Physics Flashcards
What is pressure?
Force applied per unit area
What is flow?
The quantity of a substance passing a point per unit time
What factors affect how a fluid flows?
Viscosity - The property of a fluid that causes it to resist flow (Pascal seconds, η)
- Newtonian fluid = constant viscosity (e.g. air, water)
- Non-Newtonian fluid = variable viscosity (e.g. blood)
Density - The mass of a substance per unit volume
Pressure gradient - ΔP or P1-P2
Tube radius
Tube length
What is laminar flow?
What are the most importance factors affecting laminar flow?
Streamlined flow in smooth layers passing parallel to the walls of the vessel
Overall most important factor - tube radius
Fluid factor - viscosity
What equation can be used to describe laminar flow?
The 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
Flow is inversely proportional to the fluid viscosity and length of the tube
What is turbulent flow?
Flow in which the velocity at a given point varies erratically in magnitude and direction
In turbulent flow, flow is directly proportional to the square root of pressure and inversely proportional to the square root of density (ρ).
Density is the most important fluid factor for turbulent flow
What equation gives an indication of whether flow is turbulent?
Reynold’s number= ρ ν d / η
ρ = fluid density
ν = fluid velocity
d = tube diameter
η = fluid viscosity
Higher number (>2000) indicates higher likelihood of flow being 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 number indicates whether flow is more likely to be laminar or turbulent?
Reynold’s number
<2000 = laminar flow more likely
>2000 = turbulent flow more likely
What equation defines resistance?
How does it differ for laminar and turbulent flow?
R = ΔP / ΔQ
ΔP = pressure gradient
ΔQ = flow
Laminar flow - as per the Hagen-pouiselle equation, an increase in pressure gradient produces a proportional rise in flow. Therefore the relationship is linear and resistance will be equal to the gradient of the line
Turbulent flow - Eddy currents within turbulent flow resist forward flow. Therefore a greater change in pressure is required to produce the same change in flow. There is an exponential relationship between the two
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) increase, 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
In what ways can gas flow be measured?
Rotameters - Variable orifice, constant pressure - flow rate accurate to within 2%
Vitalograph - Expired breath acts on bellows
Bell spirometer - Expiration onto inverted bell immersed in water which rises and falls according to gas volume
Pneumotachographs - Constant orifice, variable pressure
Vane meters - e.g. Wright’s spirometer. Gas flow moves a turbine linked to a dial
Mechanical flow transducers - A small disc in the gas flow is bent backwards against a strain gauge, the force is dependent on gas flow
Hot-wire anemometers - Gas flows over heated wires and the degree of cooling is related to gas flow
Ultrasonic flow meters - When gas flows in the same direction as an ultrasound signal (transmitted within gas flow) an increase in signal velocity occurs; the reverse is also true
In what ways can flow of a liquid be measured?
Syringe drivers
Peristaltic pumps
IV drip counters
What values of pressure measurements are equivalent to 1 atmosphere?
1.013 bar
101.3 kPa
14.69 PSI
760 mmHg
1020 cmH2O
101325 Pascal
