SUMMARYThe aerodynamic force and flow structure of a wing performing an unsteady motion at small Reynolds number (Re = 4000) is calculated by solving Navier-Stokes equations. Calculations were conducted for steady motion, simple unsteady motions suc as azimuth rotation, pure translation, feathering, etc., which can provide a basis for understanding the more complicated hovering flight. At Re = 4000, the delayed stall mechanism was noted during the azimuth rotation of a wing due to span wise flow. In azimuth rotation and pure translation, large value of C L could be achieved due to fast acceleration of the airfoil from rest. Large value of C L could be maintained during azimuth rotation for many chord length of travel in comparison with the pure translation due to spanwise flow that causes Dynamic Stall Vortex (DSV) to remain attached. For feathering motion, pitch oscillation motion was dominated by the azimuth rotation.

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.

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