Effect of large-scale vortex induced by a cylinder on the drag and heat transfer coefficients of smooth and dimpled surfaces

Abstract

The drag and heat transfer coefficients of smooth and dimpled surfaces located in the wake of a cylinder are experimentally determined. The cylinder, 8 mm in diameter, was placed in a 30 mm-high slot channel. In the experiments the cylinder position along the channel height and the dimpled surface geometry varied: the gap between the cylinder and the wall under consideration ranged from 0 to 21 mm and the in-line arrangement of spherical dimples and the staggered arrangements of spherical and oval dimples were considered. In determining the drag coefficients the Reynolds number based on the cylinder diameter varied in the range Red = 10.9–53.3∙103. In determining the heat transfer coefficients the Reynolds number was Red = 53.3∙103. The cylinder was unheated. The drag coefficients of the smooth and dimpled surfaces were determined by directly weighing the models on a single-component strain-gauge balance. The local values of the heat transfer coefficients were determined by means of recording the rate of the model surface cooling using an IR imager. The heat transfer and drag coefficients in the cylinder's wake divided by their values on the smooth wall in the undisturbed flow varied in the ranges 0.98–2.64 and (−1.65)-(1.95) for the smooth and 0.75–3.34 and (−3.77)-(4.5) for the dimpled surfaces. The Reynolds analogy factor for the above-mentioned models ranged from 0.45 to 7.73 depending on the particular surface and the gap between the cylinder and the wall.