These computational models contained 36 million control volumes and are so detailed that the very thin layer close to the body, called the laminar sublayer
, caused by hair on the skin of the cyclist, was recreated.
The turbulence is created by the ribs which in turn break the laminar sublayer thickness and reduces thermal resistance in the direction of flow.
Since at high Reynolds number the turbulent kinetic energy is higher, which reduces thermal resistance in the flow by breaking laminar sublayer, Nusselt number is found to be maximum at Re = 18000 and P/e = 25.
The provision of circular transverse ribs increases the intensity of turbulence that breaks the laminar sublayer. This suppression of viscous sublayer causes decrease in friction factor values as the Reynolds number increases.
When pipeline flow is turbulent, the cross section is divided into three regions: 1) laminar sublayer
, 2) buffer region and 3) turbulent core.
In order to keep the friction losses at a low level, the turbulence must be created only in the region very close to the duct surface, that is, in the laminar sublayer. Solar air heater with artificial roughness which is in the form of fine wires of different shapes, sizes, and orientations on the underside of the absorber plate is one of the important and effective design improvements that has been proposed to improve the thermohydraulic performance.
The minimum rib height, 0.7 mm, has been chosen so that the laminar sublayer is of the same order as of roughness height.
A uniform grid contained 384,678 quad cells with cell size of 0.22 mm is used to resolve the laminar sublayer as shown in Figure 3.