Flow visualization and skin friction determination in transitional channel flow

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 abstract

Effects of polymer addition on transition and length scales of flow structures in transitional channel flow

“…Figure 1 shows a typical flow visualization taken at different polymer concentrations at Re s = 1900, where the flow direction is from left to right. Similar to the results already reported by Yimprasert et al (2021), the flow in the case of pure water in Figure 1(a) has a characteristic patchwork pattern containing turbulent patches and clusters of streaks. The typical streamwise and spanwise sizes of the turbulent patches are about 10d × 5d, and the streak spacing ∆z/d is about 0.64, this value was obtained as the average of measurements that were manually obtained from ten independent photographs.…”

“…A recent study by Seki and Matsubara (2012) determined the intermittency factor from the velocity fluctuations measured by hot-wire anemometry by using a newly developed technique, and the results indicate a transitional range from Re = 1400 to Re = 2600. Pressure-drop measurements and flow visualization in transitional channel flow by Yimprasert et al (2021) supported Seki and Matsubara's transition Re range, where intermittency increases linearly with Re and the transition flow is observed to form a patchwork pattern consisting of turbulent and streaky regions. Kohyama et al (2022) performed direct numerical simulations of an 1:96 aspect ratio duct in the presence of end-walls and demonstrated that a sustained transitional flow exists above Re = 1425.…”

“…The experiments were performed in a closed-loop, nine meter long water-channel flow facility, earlier used by Yimprasert et al (2021). The water fed by a pump passes through an orifice flow meter which determines the flow rate, and then enters a stagnation box with five screens.…”

A numerical study of the side-wall effects on turbulent bands in channel flow at transitional Reynolds numbers

“…The spanwise channel size is chosen such that turbulent bands can be accommodated in the channel. The aspect ratio of 50 is much larger than that considered in DNS by [16] (up to 9) and comparable with the experimental study of [18] where the ratio was 80. Refs [7,17] used significantly larger widths in experimental studies, but they focused on the flow far from the side walls.…”

“…Below Re 800, it is rather difficult to generate a turbulent band in either numerical simulations [5] and experiments [7,18] because special perturbations are needed [5], especially when one wants to control the position and orientation (tilt angle about the streamwise direction) of the band. Song & Xiao [14] proposed an effective perturbation method for numerical simulations at low Reynolds numbers, which enables us to control the position and orientation of the band precisely.…”