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Measurement of the acceleration of scattering particles in fluids

17.11.2008

For the modelling of turbulent flows, estimating the particle lag and determining the particle size, knowledge of the momentary velocity - including the Lagrangian acceleration - is required. For this purpose, the method of quadrature signal demodulation was used and extended for any type of signals generated by laser Doppler anemometers.

Knowledge of the momentary velocity, including the Lagrangian acceleration in turbulent flows, is of great interest for the modelling of flow. Furthermore, it is helpful to know the magnitude of the acceleration to estimate the particle lag of scatterers in a flow, as well as for the determination of the particle size and for the calibration of laser Doppler anemometers (LDA). The simple method of quadrature signal demodulation was used for acceleration measurement and has been extended for any type of LDA signals.

The inline signal of an LDA is complemented by its quadrature signal using the Hilbert transformation (double FFT). The following quadrature demodulation allows the temporal position of the scatterer in the measuring volume to be calculated. A weighted 2nd order polynomial regression determines the velocity and acceleration at the centre of the signal. The weighting factors are given by the momentary power of the signal.

The method has been validated in the stagnation streamline of a pitot tube (D = 8 mm, Fig. 1). The filled black circles show the results of the new analysis. The open red circles are the result of the conventional calculation. The error bars show the standard deviations.

Acceleration in the stagnation streamline of a pitot tube

Figure 1: Acceleration in the stagnation streamline of a pitot tube

They correspond to velocity variations within the measuring volume of less than 0.3%.

Contact person:

Volker Strunck, Department 1.4, WG 1.41, e-Mail: volker.strunck@ptb.de