Flapping wing is one of the most widespread propulsion methods
found in nature. However, the current understanding of the
bird aerodynamics is incomplete. The role of unsteady motion
in the flow and its contribution to the aerodynamics is still an
open question. The current study deals with the estimation of unsteady
aerodynamic forces on freely flying birds through analysis
of wingbeat kinematics and near wake flow measurements using
long duration time-resolved particle image velocimetry. Two bird
species have been investigated, the starling and sandpiper. Using
long-time sampling data, several wingbeat cycles have been analyzed
in order to cover both the downstroke and upstroke phases
of flight. Lift and drag were obtained using the momentum equation
for viscous flows and were found to share a highly unsteady
behavior. The two birds show similar behavior during the downstroke
phase of flight, whereas the sandpiper was shown to have
a district signature during its upstroke phase. The contribution
of the circulatory lift component is shown to be significant when
estimating lift (or power) of birds in flapping flight. Moreover,
the unsteady drag term was found to have a crucial role in the
balance of drag (or thrust), particularly during transition phases.
These findings may shed light on the flight efficiency of birds by
providing a partial answer to how they minimize drag and maximize
lift during flapping flight.
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.