Session: 40-08 Compressor Flow Control Approaches
Paper Number: 124289
124289 - Mitigation of Laminar Separation Bubble Through Leading-Edge Modification of an Aerofoil With Herringbone Riblets
Flow separation is often inhabitable in many engineering applications with significant adverse consequences. In such scenarios, the boundary layer detaches from the surface due to high diffusion levels or abrupt surface curvature changes, resulting in a rapid transition and turbulent reattachment. This leads to the formation of a Laminar Separation Bubble (LSB). Once formed, an LSB significantly impacts the downstream evolution of the boundary layer, negatively affecting the stall characteristics of an aerofoil, apart from reducing the lift-to-drag ratio. Thus, an effective separation control is essential to enhance the performance of components used in aerospace or automotive applications. This, in turn, might reduce energy consumption, which is vital for optimizing efficiency and sustainability. Separation control techniques can be categorized as either active or passive, depending on the underlying principle. Active techniques involve the use of external energy input, such as synthetic jets, to manipulate the flow. In contrast, passive techniques rely on static devices like vortex generators, ribs, or porous surfaces to influence flow behaviour without the need for external energy.
In the present study, we focus on the impact of a passive technique involving the leading-edge modification using herringbone riblets. We conducted experiments involving pressure and instantaneous flow measurements by hotwire and particle image velocimetry (PIV) on an aerofoil model with a semi-circular leading edge for varying angles of attack from 0 to 5 degree. The Reynolds number based on the chord length and the inlet freestream conditions was 1.6x105, where freestream turbulence (fst) being 1.2%. The herringbone riblet engraved on the leading edge has a triangular cross-section of height (k)=0.6mm, leading to k+ =10.5. The base s was 2k, corresponding to s+=21. Further, the transverse pitch and angle between the herringbone riblet were 35k and 60 degree, respectively.
An LSB appeared on the leading edge of the aerofoil for a hydrodynamically smooth surface at varying angle of attack. Selective amplification of frequencies from background disturbances, along with exponential growth of urms, indicated that inviscid instability governed the transition. Leading-edge modification with herringbone riblets induced higher turbulence in proximity to the wall, resulting in an earlier transition of the shear layer and a reduction in bubble length by a maximum of 43%. Furthermore, the leading-edge modification bypassed the selective amplification of frequencies, leading to the algebraic growth of urms and a change in the transition mechanism, where the boundary layer was pre-transitional from the beginning. These observations were also complimented by pressure distribution, shape factor, and power spectra. However, the effectiveness of separation suppression with herringbone riblets at the leading edge diminished at higher angles of attack.
Presenting Author: Ravi Kumar Indian Institute of Technology Kanpur
Presenting Author Biography: Ph.D Scholar of IIT Kanpur
Authors:
Ravi Kumar Indian Institute of Technology KanpurPradeep Singh Vellore Institute of Technology University
Subrata Sarkar Indian Institute of Technology Kanpur
Mitigation of Laminar Separation Bubble Through Leading-Edge Modification of an Aerofoil With Herringbone Riblets
Paper Type
Technical Paper Publication