Longitudinal and spanwise vortical structures in a turbulent near wake

2002

J Vis

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The effects of initial conditions on turbulence structures of various scales in a near wake have been investigated for two wake generators with the same characteristic dimension, i.e., a circular cylinder and a screen of 50% solidity, based on the wavelet multi-resolution analysis. The experimental investigation used two orthogonal arrays of sixteen X-wires, eight in the (x, y)-plane, and eight in the (x, z)-plane. Measurements were made at x/h (x is the streamwise distance downstream of the cylinder and h is the height of the wake generator) = 20. The wavelet multi-resolution technique was applied to decomposing the velocity data, obtained in the wakes generated by the two generators, into a number of wavelet components based on the central frequencies. The instantaneous sectional streamlines and vorticity field were thus 'visualized' for each wavelet component or central frequency. It was found that the behavior of large-and intermediate-scale structures depend on the initial conditions and the smallscale structures are independent of the initial conditions. The contributions from the wavelet components to the time-averaged Reynolds stresses and vorticity were estimated. Both the large-scale and intermediate longitudinal structures make the most significant contributions to Reynolds stresses in the circular cylinder wake, but the contribution from the large-scale structures appears dominating in the screen wake. The relatively small scale structures of the circular cylinder wake contribute most to the total rms spanwise vorticity.

Section: Turbulent Structures Of Various Scalessupporting

confidence: 52%

Wavelet multi-resolution analysis of initial condition effects on near-wake turbulent structures

2002

J Vis

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“…The spectra were computed using a fast Fourier transform method. 31 As an example, Fig. 12 shows the effect of the u -control scheme on the spectral phase u 2 u 1 between u 2 and u 1 , where u 1 and u 2 were measured using hot wire 1 at x / d = 8 and y / d = −1 and hot wire 2 at x / d = 10 and y / d = −1.…”

Section: B Perturbed Vortex-airfoil Interactionsmentioning

confidence: 99%

Closed-loop controlled vortex-airfoil interactions

Zhang

2006

Physics of Fluids

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Closed-loop controlled interactions between an airfoil and impinging vortices were experimentally investigated. This work aims to minimize the fluctuating flow pressure ͑p͒ at the leading edge of the airfoil, which is a major source of the blade-vortex interaction noises commonly seen in rotorcrafts. Piezoceramic actuators were used to create a local surface perturbation near the leading edge of the airfoil in order to alter the airfoil-vortex interaction. Two closed-loop control schemes were investigated, which deployed p and the streamwise fluctuating flow velocity ͑u͒ as the feedback signal, respectively. As the control effect on p was measured using a fast response pressure transducer, the oncoming vortical flow was monitored using a particle image velocimetry and a hot wire. It was found that the control scheme based on the feedback signal u led to a pronounced impairment in the strength of oncoming vortices and meanwhile a maximum reduction in p by 39%, outperforming the control scheme based on the feedback signal p. Physics behind the observations is discussed.

“…26 The ω x -and ω y -signals display significant fluctuations in either sign between spanwise vortex centers, which could be linked to the occurrence of quasi-longitudinal rib structures. 13,26,28 As Re increases, small-scale fluctuations in the vorticity signals increase, particularly evident as Re changes from 5.0 × 10 3 to 1.0 × 10 4 . The observation is consistent with the previous report, 8 based on high-image-density particle-image velocimetry, that small-scale Kelvin-Helmholtz vortices were evident at Re = 5.0 × 10 3 and more so at Re = 1.0 × 10 4 .…”

Section: A Vorticity Signalsmentioning

confidence: 96%

“…The ratio of rms vorticity values at the centerline and y/d ≈ 7, which may be considered to be approximately in the free stream, is between 40 and 150 for different Re, indicating a good signal-to-noise ratio. For the purpose of comparison, previous measurements 9, 12,13 are also included in the figure. Zhang et al's 13 measurement was conducted at Re = 5.6 × 10 3 .…”

Section: Reynolds Stresses and Rms Vorticitymentioning

confidence: 99%

Reynolds Number Effects on Three-Dimensiional Vorticity in a Turbulent Wake

Yiu

et al. 2004

AIAA Journal

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When Reynolds number Re (≡ U ∞ d/ν, where U ∞ is the free stream velocity, d is the cylinder diameter, and ν is the kinematic viscosity of fluid) varies from 10 3 to 10 4 , there is a large change in the turbulent near-wake dynamics (e.g., the base pressure coefficient, fluctuating lift coefficient, and vortex formation length) of a circular cylinder, which has previously been connected to the generation of small-scale Kelvin-Helmholtz vortices. This work aims to investigate how this Re variation affects the three components of the vorticity vector and to provide a relatively complete set of three-dimensional vorticity data. All three components of vorticity were simultaneously measured in the intermediate region of a turbulent circular-cylinder wake using a multiwire vorticity probe. It is observed that the root-mean-square values of the three vorticity components increase with Re, especially the streamwise component, which shows a large jump from Re = 5 × × 10 3 to Re = 10 4 . At Re = 2.5 × × 10 3 , the maximum phase-averaged spanwise vorticity variance ω 2 z * , normalized by d and U ∞ , is twice as large as its counterpart for the streamwise component, ω 2x * , or the lateral component, ω 2 y * . However, at Re = 10 4 , the maximum ω 2 z * is only 55% larger than the maximum ω 2x * or 47% larger than the maximum ω 2 y * . The observation is consistent with the perception that the three-dimensionality of the flow is enhanced at higher Re due to the occurrence of Kelvin-Helmholtz vortices. The effect of Re on vorticity signals, spectra, and coherent and incoherent vorticity fields is also examined.