Ewert, R.

Siefert, M.

Institute of Transport Research:Publications

ON THE USE OF 4D SYNTHETIC TURBULENCE IN ADETERMINISTIC APPROACH TO COMPUTATIONALAEROACOUSTICSRoland Ewert, Malte SiefertDeutsches Zentrum fu¨r Luft- und Raumfahrt e.V., DLRInstitut fu¨r Aerodynamik und Stro¨mungstechnikTechnische AkustikLilienthalplatz 7, 38108 Braunschweig, Germanyroland.ewert@dlr.de, malte.siefert@dlr.deBroadband noise generated by turbulent motion and especially enhanced in the vicinity of rigid surfaces ex-hibiting sharp edges is of great technical concern. A typical problem is the sound generated at a high-lift wingof a transport aircraft, which involves low subsonic flow speed at a Reynolds number based on the wing chordlength exceeding 10 millions. A great demand exists in industry for the treatment of this type of problems withaeroacoustic design methods that allow for a reliable prediction of acoustic deltas due to geometrical shape andflow changes at turnaround times in the range of less than one day for the computation of each configuration.Large Eddy Simulation has become a promising tool to resolve the unsteady turbulent flow underlying theaeroacoustics sound generation mechanism up to moderate Reynolds numbers. Direct as well as hybrid methodshave been considered for the propagation of the generated sound field into the far-field. However, taking intoaccount the constraints imposed on design methods, it appears rather challenging to apply LES in the earlydesign process within the next 10 years.Alternatively to the time accurate resolution of turbulence via unsteady flow simulation, in physics the use ofsynthetic turbulence with deliberately imposed characteristics is considered a valuable and mature way to studyturbulence properties, e.g. the implications a distinct turbulence mechanism has for a specific turbulent flowproblem. We apply a deterministic CAA approach where the unsteady sound sources are completely prescribedfrom a 4D space-time model of synthetic turbulence. RANS modeling is used as a simplified approach to modelthe time averaged turbulent flow problem, which specifies the one- and two-point statistics to be realized bysynthetic turbulence. Such a deterministic approach is able to match the tight constraints imposed on a designmethodology in the industrial environment with todays computational resources.The model of 4D synthetic turbulence was introduced in recent works. The Fast Random Particle Mesh(FRPM) involves random particles that move in a way over a background mesh such that very efficientlyunsteady turbulence is generated on the mesh. A rich variety of different turbulence characteristics can berealized, such as inhomogeneous and anisotropic fields, where the axis of main Reynolds stresses as well as mainintegral length scales are allowed to be oriented differently, the convection property of turbulence, a turbulencetime de-correlation characteristic that obeys the sweeping hypothesis, as well as different shapes of turbulencespectra. Target values for the local length scale tensor and the turbulent stress tensor can be realized veryaccurately. Based on these essential prerequisites it becomes now possible to give a mature answer as to whichprediction quality can be expected from a RANS based aeroacoustic prediction methodology.Predictions of the sound generated at a high-lift system will be presented. One essential effect is the scaling ofsound intensities with changing flow velocity. We show that this Mach number scaling follows a power law withexponent 4.x for the slat noise problem, which is in very close agreement with experimental findings. Further-more, the shape of the predicted narrow band spectra matches those of corresponding validation experiments.The capability to predict the effect of geometrical shape variations is validated with experimental results.

On the use of 4D synthetic turbulence in a deterministic approach to computational aeroacoustics

Abstract

Similar works

Full text

Available Versions

Institute of Transport Research:Publications