Investigação numérica de escoamento e ruído gerado em corpos rombudos prismáticos.

Abstract

Many components of machines, equipments and means of transport can be represented as a bluff body whose motion through a fluid can generate noise. In this context, this thesis is focused on the study of numerical prediction of noise generated by the flow around bluff bodies. As an example of bluff body, the sound generated from flow around a circular cylinder is studied. The flow over a circular cylinder is investigated by considering the wake as tridimensional and turbulent in the subcritical regime, which is characterized by a laminar boundary layer separation. The flow over a circular cylinder is obtained by time-dependent numerical simulation considering three-dimensional computational domain. In order to cope with turbulence and three-dimensionality, the flow is solved using the Large Eddy Simulation (LES) methodology. The computational domain is discretized by the finite volume method. The noise is calculated separately using the Ffwocs Williams & Hawkings (FW-H) analogy, whose wave equation has as a source term the flow solution provided by the numerical simulation. With regard to the FW-H analogy, the acoustic pressure fluctuation is obtained in the far-field by assuming a quiescent medium between the sound sources region (near-field) and the location considered for acoustic computation. Due to the high computational cost of three-dimensional (3D) simulation, the numerical simulations were conducted with a cylinder span length limited in size, which allows taking into account part of the wake three-dimensionality. Regarding the final acoustic computation, the acoustic correction methods of Kato et al. (1993) and Seo & Moon (2007) are used in order to match the sound obtained by the short cylinder span to the correspondent sound obtained experimentally for a long cylinder span. This thesis contributed to investigate numerically the computational method of applying the Ffwocs Williams & Hawkings analogy for solving the noise generated from a threedimensional flow over a circular cylinder with high Reynolds number, particularly, at Re = 90,000 and Re = 22,000. The results show that this computational method is able to predict the far-field sound for the simulated cases, since the noise spectra obtained are found to be in agreement with the corresponding experimental data. In addition, the acoustic correction method of Kato et al. (1993) and Seo & Moon (2007) provided good predictions with regard to the adding noise computation, however, its results are dependent on accurate estimation of the spanwise coherence length of the flow. Finally, the sound spectrum obtained in the far-field is tied to the flow behavior provided by the numerical simulation, especially regarding the coherence between the pressure fluctuations over the spanwise length of the cylinder and the lift force fluctuation amplitude exerted on the cylinder wall.

Investigação numérica do ruído aerodinâmico irradiado por barras prismáticas transversais.

“…Na análise da Figura 28 verifica-se que na frequência fundamental 0 o valor somado a curva original foi em torno de 15 dB/Hz, sendo que nas demais frequências somaram-se aproximadamente 8 dB/Hz. Orselli (2012) fez o mesmo para o caso com razão de aspecto / = 6,0 e os valores de correção calculados por ele foram 8,92 dB/Hz para frequências no entorno de 0 e 4,95 dB/Hz para as demais. Nota-se que quanto maior a largura do domínio, menores são os valores das correções de Kato (1993).…”

Numerical evaluation on the effect of boss cap geometries on AUV propeller noise.

“…Lighthill (1952) developed a density-based acoustic analogy by rearranging the continuity (2.1) and momentum (2.2) equations considering the sound propagation in a uniform medium, with no sources or external forces. The following algebraic development is described according the study of Orselli (2012):…”

Otimização acústica e análise numérica do escoamento ao redor de um conjunto cilindro-placa separadora.

“…Hoje em dia, em aeronaves, houve uma grande redução do ruído proveniente das turbinas, agora comparável ao ruído causado pela estrutura do avião (ORSELLI, 2012).…”

“…Segundo Curle (1955), no caso de estruturas rombudas, como o cilindro, imersas em um escoamento com baixo número de Mach, o ruído ao longe é dominado por fontes do tipo dipolo. Tal fonte está associada à interação da esteira à jusante do cilindro com a superfície do corpo, causando uma oscilação nas forças aplicadas na estrutura e é conhecido como vortex shedding noise (ruído de esteira de vórtice, (ORSELLI, 2012) …”