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Chapter3. Implementation 38 given data framework, i.e. sideline conditions with neither flight stream nor ad- ditional broadband or tonal sound excitation, as the velocity of the exiting jets is maximal there (TP DLR 317). The SODIX calculation time can be estimated using a proprietary script that sums up the calculation time needed for each source point in every narrow band fre- quency within the simulation range. The time needed is based on Eq. 3.1, where the lower bound of summation fl is the starting frequency, the upper bound fu is the end frequency and a(TOB(i)) is the number of source points of the TOB for index frequency i. The frequency steps ∆i = 5,95 Hz result from FFT analysis and correspond to the width of a narrow band in the scope of this experiment. It has to be noted that the calculation time is merely an estimation and may vary depending on the actual simulation. fu tcalc = ∑ tpoint · a(TOB(i)) (3.1) i=fl One approach to optimise computation time is to shorten the grid for higher fre- quencies. Michel and Siller [23] observed, based on studies by Viswanathan and Czech [35], that the maximal SPL can be found for St = 0.5 . . . 1, with hot jets av- eraging at St = 0.7. A similar prediction was done by Zaporozhets et al [38]. and Bryce and Stevens [5], who located the SPL maximum at St = 1.0. According to Michel [23], the maximum moves upstream to the nozzle outlet towards higher frequencies and further downstream for lower frequencies. Generally, if the fre- quency is doubled, the distance of the SPL maximum from the nozzle is reduced by half and vice versa. For the test calculation, the SPL maximum was found to be at x/dBypass = 5 for St ≈ 1 as expected. Based on these observations, multiple iterations of weighted grids were implemented in order to reduce the number of source positions for higher frequencies (e.g. Fig. A.3 and A.4). However, all weighted grids showed problems for higher frequencies, because aliasing effects occur downstream of the original sound source. Due to the shortened source grid length for high TOB, these effects were no longer correctly registered by SODIX, because the algorithm localizes the total sound energy from outside the com- putational domain at the borders of the source grid. The SPL of the appearing clusters can exceed that of the main sound source, resulting in distorted plots, see Fig. 3.11. There is an in-depth explanation of the aliasing effect in Sect. 4.1.PDF Image | Investigation of wing installation effects on the sound field of a model jet engine
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