Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles

“…At low frequency (St = fL b /U ref 0.01, where St is the Strouhal number, f is the frequency and L b the size of the bubble defined as the distance between transitory detachment and transitory reattachment (Simpson 1989)), the TSB appears to expand and contract in a quasi-periodic breathing motion. This motion was educed using a pair of classical thermal-tuft probes in Weiss et al (2015) and later confirmed by high-speed particle image velocimetry (PIV) measurements in Mohammed-Taifour & Weiss (2016). At a medium normalized frequency of St 0.35, the unsteady behaviour of the flow is characterized by roller-like structures similar to those observed in the DNSs of Na & Moin (1998b) and Abe (2017), and with a very close convection velocity of U c 0.30U ref .…”

“…Finally, at higher frequencies (St > 1), the pressure and velocity fluctuations are caused by the turbulent nature of the flow. Mohammed-Taifour & Weiss (2016) also observed a bi-modal distribution of the pressure fluctuations, with a first peak of c p near detachment and a second near reattachment, but attributed the first peak to the low-frequency breathing motion of the TSB (see also Weiss et al (2015)). This appears to be inconsistent with the results of Na & Moin (1998b) and Abe (2017) who also observed a first peak of c p near detachment but did not resolve the low-frequency breathing motion near St 0.01 because of the necessarily limited simulation time of their DNS.…”

“…The experimental techniques used in the present work were essentially the same as in Weiss et al (2015) and Mohammed-Taifour & Weiss 2016and will only be described briefly. The average wall pressure was measured using two Scanivalve DSA3217 pressure scanners and the wall-pressure fluctuations with several Meggitt 8507C-1 piezoresistive pressure transducers.…”

“…The choice of a small TSB which is attached in the mean but still features large regions of instantaneous back flow was deliberate, so as to investigate if the low-and medium-frequency unsteadiness that was observed so far in large TSBs still occurs without any mean detachment. Our second objective is to clarify the cause of the local maximum in wall-pressure fluctuations that occurs upstream of the mean separation in the numerical simulations of Na & Moin (1998b) and Abe (2017), and in the experiments of Weiss et al (2015) and Mohammed-Taifour & Weiss (2016). Of particular interest would be to know if this local maximum is the result of the APG imposed on the attached turbulent boundary layer, as suggested by the simulations, or if it is caused by the low-frequency breathing motion.…”

Time-Dependent Effects of Glaze Ice on the Aerodynamic Characteristics of an Airfoil

“…In their measurements, they observed a large-scale unsteadiness accompanied by an enlargement and shrinkage of the bubble and a flapping motion of the shear layer near the separation line. At a turbulent separation bubble, the flow is characterized by two separate time-dependent phenomena: flapping and shedding [29]. One mode is associated with a global breathing motion of the separation bubble, described as "flapping motion" in the literature.…”