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ISABE-2015-20034

COUPLED FAN AND INTAKE DESIGN OPTIMIZATION FOR INSTALLED UHBR-ENGINES WITH ULTRA-SHORT NACELLES

Rainer Schnell* and Johakim Corroyer

German Aerospace Center (DLR) Institute of Propulsion Technology

Linder Hoehe, 51147 Cologne, Germany

*contact rainer.schnell@dlr.de

Abstract The scope of this paper is to introduce a design

methodology for coupled intake and fan systems and to present results from a design optimization study, aiming at short and ultra-short inlets in combination with a fan pressure ratio FPR=1.35 fan being representative for future UHBR engines. The first part of the paper introduces the actual design methodology that has been established and summarizes aerodynamic results and challenges resulting from substantially shortening the intake from a given and conventional reference with l/D≈0.65 to l/D≈0.35. The resulting effect on the fan aerodynamic performance will be discussed based on the performance characteristics and resulting fan inflow conditions for most of the relevant fan operating conditions in the Take-Off and Cruise regime, and will also involve more severe operating conditions such as cross wind and angle of attack, where the intake geometry has a major effect by inducing strong non-homogeneities in the fan inflow and thus enforces a strong unsteady interaction with the fan. In this context, different numerical approaches such as standard RANS mixing plane, uRANS with and without phase-lagged periodic boundary conditions and novel harmonic balance (HB) techniques shall be discussed and its advantages and limitations for this kind of applications and potential application in a design context shall be reviewed. The second part of the paper summarizes results from a coupled fan and intake design optimization study. The design study was focussing on the design of the nacelle lip and intake geometry, with the objective to ideally improve or at least recover fan efficiency and aerodynamic stability for the short intake in comparison with the datum intake. In all optimizations the entire intake and fan systems was evaluated in a coupled approach, in a first step however without modifying the fan itself. The results suggest that for the short intake, fan efficiency at Cruise conditions can be slightly improved by designing the intake part without significantly compromising the fan characteristics at Take-Off conditions, but with the current optimization setup and its limitations however, increasing the intake’s sensitivity towards high incidence flow at Take-Off static conditions. Nomenclature

AoA Angle of attack BPR Bypass Ratio

BPF Blade passing frequency (harmonic) RANS Reynolds Average Navier-Stokes EO Engine order [-] FPR Fan total pressure ratio [-] HB Harmonic Balance Amp{…} Amplitude Re{…} Real part A Area [m2] Dfan Fan diameter at fan inlet [m] Dth Intake throat area [m] d,t Nacelle thickness [m] l Intake length [m] Ma Local Mach number [-] p Pressure [Pa] r Intake lip design parameter [-] u Circ. Velocity [m/s] vax Axial velocity [m/s] x,y,z Cartesian coordinates [m] α Intake lip design parameter β Flow angle in (rotor) relative system [°] ν Fan inlet hub to tip ratio [-] η Efficiency [%]

Subscripts

1 Rotor 1 Inlet 2 Rotor 1 Outlet is Isentropic t Total quantity tip Tip

Superscripts ‘ Perturbation quantity

Introduction

Background One consequence of increasing engine bypass ratio to

further improve fuel burn and emission characteristics are increasing propulsor dimensions. This will require novel nacelle and intake concepts in order to compensate for the newly introduced weight and drag penalties. In this context, shortening the intake will increasingly lead to a strongly coupled fan and intake system, with the fan presumably becoming more and more sensitive to any incoming distortion due to its decreasing pressure ratio; its efficiency as well as its

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aerodynamic stability limit are expected to be compromised significantly [20]. Classical intake design approaches typically have been considering the intake only, without taking into account any coupling between the fan and the intake flow field using correlation based [1] as well as CFD based methods [2][11] [13][14]. Most recent efforts described in literature aim at designing the intake and fan system in a more coupled manner by taking advantage of a CFD calibrated actuator disk approach simulating the fan in a more advanced fashion [12]. The effect of any inlet distortion induced by the intake on the fan is however well investigated and was subject to a number of studies in the past [4][7][9][10]. Taking into consideration the existing studies, the demand for closely coupled design methods efficiently handling the intake induced flow characteristics on fan performance and vice versa is still high and the present paper aims at contributing to this important technical question.

Scope and Paper Structure The major questions that shall be addressed in the scope

of this study can be summarized as follows: • What effect does shortening the intake has on fan

aerodynamic performance, • what are the limitations of different, RANS and

uRANS based numerical approaches for potential use in an automated design/optimization context and

• to what extend may the fan aerodynamic characteristics be at least retained or ideally improved with a fully coupled fan and intake design/optimization approach when substantially shortening the intake (compared with a state of the art and conventional intake) ?

The following sections aim at answering those questions, firstly by introducing the underlying intake design methodology which has been established. This approach has been used in order to first of all manually design a short intake which then has been assessed aerodynamically and compared with the datum one. The results will be presented in the second part, along with a method review of current RANS and uRANS based approaches at various operating conditions using DLR’s in house flow solver TRACE [5]. The final chapter summarizes results from a design study involving an automated and evolutionary based optimization strategy. Design Process Overview

Automated and CFD based optimization procedures first of all require an adequate parameterization of the problem, along with a robust mesh generation. In our context, the nacelle and intake parameterization was split into different parts: For the lip contour, an existing and proven to be efficient leading edge parameterization as shown in Figure 1 was used; this allowed for an optimization of the lip only, while continuously retaining the connections to the fixed

intake and the outer nacelle contour. For the intake itself, up to the fan inlet face, a B-Spline based approach with an arbitrary number of control points was introduced. Although the entire geometry was considered axisymmetric for the simulations at first, all parameters were defined at different positions along the circumference and hence allowed for constructing a 3D nacelle geometry featuring inlet scarfing (drooped intake), varying thickness to account for additional devices within the nacelle and non-axisymmetric intake geometries. The outer nacelle contour was kept unchanged since its contour will be determined in a next step by assessing and optimizing the engine integration into the aircraft. To efficiently mesh the intake in a most robust fashion and ensure high quality meshes during the optimization, a topology as shown in Figure 2 was realized. The shown mesh is automatically constructed around any given intake geometry, with the grid lines always emanating orthogonal from the new intake contour. The entire process chain including automated geometry and mesh generation for the intake and resulting far field was then integrated into the available and well established turbomachinery design and optimization framework (see e.g. [1]).

DLE Intake length l

Dfan

d(θ)

Far field domain

Dth

cone angle

Turbomachinery domain

ar

lip parameterization

Figure 1: Lip and B-Spline based intake parameterization and sample geometry featuring inlet scarfing and circumferentially varying thickness as well as non-axisymmetric inlet contouring; the layout of the control points for the intake as well as the turbomachinery mesh in a meridional S2-plane are also shown in the 3D sample geometry

Figure 2: Block topology and layout for automated far field and intake meshing

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Short Intake Assessment Study

Test Case Overview The propulsor considered in this study, major

specifications of which are summarized in Table 1, is a low pressure ratio fan stage resulting from a DLR internal design study. The fan was scaled and operating conditions were chosen in order to meet the chosen reference V2500 thrust requirements (see also [16]). Figure 3 shows the resulting geometry of both fan stages, the V2500 reference and Fan135, in a direct comparison. Table 1: DLR’s low pressure ratio propulsor Fan135 – specifications summary (see also [17]) Design fan total pressure ratio FPR 1.35 Fan outer diameter Dfan 2.13 m Fan face axial Mach number 0.64 Rotor hub to tip ratio ν 0.24 Fan rotor RPM @ Takeoff / Cruise 2295 / 2632 min-1 Blade number Rotor : OGV 17 : 33 Rotor tip speed utip @ Take-Off 265 m/s

Figure 3: V2500 reference (left) and DLR’s low pressure ratio fan stage Fan135 as used for the present study (right) – both geometries to scale The initial nacelle and intake geometry was also taken from V2500 reference, for which the detailed geometry of the interior and outer contour was available [16]. In a first step this geometry was simply scaled up to fit the actual Fan135 size. For this datum intake configuration, the reference non-dimensional intake length value of l/Dfan=0.65 was retained and the initial 3D geometry was simplified to be axisymmetric. A second, short intake geometry was designed with a substantially shorter non-dimensional value of l/Dfan=0.35, taking advantage of the previously introduced design methodology. Both intake geometries are shown in Figure 4 in a direct comparison, the resulting area distributions in flow direction are shown in Figure 5 with the short intake, naturally

yielding a significantly more aggressive diffusion from the throat location up to the fan face. For both intakes, identical throat areas were defined and the fan inlet diameter was kept constant.

Figure 4: Reference inlet (conventional, low BPR V2500-type) with l/Dfan≈0.65 (top) and shortened intake with l/Dfan≈0.35 (bottom) for potential application to UHBR engines with low pressure ratio propulsors

axial position x [m]

A(x

)/Ath

roat

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

area datumarea short

fan inlet

Figure 5: Intake area distribution for datum and short intake

Numerical Approach and Operating Conditions All operating conditions are summarized in Table 2, along

with a description of the required numerical approaches and setups as they were applied in the context of the present study. Whereas the physical limitation of each method shall be discussed in the following chapters, the given table aims also at providing an overview of the computational cost associated with each approach in order to have an estimate which methodology might be the optimal choice when carrying out design related studies. The standard, axisymmetric and mixing-plane based RANS approach, applied to cases 1-4, served as a reference. In terms of computational effort, this reflects the current standard at DLR which is easily manageable with a

l

Dth

t

cone angle = 37°

Dfan

l

Dth

t

cone angle = 37°

Dfan

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reasonable amount of resources and the given hardware infrastructure. As it can be derived from the values provided in the table, a good and still manageable compromise in a design context might be achieved for non-axisymmetric cases (either due to the nacelle geometry or due to a non-axisymmetric inflow) by placing a mixing plane between the full annulus domain and the rotor passage, of course neglecting the entire unsteady interaction but however accounting for a circumferentially averaged rotor inflow variation induced by the intake (Cases 5-6). At similar costs, only harmonic balance computations might be used to cover the entire up- and downstream interactions between the inlet, the rotor and even including the OGV (Case 8) for selected harmonics. Another compromise, but probably not affordable in a design context, would be to couple the full annulus intake domain with a single passage rotor by applying phase-shifted boundary conditions in the rotor domain in order to cover the entire (periodic) intake/rotor non-linear up- and downstream interaction (Case 7) in a uRANS based manner. In our context the OGV was connected by a time-averaged mixing plane since this approach is limited to a single perturbation frequency in each blade row. Fully coupled non-linear computations of the entire intake, rotor and OGV system (Cases 9 and 10) however seem only manageable in a research context and for a limited amount of selected configurations.

If not stated otherwise, the mechanical fan rotational speed as given in Table 1 was kept constant for all Take-Off and Cruise related operating points. The boundary conditions prescribed at the outer far field boundaries were derived from standard ISA conditions (at sea level for all Take-Off related points and at 35,000 ft for the Cruise point), taking into account the corresponding flight Mach number to define absolute thermodynamic quantities. The static pressure downstream of the OGV was also kept constant e.g. when varying flight Mach number.

Intake Aerodynamics All Cases 1-8 as given in Table 2 were analyzed for the

datum as well as for the short intake as defined in Figure 4. Intake losses for different symmetric cases and at different flight Mach number levels are quantified in Figure 6. It can be seen that the low Mach number cases are associated with higher total pressure losses due to the higher acceleration and stream line curvature around the lip, with a minimum at a flight Mach number of M=0.2 Since the total pressure was evaluated at the same axial position for both, the short and the datum intake, the short intake yielded lower levels of total pressure losses due to the overall smaller wetted area. Losses at the outer nacelle surface were not considered here.

An impression of the flow field characteristics and corresponding stream lines for the different cases based on steady state RANS computations can be seen in Figure 17 and Figure 18 for the axisymmetric cases and Figure 19 for the non-axisymmetric cases. From those results, flow quantities were circumferentially averaged at the mixing plane position

within the intake and just upstream of the spinner in order to analyze the resulting change in fan aerodynamic performance. The focus shall be on Case 2 (see Table 2) as shown in Figure 7 first: In terms of axial Mach number, the strong acceleration around the short intake’s lip is still present at fan inlet. This increased axial Mach number has a detrimental effect on fan efficiency in the tip region as shown by the radial distribution of the isentropic efficiency (Figure 7); for more information about the effect of axial Mach number on fan performance see [8]. Now a non-axisymmetric and full annulus intake case shall be considered in a similar fashion: Figure 8 shows results exemplarily for a cross-wind case at Ma=0.045 (30 kts) at 90° incidence to the nacelle in comparison with the symmetric case, both for the short intake (Case 2 vs. Case 6). For the non-axisymmetric case, the resulting non-uniformity is averaged in circumferential direction within the intake, and only a mean deviation from the undisturbed fan inflow is considered. This is a strong simplification of the real scenario and it should be considered as a first step towards the assessment of the full intake and fan interaction as it will be discussed in the following section. However, an increased total pressure loss region in the nacelle tip region, mainly induced by the strong cross flow around the lee-facing intake lip and associated with a lower fan face axial Mach number, can be observed for the non-axisymmetric cross wind case. This can be better seen in Figure 9, showing a well pronounced supersonic flow regime close to the lower lip, terminated by a small shock and a strong diffusion close to separation in the intake region. The fan outlet total pressure, however, is hardly affected. It should be stressed again that major effects of the non-axisymmetry in terms of averaged, span wise fan inflow variations can be captured (which already covers an important aspect of the entire interaction), but the unsteady interaction with the fan is missing entirely. However, even this setup showed a noticeable effect on the fan performance which further will be enforced once unsteady effects are included; this effect is subject to the next chapter.

Figure 6: Intake total pressure losses (measured between far field and fan inlet upstream of the spinner)

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Figure 7: Span wise distribution of circumferentially averaged flow quantities at fan inlet – comparison between datum and short intake at cross wind reference conditions (Case 2, M=0.045)

total pressure pt [Pa]

abs. Ma [-]

rel.

radi

us[-]

99000 100000 101000 10200

0.4 0.6 0.8

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fan inlet

total pressure pt [Pa]

rel.

radi

us[-]

110000 120000 130000

0.2

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0.8

1fan outlet

Figure 8: Span wise distribution of circumferentially averaged flow quantities at fan inlet and outlet for the short intake – comparison between symmetric conditions (Case 2) and 90° cross wind (Case 6), both at flight Mach number M=0.045

Fan Performance Characteristics The major results from the entire short intake assessment

study from fan stage aerodynamic performance point of view are summarized in Figure 20. The focus will be on the Take-Off related operating points and based on the shown performance characteristics. The major observations and conclusions from this study can be summarized as follows: • As expected, with increasing flight Mach number the fan

operating points for both, datum and short intake shift towards choke (Cases 1-3)

• Shortening the intake generally lowers fan efficiency over the entire range of operation by minimum 0.5%, with an increasing effect up to 1.5% towards the low Mach number or static cases. The effect is smallest towards higher flight Mach numbers and hence better adjusted flow around the intake lip and smaller regions with an additional flow acceleration reaching the fan inlet.

• The effect on total pressure ratio is similar and the influence is significantly smaller and the resulting shift is less than 0.2%.

• All non-axisymmetric cases (cross wind as well as angle of attack, Cases 5 and 6) additionally affect the performance characteristics, with the greatest effect at low flight Mach numbers. The cross wind cases are most severe and not only significantly lower the fan efficiency (this might be of minor concern at Take-Off conditions), but also lead to a drop of fan total pressure ratio up to 0.5%-1% with a corresponding strong decline of the characteristic, presumably associated with a reduction of aerodynamic stability.

• The fan in combination with a short intake is substantially more sensitive towards any non-axisymmetric operating condition, also in particular at low Mach number or static conditions. Corresponding drops in efficiency are almost twice the values as for the datum intake and the reduction in fan total pressure is also more pronounced (Case 6)

• Considering unsteady effects for the cross wind scenario additionally lowers fan efficiency by 0.3% (datum intake) or 0.5% compared with the steady state mixing plane results (Case 6 vs. Case 7). This suggests that with a standard mixing plane approach almost 60-70% of the detrimental effect on fan performance can be covered already, considering unsteady effects additionally shifts the performance characteristics by another 20-30% in the given direction.

• Results from HB computations (Case 8) show a similar drop in efficiency than Case 7 for the cross wind scenario, with a small difference in the resulting mass flow rate, presumably due to a slight inconsistency in the blade row coupling algorithm or a mode coupling which is not covered by the HB approach. However, the results suggest that HB computations can be used as an efficient alternative to capture the main unsteady effects and resulting change in fan performance for any non-axisymmetric fan inflow.

Figure 9: Streamlines and flow around intake lip at 90° cross wind conditions – upper lip (luv facing, top) and lower lip (lee facing, bottom)

rel. flow angle β [° ]

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Short IntakeDatum Intake

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is. efficiency [%]

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wr a

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Cross-Wind Unsteady Analysis Whereas the effect of the induced unsteadiness on average

fan aerodynamic performance has already been quantified exemplarily for the cross wind case in the previous chapter (see Figure 20), results from the underlying uRANS computations shall be discussed in greater detail in this section. An impression of the resulting cross flow around the outer nacelle can be seen in Figure 21 for both, the datum and the short intake. The spinner and blade solid surfaces are colour coded with instantaneous values of the real part of the pressure perturbation induced by the cross flow around the intake (values for the first engine order are shown here). Those fluctuations already yield a stronger unsteady interaction for the short intake, in particular on the spinner, but also on the blade surfaces. The resulting temporal thrust fluctuations are in the order of 15-20% of the time averaged values as shown in Figure 10, with min-to-max amplitudes being around 10% higher for the short intake case. The general interaction pattern and the shown temporal variation is however comparable in terms of its general characteristic. As it can already be estimated by the temporal variation, the inflow distortion is not very sinusoidal in nature and thus carries a high number of harmonics which need to be taken into account to cover the entire resulting unsteadiness. This can be confirmed by looking at the spectral content of the resulting blade pressure amplitude as shown in Figure 22; it can be seen that the first four harmonics all carry a significant amount of energy, indicated by similar levels of blade pressure amplitudes. Even the 8th EO harmonic still induces significant pressure fluctuations near the blade tip, with the amplitudes however being much lower and spatially less extended. All this are important observations to be taken into account when applying frequency based solution methods such as HB (e.g. by the right choice of harmonics) or when assessing blade mechanical aspects.

time

forc

eX

[N]

-2800

-2700

-2600

-2500

-2400 Datum intakeShort intake

one rotor rev

Figure 10: Temporal distribution of a single blades’ rotor axial forces – comparison of datum and short intake at cross wind conditions (Case 7)

The resulting upstream unsteady effect induced by the fan on the intake flow was found to be of minor relevance (at least from aerodynamic performance point of view), with upstream travelling pressure perturbations out of the nacelle being in the order of 20-50 Pa and hence being of purely acoustic nature (see Figure 23). However, the acoustic signature of the short

intake at cross wind conditions is substantially different in terms of the resulting directivity compared with the datum intake as can also be seen in Figure 23 for the 1st BPF harmonic. Coupled Fan and Intake Design Optimization

Optimization Setup and Objectives A design optimization was carried by involving the DLR

optimization framework AutoOpti, which is a multi-objective approach based on an evolutionary algorithm with extensive surrogate model support [3][8]. For this initial optimization setup, only the intake geometry was variable within the design process, taking advantage of the previously described design methodology. The design space as shown in Figure 11 comprised only a very limited number of parameters (7 thereof), however allowing for a variation of the intake lip shape and the entire inlet channel including the rotor spinner angle over a wide range. The outer nacelle shape was not subject to any variation, neither was the fan geometry and no additional restrictions were applied whatsoever. Compared with the datum intake, the short intake for the optimization was further reduced in overall thickness by approximately 10% compared with the short intake previously discussed (but yielding the same l/D ratio) so performance results from the reference short intake in this section are not directly comparable to the ones shown in the first part of the paper.

The (axisymmetric) computational domain covered the intake as well as the fan stage and each individual was evaluated by 3D CFD in three operating points: Take-Off static (Case 1), Take-Off at a flight Mach number of 0.2 (Case 3) and Cruise (Case 4). The design objectives were focussing on fan aerodynamic performance in those 3 operating points only and where formulated as

• to maximize fan pressure ratio for the two given

points in the Take-Off regime as an implicit measure of fan aerodynamic stability (formulated as a weighted average of Case 1 and 3) and

• to maximize fan stage isentropic efficiency at Cruise conditions.

The long and adopted short intake geometries were taken as a reference for the optimization and the aforementioned objectives were applied to the short intake case with an l/D ratio of approximately 0.32-0.35. The expected results from this design optimization study was to learn in how far the fan stage aerodynamic performance could be at least retained or ideally improved (compared with the reference conventional and long inlet) when substantially shortening the intake.

time

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Figure 11: Minimum and maximum boundaries for the geometric parameters defining the lip and intake contour during the optimization

Results A portion of the resulting pareto-front showing both fan

related objectives is shown in Figure 12. Some 900-1000 individuals were calculated during the automated optimization in all three operating conditions and the chosen geometry with the highest fan efficiency (Member B) is compared with the reference long intake (marked as datum intake) as well as with the initial short intake. The intake geometry from Member B yielded an increased throat area as well as a thinner lip compared with the initial short intake and the datum long inlet as also can be seen in the same figure.

Figure 12: Global optimization results in terms of fan efficiency at Cruise conditions vs. average fan total pressure ratio at Take-Off conditions and resulting intake geometries1

Whereas the fan efficiency at Cruise conditions could be improved during the optimization by 0.25% (compared with the datum intake) by better adjusting the throat area and lip incidence to the incoming stream line and corresponding stagnation point, the average total pressure ratio at Take-Off

1 note that the short intake has been further reduced in thickness

compared with the short intake geometry as shown in Figure 4

conditions was slightly reduced by <0.4% and could not be improved with the given setup.

The resulting fan stage performance characteristics are shown in Figure 13 and yield a better quantitative representation of the resulting fan performance over both speed lines considered. For the chosen individual Member B, the mass flow was increased by a very small amount in all 3 operating points (presumably due to a slightly reduced intake blockage) and efficiency was increased by a similar amount, also in all 3 operating conditions. The initial short intake for the optimization yielded a much higher blockage and a corresponding reduction in fan total pressure ratio and efficiency, caused by a shock at Cruise and a separated area close to the intake throat at Take-Off conditions, as it was induced by the very thin and not well adjusted intake lip. The design objective was, however, to fully recover fan performance at given l/D ratio and reduced intake lip thickness.

corrected mass flow rate [kg/s]

isen

tropi

cef

fi cie

ncy

[%]

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erra

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Take-Off static (Case 1, OP1)

Cruise (Case 4, OP3)

Member B

Take-Off M=0.2 (Case 3, OP2)

Datum Intake

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Cruise

Figure 13: Fan performance characteristics at Take-Off and Cruise conditions for datum (blue), short (green) and optimized intake Member B (black) Since the fan geometry was not subject to any variation, a closer look at the resulting intake geometry and corresponding static pressure distribution is provided in the following figures Figure 14-Figure 16. Here it is interesting to observe how the objectives, which were formulated based on fan stage performance only, implicitly affected the intake performance. For the Take-Off static case (Figure 14), the chosen Member B yields a strong leading edge pressure peak and presumably deteriorated the intake performance. It is expected that the new intake with reduced lip thickness will be more prone to separation at severe and non-axisymmetric conditions and additional intake related objectives need to be included to better adjust the intake lip to those conditions too; results from such a refined optimization can be found in [4]. At Take-Off conditions and flight Mach number of 0.2 as shown in Figure 15 however, improved intake and fan performance went hand in hand and the resulting pressure distribution around the intake lip yielded an almost identical, well adjusted diffusion within the intake and corresponding location of the stagnation point without any excessive acceleration (contrary to the initial short intake design). A similar observation was made at Cruise condition (Figure 16): whereas the initial short intake was not at all designed for those conditions and yielded an unwanted

average FPR [-]

isen

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ncy

[%]

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optimized intake - Member Bshort intake

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min 31°max. 55°

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Center for the Relative Radius

Point of ControlFixed Points

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A(Arc = 0)

OR = 1

Arc= 0,3

R = 1Arc= 0,4

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shock within the intake, the resulting optimized individual Member B yielded a similar pressure distribution as the datum long intake being well adjusted to the Cruise regime.

axial coordinate [m]

stat

icpr

essu

re[P

a]

20000

40000

60000

80000

100000

120000

innercontour

datum intakeshort intake (initial)Member B

intakegeometry

outercontour

Figure 14: Static pressure distribution at Take-Off static conditions (Case 1)

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re[P

a]

20000

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Figure 15: Static pressure distribution at Take-Off conditions (Case 3, flight Ma=0.2)

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a]

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Figure 16: Static pressure distribution at Cruise conditions (Case 4) Summary, Conclusions and Outlook

The present paper introduced a fully coupled intake and fan design methodology for potential application to short intakes of future UHBR engines. This design method was taken advantage of at first in order to manually design a short

intake and compare its aerodynamic performance with a conventional long intake. Apart from quantifying the resulting, for most cases compromised, fan performance at different operating conditions, the complexity of the numerical setup was successively increased in order to evaluate the potential of applying different RANS und uRANS based approaches in a design context with rather high turn-around times. It was shown that even full wheel intake setups with non-axisymmetric inflow conditions such as cross wind and AoA and a simple coupling of the fan rotor were able to capture a great portion of the associated steady state phenomena affecting fan performance, neglecting however any unsteady interaction. Here more sophisticated unsteady approaches such as harmonic balance computations (HB) promise a substantial gain in computational efficiency with all predominant effects included.

The second part of the paper showed results from an initial coupled fan and intake design optimization, focussing on fan aerodynamic performance. It was shown that fan efficiency could be retained when substantially shortening and further decreasing overall thickness of the intake at different (here axisymmetric) flow conditions. This was achieved by formulating fan related objectives only, suggesting that fan and intake performance are closely coupled. Additional improvements are expected once additional, local criteria involving intake performance are combined with releasing the fan geometry. Ongoing efforts, taking full advantage of the presented methodology, are focussing on designing the entire 3D intake geometry, directly taking into account non-axisymmetric flow conditions such as cross wind and further exploiting the harmonic balance approach to also include the unsteady interaction with the fan stage in a fully coupled and automated approach. References [1] Albers, J., Miller, B.: Effect of Subsonic Inlet Lip

Geometry on Predicted Surface and Flow Mach Number Distribution, NASA Technical Note TN-D-7446, December 1973

[2] Albert, M., Bestle, D.: Aerodynamic Optimization of Nacelle and Intake, ASME Paper GT2013-94857, ASME Turbo Expo 2013 San Antonio/Texas, 2013

[3] Aulich, A.-L., Goerke, D., Blocher, M., Nicke, E., Kocian, F.: Automated Optimization Strategy on a Counter Rotating Fan, ASME Paper GT2013-94259, ASME Turbo Expo 2013 San Antonio/US

[4] Corroyer, J., Schnell, R., Nicke. E.: Fan and Intake Coupled Aerodynamic Design Optimization for UHBR Aero-Engines with Low-Pressure Ratio Fans, Deutscher Luft- und Raumfahrtkongress, 22.-24. September 2015, Rostock/Germany

[5] Fidalgo, V.J., Hall, C.A., Colin, Y: A Study of Fan-Distortion Interaction within the NASA Rotor 67

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Transonic Stage, J. Turbomach. 134(5), 051011, May 2012

[6] Frey, C., Ashcroft, G., Kersken, H.-P., Voigt, C.: A Harmonic Balance Technique for Turbomachinery Applications, ASME Paper GT-2014-25230, ASME Turbo Expo Düsseldorf/Germany, 2014

[7] Giebmanns, A., Schnell, R., Steinert, W., Hergt, A. Nicke, E., Werner-Spatz, C.: Analysing and Optimising Geometrically Degraded Transonic Fan Blades by means of 2D and 3D Simulations and Cascade Measurements, ASME Paper 2012-GT- 69064, ASME Turbo Expo Copenhagen 2012

[8] Hooker, J., Wick, A., Zeune, C., Agelastos, A.: Over Wing Nacelle Installations for Improved Energy Efficiency, AIAA-Paper 2013-2920, 2013

[9] Lengyel, T., Nicke, E., Rüd, K.-P., Schaber, R.: Optimization and Examination of a Counter-Rotating Fan Stage - The Possible Improvement of the Efficiency Compared with a Single Rotating Fan, ISABE Paper, 20th ISABE Conference Gothenburg/Sweden 2011, ISABE 2011-1232.

[10] Liou, M.-S., Lee, B.J: Minimizing Inlet Distortion for Hybrid Wing BodyAircraft, Journal of Turbomachinery Vol. 134 031020-1, May 2012

[11] Longley, J.P., Greitzer, E.M.: Inlet Distortion Effects in Aircraft Propulsion System Integration, In AGARD, Steady and Transient Performance Prediction of Gas Turbine Engines 18 p (SEE N92-28458 19-07), May 1992

[12] Prasad, D., Feng, J.: Propagation and Decay of Shock Waves in Turbofan Engine Inlets, ASME Journal of Turbomachinery, DOI 10.1115/1.1811102, Volume 127, pp 118 ff, 2005

[13] Peters, A., Sparkovsky, Z., Rose, B., Lord, W.K.: Ultra-Short Nacelles for Low Fan Pressure Ratio Propulsors, ASME Paper GT2014-26369, ASME Turbo Expo Düsseldorf, June 2014

[14] Savelyev, A.A., Mikhaylov, S.V., Zlenko, N.A.: AERODYNAMIC INLET DESIGN FOR CIVIL AIRCRAFT NACELLE, Proceedings of the 29th Congress of the International Council of the Aeronautical Sciences ICAS, St. Petersburg/Russia, September 2014

[15] Sóbester, A.: Tradeoffs in Jet Inlet Design: A Historical Perspective, JOURNAL OF AIRCRAFT Vol. 44, No. 3, May–June 2007

[16] Smith, A. R., Thorne, R. C. G., Surply, T., and Chanez, P.: Aerodynamic Aspects of Application of Negative Scarf Intake to High Bypass Ratio Civil Turbofans, AIAA Paper 2005-4205, p. 11, 2005

[17] Schnell, R., Ebel, P.-B., Becker, R. G., Schoenweitz, D.: Performance Analysis of the Integrated V2527-Engine Fan at Ground Operation, 13th Onera DLR Aerospace Symposium ODAS, Palaiseau, May 2013

[18] Schnell, R., Schoenweitz, D., Theune, M., Corroyer, J.: Integration- and Intake-Induced Flow Distortions and their Impact on Aerodynamic Fan Performance, Proceedings FOR 1066 International Symposium "Simulation of Wing and Nacelle Stall", December 1-2, Braunschweig/Germany

[19] Shorstov, V., Makarov, V., Andreev, S., Fedorchenko, J., Bekurin, D., Karnauchov, A.: Unsteady Calculation of Intake-Fan Flow of HBPR Turbofan Engine at Take-Off with Strong Cross Wind, ISABE Paper 2013-1113, 21st

ISABE Conference 9.-13. September, Busan/Korea, 2013 [20] Theune, M., Schönweitz, D., Schnell, R.: Sensitivität eines

Triebwerk-Fans gegenüber Einlaufstörungen (Inlet Distortion Sensitivity of a Low Pressure Ratio Fan), 19. DGLR-Fachsymposium der STAB, November 2014 Acknowledgements Most of the results were obtained in the context of

POWER25 (Power Plant Integration and Performance 2025), a german national joint research initiative funded by the German Ministry of Economics under the contractual agreement 20A1301B. This support is gratefully acknowledged. The authors are responsible for the content of this publication.

10

Table 2: Operating conditions and different numerical approaches pursued in the scope of the present study (FF#=Far Field, *MP=Mixing Plane,**a/s=axisymmetric, uRANS-PL=uRANS single passage rotor with phase-shifted periodic boundary conditions, HB=Harmonic Balance [5]) – Results for all cases in terms of fan stage performance are shown in Figure 20

Case

Flight Mach Symmetry Domain Numerical Approach # of cells2 Cost

FF#&Intake Rotor OGV

(million)

10 off 17 off 33 off

1 Static M=0.01 a/s** segment segment segment intake/rotor/OGV RANS w/ MP* 1,92 1

2 x-wind ref. M=0.045 a/s

3 Take-Off M=0.10 a/s

M=0.20

M=0.23

4 Cruise M=0.78 a/s

5 AoA M=0.20 non a/s full annulus segment segment intake/rotor/OGV RANS w/ MP 7,5 5

10°, 20°, 30°

6 x-wind M=0.045 non a/s

90° / 30 kts

7 x-wind M=0.045 non a/s full annulus segment segment intake/rotor fully coupled uRANS-PL 7,5 100

90° / 30 kts

rotor/OGV time averaged MP

8 x-wind M=0.045 non a/s full annulus segment segment intake/rotor/OGV fully coupled / HB 7,5 10-12

90° / 30 kts 2+2(+2) harmonics + time average

9 x-wind M=0.045 non a/s full annulus full annulus segment intake/rotor fully coupled uRANS 19 530

rotor / OGV time averaged MP

10 x-wind M=0.045 non a/s full annulus full annulus full annulus intake/rotor/OGV fully coupled uRANS 37 1200

2 Given are the total number of cells for each setup, the individual blade passages were always resolved identically and comprised 0,62 million cells (far field and

intake with low-Re formulation at all solid surfaces and corresponding y+<2), 0,76 million cells (rotor passage) and 0,54 million cells (OGV passage) – for the size of the far field domain along with the placement of its outer boundaries refer to Figure 3

11

Figure 17: Streamlines and contours of axial velocity – comparison of datum (left) and short intake (right) at fan Take-Off rotational speed and ambient static conditions, e.g. flight M=0.01 – results from RANS mixing plane computations (Case 1)

Figure 18: Streamlines and contours of axial velocity – comparison of datum (left) and short intake (right) at fan Take-Off rotational speed and flight M=0.2 – results from RANS mixing plane computations (Case 3)

Figure 19: Streamlines and contours of axial velocity for the short intake – comparison of the two non-symmetric cases at angle of

attack conditions (Case 5: AoA=20°, flight M=0.2) and cross wind (Case 6: 90°-cross wind, M=0.045 or 30 kts) – results from intake full annulus RANS computations with a mixing plane located within the intake to couple the fan rotor single passage domain

12

ISA corrected mass flow rate [kg/s]

fan

tota

lpre

ssur

era

tioFP

R[-]

610 620 630 640 650

1.21

1.22

Fan135 - Max. TakeOff (MTO)

n/√Tt=const.

M=0.01

M=0.23

M=0.2

M=0.1M=0.045

x-wind

x-wind (M=0.045)

AoA 20°

all AoA @M=0.2

AoA 20°

AoA 30°

ISA corrected mass flow rate [kg/s]

fan

stag

eis

.effi

cien

cy[%

]

610 620 630 640 65091.00

91.50

92.00

92.50

93.00

93.50

M=0.01

Max. TakeOff (MTO)

M=0.23

M=0.2M=0.1M=0.045

ISA corrected mass flow rate [kg/s]

fan

tota

lpre

ssur

era

tioFP

R[-]

610 620 630 640 650

1.21

1.22

Fan135 - Max. TakeOff (MTO)

n/√Tt=const.

M=0.01

M=0.23

M=0.2

M=0.1M=0.045

x-wind

x-wind (M=0.045)

AoA 20°

all AoA @M=0.2

AoA 20°

AoA 30°

ISA corrected mass flow rate [kg/s]

fan

stag

eis

.effi

cien

cy[%

]

610 620 630 640 65091.00

91.50

92.00

92.50

93.00

93.50

M=0.01

Max. TakeOff (MTO)

M=0.23

M=0.2M=0.1M=0.045

M=0.01

M=0.23

M=0.2

M=0.1

M=0.045

ISA corrected mass flow rate [kg/s]

fan

tota

lpre

ssur

era

tioFP

R[-]

610 620 630 640 650

1.21

1.22

Fan135 - Max. TakeOff (MTO)

n/√Tt=const.

Isolated StageReference OP ps=103000 PaFan135 + datum intakeFan135 + short intake

x

M=0.01x-wind(datum)

x-wind (short)

AoA 20°

AoA @flight M=0.2x-wind @ M=0.045 (30 kts)

AoA 10°

M=0.23

M=0.2M=0.1M=0.045

AoA 30°

AoA 20°

ISA corrected mass flow rate [kg/s]

fan

stag

eis

.effi

cien

cy[%

]

610 620 630 640 650

91.00

91.50

92.00

92.50

93.00

93.50

6

1

18

7

6

52

7

25

35

5

3

33

33

Figure 20: Fan stage performance characteristic at Take-Off conditions for all cases considered comparing datum (blue) and short intake (red) – the operating conditions comprise flight Mach numbers in the range [0.01 (static), 0.045, 0.1, 0.2, 0.23], results for the non-axisymmetric cases from RANS computations (cross wind and angle of attack) as well as time-averaged results from uRANS and HB computations for the cross wind case (diamonds) – The cases marked 1-8 in the shown figure are defined in Table 2

Figure 21: Streamlines and contours of Re{p’} (1st distortion harmonic / 1EO) for the 90° cross wind case (datum intake /left and short intake / right) – results from fully coupled non-linear uRANS computations with a rotor single rotor passage applying phase-

lagged periodic boundary conditions (Case 7)

13

Figure 22: Resulting rotor blade pressure amplitude from cross-wind induced fan inflow non-homogeneity (first inlet distortion harmonic = first EO) – Case 7

Figure 23: Streamlines (based on time average solution) and upstream running pressure pattern at first rotor BPF harmonic – Case 7