Effect of wind gust on aerodynamic characteristics of an airfoil at low reynolds number flow

Abstract

In the present study, a numerical computation of aerodynamic characteristics of Eppler 61 airfoil is studied. Eppler 61 is a thin airfoil of thickness 5.7% of the chord (c) at the location of 23.8%c from the leading edge. The camber of the airfoil is 6.4%c at 51%c. This airfoil can produce reasonable amount of lift at low Reynolds number flow (30,000 ≤ Rec ≤ 70,000). As a result, it is recommended for the design of wing of Micro Air Vehicle (MAV). Success of MAV’s application depends on its ability to operate efficiently at all weather conditions; particularly in a gusty environment. Since the size, mass and speed of a typical MAV are very small, its aerodynamics becomes vulnerable to wind gusts. It may losses both directional and attitude control due to rapid changes in lift on the wings as the effect of wind gust. As such, detail study on the effect of wind gust on aerodynamic characteristic of an airfoil is a paramount for MAV design. In the present study, the flow field around the airfoil is simulated as laminar, unsteady, two dimensional and incompressible. The governing equations for the numerical solution of the present problem are Navier-Stokes (NS) equations. The results obtained from the present numerical computation are validated with the available experimental data at the same flow conditions of Re = 46,000. It was found that the flow is inherently unsteady with frequency of 166.6 Hz at 0° angle of attack. This frequency is considered as base frequency, fb in the present study.

After validation, computational study has been carried out both in steady as well as unsteady (gust) inflow conditions. For steady flow, inlet velocity of 4.5 m/s is considered with four different angles of attack (α = 0°, 2°, 4° and 8°). For gust inflow condition, velocity varied from 4.5 m/s to 4.95 m/s and gust frequency of 166.6 Hz (fex = fb) is imposed. Later on, additional two external frequency such as fex = 0.5fb and fex = 2fb have been imposed in airfoil with two different angles of attack (α = 0° and 8°) to investigate the changes of aerodynamic characteristics of Eppler 61 airfoil. Finally, a comparison is made between the results of ‘steady inflow’ and ‘gust inflow’ condition. From the study of lift curve, it is found that airfoil aerodynamic characteristics have been changed with the presence of gust inflow. In gust condition, the value of lift coefficient, Cl is increased than that of ‘No Gust’ condition at all angles of attack studies in the present study. The lift slope is found to be increased in case of gust condition. Moreover, from the study of drag characteristics, it is found that the magnitude of drag coefficient, Cd is found to be increased than that of ‘No Gust’ condition except for two gust frequencies of 83.3 Hz and 166.6 Hz at 8° angle of attack. The increment of Cd with angle of attack is observed to be increased in gust condition. In addition to that, the fluctuating lift and drag characteristics are found non-uniform and irregular at higher gust frequencies. The presence of vortex shedding is also observed on the upper surface of airfoil at an angle of attack, α = 8° for both steady and gust inflow conditions. Maximum wake pressure fluctuation is observed at an angle of attack, α = 4° for steady flow condition. Fluctuation of Cl increases at a higher rate with the increase of gust frequency; as a result, at higher frequency of gust, MAVs might have to face serious vertical vibration due to fluctuation of lift. On the other hand, with the increase of gust frequency, the magnitude of fluctuation of Cd increases with a higher rate; as a result, at higher frequency of gust, MAVs might have to face serious longitudinal vibration due to fluctuation of drag.