Thermal effects on the wake of a heated circular cylinder operating in mixed convection regime

Abstract

The thermal effects on the wake flow behind a heated circular cylinder operating in the mixed convection regime were investigated experimentally in the present study. The experiments were conducted in a vertical water channel with the heated cylinder placed horizontally and the flow approaching the cylinder downwards. With such a flow arrangement, the direction of the thermally induced buoyancy force acting on the fluid surrounding the heated cylinder would be opposite to the approach flow. During the experiments, the temperature and Reynolds number of the approach flow were held constant. By adjusting the surface temperature of the heated cylinder, the corresponding Richardson number (Ri = Gr/Re 2 ) was varied between 0.0 (unheated) and 1.04, resulting in a change in the heat transfer process from forced convection to mixed convection. A novel flow diagnostic technique, molecular tagging velocimetry and thermometry (MTV&T), was used for qualitative flow visualization of thermally induced flow structures and quantitative, simultaneous measurements of flow velocity and temperature distributions in the wake of the heated cylinder. With increasing temperature of the heated cylinder (i.e. Richardson number), significant modifications of the wake flow pattern and wake vortex shedding process were clearly revealed. When the Richardson number was relatively small (Ri 0.31), the vortex shedding process in the wake of the heated cylinder was found to be quite similar to that of an unheated cylinder. As the Richardson number increased to ∼0.50, the wake vortex shedding process was found to be 'delayed', with the wake vortex structures beginning to shed much further downstream. As the Richardson number approached unity (Ri 0.72), instead of having 'Kármán' vortices shedding alternately at the two sides of the heated cylinder, concurrent shedding of smaller vortex structures was observed in the near wake of the heated cylinder. The smaller vortex structures were found to behave more like 'Kelvin-Helmholtz' vortices than 'Kármán' vortices, and adjacent small vortices would merge to form larger vortex structures further downstream. It was also found that the shedding frequency of the wake vortex structures decreased with increasing Richardson number. The wake closure length and the drag coefficient of the heated cylinder were found initially to decrease slightly when the Richardson number was relatively small (Ri < 0.31), and then to increase monotonically with increasing Richardson number as the Richardson number became relatively large (Ri > 0.31). The † Email address for correspondence: huhui@iastate.edu 236 H. Hu and M. M. Koochesfahani average Nusselt number (Nu) of the heated cylinder was found to decrease almost linearly with increasing Richardson number.

Separation Control on a Bridge Box Girder Using a Bypass Passive Jet Flow

“…This kind of particle seeding has been a long established method. Identical fog machines, oils, and set-ups have been used in a number of previous studies (e.g., [18,[24][25][26][27]). In addition, as proposed in Samimy and Lele [28], that particle response could be well characterized by τ , the ratio of particle response time to the flow time scale (Stokes' number), and only particles with τ > 0.05 were found to misrepresent the flow features.…”

“…The reduction of aerodynamic coefficients is defined as: Instantaneous velocity vectors can be obtained by frame-to-frame cross-correlation operation of the images obtained by the PIV system. The MATLAB codes to process the PIV images were developed by the third author and Hui Hu of Iowa State University [18,24,26,29]. There was another post-processing step for filtering/deletion of spurious vectors after image processing.…”

Unsteady Mixed Convection from Two Isothermal Semicircular Cylinders in Tandem Arrangement

“…Although the majority of these studies have focused on studying the cross-flow past bluff bodies such as cylinders of circular [1][2][3][4][5][6], elliptic [7][8][9][10], rectangular [11][12][13][14][15] and square cross-sections [16][17][18][19][20], there are fewer studies on the semicircular cylinder geometry [21][22][23][24]. Gode et al [25] studied numerically the momentum and heat transfer characteristics of a two-dimensional (2D), incompressible and steady flow over a semicircular cylinder and pointed out that the wake ceases to be steady somewhere in the range of 120 ≤ Re ≤ 130.…”

Characteristics of the wake behind a heated cylinder in relatively high Reynolds number

“…This buoyancy-induced vorticity will weaken the shear layer vorticity causing a delay in reaching the recreation point. This type of flow has been the subject of intensive investigations for low Reynolds number flows [12,13], but there are comparatively few studies for heated cylinders at moderate Reynolds numbers [14,15]; yet limited to Re $ 700. As described by Incropera [16], based on the Richardson number (Ri d = Gr/Re 2 ), heat transfer from a heated cylinder to the ambient could be free, mixed or forced convection.…”