Excitation of a Separated Flow by a Series of Protuberances Near the Leading Edge of a Model Aerofoil

This article describes the spatial development of a laminar separation bubble (LSB), its transition, and eventual breakdown under the influence of adverse pressure gradients (APGs) similar to those experienced by low-pressure turbine blades. The investigation combines a comprehensive experimental approach with a well-resolved large eddy simulation (LES). The streamwise pressure gradients were varied by manipulating the upper wall within the test section. The Reynolds number (Re), based on the plate length and inlet velocity, was 0.2 × 106 with a freestream turbulence intensity of 1.02%. The particle image velocimetry (PIV) and hotwire data were used to illustrate the vortex dynamics, growth of perturbations, and intermittency. The onset and end of transition progressively shift upstream, resulting in a reduction of the laminar shear layer length and bubble length with increasing APG. Interestingly, the flow features exhibit self-similarity in velocity profiles and the growth rate of velocity fluctuations when normalized against the bubble length. The formation of two-dimensional Kelvin–Helmholtz (K–H) rolls is apparent in the beginning, resulting in the selective amplification of frequency and exponential growth of fluctuations. Linear stability theory explains the most amplified frequency and phase speed of convective vortices, apart from the growth of disturbances. Analysis of LES data reveals intricate inviscid–viscous interactions that trigger shear layer breakdown. In brief, evolving perturbations within the braid region of vortices in the latter half interact with the advecting K–H rolls, culminating in the breakdown and the onset of turbulent flow downstream.

Features of laminar separation bubble subjected to varying adverse pressure gradients

Kumar,

Sarkar

2023

Insights into the transition of separation bubble over a rough surface at varying angles of attack

Singh,

Sarkar,

Kumar

et al. 2024

The evolution of a separated boundary layer on the rough surface in the vicinity of a leading edge of a model airfoil is documented at varying angles of attack. Particle image velocimetry and hotwire data are analyzed to elucidate the flow feature, depicting the manifestation of the shear layer, its rollup, growth of perturbations, spectral response, and intermittency. For a hydrodynamically smooth surface, a laminar separation bubble often appears near the leading edge, where the shear layer becomes inviscidly unstable. Wall roughness amplifies the near-wall perturbations, resulting in earlier transition and reattachment. This leads to a reduction in bubble length and laminar shear layer length compared to the smooth surface at the corresponding angle of attack. Notably, despite the amplification of selective frequency, the inviscid instability is bypassed on the rough surface for varying angles of attack. Moreover, the linear stability analysis proves inadequate in predicting the most amplified frequency and the growth of disturbances. Furthermore, the universal intermittency curve formulated for the bypass transition is valid for the separation-induced transition, illustrating the significance of viscous effect.

Influence of freestream turbulence on boundary layer transition over a controlled-diffusion compressor blade

Katiyar,

Sarkar

2024

The influence of inlet freestream turbulence (FST) on the boundary layer transition over the suction surface of a controlled-diffusion compressor blade is demonstrated here by employing a well-resolved large-eddy simulation. Inherent to low Reynolds number conditions, a laminar separation bubble (LSB) forms on the suction surface, attributing to substantial flow diffusion. Inlet FST levels ranging from 1.5% to 7.6% are systematically varied, while maintaining a constant Reynolds number based on axial chord and inlet velocity at 2.1 × 105. Transition of the shear layer is initiated via Kelvin–Helmholtz instability with the amplification of selective frequencies until an inlet FST of 2.3%. Secondary instability emerges in the second half of the LSB, attributed to the amplification of perturbations in the braid region, ultimately leading to breakdown near the reattachment. At a moderate FST level of 4.2%, longitudinal streaks in the first half of the blade elongate downstream, causing the LSB to disappear, while the flow becomes inflectional at the mid-chord. Thus, the boundary layer transition in the second half of the blade is attributed to the high receptivity of the inflectional layer and breakdown of streaks, leading to an exponential growth of disturbances. Finally, at an inlet FST of 7.6%, the boundary layer appears pre-transitional in the first half of the blade, exhibiting significant turbulence levels. In the latter half, excitation occurs primarily through the breakdown of streaks, reflecting an algebraic growth of disturbances. Flow features and oscillations in the Nusselt number in this case suggest the outer mode of streak-induced instability.