This thesis details an experimental investigation into the effects of elevated levels of large-scale freestream turbulence on the aerodynamics of a micro air vehicle wing in fixed, flapping, and pitching configurations Such large-scale turbulence is typi

The effect of freestream turbulence on fixed and flapping micro air vehicle wings

We present a new approach to model the complex dynamics of aerodynamic forces on an airfoil in turbulent inflow conditions. Our ansatz is based on stochastic differential equations and aims at replacing traditional look-up table methods used in wind turbine simulation systems by the effective response dynamics of lift and drag forces. The parameters of the model are derived directly from empirical data. Measurements were taken in the closed loop wind tunnel of the University of Oldenburg for an airfoil FX 79-W-151A. The turbulent inflow was generated using a fractal square grid as it is possible to generate in this way wind speed fluctuations with similar statistics as observed in nature. Forces were measured using two strain gauge force sensors at two end points of the vertically installed airfoil. The modeling is performed by applying a stochastic approach on the measured data. By estimating the first two Kramers–Moyal coefficients, a first-order stochastic differential equation called the Langevin equation is obtained. The stochastic model achieved through this approach is extended to account for oscillation effects contained in lift and drag dynamics that probably stem from unsteady aerodynamic effects. The results are optimized by applying a 2 test on the probability density functions (PDFs) of model and measurements. With the knowledge of the Langevin equation, synthetic time series are generated. Their stationary PDFs as well as conditional PDFs show good agreement with the actual measurements. A comparison of classical averaging and the stochastic approach shows that stochastic analysis achieves additional insight into the local dynamics of lift and drag forces. Copyright © 2014 John Wiley & Sons, Ltd.

Stochastic modeling of lift and drag dynamics under turbulent wind inflow conditions

A solver for the incompressible Navier–Stokes equations in which solid surfaces are represented using an immersed boundary method is presented. Adaptive Cartesian mesh refinement and coarsening are employed both to resolve immersed boundaries and to dynamically resolve important flow features as they are developing. Dynamically evolving immersed boundaries are tracked using Lagrangian marker points that are governed by direct momentum forcing using either simple force laws or explicitly prescribed motion in a framework that permits simulation of both rigid and flexible lifting surfaces. The adaptive Cartesian immersed boundary method capability presented here is well suited for low Reynolds number flows over flexible lifting surfaces as encountered in modern flapping-wing flyers, flow over complex geometries undergoing large-scale deformations, and vortex-dominated flows. The implementation of both the incompressible Navier–Stokes solver and the immersed boundary method solution algorithm are described in...

Adaptive Cartesian Immersed Boundary Method for Simulation of Flow over Flexible Geometries

Hummingbirds have been considered because they have high maneuverability. Making an example of this type of bird needs to observe the kinematic and aerodynamic issues of the bird’s flight. In this article the locus of attack angle and instantaneous elevation is studied to be used for constructing a sample of birds. At first, the charts offered in the article of bird’s flight was digitized and by using MATLAB software was fitted to the shape of a sinusoidal equation to be comparable with designed mechanisms. The designed mechanisms have three variables that by using EES software the motion equation has been approached. Then by the Lagrange Interpolation the effect of each variable in the equation of motion mechanism obtained. Finally by using EES software these equation is solved and the amount of the variables was calculated.

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Design a mechanism Like flapping hummingbird wings

: A Navier-Stokes equation solver is utilized to investigate the aerodynamics of a flapping airfoil. The roles of plunging and pitching amplitude and frequency, and Strouhal number are studied. For a NACA0012 airfoil at zero geometric angle of attack, chord Reynolds number of 20,000 and at a given plunging frequency, either drag or thrust is produced, depending on the plunging amplitude. When drag is produced, the viscous force dominates the total drag with decreasing influence as the plunging amplitude increases. At the considered plunging amplitude (from 0.0125c to 0.075c), flow history has more influence than the kinematic angle of attack in determining lift. For an airfoil experiencing combined plunge and pitch motion, both thrust and input power increase with the Strouhal number (within the range of 0.03 to 0.5). For the case studied, the thrust is induced by the lift, which follows approximately the curve of kinematic angle of attack. Leading edge vortex moves downstream and interacts with the trailing edge vortex. The impact of a gust on a stationary and flapping airfoil is also studied.

Youngsheng Lian

Papers

Computational Investigation of Unsteady Low-Reynolds Number Aerodynamics for Micro Air Vehicles