The response to temperature fluctuations of a constant-current hot-wire anemometer

The behaviour of a constant-current hot-wire anemometer system used for the measurement of temperature fluctuations in a turbulent gas is studied. The temperature-to-velocity sensitivity ratio is derived and the sensitivity and time constant behaviour are discussed. When end conduction is taken into account the dynamic sensitivity may be quite different from the static sensitivity and the difference occurs at about 1/6 Hz. A general analysis is presented which shows that at frequencies of the order of the time constant of the wire, a further step in the response occurs if the hot-wire system is compensated by an RC network. This, when combined with the low-frequency behaviour, leads to a considerable dip in the frequency response curve, the dip occurring in the centre of the energy-containing range of laboratory-produced turbulent thermal fluctuations. Radiant heat flux experiments are described which demonstrate these phenomena, and enable errors to be estimated and appropriate adjustments to the system to be made. These and other difficulties involved in measuring temperature fluctuations are discussed.