Abdul Razak Norizham

Flutter and stall flutter of a rectangular wing in a wind tunnel

The aeroelastic behavior of a rectangular wing with pitch and plunge degrees of freedom was observed experimentally using pressure, acceleration, and particle image velocimetry measurements. The wing was set at different static angles of attack and wind-tunnel airspeeds. The wing’s dynamic behavior was governed by a twoparameter bifurcation from steady to limit cycle oscillations, with the two parameters being the airspeed and the static angle of attack. At the lowest static angle, the wing underwent a classical flutter phenomenon that was transformed into a supercriticalHopf bifurcation at higher angles. The latterwas combinedwith a fold bifurcation at intermediate angles of attack. All limit cycle oscillations observed were either low-amplitude oscillations with timevarying amplitude or high-amplitude oscillations with nearly steady amplitude. They were caused by two different types of dynamic stall phenomena. During low-amplitude limit cycle oscillations the periodically stalled flow covered only the rear part of the wing. During high-amplitude limit cycle oscillations, trailing-edge and leading-edge separation occurred. Trailing-edge separation was characterized by a significant amount of unsteadiness, varying visibly from cycle to cycle. The occurrence of leading-edge separation was muchmore regular and had the tendency to stabilize the amplitude of the limit cycle oscillation motion.

Flow Visualization and Proper Orthogonal Decomposition of Aeroelastic Phenomena

The modal decomposition of unsteady flowfields was proposed in the 1990s by several authors, e.g. Hall (1994) or Dowell et al. (1998). Proper Orthogonal Decomposition (POD) is one method that can be used in order to perform this modal decomposition; it became popular for aerodynamics research in the 2000s, starting with Tang et al. (2001), although it was first proposed for use in fluid dynamics in the 1960s by Lumley (1967). The basic principle of POD is the creation of a mathematical model of an unsteady flow that decouples the spatial from the temporal variations. A 2D flowfield described by the horizontal velocity u(x, y, t) and the vertical flow velocity v(x, y, t) can thus be expressed as

Experiments on a pitch-plunge wing undergoing limit cycle oscillation

The aeroelastic behaviour of a wing oscillating in the heave and pitch degrees of freedom is examined by means of wind tunnel experiment. The phenomena of interest are classical utter and limit cycle oscillation. Classical utter is normally associated with the exponential growth of the response amplitude. Linear utter theory only predicts the critical utter speed. Any excitation or disturbance beyond the critical speed is assumed to cause exponential growth in the response amplitude. In contrast, any limited amplitude oscillations occurring post-fultter suggest the existence of nonlinear properties in the system. Such properties can originate from the aerodynamic forces in the form of ow separation and reattachment. On the structural side, damping and stiness can also contribute nonlinear properties. Furthermore, these nonlinearities can manifest themselves even at pre-utter conditions, depending on the values of some governing parameter. The focus of the present work is the transformation of classical utter into stall utter as the equilibrium angle of attack of heaving and pitching wing is increased. The interaction of stall-related nonlinearity with structural nonlinearities is also of interest. The measured aeroelastic responses are analyzed and the bifurcation behavior of the dynamic system is characterized. Structural responses as well as ow eld visualization through Particle Image Velocimetry show the origin of nonlinearity does not solely come from the manifestation of separation and the shedding of vortices, but from the structural nonlinearity which limits the response amplitude.

A Cross-Validation Approach to Approximate Basis Function Selection of the Stall Flutter Response of a Rectangular Wing in a Wind Tunnel

The stall flutter response of a rectangular wing in a low speed wind tunnel is modelled using a nonlinear difference equation description. Static and dynamic tests are used to select a suitable model structure and basis function. Bifurcation criteria such as the Hopf condition and vibration amplitude variation with airspeed were used to ensure the model was representative of experimentally measured stall flutter phenomena. Dynamic test data were used to estimate model parameters and estimate an approximate basis function.