Abstract—The unsteady wake of an EPPLER 361 airfoil in pitching motion has been investigated in a subsonic wind tunnel by hot-wire anemometry. The airfoil was given the pitching motion about the one-quarter chord axis at reduced frequency of 0182. Streamwise mean velocity profiles (wake profiles) were investigated at several vertically aligned points behind the airfoil at one-quarter chord downstream distance from trailing edge. Oscillation amplitude and mean angle of attack were varied to determine the effects on wake profiles. When the maximum dynamic angle of attack was below the static stall angle of attack, weak effects on wake were found by increasing oscillation amplitude and mean angle of attack. But, for higher angles of attack strong unsteady effects were appeared on the wake. Keywords—Unsteady wake, amplitude, mean angle, EPPLER 361 airfoil. I. INTRODUCTION HE unsteady aerodynamic theory of oscillating airfoils has received considerable attention in past years, especially problem of airfoil stall. The classical unsteady aerodynamic theory of oscillating airfoils was developed as a result of interest in aircraft flutter problems by Theodorsen [1] and Von Karman and Sears [2]. Later, Lighthill [3] and Wu [4] extended this theory using propulsion modeling of certain spaces of aquatic animal, birds and insects. In comparison with the many theoretical and numerical studies that have been devoted to the subject of oscillating airfoil, quite a few experimental results appear to be available. Experimental unsteady aerodynamic researches have very important role in both understanding the essential physics of the problem and validating the results from the computational studies. Most of the previous investigations were directed to unsteady wing loading and dynamic stall process, as reviewed by McCroskey [5] and Carr [6]. Recently, Tolouei et al [7] investigated experimentally the unsteady pressure distribution over an Sadeghi H. is with the Amir kabir University of Technology , Department of Aerospace Engineering, Tehran, Iran (e-mail: hamsadeghi@yahoo.com). Mani M., professor, Board Member, Center of Excellence in Computational Aerospace Engineering, is with the Amir kabir University of Technology , Department of Aerospace Engineering, Tehran, Iran (e-mail: mani@cic.aut.ac.ir). Ardakani M. A, assistance professor, is with Mechanical Engineering Department of Iranian Research Organization for science and technology (e- mail: ardakani@irost.org). EPPLER 361 airfoil. They found that pressure coefficients in the low angle of attack range showed little overshoot when compared with the static values, while for the large angle of attack cases the differences were significant. They considered that the large overshoot in the dynamic pressure coefficient for the high angle of attack case is probably due to the existence of the stable dynamic vortex, which is created near the leading edge and moves downstream. Soltani et al [8] showed that by increasing the oscillation frequency, the motion of the airfoil could not adjusted the free-stream flow, and the hysteresis loop of pressure coefficient will change into straight line. In addition in high angles of attack the hysteresis loop grows, so that preventing wing stall and increasing the lift. But this is just applied to smaller speeds. Ajalli et al [9] showed that at different amplitudes for unsteady airfoil, the hysteresis loops in the pressure data were both clockwise and counter clockwise when plotted against the equivalent angle of attack. It was found that heaving amplitudes had strong effects in pressure distribution, near the leading edge of the airfoil. Mani et al [10] measured Surface static pressure distribution on the upper and lower sides of the model, during the oscillating motion. It was found that reduced frequency had strong effects on the pressure distribution, near the leading edge of the airfoil. In spite of less attention about studying the characteristic of the wake in comparison with the measuring the force on oscillating airfoils, several important researches have been conducted into the problem of downstream wake. Satyanarayana [11] measured unsteady wakes of airfoils and cascades under a sinusoidally varying gust flow. Time-mean and time-dependent wake profiles at low frequency behind the airfoil were reported in his work, and the distinctions between these were discussed. Koochesfahani [12] studied experimentally the vortical flow patterns in the wake of a NACA0012 airfoil pitching at small amplitudes and showed that the oscillation wave form has an important effect in the vortical pattern shapes and mean velocity profiles in the wake. He found that there is a critical value for the oscillation frequency that the usual velocity defect profiles in the wake changes to excessive momentum similar to a jet flow and the airfoil produces trust force. Also, he showed that in special cases mean velocity profiles have two peak of velocity defect, that this is symptom of a double-wake structure. Park, Kim and Lee [13] measured the velocity field in the wake of a Effect of Amplitude and Mean Angle of Attack on Wake of an Oscillating Airfoil Sadeghi H., Mani M., and Ardakani M. A. T World Academy of Science, Engineering and Technology International Journal of Aerospace and Mechanical Engineering Vol:2, No:7, 2008 858 International Scholarly and Scientific Research & Innovation 2(7) 2008 scholar.waset.org/1307-6892/6798 International Science Index, Aerospace and Mechanical Engineering Vol:2, No:7, 2008 waset.org/Publication/6798