Aeromechanics Analysis of a Distortion-Tolerant Fan with Boundary Layer Ingestion

Abstract

A propulsion system with Boundary Layer Ingestion (BLI) has the potential to significantly reduce aircraft engine fuel burn. But a critical challenge is to design a fan that can operate continuously with a persistent BLI distortion without aeromechanical failure -flutter or high cycle fatigue due to forced response. High-fidelity computational aeromechanics analysis can be very valuable to support the design of a fan that has satisfactory aeromechanic characteristics and good aerodynamic performance and operability. Detailed aeromechanics analyses together with careful monitoring of the test article is necessary to avoid unexpected problems or failures during testing. In the present work, an aeromechanics analysis based on a three-dimensional, time-accurate, Reynolds-averaged Navier Stokes computational fluid dynamics code is used to study the performance and aeromechanical characteristics of the fan in both circumferentially-uniform and circumferentially-varying distorted flows. Pre-test aeromechanics analyses are used to prepare for the wind tunnel test and comparisons are made with measured blade vibration data after the test. The analysis shows that the fan has low levels of aerodynamic damping at various operating conditions examined. In the test, the fan remained free of flutter except at one near-stall operating condition. Analysis could not be performed at this low mass flow rate operating condition since it fell beyond the limit of numerical stability of the analysis code. The measured resonant forced response at a specific low-response crossing indicated that the analysis under-predicted this response and work is in progress to understand possible sources of differences and to analyze other larger resonant responses. Follow-on work is also planned with a coupled inlet-fan aeromechanics analysis that will more accurately represent the interactions between the fan and BLI distortion.

Conceptual aero-structural design of a fan stage under distorted flow

“…The maximum amplitude of 20° in swirl variation is considered, respectively, for as a high swirl distortion cases. The distortion intensity used in this test case is similar to the larger intensities observed in Plas et al, 46 Yao et al, 47 Bakhle et al, 23,22 Giulaini, 48 and Ferrar. 49 The distorted flow profile used in this paper (high total pressure distortion intensity and high swirl) will be denoted as the P3S3 case.…”

“…The unsteady pressure loading on each blade acts as an external forcing. 23 The periodic unsteadiness excites the fan at a certain loading frequency. When the excitation frequency comes close to one of the blade’s natural frequencies during rotation, resonance may occur.…”

“…A limited number of studies have performed structural analysis of BLI fans through finite element analysis software or actual experimentation. Bakhle et al 22,23 performed aeromechanics analysis of a BLI fan, where they used 3-D RANS CFD for aerodynamic computation. For the structural analysis, the loads were obtained from the aerodynamic analysis, the root of the blade was completely constrained, and the attachment was not included in modeling.…”

Cyclic Symmetry Finite Element Forced Response Analysis of a Distortion Tolerant Fan with Boundary Layer Ingestion

“…This computation also provided information about areas of high stress and how the relative magnitude of these stresses varies with frequency. Finally, section C, a cyclic symmetry finite element analysis model incorporated with Turbo aerodynamic analysis computer program [10] and Ansys structural dynamics computer program was developed to compute the forced response stress distribution in fan blade and as a result the blade structural safety margins for the targeted DTF blade design requirements are presented in section D. While all the equations in the following sections are represented in the cyclic symmetry coordinates, added symbols to the equations are avoided for a brevity in the descriptions.…”

Laser Displacement Measurements of Fan Blades in Resonance and Flutter During the Boundary Layer Ingesting Inlet and Distortion-Tolerant Fan Test

“…These modes were of interest to the aeroelasticity researchers since predictions had been made for these modes prior to the test. These aeroelasticity results are given in Bakhle [9] and Min [10].…”