R. A. McD. Galbraith

An experimental study of dynamic stall on a finite wing

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.

Results from a set of low speed blade-vortex interaction experiments

The results of a Blade-Vortex Interaction (BVI) experiment are presented. The experiments were the second series to be carried out at Glasgow University (see Kokkalis and Galbraith 1986, 1987), and reflect improvement to the facility. These improvements have resulted in sufficient resolution to observe clear evidence of vortex passage and propagative disturbances caused by the BVI. Additionally the quality of the pressure data permitted a meaningful assessment of force and moment coefficients, and a clear correspondence between their detailed time histories was observed. The data obtained are also compared with the results of previous studies, and qualitative and quantitative similarities are discussed.

Phenomena observed during aerofoil ramp-down motions from the fully separated state

Unsteady pressure data from seven aerofoil models performing constant pitch rate “ramp-down” motions from the fully separated state to low incidence have been analysed. It is observed that the time delay between attachment at the leading edge and the occurrence of minimum normal force is, to a first order, independent of reduced pitch rate. As a result, at sufficiently high pitch rate, negative normal force may be generated at large positive incidence. In contrast to this, the re-establishmeht of attached flow to the upper surface is strongly dependent upon pitch rate. The paper also contains a description of smoke flow visualisation tests. These were at a much lower Reynolds number than the pressure data tests, and consequently are of qualitative interest. Results of flow visualisation show that during a ramp-down test the near wake is much larger than in the corresponding static case, and that attachment is significantly delayed.

Three-dimensional vortex interactions with a stationary blade

Surface pressure measurements were obtained during a three-dimensional vortex interaction with a NACA 0015 aerofoil. The upper and lower surfaces of the blade experienced different aerodynamic loads which appear to be controlled by the impact of the vortex axial core flow on the blade surface. On the upper surface of the blade, where the vortex core flow was away from the aerofoil, the interaction was characterised by the generation of a suction peak. On the lower surface, where the axial component was towards the blade, a pressure pulse developed and seemed to be influenced by the vortex approach angle. These features resulted in rapid changes in normal force and quarter chord pitching moment during the interaction. This impulsive loading of the blade may provide some explanation for sound generation and control degradation problems associated with the tail rotor of helicopters

The aerodynamic characteristics of a GU25-5(11)8 aerofoil for low Reynolds numbers

The paper presents the results of wind tunnel tests of a GU25-5(11)8 aerofoil section over the Reynolds number range, 50,000 to 610,000. For the particular test conditions, the aerofoil exhibits severe degradation of performance below Re = 300,000; a phenomenon which is known to be quite general. This particular aerofoil section has been used for the canards of microlights where low Reynolds numbers are common.

A direct viscid-inviscid interaction scheme for the prediction of two-dimensional aerofoil lift and pitching moment in incompressible flow

This paper presents a method for assessing two-dimensional aerofoil lift and pitching moment characteristics including trailing edge and gross laminar separation. The model used is a direct viscid-inviscid interaction scheme based on a vortex panel method with boundary-layer corrections and an inviscidly modelled wake. The integral boundary-layer methods adopted behave well in the region of separation and thus, good comparisons with measured separation characteristics are obtained. Generally the predictions of lift and pitching moment may be considered to be within the experimental error, but where this is not the case, the applicability of the modelling technique is discussed.

An investigation of dynamic stall through the application of leading edge roughness

The dynamic stall characteristics of a NACA 0015 performing ramp-up motions are considered. Pressure data show the development and convection of structures at the leading and trailing edges, which indicate differing degrees of what are described as leading edge and trailing edge stall mechanisms. When a transition strip is placed at the leading edge, the characteristics of leading edge stall predominate, and the growth of the structure at the trailing edge is suppressed. In addition the stall vortex convection speed is observed to alter significantly, which is interpreted as a change in the vortex origin, size, strength and trajectory.

Flow pattern around a shrouded cylinder at Re = 5 × 103

Abstract The report describes the qualitative results of a flow visualisation study of a shrouded cylinder of the slatted form proposed by Wong [1]. A mosaic of streak patterns is drawn for the flow both internal and external to the slats. The salient features of the flow are noted, and possible reasons for such a system suppressing the vortex-excited vibrations are discussed.

The effect of wing planform shape on dynamic stall

We examine the dynamic stalling of three wing planforms and characterises the main features of the stalling process in each case. The particular data were obtained during a three year research programme in the Department of Aerospace Engineering, University of Glasgow to collect high-resolution unsteady pressure data on the dynamic stalling characteristics of finite wing planforms. In this study, which was motivated by the pressing need for a greater understanding of the strongly three-dimensional effects in the tip region of helicopter rotors, the three wing planforms considered were a straight rectangular wing, a rectangular wing with swept tips and a delta wing. The initial test programme was followed by a further three years of detailed analysis and interpretation of the test data. Results from this analysis are presented in the present paper for cases in which the wings were subject to ramp motions

Aspects of the flow in the immediate vicinity of a porous shroud

Abstract This note presents the results of a flow-visualisation study of three successful porous shrouds designed for the suppression of vortex-excited vibration. The results clearly show that, although significant detailed differences do exist, the salient features of the streak patterns in the immediate vicinity of each shroud are similar, thus confirming the previous speculations of Galbraith [1] regarding such flow patterns.