The structure of a trailing vortex from a perturbed wing

The structure of a trailing vortex from a wing undergoing small amplitude, low frequency heaving motion is investigated using space–time representations determined from stereo particle image velocimetry. The evolution of the vortex shows large fluctuations of axial velocity deficit and circulation during the oscillation cycle. Correspondingly, large variations of swirl ratio occur and onset of pronounced azimuthal vorticity arises. At a given cross-section of the vortex, the pattern of azimuthal vorticity moves around its axis in an ordered fashion as both it and the pattern of velocity defect increase in magnitude and scale. When the swirl ratio attains its minimum value during the oscillation cycle, and this value lies below the theoretically established critical threshold for amplification of azimuthal modes, the magnitude and scale of the pattern of azimuthal vorticity is maximized. Subsequent increase of the swirl ratio yields attenuation of the azimuthal vorticity. Onset of pronounced azimuthal vorticity when the swirl ratio decreases involves rapid amplification, then disruption, of axial vorticity fluctuation.

Impact of Swirling Flow Structure on Shear Layer Vorticity Fluctuation Mechanisms

Vorticity fluctuations have been identified as an important coupling mechanism during velocity-coupled combustion instability in swirl-stabilized flames. Acoustic oscillations in the combustor can cause all components of vorticity to oscillate, particularly the cross-stream, or azimuthal, vorticity that is excited in shear layer roll-up, and streamwise, or axial, vorticity that is excited during swirl fluctuations. These fluctuations can be induced by longitudinal acoustic fluctuations that oscillate across the swirler and dump plane upstream of the flame. While these fluctuations have been identified in a number of configurations, the sensitivity of this mechanism to flow configuration and boundary conditions has not been studied parametrically. In this study, we investigate the impact of time-averaged swirl level, confinement, and forcing frequency and amplitude on vorticity fluctuation dynamics in the azimuthal direction of a non-reacting swirling jet. The goal of this work is to better understand the dependence of vorticity fluctuations on these parameters as well as the vorticity conversion processes that occur in the flow. We have shown that vorticity fluctuation levels vary with time-averaged swirl number, particularly in the presence of a self-excited precessing vortex core, which dampens most acoustically-driven motion. Additionally, variations in forcing frequency excite flow response in different portions of the flow, particularly for different swirl numbers. Finally, confinement drastically changes the flow topology and unforced dynamics, resulting in significantly different response to forcing and generation of vortical fluctuations.

Experimental Study on the Receptivity of Shear Layer Separating at a Rear-Edge of Boundary-Layer Plate

Receptivity of the free shear layer developing from a 90-degrees rear-edge of boundary-layer plate to acoustic disturbances is examined experimentally to clarify the dependency of the receptivity coefficient on the rear-edge curvature. The results show that for finite rear-edge curvatures, the receptivity coefficient decreases with increasing the disturbance frequency while it is almost independent of the frequency for the sharp rear-edge over the frequency range examined. The decrease in the receptivity coefficient for the rounded rear-edge is attributed to the fact that the sound-induced Stokes layer which is the vorticity fluctuation developing into the free-shear instability mode is shed into the off-centerline of the separated shear layer.

A Multi-Sensor Hot-Wire Probe to Measure Vorticity and Velocity in Turbulent Flows

The design and construction of a miniature hot-wire probe capable of simultaneously measuring all components of the velocity and vorticity vectors in turbulent flows are presented. A brief description of the probe resolution and calibration as well as the procedure to solve the cooling equations are given. Preliminary testing in the irrotational region of the flow shows that the spurious vorticity obtained from the probe due to system measurement error does not exceed 8 percent of the RMS vorticity fluctuation levels measured over the region y+ = 15 − 45 in a turbulent boundary layer.

Characteristics of vorticity fluctuations in a turbulent wake

Measurements of the lateral components of the vorticity fluctuation have been made in the self-preserving turbulent wake of a circular cylinder. Each component was obtained separately using two X-wires separated in the appropriate lateral directions. The two velocity derivatives which make up the streamwise vorticity component were also determined but not simultaneously. An approximation to the streamwise vorticity was made from these measurements. Moments, probability density functions and spectra of the three vorticity components across the wake are presented and discussed. The high-wavenumber behaviour of the spectra is compared with calculations, based on local isotropy. Satisfactory agreement with the calculations is obtained for the lateral vorticity components over a significant high-wavenumber range. The approximated streamwise vorticity spectrum tends towards the isotropic calculation at very large wavenumbers.