Time domain impedance modelling and applications

AbstractToday, there is a high, often not fully evolved potential of noise attenuation by passive acoustic treatments. Current numerical methods are able to help developing optimal treatments. Thus, the simulation of acoustic lining in aeroengines is one of the core objectives for the development of modern CAA solvers. Here, the opportunities of the Extended Helmholtz Resonator (EHR) model of Rienstra in the time domain in this design and optimisation process are demonstrated. The optimization of a lining for a specific application as the obvious objective is still out of reach for many cases with current numerical resources. However, the model allows the optimisation towards the dissipation characteristics in an impedance flow tube measurement with a physical liner sample, which provides the numerical parameters of the liner for high fidelity CAA simulations. Moreover, the model parameters are related to the cell geometry and face sheet of the liner panel. An example is provided for the purely numerical prediction of the attenuation in the complex flow of an aeroengine duct. This is demonstrated by considering the resulting parameters in modal axisymmetric and three dimensional simulations of the rearward sound radiation from a lined bypass duct. The example demonstrates, that the optimisation of the liner properties is not achievable in a justifiable time, even if simplified two dimensional conditions are considered. A possible solution to this problem is to use the computational power of a graphics processing unit (GPU). The development of pixel shaders which implement a large number of parallel processors into the GPU, shows a much more agile growth than any CPU based system does. As an outlook, a platform independent implementation of a GPU based CAA solver with impedance boundary condition and the capability to handle axisymmetric duct geometries is presented. It demonstrates a speed up by a factor > 100