Measuring velocity of flow 2 (Measuring flow velocity of water)

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What type of experiment is this?

Experimental procedure and explanation:

• Let us measure the flow velocity of water. You will need a transparent bending straw, which can be used as a current meter by bending it to 90 °. This type of current meter is called a Pitot-tube anemometer.
• Next, create a scale to read the flow velocity with a plastic board. The scale interval is written later.
• Place the Pitot tube in the water stream, point the tip of the straw toward the upstream direction of the stream, and let the longer side of the straw stand straight vertically (in the vertical direction). In doing so, the flow is blocked by the tip of the straw, the pressure rises, and the water level inside the straw rises.
• When the height of the water level stabilizes, use the scale board to measure the flow velocity. Align the lower edge of the scale board (at height 0) with the height of the surrounding water level (the straw and scale board should be able to move separately without gluing). When you read the height of the water level with the scale board, that value becomes the flow velocity.
• This current meter can be used to measure the speed of a ship. If you put the tip of the Pitot tube into the water from the top of the ship and measure the velocity of the water flow, it will become the same value as the speed at which the ship travels (if you can assume that there is no tidal current and the surrounding water is stationary).
• Next, the scale board of this current meter is explained. The flow velocity is calculated using the formula

  

  Here, g denotes the magnitude of gravitational acceleration = 9.8 m/s² and h denotes the rising height of the water level (m). Please express the units in kg, m, s (Do not use g, cm, mm).
• It is convenient to create the scale board in advance because it is difficult to calculate during the experiment. The relationship between height and flow velocity calculated from the above equation is shown in the following table. Enter the scale of the scale board (set in cm to make the height in the table easy to understand) using these values.

Pitot tube (Flow velocity of liquid)

Flow velocity (m/s) Height (cm) 0.0 0.0 0.1 0.1 0.2 0.2 0.3 0.5 0.4 0.8 0.5 1.3 0.6 1.8 0.7 2.5 0.8 3.3 0.9 4.1 1.0 5.1

Flow velocity (m/s) Height (cm) 　 　 1.1 6.2 1.2 7.3 1.3 8.6 1.4 10.0 1.5 11.5 1.6 13.1 1.7 14.7 1.8 16.5 1.9 18.4 2.0 20.4

Flow velocity (m/s) Height (cm) 　 　 2.1 22.5 2.2 24.7 2.3 27.0 2.4 29.4 2.5 31.9 2.6 34.5 2.7 37.2 2.8 40.0 2.9 42.9 3.0 45.9

 

[Caution]	・The water inside may pop out when the flow is fast or when the flow is hit suddenly, and this can stain your body, clothes, or the inside of the house. Please note that if you lay the Pitot tube sideways or turn it upside down, the water inside will spill. ・When measuring flow velocity, be careful not to disturb the flow. For example, if a hand or part of the body is near or upstream of the measurement point, the flow will change and the flow velocity value will change.
[Keywords]	Flow velocity measurement, Pitot tube
[Related items]	Measuring velocity of flow 3
[Reference]	“The Wonders of Flow”, Japan Society of Mechanical Engineering, Kodansha Blue Backs, P110-113.

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