Transition mechanisms in laminar separation bubbles with and without incoming wakes and synthetic jet effects

Abstract

Laminar separation and transition processes of the boundary layer developing under a strong adverse pressure gradient, typical of Ultra-High-Lift turbine profiles, have been experimentally investigated for a low Reynolds number case. The boundary layer development has been surveyed for different conditions: with steady inflow, with incoming wakes and with the synchronized forcing effects due to both incoming wakes and synthetic jet (zero net mass flow rate jet). In this latter case, the jet Strouhal number has been set equal to half the wakereduced frequency to synchronize the unsteady forcing effects on the boundary layer. Measurements have been taken by means of a single-sensor hot-wire anemometer. For the steady inflow case, particle image velocimetry has been employed to visualize the large-scale vortical structures shed as a consequence of the Kelvin-Helmholtz instability mechanism. For the unsteady inflow cases, a phase-locked ensemble averaging technique, synchronized with the wake and the synthetic jet frequencies, has been adopted to reconstruct the boundary layer space-time evolution. Results have been represented as color plots, for several time instants of the forcing effect period, in order to provide an overall view of the time-dependent transition and separation processes in terms of ensemble-averaged velocity and unresolved unsteadiness distributions. The phase-locked distributions of the unresolved unsteadiness allowed the identification of the instability mechanisms driving transition as well as the Kelvin-Helmholtz structures that grow within the separated shear layer during the incoming wake interval and the synthetic jet operating period. Incoming wakes and synthetic jet effects in reducing and/or suppressing flow separation are investigated in depth. List of symbols ClJet momentum coefficient ¼

On the origin of spanwise vortex deformations in laminar separation bubbles

“…When expressed in terms of dimensionless frequency, ω * = 0.25δ w (2πf )/Ū (Ho & Huerre 1984), based on vorticity thickness (δ w ) and the average velocity of the shear bounding limits (Ū), the fundamental frequency is ω * 0 = 0.21. This value is within the expected range for Kelvin-Helmholtz instability in free shear layers (Ho & Huerre 1984) (0.21 < ω * < 0.222) and agrees well with relevant studies on LSBs (Pauley, Moin & Reynolds 1990;Watmuff 1999;Simoni et al 2012).…”

Response of a laminar separation bubble to impulsive forcing

“…Since the distance of the bubble shear layer from the wall varies with respect to streamwise position, the aforementioned scaling is not valid unless the shear layer is at sufficient distance from the wall. Consequently, correspondence with the expected range (ω * ≈ 0.21) is mainly achieved in the region of the mean maximum height (e.g Pauley et al 1990;Watmuff 1999;Simoni et al 2012a). Figure 9b depicts a comparison between ω * from the present results and the reporting of Watmuff (1999) and Diwan & Ramesh (2009).…”

“…Simoni et al 2012a). In addition, the separation bubble does not exhibit any features of long bubbles (Marxen & Henningson 2011) throughout the forcing cycle, thus, the elongation represents part of a dynamic process of a short bubble.…”

Free-stream turbulence effects on the boundary layer of a high-lift low-pressure-turbine blade

“…In a real machine, the understanding of the effects due to the FSTI is further complicated by the rotor-stator aerodynamic interaction, which strongly alters the dynamics of the separation process (if any) [22][23][24], as well as of the by-pass transition process in the case of attached flows. Indeed, incoming wakes carry high frequency velocity fluctuations that interact with the stability characteristics of the boundary layer developing between wakes [25,26].…”