Measuring velocity of flow 3 (Pitot tube)

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Experimental procedure and explanation:

• Let us make a flow measuring device called a Pitot tube using two straws.
• Connect the two straws together with Cello tape. Please wrap the Cello tape carefully so that the water does not spill from the joint.
• Put water in the connected straws. Add paint, ink, or soy sauce to color the water so that it can be easily seen from the outside. When using the Pitot tube, do not tilt the Pitot tube sideways to prevent the water inside from spilling.
• Let us measure the wind speed from the dryer as an example. Place the tip of the straw in the direction of the wind so that it faces the upstream direction of the flow; let the longer side of the straw stand straight vertically (in the vertical direction). In doing so, the flow is blocked by the tip and the pressure rises. Therefore, there is a difference in the height between the left and right water levels in the Pitot tube.
• When the height of the water level stabilizes, apply a scale board to measure the flow velocity. Align the lower edge of the scale board (at height 0) with the height of the lower water level. When you read the height of the other water level using the numerical value of the scale board, it will indicate the flow velocity.
• The scale board of this current meter is explained below. Flow velocity can be calculated by the formula

  

  Here, g denotes the magnitude of gravitational acceleration = 9.8 m/s², h denotes the rising height of the water level (m), ρ_A denotes the density of air (1 atm, 20 ° C, 1.20 kg/m³ for dry air), and ρ_W denotes the density of water (about 1000 kg/m³). 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. However, if the flow velocity is slow, the difference in the height of the water level becomes small, and it cannot be measured accurately.

Flow velocity (m/s) Height (cm) 0.0 0.0 1.0 0.0 2.0 0.0 3.0 0.1 4.0 0.1 5.0 0.2 6.0 0.2 7.0 0.3 8.0 0.4 9.0 0.5 10.0 0.6

Flow velocity (m/s) Height (cm) 　 　 11.0 0.7 12.0 0.9 13.0 1.0 14.0 1.2 15.0 1.4 16.0 1.6 17.0 1.8 18.0 2.0 19.0 2.2 20.0 2.4

• Pitot tubes are used not only to measure the speed of flow, but also to measure the speed of objects moving in air. In airplanes, racing cars, etc., a Pitot tube is attached to the body to measure the relative speed with respect to the body (however, instead of using the height of the water, the pressure difference is obtained using a pressure sensor to calculate the velocity). If the speed of the object is very fast (ignoring the wind speed of the surrounding air), the measured speed will become the approximate speed of the object (flight speed or travel speed).

[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 2, Measuring velocity of flow 4
[Reference]	“The Wonders of Flow”, Japan Society of Mechanical Engineering, Kodansha Blue Backs, P110-113.

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