Dependence of Strouhal number on the Reynolds number in the wake of two tandem square cylinders

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Dependence of Strouhal number on the Reynolds number in the wake of two tandem square cylinders

 

Author: Yang, Yichun
Title: Dependence of Strouhal number on the Reynolds number in the wake of two tandem square cylinders
Degree: M.Sc.
Year: 2013
Subject: Cylinders -- Vibration.
Fluid-structure interaction
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: x, 69 leaves : ill. (chiefly col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2576826
URI: http://theses.lib.polyu.edu.hk/handle/200/6914
Abstract: The dominant vortex frequencies, f, between two tandem square cylinders of identical cross-sectional dimension, D, or in the wake of the downstream cylinder have been measured simultaneously, using two hot wires placed behind each square cylinder. Measurements were carried out at the range of the Reynolds number from 2.5×103 to 2.8×104, where Re = U∞D/ν (U∞ is the free-stream wind velocity and ν is the kinematic viscosity of the fluid). And centre-to-centre spacing ratio of two square cylinders, L/D, varied from 1.0 to 8.0. The Strouhal number, St (=fD/U∞), exhibits a strong dependence on L/D and Re. For L/D < (L/D)critical which is a critical value and ranges between 2.3 and 3.0, at first, there is no vortex street formed in the gap between two square cylinders, and St rises rapidly with the increase of L/D. Next when L/D reaches about 1.5, the corresponding St starts to decrease. For L/D > (L/D)critical, co-shedding occurs, that is, vortices are shed from the upstream as well as the downstream square cylinder. Meanwhile, the St continues to go up gradually. Based on the distinct flow physics and their behaviors, the StRe relationship is classified into four categories. Category (a): for 1 ≤ L/D < 1.5, the shear layers separating from the upstream cylinder roll up directly behind the downstream cylinder; category 2: for 1.5 ≤ L/D < 2.2, the separating shear layers start to reattach the upper and lower surface of the downstream square cylinder and finally form vortices in the wake; category 3: for 2.2 ≤ L/D ≤ 3.0, there is a transition, that is, the roll-up shear layer reattaching on the downstream square cylinder and forming vortex street in the wake, or the vortex street produced behind the upstream cylinder; category 4: for 3.0 < L/D ≤ 8, the flow is only characterized by co-shedding. The present experiment reconfirms the previous bi-stable phenomenon occurring in the critical spacing, where two flow patterns with different vortex-shedding frequencies switch each other alternately in irregular time. Furthermore, at L/D = 1.0, the other switch phenomenon of two flow patterns is observed and sustains at high Re. In addition, the visualization of flow structure is presented by the laser-induced fluorescence technique and the behavior of St is further discussed qualitatively.

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