A numerical model for the exhaust noise radiation problem is presented. In the model, it is assumed that an incoming wave is propagating through the exhaust nozzle, or the fan duct, and radiating outside. The near-field propagation is based on the solution of the linearized Euler equations in the frequency domain: for each wave number, a linearized Euler problem is solved using a finite element method on unstructured grids for arbitrarily shaped axisymmetric geometries. The frequency-domain approach enables the suppression of the Kelvin-Helmholtz instability waves. Moreover, each single calculation, limited to a single frequency, is well suited to the exhaust noise radiation problem in which the incoming wave can be treated as a superposition of elementary duct modes. To reduce the memory requirements, a continuous Galerkin formulation with linear triangular and quadrangular elements is employed and the global matrix inversion is performed with a direct solver based on a parallel memory distributed multifrontal algorithm for sparse matrices. The acoustic near field is then radiated in the far field using the formulation of Ffowcs Williams and Hawkings. Numerical calculations for a validation test case, the Munt problem, and two turbomachinery configurations are compared with analytical solutions and experimental data.

A quasi-three-dimensional, blade-to-blade, time-accurate, viscous solver was used for a three-stage LP turbine study. Due to the low Reynolds number, transitional computations were performed. Unsteady analyses were then carried out by varying the circumferential relative position of consecutive vanes and blade rows to study the effects of clocking on the turbine's performance. A clocking strategy developed in order to limit the number of configurations to be analyzed is discussed. The optimum analytically-determined clocking position is illustrated for two different operating conditions, referred to as cruise and takeoff. The effects of clocking on wake interaction mechanisms and unsteady blade loadings is presented and discussed. For low Reynolds number turbine flows, the importance of taking transition into account in clocking analysis is demonstrated by a comparison with a fully turbulent approach.

A quasi–three–dimensional, blade–to–blade, time–accurate, viscous solver w as used for a three–stage LP turbine study Due to the low Reynolds number, transitional computations were performed. Unsteady analyses were then carried out by varying the circumferential relative position of consecutive vanes and blade rows to study the effects of clocking on the turbine’s performance. A clocking strategy developed in order to limit the number of configurations to be analyzed is discussed. The optimum analytically–determined clocking position is illustrated for two different operating conditions, referred to as cruise and takeoff. The effects of clocking on wake interaction mechanisms and unsteady blade loadings is presented and discussed. For low Reynolds number turbine flows, the importance of taking transition into account in clocking analysis is demonstrated by a comparison with a fully turbulent approach.

This paper reports the findings of a flutter investigation on a low-pressure turbine rotor having an integrally machined tip shroud with different type of constraints. Two types of tip shroud constraints, namely fully constrained and tangentially free, were used, representing two extreme conditions: (a) a typical integrated shroud design with a tight interlocking and no wear on contact surface; and (b) an extremely smooth contact surface design or the most severe wear of a fully constrained interface, or changes in the manufacturing process that result in almost no friction across the shroud surface. The tangentially free constraint is unusual in that it seeks to explore how sensitively the contact constraint could cause blades to response. The mode shapes and corresponding frequency characteristic are presented for both shroud constraints using a standard finite element analysis. The flutter analysis was firstly undertaken by considering all vibration modes of interest in a single calculation using a whole-annulus model of the rotor. It was found that the removal of the tip constraints in the tangential direction was responsible for introducing the unstable first flap family under condition of zero mechanical damping. Of considerable interest was the fact that instability in the first flap mode occurred in forward-travelling nodal diameter modes, which is considered as somewhat different from classical low-pressure turbine flutter where instability exists in backward-travelling nodal diameter modes. The flutter mechanism was verified by undertaking a detailed investigation on the forward-travelling nodal diameter modes of the first flap family using a single-passage analysis. It was concluded that tip shroud constraints are highly sensitive for turbine blade interlock designs and unusual response could be excited under extremely severe wear condition.

Wake control by boundary layer suction has been applied to a high-lift low-pressure turbine blade with the intention of reducing the wake velocity defect, hence attenuating wake-blade interaction, and consequently the generation of tonal noise. The experimental investigation has been performed in a large scale linear turbine cascade at midspan. Two Reynolds number conditions (Re = 300000 and Re = 100000), representative of the typical operating conditions of the low pressure aeroengine turbines, have been analyzed. Boundary layer suction has been implemented through a slot placed in the rear part of the profile suction side. The suction rate has been varied in order to investigate its influence on the wake reduction. Mean velocity and Reynolds stress components in the blade to blade plane have been measured by means of a two-component crossed miniature hot-wire. The wake shed from the central blade has been investigated in several traverses in the direction normal to the camber line at the cascade exit. The traverses are located at distances ranging between 5 and 80% of the blade chord from the blade trailing edge. To get an overall estimate of the wake velocity defect reductions obtained by the application of boundary layer suction, the integral parameters of the wake have been also estimated. Moreover, spectra of the velocity fluctuations have been evaluated to get information on the unsteady behaviour of the wake flow when boundary layer suction is applied. The results obtained in the wake controlled by boundary layer suction have been compared with the results in the baseline profile wake at both Reynolds number conditions for the purpose of evaluating the control technique effectiveness. The removal of boundary layer through the slot in the rear part of the profile suction side has been proved to be very effective in reducing the wake shed from the profile. The results show that a reduction greater than 65% of the wake displacement and momentum thicknesses at Re = 300000, and a reduction greater than 75% at Re = 100000 can be achieved by removal of 1.5% and 1.8% of the single passage through flow, respectively.