Nomad flutter and flow simulation acceleration for elastic wings
Creators: name: Norrison, Daniel
Publisher: name: RMIT University
Year: 2024

description: Aircraft are flexible aeroelastic structures. Therefore, in-flight they are susceptible to a self-induced oscillation known as flutter which can lead to rapid and catastrophic structural failure. The aircraft design process must ensure that flutter occurs beyond the flight envelope, yet the Government Aircraft Factories Nomad aircraft has occasionally experienced low speed flutter involving its flaperon. Past attempts to investigate the Nomad’s flaperon flutter were unsuccessful. Consequently, not much is known about the critical flutter mode except that it occurs upon landing at a speed of about 100 knots.<br><br>The objective of this research was to make a contribution towards the knowledge needed to help resolve the Nomad's flutter. The scope of this work was limited to applying computational methods rather than physical experiments. A nonlinear aeroelastic simulation was deemed necessary for an accurate flutter analysis with the Nomad’s wing geometry in its landing configuration. However, nonlinear aeroelastic methods need significant development in many areas before they can be applied to problems like the Nomad. In particular, the nonlinear aerodynamics component of nonlinear aeroelastic simulations was identified as critical area. Hence, the focus of the research work related to nonlinear computational fluid dynamics (CFD) and more specifically, turbulence modelling, grid generation and the computational cost involved.<br><br>Flow phenomena around high-lift wings, like the Nomad's, are not well understood under take-off and landing conditions. Therefore, studying the local flowfield structures around the Nomad’s wing-flaperon landing configuration would be valuable. No such examination has been reported in the literature. The steady, two-dimensional flowfield around the Nomad's wing was simulated. Results showed separated regions behind the flaperon and main wing element, and attached flow elsewhere. Pressure distributions along the flaperon were strongly influenced by turbulence model. Thus, existing Reynolds-Averaged Navier–Stokes turbulence models, coupled with wall modelling, are unreliable for this problem.<br><br>Field grid generation required for nonlinear aerodynamics computations can be laborious and may involve considerable computational resources, especially for moving geometries essential for aeroelastic simulations. Therefore, an efficient grid generation algorithm was developed. It generated a structured O-grid around a single arbitrarily shaped body in two and three dimensions by solving a system of elliptic Laplace or Poisson equations. This algorithm differs from others by implementing several acceleration techniques, including approximate factorisation and the method of false transients, to enhance the convergence rate. Computation times were drastically reduced relative to traditional methods.<br><br>In nonlinear aeroelastic simulations, over 60% of the total computational time is expended on nonlinear CFD calculations (excluding grid generation). This cost can be reduced by exploiting rapidly advancing computer technology. In this work, the scaling performance of Intel’s first general purpose quad-core processor for personal computers was studied using a CFD problem. Using a two-dimensional Euler solver developed from scratch, the results showed speedups of 350% and 256% for coarse and fine grids, respectively.