MODIFIED UNSTEADY VORTEX LATTICE METHOD FOR AERODYNAMICS OF FLAPPING WING MODELS

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Motivated by extensive possible applications of flapping-wing micro-air vehicles (MAVs) to various different areas, there has been an increasing amount of research related to this issue. In the stage of preliminary studies, one of the most important tasks is to predict the aerodynamic forces generated by the flapping motion. Studying aerodynamics of insects is an efficient way to approach the preliminary design of flapping-wing MAVs. In this paper, a modified version of an Unsteady Vortex Lattice Method (UVLM) is developed to compute aerodynamic forces appearing in flapping-wing models. A hawkmoth-like wing with kinematics based on the real motion is used for the simulations in this paper. So far, there have been several methods to estimate aerodynamic loads of a flapping-wing model in general and an insect in specific. These methods themselves always have advantages and limitations. For example, the simplest approach is based on quasi-steady theories [1] and works with each single chordwise wing section separately. This kind of method is low-cost and normally used for dynamic and control analyses, where many flapping stroke cycles are required with a limited amount of simulation time. Computation fluid dynamics (CFD), which can express even small details inside the flowfield [2], is another way to study the aerodynamics of insect flapping wings. However, this class of methods normally requires extremely large computer resources, and the high computation time is also a deterrent to their applications to several tasks. The UVLM presented in this paper is regarded a method of medium cost and fidelity. Another version of UVLM was developed by Roccia in 2013 with an application to hovering insects [3]. However, it was unable to work with the aerodynamics of several strokes due to strong wing-wake interaction. In this paper, the Squire's vortex-core growth model, which modifies Lamb-Oseen vortices by including an eddy viscosity for the turbulence [4], is implemented to overcome that troublesome problem. Moreover, the effects due to spiral leading-edge vortex structure are included by the application of the leading-edge suction analogy in the same manner as a deltawing [5]. In the current UVLM, the wing is divided into doublet panels that are equivalent to a vortex ring system [6]. Collocation points are distributed at the center of each panel, where the non-penetration boundary condition is satisfied. The Bio-Savart law is utilized for the computation of velocity field in the entire domain. The wake sheets are shed from the trailing edge and move freely in the space. The computer program for the UVLM is written in FORTRAN with the pre- and post-process interface created in MATLAB and able to work with grid-files imported from commercial drawing software. In order to ascertain the validity of the current method, several simulations have been conducted with a hawkmoth-like wing model by using the UVLM. The same wing model as that in [6] undergoes simplified wing kinematics with the similar Reynolds number to the flight of a real hawkmoth Maduca sexta. The wing has only 2 degrees of freedom composed of translational (stroke, φ) and rotational (angle of attack, a) angles. Definitions of these angles are depicted in [6]. 5 cases with respect to different values of the non-dimensional decal-acceleration times time t/TR,φ and rotational time t/TR,α of the wing kinematics are considered in this paper. More details about this are explained in [6]. The aerodynamic mesh is shown in figure 1 with the red line showing the location where the leading-edge suction analogy is applied and the wake sheets are shed from the blue line.
Publisher
KSME
Issue Date
2015-07-26
Language
English
Citation

ASME-JSME-KSME Joint Fluids Engineering Conference 2015

DOI
10.1115/AJKFluids2015-04424
URI
http://hdl.handle.net/10203/217243
Appears in Collection
AE-Conference Papers(학술회의논문)
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