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Chapter3. Implementation 34 3.2.1 Raw Data The raw data of the linear microphone array has been recorded by the DLR dur- ing the various tests. Each flow condition was sampled for 30 s at a rate of 44 kHz, using a 30 Hz highpass filter for each channel [37]. The linear array raw data con- sists of hdf5-files for each test point. The files need to be converted to mat format before source directivity calculation, because SODIX is coded in matlab. Each file contains the cross-spectral matrix (CSM) of its respective test point as well as the frequency spectrum in narrow band steps [22]. The test point (TP) naming convention is the following: Each test point is as- signed a three letter code and number by QQ in alphanumerical order. In ad- dition, each point is designated a different number by DLR. In the scope of this work the DLR denotation is used. In order to avoid confusion, DLR TP names are preceded by a "DLR" prefix (e.g. DLR 317). An exhaustive list of all TPs used in this thesis and their respective flow and working conditions can be found in Ta- ble A.3. Tests conducted with an installed wing will be referred to as "installed" cases below, whereas tests without wing installation will be referred to as "iso- lated" test cases. Furthermore, test cases with and without flight stream simula- tion are labelled with "FS" and "no FS" respectively. Data verification The results calculated by SODIX are depend on the quality of the raw data. Hence, it is advisable to perform a raw data check before the SODIX calculation. Fig- ure 3.8 shows the logarithmic auto frequency spectrum of TP DLR 317 for all microphones in the array as a function of their x position relating to the bypass nozzle diameter dBypass. The TP has been chosen representatively. The microphone frequency spectra behave as expected, being of greater sound intensity with increasing distance from bypass nozzle. It is clearly noticeable that some microphones are miscalibrated and have lower a SPL than the neigh- bouring microphones, because their SPL shows a constant offset over all narrow bands. The x/dBypass position of these microphones is marked by black triangles in Fig. 3.8. This miscalibration may result from human error on the one hand, the calibration files containing the calibration constants show no anomalies for the deviating microphones, however. On the other hand, the microphones were ex- posed to demanding flow conditions and were thus of robust construction. DuePDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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