Direct numerical simulation of heat transfer from the stagnation region of a heated cylinder affected by an impinging wake

Abstract

The effect of an incoming wake on the flow around and heat transfer from the stagnation region of a circular cylinder was studied using direct numerical simulations (DNSs). Four simulations were carried out at a Reynolds number (based on freestream velocity and cylinder diameter D) of Re D = 13 200: one two-dimensional (baseline) simulation and three three-dimensional simulations. The three-dimensional simulations comprised a baseline simulation with a uniform incoming velocity field, a simulation in which realistic wake data -generated in a separate precursor DNSwere introduced at the inflow plane and, finally, a simulation in which the turbulent fluctuations were removed from the incoming wake in order to study the effect of the mean velocity deficit on the heat transfer in the stagnation region. In the simulation with realistic wake data, the incoming wake still exhibited the characteristic meandering behaviour of a near-wake. When approaching the regions immediately above and below the stagnation line of the cylinder, the vortical structures from the wake were found to be significantly stretched by the strongly accelerating wall-parallel (circumferential) flow into elongated vortex tubes that became increasingly aligned with the direction of flow. As the elongated streamwise vortical structures impinge on the stagnation region, on one side they transport cool fluid towards the heated cylinder, while on the other side hot fluid is transported away from the cylinder towards the free stream, thereby increasing the heat transfer. The DNS results are compared with various semi-empirical correlations for predicting the augmentation of heat transfer due to free-stream turbulence.

Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

“…Numerical studies also have painted a better picture of this mechanism. For example, the DNS studies of Wissink & Rodi (2006), Wissink & Rodi (2009, 2011a showed that the increase in heat transfer occurs through an up-wash of hot fluid away from the cylinder and a down-wash of cold fluid towards the cylinder. The LES studies of Xiong & Lele (2007) and Bhaskaran & Lele (2010) observed longitudinal stream-wise vortical structures that increase heat transfer by being stretched along the surface of the turbine blade.…”

“…Secondly, the spanwise length of the grid-element is larger compared to the standard spanwise size used to compute the flow around a single cylinder. For example, the study of Venema et al (2011) had a spanwise length of 2.5D, while Wissink & Rodi (2011b) and Wissink & Rodi (2011a) had 2D and 0.6D respectively. The presence of the gridelement increases this size by a factor of 7, which in turn increases the number of cells accordingly.…”

“…The wall-normal heat-flux profiles (| u r T |) are depicted in figure 15b. The presence of the cylinder blocks the radial fluctuation u r , and therefore the heat transfer near the cylinder is mainly due to molecular diffusion (Wissink & Rodi 2011a). Moving clockwise from θ = 0 o , the wall-normal region where turbulent fluctuations are active is expanding radially outwards from the surface.…”

Heat transfer from the stagnation area of a heated cylinder at ReD=140,000 affected by free-stream turbulence

“…Intense quasi-streamwise vortices were found to develop near the leading edge leading to turbulent heat flux to peak much closer to the wall than the Reynolds stresses. Wissink and Rodi [12] performed the first DNS in which the influence of an incoming turbulent wake on stagnation heat transfer from an heated cylinder was studied. A Reynolds number of Re D = 13,200, based on the free-stream velocity and the diameter of the cylinder, was employed.…”

“…We would like to refer to this paper for an extensive literature review on laminar heat transfer induced by free stream fluctuations. Compared to Wissink and Rodi [12], the present paper studies the effect of oncoming grid turbulence, rather than turbulence in a wake, on stagnation heat transfer and employs a significantly higher Reynolds number.…”