Measuring velocity of flow 1 (Ping-pong ball anemometer)

Let's take a look!

Let us measure the speed of wind (wind speed). We will need a ping-pong ball, protractor, thread, plastic board (even a cardboard is fine), and hairpin.
Cut the plastic board into a semicircle and paste the protractor on the board. Make a hole at the center of the protractor with a pin vise or the like and insert a slightly stretched hairpin into it. Adhere the thread to the ping-pong ball, pass the other side of the thread through the hole of the protractor, and fix it with Cello tape.
Let us measure the wind speed with the created anemometer. Place the ping-pong ball in the wind of an electric fan. Check the tilt of the protractor with the hairpin so that the 90 ° direction faces directly below (vertically below). Read the scale value from the direction of the thread connected to the ping-pong ball. In the video, the wind speed is about 4–5 m/s
When you place the ping-pong ball in the wind, air resistance acts on the ping-pong ball in the direction of flow and the thread will move in the diagonal direction. Further, gravity acts on the ping-pong ball as well, and therefore, the direction of the thread is determined based on the ratio of these forces. The stronger the wind, the more the thread is pulled in the downstream direction; thus, this is used to obtain the wind speed. .
The wind velocity can be calculated using the formula

In this formula, v represents the wind velocity, m represents the mass of the ping-pong ball (kg), g denotes the magnitude of gravitational acceleration = 9.8 m/s², θ represents the angle formed between the thread and the vertical direction (direction directly below) (rad), C_ddenotes the drag coefficient of the sphere, which is approximately 0.4, ρ_A denotes the density of air (1 atm, 20 ° C, 1.20 kg/m³ for dry air), Pi π = 3.14, and d represents the diameter of the sphere (m). The units are expressed in kg, m, s (Do not use g, cm, mm).
In the video, a hardball is used for which d = 0.0397m (= 39.7mm) and m = 0.0027kg (= 2.7g). For the hardball, the diameter is d = 40mm; however, the measured value was used.
Each value must be measured and examined; however, this is a little difficult, and thus approximate values are shown in the following table. At 1 atm and 20 ° C, the hardball was assumed to be d = 39.7mm, m = 2.7g, and the large ball was assumed to be d = 44mm, m = 2.3g. These values were entered into the anemometer scale. These are only approximate values, and if you want to measure them more accurately, conditions such as actual ball diameter, mass, temperature, atmospheric pressure, and humidity must be matched.

[Caution]	When measuring flow velocity, please be careful not to disturb the flow. For example, if a hand or part of the body is near or at the upstream of the measurement point, the flow will change, and the flow velocity value will differ.
[Keywords]	Flow velocity measurement, Drag, Air resistance
[Related items]	Function of wind and rubber, Heavy Ball and Light Ball
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P54-55 and P58-59.

Last Update：2018.6.1