Investigation of natural transition in the inlet region of a two-dimensional duct flow

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Abstract
Natural transition from laminar to turbulent flow in the inlet region of a two‐dimensional duct has been investigated with the use of a surface hot film, measuring the wall shear stress fluctuation τ′w, and a single hot wire, measuring the velocity fluctuation u′. With a free‐stream turbulence level of 0.05% at the duct inlet, the transition Reynolds number Ra, based on average velocity Ua and duct width 2d, is 11 600. The transition process appears to arise from an instability which leads to the formation of turbulent regions which have features similar to those of turbulent spots in a laminar boundary layer with zero pressure gradient. The intermittency factory γ associated with the turbulent regions is a maximum at the wall and decreases to a minimum on the center line. The space‐time correlation between τ′w and u′ is consistent with the existence of an organized structure during transition. Ensemble averaged shapes of τ′w and u′ are determined for both laminar and turbulent parts of the flow. The wall shear stress increases with distance from the leading edge of the turbulent region and is a maximum near the trailing edge. At distances from the wall greater than 0.3d, the flow within the turbulent region first declerates before accelerating toward the trailing edge. The average shape of the turbulent region is, within the uncertainty of the data, symmetrical about the center line of the duct.