The control over the entire surfaces to force viscous sublayer
resolution was computationally expensive and judged unnecessary.
For the high Prandtl numbers, the grid resolution which captures the viscous sublayer can be inadequate and insufficient for the thermal conduction region.
It is well known that the viscous sublayer (0 [less than or equal to] [y.
However, with the roughness reduced on the inner wall, the small convex parts are all completely submerged in the viscous sublayer and the gas pipeline becomes "hydraulic smooth pipe" That means even though the coating surface is smoothed further, the wall friction is difficult to further reduce.
In order to improve the drag-reducing effect further, the triangle groove is designed as s=135 urn, h=100 um (s-width of the groove, h- height of the groove), where the tips of grooves stick out of the viscous sublayer, and the good drag reduction effect can also be received.
The boundary layers that cover airfoil are divided into viscous sublayer
, buffer layer and log-low region (MA, 20009).
While wall roughness hardly affects the pressure drop in laminar flow, its influence in the turbulent case is quite considerable if the mean protrusion height k is greater than the thickness of the viscous sublayer [delta].
The contribution of the roughness to the pressure loss is negligible if the protrusion height k is so small that all protrusions are inside the viscous sublayer (k < [delta]) and the effect on the resistance is negligible.
Near the wall, wall functions are used when the mesh is too coarse to resolve the viscous sublayer.
However, correct prediction of this vortex cannot be ascribed to whether the viscous sublayer is resolved.
In the simple models in common engineering use, the effect of viscosity is confined to a thin viscous sublayer near the pipe wall.
As the velocity gradient in the viscous sublayer is much steeper than that in the logarithmic zone, the sublayer has an influence on mean velocity which is disproportionate to its small thickness, and thus it is the sublayer that determines the influence of viscosity on the friction factor for Newtonian turbulent flow.