The response of hot wires in high Reynolds-number turbulent pipe flow

Issues concerning the accuracy of hot-wire measurements in turbulent pipe flow are addressed for pipe Reynolds numbers up to 6 × 10⁶ and hot-wire Reynolds numbers up to Re_w ≈ 250. These include the optimization of spatial and temporal resolution and the associated feature of signal-to-noise ratio. Very high wire Reynolds numbers enable the use of wires with reduced length-to-diameter ratios compared to those typical of atmospheric conditions owing to increased wire Nusselt numbers. Simulation of the steady-state heat balance for the wire and the unetched portion of wire are used to assess static end-conduction effects: they are used to calculate wire Biot numbers, √c₀l, and fractional end-conduction losses, σ, which confirm the 'conduction-only' theory described by Corrsin. They show that, at Re_w ≈ 250, the wire length-to-diameter ratio can be reduced to about 50, while keeping √c₀l > 3 and σ < 7% in common with accepted limits at Re_w ≈ 3. It is shown that these limits depend additionally on the choice of wire material and the length of unetched wire. The dynamic effects of end-cooling are also assessed using the conduction-only theory.