Hot Wire, Cold Wire, Hot Film, Cold Film
Hot wires and hot films are used for constant temperature anemometry. Constant temperature anemometry uses an electronic circuit to maintain the resistance (and thus temperature) of a heated probe that is being cooled by flow. The amount of power required to maintain the temperature of the probe is a measure of the wind speed across the probe. The circuit is designed to have very high gain, which allows for rapid response to changes in cooling. By using very small and thin probes that resemble wires, wedges, or films of conductive material on a ceramic body, it is possible to obtain meaningful wind speed data at several kilohertz. These systems require calibration and are often used in wind tunnel studies. Combining three sensors orthogonally allows the three components of turbulence to be measured. More details of hot wire anemometry can be found in Bruun (1995). The high-frequency, three-dimensional turbulence information can be used to determine turbulent kinetic energy or dissipation rates using an inertial estimation method or a direct dissipation method (Champagne 1978; Oncley et al. 1996; Piper and Lundquist 2004).
Cold wires and cold films are a slightly different implementation of constant temperature anemometry that use low mass sensors and high gain circuits to measure temperature fluctuations. Combining hot and cold wires allows heat fluxes to be measured at very high sampling rates.
Because of their extreme sensitivity and delicacy, hot or cold wires or films are rarely used for long duration measurements in the atmosphere, but they can deliver valuable information for short periods of time. Examples include the hot wire systems that were deployed on the CIRES tethered lifting system at the National Wind Technology Center (NWTC) (Figure 16) to measure conditions in the wake of the GE 1.5-megawatt (MW) turbine (Lundquist and Bariteau 2015). Extended campaigns can incur significant costs because of damage to the sensors, which can cost from a few hundred to a few thousand dollars.
Hot or cold wires or films are commercially available with a TRL > 8.