AERO40001 - Aerodynamics Flashcards
What are fluid properties
Density and viscosity
What are fluid variables
Pressure and velocity.
Main difference between a solid and a fluid.
In a solid, the internal stress is proportional to the strain.
In a liquid, the internal stress if proportional to the time rate of deformation.
Define the viscosity of a fluid
The ratio of internal stress to the rate of deformation.
How does viscosity change with temperature?
For a liquid, mu increases as temperature decreases.
For a gas, mu decreases as temperature decreases.
What is the boundary layer
A region in which the viscosity slows the flow.
1 knot in km/hr
1.853
1 knot in m/s
0.515 m/s
What is a chord line on an airfoil?
The straight line that connects the leading edge and the trailing edge.
Define the mean line (or the mean camber line)
A line which bisects the upper and lower airfoil.
If you have camber on an airfoil, then you can generate lift even at low incidences.
It increases the CL max.
Define inviscid flow (or ideal flow)
In the region outside the boundary layer and the wake, the rates of deformation or the rates of strain are negligible. Viscous stresses here are negligible.
The flow appears to have no viscosity here.
Define turbulence
Result of massive amplification of disturbances in the flow in boundary layers and wakes.
Define separation
The effects of viscosity slows down the flow so much near a surface that it cannot follow the surface.
What happens when stalling occurs?
A large increase in the pressure drag and a loss of lift.
Why might 2 bodies be geometrically similar?
If one is a perfect scale model of the other.
Flows and other relevant similarity parameters are the same.
- Streamline patterns will be same.
- Distribution of pressure when expressed as Cp vs. x/c will be same.
- CL, CD and CM will be same.
- The boundary layer thickness as a fraction of c will be same at the corresponding points x/c along the wing surface.
What is the principle of conservation of mass
The rate of change (in time) of mass within a control volume plus the net flux of mass out of the control volume is zero.
What happens at the stagnation point?
U = 0. Cp = 1.
What is the no-slip condition
It states that the tangential velocity at a surface has the same velocity as the surface. If the surface is stationary, then the tangential velocity will be 0.
What physical property holds between two streamlines?
Note: A streamline within a flow field is always tangential to the velocity vector.
Between two streamlines, the mass or volume flow must be constant as no flow can cross a streamline.
Explain why an aerofoil in inviscid flow produces no lift.
In inviscid flow, there is an infinite suction at the trailing edge. Therefore, there is no pressure distribution such that net forces horizontally and vertically are 0. Therefore, producing no lift or drag.
Explain the Kutta Condition. How does it give rise to lift?
It states that the flow must leave the trailing edge smoothly. Therefore, the singularity, or the infinite suction at the trailing edge can’t exist. So, there must be uniform flow.
Therefore, there must be circulation around an aerofoil. Removal of the singularity means that lift can be generated.
NOTE ABOUT SHEAR STRESS IN A CYLINDRICAL PIPE
IT ACTS AT 2PIR.
AND ITS NEGATIVE - ITS ALWAYS GOING AGAINST THE FLOW!!!
