MEASUREMENT OF VELOCITY

Generally it is the volume flow rate which is the most important

quantity to be measured and from this it is possible to calculate a mean

flow velocity across the full flow area, but in some cases it is also

important to know the velocity at a point. A good example of this is in

a river where it is essential for the captain of a boat to know whatstrength of 

current to expect at any given distance from the bank;

calculating a mean velocity from the volume flow rate would not be

much help even if it were possible to measure the exact flow area over

an uneven river bed.

It was exactly this problem which led to one of the most common

velocity measurement devices. A French engineer called Pitot was given

the task of measuring the flow of the River Seine around Paris and

found that a quick and reliable method could be developed from some

of the principles we have already met in the treatment of Bernoulli’s

equation. Figure 3.2.17 shows the early form of Pitot’s device.

The horizontal part of the glass tube is pointed upstream to face the

oncoming liquid. The liquid is therefore forced into the tube by the

current so that the level rises above the river level (if the glass tube was

simply a straight, vertical tube then the water would enter and rise until

it reached the same level as the surrounding river). Once the water has

reached this higher level it comes to rest.

What is happening here is that the velocity head (kinetic energy) of

the flowing water is being converted to height (potential energy) inside

the tube as the water comes to rest. The excess height of the column of

water above the river level is therefore equal to the velocity head of the

flowing water.