Accuracy of a laser Doppler velocimeter for instantaneous velocity measurements made on solid surfaces
A detailed analytical and experimental investigation of the accuracy of a differential laser Doppler velocimeter (LDV) for dynamic, solid surface velocity measurements made over very short averaging times of less than one millisecond is presented in this thesis. Fundamental limitations exist on the accuracy of such "instantaneous" measurements. For frequency domain signal processing (Doppler spectrum measurement) the accuracy is limited by finite transit time broadening and, in the common situation of a rotating surface, tangential velocity gradient broadening of the Doppler spectrum. For time domain processing (Doppler time period measurement) the accuracy is limited by stochastic phase fluctuations in the nominally sinusoidal signal. Several practical factors also impose limits on the accuracy. Important factors which were studied include signal-to-noise ratio, signal coherence, amplitude discrimination during signal processing, amplitude modulation due to speckle, fringe spacing variation within the illuminated spot, random velocity drift effects, stochastic phase error statistics, finite time resolution of the LDV signal processor clock and electronic noise floor in the processor. Accuracy limitations are derived for both Doppler spectrum and time period measurement techniques, along with operating conditions and signal processing methods needed to optimize LDV performance. Experiments were conducted using a differential LDV to determine the conditions needed to minimize the error in instantaneous measurements. An analysis comparing the performance of a laser speckle velocimeter relative to that of an LDV is also given. It has been demonstrated that a laser Doppler velocimeter can be employed for making instantaneous velocity measurements on solid surfaces with an accuracy on the order of 0.5% for averaging times on the order of one millisecond. ^
Major Professor: Warren H. Stevenson, Purdue University.
Engineering, Mechanical|Physics, Optics