Browsing by Author "Schönrock, Olaf"

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Open Access
Numerical prediction of flow induced noise in free jets of high Mach numbers
(2009) Schönrock, Olaf; Munz, Claus-Dieter (Prof. Dr.)
A direct aeroacoustic simulation methodology is developed on the basis of the numerical schemes implemented in the commercial tool ANSYS CFX. The focus lies upon the efficient and direct numerical prediction of the flow-induced noise generated by natural gas and pneumatic applications. The respective compressed gas related components are characterized by tiny supersonic gas jets, strong noise emissions, poor accessibility by measurement techniques and excessive simulation costs in particular. Highly resolved computational grids close to DNS requirements become necessary just in order to capture the time-averaged flow profile, tiny shocks and gradients correctly. Furthermore the coexistent supersonic flow velocity results in an exceptionally small timestepping in compliance with the CFL condition, e.g. for LES aeroacoustic simulations. Considering the assumably nonlinear noise propagation and the acoustic feedback within enclosed environments the well-established hybrid approaches cannot be employed here as well. The flow and acoustics of the whole domain rather have to be captured within a single tool instead. In fact, the corresponding simulation costs inhibit the numerical prediction and reduction of the emitted noise levels for those compressed gas components at the industrial scale. In this work the test subject is a dedicated natural gas injector in an open and a confined environment and with varying boundary conditions. Specific to the injector nozzle, four under-expanded supersonic gas jets (M=1.4, Re=30000) are formed and cause a strong flow three-dimensionality. Furthermore a turbulence cluster establishes between the jets driving jet fluctuations and aeroacoustics. To enable aeroacoustic simulations in the first place, ANSYS CFX is augmented by a transient inlet boundary condition and a non-reflective farfield boundary condition based on an implicit damping sponge layer. In order to reduce the simulation costs the scale-adaptive turbulence model (SAS-SST) recently implemented in ANSYS CFX is validated for the gas injection problem and especially for CFL numbers much larger than one. Since a degrading solution quality has to be expected then a timestep study is conducted in order to detect the limit for aeroacoustic simulations. Bottom line the different turbulence modeling allows a strongly increased global timestepping such that a net simulation costs reduction by a factor of 19 compared to LES is achieved. In spite of the generally lower solution quality the predicted noise levels, spectral distributions as well as noise sensitivities are in well agreement with own experimental data. In an alternative simulation approach the research code NSDG2D is applied to a simplified 2D setup with very promising results. The more sophisticated solver numerics based on an explicit Discontinuous Galerkin scheme allows local dynamic adaption to the problem, amongst others by local timestepping and locally adaptive element orders. These features prove to be feasible especially for locally varying unsteady compressible flows and the supersonic gas injection in particular. Considering these advantages a further reasonable simulation costs reduction compared to ANSYS CFX can be projected for the 3D application as well.