Flows near Solid Walls: LDA Measurement Instruments and Data Acquisition

In approaching solid walls it could be very difficult to recognize the velocity information from the signal of a laser Doppler system: high noise levels and low data rates are always present. The tracking and counting techniques arc not so well suited in these situations; a new generation of instruments, based on auto-correlation and FFT techniques, avoids the previous problems and allows the working also with very low signal to noise ratios. A comparison between the behavior of the old (Tracker and Counter) and new (Burst Spectrum Analyzer, BSA) signal processors has been performed, measuring;: the profiles of statistical moments and the power-spectra in a turbulent boundary layer. The Doppler signals have been processed using sampling procedures on the time history of the velocity to obtain time-equispaced data sets. Plots of the first four statistical moments and of the power-spectra in several points of a rectangular duct have been achieved. A careful comparison with previous results has been also performed.