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
Abstract. The maximum limit of water storage capacity in sewers and storm drains depends on the draining capacity of the sewer or drain. When the draining capacity is exceeded, this results in the overloading of the sewer capacity, causing flooding. This moment, which is called the "choking" phenomenon. Therefore, draining capacity will decrease, and flooding can occur in a short period of time. The purpose of the current research was to examine this phenomenon by determining the correlation between draining capacity and water storage levels in the experimental situation. The experiments were based on original techniques published in 1949 [1,2] which demonstrated an increase in the flow rate in a pipe by adding polymer solution into the fluid in a turbulent flow; this has been termed the "Drag Reduction Effect". The experiments demonstrated that the addition of the polymer solution proved to reduce the drag between the fluid and the pipe wall, and that the overloaded sewer effect can be relieved by means of polymer addition into the draining system, which enhances draining capacity during flooding situation, To conduct the experiment, an acrylic tank with a capacity of 65 litres was prepared to simulate a flooding area. Three different sizes of polyvinyl chloride (PVC) pipe (10, 18 and 20 millimeters) were used as a test section to simulate a sewer pipe. The experiment was conducted by dosing a gravity pipe flow system with Polyacrylamide (PAM), known as anionic polymer, which is normally employed in wastewater treatment processes, at different concentrations (0, 10, 30, 50, 100 wppm (part per million by weight), The results of the experiment indicate that a suitable amount of polymer addition can increase maximum enhancement of draining capacity up to 13 % and increase maximum flow rate up to 12%. It was also observed that the increased flow rates resulting from the drag reduction phenomenon occurred as an effect of the type of polymer and the concentration of the polymer.
The effect of polymer addition on transition to turbulence in a two-dimensional water-flow channel was experimentally investigated by flow visualization using reflective flakes. The flow entering the channel test section maintains a high disturbance level by expanding laterally after reaching a high Reynolds number upstream the test section. In order to obtain the intermittency factor (turbulence fraction), the visualized images were classified into nonturbulent and turbulent regions, and the streamwise scale of the streaks appearing in the non-turbulent region was estimated from the autocorrelation coefficient computed by shifting the images in the streamwise direction. The visualization results show that similar to the pure water case, intermittent flow with a patch-like distribution of turbulent and non-turbulent areas clustered by streamwise streaks is observed. The Reynolds number at which the intermittency increases shifts toward higher Reynolds numbers with increasing polymer concentration, indicating a delay of transition. The streaks appearing in the non-turbulent region elongate with increasing polymer concentration. At high concentrations, straight elongated streaks penetrate through the turbulent regions, suggesting that the polymer addition affects the stability of the streaks. These changes of the streak behavior indicate that the polymer affects not only the transition Reynolds number but also the flow structure during the transition process.
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