10. Resultados
10.4. Priorización de los controles
(a) 20 blocks,t= 0.566. (b) 32 blocks,t= 1.698.
(c) 52 blocks,t= 2.83. (d) 60 blocks,t= 11.32.
Figure 5.12: Perturbation pressure contours, m = 12, n = 2, k = 20. The computing load is distributed over 4 processors, which are denoted by different coloured areas.
Figure 5.12 illustrates the process of regridding and dynamic load balancing within four processors which are represented by different coloured areas. It can be seen that the computational load is redistributed evenly among processors along with the sound propagation and radiation. In Figure 5.13 the parallel speedup performance of the AMR code is compared with the parallel speedup performance of the SotonLEE code
Processors S p e e d u p 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8
Figure 5.13: The parallel speedup of the case of spinning mode sound radiation from an aero-engine intake with the sideline condition: (–) ideal speedup; (4) the result of the SotonLEE code; (¦) the result of the AMR code.
on a Beowulf PC cluster that consists of twenty-eight processors distributing on seven nodes connected by a Gigabit Ethernet. It is discovered that the communication cost of the AMR code is generally one to three times higher than the communication cost of the SotonLEE code. The cost is mainly contributed by the expensive communication cost of the AMR ghost construction operation. That operation consists of a lot of memory movements and network communications. Its performance is then limited by the present memory and network technology. Therefore, the speedup performance of the AMR method deteriorates slightly along with the increase of the communications. More precisely, the performance of the computation and communication costs with respect to both mean flow configurations (stationary medium and sideline condition) are recorded by the Complete System Performance Monitor [154], a graphical tool monitoring the system performance under Linux environment. Table 5.2 presents the results working on one to eight processors. When the processor number is less than eight, each node will have one working AMR process. Otherwise, the first node will have two working AMR process. To save space, only information on the first processor on the PC cluster is given. It is indicated that the communication costs for both case studies are roughly the same for the reason that the transported flow
solutions,e.g. (ρ0, u0, v0, w0, p0), are of the same size. In addition, both communication
costs grow slightly as the processors number increases from two to four. Furthermore, when the processors number reaches eight, there are two AMR processes working on the first node coincidentally so that the communication cost of the first node jumps abruptly accordingly. It subsequently affects the parallel speedup performance. In the meantime, the computation costs in Table 5.2 reveal that the case study with the stationary mean flow is simpler than the sideline case in terms of computational complexity. However, both case studies with different mean flow fields, as mentioned before, have similar communication costs. For that reason the parallel speedup of the first case study with the stationary mean flow is worse than the second case study with the sideline condition.
Table 5.2: The parallel communication and computation costs of the AMR compu- tation of the aero-engine intake radiation.
1CPU 2CPUs 4CPUs 8CPUs Communication costs of
Stationary mean flow N.A. 7.06MB 8.90MB 27.10MB Sideline mean flow N.A. 7.01MB 7.68MB 27.05MB Computation costs of
Stationary mean flow 610s 500s 469s 477s Sideline mean flow 1040s 555s 310s 270s
5.4
Summary
In this chapter the AMR method is applied to the prediction of spinning mode sound radiation from a generic engine intake. To model curved geometries, the AMR code is extended to support body-fitted grids. Both the explicit filter and artificial selec- tive damping methods are applied to absorb spurious waves generated in the AMR computation. Their effects are compared and the filter method is shown to be the preferred method for the problem studied here. The accuracy of the AMR method is demonstrated by the predicted far-field directivity, which agrees well with the LEE result computed on a uniformly fine mesh and the FEM result. In terms of com- putation efficiency, the adaptively refined mesh represents a saving of up to 40% compared with a uniform mesh. Relied on MPI, the computation loads are shown to be distributed evenly within the processors by MPI. However, the relevant com- munication cost increases along with the increase of the number of processors. The
parallel speedup performance deteriorates accordingly. In order to attain a higher efficiency on the current parallel machines, it is suggested to separate the parallel communication of the ghost construction operation from the other AMR operations to obtain an optimal performance in the future work.
Acoustic Radiation from Engine
Exhaust Duct
This chapter studies a spinning mode sound radiation from a generic aero-engine exhaust duct. The governing equations used here are extend acoustic perturbation equations (APE). The reason of using APE rather than the original LEE is explained in the second section along with the associated numerical issues. The AMR method is still applied in the computation. The associated mesh adaption procedure is dis- played and discussed in the third section. The near- and far-field solutions are then presented. Finally, a summary is given.
6.1
Introduction
In the case of radiation from either a bypass exhaust duct or a core nozzle, there are issues associated with the presence of a background mean flow with a shear layer between the exhaust flow and the external stream. Once again, Figure 5.1 displayed in the previous chapter shows the problem. Refractive effects due to the presence of a sheared flow may change noise radiation pattern. The physical process is still governed by the Navier-Stokes equations. As mentioned before, a full numerical solution of noise generation, propagation and radiation process using the Navier- Stokes equations is not feasible due to limited computational resources. However, in the duct downstream of the rotor-stator region of an aero-engine, where nonlinear and viscous noise generation effects are minimal, the propagation of the rotor-stator noise can be studied using the inviscid linearised equations about the mean flow.
Block-structured AMR has been applied to studying the radiation of spinning modes from a unflanged duct and aero-engine intake duct problems to establish far-
field directivities in the earlier chapters. The same work was also reported in [41, 97]. The results of AMR were verified by comparing to both analytical solutions and FEM results. In this chapter the block-structured AMR is applied to the general case of noise radiation from a realistic high bypass ratio engine exhaust geometry with a back- ground mean flow. A computational model used to determine the propagation and radiation of acoustic waves is outlined firstly. The computational scheme described here allows acoustic waves, propagating inside the bypass duct of a generic aircraft engine, to be admitted into a computational domain that comprises the aft duct sec- tion, the exit plane of the duct and the jet flow immediately downstream. The wave admission is realised through an absorbing non-reflecting boundary treatment which admits incoming waves and damps spurious waves generated by the numerical solu- tions. The exhaust geometry is axisymmetric and the mean flow axisymmetric with no swirl component. The acoustic disturbances are represented by a Fourier series in the circumferential direction. Subsequently, the wave propagation and diffraction can be calculated through solutions of LEE, Eq. (3.7), using a range of high-order schemes [126].
However, hydrodynamic shear layer instabilities associated with the presence of the sheared background mean flow induce unstable solutions in the computation of LEE, corrupting the desired acoustic solutions. To stabilize the solutions, it is a com- mon practice to remove some mean shear terms containing∂u0/∂rin LEE, Eq. (3.7).
The approach was validated against Munt’s analytical solution of semi-infinite duct radiation problem [155] in previous works [123, 126]. Further tests against other comparable methods are necessary on realistic geometry and flow conditions.
A set of new governing equations, APE [156, 157], are used in this work as an alternative way to validate the previous approach in computing problems with a sheared mean flow. To solve the aero-engine case problem, APE have been extended to the cylindrical coordinates. In short, the solutions of APE are compared to the previous solutions of LEE [126] through a case study of single spinning mode sound radiation from a generic engine bypass duct. The far-field directivity is estimated via an integral surface solution of the FW–H equation [118]. More details are given below.