laser多普勒风速测量中的时滞效应
Dead time effects in laser Doppler anemometry measurements
摘要：
我们提出的速度功率谱计算的所谓的直接方法从突发型激光多普勒测速仪（LDA）的数据，测量在湍流圆形射流和在计算机中产生的。使用今天的强大的计算机，我们已经能够研究更多的性能的计算光谱比以前可能的，我们注意到一些意想不到的功能的光谱，我们现在归因于一个有限的测量体积（MV）的不可避免的影响。最初引发这些研究的最突出的效应是在较高频率范围内出现阻尼振荡，由于MV的大小，在截止频率附近开始。使用模拟LDA数据的计算机生成的数据，这些效应先前已经显示出由于时滞的效应而出现，即，系统不能获取新测量值的有限时间。这些时滞时间可以追溯到突发模式LDA不能同时测量多个信号突发的事实。由于时滞时间近似等于穿过测量体积的颗粒的停留时间，因此我们处理的是变化很大的时滞时间，然而，假设对每个数据点测量停滞时间。此外，当前研究中使用的检测器和处理器引入了一定量的固定处理和数据传输时间，这进一步导致了计算光谱的失真。然而，我们表现出良好的协议之间的测量光谱和我们的建模LDA数据，从而确认我们的模型的有效性LDA突发处理器。
英文摘要：
We present velocity power spectra computed by the so-called direct method from burst type laser Doppler anemometer (LDA) data, both measured in a turbulent round jet and generated in a computer. Using today's powerful computers we have been able to study more properties of the computed spectra than was previously possible, and we noted some unexpected features of the spectra that we now attribute to the unavoidable influence of a finite measurement volume (MV). The most prominent effect, which initially triggered these studies, was the appearance of damped oscillations in the higher frequency range, starting around the cut-off frequency due to the size of the MV. Using computer generated data mimicking the LDA data, these effects have previously been shown to appear due to the effect of dead time, i.e., the finite time during which the system is not able to acquire new measurements. These dead times can be traced back to the fact that the burst-mode LDA cannot measure more than one signal burst at a time. Since the dead time is approximately equal to the residence time for a particle traversing a measurement volume, we are dealing with widely varying dead times, which, however, are assumed to be measured for each data point. In addition, the detector and processor used in the current study introduce a certain amount of fixed processing and data transfer times, which further contribute to the distortion of the computed spectrum. However, we show excellent agreement between a measured spectrum and our modeled LDA data thereby confirming the validity of our model for the LDA burst processor.
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