On management and control of turbulent shear flows

A review of helicopter rotor blade tip shapes

The reattachment of a fully turbulent, two dimensional, separated shear layer downstream of a single-sided sudden expansion in a planar duct flow was examined experimentally. The importance of changing the structure of the separated shear layer on the reattachment process itself was examined. For all cases, the Reynolds number based on step height was greater than 20,000, the expansion ratio was 5/3, and the inlet boundary layer was less than one-half step height in thickness. A crucially important phase was the development of a pulsed wall probe for measurement of skin friction in the reattachment region, thus providing an unambiguous definition of the reattachment length. Quantitative features of reattachment - including streamwise development of the mean and fluctuating velocity field, pressure rise, and skin friction - were found to be similar for all cases studied when scaled by the reattachment length. A definition of the reattachment zone is proposed.

Experimental study of flow reattachment in a single-sided sudden expansion

Experimental methods which have been developed over the last three decades to measure one or more components of the vorticity vector are described and assessed. These include methods which estimate the velocity gradients comprising the various vorticity components, methods which sense fluid rotation directly, and methods which sense other fluid properties which can be related to vorticity.

Methods of measuring vorticity in turbulent flows

The significance of vorticity and the importance of its temporally and spatially resolved measurement in turbulent flows is discussed. The measurement methods are categorized as those which use multipoint measurements of velocity to determine the required velocity gradients or circulation and those which sense these gradients directly. The characteristics and performance of these vorticity measurement methods are evaluated. Where available, measurements of some vorticity field statistics by two or more methods are compared.

The measurement of vorticity in transitional and fully developed turbulent flows

The effect of various flow manipulators on different types of swirling flows was investigated experimentally in order to determine the important mechanisms involved and to gain insight into efficient means for removing such swirls from wind tunnels and duct flows. Several swirling test flow conditions were used which had different transverse distributions of angular momentum and turbulence intensity levels. They represented typical conditions arising in flows through ducts and wind tunnels. The effectiveness of commonly used flow manipulators, such as screens, grids, and honeycombs, in removing the swirl from these flows was established. Different mechanisms dominated depending upon the scaling between the swirl size and the characteristic lengths of the manipulator, with some mechanisms being more efficient in removing the swirl than others. In many cases, similar performance was obtained by any one of several manipulators, making it possible to select a flow manipulator based on other criteria, such as maximum turbulence suppression or minimum pressure drop, which are both very important in wind tunnel design and operation.

Management of Swirling Flows with Application to Wind Tunnel Design

: An experimental investigation of techniques for generating, measuring and controlling swirling flows in confined streams has been carried out. Several typical swirling flows were generated and their careful calibration was carried out using hot-wire anemometers and miniature vane-vorticity indicators. By inserting in these flows various flow manipulators, such as honeycombs, screens and perforated plates, a comparison of the characteristics of the flow downstream of the manipulators (e.g., distribution of streamwise vorticity) to the original test flows provided a measure of the effectiveness of the manipulators in suppressing large scale vorticity as well as clues to dominant mechanism of the flow transformations. It has been found that the effectiveness of a manipulator in suppressing the swirl conditions is reduced somewhat as the strength of the swirl upstream of it is increased, and is improved as the width of the vortex is increased while its strength remains the same. Although honeycombs have a high efficiency in suppressing swirl, their optimum mesh depends on the size of the swirl. The pressure drop across the manipulator, which plays an important role in design of wind tunnels and flow ducts, appears to play a minor role in the management of swirling flows. Recommended procedures for control of large scale swirling and secondary flows are developed, and general concepts are obtained which design and test engineers can adapt for 'tailoring and manipulating' their own special flows with different rotational characteristics. (Author)

Generation and Management of Swirling Flows in Confined Streams.

Vane-type vorticity indicators of various sizes have been constructed, calibrated, and used for direct and rapid measurement of local mean streamwise vorticity in several swirling flows. The dependence of the calibration factor, which relates the vane rotational speed to the local angular velocity of the fluid, on the trees (ream velocity, the fluid rotational speed, and the transverse vorticity gradient was determined. When the vanes were calibrated carefully and checked and maintained regularly, good agreement was achieved with vorticity profiles obtained using cross-wire data. Otherwise, a vane could be used only to give an indication of vorticity. Nomenclature Nj- = angular velocity of the fluid, rpm Nv = angular velocity of the vane-vorticity indicator, rpm r = radial distance measured from the center of the test section Ux = freestream mean velocity W — vertical velocity in the fluid normal to the streamwise direction x - axial or streamwise distance y = lateral distance normal to streamwise direction and measured from the center of the test section z = vertical distance normal to streamwise direction and measured from the center of the test section a = half-angle setting between the airfoils rj = vane indicator ratio T = delay time in autocorrelation function Qv = vorticity in the streamwise direction

Vorticity Measurements Using Calibrated Vane-Vorticity Indicators and Cross-Wires

: Vane-type vorticity indicators of various sizes have been constructed, calibrated and used for direct and rapid measurement of local mean streamwise vorticity in several swirling flows. The dependence of their calibration factor, which relates the vane rotational speed to the local angular velocity of the fluid, on the free-stream velocity, the fluid rotational speed, and the transverse vorticity gradient was determined. When the vanes were carefully calibrated, and regularly checked and maintained, good agreement was achieved between vorticity profiles obtained using them and X-wire data. Otherwise, a vane can only be used to give an indication of vorticity. (Author)

R. A. Wigeland

Papers

Management of Swirling Flows with Application to Wind Tunnel Design

Generation and Management of Swirling Flows in Confined Streams.

Vorticity Measurements Using Calibrated Vane-Vorticity Indicators and Cross-Wires

Vorticity measurements using calibrated vane-vorticity indicators and comparison with X-wire data