Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements

Dynamic pitching effect on a laminar separation bubble

“…Burgmann and Schröder (2008) demonstrated experimentally a decreasing bubble size as well as an upstream shift of the separation, transition and reattachment locations, when increasing Reynolds number (2 × 10 4 -6 × 10 4 ), angle of attack (4 • -8 • ) or free stream turbulence intensity level. Similar turbulence level effects had been observed by Ol et al (2005) in three different facilities at Re = 6 × 10 4 and an incidence angle of α = 4 • .…”

“…Literature has shown that the vortex roll-up and shedding occur in the downstream part of the LSB (Burgmann et al 2007;Burgmann and Schröder 2008). Therefore, the area of interest to be observed during tomographic experiments was selected on the basis of the observations obtained from the planar TR-PIV measurements.…”

“…Furthermore, the measurement volume is dictated by the spanwise dimensions of the characteristic structures expected to occur in the LSB. Burgmann and Schröder (2008) visualised arch-like structures by considering a FOV that covers 30 % of the chord in spanwise direction. The depth of the measurement volume in the direction normal to the aerofoil chord is determined by the height of the bubble, since the characteristic vortices generated scale with this height.…”

“…The transition point is defined as the location where the normalised Reynolds stresses starts to increase exponentially along this line, i.e. the location where the logarithmic slope of this Reynolds stress curve changes dramatically; a similar approach was used by Burgmann and Schröder (2008). These results are compared to a second method used by Burgmann and Schröder (2008) and Ol et al (2005) where the transition location is determined at the chord-wise position at which Reynolds stresses reach a threshold value of 0.001.…”

“…the location where the logarithmic slope of this Reynolds stress curve changes dramatically; a similar approach was used by Burgmann and Schröder (2008). These results are compared to a second method used by Burgmann and Schröder (2008) and Ol et al (2005) where the transition location is determined at the chord-wise position at which Reynolds stresses reach a threshold value of 0.001. For simplicity these two methods are referred to as the Transition Exponential Method (TEM) and the Transition Threshold Method (TTM), respectively.…”