Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts Heat and fluid flow around two co-rotating cylinders in tandem arrangement Mohsen Darvishyadegari, Rahim Hassanzadeh * Department of Mechanical Engineering, Urmia University of Technology, Urmia, Iran ARTICLE INFO Keywords: Rotating speed (R.S) Tandem arrangement Fluid rotating zone Heat transfer Vortex shedding ABSTRACT This paper discusses on the heat and fluid flow around two co-rotating cylinders in the tandem arrangement. The non-dimensional rotating speed (R.S) varies in the range of 0 ≤ R.S ≤ 4 and different non-dimensional gap spaces such as G/D = 1.5, 2.0, and 3.0 are considered between the cylinders. Computations are performed at the Reynolds number of 200 with constant Prandtl number of 7.0. It is demonstrated that rotating the cylinders deforms the recirculating regions of both upstream and downstream cylinders in which the rate of this de- formation changes as a function of the R.S and G/D. On the other hand, co-rotating the cylinders shows some additional events such as the azimuthal displacement of the front stagnation points and development of the negative lift coefficient for both cylinders. It is found that the instabilities of the shear layer for both upstream and downstream cylinders are maximum at R.S = 1 and with increasing the R.S, the vortex shedding suppresses around the cylinders due to dominating the fluid rotating zone. Finally, it is revealed that at higher R.S values, a uniform Nusselt number distribution can be observed on both cylinders regardless of the gap space between the upstream and downstream cylinders. 1. Introduction Flow around a stationary cylinder in the free-stream is a funda- mental and classical problem to study the nature of flow around a bluff body. The flow around a bluff body is the subject of several serious applications in structural and environmental problems such as tall buildings, offshore structures, bridge piers, heat exchangers, cooling towers, chimney stacks, etc. To date, numerous publications have been presented on the flow around a single cylinder [1–10] or multiple cy- linders [11–20] with various arrangements not only by means of dif- ferent numerical simulations but also using the various experiments. Therefore, it can be said that the flow around the stationary cylinders is well documented in terms of the vortex shedding process, flow struc- tures, and heat transfer characteristics. Recently, the flow around a rotating cylinder embedded in the free-stream or in the proximity of a flat plate has been of interest among the researchers. For instance, Rao et al. [21] investigated the flow around a rotating cylinder translating along a flat plate at different heights. They considered both clockwise and counter-clockwise rotations for the cylinder and reported several results. In a two-dimensional simulation performed by Rao et al. [22] for flow around the single and two rotating cylinders on the flat wall, it was revealed that reverse rotation of the cylinder can completely sup- press the vortex shedding process. In a similar work, Afroz et al. [23] indicated that it is possible to generate adverse pressure gradients over a flat wall using the rotating cylinder. Thakur et al. [24] published a work on the motion of a rotating cylinder in the free-stream Bingham plastic fluid. They found that, except at high values of the R.S, the drag coefficient is positive, whereas, the lift coefficient is negative over the whole range of applied parameters. Karabelas [25] used the large eddy simulation technique in order to study the flow around a rotating cir- cular cylinder in the free-stream at Re = 140,000. It was demonstrated that all vortex shedding process, location of the front stagnation point, and values of the lift and drag coefficients are under the influence of the R.S. Lam [26] showed that the vortex formation length for a rotating cylinder exposed the free-stream decreases with increasing the rotating speed. Dol et al. [27] experimentally investigated the vortex shedding process around a rotating cylinder at Re = 9000 and found that at R.S = 2.7, the vortex shedding suppression occurs. In the case of the heat transfer from a rotating cylinder, Ghazanfarian and Nobari [28] stated that the rate of heat transfer diminishes rapidly by increasing the rotating speed. Ikhtiar et al. [29] investigated the effects of a single gust impulse on the free-stream and forced convection of a rotating cylinder. They compared the obtained results of the gust condition with those of without dust condition at the Reynolds numbers in the range of 80 and 160. Chatterjee and Sinha [30] and Paramane and Sharma [31] stated that the cylinder rotation has a negative effect on the heat transfer rate. Table 1 summarizes some contributions of the previous works regarding the heat and fluid flow around a single rotating cylinder. https://doi.org/10.1016/j.ijthermalsci.2018.09.014 Received 23 May 2017; Received in revised form 27 July 2018; Accepted 7 September 2018 * Corresponding author. E-mail addresses: mdarvishyadegari68@gmail.com (M. Darvishyadegari), r.hassanzadeh@uut.ac.ir (R. Hassanzadeh). International Journal of Thermal Sciences 135 (2019) 206–220 1290-0729/ © 2018 Elsevier Masson SAS. All rights reserved. T