Recent progress in flapping wing aerodynamics and aeroelasticity

Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

New insights into large eddy simulation

Classifications, applications, and design challenges of drones: A review

4-10 5 . In order to gain better understanding of the fluid physics and associated aerodynamics characteristics, we have coupled (i) a NavierStokes solver, (ii) the e N method transition model, and (iii) a Reynolds-averaged two-equation closure to study the low Reynolds number flow characterized with laminar separation and transition. A new intermittency distribution function suitable for low Reynolds number transitional flow is proposed and tested. To support the MAV applications, we investigate both rigid and flexible airfoils, which has a portion of the upper surface mounted with a flexible membrane, using SD7003 as the configuration. Good agreement is obtained between the prediction and experimental measurements regarding the transition location as well as overall flow structures. In the current transitional flow regime, though the Reynolds number affects the size of the laminar separation bubble, it does not place consistent impact on lift or drag. The gust exerts a major influence on the transition position, resulting in the lift and drag coefficients hysterisis. It is also observed that thrust instead of drag can be generated under certain gust condition. At α=4 o , for a flexible wing, self-excited vibration affects the separation and transition positions; however, the time-averaged lift and drag coefficients are close to those of the rigid airfoil.

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Laminar-Turbulent Transition of a Low Reynolds Number Rigid or Flexible Airfoil

This is an ideal book for graduate students and researchers interested in the aerodynamics, structural dynamics and flight dynamics of small birds, bats and insects, as well as of micro air vehicles (MAVs), which present some of the richest problems intersecting science and engineering. The agility and spectacular flight performance of natural flyers, thanks to their flexible, deformable wing structures, as well as to outstanding wing, tail and body coordination, is particularly significant. To design and build MAVs with performance comparable to natural flyers, it is essential that natural flyers' combined flexible structural dynamics and aerodynamics are adequately understood. The primary focus of this book is to address the recent developments in flapping wing aerodynamics. This book extends the work presented in Aerodynamics of Low Reynolds Number Flyers (Shyy et al. 2008).

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An Introduction to Flapping Wing Aerodynamics

Experimental measurements and unsteady Reynolds-averaged Navier-Stokes simulations of the low-Reynolds-number flow past an SD7003 airfoil with and without plunge motion at Re = 60 k are presented, where transition takes place across laminar separation bubbles. The experimental data consist of high-resolution, phase-locked particle image velocimetry measurements in a wind tunnel and a water tunnel. The numerical simulation approach includes transition prediction which is based on linear stability analysis applied to unsteady mean-flow data. The numerical results obtained for steady onflow are validated against particle image velocimetry data and published force measurements. Good agreement is obtained for specific turbulence models. Flows with plunge motion reveal strong effects of flow unsteadiness on transition and the resulting laminar separation bubbles which are well captured in the simulations.

Numerical and Experimental Flow Analysis of Moving Airfoils with Laminar Separation Bubbles

Validation of the RANS-simulation of laminar separation bubbles on airfoils

This paper presents an investigation of low-Reynolds-number flows past an SD7003 airfoil at Re = 60K, where transition takes place across a laminar separation bubble (LSB). Results of experimental measurements and numerical calculations are analyzed and discussed. In particular, reasonably good results were obtained using three different numerical approaches: Large-eddy simulation (LES) that demonstrated vortical structures at different transition stages, and where the transition occurred naturally; unsteady Reynolds-averaged Navier-Stokes (URANS) simulations for several turbulence models based on the ω-length-scale equation, coupled to a linear stability solver to predict transition point; URANS simulations using a low-Re k-e turbulence model, where eddy viscosity was suppressed in the near-wall laminar region by damping functions.

An Investigation of Low-Reynolds-number Flows past Airfoils

This paper presents investigations of low-Reynolds-number flows past an SD7003 aerofoil at Re = 60k, where transition takes place across a laminar separation bubble (LSB). Results of experimental measurements and numerical calculations are analysed and discussed. In particular, reasonably good results were obtained using two different numerical approaches: Large-eddy simulation (LES) that demonstrated vortical structures at different transition stages, and where the transition occurred naturally; unsteady Reynolds-averaged Navier-Stokes (URANS) simulations for several turbulence models based on the ω-length-scale equation, coupled to a linear stability solver to predict the transition position.

Jan Windte

Papers

Numerical and Experimental Flow Analysis of Moving Airfoils with Laminar Separation Bubbles

Validation of the RANS-simulation of laminar separation bubbles on airfoils

An Investigation of Low-Reynolds-number Flows past Airfoils

Numerical and Experimental Flow Analysis of Moving Airfoils with Laminar Separation Bubbles

Computational and experimental investigations of low-Reynolds-number flows past an aerofoil