Improvement of cold-wire response for measurement of temperature dissipation

This work aimed at improving fine-scale measurements using cold-wire anemom etry. The dissipation ev of the temperature variance <(<theta>' (2))over ba r> was measured on the axis of a heated turbulent round jet. The measuremen ts were performed with a constant current anemometer (CCA) operating fine P t-10%Rh wires at very low overheat. The CCA developed for this purpose allo wed the use of the current injection method in order to estimate the time c onstant of the wire. In the first part of the paper, it is shown that the t ime constants obtained for two wire diameters - d = 1.2 and d = 0.58 pin - compare well with those measured at the same time using two other methods ( laser excitation and pulsed wire). Moreover, for these two wires, the estim ated time constants were in good agreement with those obtained from a semi- empirical relation. In the second part of the paper, a compensation procedu re - post-processing filtering - was developed in order to improved the fre quency response of the cold-wire probes. The measurements carried out on th e axis of the jet (Re-nu) = 16 500, Re-lambda similar or equal to 167) show ed that the frequency response of the 1.2 mini wire was significantly impro ved. In fact, the spectral characteristics of the compensated signal obtain ed with the 1.2 mum wire compared fairly well with those from the 0.58 tm w ire. Moreover, the results indicated that the compensation procedure must b e applied when the cut-off frequency of the cold-wire f(c) is lower than tw o times the Kolmogorov frequency f(K). In the case where f(c) similar or eq ual to 0.6f(K), the compensation procedure can reduce the error in the esti mate of co by more than 20%. When f(c) similar or equal to 2f(K), the effec t of the compensation is reduced to about 5%.