Milind A. Bakhle

Glenn Research Center, Cleveland, Ohio

T.S.R. Reddy

University of Toledo, Toledo, Ohio

Gregory P. Herrick 

Glenn Research Center, Cleveland, Ohio

Aamir Shabbir and Razvan V. Florea

United Technologies Research Center, East Hartford, Connecticut 

Aeromechanics Analysis of a 

Boundary Layer Ingesting Fan

NASA/TM—2013-217730

January 2013

AIAA–2012–3995
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Milind A. Bakhle

Glenn Research Center, Cleveland, Ohio

T.S.R. Reddy

University of Toledo, Toledo, Ohio

Gregory P. Herrick 

Glenn Research Center, Cleveland, Ohio

Aamir Shabbir and Razvan V. Florea

United Technologies Research Center, East Hartford, Connecticut 

Aeromechanics Analysis of a 

Boundary Layer Ingesting Fan

NASA/TM—2013-217730

January 2013

AIAA–2012–3995

National Aeronautics and

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Glenn Research Center

Cleveland, Ohio 44135

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Acknowledgments

The work described in this paper was performed at NASA Glenn Research Center (GRC) in collaboration with United Technologies 

Research Center (UTRC). The authors would like to thank Ms. Rula Coroneos (GRC) for helping with MSC Nastran runs. The 

authors would like to thank Mr. David Arend (GRC Team Lead, Robust Design of Embedded Engine Systems) and Dr. Gregory 

Tillman (UTRC Team Lead). The authors are grateful for the support of this work provided by the Subsonic Fixed Wing Project 

(Dr. Rubén Del Rosario, Project Manager; Dr. Michael Hathaway, Technical Challenge Lead), and the Environmentally Responsible 

Aviation Project (Dr. Fayette Collier, Project Manager; Dr. Kenneth Suder, Propulsion Sub-project Manager).

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-217730 1

Aeromechanics Analysis of a Boundary Layer Ingesting Fan

Milind A. Bakhle

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

T.S.R. Reddy 

University of Toledo 

Toledo, Ohio 43606 

Gregory P. Herrick 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Aamir Shabbir and Razvan V. Florea 

United Technologies Research Center 

East Hartford, Connecticut 06108 

Abstract 

Boundary layer ingesting propulsion systems have the potential to significantly reduce fuel burn but 

these systems must overcome the challenges related to aeromechanics—fan flutter stability and forced 

response dynamic stresses. High-fidelity computational analysis of the fan aeromechanics is integral to 

the ongoing effort to design a boundary layer ingesting inlet and fan for fabrication and wind-tunnel test. 

A three-dimensional, time-accurate, Reynolds-averaged Navier Stokes computational fluid dynamics 

code is used to study aerothermodynamic and aeromechanical behavior of the fan in response to both 

clean and distorted inflows. The computational aeromechanics analyses performed in this study show an 

intermediate design iteration of the fan to be flutter-free at the design conditions analyzed with both clean 

and distorted in-flows. Dynamic stresses from forced response have been calculated for the design 

rotational speed. Additional work is ongoing to expand the analyses to off-design conditions, and for onresonance conditions. 

Introduction 

Boundary layer ingestion (BLI) propulsion systems have the potential for significant reduction in 

aircraft fuel burn (Ref. 1); previous system studies (Refs. 2 to 6) have shown that 5 to 10 percent 

reduction in fuel burn is possible. Recent work (Ref. 7) has confirmed this potential benefit through a 

system study focused on the propulsion system and its integration into the Blended Wing Body (BWB) 

aircraft. The system study indicated that low-loss inlets and high-performance, distortion-tolerant 

turbomachinery are key technologies required to achieve a 3 to 5 percent BLI fuel burn benefit for future 

aircraft relative to a baseline high-performance, pylon-mounted, propulsion system. The study further 

identified the key sensitivity parameters and their desired targets. However, this system-level benefit 

could be diminished by not achieving the targets of inlet total pressure loss, fan efficiency reduction, and 

fan stall margin reduction. Further, the fan must meet aeromechanics requirements regarding dynamic 

stresses and flutter stability. Efforts (Ref. 8) are currently in progress to design and fabricate the inlet and 

fan for a BLI propulsion system that will be tested in a wind-tunnel to simulate cruise conditions of a 

representative large commercial transport aircraft. This paper describes in detail preliminary 

computational aeromechanics analyses performed on the boundary layer ingesting fan. The objectives of 
NASA/TM—2013-217730 2

the aeromechanics analyses are to determine the dynamic stresses in the fan blade due to operation in a 

distorted inflow and to determine the flutter stability of the fan. The present aeromechanics analyses are 

performed at flight conditions and further studies are required at more challenging conditions including 

take-off. 

The inlet duct that brings the flow to the fan face is a very important element of this propulsion 

system. Under a separate effort, the NASA Inlet A (Ref. 9) has been used as the starting point for the 

design of the inlet used in the current study. An optimization-based parametric inlet design system was 

developed and applied to considerably reduce the total pressure loss and the distortion harmonic 

amplitudes (Ref. 8). 

Fan Aerodynamic Analysis 

Fan aerodynamic analysis in the present work is based on the time-domain solution of Reynoldsaveraged Navier Stokes (RANS) equations using the TURBO aeroelastic analysis code (Refs. 10 and 11). 

Briefly, this is an implicit finite-volume RANS solver that uses structured multi-block grids to model 

flow through one or more blade rows. Previous applications of TURBO in the study of flutter and forced 

vibration are presented in References 12 to 14. Further validation of TURBO in the study of 

aeromechanics is available in References 15 to 19. An inlet distortion boundary condition is used at the 

upstream boundary to prescribe the flow entering the fan domain since the inlet duct is not modeled in 

this part of the work. With this inlet distortion boundary condition, total conditions are held (at each 

respective radial and azimuthal location where prescribed) as governed by the characteristics; Herrick 

(Ref. 20) implemented and applied this boundary condition in previous work. A throttle exit boundary 

condition is used at the downstream boundary to avoid over-constraining the exit flow; this boundary 

condition maintains only the integrated corrected mass flow through the exit plane at a prescribed value, 

while local flow quantities are free to attain local equilibrium. For fan aeromechanics analysis with blade 

vibrations, dynamic grid deformation is used to prescribe harmonic blade vibrations and the energy 

method is used to evaluate aerodynamic damping and flutter stability.

Since the design of the BLI fan is in progress, the analyses presented in this paper are performed on 

an aerodynamic design iteration. The computational meshes used in this study of the BLI fan include a 

combination of H, O, and C block meshes. Figure 1(a) shows the computational domain for the BLI fan 

(one blade passage); Figure 1(b) shows the fan rotor blade and Figure 1(c) shows a cross-sectional view at 

a fixed radial location. 

 (a) Overview of computational domain (one blade passage) (b) Fan rotor (c) Cross-sectional view 

Figure 1.—Computational domain used in fan aerodynamic analysis. 
NASA/TM—2013-217730 3

 

 a) Total pressure ratio b) Adiabatic efficiency 

Figure 2.—Performance characteristics of fan with clean inflow. 

Initial analysis is carried out with clean inflow prescribed at the inlet boundary. This condition 

represents typical operation at cruise conditions for a fan operating without boundary layer ingestion. For 

example, an engine in a typical under-wing mount would experience clean inflow if no crosswind were 

present. The inlet profile has only radial variations of total pressure, total temperature, radial flow angle, 

and tangential flow angle. One blade passage is used for the computations and periodic boundary 

conditions are imposed in the circumferential direction based on an assumed symmetry of the flowfield in 

each blade passage. A converged solution is obtained for a prescribed exit boundary condition and the 

speedline is obtained by varying the exit boundary condition, keeping the rotational speed fixed. Figure 2 

shows the performance characteristics of the fan, adiabatic efficiency and total pressure ratio, for the 

design speed. No specific attempt is made to locate the stall boundary, rather the results shown Figure 2 

are those for which fully converged flowfields were obtained. The performance characteristics with clean 

inflow serve as a baseline for the following computations with distorted inflow. 

As mentioned previously, an optimization-based parametric inlet design process (Ref. 8) was used to 

design the inlet with reduced inlet total pressure loss and reduced distortion harmonic amplitudes as 

compared to the NASA Inlet A. The inlet flowfield is not part of the current computations. Instead, the 

flowfield from an intermediate inlet design iteration, identified as UTRC P3 inlet in Reference 8, provides 

the inlet boundary condition for the fan computations. Due to the shape of the inlet, this flowfield is not 

axisymmetric. Rather, it contains both radial and circumferential variations in flow quantities. The relevant 

flow quantities required to fully prescribe the inlet boundary condition for the fan computational domain are 

shown in Figure 3. Note that for this computation of the 18-bladed fan, all 18 blade passages are included in 

the computational domain. The time-domain computations are advanced with 100 time steps per passage, or 

1800 time steps per rotor revolution; the solution is converged to periodicity after 6 revolutions. Figure 4 

shows the variation of the instantaneous total pressure ratio for various blade passages as these passages 

traverse the inlet distortion. The repeating variation for any particular passage indicates the periodicity of the 

flowfield in time, and the shift of the results along the time axis for different passages indicates that each 

passage has the same flowfield when it is at a particular circumferential location at different times. The 

instantaneous average value over 18 passages is also shown for comparison. 

The variation of the total pressure ratio for different passages as a function of the mass flow rate 

through each passage is shown in Figure 5. The instantaneous average value over 18 passages is also 

shown for comparison. The passage mass flow rates have been multiplied by the number of passages (18) 

to represent values for the full rotor. The overlay of the values for two revolutions and for different 

passages indicates once again the periodicity in time and across the passages. The excursion of the 

1.05

1.15

1.25

1.35

1.45

1000 1100 1200 1300

Total pressure ratio

Inlet corrected mass flow rate, lbm/s

60

70

80

90

100

1000 1100 1200 1300

Adiabatic efficiency, %

Inlet corrected mass flow rate, lbm/s
NASA/TM—2013-217730 4

 

Figure 3.—Inlet boundary conditions for fan computational domain. 

Figure 4.—Periodicity of the rotor flowfield. Figure 5.—Variation of performance due to inlet 

distortion. 

instantaneous value also provides an indication of the variation in the flowfield in each passage as it 

traverses the inlet distortion. The nominal characteristics for clean inflow are repeated from Figure 2 for 

comparison. Note that a simplified approximation is used to calculate the instantaneous pressure ratio and 

mass flow rate in each blade passage using the grid block in place of computed stream tubes. 

Fan Blade Structural Analysis 

The aerodynamic design iteration of the BLI fan is used to create a structural model. It is assumed that 

the blades are made of Titanium alloy Ti-6Al-4V, which is typically used for scale model fan blades. In the 

structural model, 8-node hexahedral or brick elements are used and the blade root section is fully 

constrained in the analysis (no blade attachment is modeled). In addition to the centrifugal loads, nominal 

pressure load is prescribed. Nonlinear static and modal analyses are performed using commercial finite 

element analysis software to calculate the blade static deflection, static stress, and modal deflections(mode 

shapes), stresses, and frequencies. The analyses include the effects of centrifugal stiffening and softening. 

The mode shape and frequency results for the first four modes are shown in Figure 6. It can be noted that the 

first mode is predominantly bending and the third mode is predominantly torsion; these two modes are 

considered for the flutter stability analysis in a following section. 

1.30

1.35

1.40

1.45

1.50

1.55

29400 30400 31400 32400 33400 34400

passage 1

passage 5

passage 10

passage 15

average

Total pressure ratio

Time step counter

1 rotor

revolution

1.05

1.15

1.25

1.35

1.45

1000 1100 1200 1300

steady

passage 1

passage 5

passage 10

passage 15

average

Total pressure ratio

Inlet corrected mass flow rate, lbm/s
NASA/TM—2013-217730 5

Figure 6.—Fan blade vibration mode shapes and frequencies at design speed. 

Fan Blade Forced Vibration Response 

The blade structural dynamics can be described in modal form by the following equation: 

[ ]{ } [ ]{} { qKqM =+ AD} (1) 

where [M] is the modal mass matrix, [K] is the modal stiffness matrix, {q} is the generalized 

displacement vector, {AD} is the motion-independent aerodynamic load vector that is referred in this 

context as the modal force vector, and the overdot denotes time derivative. Note that since damping is not 

included, this analysis is expected to provide conservative results. 

For a frequency of excitation ω, the generalized displacement vector of an undamped blade can be 

written as: 

{ } [ ] [] [ ] MKq { } AD

1 2 −

= ω− (2) 

The components of modal force can be defined by the following equation: 

∫ AD = nn ⋅δ pdA



 (3) 

where δ



 is the modal displacement vector, p is the aerodynamic pressure, A



 is the blade surface area 

vector, and the subscript n denotes the mode index. 

The inlet distortion provides a periodic excitation for each blade with a fundamental period of one 

rotor revolution. The unsteady pressure from the unsteady flowfield computed by TURBO is used with 

the blade modal displacement and area to calculate the modal force. Figure 7 shows this time variation of 

modal force for the first four modes. Since the flowfield has converged to periodicity, the modal force is 

seen in Figure 7 as a periodic function of time. Figure 8 shows the Fourier components of modal force. 

The first and second harmonics are significant in magnitude, with a rapid drop-off beyond the fourth 

harmonic. However, it can be noted from Equation (2) that the forced response vibration amplitude will 

be large if the excitation frequency is nearly equal to the natural frequency. To determine conditions at 

which the excitation frequency is nearly equal to the natural frequency, a Campbell diagram is 

constructed by repeating the modal analysis at different rotational speeds. Figure 9 shows the Campbell 

diagram for the fan rotor blade. The rotational speed is non-dimensionalized by the design value. The 

lines of constant slope are the “engine order (EO)” excitation lines that correspond to the harmonic 

content of the excitation. For example, the third harmonic in Figure 8 acts as a 3 EO excitation that has a 

frequency nearly equal to the mode 2 frequency at design rotational speed (highlighted with a circle on 

Figure 9). Similarly, the 7 EO excitation frequency is nearly equal to the mode 4 frequency. 
NASA/TM—2013-217730 6

The forced vibration response or dynamic stress at design speed can be calculated to include 

contributions from various modes for any given engine order excitation. Rewriting Equation (2) for each 

harmonic (engine order), the generalized displacement for n

th mode due to rth harmonic excitation can be 

written as: 

{ } nr [ ] [] [ ] MKq nrn { } ADnr

1 2 −

= ω− (4) 

Then, the blade displacements ur resulting from the r

th harmonic excitation are given by: 

n nr n r = φΣ qu (5) 

where φn is the n

th mode shape. Similarly, the dynamic stress response amplitudes σr for the rth harmonic 

excitation can be obtained from the modal stress sn as: 

n nr n r σ = Σ qs (6) 

Figure 7.—Modal force variation. Figure 8.—Fourier components of modal force. 

Figure 9.—Campbell diagram for BLI fan blade. 

-4000

-2000

0

2000

4000

0 600 1200 1800

mode 1

mode 2

mode 3

mode 4

Modal force, lbf

Time step counter

0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10

mode 1

mode 2

mode 3

mode 4

Modal force magnitude, lbf

Harmonic number

0

100

200

300

400

500

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Frequency, Hz

Non-dimensional rotational speed

1 EO

2 EO

3 EO

4 EO

8 EO

7 EO

6 EO

5 EO

mode 1

mode 2

mode 3

mode 4

mode 5
NASA/TM—2013-217730 7

Figure 10.—Fan blade forced vibration response 

calculated from Equation (4). 

The results of such calculations are shown in Figure 10 and Table I. Four modes are included in the 

analysis corresponding to eight engine order excitations. Figure 10 shows the increased generalized 

displacement in mode 2 for engine order 3 excitation (q23), resulting from an excitation frequency close to 

the natural frequency (as seen in Figure 9). Also, Table I shows the resulting increased dynamic stress 

from engine order 3 excitation. Similarly, the small frequency margin for mode 4 and engine order 7 

(Figure 9) is seen to result in an increased dynamic stress from engine order 7 excitation (Table I). Note 

that the stress values listed in Table I are the von Mises stresses. Also, note that the analysis was done for 

the design rotational speed. It is expected that the on-resonance analysis near the design speed will result 

in significantly higher dynamic stresses. Work is in progress to perform this on-resonance analysis. 

TABLE I.—VIBRATION AMPLITUDE AND DYNAMIC 

STRESS FOR DIFFERENT ENGINE ORDER 

EXCITATIONS AT DESIGN SPEED 

Harmonic or 

engine order 

Vibration amplitude 

(at tip trailing edge), 

in. 

Dynamic stress amplitude 

(at root of blade), 

psi 

1 5.5×10–2 273 

2 3.0×10–2 290 

3 1.9×10–2 666 

4 3.1×10–3 308 

5 2.6 ×10–3 169 

6 2.7×10–4 33 

7 7.0×10–4 427 

8 6.0×10–5 19 

0.000

0.002

0.004

0.006

0.008

1234

qn1

qn2

qn3

qn4

qn5

qn6

qn7

qn8

Generalized displacement, in.

Mode number
NASA/TM—2013-217730 8

Fan Blade Flutter Stability 

The flutter stability of the fan is evaluated using the TURBO aeroelastic analysis code. The energy 

method (Ref. 21) or work-per-cycle approach is used to calculate aerodynamic damping to determine 

flutter stability as described in Reference 22. For a clean inflow prescribed at the inlet boundary, the 

flowfield through the blade row is first calculated with no prescribed blade vibration. Starting with this 

converged steady flowfield, blade vibrations are prescribed in a selected mode, frequency, and nodal 

diameter pattern or phase angle. Note that the number of different possible nodal diameter patterns is 

equal to the number of blades that are on the fan rotor. After the transients in the flowfield decay, and a 

periodic flowfield is obtained, the work done on the vibrating blade is calculated for a cycle of blade 

vibration as follows: 

W pd ( )tXA dt

surface

= ∂∂⋅− ∫ ∫   (7) 

where, p is the aerodynamic pressure due to blade vibration, A is the blade surface area vector, X



 is the 

displacement vector on the blade surface, and t denotes time. The aerodynamic damping ratio (ζ) can be 

approximately related to the work-per-cycle (W) and the average kinetic energy (KE) of the blade over one 

cycle of vibration through the following expression: 

π−≈ζ KW E 8 (8) 

If aerodynamic damping is negative, flutter can occur. Note that the structural damping (material and 

mechanical damping) has not been considered. 

Clean Inflow 

Initial fan flutter calculations are performed at design operating speed near peak efficiency condition with 

clean inflow prescribed at the inlet boundary. The computational domain consisted of all 18 blade 

passages (full rotor analysis). Figure 11 shows the results of the calculations with all blades vibrating inphase (0 nodal diameter) in mode 1 (bending). Individual blade values are shown to converge to the 

average value within 10 cycles of vibration. The converged value is positive indicating no flutter at the 

condition analyzed. Similar calculations are performed with vibrations in other nodal diameter patterns 

(different interblade phase angles). The converged results are summarized in Figure 12. It can be noted 

that aerodynamic damping has a fairly large range of variation with nodal diameter pattern, with low 

values for low nodal diameter patterns. For all nodal diameter patterns considered, aerodynamic damping 

is positive, indicating no flutter for the conditions analyzed, namely, design operating speed near peak 

efficiency condition. Since decreasing mass flow rate along the speedline (moving towards stall) can 

sometimes result in reduced aerodynamic damping and possibly flutter, calculations are performed at the 

design operating speed near stall condition (approximately 11 percent lower mass flow rate). Only results 

for the selected range of nodal diameter patterns are shown in 

Figure 13. The lowest aerodynamic damping value is seen to decrease as the operating point is 

changed from near peak efficiency to near stall. For mode 1 (bending) vibrations, aerodynamic damping 

reaches low values for the 1 nodal diameter pattern, although all calculated values remain positive. 

The results of calculations performed for mode 3 (torsion) at design operating speed near peak 

efficiency condition with clean inflow are shown in Figure 14. In contrast to mode 1, the aerodynamic 

damping stays within a narrow range of values. All calculated aerodynamic damping values are positive, 

indicating no flutter for the conditions analyzed; minimum values are higher for mode 3 as compared to 

mode 1. Calculations are also performed at the design operating speed near stall condition for a selected 
NASA/TM—2013-217730 9

range of nodal diameter patterns. No significant change in aerodynamic damping is observed as the 

operating point is changed from near peak efficiency to near stall. 

Distorted Inflow 

Typically, in a linear formulation, fan flutter stability and forced response due to aerodynamic 

excitations are considered separately. Moreover, fan inlet distortion is typically considered only in the 

context of cross-wind operation at low forward speed operation on the ground. In the present case of the 

BLI fan, the concern is that the inlet distortion is persistent at all operating conditions and includes 

multiple harmonics. Previous studies (Refs. 23 to 25) have indicated that nonlinear interactions between 

Figure 11.—Aerodynamic damping with clean inflow at 

design operating speed, near peak efficiency condition, 

mode 1 vibration, 0 nodal diameter pattern.

 

Figure 12.—Aerodynamic damping with clean inflow at 

design operating speed, near peak efficiency condition, 

mode 1 vibration, various nodal diameter patterns.

 

Figure 13.—Effect of operating point on aerodynamic 

damping with clean inflow at design operating speed, 

mode 1 vibration, various nodal diameter patterns.

Figure 14.—Aerodynamic damping with clean inflow at 

design operating speed, near peak efficiency condition, 

mode 3 vibration, various nodal diameter patterns.

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10 12

all blades

average

Aerodynamic damping, %

Vibration cycle number

no flutter

0

1

2

3

4

5

-9 -6 -3 0 3 6 9

Aerodynamic damping, %

Nodal diameter

no flutter

0.0

0.5

1.0

1.5

2.0

-3 -2 -1 0 1 2 3

near peak efficiency

near stall

Aerodynamic damping, %

Nodal diameter

no flutter

Decreasing mass flow 

rate along speedline

0

1

2

3

4

5

-9 -6 -3 0 3 6 9

near peak efficiency

near stall

Aerodynamic damping, %

Nodal diameter

no flutter
NASA/TM—2013-217730 10

inlet distortion and blade vibrations may be important. The influence of inlet distortion on flutter stability 

is related, in part, to the changes in operating conditions of individual blades as they move through the 

distorted inlet flow as can be seen in Figure 5. In addition, since the fan blade rotational frequency is 

typically not an integral multiple of the blade vibration frequency, each blade vibration cycle takes place 

in a different flowfield depending on the location of the blades relative to the inlet distortion. 

Various approaches are considered to evaluate flutter stability with distorted inflow: (i) circumferentially 

average the distorted inflow to obtain an equivalent radial profile and use work-per-cycle analysis, (ii) select 

a portion of the inlet distortion to represent a “worst-case” inflow condition that is used at all circumferential 

locations and use work-per-cycle analysis, (iii) prescribe blade vibrations and distorted inflow 

simultaneously, use work-per-cycle analysis, average the results over all blades, and over multiple blade 

vibration cycles, (iv) use tightly-coupled aeroelastic analysis with distorted inflow; blade vibrations are 

determined as part of the computations; post-process the time history to estimate average damping over all 

blades and multiple vibration cycles. Upon considering the above approaches, approach (iii) is selected as 

the preferred approach that would avoid having to make large changes to the computational code, not be 

overly conservative, and not average out the effects of the inlet distortion entirely. It is recognized that due 

to the varying upstream conditions, each vibration cycle of each fan blade will be unique, and thus there will 

not be a convergence to a single value for all blades after running multiple vibration cycles. Instead, one 

would expect to obtain results that represent a range of aerodynamic damping values. 

The calculated aerodynamic damping with distorted inflow for design operating speed, near peak 

efficiency condition, mode 1 vibration, and 0 nodal diameter pattern is shown in Figure 15. The results 

for individual blades are plotted, along with the average over all the blades. Since the period of blade 

vibration is not the same as the period of blade rotation, each vibration cycle for each blade occurs in a 

different flowfield, resulting in a different aerodynamic damping. This plot shows a variation of less than 

0.3 percent in aerodynamic damping for different blades resulting from the circumferential variations in 

the flowfield due to the distorted inflow. The average over all blades is nearly unchanged after the first 

vibration cycle. Figure 16 shows the same results plotted as a variation with blade number; the average 

over all vibration cycles is seen to be nearly the same for all blades. 

Figure 15.—Aerodynamic damping with distorted inflow 

at design operating speed, mode 1 vibration, 0 nodal 

diameter pattern.

Figure 16.—Aerodynamic damping for all blades with 

distorted inflow at design operating speed, mode 1 

vibration, 0 nodal diameter pattern.

0.0

0.4

0.8

1.2

1.6

2.0

0 2 4 6 8 10

0 nodal diameter, all blades

average

Aerodynamic damping, %

Vibration cycle number

0.0

0.4

0.8

1.2

1.6

2.0

0 3 6 9 12 15 18

0 nodal diameter, all cycles

average

Aerodynamic damping, %

Blade number
NASA/TM—2013-217730 11

Figure 17.—Aerodynamic damping with distorted inflow 

at design operating speed, mode 1 vibration, 1 nodal 

diameter pattern.

Figure 18.—Aerodynamic damping for all blades with 

distorted inflow at design operating speed, mode 1 

vibration, 1 nodal diameter pattern.

The calculated aerodynamic damping with distorted inflow for design operating speed, near peak 

efficiency condition, mode 1 vibration, and 1 nodal diameter pattern is shown in Figure 17 and Figure 18. 

These results are of particular interest since the 1 nodal diameter pattern showed the lowest aerodynamic 

damping for clean inflow (Figure 13). Figure 17 shows that the range of variation in aerodynamic 

damping over all blades is approximately 0.3 percent, which is slightly larger than the range for 0 nodal 

diameter. The average value is nearly unchanged after the first vibration cycle. Figure 18 shows a slightly 

higher variation in average aerodynamic damping with blade number as compared to the 0 nodal diameter 

results (Figure 16), nonetheless this variation of the average value is quite small. As compared to the 0 

nodal diameter pattern, these results show lower average values of aerodynamic damping. The differences 

between the average values for clean inflow and distorted inflow are quite small and all calculated values 

of aerodynamic damping are positive. 

Concluding Remarks 

To realize the benefits of Boundary Layer Ingestion (BLI) propulsion, the fan must have acceptable 

aeromechanical characteristics to continually operate in the persistent inlet distortion coming from the 

BLI inlet. In this study, an intermediate design of a fan operating with a BLI inlet has been analyzed for 

its aeromechanical characteristics in the presence of distorted in-flow. The inlet duct was not modeled as 

part of this study. Instead, the inlet distortion from a separate flow calculation was imposed as the inlet 

boundary condition for the fan analysis. 

A structural model was created to analyze the structural dynamics characteristics of the fan, and a 

Campbell diagram was constructed. The forced response analysis provided the dynamic stresses at design 

rotational speed. Further analysis is required to examine the forced response at other rotational speeds 

where the engine order excitation frequency coincides with a blade natural frequency (on-resonance 

conditions). Further work is also needed to understand the effects of blade root fixity modeling on blade 

natural frequencies and mode shapes. 

Baseline fan flutter analysis was performed at design speed using clean inflow conditions. The 

aerodynamic damping near peak efficiency condition, calculated using the energy method, was seen to 

have low values for low nodal diameters for the first structural mode. For operating conditions near stall, 

the aerodynamic damping was seen to drop to very low values. However, none of the results showed 

negative aerodynamic damping that would indicate flutter. Similar analysis of the third mode also showed 

0.0

0.4

0.8

1.2

1.6

2.0

0 2 4 6 8 10

1 nodal diameter, all blades

average

Aerodynamic damping, %

Vibration cycle number

0.0

0.4

0.8

1.2

1.6

2.0

0 3 6 9 12 15 18

1 nodal diameter, all cycles

average

Aerodynamic damping, %

Blade number
NASA/TM—2013-217730 12

no flutter at design speed, both near peak efficiency and stall conditions. Further analysis is required at 

off-design conditions where flutter is more frequently encountered. Also, additional analysis is required to 

understand the sensitivity of the flutter stability results to physical and numerical parameters. 

Different approaches were considered to analyze flutter stability in the presence of the BLI distortion. 

Analysis was performed with both the inlet distortion and blade vibrations simultaneously modeled. No 

flutter was encountered in the limited number of cases examined for the first vibration mode. To reduce 

the possibility of aeromechanical difficulties during the operation of the BLI fan, many possible 

combinations of rotational speed, operating condition, vibration mode, and nodal diameter pattern need to 

be analyzed for flutter stability. 

Current aeromechanical analyses of the intermediate design of the BLI fan have not shown any 

problems of flutter or forced response. The analyses completed thus far represent initial work towards risk 

reduction in the design and testing of the BLI fan. Future work will focus on completing the analysis for 

this intermediate design and performing similar analyses for the final design that will be tested in the 

wind tunnel. Additional follow-on work may be related to coupled inlet-fan analyses, which would model 

with greater accuracy the operation of this BLI fan. Further, a tightly coupled aeroelastic analysis may be 

considered in the future to provide more detailed and accurate representation of the interactions between 

blade vibrations and inlet distortion. 

References 

1. Smith, L. H., “Wake Ingestion Propulsion Benefit,” AIAA Journal of Propulsion and Power, Vol. 9, 

No. 1, Jan.-Feb., 1993, pp. 74-82 

2. Daggett, D., Kawai, R., and Friedman, D., “Blended Wing Body Systems Studies: Boundary Layer 

Ingestion Inlets with Active Flow Control,” NASA/CR—2003-212670, December 2003. 

3. Kawai, R., Friedman, D., and Serrano, L., “Blended Wing Body (BWB) Boundary Layer Ingestion 

(BLI) Inlet Configuration and Systems Studies,” NASA/CR—2006-214534, December 2006. 

4. Plas, A. P., “Performance of a Boundary Layer Ingesting (BLI) Propulsion System, AIAA-2007-450, 

January 2007. 

5. Nickol, C. L., “Silent Aircraft Initiative Concept Risk Assessment,” NASA/TM—2008-215112, 

February 2008. 

6. Nickol, C. L., and McCullers, L. A., “Hybrid Wing Body Configuration System Studies,” AIAA2009-931, January 2009. 

7. Tillman, T. G., Hardin, L. W., Moffitt, B. A., Sharma, O. P., Lord, W. K., Berton, J., and Arend, D., 

“System-Level Benefits of Boundary Layer Ingesting Propulsion,” invited presentation, AIAA 49th 

Aerospace Sciences Meeting, January 2011. 

8. Florea, R. V., Matalanis, C., Hardin, L. W., Stucky, M., and Shabbir, A., “Parametric Analysis and 

Design for Embedded Engine Inlets,” AIAA Paper 2012-3994, July 2012. 

9. Berrier, B. L., Carter, M. B., and Allan, B. G., “High Reynolds Number Investigation of a FlushMounted, S-Duct Inlet with Large Amounts of Boundary Layer Ingestion,” NASA/TP—2005-

213766, 2005. 

10. Chen, J.-P., and Whitfield, D. L., “Navier-Stokes Calculations for the Unsteady Flowfield of 

Turbomachinery,” AIAA Paper 1990-0676, January 1990. 

11. Chen, J.-P. and Briley, W. R., “A Parallel Flow Solver for Unsteady Multiple Blade Row 

Turbomachinery Simulations,” ASME Paper GT2001-0348, June 2001. 

12. Srivastava, R., Bakhle, M. A., and Keith, T. G., Jr., “Numerical Simulation of Aerodynamic 

Damping for Flutter Analysis of Turbomachinery Blade Rows,” AIAA Journal of Propulsion and 

Power, Vol. 19, No. 2, March 2003. 

13. Srivastava, R., Bakhle, M. A., Keith, T. G., Jr., and Stefko, G. L., “Flutter Analysis of a Transonic 

Fan,” ASME Paper GT-2002-30319, June 2002. 
NASA/TM—2013-217730 13

14. Bakhle, M. A., Liu, Jong S., Panovsky, Josef, Keith, Theo G. Jr., and Mehmed, Oral, “Calculation 

and Correlation of the Unsteady Flowfield in a High Pressure Turbine,” ASME Paper GT-2002-

30322, June 2002. 

15. Panovsky, Josef, James, Darrell K., and Hassan, Khaled K., “An Assessment of Fan Flutter Using 

TURBO,” 5th National Turbine Engine High Cycle Fatigue Conference, published by Universal 

Technology Corporation, Dayton, Ohio, April 2000. 

16. Panovsky, J., Liu, J. S. and Bakhle, M. A., “Comparisons of Experimental and Computational 

Forced Response in a High Pressure Turbine,” Proceedings of the 7th National Turbine High Cycle 

Fatigue Conference, published by Universal Technology Corporation, Dayton, Ohio, April 2002. 

17. Kielb, Robert E., Barter, John W., Thomas, Jeffrey P., and Hall, Kenneth C., “Blade Excitation by 

Aerodynamic Instabilities,” ASME Paper GT2003-38634, June 2003. 

18. Sanders, A. J., Hassan, K. K. and Rabe, D. C., “Experimental and Numerical Study of Stall Flutter in 

a Transonic Low-Aspect Ratio Fan Blisk,” ASME Journal of Turbomachinery, Vol. 126, No. 1, pp. 

166-174, January 2004. 

19. Sanders, A. J., “Stall Flutter Assessment of and Advanced Design Transonic Fan Using TURBOAE,” Proceedings of the 6th National Turbine High Cycle Fatigue Conference, published by 

Universal Technology Corporation, Dayton, Ohio, April 2001. 

20. Herrick, G. P., “Effects of Inlet Distortion on Aeromechanical Stability of a Forward-Swept HighSpeed Fan,” AIAA Paper 2010-6711, August 2010. 

21. Carta, F. O., “Coupled Blade-Disk-Shroud Flutter Instabilities in Turbojet Engine Rotors,” Trans 

ASME Journal of Engineering for Power, Vol. 89, No.3, pp. 419- 426, July 1967. 

22. Bakhle, M. A., Reddy, T. S. R., and Stefko, G. L., “Comparisons of Flutter Analyses for an 

Experimental Fan,” NASA/TM—2010-216221, April 2010. 

23. Li, H. D., and He, L., “Single-Passage Analysis of Unsteady Flows Around Vibrating Blades of a 

Transonic Fan Under Inlet Distortion,” ASME Journal of Turbomachinery, Vol. 124, No. 2, pp. 285-

292, April 2002. 

24. Ekici, K., and Hall, K. C., “Nonlinear Frequency-Domain Analysis of Unsteady Flows in 

Turbomachinery with Multiple Excitation Frequencies,” AIAA Journal, Vol. 46, No. 8, pp. 1912-

1920, August 2008. 

25. Vahdati, M., Sayma, A. I., Bréard, C., and Imregun, M., “Computational study of intake duct effects 

on fan flutter stability,” AIAA Journal, Vol. 40, No. 3, pp. 408-418, March 2002. 
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Aeromechanics Analysis of a Boundary Layer Ingesting Fan 

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Bakhle, Milind, A.; Reddy, T., S., R.; Herrick, Gregory, P.; Shabbir, Aamir; Florea, Razvan, 

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14. ABSTRACT

Boundary layer ingesting propulsion systems have the potential to significantly reduce fuel burn but these systems must overcome the 

challenges related to aeromechanics-fan flutter stability and forced response dynamic stresses. High-fidelity computational analysis of the 

fan aeromechanics is integral to the ongoing effort to design a boundary layer ingesting inlet and fan for fabrication and wind-tunnel test. A 

three-dimensional, time-accurate, Reynolds-averaged Navier Stokes computational fluid dynamics code is used to study aerothermodynamic 

and aeromechanical behavior of the fan in response to both clean and distorted inflows. The computational aeromechanics analyses 

performed in this study show an intermediate design iteration of the fan to be flutter-free at the design conditions analyzed with both clean 

and distorted in-flows. Dynamic stresses from forced response have been calculated for the design rotational speed. Additional work is 

ongoing to expand the analyses to off-design conditions, and for on-resonance conditions.

15. SUBJECT TERMS

Aeroelasticity; Inlet distortion; Flutter; Unsteady flow; Forced response; Navier-Stokes equation; Turbomachinery 

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