The flow field of an airfoil oscillated periodically over a reduced frequency range, 0 ≤ k ≤ 1.6, is studied experimentally at chord Reynolds numbers of Rc = 22000 and 44000. For most of the data, the NACA0012 airfoil is pitched sinusoidally about one quarter chord between angles of attack α of 5° and 25°. The cyclic variation of the near wake flow field is documented through flow visualization and phase-averaged vorticity measurements. In addition to the familiar dynamic stall vortex (DSV), an intense vortex of opposite sign is observed to originate from the trailing edge just when the DSV is shed. The two together take the shape of the cross-section of a large ‘mushroom’ while being convected away from the airfoil. The phase delay in the shedding of the DSV with increasing k, as observed by previous researchers, is documented for the full range of k. It is observed that the sum of the absolute values of all vorticity convected into the wake over a cycle is nearly constant and is independent of the reduced frequency and amplitude of oscillation but dependent on the mean α. The time varying component of the lift is estimated in a novel way from the shed vorticity flux. The analytical foundation of the method and the various approximations are discussed. The estimated lift hysteresis loops are found to be in reasonable agreement with available data from the literature as well as with limited force balance measurements. Comparison of the lift hysteresis loops with the corresponding vorticity fields clearly shows that major features of the lift variation are directly linked to the evolution of the large-scale vortical structures and the phase delay phenomenon.

Experimental investigation of the flow field of an oscillating airfoil and estimation of lift from wake surveys

Using a two-dimensional airfoil, a disturbance was introduced into a plane mixing layer some distance
downstream of the splitter plate trailing edge. Results indicate that it is possible to induce very large changes in
the layer growth rate downstream of the disturbance location, while leaving the portion of the shear layer
between the splitter plate and the disturbance source essentially unaffected. Furthermore, the use of forcing for
modification of the mixing layer in the region upstream of the disturbance is demonstrated. It Is shown that two
different mechanisms are responsible for coupling such disturbances to the flow in the present forcing of
upstream and downstream regions.

/pdf/effects-of-a-downstream-disturbance-on-the-structure-of-a-1c8uykqh48.pdf

Effects of a downstream disturbance on the structure of a turbulent plane mixing layer

The flowfield of an airfoil oscillated periodically over a wide range of reduced frequencies, 0 less than or = k less than or = 1.6 is studied experimentally at chord Reynolds numbers of R sub c = 22,000 and 44,000. The NACA0012 airfoil is pitched sinusoidally about one quarter chord between angles of attack (alpha) of 5 and 25 degrees. Detailed flow visualization and phase averaged vorticity measurements are carried out for k = 0.2 to document the evolution and the shedding of the dynamic stall vortex (DSV). In addition to the DSV, an intense vortex of opposite sign originates from the trailing edge just when the DSV is shed. After being shed into the wake, the two together take the shape of a large 'mushroom' while being convected away from the airfoil. The unsteady circulation around the airfoil and, therefore, the time varying component of the lift is estimated in a novel way from the shed vorticity flux and is found to be in good agreement with the lift variation reported by others. The delay in the shedding of the DSV with increasing k, as observed by previous researchers, is documented for the full range of k. The DSV, for example, is shed nearly at the maximum alpha of 25 degrees at k = 0.2, but is shed at the minimum alpha of 5 degrees at k = 0.8. At low k, the flowfield appears quasi-steady and the bluff body shedding corresponding to the maximum alpha (25 degrees) dominates the unsteady fluctuations in the wake.

Experimental investigation of the flowfield of an oscillating airfoil

L'influence d'un ecoulement tourbillonnaire sur un profil oscillant dans une soufflerie subsonique

Vortex influence on oscillating airfoils at high angle of attack

This the final report of research conducted at the California Institute of Technology,
by Paul E. Dimotakis, in collaboration with Dr. M. M. Moochesfahani as co-investigator,
and with the assistance of Mr. P. Tokumaru during the last year. The primary goal was to
explore ways in which open loop and closed feedback loop control methods can be utilized to affect the qualitative and quantitative behavior of turbulent shear layers. In particular, we attempted to

i. investigate the dynamic behavior and response of these flows through a study of the feedback control schemes required to produce a given desired outcome,

ii. explore the extent to which specific properties of turbulent shear layer flows, such as growth rate profile and mixing, can be manipulated and altered by such means,

and, 

iii. devise schemes for producing turbulent shear layer flows with specific desirable properties, as might be dictated, for example, by the flow specifications for the
efficient operation of a combustion device.

In the course of this work, other derivative and closely related efforts were also undertaken,
some of which will be described below.

The work conducted under the sponsorship of this Grant was primarily experimental and in close collaboration with a broader experimental, numerical and theoretical effort at
Caltech to study unsteady separated flows, and the evaluation and use of control techniques
in these flows in particular.

D. Mook

Papers

On the wake of a pitching airfoil