Flutter Behaviour of a Laminar Supercritical Airfoil

Abstract

Today a lot of attention is paid to the development of environmental friendly technologies. In the aircraft industry, emissions need to be reduced as well. Fuel can be saved by ensuring that the flow over the wings of the aircraft is completely or partly laminar. A laminar boundary layer creates less skin friction drag than a turbulent boundary layer. It is however more prone to separation. Nevertheless, at the cruise speeds of transport aircraft a laminar boundary layer can be beneficial (because of the small angle of attack). At these speeds a shock wave can however be present on the airfoil at off-design conditions. This shock wave can cause a local separation of the boundary layer, if the pressure gradient is however not too strong, the flow will reattach after the shock and hence the drag of the aircraft will be reduced. During flight disturbances in the flow can set the aircraft into motion. This motion can be damped or amplified. The boundary between these types of motion is called the flutter boundary. This boundary is an important constraint of the flight envelope of aircraft. The influence of boundary layer transition on the flutter behaviour of transonic aircraft has not been investigated yet, therefore a first step has been taken in this thesis. The objective of this thesis is to: Investigate the influence of laminar to turbulent boundary layer transition on the flutter boundary and damping characteristics of a supercritical laminar airfoil (the CAST-10 airfoil) in transonic flow using numerical simulations. In order to do so numerical simulations are performed with the CAST-10 airfoil with two RANS codes: the DLR TAU code and the ANSYS CFX code. The DLR TAU code uses the eN-method for transition prediction, whereas CFX uses the γ − Re_ transition model. Steady and unsteady flow simulations have been performed with both codes. The steady flow simulations are used to initialise the unsteady flow simulations. In these unsteady flow simulations, the airfoil is forced to perform a sinusoidal pitching or plunging motion. The response of the airfoil to these applied motions serves as input to a flutter program, which uses the k-method to compute the flutter boundary. Simulations with both free and fixed transition are carried out. In case of fixed transition, transition was fixed at the leading edge of the airfoil.