טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentWeidenfeld Michael
SubjectApproaches for the Monitoring of Airfoil Aerodynamic
Noise
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Avshalom Manela
Dr. Eran Arad


Abstract

Airfoil sound is a dominant noise component in the acoustic signature of a wide variety of aerodynamic systems. In line with current interest in reducing aircraft noise, the present work studies means by which airfoil aerodynamic radiation may be monitored. Motivated by biomimetic observations of the quiet flight of owls, the principal approach focuses on the effects of airfoil porosity, elasticity, and material non-homogeneity on the acoustic field of an otherwise rigid, impermeable and homogeneous airfoil. Considering low Mach and high Reynolds number flow conditions, the fluid structure interaction of a  two-dimensional airfoil is specified using thin-airofil theory and a discrete-vortex wake model. Applying the Powell-Howe acoustic analogy, the system hydrodynamic field is substituted as a source term into the vortex sound equation using compact-body approximation, where the leading order dipole-type noise combines the vortex-airfoil interactions, airfoil seepage flow, and airfoil motion source contributions.


We start by examining the effects of permeability on the system sound, and identify a mechanism for sound attenuation via the counteracting motion- and seepage-noise signals. We then consider the impact of inhomogeneous structural properties on the sound radiation of a flapping airfoil, and delineate optimal conditions for sound attenuation while maintaining wing aerodynamic efficiency. The first part of the work ends with a study on the near and far fields of a ``hanging'' flexible filament subject to gravity-induced tension force. Focusing on the limit of small bending rigidity, the dynamics of a highly elastic structure is examined and compared with a membrane.

Differences are identified in the vicinity of the filament end points, where semi-analytic approximations are obtained. The ``hanging filament'' setup is further used to study the far-field radiation of an actuated filament over the entire range of flexural rigidities.


In a secondary effort, the broad-band aerodynamic noise of a NACA0012 airfoil is calculated using high-fidelity large eddy simulations in conjunction with the Ffowcs Williams-Hawkings acoustic analogy. Both near- and far-field results are validated,

followed by preliminary investigation of the effect of trailing-edge temperature variations on sound radiation. The setup consists of time-harmonic temperature-dipole actuators, fixed in the vicinity of the airfoil downstream end. The altered sound spectrum obtained suggests that temperature dipoles may be used as an effective means for airfoil noise control.