Fluid dynamics Flashcards
What is flow?
The movement of a gas or liquid through a tube or other system, in volume per unit time
Flow (Q) = ΔVolume / ΔTime
Flow (Q) = Velocity x cross-sectional area
What is laminar flow?
Orderly movement of a fluid that complies with a model in which parallel layers have different velocities relative to each other
Flow rate is directly proportional to the pressure driving the flow
Laminar flow is likely when Reynolds number is <2000
Hagen-Poiseuille equation
Q = (π ΔP r^4) / 8ηl
Q = flow
ΔP = pressure gradient P1-P2
r = radius of tube
η = fluid viscosity
l = length of tube
Flow is directly proportional to the pressure gradient and radius of the tube. If the radius doubles, flow rate will increase by 16x
Flow is inversely proportional to the fluid viscosity and length of the tube
What is turbulent flow?
Fluid motion that has local velocities and pressures that fluctuate randomly, resulting in disorganized flow and eddy currents
What is Reynolds number?
Reynold’s number= ρ ν d / η
ρ = fluid density
ν = fluid velocity
d = tube diameter
η = fluid viscosity
Higher number indicates higher likelihood of flow being turbulent
<2000 = likely laminar
2000 - 4000 = transitional
>4000 = likely turbulent
Therefore turbulent flow is more likely with higher density fluid, increased fluid velocity and wider tubes
Laminar flow is more likely with more viscous fluid
What is Bernoulli’s principle?
An increase in velocity of an ideal fluid undergoing laminar flow is accompanied by a simultaneous reduction in its pressure. This may be seen when introducing a constriction into the path of fluid flow
This occurs because of the law of conservation of energy - in a closed system energy cannot be created or destroyed. Therefore as velocity (kinetic energy) increases, pressure (potential energy) must decrease in order to maintain the total energy within the system
What is the Venturi effect?
According to the Bernoulli principle - when a fluid / gas passes through a constriction point the velocity will increase and the pressure decrease. The reduction in pressure can be used to entrain a second fluid into the main fluid flow
Examples:
Venturi mask
Nebulizers
Suction
What is the Coanda effect?
The tendency of a stream of fluid flowing close to a convex surface to follow the line of the surface, rather than its original course
What is Laplace’s law?
The larger the radius of a vessel, the greater the wall tension required to withstand a given internal fluid pressure
This explains why larger alveoli do not collapse, and there is a tendency of small alveoli to empty into larger ones
