuating the frequency and amplitude of the oscillation. The unsteady fluctuation of surface static pressure is calculated using an appropriate statistical approach. This analysis reveals that a bump with a low crest height is the most effective to suppress pressure fluctuation on an aerofoil globally. Finally, the overall aerodynamic performance is assessed by evaluating the lift curves, the effect of drag-divergence, lift-to-drag ratio and the parasitic drag. This assessment concludes that the bumps can lag the stall angle of attack, as well as delaying the drag-divergence Mach number. The bumps can offer an effective reduction of the total drag but incur a little penalty in the viscous drag. Recommendation for future work has been proposed with an emphasis on the shape optimisation of the bump geometry and the prediction of the buffet boundary for an optimum design configuration.