Aircraft concepts using boundary layer ingestion (BLI) are promising future flight systems aiming at reduced mission fuel burn. In this scenario the fan operates at inhomogeneous inflow conditions around the circumference. The accurate calculation of the resulting flow phenomena using uRANS approaches is highly time-consuming and therefore not suitable for fan design at early stages of the design process. In this paper, we introduce a throughflow based design methodology for BLI. The methodology includes the detailed design of the annulus geometry, the design of rotor and stator blades as well as the performance assessment in the BLI scenario. The throughflow approach is extended and calibrated to cover major BLI flow physics. The calibration data set consists of 12 fans with different fan pressure ratios (FPR) and meridional Mach numbers. The new design methodology is applied to design three specific fans and the effect of radial FPR is investigated. All designs are verified using 3D CFD uRANS approaches and the results indicate that either ascending or constant radial FPR should be favored. In addition an exploration study is conducted using the design methodology. This study covers 72 fan stages in the parameter space tip speed and meridional Mach number. The results suggest that there is a favorable tuple of design parameters to design a distortion tolerant fan by trading tip speed and meridional Mach number. In the end the newly gained knowledge is transferred to the DLR UHBR fan in order to improve the tolerance to BLI.

Due to a high degree of complexity and computational effort, overall system simulations of jet engines are typically performed as 0-dimensional thermodynamic performance analysis. Within these simulations and especially in the early cycle design phase, the usage of generic component characteristics is common practice. Of course these characteristics often cannot account for true engine component geometries and operating characteristics which may cause serious deviations between simulated and actual component and overall system performance. This leads to the approach of multi-fidelity simulation, often referred to as zooming, where single components of the thermodynamic cycle model are replaced by higher-order procedures. Hereby the consideration of actual component geometries and performance in an overall system context is enabled and global optimization goals may be considered in the engine design process. The purpose of this study is to present a fully automated approach for the integration of a 3D-CFD component simulation into a thermodynamic overall system simulation. As a use case, a 0D-performance model of the IAE-V2527 engine is combined with a CFD model of the appropriate fan component. The methodology is based on the DLR in-house performance synthesis and preliminary design environment GTlab combined with the DLR in-house CFD solver TRACE. Both, the performance calculation as well as the CFD simulation are part of a fully automated process chain within the GTlab environment. The exchange of boundary conditions between the different fidelity levels is accomplished by operating both simulation procedures on a central data model which is one of the essential parts of GTlab. Furthermore iteration management, progress monitoring as well as error handling are part of the GTlab process control environment. Based on the CFD results comprising fan efficiency, pressure ratio and mass flow, a map scaling methodology as it is commonly used for engine condition monitoring purposes is applied within the performance simulation. Hereby the operating behavior of the CFD fan model can be easily transferred into the overall system simulation which consequently leads to a divergent operating characteristic of the fan module. For this reason, all other engine components will see a shift in their operating conditions even in case of otherwise constant boundary conditions. The described simulation procedure is carried out for characteristic operating conditions of the engine.

Civil aviation is aiming at fuel efficient aircraft concepts. Propulsion systems using boundary layer ingestion (BLI) are promising to reach this goal. The focus of this study is on the DLR UHBR fan stage of a tube and wing aircraft with rear-integrated engines. In this integration scenario the propulsion system and especially the fan stage receives distorted inflow in steady-state flight conditions. The distortion pattern and distortion intensity are dependent on the operating conditions. Consequently, the interaction of the fan and the distortion changes over the flight envelope. The first part of the paper aims at gaining knowledge of the BLI fan performance in the operating points end of field, approach, cruise (CR) and top of climb (TOC) using high-fidelity, unsteady RANS approaches. The analysis includes fan map performance metrics and a deeper insight into the flow field at CR and TOC. The preliminary design of a fan stage requires fast turn-around times, which are not fulfilled by high-fidelity approaches. Therefore, a fast, throughflow-based methodology is developed, which enables aerodynamicists to design distortion-tolerant fans. The main characteristics of the methodology is outlined in the second part. Consequently, the methodology is taken advantage of to investigate parameter sensitivities in terms of tip speed, blade thickness, solidity, the annulus geometry and a non-axisymmetric stator. This study suggests that distortion-tolerant fans should be designed at higher tip speeds than conventional design experience recommends to limit the local operating point excursion.

The design process of modern aircraft engines extensively makes use of computational fluid dynamics (CFD). During the design process fan, compressor and turbine stages are typically evaluated using single passage CFD-simulations. For the design of an inlet distortion tolerant fan, the inhomogeneity of the inflow in circumferential direction does not allow the application of periodic boundary conditions and hence single passage simulations. Accordingly transient full annulus simulations have to be applied to correctly resolve the underlying physical interactions. The associated computation time for such a high fidelity simulation prohibit its application in today’s design procedures. This paper compares the simulation results of the ingestion of a mild and generic fuselage boundary layer of a blended wing body aircraft into an UHBR engine on the basis of four different simulation methods. The four different simulations include: transient full annulus CFD, Harmonic Balance frequency domain based CFD, steady full annulus CFD (Frozen Rotor) and steady single passage CFD for each blade passage. The results of the simulations are assessed using a newly developed stream tube based post-processing procedure. For the ingestion of the fuselage boundary layer the four numerical setups result in different aerodynamic conditions in the fan stage. For different aerodynamic measures different discrepancies are encountered for the different simulation methods. While the Harmonic Balance method can reproduce most features well, the simpler models overestimate the variations in the different flow features.