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Research Papers

J. Turbomach. 2018;140(8):081001-081001-13. doi:10.1115/1.4040550.

The effects of water ingestion on the performance of an axial flow compressor are experimentally studied with and without endwall treatment. The background to the work is derived from the assessment of airworthiness for an aero-engine. The stability-enhancing effects with endwall treatments under rain ingestion are not previously known. Moreover, all the endwall treatments are designed under dry air conditions in the compressor. Water ingestion at 3% and 5% relative to the design mass flow proposed in the airworthiness standard are applied to initially investigate the effects on the performance under smooth casing (SC). Results show that the water ingestions are mainly located near the casing wall after they move through the rotor blade row. The pressure rise coefficient increases, efficiency declines, and torque increases under the proposed water ingestion. The increase of the inlet water increases the thickness of the water film downstream the rotor blade row and aggravates the adverse effects on the performances. Subsequently, three endwall treatments, namely circumferential grooves, axial slots, and hybrid slots–grooves, are tested with and without water ingestion. Compared with no water ingestion, the circumferential grooves basically have no resistance to the water ingestion. The axial slots best prevent the drop of the pressure rise coefficient induced by water ingestion, and hybrid slots–grooves are the second-best place owing to the contribution of the front axial slots. Therefore, the hybrid slots–grooves can not only extend the stall margin with less efficiency penalty compared with axial slots, but also prevent rain ingestion from worsening the compressor performance.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(8):081002-081002-9. doi:10.1115/1.4040678.

This paper aims at underlining the existence of some generic properties of windmilling flows, partially spread in the literature but never clearly stated. Two kinds of axial machines are investigated from compressor mode to highly loaded windmill: a conventional fan with poor turbine performance and an optimized fan able to reach high efficiencies in both compressor and windmilling operations. Both simulations and experiments are used to perform the analysis. Three particular behaviors were identified as typical of fans operating at windmill: the inverse stacking of the speed lines visible in (Π, Qm) turbine maps, the appearance of a slope change on the loading-to-flow coefficient diagram at windmill, and a threshold effect occurring at highly loaded windmill.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(8):081003-081003-12. doi:10.1115/1.4040551.

A novel theoretical model of the internal flow field in multistage axial compressors based on an eigenvalue approach is developed, in order to predict the onset of acoustic resonance in aircraft engines. Using an example high-speed four-stage compressor, it is shown that one of the resultant frequencies is in excellent agreement with the experimental data in terms of acoustic resonance. On the basis of the computed natural frequency of the whole compression system and the measured spanwise distribution of static pressure, the location of the acoustic excitation source can be found in the third stage. Unsteady flow simulations of the full annulus of this stage reveal two criteria for acoustic excitation at the rotor-blade tip, reversed flow near the suction surface and flow impingement on the pressure surface. Additionally, a fast Fourier transform of the unsteady pressure field at the upper rotor-blade span verifies the existence of the computed unstable frequency of the oscillating tip leakage flow. Using this novel theory, which combines a theoretical calculation of flow-instability frequency of the global system with the computational simulation of a single stage, the onset mechanism and location of the excitation source of acoustic resonance in multistage turbomachinery can be explained at acceptable computational cost.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(8):081004-081004-11. doi:10.1115/1.4040113.

Current design-cycle Reynolds-averaged Navier–Stokes (RANS) based computational fluid dynamics (CFD) methods have the tendency to over-predict corner-stall events for axial-flow compressors operating at off-design conditions. This shortcoming has been demonstrated even in simple single-row cascade configurations. Here we report on the application of hybrid RANS/large eddy simulation (LES), or detached eddy simulation (DES), for simulating the corner-stall data from the linear compressor cascade work conducted at Ecole Centrale de Lyon. This benchmark data set provides detailed loss information while also revealing a bimodal behavior of the separation which, not surprisingly, is also not well modeled by RANS. The hybrid RANS/LES results presented here predict bimodal behavior similar to the data only when special treatment is adopted to resolve the leading-edge endwall region where the horseshoe vortex (HV) forms. The (HV) is shown to be unstable, which produces the bimodal instability. The DES simulation without special treatment or refinement in the HV region fails to predict the bimodal instability, and thus the bimodal behavior of the separation. This, in turn, causes a gross over-prediction in the scale of the corner-stall. The HV region is found to be unstable with rolling of the tertiary vortex (TV) over the secondary vortex and merging with the primary HV. With these flow dynamics realized in the DES simulations, the corner stall characteristics are found to be in better agreement with the experimental data, as compared to RANS and standard DES approaches.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(8):081005-081005-10. doi:10.1115/1.4040110.

Discrepancies between rig tests and numerical predictions of the flutter boundary for fan blades are usually attributed to the deficiency of computational fluid dynamics (CFD) models for resolving flow at off-design conditions. However, as will be demonstrated in this paper, there are a number of other factors, which can influence the flutter stability of fan blades and lead to differences between measurements and numerical predictions. This research was initiated as a result of inconsistencies between the flutter predictions of two rig fan blades. The numerical results agreed well with rig test data in terms of flutter speed and nodal diameter (ND) for both fans. However, they predicted a significantly higher flutter margin for one of the fans, while measured flutter margins were similar for both blades. A new set of flutter computations including the whole low-pressure system was therefore performed. The new set of computations considered the effects of the acoustic liner and mistuning for both blades. The results of this work indicate that the previous discrepancies between CFD and tests were caused by, first, differences in the effectiveness of the acoustic liner in attenuating the pressure wave created by the blade vibration and second, differences in the level of unintentional mistuning of the two fan blades. In the second part of this research, the effects of blade mis-staggering and inlet temperature on aerodynamic damping were investigated. The data presented in this paper clearly show that manufacturing and environmental uncertainties can play an important role in the flutter stability of a fan blade. They demonstrate that aeroelastic similarity is not necessarily achieved if only aerodynamic properties and the traditional aeroelastic parameters, reduced frequency and mass ratio, are maintained. This emphasizes the importance of engine-representative models, in addition to accurate and validated CFD codes, for the reliable prediction of the flutter boundary.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(8):081006-081006-11. doi:10.1115/1.4040564.

In turbomachinery, the steady adjoint method has been successfully used for the computation of derivatives of various objective functions with respect to design variables in gradient-based optimization. However, the continuous advances in computing power and the accuracy limitations of the steady-state assumption lead toward the transition to unsteady computational fluid dynamics (CFD) computations in the industrial design process. Previous work on unsteady adjoint for turbomachinery applications almost exclusively rely upon frequency-domain methods, for both the flow and adjoint equations. In contrast, in this paper, the development the discrete adjoint to the unsteady Reynolds-averaged Navier–Stokes (URANS) solver for three-dimensional (3D) multirow applications, in the time-domain, is presented. The adjoint equations are derived along with the adjoint to the five-stage Runge–Kutta scheme. Communication between adjacent rows is achieved by the adjoint sliding interface method. An optimization workflow that uses unsteady flow and adjoint solvers is presented and tested in two cases, with objective functions accounting for the transient flow in a turbine vane and the periodic flow in a compressor three-row setup.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(8):081007-081007-11. doi:10.1115/1.4040679.

A major concern for new generations of large turbine blades is forced and self-excited (flutter) vibrations, which can cause high-cycle fatigue (HCF). The design of friction joints is a commonly applied strategy for systematic reduction of resonance amplitudes at critical operational conditions. In this paper, the influence of geometric blade design parameters onto the damped system response is investigated for direct snubber coupling. A simplified turbine blade geometry is parametrized and a well-proven reduced-order model for turbine blade dynamics under friction damping is integrated into a 3D finite element tool-chain. The developed process is then used in combination with surrogate modeling to predict the effect of geometric design parameters onto the vibrational characteristics. As such, main and interaction effects of design variables onto static normal contact force and resonance amplitudes are determined for a critical first bending mode. Parameters were found to influence the static normal contact force based on their effect on elasticity of the snubber, torsional stiffness of the airfoil and free blade untwist. The results lead to the conclusion that geometric design parameters mainly affect the resonance amplitude equivalent to their influence on static normal contact force in the friction joint. However, it is demonstrated that geometric airfoil parameters influence blade stiffness and are significantly changing the respective mode shapes, which can lead to lower resonance amplitudes despite an increase in static contact loads. Finally, an evolutionary optimization is carried out and novel design guidelines for snubbered blades with friction damping are formulated.

Commentary by Dr. Valentin Fuster

Errata

J. Turbomach. 2018;140(8):087001-087001-1. doi:10.1115/1.4040565.

There is a mistake in Eq. (5) which could misguide the reader; the correct formulation is

(P̃i+Rei2/2)(Rei+1/Rei)(P̃i+1+Rei+12/2)(SRiAR)(P̃h+SRi2Rei2/2)=f¯L/DhRei2/2

Commentary by Dr. Valentin Fuster

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