Chapter 1 Flashcards
(33 cards)
What are the sources of aerodynamic forces and moments?
- Pressure distribution over the body source.
2. Shear stress distribution over the body surface.
Pressure Direction
Normal to surface
Shear stress direction
Tangential to surface
How did shear stress formed?
Due to “tugging action” on the surface which is caused by FRICTION betwen the body and air.
What causes normal and axial forces on the body?
Due to the distributed loads imposed by the pressure and shear stress distributions.
If the aerodynamic force on a body is specified in terms of a resultant single force R
Where in the body should the resultant force be?
The resultant force should be cated on the body such that it produces the same effect as the distributed loads.
Location of A’ and N’
A’ and N’ must be placed at a location it such to generate the same moment about the leading edge
A’ is placed at chord line
N’ is located at a distance Xcp downstream of the leading edge.
How to determine whether aerodynamicmoments are positive or negative?
Positive if it they tend to increase the angle of attack.
Center of pressure definition.
- The location where the resultant of a distributed load effectively acts on the body.
- Point at which the aerodynamic moment is zero.
What are the criteri to ensure that two forces are dynamically similar?
From the results of the dimensional analysis
- The bodies and any other solid boundaries are geometrically similar for oth flows.
- The similrity parameters are the same for both flows -> Re and M.
Hence, lift, drag and moment coefficients will be the same
Key point in the validity of windtunnel testing
The measured lift, drag and moment coefficients will be the same for free flight as long as MACH and REYNOLDS numbers for the wind tunnel are the same as for free-flight case.
Common difficulty in wind tunnel testing
- Difficulty of simulating both Mach numbers and Reynolds number simultaneously in the same tunnel.
- Wind tunnel unable to pressuurized far above atmospheric pressure due to the large extra financial cost.
Vehicles that depend on buoyancy force
- Naval vehicles.
- Lighter than air vehicles such as blimps and hot air balloons. They obtain sufficient buoyancy force by by displacing huge volumes of air.
Mean-free path
The mean distance that a molecule travels between collisions with neighbouring molecules.
Continuum flow
The molecules impact the body surface so frequently that the body cannot distinguish the individual molecules conditions and the surface feels the fluid as continuous motion.
Free molecular flow
The gas molecules are spaced so far apart, that collisions become infrequent and the body can feel distinctly each molecular impact.
Eg : Space shuttle at outer edge of atmosphere.
Low-density flows
Characteristics of both continuum + free molecule flows
Inviscid flow
A flow that assumed to have no friction. Do not truly exist in nature.
Some flows that are dominated by viscous effects
High angle of attack, boundary layer will tend to separate from the top surface and a large wake is formed downstream.
Incompressible flow
A flow in which density is constant. Truly incompressible does not occur in nature.
M < 0.3
Compressible flow
A flow where the density is variable is called compressible.
Subsonic flow
- M < 1
- Have smooth streamlines
- M < 0.8 for slender bodies
- For blunt bodies even lower Mach number.
For the vast region of the flow field away from the body
The velocity gradient are small, and friction no role.
For the thin region of the flow
The velocity gradient is large and friction plays a defining role.
