What is volume strength

Volume flow rate

Synonyms: volume flow, flow rate
English: volumetric flow rate

1 definition

The Volume flow rate is a physical quantity that indicates the volume that flows through a defined cross-section (e.g. a tube) within a certain time.

2 background

In physiology, the volume flow strength of the blood in the body's circulation is also used Q and the volume flow rate of the breathing air with V. although both describe the same physical term. Physicists often use the term I., in analogy to the electrical current strength. A point is drawn over the symbols V and Q, the Volume change per unit of time expresses.

3 physics

The formula for calculating the volume flow rate is:

V = V / t
V = volume

t = time

The displaced volume is calculated using the cylinder volume formula:

Since we are dealing with a tube in this case, we take the length l instead of the height h, i.e.:

The volume flow rate is related to the flow velocity v and can be specified as the flow velocity multiplied by the cross-sectional area.

V = A * v
A = cross-sectional area

v = flow velocity

  • The volume flow rate is proportional to the fourth power of the radius (Hagen-Poiseuille law).
    • If the diameter is halved, the volume flow rate decreases by 16 times (at constant pressure).
  • The volume flow rate is proportional to the pressure.
    • If the volume flow rate is reduced by 16 times, then 16 times the pressure is required to transport the same amount.
  • The volume flow rate is proportional to the square of the cross-sectional area A.
    • If the cross-sectional area is halved, the volume flow rate decreases four-fold.

4 equation of continuity

Since liquids are generally incompressible, the same amount of liquid must flow through each cross-section of a pipe per unit of time. In narrower places this means that the flow rate has to increase. The speed decreases in places with a larger cross-section. This leads to the following continuity equation:

The speeds are inversely proportional to the cross-sectional areas.