fluid dynamics

Physical Principles of Respiratory CareI. States of MatterII. Change of StateIII. Gas Behavior Under Changing

ConditionsIV. Fluid Dynamics

Fluid Dynamics1. Pressure in Flowing Fluids2. Patterns of Flow

Laminar Flow Turbulent Flow Transitional Flow

3. Flow, Velocity, and Cross-Sectional Area4. Bernoulli Effect5. Fluid Entrainment6. Fluidics and the Coanda Effect

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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.

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Patterns of Flow Patterns of flow

Laminar flowfluid moving in discrete cylindrical layers or streamlinesPoiseuille’s lawpredicts pressure

required to produce given flow using ΔP = 8nl V./ πr4

Conditions that cause laminar flow to become turbulent1. High linear gas velocity2. High gas density3. Low gas viscosity4. Large tube diameter

Patterns of flow Turbulent flowloss of regular

streamlines; fluid molecules form irregular eddy currents in chaotic pattern.Predicted by using Reynold`s number

(NR)NR = v d2r / h

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Patterns of Flow Transitional Flow

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Poiseuille’s Law(only applies to laminar flow) Flow of fluid through a tube:

Driving pressure Resistance

ViscosityLength of the tubeRadius of the tube

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Poiseuille’s Law1. The more viscous the fluid the more

pressure is required to cause it to move through a given tube

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Poiseuille’s Law2. 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

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Poiseuille’s Law3. 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

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Poiseuille’s LawRespiratory Care Application:

ETT

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Poiseuille’s Law Asthma

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Pressure in Flowing Fluids

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

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Law of Continuity

If the area of flow is decreased, then the velocity must increase

If the area of flow in increased, then the velocity must decrease

2.54

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Law of Continuity

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The Bernoulli EffectAs the speed of the fluid increases, the pressure in a fluid decreases

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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

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Fluid EntrainmentRespiratory Care Application:

Air injector

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Fluid EntrainmentRespiratory Care Application:

Entrainment mask

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Fluid EntrainmentRespiratory Care Application:

Small Volume Nebulizer

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Fluid EntrainmentRespiratory Care Application:

Large volume jet nebulizer

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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.

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AdvantagesOperate without moving parts, minimizing maintenance expenses

Generally cost less than electronic counterparts

Don’t break down as often as their electronic counterparts

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DisadvantagesNot easily interfaced with microprocessors

Not as accurate as their electrical counterparts

Difficult to measure tidal volume because tidal volume exits with source gas