Fluid Flow Flashcards
Define Flow and give the equation
Flow is the quantity of a fluid (gas or liquid) passing a point per unit time
F = Q/t
What is laminar flow and how does it differ from turbulent flow
Laminar flow occurs when a fluid moves in a steady manner without eddies or turbulence. This type of flow is usually seen in smooth tubes and at low rates of flow.
- flow directly proportional to pressure difference
- P/Q = R (V = IR)
- Resistance is independent of the flow
Turbulent flow, fluid no longer flows in a smooth fashion but swirls in eddies and the resistance is higher than for the same laminar flow.
- flow no longer directly proportional to the pressure
- resistance is not independent of the flow
Draw a graph to demonstrate the relationship between Pressure, Flow and Resistance in laminar flow.
Page 12 kenny and davis
X - axis: Q = Flow
Y - axis: P = Pressure
Gradient: Resistance
P/Q = R
Define and describe the Hagen-Poiseuille equation
The Hagen-Poiseuille equation describes the relationship between the flow, change in pressure and viscosity of a fluid flowing in a smooth pipe of a specific radius and length undergoing laminar flow.
Q = π (P1 - P2) r^4
_________
8 η l
Q: Flow r: Radius P1 - P2: Pressure gradient η: Viscosity l: length
How can the onset of turbulent flow be predicted?
Using Reynolds number:
Reynold's number = v ρ d
\_\_\_\_
η
v: velocity
ρ: density
d: diameter
η: viscosity
R < 2000 –> laminar flow
R 2000 - 4000 –> laminar or turbulent flow
R > 4000 –> turbulent flow
What is the formula for density and what are its SI units
density = mass / volume
ρ = m / V
Units: kg/m^3
In turbulent flow, what is the relationship between pressure and 1) Flow, 2) length and 3) density.
Draw the graph illustrating the relationship between Pressure and turbulent flow
P is proprotional to Q^2
P is proportional to L (length)
P is proportional to ρ (density)
Define critical flow in the context of flow of gases through an endotracheal tube
Critical flow is the flow rate and transition point at which laminar flow becomes turbulent flow. This point can be predicted using Reynold’s number.
What is the effect of warming gases on the critical flow
Increases the critical flow as the density is reduced. If density is reduced therefore Reynolds number reduced. The transition point between laminar and turbulent flow is hence increased (as Reynolds number will decrease)
Define La Place’s law for a tube and draw a diagram to illustrate the variables. What is critical closing pressure in arterioles and bronchioles.
The pressure gradient needs to remain balanced across the wall of a tube to prevent progressive distention and collapse.
P = thickness x T
____________
r
T: Tension
P: Pressure gradient across the wall of the tube
r: radius
If a fall in pressure distends the arteriole less –> radius falls but often tension is maintained by smooth muscle –> collapse of lumen of arteriole (or bronchiole). The pressure below which collapse occurs is called the critical closing pressure
Describe Laplace’s law as it would apply to an alveolus. Explain the mechanism of action of surfactant
As an alveolus is spherical, there are two planes of curvature and hence tension should be multiplied by 2:
P = 2 T
___
r
During expiration, air and pressure within the alveolus decrease leading to a reduction in radius. If pressure across the wall of the alveolus is to remain constant, the tension should reduce an equivalent amount to prevent collapse. Surfactant reduces the surface tension present in the fluid lining the alveoli and renders the SURFACE TENSION VARIABLE. As the alveolus increases in size –> surface tension increases. As the alveolus decreases in size –> surface tension decreases. P is kept constant and collapse prevented.
List the factors that increase and decrease blood viscosity and how is it measured
Increase
- Increased HCT or other viscous blood component (paraproteinaemia, polycythaemia)
- Decrease Temperature
- Older age
- Cigarette smoking (polycythaemia)
Decrease
1. Low molecular weight Dextran
Measurement:
Viscometer
What is the Bernoulli principle?
The fall of pressure at a constriction in a tube.
Tube narrowing –> increased kinetic energy (increased velocity) with simultaneous fall of potential energy (decreased pressure). Law of conservation of energy. This leads to entrainment of air (at the point of lower pressure). This is how a Venturi mask with injector works.
What is jet entrainment?
Friction between oxygen moving at high speed and the air pulls more air along with the flow of oxygen.
What principles are employed by a Venturi Mask to achieve clinically useful concentration of administered O2
Bernoulli principle
Jet entrainment
