Abstract
We present direct numerical simulations of developing turbulent channel flows subjected to thermal expansion or contraction downstream of a heated or cooled wall. Using different constitutive relations for viscosity we analyse the response of variable property flows to streamwise acceleration/deceleration by separating the effect of streamwise acceleration/deceleration from the effect of wall-normal property variations. We demonstrate that, beyond a certain streamwise location, the flow can be considered in a state of ‘quasi-equilibrium’ regarding semilocally scaled variables. As such, we claim that the development of turbulent quantities due to streamwise acceleration/deceleration is localized to the region of impulsive heating/cooling, while changes in turbulence occurring farther downstream can be attributed solely to property variations. This finding allows us to study turbulence modulation in accelerating/decelerating flows using the semilocal scaling framework. By investigating the energy redistribution among the turbulent velocity fluctuations, we conclude that a change in bulk streamwise velocity has a non-local effect which originates from the change in mean shear and modifies the energy pathways through velocity-pressure-gradient correlations. On the other hand, the wall-normal property gradients have a local effect and act through the modification of the viscous dissipation. We show that it is possible to superimpose and compare the two different effects when using the semilocal scaling framework.
Highlights
Expanding and contracting flows are found in many fields and engineering applications that deal with heat transfer to a fluid, such as chemical reactors, energy
Patel et al (2016) shows that the effects of an increasing semilocal Reynolds number can be compared to an adverse pressure gradient flow, while for a decreasing Reynolds number, the effects are similar to a flow with a favourable pressure gradient (FPG)
We have shown turbulence modulation for a variety of streamwise accelerating and decelerating flows characterized by different property variations
Summary
Expanding and contracting flows are found in many fields and engineering applications that deal with heat transfer to a fluid, such as chemical reactors, energy. Most of the knowledge on turbulence modulation by variable properties in wall turbulence has been gathered in the framework of fully developed flows, such as supersonic boundary layers (Spina & Smits 1987; Pirozzoli, Bernardini & Grasso 2008; Ringuette, Wu & Martin 2008; Pirozzoli & Bernardini 2011) and turbulent channel flows (Foysi, Sarkar & Friedrich 2004; Zonta, Marchioli & Soldati 2012; Patel et al 2015; Patel, Boersma & Pecnik 2016; Sciacovelli, Cinnella & Gloerfelt 2017) In his early work Morkovin (1961) reported that in a compressible flow, property fluctuations can be neglected as turbulence modulation occurs mainly due to mean property variations. We will make extensive use of the semilocal scaling framework, to contextualize streamwise acceleration/deceleration in variable property flows
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