Hot wire anemometer is the instrument preferred in the measurement of highly fluctuating velocities in air flows. This is because a hot wire anemometer offers several advantages over other types of instruments in accurately measuring rapidly changing air velocities.

The Working Principle of Hot Wire Anemometer:
A hot wire anemometer works on the principle of convective heat transfer. It consists of a thin wire, typically made of platinum or tungsten, which is heated to a constant temperature using an electric current. When the wire is placed in a moving fluid (in this case, air), the fluid flow causes convective heat transfer from the wire, leading to a change in its temperature. This change in temperature is directly proportional to the velocity of the fluid flow.

Advantages of Hot Wire Anemometer:
1. High Sensitivity: Hot wire anemometers are highly sensitive instruments, capable of accurately measuring even the slightest changes in air velocities. This makes them ideal for measuring highly fluctuating velocities in air flows.

2. Fast Response Time: Hot wire anemometers have a very fast response time, allowing them to quickly detect and measure rapid changes in air velocities. This is crucial when dealing with highly fluctuating flows where traditional instruments may not be able to provide accurate readings.

3. Wide Measurement Range: Hot wire anemometers have a wide measurement range, making them suitable for a variety of applications. They can accurately measure both low and high air velocities, providing versatility in different airflow conditions.

4. Non-Intrusive: Hot wire anemometers are non-intrusive instruments, meaning they do not disturb the flow of air being measured. This is important when dealing with highly fluctuating velocities, as any disturbance to the flow can affect the accuracy of the measurements.

5. Compact and Portable: Hot wire anemometers are compact and portable, making them easy to use in different locations and environments. They are commonly used in laboratory settings as well as in field measurements.

Overall, the hot wire anemometer is the preferred instrument for measuring highly fluctuating velocities in air flows due to its high sensitivity, fast response time, wide measurement range, non-intrusive nature, and portability. Its ability to accurately capture rapidly changing air velocities makes it a valuable tool in various industries, including HVAC, aerospace, and environmental monitoring.