Hot-wire anemometry

Hot-wire anemometry measures fluid velocity by electrically heating a fine wire and inferring flow speed from the wire’s convective cooling.

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King's law: An empirical relation between a heated wire’s electrical heat loss and fluid velocity. It links wire cooling to flow speed in a commonly used calibration model.
Electrical resistance: A material’s opposition to electric current, expressed as the ratio of voltage to current. The wire’s resistance changes with temperature and serves as an electrical measure of its heating.
Turbulence: Irregular fluid motion marked by fluctuating velocities and interacting eddies. Hot wires can resolve rapid velocity fluctuations within turbulent flows.
Laser Doppler velocimetry: An optical method that measures flow velocity from the Doppler shift of light scattered by particles. It measures particle motion without inserting a heated wire into the flow.
Constant-temperature anemometry: A hot-wire operating mode that adjusts electrical current to keep the sensor at a fixed temperature. Its feedback circuit measures the power needed to offset flow-dependent cooling.
Temperature coefficient of resistance: A quantity describing how a material’s electrical resistance changes with temperature. A suitable coefficient lets the circuit infer wire temperature from resistance.
Boundary layer: A thin region near a surface where viscosity strongly affects fluid velocity. Small probes measure steep velocity changes across this near-wall region.
Particle image velocimetry: An optical method that estimates fluid velocity by tracking seeded particles across images. It provides spatial velocity fields, unlike a hot wire’s local point measurement.
Constant-current anemometry: A hot-wire operating mode that holds electrical current fixed and measures changes in wire temperature or resistance. It infers velocity from cooling without actively maintaining wire temperature.
Nusselt number: A dimensionless ratio of convective to conductive heat transfer across a fluid boundary layer. It characterizes convective heat loss from the wire.