Fluid forces and vortex patterns of an oscillating cylinder pair in still water with both fixed side-by-side and tandem configurations

Models of cylinders in the oscillatory flow can be found virtually everywhere in the marine industry, such as pump towers experiencing sloshing load in a LNG ship liquid tank. However, compared to the problem of a cylinder in the uniform flow, a cylinder in the oscillatory flow is less studied, let alone multiple cylinders. Therefore, we experimentally and numerically studied two identical circular cylinders oscillating in the still water with either a side-by-side or a tandem configuration for a wide range of Keulegan-Carpenter number and Stokes number β. The experiment result shows that the hydrodynamic performance of an oscillating cylinder pair in the still water is greatly altered due to the interference between the multiple structures with different configurations. In specific, compared to the single-cylinder case, the drag coefficient is greatly enhanced when two cylinders are placed side-by-side at a small gap ratio, while dual cylinders in a tandem configuration obtain a smaller drag coefficient and oscillating lift coefficient. In order to reveal the detailed flow physics that results in significant fluid forces alternations, the detailed flow visualization is provided by the numerical simulation: the small gap between two cylinders in a side-by-side configuration will result in a strong gap jet that enhances the energy dissipation and increase the drag, while due to the flow blocking effect for two cylinders in a tandem configuration, the drag coefficient decreases.

Numerical Study of Aerodynamic Forces of Two Airfoils in Tandem Configuration at Low Reynolds Number

For a tandem airfoil configuration, an airfoil is placed in the wake of an upstream airfoil. This interaction affects the aerodynamic forces of the airfoils, especially the downstream one. In the present study a tandem configuration consists of an upstream pitching airfoil and a downstream stationary airfoil is investigated. This study aims to investigate the role of reduced frequency and pitch amplitude of the upstream airfoil’s motion on lift and drag coefficients of two airfoils. These two parameters play an important role in the formation of vortices. The investigation is done for Selig-Donovan 7003 (SD7003) airfoils at low Reynolds number of 30,000 using a computational fluid dynamics. Incompressible URANS equations were employed for solving the flow field. It was found that for a fixed reduced frequency of 0.5 thrust is produced on the hindfoil for a part of cycle for different pitch amplitudes from light to deep stall while for a fixed pitch amplitude at different reduced frequencies high level of thrust or drag can be produced. The reason is related to the type and intensity of vortex-blade interaction.

Dominant Processing Factors in Two-Step Fabrication of Pure Sulfide CIGS Absorbers

Pure sulfide CIGS solar cells are interesting candidates for standalone solar cells or top cells in a tandem configuration. To understand the limits and improve the power conversion efficiency of these devices, a comprehensive approach aimed at composition, interface, and process engineering should be employed. Here, the latter was explored. Using a two-step fabrication technique and one-variable-at-a-time methodology, we found the four processing factors affecting the absorber the most. While two were already backed by the previous literature, we found new and statistical evidence for two other important factors as well. The impact of alkali barrier diffusion was also established with statistical significance and under various processing conditions. Furthermore, the absorber roughness for samples without a barrier indicated a significant negative linear correlation with the devices’ efficiency. This contribution could aid engineers in more efficient process designs.