Lecture 7 - Forward flight of birds and insects Flashcards
What is camber?
A measure of the curvature of the aerofoil.
Describe aerofoil stall.
Due to viscous effects, a boundary layer forms on the aerofoil
surface
As the stream wise adverse pressure gradient increases, the
boundary layer reduces its momentum
The boundary layer separates beyond certain adverse pressure
gradient
After the boundary layer fully separates from the upper surface,
the lift drops and the drag increases. The aerofoil is said to be
stalled.
What is the alula?
A small projection on the anterior edge of the wing of modern birds, to prevent stalling.
What “devices” do birds use to achieve lift?
- The “alula” a leading edge slot
- The drooping leading edge
clump - The swept forward position of
the tail - The layered wing feathers as a
multi-surface aerofoil - The upward deflected flight
feathers of the wing tips
Pressure coefficient equation
C_p0 = 1 - (V/V_inf)^2
If C_p is positive?
p>p_inf and V<V_inf
If C_p is zero?
p=p_inf and V=V_inf
if C_p is negative?
p is less than p_inf and V is greater than V_inf
What is the d’Alambert paradox?
where drag = 0.
This would not be possible if viscous effects were added.
These effects generate frictional shear stress at the body surface and cause the flow to separate from the surface on the back of the body creating a wake downstream of the body (destroying the symmetry about the vertical axis).
What is creeping/stokes flow?
At very low Re the flow is smooth and unseparated and behaves as if
it were purely viscous with negligible inertia.
The streamlines are symmetrical.
The streamline pattern is similar to that for potential flow.
As Re increases the flow remains attached.
What is Kutta-Joukowski Theorem?
An isolated two-dimensional aerofoil or closed cylinder of arbitrary cross-section in an incompressible inviscid flow generates a lift that is directly related to the circulation about the object.
lift per unit span on an aerofoil?
l = p_inf * V_inf * Γ
