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

Abstract

Accurate prediction of the blade vibration stress is required to determine overall durability of fan blade design under Boundary Layer Ingestion (BLI) distorted flow environments. Traditional single blade modeling technique is incapable of representing accurate modeling for the entire rotor blade system subject to complex dynamic loading behaviors and vibrations in distorted flow conditions. A particular objective of our work was to develop a high-fidelity full-rotor aeromechanics analysis capability for a system subjected to a distorted inlet flow by applying cyclic symmetry finite element modeling methodology. This reduction modeling method allows computationally very efficient analysis using a small periodic section of the full rotor blade system. Experimental testing by the use of the 8-foot by 6-foot Supersonic Wind Tunnel Test facility at NASA Glenn Research Center was also carried out for the system designated as the Boundary Layer Ingesting Inlet/Distortion-Tolerant Fan (BLI 2 DTF) technology development. The results obtained from the present numerical modeling technique were evaluated with those of the wind tunnel experimental test, toward establishing a computationally efficient aeromechanics analysis modeling tool facilitating for analyses of the full rotor blade systems subjected to a distorted inlet flow conditions. Fairly good correlations were achieved hence our computational modeling techniques were fully demonstrated. The analysis result showed that the safety margin requirement set in the BLI 2 DTF fan blade design provided a sufficient margin with respect to the operating speed range.

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

“…The blade vibration mode shape and frequency specific to a selected nodal diameter pattern are used in each flutter calculation. For mode 1, there is no significant change in the mode shape and frequency with the nodal diameter 20 . As the nodal diameter pattern changes, a comparison of the blades with the largest displacements indicates that the mode shapes remain very nearly the same as illustrated in Fig.…”

“…Figure 14 shows that there is a significant change in the mode shape for mode 2 with the nodal diameter pattern. The changes in mode shape and frequency with nodal diameter pattern are a result of the blade-disk dynamics captured by the structural model that includes the blade airfoil, platform, dovetail attachment and disk sector modeled with cyclic symmetry boundary conditions 20 . Note the differences in the shapes of the contour lines as well as the range of values particularly for the 1 ND pattern.…”

“…The fan blade forced response near a resonance is dominated by a single mode unless the modes are closely spaced. Since the lower modes of the DTF fan blade are well separated from each other [20][21][22] , it is not necessary to include other modes in the resonant response analysis. Further, the BLI distortion produces aerodynamic excitations that act as (backward) traveling waves on the fan blades and excite only specific blade modes with particular nodal diameter patterns when the dynamics of blade to blade differences in structural characteristics (mistuning) is not included in the formulation.…”

A Review of Distributed Electric Propulsion Concepts for Air Vehicle Technology

“…The aforementioned STARC-ABL aircraft is another notable aircraft concept under development that extensively leveraged BLI benefits, where an electric fan at the aft section of aircraft ingests the fuselage-generated boundary-layer flow, similar to the propulsion system commonly found in submarines [21]. Experimental and numerical work has also been performed to assess the effects of BLI on propulsion system performance and inlet distortion problems [38]. Finally, the E-Thrust and Lilium aircraft propulsion systems also utilize BLI as a performance improving mechanism.…”

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].…”