Concerning time and length scale estimates made from burst-mode LDA autocorrelation measurements

Abstract:  An investigation was performed in grid-generated turbulence to highlight the difficulties in estimating integral and microscales from discrete autocorrelation measurements made using a burst-mode laser Doppler anemometer (LDA). Comparison was made to hot-wire results and the theoretical growth laws determined by the decay of turbulent energy. The chief concern was the presence of a spike in the measured autocorrelation coefficient functions at t=0. It was found that renormalizing the autocorrelation to a value at t=0 determined by “backfitting” a quadratic equation to the slots near t=0 led to microscale estimates that showed the same trend as the decay law and were in agreement with those determined using a hot-wire. Additionally, practical guidelines are presented for making accurate autocorrelation measurements by LDA.