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The development of a wing-tip vortex of a rectangular, square-tipped wing having a NACA 0012 airfoil at a chord Reynolds number Recw = 2 × 105, under the effect of synthetic jet actuation, was experimentally studied. Five control configurations were considered: case C1 with momentum coefficient Cμ = 0.001 and actuation frequency F+ = 0.075; case C2 with Cμ = 0.001 and F+ = 0.15; case C3 with Cμ = 0.001 and F+ = 0.3; case C4 with Cμ = 0.001 and F+ = 0.6; and case C5 with Cμ = 0.001 and F+ = 1.2. Under the most effective configuration, case C3, the vortex was stretched and appeared to be diffuse with a nearly 40 % decrease in the peak circumferential velocity and 50 % decrease in the core axial vorticity. The vortex core radius largely broadened suggesting that the lower-frequency control configuration allowed the synthetic jet to travel larger distances into the vortex bringing turbulent structures within its core resulting in increased mixing and subsequently a more diffuse vortex. [ABSTRACT FROM AUTHOR]

In this experimental investigation, hotwire measurements were used to study the development of a separating shear layer in the near wake of a circular cylinder. The main purpose was to investigate the effects of the free stream turbulence intensity, Ti , on the separating shear layer. The measurements were performed over a range of Re , based on the cylinder diameter and mean free-stream velocity, between 4.5×10 3 and 4.7×10 4 , and free stream turbulence intensities of 0.25%, 2.2%, 3.4% and 6.2%. Mean and turbulent velocities in the separating shear layer plotted in self-similar coordinates showed that the shear layer evolved downstream approximately as a plane mixing layer when Ti ≤2.2%, while for high Ti ( Ti ≥3.4%) a departure from similarity was observed for downstream locations x / D ≥1.1. The shear layer transition point moved closer to the cylinder and the longitudinal distance at which the separating shear layer can no longer be approximated by a mixing layer decreased as Ti increased. Power spectra measurements showed peaks related to the shear layer frequency, f sl , and its harmonics. In agreement with the self-similarity study, these peaks became broader and indiscernible as Ti was increased, and at the highest Ti of 6.2%, peaks related to f sl and its harmonics could no longer be seen in the power spectra at all Re investigated in this work. The present measurements suggested that the appearance of peaks at f sl and its harmonics in the power spectra is well tied to the extent by which the separating shear layer behaves as a mixing layer. The free stream turbulence seemed to amplify the turbulence content of the separating shear layer thus promoting and accelerating the breakdown of the shear layer vortices. It also decreased the large vortex formation length which limited the downstream development region of the separating shear layer hence preventing the latter from developing in a manner similar to that of a mixing layer. The overall quantitative effect of the free stream turbulence on the development of the separating shear layer was measured by the change in the exponent of the functional relationship relating f sl and Re (i.e. f sh / f v ~ Re n where f v is the von Kármán vortex shedding frequency) with Ti . The present measurements have shown that the exponent n increased with increasing Ti namely, n was found to increase from 0.56 to 0.65 as Ti increased from 0.25% to 3.4%. Finally, a model has been developed showing that an increase in the free-stream turbulence intensity has, qualitatively, similar effects on the separated shear layer as an increase in Re . [ABSTRACT FROM AUTHOR]

Stationary and flying single hot-wire measurements were made to investigate the near to intermediate field in a round free turbulent jet. Measurements were carried out at Reynolds numbers ReD based on the jet exit mean velocity and the nozzle diameter, between 6000 and 100 000. The objective of this study was to investigate the concept of a mixing transition proposed by Dimotakis [“The mixing transition in turbulent flows,” J. Fluid Mech. 409, 69 (2000)]. This was done through the measurement of the velocity spectra and the calculation of Kolmogorov λK, Taylor microscale λT, the Liepmann–Taylor microscale λL, and the viscous length scale λν at different positions in the near to intermediate regions of a round free jet. The present results demonstrate the local nature of mixing transition. It is found that the Kolmogorov, Taylor, and viscous length scales all decrease in magnitude with the local Reynolds number Reδ based on the local center line mean velocity and the local time-averaged diameter of the jet and appear to be only weakly dependent on the radial position, although the ratio ReT/Reδ1/2 varies nonlinearly across the jet radius. The ratio of the laminar to viscous length scale exceeds unity beyond the potential core of the jet. Moreover, the skewness of the axial velocity is approximately -0.4 over 30

The development of a wing tip vortex under the effect of synthetic jet (SJ) actuation was examined for a rectangular, square-tipped wing having a NACA 0012 airfoil using hotwire anemometry at a chord Reynolds number Re c = 8 × 10 4 . Three control configurations were considered for a comparative study, namely case C1 with momentum coefficient C μ = 0.004 and an actuation frequency F + = 0.96 , case C2 with C μ = 0.04 and F + = 0.96 and case C3 with C μ = 0.04 and F + = 0.29 . Under case C3, the vortex was stretched into an ellipsoid shape with a nearly 60% decrease in the peak circumferential velocity and the core axial vorticity. The vortex core radius largely broadened suggesting that the lower frequency control configuration allowed the synthetic jet to travel larger distances into the vortex bringing turbulent structures within its core. This resulted in an increased mixing and subsequently a more diffuse vortex. Measurements of the vortex development in the near and mid-wake regions (up to x / c = 5 ) showed that the circumferential velocity was largely reduced under the effect of the SJ actuation. The enhanced turbulent mixing at the inner region of the vortex resulted in an accelerated outward diffusion of its core even under non-optimal control configurations. [ABSTRACT FROM AUTHOR]

The interaction of a wing-tip vortex of a rectangular, square-tipped wing having a NACA 0012 airfoil section with a grid-generated turbulent flow was investigated in this paper. The experiments were conducted in the near and mid-wake regions at three free stream turbulence (FST) intensities of 0.5%, 3% and 6%, and at two Reynolds numbers, Re, based on the wing chord length, c w , of 2 × 10 5 and 3 × 10 5. Stereoscopic Particle Image Velocimetry (SPIV) and hot-wire measurements were carried out at four downstream positions, namely x / c w = 1.25 , 3.25 , 6.25 and 7.75. Streamwise velocity contours showed that the wing-tip vortex decayed with increased FST and downstream distance. In the vortex core region, the streamwise velocity decelerated while the vortex adopted a wake-like profile. FST was found to decrease the vortex circulation and to increase the vortex radius and vortex meandering amplitude. When increasing Reynolds number, the grid cases showed little variation of the vortex radius and peak vorticity levels, particularly at larger downstream positions, suggesting that the effects of FST and R e number on the vortex development are nearly independent. The measured total turbulent kinetic energy (TKE) was found to be mostly due to vortex meandering. In that, total TKE levels devoid of meandering showed a virtually turbulence-free vortex core. • The effect of free stream turbulence on a wing-tip vortex development was investigated in the wind tunnel. • Stereoscopic Particle Image Velocimetry (SPIV) and hot-wire measurements were carried out at four downstream positions. • Non-linear interaction between the effect of Reynolds number and free-stream turbulence was quantified. • Vortex meandering were evaluated and its induced turbulence was removed from the total turbulent kinetic energy. [ABSTRACT FROM AUTHOR]