Numerical investigation of flow through a triangular duct: The coexistence of laminar and turbulent flow

• Flow laminarization in corner region of non-circular ducts is observed.
• Coexistence of laminar and turbulent flow leads to a reduced friction factor.
• Analytical considerations for statistically axisymmetric turbulence are used.
• These considerations show mechanisms responsible for flow laminarization.
• The DNS results support these findings.

Experimental studies of turbulent flow through a triangular duct with small apex angle of 11.5° performed by Eckert and Irvine (1956) show flow laminarisation in the corner region of the duct. This effect is re-investigated using direct numerical simulation (DNS). In order to analyze the impact of duct corners on the flow behavior, results for the friction factor and the mean velocity profiles arising from different non-circular duct geometries, namely a square duct, an equilateral triangular duct and isosceles triangular ducts with an apex angle of 11.5° and 4° are compared with the results for circular pipe flow. Within the cross-sections of the ducts with the small apex angles, regions of essentially laminar and turbulent properties are found to exist simultaneously. From analysis of the turbulent quantities performed exemplary for the 11.5°-triangle we gain further insights into the mechanisms responsible for flow laminarization which are supported by analytical considerations for statistically axisymmetric turbulence.