William J. Devenport

The structure and development of a wing-tip vortex

Experiments have been performed on the tip vortex trailing from a rectangular NACA 0012 half-wing. Preliminary studies showed the vortex to be insensitive to the introduction of a probe and subject only to small wandering motions. Meaningful velocity measurements could therefore be made using hot-wire probes. Detailed analysis of the effects of wandering was performed to properly reveal the flow structure in the core region and to give confidence in measurements made outside the core. A theory has been developed to correct mean-velocity profiles for the effects of wandering and to provide complete quantitative estimates of its amplitude and contributions to Reynolds stress fields. Spectral decomposition was found to be the most effective method of separating these contributions from velocity fluctuations due to turbulence. Outside the core the flow structure is dominated by the remainder of the wing wake which winds into an ever-increasing spiral. There is no large region of axisymmetric turbulence surrounding the core and little sign of turbulence generated by the rotational motion of the vortex. Turbulence stress levels vary along the wake spiral in response to the varying rates of strain imposed by the vortex. Despite this complexity, the shape of the wake spiral and its turbulent structure reach an approximately self-similar form. On moving from the spiral wake to the core the overall level of velocity fluctuations greatly increases, but none of this increase is directly produced by turbulence. Velocity spectra measured at the vortex centre scale in a manner that implies that the core is laminar and that velocity fluctuations here are a consequence of inactive motion produced as the core is buffeted by turbulence in the surrounding spiral wake. Mean-velocity profiles through the core show evidence of a two-layered structure that dies away with distance downstream.

Time-depeiident and time-averaged turbulence structure near the nose of a wing-body junction

The behaviour of a turbulent boundary layer on a flat surface as it encounters the nose of a cylindrical wing mounted normal to that surface is being investigated. A three-component laser anemometer has been developed to measure this highly turbulent three-dimensional flow. Measurements of all the non-zero mean-velocity and Reynolds-stress components have been made with this instrument in the plane of symmetry upstream of the wing. These data have been used to estimate some of the component terms of the turbulence kinetic energy equation. Histograms of velocity fluctuations and short-time cross-correlations between the laser anemometer and a hot-wire probe have also been measured in the plane of symmetry. In all, these results reveal much of the time-dependent and time-averaged turbulence structure of the flow here. Separation occurs in the plane of symmetry because of the adverse pressure gradient imposed by the wing. In the time mean the resulting separated flow consists of two fairly distinct regions: a thin upstream region characterized by low mean backflow velocities and a relatively thick downstream region dominated by the intense recirculation of the mean junction vortex. In the upstream region the turbulence stresses develop in a manner qualitatively similar to those of a two-dimensional boundary layer separating in an adverse pressure gradient. In the vicinity of the junction vortex, though, the turbulence stresses are much greater and reach’ values many times larger than those normally observed in turbulent flows. These large stresses are associated with bimodal (double-peaked) histograms of velocity fluctuations produced by a velocity variation that is bistable. These observations are consistent with large-scale low-frequency unsteadiness of the instantaneous flow structure associated with the junction vortex. This unsteadiness seems to be produced by fluctuations in the momentum and vorticity of fluid from the outer part of the boundary layer which is recirculated as it impinges on the leading edge of the wing. Though we would expect these fluctuations to be produced by coherent structures in the boundary layer, frequencies of the large-scale unsteadiness are substantially lower than the passage frequency of such structures. It therefore seems that only a fraction of the turbulent structures are recirculated in this way.

Effects of a Fillet on the Flow Past a Wing-Body Junction

Measurements are presented to demonstrate the effects of a single fillet on the flow of a turbulent boundary layer past an idealized wing-body junction. The time-averaged flow structure in the vicinity of the wing and in the wake of the wing-body junction is considered, as well as the unsteadiness of the horseshoe vortex. The effects of angle of attack and approach boundary-layer thickness are also examined.

The structure and development of a counter-rotating wing-tip vortex pair

Experiments have been performed to examine the turbulence structure and development of a pair of counter-rotating wing-tip vortices. The vortices were generated by two rectangular NACA 0012 half wings placed tip to tip, separated by 0.25 chordlengths. Preliminary studies showed the vortices to be insensitive to the introduction of a probe and subject only to small wandering motions. Meaningful measurements could therefore be made using hot-wire probes. Three-component velocity measurements were made 10 and 30 chordlengths downstream of the wing leading edges for a chord Reynolds number of 260000. At 10 chordlengths the vortex cores are laminar. True turbulence levels within them are low and vary little with radius. The turbulence that surrounds the cores is formed by the roll-up of and interaction of the wing wakes that spiral around them. This turbulence is stretched and organized but apparently not produced by the circulating mean velocity fields of the vortices. At 30 chordlengths the vortex cores have become turbulent. True turbulence levels within them are larger and increase rapidly with radius. The turbulent region surrounding the cores has doubled in size and turbulence levels have not diminished, apparently being sustained by outward diffusion from the core regions. The distribution of the turbulence has also changed, the wake spirals having been replaced by a much more core-centred turbulence field. This change in flow structure contrasts sharply with what is seen in the equivalent isolated tip vortex, produced when one of the wings is removed. Here the vortex core remains laminar and the turbulence surrounding it decays rapidly with downstream distance. This implies that the transition to turbulence in the cores of the vortex pair is stimulated by interaction between the vortices. Spectral measurements at 10 chordlengths suggest that short-wave instability may be the cause.

Wake of a Compressor Cascade with Tip Gap, Part 1: Mean Flow and Turbulence Structure

The purpose of this work is to understand the structure of the vortex-dominated endwall flows found in aircraft engine fans and compressors. Our approach is to model the flow using a linear cascade where detailed turbulence measurements can be made and the relative motion between blade tip and casing can be simulated. The cascade consists of eight 4% thick modified circular-arc blades and operates at a chord Reynolds number of 3.88 × 10 5 with a thin inlet boundary layer and 12.5 deg of turning. We present baseline results for the tip-leakage flow with stationary endwall. Three component velocity and turbulence measurements are used to reveal the evolution of the flow as a function of distance downstream of the blades for a tip gap of 1.6% chord and as a function of tip gap from 0.8 to 3.3%

Near-wall behavior of separated and reattaching flows

Two separated and reattaching flows produced by a sudden expansion in a pipe have been studied experimentally. Velocity measurements were made close to the reattachment surface using a new type of pulsed-wire probe. These data show the near-wall flow to be very different from a normal turbulent boundary layer. Mean-velocity profiles do not obey the law of the wall and cannot be correlated outside the linear sublayer using the friction velocity. However, they do contain semilogarithmic regions that appear to form tangents to the linear sublayer profile. A satisfactory description of the backflow mean velocity profile is obtained by incorporating this observation into Simpsons model. The distribution of streamwise turbulence intensity in the near-wall region appears independent of the mean skin friction. It is, however, related to the root-mean-square of skin friction fluctuations expressed as a friction velocity. A simple one-dimensional model of the near-wall flow suggests that the form of the turbulence intensity profile is dependent on the frequency of large-scale velocity fluctuations.

Effects of a Leading-Edge Fillet on the Flow Past an Appendage-Body Junction

Oil-flow visualizations and pressure and velocity measurements are presented to demonstrate the effects of a leading-edge fillet on the flow of a turbulent boundary layer past an idealized appendage-body function. The fillet, a large fairing in the corner between the appendage nose and body surface, modifies the flow in a way that is desirable in many applications. With the appendage at zero angle of attack, it eliminates leading-edge separation and thus the formation of a horseshoe vortex around the wing nose. It greatly improves the stability of the flow close to the junction and the nonuniformity of its wake

Wake of a Compressor Cascade with Tip Gap, Part 2: Effects of Endwall Motion

The effects of relative motion between the blade tips and endwall on the flow downstream of a linear compressor cascade have been studied. Endwall motion was simulated by using a 0.25-mm-thick Mylar belt propelled ove ra sliding surface beneath the tips of the cascade blades. Three-component mean-velocity and turbulence measurements made in cross sections downstream of the cascade reveal that the wall motion flattens and shears the turbulence and mean-velocity distributions of the vortex. Mean-helicity density plots also show that endwall motion smears the vortex center from a single point (when seen in cross section) into a ribbon that makes an angle of some 30 deg with the endwall. Despite these effects, many critical features of the vortex are almost unaffected by the endwall motion. The vortex produces almost the same magnitude of streamwise mean-velocity deficit, and this deficit still dominates both the mean velocity field and the production of turbulence. Thus, although endwall motion distorts and displaces the leakage vortex it does not fundamentally alter the mechanisms that govern the development of its mean flow and turbulence structure.

Flow structure produced by the interaction and merger of a pair of co-rotating wing-tip vortices

Experiments have been performed to study the co-rotating wing-tip vortex pair produced by a pair of rectangular wings in a split-wing configuration. Detailed measurements made in cross-sections upstream and downstream of merger reveal, for the first time, the complex turbulence structure of this flow. The vortices spiral around each other and merge some 20 chordlengths downstream of the wings. As merger is approached the vortices lose their axisymmetry – their cores develop lopsided tangential velocity fields and the mean vorticity field is convected into filaments. The cores also become part of a single turbulence structure dominated by a braid of high turbulence levels that links them together. The braid, which quite closely resembles the structure formed between adjacent spanwise eddies of transitional mixing layers, grows in intensity with downstream distance and extends into the vortex cores. Unlike a single tip vortex, the unmerged cores appear turbulent. The merging of the vortices wraps the cores and the flow structure that surrounds them into a large turbulent region with an intricate double spiral structure. This structure then relaxes to a closely axisymmetric state. The merged core appears stable and develops a structure similar to the laminar core of a vortex shed from a single wing. However, the turbulent region formed around the vortex core during the merger process is much larger and more axisymmetric than that found around a single wing-tip vortex.

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

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.