The general equation governing the operation of hotwire can be now be simplified to
In isothermal flow measurements Equation 7 can be further simplified as
A relationship between Nusselt number and Reynolds number with  as a parameter that can be reliably used for converting the voltage output of the anemometer into velocity cannot be analytically derived. The analytical solution of flow past a circular cylinder for a wire-diameter based Reynolds number less than 40 shows that a power law relation is generally valid. The value of the exponent is 0.5 when the wire is taken to be infinitely long, flow is two dimensional and fluid properties are independent of temperature. However, the exponent depends on the geometric and physical properties of the wire, physical properties of fluid, and interference effects due to the prongs. A value of 0.45 is generally preferred; other empirical correlations derived using individual calibration of the wires is recommended in hotwire applications. (Chew and Simpson, 1988)
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