Effect of an adverse pressure gradient on the streamwise Reynolds stress profile maxima in a turbulent boundary layer

It is widely accepted that in a turbulent boundary layer （TBL） with adverse pressure gradient （APG） an outer peak usually appears in the profile of streamwise Reynolds stress. However, the effect of APG on this outer peak is not clearly understood. In this paper, the effect of APG is analysed using the numerical and experimental results in the literature. Because the effect of upstream flow is inherent in the TBL, we first analyse this effect in TBLs with zero pressure gradient on flat plates. Under the individual effect of upstream flow, an outer peak already appears in the profile of streamwise Reynolds stress when the TBL continues developing in the streamwise direction. The APG accelerates the appearance of the outer peak, instead of being a trigger.

Detailed Analysis of the Flow in the Inducer of a Transonic Centrifugal Compressor

Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. A detailed analysis of the flow in the inducer (i.e. the entry zone of the impeller between the main blade leading edge and the splitter blade leading edge) is proposed from choke to surge. Steady and unsteady simulations were performed using the code elsA, which uses a multi-domain approach on structured meshes and solves the compressible RANS equations, associated with a two-equation turbulence model k-l in the rotating frame of reference. The 1MW LMFA-ECL test rig was used for carrying out the tests in the compressor stage. Unsteady pressure measurements up to 150 kHz and Laser Doppler Anemometry measurements were performed in the inducer. A good agreement is obtained between the experimental and numerical data even if an over dissipation is noticed in the numerical results. The change in flow pattern from choke to surge is mainly due to a change in the tip leakage flow trajectory which straightens, leading to a flow blockage of an individual passage near shroud. A spectral analysis shows that only the blade passing frequency and its harmonics compose the various spectra obtained from choke to surge.

Numerical Investigation of Flutter Stability in Subsonic Space Turbine Blisk with Emphasis on Cut-on/Cut-off Modes and Interblade Phase Angles

The so-called blisks,i.e.integrally bladed disks,are characterized by very low viscous material damping and make the flutter prediction much more critical.In that framework,a two-dimensional numerical study of a space turbine blisk featuring complex deformation of blades and high eigenfrequency(>40kHz)is performed.The simulations are based on unsteady Reynolds Averaged Navier Stokes computations linearized in the frequency domain and consist in the superposition of an unsteady linear(in time)pressure field,generated by a harmonic perturbation,upon a steady nonlinear(in space)flow.The aerodynamic damping coefficient is calculated over a range of nodal diameters,and the blades are predicted aeroelastically stable.However,violent changes occur and are rather critical since sudden and large deviations in stability appear.In that context,the nature of the waves propagating from the cascade are evaluated.Such an approach provides fundamental knowledge about the perturbations which can either propagate to the far-field(cut-on mode)or decay(cut-off mode).It is expected that the ability of the flow to damp or to amplify the blade motion is strongly affected by the way unsteady perturbations are transferred from the cascade to the far-field.The nature of the waves are first assessed from the aforementioned linearized results,then they are evaluated analytically and finally compared.A good agreement is found despite the strong assumptions of the analytical model.The results show a clear correlation between the cut-on/cut-off conditions and stability.The least stable configuration corresponds to cut-off mode at the inlet and no wave at the outlet.Without outgoing waves from the cascade,the blade is prone to be less stable:the energy from the blades vibration is necessarily dissipated or sent out by the cascade.

An Extraction of the Dominant Rotor-Stator Interaction Modes by the Use of Proper Orthogonal Decomposition (POD)

合适的直角的分解方法在转子定子以内被用于即时速度地一台高速度的高压的离心的压缩机的内部排的区域。处理数据来自实验和数字模拟。与 Fourier 变换比较， POD 为两块起始的地给最好的形式的近似，以在要考虑的模式的任何给定的数字上表示的精力：到速度地的活动范围 98% 全部的精力，邮政部门模式的要求的数字比 Fourier 泛音的数字低多达九倍。单个邮政部门模式被给并且证明不稳定的转子定子相互作用已经是在第一个模式的现在。关键词转子定子相互作用 - 合适的直角的分解 -

Temporal Behaviour of a Corner Separation in a Radial Vaned Diffuser of a Centrifugal Compressor Operating near Surge

The temporal behaviour of a flow separation in the hub-suction side corner of a transonic diffuser is studied thanks to unsteady numerical simulations based on the phase-lagged approach. The validity of the numerical results is confirmed by comparison with experimental unsteady pressure measurements. An analysis of the instantaneous skin-friction pattern and particles trajectories is presented. It highlights the topology of the separation and its temporal behaviour. The major result is that, despite of a highly time-dependent core flow, the separation is found to be a "fixed unsteady separation" characterized by a fixed location of the main saddle of the separation but an extent of the stall region modulated by the pressure waves induced by the impeller-diffuser interaction.

Modal Decomposition for the Analysis of the Rotor-stator Interactions in Multi-stage Compressors

A modal analysis method of the rotor-stator interactions in multistage compressors has been developed by LMFA.This method,based on a double modal decomposition of the flow over space and time,has been applied to nu-merical and experimental results of the high-speed 3?-stage compressor CREATE based at LMFA,Lyon-France.It reveals the presence of a very strong rotor-stator interaction which completely drives the flow at casing behind all the rotors.This modal analysis method applied to an unsteady RANS simulation permits to calculate the en-ergy of the rotor-stator interactions and to plot energetic meridian maps to explain experimental results and to analyze the interaction in the whole machine.

Topological Studies of Three-dimensional Flows in a High Pressure Compressor Stator Blade Row without and with Boundary Layer Aspiration

This paper presents a numerical study of the flow topologies of three-dimensional （3D） flows in a high pressure compressor stator blade row without and with boundary layer aspiration on the hub wall. The stator blade is representative of the first stage operating under transonic inlet conditions and the blade design encourages development of highly complex 3D flows. The blade has a small tip clearance. The computational fluid dynamics （CFD） studies show progressive increase of hub corner stall with the increase in incidence. Aspiration is implemented on the hub wall via a slot in the comer between the hub wall and the suction surface. The CFD studies show aspiration to be sensitive to the suction flow rate; lower rate leads to very complex flow struc- tures and increased level of losses whereas higher rate renders aspiration effective for control of hub comer separation. The flow topologies are studied by trace of skin friction lines on the walls. The nature of flow can be explained by the topological rules of closed separation. Furthermore, a deeper analysis is done for a particular case with advanced criterion to test the non-degeneracy of critical points in the flow field.

Unsteady Pressure Measurements in a High-Speed Centrifugal Compressor

This paper presents the unsteady data acquisition system used to measure the pressure field in high speed compressors.Details and electronic sketches are given for the conditioners developed in-house that have been used to amplify and to filter the pressure signal with the aim of acquiring data up to 150 kHz.A discussion of the experimental results carried out in a centrifugal compressor is proposed.Through different processing of the pressure signals and a comparison with URANS simulations,the excitation of the pressure transducers by the pressure waves generated by shock waves that occur between the impeller and the diffuser is highlighted.The levels of pressure fluctuations measured when entering into surge are also presented and reveal very repetitive behaviour of the flow instabilities.

全场离散Tophat过滤操作的快速算法

Tophat是一种常见的过滤器,但是在实际计算机应用中,较大过滤尺度的全场过滤操作效率很低。本文针对全场离散Tophat过滤操作设计了新型快速算法,分别在三维和二维情形下给出了算法描述,在三维情形下,将普通运算的复杂度O(n^3△~3)降为O(n^3);二维情形下,将普通运算的复杂度O(n^2△~2)降为O(n^2),即复杂度与过滤尺度无关,只与过滤场的大小有关,该算法可极大提高过滤计算的效率,在一些大规模数据库(如Johns Hopkins大学的湍流数据库)服务中具有广泛的应用前景。