English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We present highly resolved measurements of the near-wall temperature field inthermally driven convection at a rough surface. Our measurements have beenundertaken in a very large experimental facility called the "Barrel ofIlmenau". They provide a unique insight into the local transport process at theinterface between a hot solid surface and a surrounding fluid. In order toprobe the near-wall temperature field, we used a tiny micro-thermistor of 130$\mu$m in diameter and 330 $\mu$m in length with a response time $\tau_{70}$ ofless than 150 ms. This sensor is forty times smaller than the thickness of theboundary layer, and it permits a resolution better than the typical Kolmogorovmicro-scales that appear in our experiment. We demonstrate that the heat fluxenhancement generally observed at rough surfaces, basically results from anincrease of the local heat transfer coefficient at the top of the roughnesselements. We also shed light on the detailed mechanism of the transition in thescaling of the global heat transfer relation $Nu\sim Ra^{\gamma}$ that has beenobserved beyond a critical Ra number $Ra_c$. In turbulent Rayleigh-B\'enardconvection with rough surfaces, it already appears at $Ra_c\approx 10^{10}$[Tisserand et al.{\em Phys. Fluids \/} 23, 015105(2011)] while, for smoothplates, it has been predicted at $Ra_c\approx 10^{14}$ [[Grossmann \& Lohse{\em Phys. Rev. Lett.\/} {\bf 86}, 3316(2001)]. We found this transition atrough surfaces to be attributed to an earlier laminar-turbulent transition ofthe boundary layer at the top of the roughness elements as well as to amodification of the temperature field in between the obstacles.

Thermal boundary layers in turbulent Rayleigh-B\'enard convection with rough surfaces