An Experimental Investigation on the Characteristics of Turbulent Boundary Layer Flows Over a Dimpled Surface

Abstract

An experimental investigation was conducted to quantify the characteristics of the turbulent boundary layer flows over a dimpled surface in comparison to those over a conventional flat plate. In addition to measuring surface pressure distributions to determine the friction factors of the test plates and to map the surface pressure inside the dimple cavity, a high-resolution digital particle image velocimetry (PIV) system was used to achieve detailed flow field measurements to quantify the characteristics of the turbulent boundary layer flows over the test plates and the evolution of the unsteady vortex structures inside the dimple cavity at the middle of the dimpled test plate. It was found that the friction factor of the dimpled plate would be about 30–80% higher than that of the flat plate, depending on the Reynolds number of the test cases. In comparison with those over a conventional flat surface, the flow characteristics of the turbulent boundary layer flows over the dimpled surface were found to be much more complicated with much stronger near-wall Reynolds stress and higher turbulence kinetic energy (TKE) levels, especially in the region near the back rims of the dimples. Many interesting flow features over the dimple surface, such as the separation of oncoming boundary layer flow from the dimpled surface when passing over the dimple front rim, the formation and periodic shedding of unsteady Kelvin–Helmholtz vortices in the shear layer over the dimple, the impingement of the high-speed incoming flow onto the back rim of the dimple, and the subsequent generation of strong upwash flow in the boundary flow to promote the turbulent mixing over the dimpled surface, were revealed clearly and quantitatively from the PIV measurement results. The quantitative measurement results are believed to be the first of its nature, which depict a vivid picture about the unique flow features over dimpled surfaces and their correlations with the enhanced heat transfer performance reported in previous studies.

Calculation and Experimental Study of Heat Exchange in a System of Plane-Parallel Channels with Surface Intensifiers

“…The parameters of the surface relief do not influence this specifically or immediately since they do not prescribe the vortex structures to deviate in a coordinated fashion in one or the other direction. The matching of states is due to nonlinearity of the equations of motion of the continuous medium, which also manifests itself when large-scale vortex structures [19,20] arise.…”

Heat transfer characteristics of a dimpled/protrusioned pin fin wedge duct with different converging angles for turbine blades

“…After passing through the pin fin, part of the airflow nearby the dimple separates and is involved in the flow recirculation at the front half of the dimple where a relatively lower TKE region is born, which will be discussed later. The remaining airflow shifts downwards by the flow recirculation and impinges on the rear half of the dimple [38]. The impingement on the rear half of the dimple can make the boundary layer thinner and this is remarkably beneficial for the heat transfer augmentation.…”

Investigations of water-cooled tubes with protrusions for flow and heat transfer characteristics

“…Li et al (2018) conducted a numerical study on the heat transfer performance of water-cooled micro channel radiators with protrusion and pin-fins, and further optimized them. Zhou et al (2016) investigated experimentally the flow characteristics of turbulent boundary layer flows over a convex and flat plate surface. The results showed that the flow characteristics of turbulent boundary layer over the convex surface were much more complex, especially in the region near the back rims of the protrusion.…”