Characteristics of flow configurations around side-by-side twin wind blades

The effects of gap ratio (g∗) and angle of attack (α) on side-by-side twin wind blades were investigated in an open-channel wind tunnel. Characteristic wake-flow patterns and aerodynamic performance were analyzed using smoke-streak flow visualization, hot-wire velocimetry, and six-force balancer. Seven smoke-streak flow patterns were defined - attached surface flow, wake instability wave, vortical wake, gap flow, bluff-body wake, anti-phase vortex shedding, and in-phase vortex shedding. For g∗ ≈ 0, the flow characteristics were similar to those of a single wind blade. As g∗ increased, these two wind blades induced the vortical wake, gap flow, and anti-phase vortex shedding modes. With further increase in g∗, the wake-flow patterns were similar to those behind a single wind blade. The hot-wire velocimeter detected that the maximum velocity fluctuation occurred at g∗ = 0.083. This velocity fluctuation decreased toward that of free stream as g∗ increased. The vortex-shedding frequency decreased as α increased. For a single wind blade, the maximum lift occurred at α = 10° and the drag increased with α. The pitching momentum increased with α when α < 45°. The lift, drag, and pitching momentum on the lower wind blade decreased significantly due to the existence of upper wind blade. The effect of upper wind blade on the lower one deceased as g∗ increased.