Delayed stall is the most dominant lift enhancing factor in insect flapping motion. Micro air vehicle operates at Reynolds number 10 4 -10 5 ; slightly higher than the insects' Reynolds number (Re). In the present research, thefocus is to investigate "stall-absent" phenomenon at Re representative of the micro air vehicles, the effect of spanwiseflow on the leading edge vortex and also to study the effect of geometry variations on the aerodynamic performance of the wing in unsteady motion. Corrugated dragonfly airfoil with rectangular wing planform is used, however, with wing kinematics restricted to azimuth rotation only. Three-dimensionalfinitevolume method is used, through commercial software Fluent, to numerically solve time-dependent incompressible Navier-Stokes equations. Computed results at Re 34000 and 100,000 reveal the same phenomenon of delayed stall, as observed in the case of insects. Furthermore, the performance of flat plate, profiled and corrugated wing in a sweeping motion at a high angle of attack is also compared.

The present study investigates the use of various wingtip devices to analyse the parameters of lift and drag for an aircraft wing. The coefficients of lift and drag are investigated in this research to optimize the wing design for enhancing the aircraft performance. A reduction in the drag produced due to wingtip vortices leads to reduced fuel consumption which contributes to the reduction in fuel emissions. The two-dimensional analysis is carried out for the selection of an apposite aerofoil by comparing the lift/drag characteristics of NACA 0012, 2415 and 23015 respectively at the velocity of 79.16 m/s at the angles of attack of 0°,4°,8°,12°,16° and 20°. The aerofoil section NACA 2415 is used to design the three-dimensional aircraft wing. For the analysis of the various wing tip devices the three-dimensional wing is incorporated with the spiroid winglet, blended winglet, wingtip fence and a mini-winglet. The CFD analysis for the wing designs is carried out for the take-off and landing phases of an aircraft’s flight because the effect of vortices is the highest during these flight phases. The angles of attack range from 0° to 20°. The CFD results reveal that for the wing designs, the plain wing produced the highest drag and the blended winglet proved to be the wingtip device with the most beneficial design. The results obtained for the 30° cant angled blended winglet and 60° cant angled wingtip fence produces additional lift when compared to the results obtained for the counterpart designs. The results obtained from the analysis are in close correlation to the established use of the wingtip devices.

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An Unsteady force generation mechanisms (delayed stall, wake capture and rotational lift) during idealized hovering of insect flight at Reynolds number (Re) of 136 have been identified in this research. Dependence of flow physics on Re forms the basis of present study to observe the dependence of unsteady force generation mechanisms on Re. A systematic study has been carried out by increasing Re from 136 to 4000 to investigate persistence of delayed stall, wake capture and rotational lift phenomenon. Using the solution of 3D Navier-Stokes equations, the aerodynamic force and the detailed flow structure around the wing are obtained which can provide useful insights into mechanism of unsteady force generation during idealized hovering at Re=4000. After grid and Mach number sensitivity analysis, the results are compared with previous studies at Re=136 for the code validation. The aerodynamic force and flow structure of a wing performing hovering motion at Re=4000 is calculated by solving Navier-Stokes equations. Re=4000 is selected on the premise that the length scale (mean aerodynamic chord) becomes closer to a Micro Air Vehicle (MAV); furthermore 30 times increase in Re (from 136 to 4000) is considered sufficient to assess changes in flow physics while remaining in laminar flow regime. Calculations are conducted for idealized hovering motion during which stroke 1 is initiated in still air, followed by flipping motion for reversing the direction and then stroke 2 (similar to stroke 1 but in opposite direction). Results obtained from this research are helpful for future work where they can be compared with those obtained from actual wing kinematics to assess the impact of kinematics on unsteady mechanisms.

The aerodynamic force and the flow structure of a wing performing hovering motion at small Reynolds number (Re=4000) is calculated by computationally solving the 3D Navier-Stokes equations. The computations are performed for the hovering motion which consists of stroke 1, followed by the flipping motion for reversing the direction and then the stroke 2 (similar to stroke 1 but in the opposite direction). The intent of the study is to research the effects of different scheduling of the flip motion between the two strokes. At Re=4000, the delayed stall mechanism is noted during the azimuth rotation of a wing with a high value of CL due to stabilized Leading Edge Vortex. The lift contribution during the flip (pitch rotation for reversing the direction) for the complete stroke is not substantial. During a stroke, the wing encountered the wake from the previous stroke in which, the wake does not contribute positively.