Fan Optimization Under Distorted Boundary Layer Ingesting Flow

Aviation sustainability is a significant consideration in the designs of future generation aircraft. One way to achieve aviation sustainability is by reducing the overall drag acting on the aircraft. One approach being researched to achieve this goal is the implementation of boundary layer ingesting (BLI) technique. BLI technique has been shown by many researchers around the globe to be able to significantly improve the overall aircraft performance by reducing the drag and noise generated by the aircraft. However, inlets designed to ingest boundary layers tend to develop a large region of low-pressure flow which can drastically harm the overall performance of an engine placed behind the distorted flow. The success of BLI techniques heavily depends on the design of an engine fan that can deliver high aerodynamic performance even under the influence of the distorted inlet flow. The study of this paper outlines a procedure to achieve this goal by utilizing CFD-based design optimization methods. The study was broken up into three primary phases which iterated the design of an initial baseline geometry with the primary objective function being the adiabatic efficiency of the fan. The results of this study show that the optimized fans can improve the adiabatic efficiency by around 4–5% when compared to the baseline design.

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.

This paper presents the recent investigations of the Boundary Layer Ingestion (BLI) concept conducted at ONERA. Both numerical and experimental investigations of a BLI propulsion system are conducted synergistically with the objectives of preparing and validating both the numerical simulations tools and the experimental methodologies that will be necessary for the design of a future transport aircraft incorporating a BLI propulsive system. A test rig dedicated to the investigation of BLI has been specifically designed manufactured and tested in the ONERA L1 wind tunnel in Lille in low-speed conditions. This experimental rig embeds an electrically powered fan and several measurements capabilities including separate measurements of the forces acting on the different model components, thus allowing a in depth investigations and quantification of the mechanisms of BLI. Numerical simulations of this test rig, based on RANS CFD calculations using the Actuator Disk (AD) approach to model the effect of the fan absorbing a boundary layer flow have been used, first to help defining the experimental test rig and then, using the experimental results to establish and validate a numerical simulation approach which could be used in the future to facilitate the integration of a BLI propulsion system on a future transport aircraft.

Boundary layer ingestion (BLI) propulsion offers notable benefits for blended wing body (BWB) aircraft, and understanding the interrelated effects of BLI and propulsion–airframe integration (PAI) is critical for early-stage design decisions. This study numerically applies combined momentum- and energy-based analyses to a closely coupled but non-integrated BWB–turbofan configuration enabling a continuous transition from non-BLI to BLI conditions. By introducing an idealized capture streamtube–airframe interaction force, the drag of BLI layout is decomposed into additional and external components, enabling quantification of a lift-to-drag ratio improvement of 1.7–2.6, corresponding to a 7.14–8.27% gain in power saving coefficient (PSC). Additional drag reduction, the primary contributor to total drag savings, is analytically attributed to inlet total pressure loss. The resulting decrease in required thrust under BLI shows strong mathematical correlation with jet dissipation reduction, revealing an intrinsic link between drag reduction and power saving. PAI exerts a significant influence on the BLI benefits, including nacelle cowl drag penalties, significant variations in shock wave location and strength, and notable suppression of both boundary layer and wake dissipation for the portion of cowl immersed in the airframe wake. These findings inform the transition from podded to BLI engine layouts.

A distributed fan propulsion system can potentially achieve a 10%-20% fuel burn reduction compared with traditional podded engines. However, the ducted fan is continuously working under inflow distortion conditions caused by boundary layer ingestion, thus resulting in an increase of aerodynamic loss and decreasing the aircraft overall aerodynamic benefit achievement. To control the loss in the boundary layer ingestion ducted fan, a loss reduction design strategy based on the lip-installed bump is developed. The computational results show that the lip-installed bump can effectively suppress the intensity of the distorted inflow with a 9.7% reduction at the rotor inlet. The reason is that a high-pressure area will be presented on the mount of bump and a low-pressure on both sides of the bump compression surface which pushes the low-momentum flow in the boundary layer away from lip. In this way, the bump energizes the low-momentum flow on the downer wall of the lip and reduces the local boundary layer momentum loss thickness. In the meanwhile, as the distortion intensity is decreased near the downer wall at rotor inlet, the local blade load is also reduced, especially near the blade tip. Further, the decrease of the blade load near the tip leads to a reduction of the normal pressure difference between the pressure and suction surfaces which can eliminate the interaction of the main flow and the tip leakage flow, resulting in a 6% decrease of flow loss in the rotor blade tip region. In comparison to the baseline ducted fan, the fan with a lip-installed bump has achieved a 1.3% enhancement of adiabatic efficiency at the re-design point under inflow distortion condition, without a sacrifice of total pressure ratio.

Recent innovative engine integration systems present interesting prospects for considerable aircraft fuel burn reduction. In this context, the Airbus Nautilius is a patented engine integration concept embedding two Ultra High Bypass Ratio (UHBR) turbofans at the rear of the fuselage so as to take the full benefit of the boundary layer ingestion (BLI). This concept presents very promising power savings compared to classical podded configurations when neglecting the penalty on fan performance. However, as the engine is now working in distorted boundary layer flow conditions, these benefits can be partially offset by an additional fan performance penalty. In order to assess these additional losses and therefore to refine the potential of this concept, a numerical fan design activity has been conducted by Onera on behalf of and in collaboration with Airbus. First, blade design modifications have been applied to a pre-existing UHBR fan stage originally designed for “classical” podded configurations in order to maximize the isentropic efficiency in cruise conditions. The results of this first activity shows that more than half of the fan isentropic efficiency penalty due to the distortion at the engine inlet can be recovered by few design iterations. Secondly, an operability assessment at take-off high-lift conditions has been conducted on the optimized geometry and ensures that the fan stall margin remains acceptable. In conclusion, these results confirm the initial potential and mark an additional step towards a broader assessment of the Nautilius engine integration concept, which shall be further completed by acoustic and aeroelastic studies.

<div class="htmlview paragraph">A region with floor boundary-layer suction upstream of the vehicle to remove the oncoming boundary layer is often used in automotive wind tunnels. These suction systems inevitably change the empty-tunnel pressure gradient. In this paper, the empty-tunnel pressure gradient created by the use of boundary layer suction and its effect on measured drag are investigated. By using excess suction - more suction than necessary to remove the floor boundary layer – it was possible to show experimentally that the major part of the drag increase due to boundary layer suction is created by unintended gradient effects. Only a minor part of the drag increase is due to the increased flow velocities at the lower parts of the vehicle, or in other words, due to the improved ground simulation. A theoretical model, using the concept of horizontal buoyancy to predict the gradient effect, is proposed. The model is compared to the experimental results as well as to CFD calculations. The use of the analytical correction method to correct for the unintended gradient effects is expected to be a significant improvement for results obtained using boundary layer suction. Alternative methods to control the boundary layer thickness, which do not introduce strong empty-tunnel gradients, are recommended.</div>

PurposeThis research aims to present an aero-propulsive interaction model applied to conceptual aircraft design with distributed electric propulsion (DeP). The developed model includes a series of electric ducted fans integrated into the wing upper trailing edge, taking into account the effect of boundary layer ingestion (BLI). The developed model aims to estimate the aerodynamic performance of the wing with DeP using an accurate low-order computational model, which can be easily used in the overall aircraft design's optimization process.Design/methodology/approachFirst, the ducted fan aerodynamic performance is investigated using a low-order computational model over a range of angle of attack required for conventional flight based on ducted fan design code program and analytical models. Subsequently, the aero-propulsive coupling with the wing is introduced. The DeP location chordwise is placed at the wing's trailing edge to have the full benefits of the BLI. After that, the propulsion integration process is introduced. The nacelle design's primary function is to minimize the losses due to distortion. Finally, the aerodynamic forces of the overall configuration are estimated based on Athena Vortex Lattice program and the developed ducted fan model.FindingsThe ducted fan model is validated with experimental measurements from the literature. Subsequently, the overall model, the wing with DeP, is validated with experimental measurements and computational fluid dynamics, both from the literature. The results reveal that the currently developed model successfully estimates the aerodynamic performance of DeP located at the wing trailing edge.Originality/valueThe developed model's value is to capture the aero-propulsive coupling accurately and fast enough to execute multiple times in the overall aircraft design's optimization loop without increasing runtime substantially.

This paper considers the application of two types of performance accounting systems to a boundary layer ingesting (BLI) propulsion configuration to identify features that arise due to propulsion-airframe integration. To do this, a canonical, two-dimensional BLI configuration consisting of a fuselage element propelled by an integrated ducted propulsor has been developed, and aerodynamic forces and power for a range of angle of attack and propulsor mass flow has been calculated using MSES, a viscous-inviscid flow solver for multi-element airfoils. In a traditional thrust-drag accounting, BLI manifests as a negative “interference drag,” equal to 46% of the isolated fuselage element drag at zero angle of attack and design mass flow. The results also show that accounting for changes in vehicle angle of attack before changes in throttle setting reduces variation in the throttle-dependent force and power increments with respect to the angle of attack, suggesting that such an approach may be appropriate for BLI configurations. The second type of accounting system was the Power Balance Method, which has been suggested to be more suitable for the analysis of integrated geometries with BLI. The results show that the boundary layer, wake, and jet dissipation of the BLI configuration vary with configuration angle of attack and mass flow, a behavior that must be considered when using power-based accounting. The analysis also highlights the difficulty in defining separate contributions to viscous dissipation from adjacent surface wake and jet flows that are inherent to BLI configurations.

Influence of coupled boundary layer suction and bowed blade on flow field and performance of a diffusion cascade

(ABSTRACT) In 2005, NASA released plans of next generation commercial airplane for 2030, with a crossdisciplinary effort on: reduced fuel consumption, aviation reliability, fundamental noise reduction and shorter take-off length. Meeting these requirements will need a fundamental shift in aircraft and engine design. Turboelectric distributed propulsion system was chosen to achieve these targets. Different from traditional turbofan, distributed propulsion system employs a large number of fans embedded on upper surface of the airframe and two turbogenerators at wing tip. This novel configuration benefits from boundary layer ingestion and distributed fans to achieve higher bypass ratio but lower fuel burn. The N3-X hybrid-wing-body is used as a baseline aircraft for the study. This paper gives basic simulation methods, as well as computational models for turboelectric distributed propulsion system. Initially, a boundary layer ingesting model has been built from computational results for embedded propulsor at different inlet conditions. In a further step, a weight estimation model of propulsors was concluded to estimate propulsors’ weight and size. Then, thermal cycle model was built to calculate engine’s performance at both design point and off design conditions. Finally, effects of boundary layer ingestion on the propulsion system were examined. The boundary layer ingesting model showed mass-average inlet pressure and Mach number are function of flight Mach number and fan inlet mass flow, on the N3-X airframe. The weight estimation model shows the overall system weight decreased with increased number of propulsors, which also caused total inlet width of propulsors increasing. So for a given total inlet width, the propulsor should be used as many as possible to reduce weight. Thermal cycle results show that fan shaft speed should be chosen as high as possible before reaching the fan tip speed limitation, and fan pressure ratio (FPR) between 1.3 and 1.35 yields minimum thrust specific fuel consumption (TSFC) at the aerodynamic design point. A fan pressure ratio of 1.3 is chosen for its potential effects on noise control. In the end, a turboelectric distributed engine was simulated to satisfy NASA N+3 subsonic commercial airplane goals.

Propulsors with boundary layer ingestion (BLI) generate a propulsive force with lower flow power input than conventional engines. This aerodynamic benefit can be traced back to its sources: reductions in jet, surface, and wake dissipation. A framework for BLI analysis is developed based on the power balance method: parametric expressions are derived for the net streamwise force on an aircraft and for the mechanical flow power required, as well as relations for the dissipation components. They illustrate the range of possible comparisons between BLI and non-BLI engine installations, and show how the benefit varies with design choices. Applying the framework to wind tunnel data from powered D8 aircraft model tests, the sources of aerodynamic BLI benefit are quantified for a realistic transport aircraft configuration. With the same propulsors (equal nozzle area) BLI reduces cruise power by 8.2%, of which 5.2% comes from a reduction in jet dissipation, 2.4% from reduced surface dissipation, and 0.6% from reduced wake dissipation. The jet dissipation reduction is equivalent to a 3.4 percentage points increase in propulsive efficiency. If the installations are compared at equal mass flow, the benefit amounts to a 9% lower cruise power.

This article describes the conceptual design of a turbo-electric, single-aisle, mid-range aircraft with tail-integrated distributed propulsors for boundary layer ingestion (BLI). Six tail-BLI propulsors, driven by 0.5 MW electric motors, are installed on the aircraft, with a nonaxisymmetric aft fuselage and a T-tail, in addition to two underwing turbofan engines. The estimated fuel burn benefit is 8.5% in payload range fuel consumption at a cruise Mach number of 0.8, compared to the baseline. The nonaxisymmetric aft fuselage creates swirl into the tail-BLI propulsors, with the fan rotational direction chosen for co-swirl to reduce incidence. The number and size of the tail propulsors are set by the fuel burn trade, between maximizing boundary layer kinetic energy defect ingestion and minimizing propulsion system weight. The propulsor fan pressure ratio is set by the trade between propulsor weight and propulsive efficiency. The article presents a rigorous assessment of the potential for fuel burn reduction with tail-integrated, BLI, distributed propulsors and gives guidelines concerning the design features that enable this improvement in fuel burn. The distributed aft fuselage BLI propulsion system is suggested to offer a relevant pathway for tube-and-wing configurations with a potential major advancement in fuel burn reduction.

Steady, linear hydromagnetic boundary layer at the free surface of a rotating baroclinic fluid is analysed under Boussinesq approximation. The flow is driven by the buoyancy forces which exist due to the density variations associated with baroclinicity. Meridional circulations of mass flux and electric current flux occur due to the boundary layer suction as well as due to the hydromagnetic effects. The boundary layer suction dominates and the dynamics of the flow reduce to that of nonmagnetic case for α2<O(E), whereE is the Ekman number and α2 (=σB 0 2 /ϱΩ) is the magnetic interaction parameter, which represents the ratio of hydromagnetic to Coriolis forces. When α2>O(E), meridional circulation induced in the interior due to hydromagnetic effects is stronger than that can be supported by the boundary layer. The electric current flux through the interior is stronger for any value of α than that can be supported by the boundary layer. Further it is argued that hydromagnetic effects suppress the laminated flow (∂v/∂z≠0) in the interior. As a result, the meridional circulations are strong when α=O(1).

When creating new or modernizing existing aircraft models, a lot of attention is paid to the economic indicators of their operation. A parameter characterizing these indicators is the fuel consumption of gas turbine engines. In turn, the value of this parameter is significantly influenced by the external aerodynamic resistance of the aircraft and, in particular, the nacelle of the engine. The formed boundary layer on the surface of the nacelle directly affects its aerodynamic resistance. This study was devoted to determining the influence of the boundary layer suction from the outer surface of the engine nacelle of the gas generator of a bypass engine with a rear fan attachment. The suction of the boundary layer is carried out through the holes in the surface of the gas generator nacelle under the influence of the pressure drop created by the sleeve of the vane crown of the fan contour. The boundary layer is sucked into the internal cavity of the nacelle and enters the inlet into the fan contour of the turbofan attachment, where it participates in creating the internal thrust of the turbofan attachment. The influence of the boundary layer suction on the external resistance was evaluated on the basis of known methods for calculating turbulent jets and boundary layer theory. During the simulation of the flow on the surface of models of engine nacelles with boundary layer suction and the determination of the external resistance of the engine nacelle, the suction coefficient was used, which depends on the geometric dimensions of the suction holes and the flow parameters on the surface of the engine nacelle and in the suction channels. The simulation was performed in the range from 0 to 1. At the coefficient of suction of the boundary layer , the resistance of the power unit reached the smallest value and was determined by the resistance of the fan attachment. A comparative analysis of the external resistance of engine nacelles with the conventional configuration of a bypass turbojet engine (with a front fan arrangement) and an engine nacelle with a rear turbofan attachment was also carried out. As previous studies have shown, the effect of applying a suction of the boundary layer on the surface of the nacelle of the gas generator also depends on the degree of bypass ratio of the turbofan attachment.