The main uses and working principles of an anemometer
1. Measure the average flow velocity and direction.
2. Measure the pulsating velocity and its frequency spectrum of the incoming flow.
3. Measure the Reynolds stress in turbulence and the velocity and time dependencies between two points.
4. Measure wall shear stress (usually using a hot film probe placed flush with the wall, similar to speed measurement).
5. Measure fluid temperature (measure the curve of probe resistance changing with fluid temperature in advance, and then determine the temperature based on the measured probe resistance.
Working principle of anemometer
The basic principle of an anemometer is to place a thin metal wire in a fluid, apply current to heat the wire, and make its temperature higher than the temperature of the fluid. Therefore, the metal wire anemometer is called a "hot wire". When the fluid flows through the metal wire in a vertical direction, it will take away some of the heat from the wire, causing the temperature of the wire to decrease. According to the theory of forced convective heat exchange, a relationship can be derived between the heat dissipated by the hot wire Q and the velocity v of the fluid. A standard hot wire probe consists of two brackets tensioned with a short and thin metal wire. Metal wires are usually made of metals with high melting points and good ductility, such as platinum, rhodium, tungsten, etc. The commonly used wire diameter is 5 μ m and the length is 2 mm; the smaller probe diameter is only 1 μ m and the length is 0.2 mm. According to different purposes, the hot wire probe is also made into double wire, triple wire, oblique wire, V-shaped, X-shaped, etc. In order to increase strength, metal film is sometimes used instead of metal wire. Usually, a thin metal film is sprayed on a thermally insulated substrate, which is called a hot film probe. The hotline probe must be calibrated before use. Static calibration is carried out in a specialized standard wind tunnel, measuring the relationship between flow velocity and output voltage and drawing it into a standard curve; Dynamic calibration is performed in a known pulsating flow field or by adding a pulsating electrical signal to the heating circuit of an anemometer to verify the frequency response of the hot wire anemometer. If the frequency response is poor, corresponding compensation circuits can be used to improve it.
