FLuid mechanics Flashcards
What is pressure
force/unit area
What is a fluid?
A fluid is a substance that continually deforms (or flows) under an applied shear stress.
‘Fluid’ can be used to describe different phases of matter that behave similarly under flow conditions:
Gases
Liquids
Plasma
Even some plastic solids
What is the fluid rpessure?
THe pressure at some point in a fluid
What is flow
Flow is the quantity of fluid (gas, vapour or liquid) passing a point per unit time. The units typically used in medicine are L/min.
Flow is represented by the equation: Flow = Quantity/Time
what is the most important fluid factor in laminar flow?
viscosity
What is the most important fluid component of turbulent flow?
Density
What is the significance of reynolds number
Re <2000 flow is likely to be laminar
Re >2000 flow is likely to be turbulent
resistance equation
delta P/delta Q
What devices use the venturi effect
Venturi oxygen masks - driving oxygen entrains room air, which dilutes the 100% oxygen driving gas to a value calibrated by the entrainment ratio of the nozzle aperture, Fig 1
Nebulizers - gas entrains liquid as droplets small enough to be inhaled
Sanders injector for ventilation
Suction
What is a vitalograph
Single expired breth acts on bellows. marks paper moving at a constant speed
What is a bell spirometer
Gas expired into a bell in water- rises and falls depending on gas volume
What is a rotameter?
VAriable orifice and constant pressure- accurate to within 2%
Pneumotachograph
Variable pressure and constant orifice. Can use different heads that cause a pressure drop across a transducer
Vane meter?
eg wrights spirometer- flow moves a turbine that is linked to a dial
MEchanical flow transducers?
small disc in the flow is bent against a strain gauge- force depends on flow
Hot wire anemometers?
gas flows over heated wires, degree of cooling is realted to gas flow
Ultrasonic flow meters
gas flows in the same direction, an increase in signal velocity occurs
how do rotameters work?
valve opening causes mroe flow which acts on a bobbin. te bobbin rises to a position where the force of the gas upwards is equal to the gravity pulling down
how do rotameters work at low flow?
Narrow end of tube, flow is laminar
HOw does a rotameter behave at high flow
bobbin rises higher, obstruction is greater diameter and reduced length- turbulent flow
WHy are rotameters gas specific
Because gases have different densities and viscosities, which in turn affect flow under different conditions.
What might affect rotameter accuracy?
Warmer gas is less dense and viscous, may cause overreading
