Aerodynamics of permeable wings

Abstract

This research investigates the effect of permeability on the aerodynamics of airfoils and wings. The aerodynamic performance of these airfoils and wings were studied experimentally in the IIUM-low speed wind tunnel. From the literature, it appears that a comprehensive experimental study on permeable wings and airfoils is needed. The research comprises two main objectives; the first of which is to investigate the aerodynamic performance of permeable wings and airfoils using experimental and simulation methods (CFD). The second objective is to investigate experimentally the effect of permeable wingtips on the flow field over the wingtip and its effect on the wake vortex flow downstream using particle image velocimetry (PIV). A permeable thin flat plate, representing a thin symmetric airfoil, as well as a finite wing of the same cross section is used. Permeability is introduced by using a honeycomb structure. The experiment was performed for a range of different porosity values. The results are presented in terms of lift slope versus permeability. The lift slope reduces as the permeability increases for both wings and airfoils. The behaviour/trend of the lift slope is similar to the analytical results available in the literature. The effect of permeability on the aerodynamic center is plotted as well. As the permeability increases the aerodynamic center moves towards the impermeable region. The investigation on the applicability of the standard equation for calculating the lift slope of a wing from an airfoil is applied to permeable wings and airfoils. The result shows that this equation is applicable to both conventional impermeable as well as permeable wings and airfoils. The CFD work is carried out on a thin symmetric airfoil using NACA008 as its cross section. The results of the variation of the lift slope with permeability show a similar behavior as in the experimental study. The results of permeability from CFD shows that a low value of permeability reduces the drag coefficients and thus increases the lift to drag ratio by a large amount. The effect of directional porosity of wing tips on the flow field on the wing surface and in the nearfield of the wing is investigated through PIV. The PIV experiment was performed on seven models of wingtips including the base model. An impermeable wing with a NACA 653218 section was used in this study. Directional porosity is used in five wing tip configurations and one wing tip was made of a honeycomb structure. Configurations 4 – 7 have the highest porosity and the porosity direction in configurations 4 and 5 is 90º, and configurations 6 and 7 have a directional porosity of 95º and 100º, respectively, the directional porosity angle is measured from the chord line., have the highest effect on flow vortex downstream and the reduction in vorticity can reach up to 90% and reduction in tangential velocity can reach up to 74%. These directional porosity wing tips have a great impact on the flow field over the wingtip surface as shown by studying the flow field over the upper surface of wingtips using PIV measurements. These configurations have a porosity perpendicular to the chord line. Configuration 5 has the highest impact as it has the highest porosity value. Configurations 2 and 3 result in a lesser effect on vorticity and tangential velocity as they have porosity inclinations of 30º and 45º respectively. The PIV results over the upper surface of the wingtip show a high disturbance of the flow on the upper surface which results in a reduction in wake vortex downstream. The aerodynamic performances of permeable wingtips were obtained as well and they show a negligible reduction in lift but increase in drag coefficients in some configurations can reach up to 18% at angles of attack [10º - 15º]. These permeable wing tip configurations can be used to alleviate the wake vortex as they are not add-on devices and they are easy to deploy. Thus, this research investigated the behavior of permeable airfoils and wings and compared their behavior with analytical results. It also verified the applicability of the standard equation of calculating lift slope of wing from airfoil lift slope for permeable wings and airfoils. The research also introduced new directionally permeable wingtips which have high impact on vorticity reduction downstream in the near wake field. Last, it investigated the flow behavior over the porous wingtip surface to investigate its role in wake vortex strength reduction downstream.