ALBERT

Description: Spanwise scale changes of the streamwise vortical structure in a plane forced mixing layer have been investigated through direct measurements. Detailed three-dimensional phase-averaged measurements were obtained of the spanwise and streamwise vorticity in a forced mixing layer undergoing three spanwise roller pairings. A two-stream mixing layer with a velocity ratio (U2/U1) of 0.6 and laminar initial boundary layers was generated in a mixing-layer wind tunnel. Acoustic forcing, consisting of a fundamental roll-up frequency and its first, second and third subharmonics, was used to phase-lock the initial development and the first three pairings of the spanwise rollers. Although the overall spanwise scale remained unchanged through the first two roller pairings, some (cyclic) ‘readjustment’ of the weaker streamwise structures was observed. The overall spanwise scale doubled during the third roller pairing. For the first time, one of the proposed mechanisms for the scale change has been identified and its details measured directly. The weakest (positive) streamwise vortex is split into two and displaced by stronger neighbouring (negative) vortices. These two vortices (of the same sign) then merge together, thus doubling the spanwise scale and circulation of the resulting streamwise vortical structure. © 1995, Cambridge University Press. All rights reserved.

Description: An eXperimental study has been conducted to investigate the three-dimensional structure of a plane, two-stream miXing layer through direct measurements. A secondary streamwise vorteX structure has been shown to ride among the primary spanwise vortices in past flow visualization investigations. The main objective of the present study was to establish quantitatively the presence and role of the streamwise vorteX structure in the development of a plane turbulent miXing layer at relatively high Reynolds numbers (Re8~ 2.9 X 104). A two-stream miXing layer with a velocity ratio, u2/u1= 0.6 was generated with the initial boundary layers laminar and nominally two-dimensional. Mean flow and turbulence measurements were made on fine cross-plane grids across the miXing layer at several streamwise locations with a single rotatable cross-wire probe. The results indicate that the instability, leading to the formation of streamwise vortices, is initially amplified just downstream of the first spanwise roll-up. The streamwise vortices first appear in clusters containing vorticity of both signs. Further downstream, the vortices re-align to form counter-rotating pairs, although there is a relatively large variation in the scale and strengths of the individual vortices. The streamwise vorteX spacing increases in a step-wise fashion, at least partially through the amalgamation of like-sign vortices. For the flow conditions investigated, the wavelength associated with the streamwise vortices scales with the miXing-layer vorticity thickness, while their mean strength decays as approXimately 1/X1 .5. In the near field, the streamwise vortices grossly distort the mean velocity and turbulence distributions within the miXing layer. In particular, the streamwise vorticity is found to be strongly correlated in position, strength and scale with the secondary shear stress (u‘w’). The secondary shear stress data suggest that the streamwise structures persist through to what would normally be considered the self-similar region, although they are very weak by this point and the miXing layer otherwise appears to be two-dimensional. © 1992, Cambridge University Press. All rights reserved.

Description: The origin and evolution of spatially stationary streamwise vortical structures in plane mixing layers with laminar initial boundary layers were recently examined quantitatively (Bell & Mehta 1992). When both initial boundary layers were made turbulent, such spatially-stationary streamwise structures were not measured which is indicative of the high sensitivity of these structures to initial conditions. In the present study, the effects of four different types of spanwise perturbations at the origin of the mixing layer were investigated. The wavelengths of the imposed perturbations were chosen to be comparable to the initial Kelvin-Helmholtz wavelength. For the first two perturbations, the boundary layers were otherwise left undisturbed. A serration on the splitter plate trailing edge was found to have a relatively small effect on the formation and development of the streamwise structures. The introduction of cylindrical pegs in the high-speed side boundary layer not only generated a regular array of vortex pairs, but also affected the mixing-layer growth rate and turbulence properties in the far-field region. For the other two perturbations, the initial boundary layers were tripped on the splitter plate. An array of vortex generators mounted in the high-speed boundary layer and a corrugated surface attached to the splitter plate trailing edge had essentially the same effects. Both imposed a regular array of relatively strong streamwise vortices in counter-rotating pairs upon the mixing layer. This resulted in large spanwise distortions of the mixing layer mean properties and Reynolds stresses. While the vorticity injection increased the growth rate in the near-field region as expected, the far-field growth rate was reduced by a factor of about two, together with the peak Reynolds stress levels. This result is attributed to the effect of the relatively strong streamwise vorticity in making the spanwise structures more three-dimensional and hence reducing entrainment during the pairing process. The imposed streamwise vorticity did not follow the pattern of increasing spanwise spacing seen in the ‘naturally occurring ’ streamwise vorticity. The mean streamwise vorticity decayed with increasing streamwise distance in all cases, albeit at different rates. © 1993, Cambridge University Press. All rights reserved.

Description: The three-dimensional structure and streamwise evolution of two-stream mixing layers at high Reynolds numbers (Reδ ∼ 2.7 × 104) were studied experimentally to determine the effects of mild streamwise curvature ([formula omitted] 〈 3%). Mixing layers with velocity ratios of 0.6 and both laminar and turbulent initial boundary layers, were subjected to stabilizing and destabilizing longitudinal curvature (in the Taylor–Görtler sense). The mixing layer is affected by the angular momentum instability when the low-speed stream is on the outside of the curve, and it is stabilized when the streams are reversed so that the high-speed stream is on the outside. In both stable and unstable mixing layers, originating from laminar boundary layers, well-organized spatially stationary streamwise vorticity was generated, which produced significant spanwise variations in the mean velocity and Reynolds stress distributions. These vortical structures appear to result from the amplification of small incoming disturbances (as in the straight mixing layer), rather than through the Taylor–Görtler instability. Although the mean streamwise vorticity decayed with downstream distance in both cases, the rate of decay for the unstable case was lower. With the initial boundary layers on the splitter plate turbulent, spatially stationary streamwise vorticity was not generated in either the stable or unstable mixing layer. Linear growth was achieved for both initial conditions, but the rate of growth for the unstable case was higher than that of the stable case. Correspondingly, the far-field spanwise-averaged peak Reynolds stresses were significantly higher for the destabilized cases than for the stabilized cases, which exhibited levels comparable to, or slightly lower than, those for the straight case. A part of the Reynolds stress increase in the unstable layer is attributed to ‘extra’ production through terms in the transport equations which are activated by the angular momentum instability. Velocity spectra also indicated significant differences in the turbulence structure of the two cases, both in the near- and far-field regions. © 1994, Cambridge University Press. All rights reserved.

Description: The formation and evolution of the three-dimensional structure of straight and mildly curved (b/R〈2%) flat plate wakes at relatively high Reynolds numbers (Reb = 28 000) have been studied through detailed measurements of the mean and fluctuating velocities. In both cases, the role of initial conditions was examined by generating wakes from untripped (laminar) and tripped (turbulent) initial boundary layers. The curved wake was affected by the angular momentum instability such that the inside half of the wake was unstable, whereas the outside half was stable. In both the straight and curved untripped wakes, large spanwise variations, in the form of ‘pinches’ and ‘crests’, were observed in the contours of mean velocity and Reynolds stresses. Well-organized, ‘spatially stationary’ streamwise vorticity was generated in the near-field region in the form of quadrupoles, to which the spanwise variations in the velocity contours were attributed. The presence of mean streamwise vorticity had a significant effect on the wake growth and defect decay rates, mainly by providing additional entrainment. In the straight wake, the mean streamwise vorticity decayed on both sides of the wake such that it had decayed completely by the far-field region. However, in the curved case, the mean streamwise vorticity on the unstable side decayed at a rate significantly lower than that on the stable side. Despite the decay of mean streamwise vorticity, the spanwise variations persisted into the far wake in both cases. The effects of curvature were also apparent in the Reynolds stress results which showed that the levels on the unstable side were increased significantly compared to those on the stable side, with the effect much stronger in the initially laminar wake. With the initial boundary layers tripped, spatially stationary streamwise vortex structures were not observed in either the straight or curved wakes and the velocity contours appeared nominally two-dimensional. This result further confirms the strong dependency of the three-dimensional structure of plane wakes on initial conditions. © 1995, Cambridge University Press. All rights reserved.

Description: Detailed three-dimensional phase-averaged measurements of the spanwise and streamwise vorticity formation and evolution in a forced mixing layer have been obtained. A plane two-stream mixing layer with a velocity ratio (U2/U1) of 0.6, a maximum Reynolds number (Reδ) of about 3150 and laminar initial boundary layers was generated in a mixing layer wind tunnel. Acoustic forcing, consisting of a fundamental roll-up frequency and its subharmonic, was used to phase-lock the initial development and first pairing of the spanwise vortical structures. For the first time, phase-averaged measurements of all three velocity components have been obtained on a three-dimensional grid, yielding the spanwise and streamwise vorticity distributions without invoking Taylor's hypothesis. The phase-averaged results show that the streamwise vorticity first appears in the form of 'ribs' just upstream of the first spanwise vortex roll-up. At the same time, the first spanwise roller becomes kinked, thus also contributing to the streamwise vorticity. As a result, in cross-stream cuts through the spanwise rollers, the streamwise vorticity appears in a 'three-tier' arrangement with opposite-signed vorticity in the centre. In terms of phase-averaged quantities, the maximum streamwise vorticity in the initial ribs is equivalent to about 10-15% of the peak spanwise vorticity and the streamwise rib circulation is equivalent to about 5-10% of the spanwise circulation. Further downstream, the peak streamwise vorticity decreases with increasing distance, while the average circulation remains approximately constant. Downstream of the pairing, the streamwise vorticity levels in the spanwise rollers are reduced. However, the spanwise spacing of the streamwise vortices does not increase within the measurement domain. Phase-averaged Reynolds stress measurements show that relatively high stress levels (periodic and random) were generated in the cores of the spanwise vortices.