Wingtip Vortex Behavior in the Vicinity of the Maximum Lift to Drag Ratio Lift Condition

Adverse effects of lift induced drag on the aerodynamic efficiency of aircraft are well known. Lift induced drag is generated as a byproduct of downwash from the wingtip vortices. The flow physics associated with wingtip vortex core axial flow transformation from wake-like (velocity less-than the freestream) to jet-like (velocity greater-than the freestream) behavior in the vicinity of the maximum lift to drag ratio (L/D) lift condition is explored.

Particle Image Velocimetry (PIV) experiments were performed in the UD Low Speed Wind Tunnel in the near wake of an AR 6 wing with a Clark-Y airfoil to investigate the characteristics of the wingtip vortex at angles of attack ranging from 2 and 8 degrees. Results showed changes in the velocity distributions in the vortex inner and outer cores. Vorticity and exergy distributions indicated the existence of the wake-like to jet-like transformation in the range of 4° to 6° angle of attack. This range corresponds with the maximum L/D angle of attack of the Clark-Y tested. A relationship between the vortex core axial velocity profile changeover and the angle of attack at maximum L/D was identified. Improved understanding of this relationship could be extended not only to improve aircraft performance through the reduction of lift induced drag, but also to air vehicle performance in off-design cruise conditions.

Wind Tunnel Blockage Corrections: Review and Application to Savonius Vertical-Axis Wind Turbines

An investigation into wake and solid blockage effects of vertical axis wind turbines (VAWTs) in closed test-section wind tunnel testing is described. Static wall pressures have been used to derive velocity increments along wind tunnel test section which in turn are applied to provide evidence of wake interference characteristics of rotating bodies interacting within this spatially restricted domain. Vertical-axis wind turbines present a unique aerodynamic obstruction in wind tunnel testing, whose blockage effects have not yet extensively investigated. The flowfield surrounding these wind turbines is asymmetric, periodic, unsteady, separated and highly turbulent. Static pressure measurements are taken along a test-section sidewall to provide a pressure signature of the test models under varying rotor tip-speed ratios (freestream conditions and model RPMs). Wake characteristics and VAWT performance produced by the same vertical-axis wind turbine concept tested at different physical scales and in two different wind tunnels are investigated in an attempt to provide some guidance on the scaling of the combined effects on blockage. This investigation provides evidence of the effects of large wall interactions and wake propagation caused by these models at well below generally accepted standard blockage figures.

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Low Speed Wind Tunnel, Dr. Aaron Altman, Director

Kettering Laboratories 
300 College Park 
Dayton, Ohio 45469