Concepts Flashcards
Transition
The point where laminar flow becomes turbulent
Invicid / Ideal Flow
Flow with no viscosity
Separation
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
No-Slip Condition
Where any viscous fluid adjacent to a surface adopts the surface’s velocity
Internal Stress of a fluid
This is proportional to the time rate of deformation of the fluid
(Dynamic) Viscosity
The constant of proportionality between shear stress and strain of a fluid. Often called dynamic viscosity
Skin Friction
The sum of all shear stresses - not fully equal to the drag however
Turbulence
The result of massive amplifications of disturbances in the flow in the boundary layers and wakes
Stall
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
Lagrangian/Eulerian Frame of Reference
Elements moving with the fluid / fixed control volume with fluid moving
Reynold’s Number
It is defined by ρuD/μ and is the ratio of inertial forces (ρu^2) to viscous forces (μ δu/δy ~ μ u/D)
Mach Number
Defined by M = (speed of flow)/(speed of sound). It is not very relevant for M<0.3
Kinematic Viscosity
Defined as ν=μ/ρ , it is the ratio of the flow’s dynamic viscosity and its density
Compressibility
The relative volume change due to a pressure change
Buckingham’s Rule
The no. of variables - the no. of dimensions = least no. of dimensionless groups
Geometric Similarity
When a body is a perfect scale model of another
Dynamic Similarity
When a body is geometrically similar and all relevant parameters are the same (eg. Re, M)
Vortex Shedding
The formation of alternating low-pressure vortices (oscillation) after a fluid passes a body
Fully Developed Flow
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
Impermeability
When there is no slip and no penetration of a fluid in contact with a surface
Manometer equation
ΔP = ρgΔh
Alternatively Δh= L sin θ
Dimensionless Numbers
The product and ratios of flow variables which are independent of a system of units being applied
Irrotational Flow
Flow where the vorticity is zero everywhere:
ω(z) = δv/δx - δu/δy = 0
Pressure is constant throughout the flow
Static Pressure
The pressure of a fluid associated with its state (potential energy), not its motion
Dynamic Pressure
The pressure of a fluid associated with its motion (kinetic energy), given by
P = 0.5 ρu^2
Bernoulli’s Equation
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
Equations of Conservation of Momentum in 3D
Eulerian form for x direction
δu/δt + u δu/δx + v δu/δy + w δu/δz = - (1/ρ) δp/δx
or
Du/Dt = - (1/ρ) δp/δx
Velocity Gradient
Defined as δu/δy , where δu/δy = δθ/δt = δε/δt = rate of shear strain
Occurs at a surface where no slip condition applies
Shear Stress
The product of the viscosity and the time rate of shear strain:
τ = μ δu/δy
Boundary Layer
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.
Newtonian Fluid
A fluid where the stress is proportional to the time rate of change;
σ is proportional to δε/δt
Free Stream
The stream when it is not deflected, compressed or slowed down by an object
Definition of Pressure on a Surface
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
Incompressible Flow
Flow which has a constant viscosity and density.
Flows with low speeds and constant temperature can be considered incompressible.
Pressure Coefficient, Cp
Cp = (Total pressure - static pressure)/(dynamic pressure)
Lift coefficient, Cι
Cι = (lift force)/(dynamic pressure x area)
Drag coefficient, Cd
Cd = (drag force)/(dynamic pressure x area)
Pitching Moment Coefficient, Cm
Cm = (pitching moment)/(dynamic pressure x area x chord length)
Wake
The region of flow affected by viscosity downstream of a body
Definition of conservation of mass
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.
