Two-point LDV measurements in a plane mixing layer

A completely non-intrusive technique, utilizing synchronized flow visualization and one-point, two-component laser-Doppler velocimetry, has been developed to examine the vortical structure of a homogeneous plane shear layer at a moderate Reynolds number. Results include ensemble-averaged velocity and vorticity maps and, derived from these data, zone-averaged statistics of the basic structure, at 6.2 initial wavelengths downstream of the splitter-plate trailing edge. The velocity field data paint a clear picture of the processes involved in the engulfment or entrainment of free-stream irrotational fluid by the vortices, and the evolution of the material interface separating fluid from the two free streams. The ensemble-averaged vorticity distribution is seen to be of an elliptical cross-section, closely resembling that of the Stuart vortex for a vorticity distribution parameter of 0.4. The effect of jitter which, in the case of zone averages is effectively removed, is assessed by comparison to conventional mean flow properties. It is found that vortex jitter greatly influences u′ and Ruu and, in confirmation of previous results, that a passive vortex does not contribute to the production of large-scale Reynolds stress.

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“…-0.025h0) the local transverse location at which ( U > = +(U,+ U z ) ; this is consistent with the results of Koochesfahani et al (1979).…”

Section: Discussionsupporting

confidence: 88%

Vortex dynamics in a plane, moderate-Reynolds-number shear layer

Panides¹,

Chevray

1990

J. Fluid Mech.

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“…Also, it is interesting to consider the total circulation of the shear layer per unit wavelength; that is, Γ∕λ Γ o;v ∕λ nΓ o;vs ∕λ. Based on the circulation values in Table 1, and λ∕θ 17.8, Γ∕λ 0.95ΔU, which is consistent with Γ∕λ ∼ 1 for shear layers (e.g., see [14]). Figure 9 displays a comparison between the computed and the measured streamwise velocity profiles at 10 different phases of the oscillation cycle (Ψ 0 to 1 in steps of 0.1).…”

Section: Resultssupporting

confidence: 75%

“…The corresponding induced velocity decays at least quadratically with distance, and hence these differences should have minimal, if any, influence on the computed velocity. The negligible influence of higher-order moments on the induced velocity is also discussed in reference [14] for the specific case of a two-stream shear layer.…”

Section: Vortex-array Modelmentioning

confidence: 97%

Structure-Based Model of a Forced Shear Layer

2019

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The focus of this work is on assessing the ability of an array of discrete finite-core vortices to model vorticitydominated flows where vortex sheets that have not fully rolled up into isolated vortices are present. The study extends our earlier work of modeling spatially periodic isolated vorticity in the wake of an oscillating airfoil. The present extension of the model is carried out by taking advantage of existing measurements in the unsteady flow of a forced two-stream shear layer. The shear-layer Reynolds number is 1.55 × 10 5 , based on the velocity difference between the two streams and the streamwise location of the measurements. Single-component laser Doppler velocimetry is used to capture the transverse profiles of the streamwise velocity in the shear layer. Overall, the results show that the model provides good agreement with the experiments. The utility of the model is demonstrated by using it to establish possible physical connections between the flow features and the characteristics of the cross-stream profiles of the root-mean-square streamwise and transverse velocity. The model predictions are also found to exhibit small change with significant reduction in the number of vortex elements, highlighting the efficiency of the model and its potential usability in real-time flow control applications.

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“…There is also some evidence that, in mixing layers which have developed from laminar initial conditions, the streamwise vortices survive in the post-transition flow with an average spacing which increases in step with that of the spanwise structures (Jimenez, Cogollos & Bernal 1985;Bernal & Roshko 1986;Bell & Mehta 1992). It seems that the structures are not always present however and, even in situations where their presence has been detected (Koochesfahani et al 1979;Hussain & Zaman 1985), velocity correlation measurements show no evidence of the large-amplitude antisymmetric induced fluctuations characteristic of the vortex amalgamations in the pre-transition flow (Browand & Weidman 1976).…”

Section: Introductionmentioning

confidence: 99%

Organized large structure in the post-transition mixing layer. Part 1. Experimental evidence

D’Ovidio

Coats

2013

J. Fluid Mech.

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New flow-visualization experiments on mixing layers of various velocity and density ratios are reported. It is shown that, in mixing layers developing from laminar initial conditions, the familiar mechanism of growth by vortex amalgamation is replaced at the mixing transition by a previously unrecognized mechanism in which the spanwise-coherent large structures individually undergo continuous linear growth. In the organized post-transition flow it is this continuous linear growth of the individual structures that produces the self-similar growth of the mixing-layer thickness, with the occasional interactions between neighbouring structures occurring as a consequence of their growth, not its cause. It is also observed that periods during which the post-transition mixing layer comprises orderly processions of large structures alternate with periods during which no large-scale organization is apparent downstream of the transition location. These two fully turbulent flow states are characterized by different growth rates, entrainment ratios and orientations of the mixing layer relative to the free streams. The implications of these findings are discussed.

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Two-point LDV measurements in a plane mixing layer

Cited by 26 publications

References 5 publications

Vortex dynamics in a plane, moderate-Reynolds-number shear layer

Vortex dynamics in a plane, moderate-Reynolds-number shear layer

Structure-Based Model of a Forced Shear Layer

Organized large structure in the post-transition mixing layer. Part 1. Experimental evidence

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