8. Flow Flashcards
What is flow?
> Flow (F) is the quantity (Q)
of a liquid or gas
passing a point
per unit time (t).
> A simplified equation for this is:
F = Q/t.
> There are two main types of flow –
laminar and turbulent flow,
both of which have
very different physical characteristics.
What are the characteristics of laminar flow?
> Fluid moves in a
steady manner
(no eddies or turbulence).
> Flow rate is greatest at the centre
of the flow stream
(2 × flow rate at side of tube).
> A pressure difference
must exist for fluid to flow.
> Flow is directly proportional
to this pressure difference.
> Resistance of the tube is calculated
by the ratio of pressure to flow.
What equation can be used to calculate laminar flow?
Hagen–Poiseuille equation is used to calculate laminar flow:
π × ΔP × r4
Flow = ___________
8 × L × n
Where:
π =
Pi
(mathematical constant – the ratio of any circle’s circumference
to its diameter)
ΔP = Pressure drop
r = Radius
L = Length of tube
n = Viscosity of fluid
Give an example where the
Hagen–Poiseuille equation is
clinically applied.
Administering a unit of blood rapidly to a patient:
> Use a short,
wide-bore cannula
(i.e. a 16 G short cannula is better than
using the 16 G distal port of a long central line)
> Raise the height of the giving set.
> Apply a pressure bag to the unit of blood.
> Warm the blood (reduces viscosity).
What are the characteristics of
turbulent flow?
> Flow characterised by swirls and eddies.
> Transition of laminar to turbulent flow
may occur at constrictions.
> Fluid velocity varies across the tube.
> Flow is proportional to the
square root of pressure
(i.e. to double flow,
the pressure must be increased by a factor of 4).
> Resistance is no longer constant
because the relationship between
pressure and flow is no longer linear.
> Density of the fluid is the important
determinant in turbulent flow
(as opposed to fluid viscosity in laminar flow).
Wht is turbulent flow proportional to
Thus:
> Flow is proportional to the square root of pressure.
> Flow is proportional to the radius squared.
> Flow is inversely proportional to the square root of the tube length.
> Flow is inversely proportional to the square root of fluid density.
What is Reynolds number?
This is a number that predicts the
onset of turbulent flow of a fluid:
Velocity × Density × Tube diameter Reynolds number = \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
Viscosity
> Reynolds number < 2000 predicts laminar flow.
> Reynolds number > 2000 predicts turbulent flow.
(NB: Reynolds number does not have any units – it is a dimensionless number.)
Give an example where the concept of Reynolds number
is used clinically.
Heliox is a mixture of 21% oxygen and 79% helium. Helium is much less
dense than nitrogen, making heliox much less dense than air (about three
times less dense). This reduction in density lowers the Reynolds number,
which changes turbulent flow to laminar flow. Laminar flow is known to
reduce the work of breathing.
