The Measurement of Turbulence with the Laser-Doppler Anemometer

The power spectrum and the correlation of the laser Doppler velocimeter velocity signal obtained by sampling and holding the velocity at each new Doppler burst are studied Theory valid for low fluctuation intensity flows shows that the measured spectrum is filtered at the mean sample rate and that it contains a filtered white noise spectrum caused by the steps in the sample and hold signal In the limit of high data density, the step noise vanishes and the sample and hold signal is statistically unbiased for any turbulence intensity

Power spectra of fluid velocities measured by laser Doppler velocimetry

Turbulent burning velocities have been measured in an explosion bomb equipped with four high speed fans. Turbulent parameters were measured by laser doppler anemometry. The turbulent Reynolds numbers were significantly higher than in most previous measurements and high rates of strain were achieved until, ultimately, several of the flames quenched. Results are presented in terms of previously used dimensionless parameters plus a Lewis number and a dimensionless activation energy. The two-eddy theory of burning can allow for flame straining reductions in laminar burning velocity and experimental values of u t / u 1 were compared with those from such a theory.

Turbulent Burning Velocities and Flame Straining in Explosions

We review the problem of spectral estimation from velocity data sampled irregularly in time by a laser Doppler anemometer (LDA) from very early estimators based on slot correlation to more refined estimators, which build upon a signal reconstruction and an equidistant re-sampling in time. The discussion is restricted to single realization anemometry, i.e. excluding multiple particle signals. We classify the techniques and make an initial assessment before describing currently used methods in more detail. An intimately related subject, the simulation of LDA data, is then briefly reviewed, since this provides a means of evaluating various estimators. Using the expectation and variance as figures of merit, the advantages and disadvantages of several estimators for varying types of turbulent velocity spectral distributions are discussed. A set of recommendations is put forward as a conclusion.

http://iopscience.iop.org/article/10.1088/0957-0233/11/8/301/pdf

Estimation of turbulent velocity spectra from laser Doppler data

Role of entrainment in the stabilisation of jet-in-hot-coflow flames

A new method is introduced for correcting the biased statistics of measurements with laser‐Doppler velocimeters operating in the individual realization mode and using counters for signal processing. Two variations of the new correction method are proposed. One is suitable for all uniformly seeded flows; prior to averaging, the individual velocity measurements are weighted with the corresponding residence time of the seed particles in the probe volume. The other variation is suitable for flows with nonuniform seeding in which the average particle separation time is small compared with the time scale of the turbulent fluctuations, and in this method the particle separation time is used to weight the individual measurements. In both cases, a large number of samples is required. An experimental setup for realizing the new biasing correction method is described for each variation. The residence‐time weighting method is applied to mean‐velocity measurements in a uniformly seeded round jet. The results are compared with uncorrected measurements and with results using the McLaughlin–Tiederman correction. The uncorrected results show biasing errors of up to 25%, and the McLaughlin–Tiederman correction works only in the center portion of the jet where the instantaneous flow direction is nearly constant. The McLaughlin–Tiederman correction is restricted conceptually to this flow situation and to uniformly seeded flows. In contrast, the new correction method is not restricted to any particular flow situation. It supplements the new burst‐gating counter system described in a parallel paper to form an especially accurate and versatile LDV‐signal processor.

W. Rodi

Papers

New biasing elimination method for laser‐Doppler velocimeter counter processing