Leonidas Siozos-Rousoulis

Analytical acoustic pressure gradient prediction for moving medium problems

In this paper, an acoustic pressure gradient formula capable of accounting for constant uniform flow effects is suggested. Acoustic pressure gradient calculation is key for acoustic scattering problems, because it may be used to evaluate the hardwall boundary condition. Realistic cases of rotating machines may be evaluated in a moving frame of reference and as such, an acoustic pressure gradient formula capable of accounting for constant uniform flow effects finds significant application. A frequency domain formulation was thus derived for periodic noise source motion located in a moving medium. The suggested formula is mathematically compact and easy to implement. It may offer us significant advantages when tonal noise emissions are dominant, thus finding application potential in acoustic scattering problems in rotating machines in a constant uniform flow. Moreover, the formula contains no Doppler factor, thus facilitating noise prediction for sources in supersonic motion.

A time-domain Kirchhoff formula for the convective acoustic wave equation

Kirchhoff’s integral method allows propagated sound to be predicted, based on the pressure and its derivatives in time and space obtained on a data surface located in the linear flow region. Kirchhoff’s formula for noise prediction from high-speed rotors and propellers suffers from the limitation of the observer located in uniform flow, thus requiring an extension to arbitrarily moving media. This paper presents a Kirchhoff formulation for moving surfaces in a uniform moving medium of arbitrary configuration. First, the convective wave equation is derived in a moving frame, based on the generalized functions theory. The Kirchhoff formula is then obtained for moving surfaces in the time domain. The formula has a similar form to the Kirchhoff formulation for moving surfaces of Farassat and Myers, with the presence of additional terms owing to the moving medium effect. The equation explicitly accounts for the influence of mean flow and angle of attack on the radiated noise. The formula is verified by analytical cases of a monopole source located in a moving medium.

Acoustic Velocity Formulation for Kirchhoff Data Surfaces

This paper deals with the derivation of an analytical time-domain formulation for the prediction of the acoustic velocity field generated by moving bodies in a medium at rest, according to the Kirchhoff method. The present formulation can be implemented in acoustic pressure codes based on the Farassats Kirchhoff formula for arbitrary moving bodies, thus allowing direct and fast calculation of the acoustic velocity field in scattering problems. For validation purposes, four test cases are considered, namely a three-dimensional monopole, dipole and quadrupole source, as well as a monopole in uniform flow. Comparison of the results with the analytical solutions proves the remarkable accuracy of the present formulation.

A time-domain method for prediction of noise radiated from supersonic rotating sources in a moving medium

This paper presents a time-domain method for noise prediction of supersonic rotating sources in a moving medium. The proposed approach can be interpreted as an extensive time-domain solution for the convected permeable Ffowcs Williams and Hawkings equation, which is capable of avoiding the Doppler singularity. The solution requires special treatment for construction of the emission surface. The derived formula can explicitly and efficiently account for subsonic uniform constant flow effects on radiated noise. Implementation of the methodology is realized through the Isom thickness noise case and high-speed impulsive noise prediction from helicopter rotors.

On the refinement of the rotation rate based Smagorinsky model using velocity field gradients

In this paper, a gradient-based refinement of the rotation rate-based Smagorinsky (RoSM) subgrid scale (SGS) model is presented. The refined model satisfies the Galilean invariance condition generally, without any assumptions. The suggested model retains the advantages offered by the original RoSM, thus being simple and efficient. It provides a Smagorinsky model constant that is always positive, with low fluctuations in space and time, without the need for any numerical stability control algorithms. The validity of the proposed SGS model is shown through three test cases, namely, a turbulent channel flow, subcritical flow past a stationary cylinder, and a spatially developing free round jet. The refined RoSM provides comparable results with the dynamic Smagorinsky, while matching well to reference data. The refined RoSM is shown to be computationally efficient, being 20% faster than the dynamic Smagorinsky model.