Vortex structures behind twin, highly heated cylinders in parallel arrangements have been investigated experimentally. The experiments were conducted under the following conditions: cylinder diameter, D = 4 mm; mean flow velocity, U∞= 1.0 m/s; Reynolds number, Re = 250; cylinder clearance, S/D = 0.5 to 1.4; and cylinder heat flux, q = 0 to 72.6 kW/m2. For S/D > 1.2, the Karman vortex street is formed alternately behind each cylinder divided on the slit flow. The slit flow velocity increases with a decrease in S/D and decreases with increasing heat flux. For S/D < 1.2, the wake vortexes become asymmetric having small and large scale vortexes divided by the slit flow. In the small scale vortexes, the symmetric counter-rotating twin vortexes are formed just behind the cylinders. In the large scale vortexes, the generated vortexes have a similar structure to a Karman vortex even though the Strouhal number is approximately half of the ordinary single cylinder vortex. For isothermal conditions, the transition phenomena from symmetric to asymmetric wake structures are observed in the range of 0.9 < S/D < 1.2. In addition, the asymmetric vortexes are irregularly switched up and down in the case of isothermal conditions. In the highly heated condition, the switching phenomena and the transition phenomena could not be observed and the small scale vortexes always formed behind the upper cylinder. The critical S/D increases approximately 30% in the heated condition (q = 72.6 kW/m2). As a result, the increased local kinematic viscosity and S/D play a key role for the vortex structure and formation behind arrangements of two parallel cylinders.
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