Relationship between the free shear layer, the wingtip vortex and aerodynamic efficiency

Date of Award

2016

Degree Name

Ph.D. in Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Advisor: Aaron Altman

Abstract

The overarching objective of this experimental investigation is to explore the relationship between the aerodynamic efficiency of the wing and its turbulent wake (both the free shear layer and the wingtip vortex). Recent evidence of unique turbulent signatures in the free shear layer of a turbulent generator provided the motivation behind this research. The balance of induced drag and the parasite drag was hypothesized to be mirrored in the properties of the wingtip vortex and the free shear layer respectively expanding from classical theoretical descriptions. Experimental investigations were focused on the wake of wings to understand this balance in the parasite and the induced drag and to explore the use of the properties in the turbulent wake to increase the aerodynamic efficiency of the wing. Because of the highly complex nature of the wake, the research is broken down into several individual sub-studies which explore a) the relationship between the aerodynamic efficiency and the free shear layer, b) the relationship between the aerodynamic efficiency and the wingtip vortex, and c) the relationship between the free shear layer and the wingtip vortex and their correlation to the aerodynamic efficiency. Particle Image Velocimetry (PIV) was used to measure the velocity in the wake of an SD 7003 wall-to-wall model and an AR 4 flat plate with and without a spanwise boundary layer trip in the Horizontal Free Surface Water Tunnel (HFWT) at the Air Force Research Labs (AFRL) and in the Low Speed Wind Tunnel at the University of Dayton (UD-LSWT). The results from experimental investigations were Reynolds decomposed to study the mean and fluctuating quantities in the wake of the wing. The initial prediction of these quantities in the wake of SD 7003 wall-to-wall model and AR 4 flat plate were made using the existing momentum deficit and Reynolds stress models (which are derived from simplified Navier-Stokes equations). Even though the momentum deficit model yielded a good match with the experimental data, the Reynolds stress model was not able to predict the experimental data because of the asymmetry in the distribution. The eddy viscosity parameter in the algebraic models was then identified and incorporated in the algebraic models. The variation in the surrogate eddy viscosity parameter when compared to the experimental data showed direct correlation with the variation in the aerodynamic efficiency of the wing. In order to fortify the relationship between the turbulent properties in the free shear layer and the aerodynamic efficiency, the energy loss in the wake of the SD7003 wall-to-wall model was quantified by determining the viscous dissipation (Exergy) as a function of initial conditions upstream. The changes in Exergy mirrored the aerodynamic efficiency of the SD 7003 wall-to-wall model. But in the AR 4 wing wake, there existed a net spanwise momentum due to the formation of the wingtip vortices. Using orthogonal PIV planes of interrogation in the wingtip vortex station across several distances downstream, the evolution of the wingtip vortex and its relationship with aerodynamic efficiency of the wing were investigated. The wake-like to jet-like transition in the core of the wingtip vortex was not observed at the angle of attack of maximum aerodynamic efficiency. However the maximum viscous dissipation and the Reynolds stress in the wingtip vortex shows changes in the slope at the maximum (L/D) location. In the presence of a spanwise boundary layer trip, the location of the change of slope in the viscous dissipation and Reynolds stress was changed indicating a direct correlation to the properties in the wingtip vortex and the aerodynamic efficiency of the wing. Significant changes in the boundary layer of the flat plate with the boundary layer trip were observed at lower angles of attack. The resulting changes in the turbulence character of the wingtip vortex and the free shear layer were investigated for evidence of an interaction between the free shear layer and the evolution of the wingtip vortex. The streamwise, cross-stream and spanwise oriented PIV of the wingtip vortex shows definitive evidence of the free shear layer interaction with the wingtip vortex at angles of attack lower than maximum (L/D). This interaction was reflected in the normalized azimuthal velocity profile of the wingtip vortex as well. The composite of the velocity profiles from the multiple different planes showed a transfer of momentum from the free shear layer to the wingtip vortex in the vicinity of maximum (L/D) angle of attack. This suggests that by manipulating the cross-stream flow in the wake of the wing from the wing root to the wingtip, the balance of induced drag and parasite drag can be altered given initial conditions and the aerodynamic efficiency can be improved in off-design conditions.

Keywords

Airplanes Wings Design and construction, Wakes (Aerodynamics), Shear flow, Aerospace Engineering, Fluid Dynamics, Wingtip Vortex, Free Shear Layer, Turbulence, Exergy, Wingtip Vortex Shear Layer Interaction, Aerodynamics, Boundary Layer Trip, Serrated Leading Edge, Streamwise Wingtip Vortex Particle Image Velocimetry, Momentum Transfer

Rights Statement

Copyright © 2016, author

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