Efficient Hybrid CAA Method for Jet Noise Prediction of Isolated Nozzles

Kurzfassung

The present work deals with the jet noise prediction of isolated nozzles using the means of computational aeroacoustics (CAA). The axisymmetry of the isolated nozzles permits to exploit the Fourier series decomposition in azimuthal direction and thus to achieve higher computational efficiency. Instead of 3-D computation, a limited number of azimuthal modes are solved in the complex 2-D space. In the post-processing, all of the computed azimuthal modes are summed up to one total spectrum, which is equivalent to a spectrum in 3-D space. The results of this approach have demonstrated the capability to deliver representative spectra for specific nozzle geometries. The CAA computations solve the acoustic near-field of the jet for the first azimuthal modes, the number and order of which is defined by the user. In the performed computations azimuthal modes up to an order of ten are considered which were found to represent the spectra sufficiently up to the highest frequencies of technological interest. The entire process-chain consists of CFD solver TAU solving the RANS solution, the RPM code generating the acoustic sources, the PIANO code solving the sound propagation and eventually the extrapolation to the far-field with the APSIM+ code (Ffowcs-Williams & Hawkings method) as an optional post-processing tool. The extrapolation to the far-field is required for a direct comparison of the computed spectra with the measurements, which are usually available as far-field spectra. On the other hand, in combination with the extrapolation the computational effort for the near-field solution can be significantly reduced to the vicinity of the jet flow. The proposed method has been implemented in the PIANO/RPM code and subsequently verified and validated in combination with the fine-scale jet mixing noise source model of Tam & Auriault. For the consideration of jet configurations with heated exhaust, an extended variant of the cold fine-scale noise model, which has been proposed by Tam, Pastouchenko and Viswanathan is included in the same approach. The nozzle geometry of a single stream jet from the EU project JEAN has been used for the verification/validation. Considering the cases with more practical relevance, the once established method has been applied to different dual-stream nozzle geometries. The used geometries as well as the corresponding experimental spectra were provided by Rolls-Royce Deutschland. The well matching comparisons between the computations and measurements have proven the capability of the approach to deliver accurate results for complex nozzle geometries, too. Judging on the basis of the performed simulations, altogether the applied methodology has shown an efficient computational performance for the isolated single stream nozzles as well as for the complex geometries such as dual-stream nozzles with/without co-flow. The advantage hereby is that a large number of different nozzle geometry configurations can be predicted within short term as long the required RANS solutions are available. From the side of the CAA computation, the same CAA grid can be reused for varying nozzle geometries exchanging only the RANS solution and the input data for RPM. For configurations with different nozzle diameter, the grid can be scaled to the changed diameter retaining the non-dimensional grid resolution. Thus, it is well appropriate for industrial application as an aeroacoustical design tool for nozzle geometry optimization.