Conpressibility Effects on Dynamic Stall of Oscillating Airfoils

A research proposal to investigate the "Compressibility Effects on Dynamic Stall of Oscillating Airfoils" was submitted to ARO and the project was funded in April 1986. The aim was to obtain a basic understanding of the effect of compressibility on the phenomenon of dynamic stall under typical flight conditions encountered by a helicopter in forward flight., so that eventually a means for its control can be devised and thus, its flight envelope can be expanded. The initial phase of the study was devoted to building a drive system to produce the necessary unsteady airfoil motion. A novel design was arrived at after reviewing the various possibilities and was built. It uses a four-bar chain mechanism of which the airfoil is one of the links.U.S. Army Research Office ARO 23394.10-EG, MIPR ARO 137-86U.S. Army Research Office ARO 23394.10-EG, MIPR ARO 137-8

Oscillating Airfoil Velocity Field During Large Amplitude Dynamic Stall

The leading edge flow field of an airfoil executing a. sinusoidal oscillatory motion and experiencing dynamic stall under compressibility conditions has been studied using a two component LDV system. Phase averaged mean velocity measurements and some flow quantities derived from it are presented and discussed. The results indicate extremely large accelerations of the flow are present around the leading edge with mean velocity values 60% higher than and instantaneous velocities as large as 1. 75 times the free stream velocity. The velocity profiles at certain locations over the airfoil resemble that of a wake

Interferometry and Computational Studies of an Oscillating Airfoil Compressible Dynamic Stall

Proc. of the 5th Asian Congress of Fluid Mechanics, Taejon City, Korea, Aug. 1992, Vol. II, pp. 1047 - 1050

Comparison of pitch rate history effects on dynamic stall

Presented at the NASA/AFOSR/ARO Workshop on Physics of Forced Unsteady Separation April 17-19, 1990 NASA Ames Research Center, Moffett Field, CA 9403

Compressible Dynamic Stall Control using a Variable Droop Leading Edge Airfoil

(AIAA Paper 2003-0048), Journal of Aircraft, Vol. 41, No. 4, pp. 862-969, Jul. - Aug. 2004.The control of compressible dynamic stall using a variable droop leading edge airfoil is described. The leading 25% of a VR-12 airfoil is drooped as it executes sinusoidal pitch oscillations such that the leading portion of the airfoil is always at a low effective incidence to the flow. Airfoil performance data determined for freestream Mach numbers ranging from 0.2 to 0.4, at reduced frequencies from 0 to 0.1, and using unsteady pressure transducer measurements, show that droop reduces the tendency of the airfoil to enter the dynamic stall state. Even when it does, the strength of the dynamic stall vortex is significantly reduced, which is reflected in the 40 to 50% smaller negative peak pitching-moment values, with positive damping of the airfoil. Also, the airfoil drag when the droop is dynamically varied is reduced by up to 75% relative to a nondrooped airfoil, making a strong case for the use of this concept for dynamic stall control.U.S. Army Research Offic

Compressible Dynamic Stall Control Using Dynamic Shape Adaptation

(AIAA Paper 99-0655), AIAA Journal, Vol. 39, No. 10, pp. 2021-2024, Oct. 2001.U.S. Army Research Office, Research Grant MIPR8BNP-SARO0

A Quantitative Study of Unsteady Compressible Flow on an Oscillating Airfoil

(AIAA Paper 91-1683), Journal of Aircraft, Vol. 31, No. 4, Jul.- Aug. 1994, pp. 892 - 898

A Holographic Animation of Compressible Flow Interferograms

Prize Winning Entry in ﾓGallery of Fluid Motionﾔ, Physics of Fluids, Vol. 4, No. 9, Sep. 1992, pp. 1869 - 1882

Schlieren Studies of Compressibility Effects on Dynamic Stall of Airfoils in Transient Pitching Motion

Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0 - 60 degrees over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experiments] documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers, the flow was dominated by a single vortex. The delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value. Increasing the pitch rate monotonically delayed stall to angles of attack as large as 27 degrees.AFOSR-MIPR-87-0029 and 88-0010NAVAIRAR

Design and Development of a Dynamically Deforming Leading Edge Airfoil for Unsteady Flow Control

ICIASF '97 RECORD, IEEE Publication No. 97CH36121, pp. 132-140.A novel approach to unsteady flow separation and dynamic stall control using a dynamically deforming leading edge airfoil is described. The design details of a carbon-fiber composite skin airfoil having a thickness of 0.002 in. at the leading edge and capable of deforming at 20 Hz in unsteady flow at freestream Mach numbers of up to 0.45, are discussed. Implementation of the scheme at model scales places extraordinary demands on the design, material and fabrication of such an airfoil. Rate scaling further requires very-rapid-response instrumental ion, measurement techniques and data acquisition schemes. The special instrumentation control system developed for these experiments as well as the fluid dynamic results of successful flow control that was achieved using this method, are also discussed.U.S. Army Research Office ARO MIPR 133-94, ARO 32480.11-EGU.S. Army Research Office ARO MIPR 133-94, ARO 32480.11-E