Study of flow instability in turbine engine compressors is crucial to understand the inception and evolution of engine stall. Aerodynamics experts have been working on detecting the early signs of stall in order to devise novel stall suppression technologies. A state-of-the-art Navier-Stokes based, time-accurate computational fluid dynamics simulator, TURBO, has been developed in NASA to enhance the understanding of flow phenomena undergoing rotating stall. Despite the proven high modeling accuracy of TURBO, the excessive simulation data prohibits post-hoc analysis in both storage and I/O time. To address these issues and allow the expert to perform scalable stall analysis, we have designed an in situ distribution guided stall analysis technique. Our method summarizes statistics of important properties of the simulation data in situ using a probabilistic data modeling scheme. This data summarization enables statistical anomaly detection for flow instability in post analysis, which reveals the spatiotemporal trends of rotating stall for the expert to conceive new hypotheses. Furthermore, the verification of the hypotheses and exploratory visualization using the summarized data are realized using probabilistic visualization techniques such as uncertain isocontouring. Positive feedback from the domain scientist has indicated the efficacy of our system in exploratory stall analysis.
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.
The development of the plastic and viscoelastic properties and the corresponding failure limits of the acetal homopolymer polyoxymethylene were studied in unidirectional cyclic fatigue. Samples with molecular weights (MWs) ranging from 90 to 142 kg/ kmol were tested in displacement-controlled conditions, resulting in maximum stress amplitudes between 30 and 59 MPa and strain amplitudes between 35.8 and 92.6 le. The zero-hour material properties and the cycle-dependent property development were predominantly dictated by deformations in the crystalline regions and showed a negligible dependence on MW. However, the final failure limits were found to be primarily dependent on the length of the amorphous tie chains that connect the crystallites. As such, fatigue life analysis showed a strong dependence on MW. Results are interpreted in light of the primary mechanical failure mechanisms and the corresponding molecular deformations.
Boundary layer ingestion has significant potential to reduce fuel burn in aircraft engines. However, designing a fan that can operate in an environment of continuous distortion without aeromechanical failure is a critical challenge. Capturing the requisite aeromechanical flow features in a high-fidelity computational setting is necessary in validating satisfactory designs as well as determining possible regions for overall improvement. In the current work, a three-dimensional, time-accurate, Reynolds-averaged Navier-Stokes computational fluid dynamic code is utilized to study a distortion-tolerant fan coupled to a boundary layer ingesting inlet. The comparison between this coupled inlet-fan and a previous fan-only simulation will provide insight into the changes in aeromechanic response of the fan blades. Additionally, comparisons to previous wind tunnel tests are made to provide validation of inlet distortion as seen by the distortion-tolerant fan. A resonant crossing was also investigated for the 85% speed operational line condition to compare resonant response between the inlet-fan, fan-only, and experiment. A decrease in maximum tip displacement is observed in the forced response of the coupled inlet-fan compared to the fan-only simulation. The predicted maximum tip displacement was still below the upper limit on the range observed in the wind tunnel tests but matched well with the average tip displacement value of 27.6 mils. A single mode was chosen at the 100% speed condition to provide insight into the effects that the inlet duct has on fan stability. Near stall and near choke conditions were also simulated to observe how the changes of progressing along the speed line affects flutter stability prediction. The analysis shows the fan has low levels of aerodynamic damping at all the conditions tested. However, the coupled inlet-fan shows a decrease in the level of aerodynamic damping over what was observed with the fan-only simulation. Some of the blades experienced single cycles of negative aerodamping which indicate a possibility of increased blade vibration amplitude but were followed by positive aerodamping cycles. Work is continuing to understand possible sources to account for the differences observed between the two simulation cases as well as with the experiment.
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