Study On The Frequency Modulation Doppler Global Velocimetry And Testing On A FMDGV System

Doppler global velocimetry (DGV) is a recently developed flow fieldvelocity measurement technique which works on the basis of molecular filterprinciple and laser Doppler shift phenomenon. The DGV technique is suitable for thecase of hypersonic flow and large-size wind tunnel flow field measurements. In orderto remove the light intensity changes besides Doppler frequency shift, a referencecamera is necessary in the traditional DGV systems. This will bring out some newerrors, such as images misalignment error and beam splitting error. In order toincrease the measurement accuracy, frequency modulation DGV was developed，inwhich the reference detector is needless and Doppler frequency shift is determinedby the ratio of the first and second harmonics of the light intensity modulated signal.The main purpose of this paper is to develop a FM-DGV system, and to examine thesystem. The performance of the FM-DGV system will be analyzed based on themeasurement results.In this thesis, the background and developing history of FM-DGV technique arestated, and some significant research results in FM-DGV works are presented. Inorder to know this technique better, the working principle and typical systemcomposition of FM-DGV are elaborated. A FM-DGV system was developed in thestudy. Experimental test works have been carried out on the system. The effects oftemperature about absorption line of the molecular filter have been investigated. Thecalibration curve of the relationship between the centre frequency difference andquotient of the first and the second harmonics has been performed based on theabsorption curve of the iodine filter. Measurements have been taken on the velocitydistributions of the rotating wheel test, where peripheral velocity varied from10m/sto30m/s. The rotational wheel test results show that the maximal speed error issmaller2m/s and root-mean-square value of measured speed bias is less than1m/s,the speed distributions measured by FM-DGV are consistent with the real speed,which points out that the FM-DGV system is reliable. Comparing the conventional DGV measurement results, it’s found that the measurement accuracy of FM-DGV ishigher. Measurements have also been carried out on an actual jet flow generated by anebulizer. The plane velocity distributions being got in the measurement arecoincident with those of theoretical predict. This verified the availability of theFM-DGV system for the real flows. In the end of this paper, uncertainty of thisFM-DGV system measurement results is analyzed, and discussion has been made interms of several possible error sources. The measured results and the discussions arevaluable for the FM-DGV system improvements.