The control of flow separation by periodic excitation

The vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes are studied in a low speed water channel. it is shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude and also the shape of the oscillation waveform. An important observation in this study has been the existence of an axial flow along the cores of the wake vortices. Estimates of the magnitude of the axial flow suggest a linear dependence on the oscillation frequency and amplitude.

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Vortical patterns in the wake of an oscillating airfoil

It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.

Flapping Wing Aerodynamics: Progress and Challenges

The evolution of particle image velocimetry (PIV) from its various roots is discussed. The importance of these roots and their influence on different trends in the speciality are described. The state-of-the-art of the technique today is overviewed and illustrated by reference to recent, seminal publications describing both the development and application of PIV.

Particle image velocimetry: A review

Optimum design configuration of Savonius rotor through wind tunnel experiments

The Beddoes/Leishman dynamic-stall model has become one of the most popular for the provision of unsteady aerofoil data embedded in much larger codes. The underlying modeling philosophy was that it should be based on the best understanding, or description, of the associated physical phenomena. Even though the model was guided by the flow physics, it requires significant empirical inputs in the form of measured coefficients and constants. Beddoes provided these for a Mach number range of 0.3–0.8. This paper considers one such input for a Mach number of 0.12, where, from the Glasgow data, it is shown that the current stall-onset criterion, and subsequent adjustments, yield problematic results. A new stall criterion is proposed and developed in the best traditions of the model. It is shown to be very capable of reconstructing the Glasgow’s data for stall onset both the ramp-up and oscillatory tests.

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A New Stall-Onset Criterion for Low Speed Dynamic-Stall

Results of a blade-vortex interaction (BVI) study incorporating particle image velocimetry (PIV) are presented. Test cases encompass a variety of intersection geometries, including head-on parallel interactions. Results provide quantified flow visualization images of the flows generated during interactions. The deformation and division of the interaction vortex during head-on interactions are documented. In addition, secondary vortical structures generated during interactions are observed and their effect upon the interaction vortex is examined. All PIV results are examined in the light of understanding gained from previous pressure measurements made from the same BVI facility.

Examination of vortex deformation during blade-vortex interaction

This paper examines the dynamic stalling of a finite wing of aspect ratio 3·0 when subject to constant pitch motions up to and beyond stall. In particular, unsteady surface pressure data were obtained at 192 locations on the wing surface and these were then analysed to provide information on the nature and phasing of dynamic stall events in both the chordwise and spanwise directions. It was also possible to obtain sectional force and moment coefficients by integration of the pressures measured on specific chordal arrays. This provided valuable insight into the load distribution on the wing throughout the range of motion. On this basis, it was established that the wing loading distribution was consistent with conventional understanding of steady wing loading up to the incidence at which the dynamic stall vortex was initiated. Beyond this point, the formation and subsequent convection of the vortex structure was found to be strongly three-dimensional but, nevertheless, exhibited many of the features of two-dimensional dynamic stall.

An experimental study of dynamic stall on a finite wing

Numerical simulation of 3-D dynamic stall has been undertaken using computational fluid dynamics. As a first
step, validation calculations have been performed for cases in which experimental data were available. Although the
amount and quality of the experimental data available for 3-D dynamic stall does not match what is available for 2-D
cases, the computational fluid dynamics was found capable of predicting this complex 3-D flow with good accuracy.
Once confidence on the computational fluid dynamics method was established, further calculations were conducted
for several wing planforms. The calculations revealed the detailed structure of the 3-D dynamic stall vortex and its
interaction with the tip vortex. Remarkably, strong similarities in the flow topology were identified for wings of very
different planforms.

Computational Fluid Dynamics Study of Three-Dimensional Dynamic Stall of Various Planform Shapes

Results of a comparison of dynamic stall onset are presented, as assessed from low Mach number windtunnel data contained in the University of Glasgow's database and that from the well established Beddoes' model. The model, which was originally developed to reconstruct higher Mach number dynamic stall characteristics, exhibited a lower stall onset incidence than that assessed from the windtunnel data. The differences and speculations on the physical reasoning underlying the two assessments are discussed. An addition to the Beddoes model is proposed which yields an improved reconstruction of the Glasgow windtunnel data.

R. A. McD. Galbraith

Papers

A New Stall-Onset Criterion for Low Speed Dynamic-Stall

Examination of vortex deformation during blade-vortex interaction

An experimental study of dynamic stall on a finite wing

Computational Fluid Dynamics Study of Three-Dimensional Dynamic Stall of Various Planform Shapes

Modelling dynamic stall vortex inception at low Mach numbers