Abstract
The present study experimentally determines the transitional Reynolds number range for plane channel flow and characterizes its transitional state. The pressure along the channel is measured to determine the skin friction coefficient as function of Reynolds number from the laminar state, through the transitional region into the fully turbulent state. The flow structure was studied through flow visualisation which shows that as the Reynolds number increases from the laminar state the transitional region starts showing randomly occurring turbulent spots. With increasing Reynolds number the spots shift into oblique patches and bands of small scale turbulence that form across the channel width, together with large-scale streaky structures found in areas between the turbulent regions. An image analysing technique was used to determine the intermittency factor, i.e. the turbulence fraction in the flow, as function of Reynolds number. It is found that the skin friction coefficient reaches its turbulent value before the flow is fully turbulent (the intermittency factor is still below one). This suggests that the observed streaky structures in non-turbulent regions contribute to the enhancement of the wall-normal transfer of momentum. Also above the Reynolds numbers where the turbulent skin friction coefficient has been established large-scale features consisting of irregular streaky structures are found. They have an oblique shape similar to the non-turbulent and turbulent patches in the transitional flow indicating that the transition process is not fully complete even above the Reynolds number where the skin friction reaches its turbulent level.Graphic abstractProcessing of flow visualization images to determine the turbulenceintermittency in the transitional region. Flow is from left to right, (a)unprocessed, (b) filtered (c) standard deviation of (b), (d) binary picture
Highlights
The phenomenon of transition to turbulence in shear flows depends on the stability of the flow itself and on initial, upstream and background disturbances
The determination of the skin friction coefficient and the flow visualization were carried out in order to investigate the transition from laminar to turbulent flow in a wide aspect ratio channel with a strong inlet disturbance
The fact that the transition to the turbulent value of the skin friction coefficient is faster than in previous experiments indicates the importance of a high upstream disturbance level in order to determine the low-end marginal Reynolds number
Summary
The phenomenon of transition to turbulence in shear flows depends on the stability of the flow itself and on initial, upstream and background disturbances. In the two previously described flow visualization studies the channel inlet flow was controlled to have a low turbulence level so it was possible to introduce a deterministic disturbance in a laminar flow to generate turbulent spots. A so called ‘single slope method’ was developed to obtain the intermittency factor without determination of a threshold and they presented a curve of the intermittency factor as function of Reynolds number They found that the upper marginal Re defined as the lowest Re for fully developed turbulent channel flow is 2600, and that the lower marginal Re defined as the lowest Re for intermittent channel flow with laminar and turbulent parts is 1400 (this would correspond to the transition Reynolds number). The inclination of the turbulent band with respect to the streamwise direction is 24◦ and corresponds to the ratio
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