The impedance boundary condition for acoustics in swirling ducted flow

The acoustics of a straight annular lined duct containing a swirling mean flow is considered. The classical Ingard–Myers impedance boundary condition is shown not to be correct for swirling flow. By considering behaviour within the thin boundary layers at the duct walls, the correct impedance boundary condition for an infinitely thin boundary layer with swirl is derived, which reduces to the Ingard–Myers condition when the swirl is set to zero. The correct boundary condition contains a spring-like term due to centrifugal acceleration at the walls, and consequently has a different sign at the inner (hub) and outer (tip) walls. Examples are given for mean flows relevant to the interstage region of aeroengines. Surface waves in swirling flows are also considered, and are shown to obey a more complicated dispersion relation than for non-swirling flows. The stability of the surface waves is also investigated, and as in the non-swirling case, one unstable surface wave per wall is found

On the Kutta condition for the sound transmission through outlet guide vanes

The present work proposes an analytical formulation including a Kutta condition for the acoustic wave-scattering by a linear cascade of outlet guide vanes for turbomachinery applications. The Kutta condition has an increasing effect on the structure and on the magnitude of the acoustic response of the cascade as the Mach number of the mean flow is increased. The present analytical approach is believed a consistent alternative to numerical methods, especially at the early design stage. The analytical formulation relies on a mode-matching technique. The methodology is applied in a two-dimensional context and for an oblique acoustic wave. Results, focusing on the radiated acoustic power and the pressure fields, highlight the effect of the Kutta condition for various Mach numbers

Experimental validation of a cascade response function for fan broadband noise predictions

International audienc

Fonctions de réponse de grille d'aubes et effet d'écran pour le bruit à large bande des soufflantes

LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Broadband noise prediction of fan outlet guide vane using a cascade response function

International audienc

On a uniformly valid analytical rectilinear cascade response function

International audienceThis paper extends an existing analytical model of the aeroacoustic response of a rectilinear cascade of flat-plate blades to three-dimensional incident vortical gusts, by providing closed-form expressions for the acoustic field inside the inter-blade channels, as well as for the pressure jump over the blades in subsonic flows. The extended formulation is dedicated to future implementation in a fan-broadband-noise-prediction tool. The intended applications include the modern turbofan engines, for which analytical modelling is believed to be a good alternative to more expensive numerical techniques. The initial model taken as a reference is based on the Wiener-Hopf technique. An analytical solution valid over the whole space is first derived by making an extensive use of the residue theorem. The accuracy of the model is shown by comparing with numerical predictions of benchmark configurations available in the literature. This full exact solution could be used as a reference for future assessment of numerical solvers, of linearized Euler equations for instance, in rectilinear or narrow-annulus configurations. In addition, the pressure jump is a key piece of information because it can be used as a source term in an acoustic analogy when the rectilinear-cascade model is applied to three-dimensional blade rows by resorting to a strip-theory approach. When used as such in a true rectilinear-cascade configuration, it reproduces the exact radiated field that can be derived directly. The solution is also compared to a classical single-airfoil formulation to highlight the cascade effect. This effect is found important when the blades of the cascade overlap significantly, but the cascade solution tends to the single-airfoil one as the overlap goes to zero. This suggests that both models can be used as the continuation of each other if needed

On the use of a uniformly valid analytical cascade response function for fan broadband noise predictions

International audienc