## 3 Results and Discussion  
Figure 2a depicts the effects of the PIB concentration on the DR performance as a function of disk rotational speed. By contrast, Figure 2b shows the effect of Reynolds number on %DR with different polymer concentrations. The rotational speed of the disk varied from 2000 rpm at 100 intervals to approximately 3000 rpm, which corresponding to a \(Re\) of 302,227 to 453,341. From the Figure 2a, the DR percentage of the additive is favored by the polymer concentration. The %DR increased for the 50 and 100 ppm of the sample. However, DR decreased with 150 ppm. The decrease was considerably pronounced at rotational speeds ranging from 2000 to 2200 rpm. Such a point could be the critical concentration point of this polymer. Similar observations have been reported by [PERSON] et al. [3; 19; 36]. The authors concluded that the two competitive mechanisms or effects relate to the polymer concentration on %DR. They also suggested that the percentage DR could increase with increasing concentration because of the increased number of drag reducers available. Nevertheless, further increasing the polymer concentration leads to a drastic increase in the eddy viscosity, and the degree of turbulence decreases (i.e., increase in frictional drag and reduction of \(Re\)). In addition, %DR increased with the rotational speed, as evidently shown in Figure 2b. By increasing the rotational speed (i.e., increasing the \(Re\)), the degree of turbulence above the disk surface will increase. This feature will provide a considerably suitable environment for the polymer molecules to perform. Consequently, the additive interaction with turbulent structures formed inside RDA could be controlled. Hence, numerous polymer molecules interfere within the turbulent structures, suppress the formation of eddies on the disk surface, thereby increasing %DR. This study obtained approximately 19.197% DR at 150 ppm of PIB and 3000 rpm (\(Re\) = 453,341). Figure 1: Schematic diagram of rotating disk apparatus (RDA). Legend: (1) Outside frame, (2) fluid container, (3) rotating disk, (4) disk holding shift, (5) torque sensor, (6) electric motor, (7) thermocouple, (8) controller interface, and (9) PC. Figure 3a depicts the effects of the SDS concentration on %DR as a function of rotational velocity, while Figure 3b shows the effect of the Re on %DR with different surfactant concentrations. Evidently, %DR increased with the SDS concentration because surfactant working mechanism could be based on the increase in drag reducer molecules, which is favored by the increase in their respective concentrations. Apart from such observation, %DR evidently increased with the rotational speed. This quality could be attributed to the capability of the surfactant to self-repair (micelles formation) and return to its original form after passing through the high shear stress regions. The average distance between micelles is substantially small and difficult to break by strong turbulence eddies and shear stress. In addition, no DR was observed at 200 ppm for all rotational speeds except at 3000 rpm, where %DR was approximately 1.5%. The negative DR is due to the relation between the degree of turbulence and the DR effect of the polymer or surfactant additives. DR occurs when an external or additional effect (additives) acting within the flow media will change the flow behavior and degree of turbulence in a manner that can reduce power dissipation. Such activity happens when the external effect balances overcome the internal effect (turbulence). In many cases, the DR technique fails to act because this balancing effect is not reached, although the degree of turbulence in many cases plays a significant role in controlling the DR performance. Hence, the negative readings were observed in cases where the surfactant concentration was unsuitable to suppress turbulence and started to show negative performance by creating considerable turbulent and resistive motion.