TRANSIENT CONTROL OF THE SEPARATING FLOW OVER AN AIRFOIL

6th International Symposium on Turbulence and Shear Flow Phenomena

George Tak Kwong Woo ; Ari Glezer

PID  :  127772

来源: CEUR

【 摘 要 】

The effects of pulsed actuation on stalled NACA 4415airfoil is investigated in wind tunnel experiments.Theactuation results in transitory flow attachment that ismanifested by rapid changes in the global circulation andaerodynamic forces.Actuation is applied by a momentary[O(1 msec)] jet produced by a combustionbased actuatorsuch that the characteristic duration of the impulse is anorder of magnitude shorter than the characteristicconvective time over the airfoil.The present work hasshown that largescale changes in vorticity accumulationand flux can be effected by successive repetitions of a singleactuation pulse and are accompanied by significantshedding of CCW vorticity concentrations on the pressureside coincidently with the trapping of CW vorticityconcentrations, hence extending the streamwise domain ofthe attached vorticity layer towards the trailing edge.BACKGROUNDTraditional approaches to control of separation onstalled airfoil have focused on quasisteady actuation withintwo distinct frequency regimes. “Lowfrequency” actuationhas relied on receptivity of the separated, wakedominatedflow to external actuation within a narrowband of Strouhalnumbers that effectively correspond to unstable frequenciesof the near wake, Stact ~ O(1) (e.g., Neuburger andWygnanski, 1987, and Seifert et al., 1996).“Highfrequency” actuation is decoupled from globalflow (wake) instabilities and emphasizes fluidicmodification of the “apparent” aerodynamic shape of thesurface upstream of separation at actuation frequencies thatare at least an order of magnitude higher than thecharacteristic wake frequency [i.e., Stact ~ O(10)] (e.g.,Honohan et al., 2000, and Glezer et al., 2005).Actuation iseffected by forming a controlled interaction domain oftrapped vorticity between a surfacemounted fluidic actuatorand the cross flow above the surface that displaces the localstreamlines of the cross flow and thereby induces a ‘virtual’change in the shape of the surface.The separated flow is extremely susceptible to transitoryactuation such that substantial control authority can beachieved when the actuation input is applied on time scalesthat are significantly shorter than the characteristicadvection time over the separated flow domain. Brzozowski and Glezer (2006) exploited the receptivity ofseparated flow over a stalled airfoil and showed that a singleactuation pulse [O(0.05Tconv)] could lead to brief, partialcollapse of the separated flow domain and a momentaryincrease in circulation on time scale of 10Tconv. The recentwork of Woo et al. (2008) demonstrated significant pressureand lift recovery of a stalled airfoil with successively pulsedactuation that are applied Tconv apart, and with burst modulated actuation. The present work is motivated by theprevious investigations of pulsed actuation. The major focusof the present work is on the transient aerodynamic effects