Chapter 9

Question 1

Drag

Answer 1

The resultant force in the direction of the upstream velocity.

Question 2

Lift

Answer 2

The resultant force normal to the upstream velocity.

Question 3

Lift Coefficient

Answer 3

C_L=L/((1/2)ρU2A)

Question 4

Drag Coefficient

Answer 4

C_D=D/((1/2)ρU2A)

Question 5

Wake Region

Answer 5

Region of recirculating flow immediately behind a moving or stationary solid body, caused by the flow of surrounding fluid around the body.

Question 6

Region of disturbed flow (usually turbulent) downstream of a solid body moving through a fluid, caused by the flow of the fluid around the body.

Answer 6

Wake Region

Question 7

Boundary Layer

Answer 7

The layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant.

Question 8

Laminar Boundary Layer

Answer 8

A laminar boundary layer is one where the flow takes place in layers, i.e., each layer slides past the adjacent layers.

Question 9

Turbulent Boundary Layer

Answer 9

There is an intense agitation. A turbulent boundary layer is marked by mixing across several layers of it.

Question 10

boundary layer thickness

Answer 10

The boundary layer thickness, δ, is the distance across a boundary layer from the wall to a point where the flow velocity has essentially reached the ‘free stream’ velocity.

Question 11

transition

Answer 11

is where the laminar B.L. is going to a turbulent B.L. and occurs at a critical value of the Reynolds number, Re_xcr on the order of 2x10⁵ to 3x10⁶ which is dependant on the roughness of the surface and the amount of turbulence in the upstream flow.

Question 12

free-stream velocity

Answer 12

The air/fluid far upstream of an aerodynamic body, that is, before the body has a chance to deflect, slow down or compress the air.

Question 13

favorable pressure gradient

Answer 13

A decrease of pressure in the direction of flow.

Question 14

adverse pressure gradient

Answer 14

Occurs when the static pressure increases in the direction of the flow.

Question 15

boundary layer seperation

Answer 15

Flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer relative to the object falls almost to zero. The fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices.

Question 16

friction drag

Answer 16

Caused by the friction of a fluid against the surface of an object that is moving through it. It is directly proportional to the area of the surface in contact with the fluid and increases with the square of the velocity.

Question 17

pressure drag

Answer 17

The dominant component in the case of vehicles with regions of separated flow, in which the pressure recovery is fairly ineffective. The friction drag force, which is a tangential force on the aircraft surface, depends substantially on boundary layer configuration and viscosity.

Question 18

stall

Answer 18

A reduction in the lift coefficient generated by a foil as angle of attackincreases. This occurs when the critical angle of attack of the foil is exceeded.

Question 19

circulation

Answer 19

Is the line integral around a closed curve of the velocity field.

Question 20

Magnus Effect

Answer 20

A particular manifestation of Bernoulli’s theorem: fluid pressure decreases at points where the speed of the fluid increases. In the case of a ball spinning through the air, the turning ball drags some of the air around with it.