Heat is transferred at smaller scales than momentum in turbulence

What is it about?

Turbulent flow consists of innumerable vortices with various sizes, and fluid motions caused by these vortices at various different scales are responsible to significant heat and momentum transfer by turbulent flow. In this study, we carry out computer simulation of a very simple turbulent flow configuration, called plane Couette turbulence, and investigate how momentum and heat are transferred by turbulent shear flow at different scales. We show that the size of vortices that mainly transfer heat is notably smaller than that of vortices responsible for momentum transfer. It is further shown by a detailed analysis on the simulation data that energy transfer between different scales in temperature field is more significant than that in velocity field at relatively small scales. Such difference between the interscale energy transfer in velocity and temperature field leads to the dissimilarity between the turbulent heat and momentum transfers.

Featured Image

Photo by Borys Liechti on Unsplash

Photo by Borys Liechti on Unsplash

Why is it important?

Transfer of momentum in turbulent flow is directly related to friction drag, and heat transfer by turbulent flow is essentially important for efficiency of thermal fluid heat exchanger. Therefore, for achieving better energy efficiency it is important to control turbulent flow so that momentum transfer (friction drag) is suppressed while heat transfer is enhanced. However, it is widely know that transfer of momentum and heat by turbulence are basically similar (Reynolds analogy), and therefore the dissimilar flow control (i.e., decreasing drag with increasing heat transfer at the same time) is considered difficult. The importance of this study is that the scales at which momentum and heat transfers by turbulence occur are found notably different despite the Reynolds analogy, and the mechanism is also certainly unveiled. Such knowledge regarding the difference between turbulent momentum and heat transfers may lead to development of a new technique for dissimilar control of turbulent flow.