Wavelet time-frequency localization of bi-stable flows in tube banks

In this paper, the presence of nonstationary wakes in tube banks is studied by means of wavelet analysis of hot wire time series. The measurements were performed in a tube bank with square arrangement and pitch-to diameter ratio of 1.26, at steady and transient conditions. The Reynolds number at steady state flow, computed with the tube diameter and the flow velocity in the narrow gap between tubes is 8 x 104. Air is the working fluid. Velocity measurements were performed after the third row using hot wire anemometry. Discrete and continuous wavelet transforms were applied to the time series to allow the analysis of phenomena in a time-frequency domain. Flow visualizations in a water channel in an enlarged arrangement with the same P/D were also used for the interpretation of the results. Results show that wavelet analysis is an adequate tool to analyze this class of phenomena, since the classical approach through Fourier transform performs an average over the entire time series and the information in time domain is lost. Results evidence the presence of a nonstationary phenomenon characterized by the alternation in the magnitude of the mean velocity measured behind the third row. This phenomenon can be very important in the heat transfer process by promoting the flow redistribution inside the bank. It can be very important as well in the flow induced vibration process by influencing the characteristic frequencies and the fluctuating pressure distribution on the tubes of the bank. (authors)