Abstract:The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied with the help of Large-eddy simulations (LES). A flat plate with a semicircular leading edge is employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case an infinite row of flat plates). This setup is a simplified repre… Show more
“…50 The mean bubble length ( l ) was reported as 2.75 D in the experiment, while LES predicted 2.77 D at a comparable level of fst. Furthermore, discrete spanwise vortical structures are formed due to excitation of the separated boundary layer 40,42 and scale of these eddies is 20–30% of the mean bubble length. 51 Thus, the scale of the spanwise eddies will be in between 0.55 and 0.83 D considering a bubble length of 2.77 D without injection.…”
Section: Computational Detailsmentioning
confidence: 99%
“…Examples are transitional flow over a low-pressure turbine blade subjected to periodic passing wakes, 53–56 vortex shedding behind a circular cylinder in proximity to a boundary layer, 57,58 transition of a laminar separation bubble and film cooling under the passing wakes. 16,42 These simulations involve resolving the phenomena, such as, bypass transition, wake-boundary layer interactions, separation-induced transition and shear layer instability leading to breakdown. Figure 4.Streamwise variation of time averaged (a) centreline and (b) span-averaged film-cooling effectiveness of steady jets: Solid line represents LES and circle denotes experiment.…”
Section: Computational Detailsmentioning
confidence: 99%
“…The sinusoidal oscillation may be considered as a generic jet unsteadiness that even occurs by periodic passing wakes. Earlier studies 40–42 elucidated that the laminar boundary layer, after separating near the leading edge, has undergone transition due to amplification of fst , eventually become turbulent and reattached forming a separation bubble. Thus, the novelty of the present study is to demonstrate the interactions of oscillatory jets with the separated boundary layer.…”
Influence of external modulation on unsteady flow and heat transfer near the leading edge of a constant thickness aerofoil has been described through large eddy simulation. This is a simplified approach to study film cooling activities near the leading edge of a turbine blade. Discrete jets, which are forced at a Strouhal number (St) of 0.37 with an averaged blowing ratio of unity, are ejected normally from a series of film cooling holes to a separated boundary layer. The results are compared against the corresponding steady injection. Larger coherent structures appear for a forced jet with an augmented vortex dynamics resulting in high jet lift-off, earlier break down, enhanced mixing with the cross flow and dilution of coolant layer. Resolved hairpins, which are the signature of coherent structures, illustrate that the vorticity and thermal field are highly correlated. Furthermore, evolution of hairpins and their advection control scalar transport and mixing. In brief, the modulation of coolant jet near the leading edge appears not beneficial for the combination of blowing ratio and frequency considered here.
“…50 The mean bubble length ( l ) was reported as 2.75 D in the experiment, while LES predicted 2.77 D at a comparable level of fst. Furthermore, discrete spanwise vortical structures are formed due to excitation of the separated boundary layer 40,42 and scale of these eddies is 20–30% of the mean bubble length. 51 Thus, the scale of the spanwise eddies will be in between 0.55 and 0.83 D considering a bubble length of 2.77 D without injection.…”
Section: Computational Detailsmentioning
confidence: 99%
“…Examples are transitional flow over a low-pressure turbine blade subjected to periodic passing wakes, 53–56 vortex shedding behind a circular cylinder in proximity to a boundary layer, 57,58 transition of a laminar separation bubble and film cooling under the passing wakes. 16,42 These simulations involve resolving the phenomena, such as, bypass transition, wake-boundary layer interactions, separation-induced transition and shear layer instability leading to breakdown. Figure 4.Streamwise variation of time averaged (a) centreline and (b) span-averaged film-cooling effectiveness of steady jets: Solid line represents LES and circle denotes experiment.…”
Section: Computational Detailsmentioning
confidence: 99%
“…The sinusoidal oscillation may be considered as a generic jet unsteadiness that even occurs by periodic passing wakes. Earlier studies 40–42 elucidated that the laminar boundary layer, after separating near the leading edge, has undergone transition due to amplification of fst , eventually become turbulent and reattached forming a separation bubble. Thus, the novelty of the present study is to demonstrate the interactions of oscillatory jets with the separated boundary layer.…”
Influence of external modulation on unsteady flow and heat transfer near the leading edge of a constant thickness aerofoil has been described through large eddy simulation. This is a simplified approach to study film cooling activities near the leading edge of a turbine blade. Discrete jets, which are forced at a Strouhal number (St) of 0.37 with an averaged blowing ratio of unity, are ejected normally from a series of film cooling holes to a separated boundary layer. The results are compared against the corresponding steady injection. Larger coherent structures appear for a forced jet with an augmented vortex dynamics resulting in high jet lift-off, earlier break down, enhanced mixing with the cross flow and dilution of coolant layer. Resolved hairpins, which are the signature of coherent structures, illustrate that the vorticity and thermal field are highly correlated. Furthermore, evolution of hairpins and their advection control scalar transport and mixing. In brief, the modulation of coolant jet near the leading edge appears not beneficial for the combination of blowing ratio and frequency considered here.
The evolution of a separated boundary layer over a model airfoil with semicircular leading-edge has been illustrated for angles of attack (α) varying from −3 deg to 10 deg, where the Reynolds number (Rec) based on chord is 1.6 × 105 and the inlet freestream turbulence (fst) being 1.2%. The features of boundary layer are described through measurements of velocity and surface pressure besides the flow visualization using a planar particle image velocimetry (PIV). Freestream perturbations are amplified because of enhanced receptivity of the separated boundary layer resulting in pockets of disturbances, which then propagate downstream attributing to random fluctuations near the reattachment. The separation and reattachment locations including the onset and end of transition are identified for changing α. The reattachment point changes from 18.8% to 47.7% of chord with the onset of separation at almost 7%, whereas the onset of transition moves upstream from 13.2% to 9% with increasing α. The bubble bursting occurs at α = 10 deg. The transition in the separated boundary layer occurs through Kelvin–Helmholtz (K–H) instability for α = 0 deg and 3 deg, whereas the K–H mechanism is bypassed for higher α with significant viscous effect.
The paper documents the manifestation of a shear layer under the excitation of a series of hemispherical protuberances near the leading edge of a constant-thickness airfoil. The experiments are performed at a Reynolds number of 1.6 × 105 based on the chord length and inlet velocity, where freestream turbulence is 1.2%. The hotwire and particle image velocimetry data are analyzed to appreciate the flow feature, illustrating the growth of perturbations, vortex dynamics, intermittency, and spectral response. A laminar separation bubble (LSB) appears near the leading edge for a smooth surface, and the shear layer is inviscidly unstable. The evolution of the shear layer significantly changes with a series of protuberances. The breakdown of the shear layer occurs almost immediately, triggering local turbulence resulting in a considerable reduction of bubble length. A separation bubble, which is highly asymmetric in the spanwise direction, is formed in this case. Although the power spectra of velocity fluctuations reveal the selective amplification of frequencies even with protuberances, the immediate augmentation of turbulence followed by faster decay suggests the transient growth of turbulence. The study has documented insight into features of a separation bubble subjected to leading-edge perturbations and might influence future studies on separation control over an airfoil.
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