The near-field behavior of a wingtip vortex flow has been studied computationally and experimentally in an interactive fashion. The computational approach involved using the method of artificial compressibility to solve the three-dimensional, incompressible, Navier-Stokes equations with experimentally determined boundary conditions and a modified Baldwin-Barth turbulence model. Inaccuracies caused by the finite difference technique, grid resolution, and turbulence modeling have been explored. The complete geometry case was computed using 1.5 million grid points and compared with mean velocity measurements on the suction side of the wing and in the near wake. Good agreement between the computed and measured flowfields has been obtained. The velocity distribution in the vortex core compares to within 3% of the experiment.

Numerical/experimental study of a wingtip vortex in the near field

Incompressible swirling jets with Reynolds numbers of 1.0 × 10 5 have been studied using a miniature five-hole probe. The jet facility in which these measurements have been made has been specifically designed to produce a highly conditioned, swirling jet flow. To provide a comprehensive investigation of the effect of low-to-moderate swirl (well below vortex breakdown) on jet growth rate, two tangential velocity profiles, a solid-body type and a q-vortex type with swirl numbers of both 0.10 and 0.23, have been investigated and compared with a nonswirling jet

Experimental Study of Incompressible Jets with Different Initial Swirl Distributions: Mean Results

We present the development of miniature multihole pressure probes and a novel neural-network-based calibration algorithm for them. Seven-hole probes of tip diameters as low as 0.035 in. (0.9 mm) were successfully fabricated with high tip surface quality. Any of the typical probe tip geometries, i.e., hemispherical, conical, or faceted, could be fabricated. The miniature probes were calibrated and tested in a wind tunnel. A backpropagation-based neural-network calibration algorithm was developed for these probes, with flexibility in network architecture design and network self-optimization capabilities. In the feedforward mode the algorithm yields computational speeds an order of magnitude higher than those typically achieved by similar accuracy interpolation algorithms. The new algorithm has prediction accuracies of 0.28 deg in the flow angles and 0.35% in the velocity magnitude

Miniature Multihole Pressure Probes and Their Neural-Network-Based Calibration

An extensive parametric study of vortices shed from airfoil vortex generators has been conducted to determine the dependence of initial vortex circulation and peak vorticity on elements of the airfoil geometry and impinging flow conditions. These elements include the airfoil angle of attack, chord length, span, aspect ratio, local boundary layer thickness, and free stream Mach number. In addition, the influence of airfoil-to-airfoil spacing on the circulation and peak vorticity has been examined for pairs of co-rotating and counter-rotating vortices. The vortex generators were symmetric airfoils having a NACA-0012 cross-sectional profile. These airfoils were mounted either in isolation, or in pairs, on the surface of a straight pipe. The turbulent boundary layer thickness to pipe radius ratio was about 17 percent. The circulation and peak vorticity data were derived from cross-plane velocity measurements acquired with a seven-hole probe at one chord-length downstream of the airfoil trailing edge location. The circulation is observed to be proportional to the free-stream Mach number, the angle-of-attack, and the span-to-boundary layer thickness ratio. With these parameters held constant, the circulation is observed to fall off in monotonic fashion with increasing airfoil aspect ratio. The peak vorticity is also observed to be proportional to the free-stream Mach number, the airfoil angle-of-attack, and the span-to-boundary layer thickness ratio. Unlike circulation, however, the peak vorticity is observed to increase with increasing aspect ratio, reaching a peak value at an aspect ratio of about 2.0 before falling off again at higher values of aspect ratio. Co-rotating vortices shed from closely spaced pairs of airfoils have values of circulation and peak vorticity under those values found for vortices shed from isolated airfoils of the same geometry. Conversely, counter-rotating vortices show enhanced values of circulation and peak vorticity when compared to values obtained in isolation. The circulation may be accurately modeled with an expression based on Prandtl's relationship between finite airfoil circulation and airfoil geometry. A correlation for the peak vorticity has been derived from a conservation relationship equating the moment at the airfoil tip to the rate of angular momentum production of the shed vortex, modeled as a Lamb (ideal viscous) vortex. This technique provides excellent qualitative agreement to the observed behavior of peak vorticity for low aspect ratio airfoils typically used as vortex generators.

/pdf/initial-circulation-and-peak-vorticity-behavior-of-vortices-11g0s4kuid.pdf

Initial Circulation and Peak Vorticity Behavior of Vortices Shed from Airfoil Vortex Generators

A two-step calibration and measurement procedure for seven-hole pressure probes has been developed that allows simple and accurate real-time processing. The method adds an extra step to traditional least-squares calibration schemes such as Gallington’ s method (Gallington, R. W., “ Measurement of Very Large Flow Angles with Non-Nulling Seven Hole Probes,”Proceedings of the 27th International Instrumentation Symposium , Instrumentation Society of America, Triangle Park, NC, 1981, pp. 115 ‐130)involving the interpolation of error look-up tables. A measurement system employing the two-step calibration scheme is described. Measurements are made in pipe and wind-tunnel e ows, and a detailed uncertainty analysis is performed, to demonstrate the accuracy of the new scheme. Independent of the new scheme, the measurements also show some effects of Reynolds number, velocity gradient, and turbulenceon theprobeaccuracy. TheReynoldsnumbereffect was notanticipated and may indicate the need for multiple calibrations in some circumstances.

Seven-Hole Pressure Probe Calibration Method Utilizing Look-Up Error Tables

This report describes the calibration of a non-nulling, conical, seven-hole pressure probe over a large range of flow onset angles. The calibration procedure is based on the use of differential pressures to determine the three components of velocity. The method allows determination of the flow angle and velocity magnitude to within an average error of 1.0° and 1.0% respectively. Greater accuracy can be achieved by using high quality pressure transducers. Also included is an examination of the factors which limit the use of the probe, a description of the measurement chain, an error analysis, and a typical experimental result. In addition, a new general analytical model of pressure probe behavior is described and the validity of the model is demonstrated by comparing it with experimentally measured calibration data for a three-hole yaw meter and a sevenhole probe.

Modelling, calibration, and error analysis of seven-hole pressure probes

A new approach to experimental data acquisition has been demonstrated. The results of the proof-of-concept experiment show that by incorporating a degree of specific fluid dynamics knowledge to the data acquisition process, it is possible to increase the resolution of the experiment, or alternately, reduce the total number of data points required to achieve parity with the results of most conventional data acquisition approaches.

G. G. Zilliac

Papers

Modelling, calibration, and error analysis of seven-hole pressure probes

An intelligent data acquisition system for fluid mechanics research