Computational fluid dynamic simulation of airfoils in unsteady low Reynolds number flows
- Computational fluid dynamic simulation of airfoils in unsteady low Reynolds number flows
The inherent complexity of low Reynolds number (LRN) flows and their respective viscous vortical patterns demand an accurate solution method to achieve the desired accuracy. This complicated flow field needs even more robust methods when the flow is unsteady. The flow field of unsteady airfoils and wings in LRN regime is challenging to solve and Computational Fluid Dynamics (CFD) simulations stand out as solid solution techniques in this area. This thesis is motivated by an existing rotating-flapping mechanism, whose kinematics components can be broken into pitching, plunging and a novel figure-of-eight-like flapping motion of its blades and each blade's cross section. The focus is on two-dimensional low Reynolds number (LRN) flows using Computational Fluid Dynamics (CFD) and a Finite Volume Method (FVM). As one of the targets is to simulate a pair of blades, and consequently a pair of airfoils, a mesh motion library is developed to perform rotational and translational motions of multi-body configurations. The library and its sub-routines are tested on pairs of pitching, plunging and flapping airfoils, where the moving mesh problem is performed with a significant gain in the computational time compared to other moving mesh techniques such as Laplacian smoothing algorithm. The simulations of a single airfoil under harmonic and the novel figure-of-eight-like flapping motions, respectively, are conducted within 67% and 80% time it took to obtain a steady solution using the Laplace smoothing mesh motion algorithm, while the calculated force coefficients were in reasonably close agreement. Flow fields of single unsteady airfoils under pitching, plunging and figure-of-eight flapping motions are also simulated in this thesis accompanied with extensive parametric studies. The simulations of the considered figure-of-eight flapping pattern shows that its highly inclined asymmetrical kinematics results in higher vertical lift coefficients than the existing flapping patterns in the literature, useful for stable hovering flight. The studies over paired-airfoils arrangements under pitching and plunging and the figure-of-eight flapping motion show that the airfoil-airfoil interaction affects the fluid forces noticeably. The multi-plunging analysis, for example, reveals that the maximum lift coefficients is higher than that of a single plunging airfoil, while minimum drag coefficients is lower, showing the favorable effect of airfoil-airfoil interaction in the studied multi-plunging cases.