Measuring the average velocity of flow
METHODS OF MEASURING THE AVERAGE VELOCITY OF FLOW

Measuring the average velocity of flow

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METHODS OF MEASURING THE AVERAGE VELOCITY OF FLOW

Following six methods can be used for determining the average velocity of flow:

  1. Surface floats or simply floats,
  2. Double floats,
  3. Velocity rods,
  4. Current meter,
  5. Pitot tube, and
  6. Travelling screens.

1. Surface Floats.

  • A float or surface float is nothing but a piece of cork or anything which can float on water.
  • Such a float gives surface velocity of flow only.
  • The distance travelled by such a float divided by the time taken to travel a

Fig. 10.4. Single, double floats and velocity rod.

 

  • specified distance, gives the surface velocity of flow.
  • Surface floats are simultaneously released in each of the seven compartments of the river and time taken by the float of each compartment in reaching section EF from AB is noted.
  • Surface velocity for each compartment can then be computed by dividing the distance between AB and EF, by the time taken by the float of the corresponding compartment.
  • After having found out the surface velocities, the mean velocities can be calculated for each compartment by multiplying the surface
  • velocities by a suitable coefficient 0.85. Average of all the mean velocities can then be found out, which is overall average velocity (V) of the flow.

2. Double Floats.

  • It is just like a surface float with the difference that it is attached with a steel ball.
  • The steel ball remains suspended in water.
  • The ball is maintained at such depth that the resulting velocity of the float is not the surface velocity but average velocity.
  • Time of travel of the float is noted for all the compartments dividing width of the river and average of the velocities for all the zones is found out which is considered as average velocity of flow.

3. Velocity Rods.

  • These are wooden rods painted with some water proofing paint.
  • A weight is attached to the bottom of the rod.
  • The weight is so adjusted that rod length equal to 0.94 × depth remains inside the water.
  • The top about 4 cm length of the rod remains out of water and remains visible.
  • These rods when floated in flowing water give the mean velocity of flow directly.
  • The use of floats and velocity rods is generally restricted to straight rivers having almost uniform cross-section throughout.
  • They are not of much use for natural stream, especially when depth of water exceeds 1.5 m or so.

4. Current Meter.

  • It is one of the best methods for measuring the velocity of natural stream.
  • Various types of current meters are in use these days,

Fig. 10.5. Current meter.

  • It is a differential type of meter and consists of a horizontal wheel carrying a series of cups that rotate on a vertical axis.
  • Half of the cups behave convex and half as concave to the current.
  • There is a tail-vane and a counter weight at bottom to balance the meter and to keep it steady.
  • When current meter is suspended in water, the velocity of flow causes the wheel to rotate.
  • The current meter is fitted with a device so as to record the number of revolutions of the horizontal wheel due to velocity of flow.
  • The rate of revolutions is proportional to the velocity of flow.
  • Since the velocity for different compartments is different, hence the meter is lowered in the centre of each compartment separately and individually.
  • The wheel of the meter should face the current and is generally kept at a depth of 0.6 depth from the free surface of water.
  • The velocity measured by keeping the meter at a depth of 0.6 depth, gives the mean velocity.
  • Sometimes the velocities at a depth of 0.2 depth and 0.8 depth are measured and their average value is taken to be the mean velocity.
  • The current meter is used by suspending it with the help of a boat.
  • For narrower streams the meter can be suspended by a cable stretched across the river and at the mid-points of each compartment.

Fig. 10.6. Current meter being used by boat.

 

  • he boat can be conveniently use for small velocities of the order of 1 m/sec but for higher velocities and rough waters, it becomes difficult to keep the boat steady.
  • In such a case the boat is generally anchored.

5. Pitot Tube.

  • It is a device with the help of which velocity head of flowing water is converted into pressure head.
  • The pressure head can be measured and the velocity of flow calculate by equating their pressure head to\[ \frac{V^{2}}{2g}\] Pitot tube is an ordinary tube having both ends open.

6. Travelling Screen Method.

  • This method cannot be used for rivers or streams.
  • This method can be used for canals and that too for lined canals.
  • The method consists of a canvass screen mounted on a frame work of iron angles.
  • The size of the screen is so adjusted that some open space remains between bottom and banks, and the canvass bottom and edges respectively.
  • The whole frame work is mounted on a travelling trolley which can move on the rails laid at banks and parallel to the canal.

Fig. 10.7. Pilot tubes

  •  When canvass screen is put against the current of flowing water it starts moving because of thrust of water.
  • The velocity of moving screen is taken as the average velocity of flow.
  • The method is not very common.

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