1. Physical Principles of Respiratory Care
States of Matter
Change of State
Gas Behavior Under Changing Conditions
Fluid Dynamics

2. Fluid Dynamics Pressure in Flowing Fluids Patterns of Flow
Laminar Flow
Turbulent Flow
Transitional Flow
Flow, Velocity, and Cross-Sectional Area
Bernoulli Effect
Fluid Entrainment
Fluidics and the Coanda Effect

3. Fluid Dynamics The study of fluids in motion is called hydrodynamics.
The pressure exerted by a liquid in motion depends on the nature of the flow itself.
A progressive decrease in fluid pressure occurs as the fluid flows through a tube due to resistance.

4. Patterns of Flow Patterns of flow
Laminar flowfluid moving in discrete cylindrical layers or streamlines
Poiseuille’s lawpredicts pressure required to produce given flow using
ΔP = 8nl V./ πr4

5. Conditions that cause laminar flow to become turbulent
High linear gas velocity
High gas density
Low gas viscosity
Large tube diameter

6. Patterns of flow
Turbulent flowloss of regular streamlines; fluid molecules form irregular eddy currents in chaotic pattern.
Predicted by using Reynold`s number (NR)
NR = v d2r / h

7. Patterns of Flow Transitional Flow
Transitional Flow: A mixture of laminar and turbulent flow
Occurs at points where tubes divide into one or more branches
Where airways divide

8. Poiseuille’s Law (only applies to laminar flow)
Flow of fluid through a tube:
Driving pressure
Resistance
Viscosity
Length of the tube
Radius of the tube

9. Poiseuille’s Law
The more viscous the fluid the more pressure is required to cause it to move through a given tube

10. Poiseuille’s Law
Resistance to flow is directly proportional to the length of the tube
If the length of a tube is increased four times, the driving pressure to maintain a given flow must be increased four times

11. Poiseuille’s Law
Resistance to flow is inversely proportional to the fourth power of the radius of the tube
If the inside diameter of the tube is decreased by one half, the driving pressure must be increased 16 times to maintain original flow

12. Respiratory Care Application: ETT
Poiseuille’s Law
Respiratory Care Application:
ETT

13. Poiseuille’s Law
Asthma

14. Pressure in Flowing Fluids

15. Law of Continuity
The speed of flow in a closed system will be inversely proportional to the area of the tubes through which it flows

16. Law of Continuity
2.54
If the area of flow is decreased, then the velocity must increase
If the area of flow in increased, then the velocity must decrease
This fact is based upon the observation that the total fluid flowing through the system remains constant
For example:
if 5 L/min flows past point A, 5 L/min must flow past point B, and point C
Since the area at point B is one-half that of point A, the fluid must flow twice as fast. If the area of flow is decreased, then the velocity must increase.
At point C, the total area is five times as large as at point A, so the average speed is slower by a factor of 5. If the area of flow is increased, then the speed must decrease
A garden hose is a good example of this phenomenon

17. Law of Continuity

18. Review
We already know that when a fluid flows through a tube of uniform diameter, there will be a gradual drop in pressure
We also know that as the area of flow is decreased, then the speed must increase (Law of Continuity)

19. As the speed of the fluid increases, the pressure in a fluid decreases
The Bernoulli Effect
As the speed of the fluid increases, the pressure in a fluid decreases
The first three columns in this figure demonstrate this gradual pressure drop. As the pressure drops, the height of the liquid in the column will decrease
Notice that as the fluid flows through a constriction in the tube, the pressure of the fourth tube shows an even greater drop in pressure.
If it is assumed that the total flow of liquid is the same before and after the constriction, then the flow of liquid must accelerate as it enters the constriction
According to the Law of Continuity, flow must increase in order to transport the same volume of fluid in a given time
Thus it is reasonable to assume that the drop in fluid pressure is directly related to the increase in fluid speed.
When the fluid passes through a constriction, the pressure drop is much greater. As can be observed in the fourth column.
This drop in fluid pressure is directly related to the increase in fluid speed.
The strict proportions between fluid speed and area is true only for incompressible fluids in a closed system, but the qualitative features are similar for flow in open systems, and also for gas flow.

20. Review
We already know from the Law of Continuity that when a flowing fluid encounters a narrow passage the velocity increases.
We also know from Bernoulli’s Principle that this increase in velocity causes a decrease in pressure

21. Venturi Principle of Fluid Entrainment
If the increase in velocity at a constriction is so great that is causes the pressure of the fluid to fall below atmospheric (becoming negative) it can pull another fluid into the primary flow
If we place an open tube distal to such a constriction,
this negative pressure can actually pull another fluid into the primary flow system,
this effect is called fluid entrainment

22. Respiratory Care Application: Air injector
Fluid Entrainment
Respiratory Care Application:
Air injector
A, The basic design.
B, Greater entrainment and total flow occurs with larger entrainment ports.
C, Alternatively, a smaller jet increases source gas velocity and entrains more air

23. Respiratory Care Application: Entrainment mask
Fluid Entrainment
Respiratory Care Application:
Entrainment mask

24. Respiratory Care Application: Small Volume Nebulizer
Fluid Entrainment
Respiratory Care Application:
Small Volume Nebulizer
This is a typical small volume nebulizer which is
powered by a high-pressure stream of gas directed through a restricted orifice (or jet).
The gas stream leaving the jet passes by the opening of a capillary tube immersed in solution.
Because the high jet velocity produces low lateral pressure at its outlet, it draws the liquid up the capillary tube and into the gas stream, where it is sheared apart into droplets.
These droplets are directed toward a baffle (a baffle is a surface upon which large particles impact and fall out of suspension) so that only small particles are sent to the patient’s airways allowing for better deposition

25. Respiratory Care Application: Large volume jet nebulizer
Fluid Entrainment
Respiratory Care Application:
Large volume jet nebulizer

26. Fluid Dynamics Fluidics and the Coanda effect
Fluidics is a branch of engineering that applies hydrodynamics principles in flow circuits.
The Coanda effect (wall attachment) is observed when fluid flows through a small orifice with properly contoured downstream surfaces.

29. Operate without moving parts, minimizing maintenance expenses
Advantages
Operate without moving parts, minimizing maintenance expenses
Generally cost less than electronic counterparts
Don’t break down as often as their electronic counterparts

30. Not easily interfaced with microprocessors
Disadvantages
Not easily interfaced with microprocessors
Not as accurate as their electrical counterparts
Difficult to measure tidal volume because tidal volume exits with source gas