M.Sc Thesis


M.Sc StudentChen Friedman
SubjectNumerical Investigation of Circulation Control
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Arieli Rimon
Dr. Levy Yuval
Full Thesis textFull thesis text - English Version


Abstract

In 1932, Henri Coanda discovered the natural tendency of a fluid to adhere to curved surfaces. Named after him, the Coanda effect applies to highly-curved surfaces, provided that the resulting radial pressure gradient is strong enough to produce a centripetal force that keeps the fluid attached. The Coanda effect enables changes in the flow by using jets ejected tangentially to curved surfaces. Among the possible applications are: separation onset delay, thrust vectoring, and circulation control over airfoils, enabling higher flap efficiencies, and reducing the number of moving surfaces. Already in the 60's, lift coefficients of over 3 have been measured for an elliptic airfoil. Later on, jet-flap-configurations produced lift coefficients of almost 8.

The thesis outlines the physical principles of the Coanda effect and describes possible aerodynamic applications. Jet-dominated flows are mainly characterized by the dimensionless jet momentum coefficient, which is defined as the ratio of jet momentum to freestream momentum. Numerical simulations have been conducted to study basic flow mechanisms of wall-jets both on straight and curved wall boundaries. The good agreement with the results allowed to validate the flow solver and the turbulence model for these flows.

The work focuses on unsteady circulation control effects that occur as a result of changes in the jet momentum coefficient. Simulations were conducted for a 20% thickness symmetric elliptical airfoil with circulation control, and the steady-state results show a reasonable agreement with experimental measurements from the late 60's. The unsteady investigation presents a comparison between the response to a step change in momentum coefficient and the response to a step change in angle of attack (analytical Wagner function). The time dependent lift response is characterized by a minor initial drop followed by some oscillations which decay rather fast before the new steady state develops. Physical explanations are presented for all phases of the airfoil flow response.