On the periodically excited plane turbulent mixing layer, emanating from a jagged partition

Abstract

The flow in a turbulent mixing layer resulting from two parallel different velocity streams, that were brought together downstream of a jagged partition was investigated experimentally. The trailing edge of the partition had a short triangular ‘chevron’ shape that could also oscillate up and down at a prescribed frequency, because it was hinged to the stationary part of the partition to form a flap (fliperon). The results obtained from this excitation were compared to the traditional results obtained by oscillating a two-dimensional fliperon. Detailed measurements of the mean flow and the coherent structures, in the periodically excited and spatially developing mixing layer, and its random constituents were carried out using hot-wire anemometry and stereo particle image velocimetry.The prescribed spanwise wavelength of the chevron trailing edge generated coherent streamwise vortices while the periodic oscillation of this fliperon locked in-phase the large spanwise Kelvin–Helmholtz (K-H) rolls, therefore enabling the study of the inter- action between the two. The two-dimensional periodic excitation increases the strength of the spanwise rolls by increasing their size and their circulation, which depends on the input amplitude and frequency. The streamwise vortices generated by the jagged trailing edge distort and bend the primary K-H rolls. The present investigation endeavours to study the distortions of each mode as a consequence of their mutual interaction. Even the mean flow provides evidence for the local bulging of the large spanwise rolls because the integral width (the momentum thickness, θ), undulates along the span. The lateral location of the centre of the ensuing mixing layer (the location where the mean velocity is the arithmetic average of the two streams,y0), also suggests that these vortices are bent. Phase-locked and ensemble-averaged measurements provide more detailed information about the bending and bulging of the large eddies that ensue downstream of the oscillating chevron fliperon. The experiments were carried out at low speeds, but at sufficiently high Reynolds number to ensure naturally turbulent flow.

The application of boundary layer independence principle to three-dimensional turbulent mixing layers

“…Thus the flow emanating from a chevron nozzle is still poorly understood, and the application of the independence principle to this flow could simplify the analysis. The flow field far downstream of a two-dimensional chevron splitter plate with cutouts swept-back at 38 • was measured by Kit et al (2007) for a velocity ratio between the two mixing streams of 0.4. They observed that the location of the centre of mixing layers undulates between the notch and cusp in the splitter plate.…”

On a Turbulent Mixing Layer Created Downstream of a “Λ” Notch Simulating One Wavelength of a Chevron Nozzle

“…4. Lateral distances, y, are measured relative to the location of y 0.5 where the mean velocity is the average of the two streams (y o in the definition of Kit et al Along the center line (z = 0 mm) for 20 mm ≤ x ≤ 300 mm and 400 mm ≤ x ≤ 1,100 mm respectably (inset in a defines δ ω ); c, d along the mid span (z = 150 mm) for 300 mm ≤ x ≤ 700 mm and 800 mm ≤ x ≤ 1,100 mm respectably [6]) and these distances are normalized by the vorticity thickness δ ω that is determined in the traditional way, δ ω = (U 2 − U 1 )/(∂U/∂ y) max , as shown schematically in Fig. 4.…”

“…investigation carried out by Kit et al [6] where a small flap having a triangular sawtooth shape was hinged to the trailing edge of the partition, and was able to oscillate uniformly in a direction normal to the merging streams.…”

“…1b). Kit et al [6] attempted to control the interactions between streamwise and spanwise eddies in the far field of this flow by using an oscillating "chevron" type fliperon in otherwise a two-dimensional mixing layer. This enabled them to compare their results to data obtained using a 2D fliperon that generated only 2D eddies.…”

“…Periodic oscillation of the saw-tooth fliperon created large spanwise KelvinHelmholtz (K-H) rolls that were locked in phase to the oscillating solid boundary and the variations of y o with z were virtually eliminated [6]. Thus the periodic motion of the chevron-fliperon inhibited the bending of the large spanwise eddies while concomitantly increasing their local swelling (bulging) downstream of the trailing edge (Fig.…”