Correction of cold-wire response for mean temperature dissipation rate measurements

Abstract

This work is aimed at improving dissipation rate measurements using cold-wire anemometry. The mean dissipation rate of the temperature variance is measured on the axis of a heated turbulent round jet. Measurements are performed with a constant current anemometer (CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose uses a current injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the time constants obtained for two wire diameters ( d=1.2 and 0.58 μm) compare well with those measured at the same time using two other methods (laser excitation and pulsed wire method). Moreover, for these two wires, the estimated time constants are in good agreement with those obtained from theory. In the second part of the paper, a compensation procedure, involving post-processing filtering, is developed in order to improve the frequency response of the cold-wire probes. Measurements on the jet axis (Re D=16 500 , Re λ =167) show that the frequency response of the 1.2 μm wire is indeed significantly improved after compensation. The spectrum of the compensated signal for the 1.2 μm wire compares fairly well with that for the 0.58 μm wire. The results also indicate that the compensation should be applied when the cut-off frequency of the cold-wire f C is less than about 2 f K, where f K is the Kolmogorov frequency. When f C≈0.6 f K, the compensation can reduce the error in the estimated mean dissipation rate by more than 20%. When f C=2 f K, the reduction is about 5%.