Unsteady forces and flows in low Reynolds number hovering flight: two-dimensional computations vs robotic wing...

Area of Science:

• Fluid dynamics
• Aerodynamics
• Robotics

Background:

• Hovering flight in insects and robotic devices relies on complex wing kinematics.
• Understanding unsteady aerodynamic forces is crucial for efficient flapping wing design.

Purpose of the Study:

• To compare computational, experimental, and quasi-steady forces on a generic hovering wing.
• To investigate unsteady effects by comparing 2D computations and 3D experiments.
• To analyze kinematic patterns and their influence on aerodynamic forces.

Main Methods:

• Sinusoidal motion of a wing along a horizontal stroke plane.
• Comparison of two-dimensional (2D) computations with three-dimensional (3D) experiments.
• Analysis of forces (lift and drag) across different kinematic patterns, amplitudes, and phases.

Main Results:

• Computed drag closely matches experimental results.
• Computed lift agrees with experiments for symmetrical or advanced angle of attack changes, but lags in delayed cases.
• Force coefficients show weak dependence on stroke amplitude (3-5 chords) but are sensitive to phase, suggesting maneuver strategies.
• Unsteady forces rapidly reach a periodic state; fluid forces are dominated by pressure.
• 2D unsteady forces approximate 3D forces well before flow separation, especially in symmetrical and advanced rotation cases.

Conclusions:

• Two-dimensional (2D) models can effectively approximate three-dimensional (3D) unsteady aerodynamic forces in hovering wings under specific conditions.
• The phase between wing stroke and angle of attack is critical for force generation and suggests simple maneuver strategies for flapping wing devices.
• Understanding the conditions for flow separation is key to determining the validity of 2D approximations for 3D flapping wing dynamics.