Measuring velocity of flow 4 (Inclined Pitot tube)

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• The Pitot tube used in “Measuring velocity of flow 3” is good when the flow is fast; however, when the flow is slow, the difference in the height of the water level is small and cannot be measured well. Therefore, let us attempt to improve it so that it can measure even a slightly slower flow by tilting the Pitot tube.
• A platform inclined at a ratio of 4 horizontally and 1 vertically is made and used in the experiment. The heights of the left and right water levels are expanded, which makes it easier to read the distance x between the two water levels and measure in slow flow.
• You can see that the distance between the two water levels increases if you point the tip of the Pitot tube in the upstream direction of the flow. Align the lower edge of the scale board (the position where the flow velocity is 0) with the position of the water level of the lower side, and read the distance up to the other water level. Make a scale board so that the value at that time becomes the flow velocity (how to make the scale board is explained later).
• If the angle formed between the water level and the slope is θ, the flow velocity can be calculated by the formula.

where g denotes the magnitude of gravitational acceleration = 9.8 m/s², x denotes the distance between the two water levels (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). For the inclined surface of horizontal 4 and vertical 1 used in this experiment, sinθ = 1/√17 = 0.243.

• 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.

Inclined Pitot tube (Inclined surface of horizontal 4 and vertical 1)

Flow velocity (m/s) Height (cm) 0.0 0.0 1.0 0.0 2.0 0.1 3.0 0.2 4.0 0.4 5.0 0.6 6.0 0.9 7.0 1.2 8.0 1.6 9.0 2.0 10.0 2.5

Flow velocity (m/s) Height (cm) 　 　 11.0 3.1 12.0 3.6 13.0 4.3 14.0 4.9 15.0 5.7 16.0 6.5 17.0 7.3 18.0 8.2 19.0 9.1 20.0 10.1

 

[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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