Flow patterns and characteristics of two tandem wing-blades at low Reynolds numbers

Abstract

Flow pattern, aerodynamic performance, vortex-shedding frequency and turbulence intensity were compared between a single wing-blade and two tandem wing-blades for various Reynolds numbers (Re), gap ratios (g*) and angles of attack (α). The wing-blade profile was NACA 0012 which was fabricated from stainless steel. The effects of α comprised the effect of the angle of attack of the front wing-blade (α₁) and that of the rear wing-blade (α₂). Flow behaviors and flow patterns were visualized using the smoke-wire method. The vortex-shedding frequency and turbulence intensity were measured using a hot-wire anemometer. The smoke-streak flow pattern around the single wing-blade was categorized as five characteristic flow patterns, which are attached surface flow, instability wave in wake, vortical wake, separation from near-leading-edge and bluff-body wake. The smoke-streak flow patterns around the two tandem wing-blades (changing α₁ and g*) were classified as attached flow, separation, turbulent flow and vortex street for α₂ = 0°. The flow patterns for α₁ = 0° (with varying α₂ and g*) were attached flow, vortical wake, bluff-body wake and bi-vortex street As α₁, α₂ and g* changed, the flow patterns were classified as attached flow, vortical wake, bluff-body wake and turbulent flow. The characteristics of St₁, St₂, I₁ and I₂ parameters (measured around the tandem wing-blades) were compared with St_S and I_S (detected around the single wing-blade). The aerodynamic performance was measured using a six-force balancer. For α₂ ≥ 30°, the maximum lift coefficient (C_L) was reached at g* = 0 because of the equivalent flap effect that was caused from the existence of rear wing-blade.