Concepts

Question 1

Transition

Answer 1

The point where laminar flow becomes turbulent

Question 2

Invicid / Ideal Flow

Answer 2

Flow with no viscosity

Question 3

Separation

Answer 3

Where the air flow leaves the surface (of the aerofoil) because the effects of viscosity slow down the fluid so much that it cannot follow the surface

Question 4

No-Slip Condition

Answer 4

Where any viscous fluid adjacent to a surface adopts the surface’s velocity

Question 5

Internal Stress of a fluid

Answer 5

This is proportional to the time rate of deformation of the fluid

Question 6

(Dynamic) Viscosity

Answer 6

The constant of proportionality between shear stress and strain of a fluid. Often called dynamic viscosity

Question 7

Skin Friction

Answer 7

The sum of all shear stresses - not fully equal to the drag however

Question 8

Turbulence

Answer 8

The result of massive amplifications of disturbances in the flow in the boundary layers and wakes

Question 9

Stall

Answer 9

The point when the flow cannot remain attached to the surface, resulting in a large increase in pressure drag and loss of lift.

Trailing edge stall and leading edge stall are two types that can occur

Question 10

Lagrangian/Eulerian Frame of Reference

Answer 10

Elements moving with the fluid / fixed control volume with fluid moving

Question 11

Reynold’s Number

Answer 11

It is defined by ρuD/μ and is the ratio of inertial forces (ρu^2) to viscous forces (μ δu/δy ~ μ u/D)

Question 12

Mach Number

Answer 12

Defined by M = (speed of flow)/(speed of sound). It is not very relevant for M<0.3

Question 13

Kinematic Viscosity

Answer 13

Defined as ν=μ/ρ , it is the ratio of the flow’s dynamic viscosity and its density

Question 14

Compressibility

Answer 14

The relative volume change due to a pressure change

Question 15

Buckingham’s Rule

Answer 15

The no. of variables - the no. of dimensions = least no. of dimensionless groups

Question 16

Geometric Similarity

Answer 16

When a body is a perfect scale model of another

Question 17

Dynamic Similarity

Answer 17

When a body is geometrically similar and all relevant parameters are the same (eg. Re, M)

Question 18

Vortex Shedding

Answer 18

The formation of alternating low-pressure vortices (oscillation) after a fluid passes a body

Question 19

Fully Developed Flow

Answer 19

When there is no change of momentum of fluid in the direction of flow (δu/δx=0), but a momentum change perpendicular to the flow direction

Question 20

Impermeability

Answer 20

When there is no slip and no penetration of a fluid in contact with a surface

Question 21

Manometer equation

Answer 21

ΔP = ρgΔh

Alternatively Δh= L sin θ

Question 22

Dimensionless Numbers

Answer 22

The product and ratios of flow variables which are independent of a system of units being applied

Question 23

Irrotational Flow

Answer 23

Flow where the vorticity is zero everywhere:
ω(z) = δv/δx - δu/δy = 0
Pressure is constant throughout the flow

Question 24

Static Pressure

Answer 24

The pressure of a fluid associated with its state (potential energy), not its motion

Question 25

Dynamic Pressure

Answer 25

The pressure of a fluid associated with its motion (kinetic energy), given by
P = 0.5 ρu^2

Question 26

Bernoulli’s Equation

Answer 26

The sum of the pressure energy (total pressure), kinetic energy per unit volume (dynamic pressure) and potential energy per unit volume (static pressure) is constant throughout the fluid for incompressible flows

Question 27

Equations of Conservation of Momentum in 3D

Eulerian form for x direction

Answer 27

δu/δt + u δu/δx + v δu/δy + w δu/δz = - (1/ρ) δp/δx

or

Du/Dt = - (1/ρ) δp/δx

Question 28

Velocity Gradient

Answer 28

Defined as δu/δy , where δu/δy = δθ/δt = δε/δt = rate of shear strain
Occurs at a surface where no slip condition applies

Question 29

Shear Stress

Answer 29

The product of the viscosity and the time rate of shear strain:
τ = μ δu/δy

Question 30

Boundary Layer

Answer 30

The region in which viscosity slows the flow. It is the zone over which the average fluid velocity decreases from a free stream value to a value at the surface.

Question 31

Newtonian Fluid

Answer 31

A fluid where the stress is proportional to the time rate of change;
σ is proportional to δε/δt

Question 32

Free Stream

Answer 32

The stream when it is not deflected, compressed or slowed down by an object

Question 33

Definition of Pressure on a Surface

Answer 33

It is the normal force per unit area exerted on a surface due to the time rate of change of momentum of the gas molecules impacting on (or crossing) the surface

Question 34

Incompressible Flow

Answer 34

Flow which has a constant viscosity and density.

Flows with low speeds and constant temperature can be considered incompressible.

Question 35

Pressure Coefficient, Cp

Answer 35

Cp = (Total pressure - static pressure)/(dynamic pressure)

Question 36

Lift coefficient, Cι

Answer 36

Cι = (lift force)/(dynamic pressure x area)

Question 37

Drag coefficient, Cd

Answer 37

Cd = (drag force)/(dynamic pressure x area)

Question 38

Pitching Moment Coefficient, Cm

Answer 38

Cm = (pitching moment)/(dynamic pressure x area x chord length)

Question 39

Wake

Answer 39

The region of flow affected by viscosity downstream of a body

Question 40

Definition of conservation of mass

Answer 40

The rate of change in time of mass within a control volume plus the net flux of mass out out of the control volume is zero.