Effects of system rotation on turbulent thermal flow in a circular pipe

Abstract

This thesis reports a set of numerical studies designed to advance the understanding of turbulent flow and heat transfer in a circular pipe under either radial or axial system rotation. To achieve this, direct numerical simulation (DNS) is used, enabling full resolution of the spatial and temporal scales inherent to the turbulent flow. The first DNS focuses on the evolution of vortical structures and enstrophy in a turbulent thermal pipe flow under radial system rotation, and the findings are systematically compared with those from the non‑rotating case. The dynamics of vortical structures are examined through analyses of the instantaneous axial vorticity field, swirling strength, vorticity fluctuation intensities and fluctuating enstrophy, vorticity spectra, and the transport of fluctuating enstrophy in both physical and spectral spaces. It is observed that, in response to radial system rotation, large‑scale secondary vortices develop near the suction side of the pipe. At higher rotation numbers, the radial rotation strongly suppresses the turbulent production of fluctuating enstrophy by weakening the vortex stretching mechanisms. The influence of the large-scale secondary flow structures on the heat convection is analysed based on statistical moments of the temperature field and budget balance of the transport equation of turbulent heat fluxes. The second DNS investigates the effects of axial system rotation on turbulent thermal pipe flows. To ensure that energetic turbulent eddies are properly captured at high rotation numbers, very long pipe lengths significantly exceeding those used in previous studies are employed. Two distinct types of energy‑containing flow structures are observed. The first type is streaks (or hairpin structures), which are characteristic of the turbulent boundary layer developing over the pipe wall for both non-rotating and axially rotating flows. The second type is Taylor columns, which emerge at moderate and high rotation numbers. Because the DNS spans a wide range of rotation numbers and utilises extended pipe lengths, a large dataset of velocity fields is analysed to elucidate the influence of axial rotation on turbulent structures. This analysis includes examinations of the mean velocity field, Reynolds stresses, two‑point autocorrelations, velocity spectra, skewness and flatness factors, and joint probability density functions (JPDF). To further study the effects of axial system rotation on turbulent transport in a circular pipe, a detailed investigation of the Reynolds stress and turbulence kinetic energy (TKE) transport processes is conducted in both physical and spectral spaces. Finally, the turbulent heat transfer in a circular pipe under axial system rotation is investigated through the analyses of the mean temperature field, Nusselt number, temperature variance and turbulent heat flux.