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International Journal of Impact Engineering ] (]]]]) ]]]–]]]

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FE analysis of geometry effects of an artificial bird striking anaeroengine fan blade

S.A. Meguid�, R.H. Mao, T.Y. Ng

Division of Aerospace Engineering, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue,

Singapore 639798, Singapore

Received 9 January 2007; received in revised form 24 April 2007; accepted 27 April 2007

Abstract

Bird strike resistance of aeroengines is a strict certification requirement. Apart from costly experimental bird strike tests, explicit

numerical modeling techniques have been employed. However, due to the complicated bird geometry, artificial bird models are still not

well defined and it is a perennial problem selecting an appropriate representative artificial bird geometry for the simulations. To examine

the relative effects of the artificial bird geometry, explicit 3-D finite element analyses are conducted herein using the commercial code LS-

DYNA. As a validation test, we first studied the nonlinear transient dynamic response of an artificial bird striking a rigid flat target.

Following the validation, we studied the impact behavior of an artificial bird impinging a flexible aeroengine fan blade. The study focused

on the three most-frequently used configurations in the literature: namely, hemispherical-ended cylinder, straight-ended cylinder, and

ellipsoid, at various length-to-diameter aspect ratios. The results show that the initial contact area between the bird and target in the early

phase of the impact event would have a significant effect on the peak impact force. The aspect ratio of the bird striking both rigid panel

and flexible fan blade was found to have little influence on the normalized impact force and impulse.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Bird strike; Artificial bird configuration; Aspect ratio; Rigid panel; Flexible fan blade

1. Introduction

Ever since they began to share the sky with the birds acentury ago, aircrafts have been perpetually suffering frombird strikes. In fact, about 90% of all foreign objectdamage (FOD) can be traced to avian origins. Presently, allavailable evidence suggests that the bird strike hazard isincreasing, and this can be attributed to globalization andconservation, which has resulted in the steady increase inair traffic density levels as well as the dramatic expansion ofwild bird populations [1], Fig. 1. These strikes pose a realdanger to the lives of aircraft crew members and theirpassengers, and the potential for severe consequencesfollowing a strike is becoming more and more significant.This is because modern jet airliners are carrying more andmore passengers, and it is well known that even minordamage due to FOD can easily lead to a catastrophic chain

ee front matter r 2007 Elsevier Ltd. All rights reserved.

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ess: [email protected] (S.A. Meguid).

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of events. It is conservatively estimated by the Interna-tional Birdstrike Research Group [2] that collisionsbetween aircraft and birds cost the aviation industryworldwide over US$1.2 billion each year. As a result,certification authorities require that all exposed aircraftcomponents must be tested to prove their capability towithstand the most adverse impact loading. Specialattention has always been given to the aeroengine, which,alongside being the most vulnerable aircraft component orsystem to bird strikes, is also the only power plant of a jetairliner. The fan blade is the first component of anaeroengine that comes in contact with the bird in a birdstrike event, and the resulting plastic deformation andfracture of the fan blade may lead to partial or total loss ofthrust, as well as possible containment failure which isstrictly prohibited by aviation regulatory authorities, suchas the FAA [3]. In fact, even a minor damage to fan bladesmay introduce engine unbalance.In the 1970s, the validation of the structural integrity

and resistance to bird strike of aircraft components was

f an artificial bird striking an aeroengine fan.... Int J Impact Eng (2007),

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Nomenclature (in SI units)

Across average cross-sectional area of bird, Across ¼

m=r0 L ðm2Þ

D diameter of bird (m)E Young’s modulus of the blade material (Pa)F impact or contact force between bird and target

(N)I impulse between bird and target, I ¼R1

0F dt ðNsÞ

Iad normalized impulse, Iad ¼ I=m _w0

L length of bird (m)L/D aspect ratio (or length-to-diameter ratio) of

bird geometrym mass of bird, m ¼ 1.82 kgP pressure (Pa)PsTH theoretical stagnation pressure, pTH

s ¼

ð1=2Þr0 _w20ðPaÞ

Pad normalized impact pressure, Pad ¼

ðF=AcrossÞ=PTHs

T normalized time, T ¼ t=T0 ¼ t=ðL= _w0Þ

T0 nominal impact duration, T0 ¼ L= _w0ðsÞt time (s)u,v,w displacements in X, Y, and Z directions (m)_w0 initial velocity of bird, _w0 ¼ 225m/sX,Y,Z cartesian coordinates (m)sij stress tensor (Pa)_eij strain rate tensor (s�1)r instantaneous mass density of bird (kg/m3)r0 initial mass density of bird, r0 ¼ 934.3 kg/m3

m mass density changing ratio of bird, m ¼ðr=r0Þ � 1

V Poisson ratio of the fan blade materialg kinematic viscosity coefficient of fluidic bird

material (m2/s)

S.A. Meguid et al. / International Journal of Impact Engineering ] (]]]]) ]]]–]]]2

solely dependent on experiments. It was established byBarber et al. [4] that the loads generated by a high-speedimpacting bird were adequately duplicated by representingthe bird as a circular cylinder with the same mass, density,and compressibility as the bird tissue. Shortly thereafter,Wilbeck [5] noted the fact that, in case of high-speedimpact, the response of the bird is similar to that of a fluid(such as water) where the strength of the bird material isextremely small compared with the impact loads. Inaddition, high-speed photography was employed by Gaoand Li [6] and Teichman and Tardos [7] to record theevolution of the bird torso and the large deformations ofthe targets such as aeroengine fan blades, through whichthe fluidic property hypothesis of the bird tissue under

Fig. 1. Bird-strike frequency in

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high-speed impact scenarios was further strengthened.These experimental tests, however, are very expensive,time-consuming and difficult to perform. Besides, due tothe extremely high velocities and energies involved, theexperimental registration of the impact parameters canoften prove difficult and complex.With the development of advanced numerical techniques

and the advent of high-performance computing, numericalsimulations have been more widely used since the 1980s toevaluate the impact capability of different aircraft compo-nents. Explicit nonlinear finite element (FE) codes, whichare available in several high-end commercial FE solvers,have been used to treat this class of problems. An accuratenumerical model can reveal a significant amount of useful

recent years in the USA [1].



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Fig. 2. The three configurations considered for the artificial bird model: (a) straight-ended cylinder, (b) hemispherical-ended cylinder, and (c) ellipsoid.

S.A. Meguid et al. / International Journal of Impact Engineering ] (]]]]) ]]]–]]] 3

information to the designer with regard to the mechanismsinvolved in the high-speed soft body impact event. Havingprivy to these information prior to conducting experi-mental tests enables the rapid and economical design ofaircraft structures with enhanced impact resistance. Inconjunction with the broadened application of numericaltechniques to bird strike modeling and simulation, theartificial bird has substituted real birds, mainly for betterrepeatability and convenience [8].

For all its promise, the reliability of the numerical resultsis critically dependent on the accurate modeling of thetemporal and spatial distribution of the impact forcebetween the bird and target [9,10]. In connection with this,the complex and intricate geometrical configurations ofdifferent bird species have perpetually posed a problem fordeveloping a sufficiently simplified and consistent birdmodel. Many researchers such as Frischbier [11], Langrandet al. [12], McCarthy et al. [13], and Airoldi and Cacchione[14], have simplified the bird torso as a hemispherical-ended cylinder. The ellipsoid geometry is also a well-accepted choice, which has been suggested by the Interna-tional Birdstrike Research Group [15], and has been usedby Guan et al. [16]. Besides these two configurations, thestraight-ended cylinder has also been adopted by Brock-man and Held [17], but its application remains somewhatinfrequent. These three configurations, which are typical ofartificial bird geometries, are shown schematically in Fig. 2.Nevertheless, the differences among the impact behaviorsassociated with these different configurations have notbeen reported in the open literature. Furthermore, theeffect of the length-to-diameter aspect ratio (representingthe biometric property of different bird species) is alsoexamined.

It has been highly recommended by the InternationalBirdstrike Research Group that the bird model, oncestandardized, should become the norm for all bird impacttesting thereafter [18]. From the simulation viewpoint,standardizing the geometry of the artificial bird model iscorrespondingly important. Both the bird configurationand aspect ratio have been found to be importantparameters that influence the impact response of the target[19]. Thus, the aim of the present investigation is tocompare and analyze the numerical results for the above-mentioned three different bird configurations, and variousaspect ratios will also be considered. The simulation will becarried out using highly accurate Lagrangian-based explicit


doi:10.1016/j.ijimpeng.2007.04.008

nonlinear FE analysis. The flexible fan blade adopted in thepresent investigation is a typical metallic wide-chordaeroengine fan blade.

2. Finite element modeling in a Lagrangian framework

The finite element method (FEM) has been found to bevery powerful in the numerical simulation of the crash-worthiness and failure analyses [20]. The coupling betweenthe bird and the target can be achieved by a Lagrangianformulation. During high-speed impact, large straindistortions will inevitably occur to the discretized Lagran-gian bird, leading to a decrease in solution time-step andpossible negative elemental volumes. The excessivelydistorted elements that possess negative volumes are theneliminated after each time step. However, this automaticelimination procedure usually introduces artificial oscilla-tions in the contact force between the bird and target.Fortunately, the strategy of employing highly refinedmeshes that encounter element elimination can be used toalleviate this problem [21]. Modern high-performancecomputing systems can tolerate very small time steps inthe order of 10�9 s or even 10�1 s, where it used to be10�6–10�7 s, just a few years ago. Thus, by refining themesh of the Lagrangian bird, the artificial oscillationsassociated with the impact force can be dramaticallyreduced.

2.1. Bird properties

The bird’s mechanical property actually changes fromthe low-velocity to the high-velocity regimes. Generally, themechanical property of typical avian tissues at lowspeeds is neither uniform nor homogeneous. However, atprogressively higher speeds, this nonuniformity andinhomogeneity become increasingly negligible, and thebird can safely be considered as a homogeneous jetof fluid impinging a structure [5]. Thus, the constitu-tive material law of homogenized fluidic materials can beused:

sij ¼ �Pdij þ 2rg _eij . (1)

There are different hydrodynamic models which havebeen successfully used to describe the material proper-ties of the bird in compression and among whichthe most popularly used is the polynomial fitted pressure



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Fig. 3. Discretized geometries of the three bird configurations used.

Fig. 4. A schematic diagram of a bird striking a flexible fan blade: (a)

isometric view, and (b) top view.


equation:

p ¼ C0 þ C1mþ C2m2 þ C3m3, (2)

where m is the nondimensional mass density changing ratioof the bird tissue earlier defined in the nomenclature. Here,we model the loading from the bird as a pressure pulse onthe structure. This is a feasible approach and we can alsoavoid modeling the bird disintegrating into numerousdebris particles. The Brockman compressible modules [17]are employed in the present simulations, whereby

C0 ¼ 0;

C1 ¼ 2323MPa;

C2 ¼ 5026MPa;

C3 ¼ 15180MPa:

8>>><>>>:

(3)

The total mass of the bird is 4 lb (or 1.82 kg), which ispresently used as the upper limit of the bird mass criteria inthe bird–aircraft strike scenarios [22]. Its initial massdensity is set at 934.3 kg/m3, while the initial velocity ofthe bird is taken to be 225m/s in the normal Z-direction.Table 1 lists the detailed parameters of three geometrieswith the commonly used aspect ratio of 2.0:1. Table 2 listsall diameters of hemispherical-ended cylindrical birds withaspect ratios 1.5:1, 2.0:1, and 2.5:1. The bird model ismeshed with 3-D 8-node fully integrated solid elements(Solid-164 in LS-DYNA) with a characterized ratio of birddiameter to mesh size of 32, as shown in Fig. 3, which isfound to provide efficient simulation runs without com-promising on accuracy. In addition, the hourglass energy isfound to be well controlled in the present simulation.

2.2. The flexible fan blade

A sector of the fan disk, composed of a single blade anda hub section, as shown in Figs. 4(a) and (b), is used in thepresent simulation. The hub is assumed to be fixed andrigid in comparison with the blade. The deformable bladeis discretized using the 2-D shell elements (Shell-163 in LS-

Table 1

Details of the different bird configurations considered (aspect ratio is

2.0:1)

Configuration

type

Straight-ended

cylinder

Hemispherical-

ended cylinder

Ellipsoid

Length L (m) 0.214 0.228 0.246

Diameter D (m) 0.107 0.114 0.123

Table 2

Details of the different bird aspect ratios considered (configuration is

hemispherical-ended cylinder)

Aspect ratio L/D 1.5:1 2.0:1 2.5:1

Length L (m) 0.192 0.228 0.260

Diameter D (m) 0.128 0.114 0.104


doi:10.1016/j.ijimpeng.2007.04.008

DYNA), with 31 nodes in the axial direction and 61 nodesin the radial direction found to provide sufficientlyconverged results. In addition, the Hughes–Liu formula-tion was used to eliminate the hourglass modes. The

Fig. 5. Velocity vectors of the bird at T ¼ 0.05 when impacting a rigid

target: (a) side view, and (b) isometric view.



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Fig. 6. Deformation history of a bird impacting a rigid target: (a) T ¼ 0,

(b) T ¼ 0.25, (c) T ¼ 0.5, (d) T ¼ 0.75, and (e) T ¼ l.


hourglass coefficient was set to 0.1, quadratic bulk viscosityto 1.5, and linear bulk viscosity to 0.06.

Fan blades of modern aeroengines are typically made oftitanium alloy Ti–6A1–4V. Due to the high strain ratesassociated with this problem, the constitutive law used is ofthe viscoplastic type originally devised by Perzyna [23]

syð�peff ; _�

peff Þ ¼ syð�

peff Þð1þ ð_�

peff=CÞ1=p

Þ, (4)

where _�peff is the effective plastic strain rate, and syð�

peff Þ is

the initial quasi-static yield stress of the blade material. C

and P are strain rate sensitive parameters determined byexperiments. The magnitudes of the relevant parametersare listed as follows:

E ¼ 1:14� 1011 Pa;

v ¼ 0:33;

rblade ¼ 4:429� 103 kgm�3;

syð�peff Þ ¼ 1:14� 108 Pa;

C ¼ 40:0 s�1;

P ¼ 5:0:

8>>>>>>>>><>>>>>>>>>:

(5)

3. Results and discussion

3.1. Bird striking a rigid panel

Fig. 5 shows a typical example of the velocity vectors forthe hemispherical-ended cylindrical bird model striking arigid target at a normalized time J ¼ t/T0 ¼ 0.05. Thesimulations begin at the instant the traveling bird impingesthe target. Both side and isometric views are shown in thatfigure. It shows that the bird has been deformed, with itsright tip contacting the target (Fig. 5(a)). The maximumvelocity of the fluidic bird elements, which appears near thefrontal surfaces of the bird close to the target, is found tobe approximately 123m/s at this instant of impact.

The time histories of the bird impacting a rigid panel fordifferent phases are depicted in Fig. 6. The hydrodynamicfluid-like behavior of the bird can be clearly observed fromT ¼ 0.5 onwards. During the impact process, the momen-tum of the bird will be progressively absorbed by the target;whereas the kinetic energy of the bird will be dissipated interms of heat, as well as manifested by the elimination ofsome of the bird elements. Finally, the bird elements, withthe umbrella-like configuration, do not possess additionalforward momentum, and the impact process is completedat the normalized time T ¼ l. Nevertheless, following thisimpact event, the bird elements continue to deform andexpand outwards.

Fig. 7 shows the variation of the normalized impactpressure over time. It is compared with the experimentaldata from the GARTEUR Bird Strike Group [24] as wellas the numerical results of Langrand et al. [12]. It is clearlyobserved that the present numerical results correspond wellwith the experimental data. When compared with the


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simulation results reported in the literature, the presentresult exhibits fewer oscillations than those prevalent inearlier studies, and is clearly more stable. This can beattributed to the fine mesh density used in discretizing thebird geometry. Specifically, we used a bird-to-elementlength ratio of about 32, compared with 8 used by Stoll andBrockman [25], and Airoldi and Cacchione [14]. The peakvalue of the pressure from the present simulation is about20% lower than that of the experimental data, and thisdifference is probably due to the use of water-compressiblemodules for the bird’s material model, which under-estimates the behavior of a real bird.

3.1.1. Effects of bird configuration

Fig. 8(a) shows the impact force variations for differentconfigurations of the bird model. It is found that thestraight-ended cylinder reaches its maximum impact forceof 7.99� 105N at about 0.049ms after initial contact, andthere is only one dominant peak in its contact force profile.However, there are two quite distinct peaks correspondingto the hemispherical-ended cylinder and ellipsoid models.The hemispherical-ended cylinder reaches its maximumforce of 5.27� 105N at 0.054ms, and its second peak of3.33� l05N at 0.134ms, although this second peak is notas significant as the first one. The ellipsoid model, on theother hand, reaches its first peak of 3.60� 105N at0.111ms, and its second peak of 3.78� l05N at 0.28ms.Interestingly, this second peak is found to be the maximummagnitude in its force variation time history. Among thethree different bird geometries, the maximum impact forcefor the straight-ended cylindrical bird is the highest. This isbecause at the instant the three types of bird configurationsimpinge the rigid target, the straight-ended cylindrical birdhas comparatively the largest instantaneous contact area.



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Fig. 7. Variation of normalized impact pressure versus normalized time for a hemispherical-ended cylindrical bird impinging a rigid panel.

Fig. 8. Effect of artificial bird configurations on rigid impact forces and pressures: (a) impact forces, and (b) normalized impact pressures.


Please cite this article as: Meguid SA, et al. FE analysis of geometry effects of an artificial bird striking an aeroengine fan.... Int J Impact Eng (2007),

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Fig. 9. Effect of aspect ratios of artificial birds on rigid impact forces and pressures: (a) impact forces, and (b) normalized impact pressures.


This also explains why there is only one significant peak inits force variation, and it is because all the elements of thisbird model will decelerate immediately and dramaticallyonce it impacts the target. However, this situation is quitedifferent for the hemispherical-ended cylinder and ellipsoi-dal birds. Since the bird in the high-speed impact scenariosbehaves as a liquid, the shear stress between the bird’selements flowing on neighboring streamlines, comparedwith the local high pressure, is comparatively negligible.This has been experimentally observed by Barber et al. [4],Wilbeck [5] and Gao and Li [6]. The present investigationtreats the bird hydro-dynamically, and neglects the fluidicviscous effect of the bird tissue. Therefore, for thehemispherical-ended cylinder and ellipsoidal birds, onlythe elements directly downstream of those in contact withthe target will experience the dramatic deceleration,whereas the other elements will not decelerate significantlyuntil their frontal elements come into contact with the


doi:10.1016/j.ijimpeng.2007.04.008

target. Thus, when impact progresses for the hemisphe-rical-ended cylinder or ellipsoidal birds, the contact forcesdecrease after the initial peaks, but will increase again,leading to their respective second peaks. This second peakis quite distinct in the ellipsoidal case, but rather lesspronounced in the hemispherical-ended cylindrical birdcase. In the ellipsoid model, the contact area reaches itsmaximum value somewhat late into the impact event. Itoccurs at approximately the midpoint of the entire impactduration, which explains the presence of its second peak.The normalized impact pressure evolutions for the three

bird configurations are plotted in Fig. 8(b). It should benoted that because the cross-sectional areas vary along thegeometrical axes of the hemispherical-ended cylinder andthe ellipsoid models, the normalization is based on theirrespective averaged cross-sectional areas. The maximumnormalized impact pressure is found to be 3.73 atT ¼ 0.055 for the straight-ended cylinder, 2.61 at



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Fig. 11. Deformation history for a hemispherical-ended cylindrical bird

impinging a flexible fan blade: (a) T ¼ 0, (b) T ¼ 0.25, (c) T ¼ 0.5, (d)

T ¼ 0.75, (e) T ¼ l, and (f) T ¼ 1.34.

Fig. 10. Effect of aspect ratios of artificial birds on rigid impact impulse.


T ¼ 0.053 for the hemispherical-ended cylinder, and 2.00 atT ¼ 0.27 for the ellipsoid model. Thus, the maximumimpact pressure for the straight-ended cylinder impact caseis about 43% higher than the hemispherical-ended cylindercase, and the latter is about 30% higher than the ellipsoidcase.

3.1.2. Effects of bird aspect ratio

In this section, the 4 lb bird with mass density 934.3 kg/mwas simulated at different aspect ratios of 1.5:1, 2.0:1, and2.5:1. We selected the hemispherical-ended cylinder geo-metry for the simulations. The impact force evolutions areplotted in Fig. 9(a). Due to its largest cross-sectional areas,the model with 1.5:1 aspect ratio has a maximum impactforce of 6.57� l05N, and it drops to zero first at 0.62ms.The peak impact forces for the 2.0:1 and 2.5:1 cases are5.27� l05 and 4.62� l05N, respectively. This is due to thevariance in their respective cross-sectional areas. Fig. 9(b)shows that the peak values for the three cases are much thesame after normalization, being 2.75 for aspect ratio 1.5:1,2.62 for aspect ratio 2.0:1, and 2.61 for aspect ratio 2.5:1.Thus, we note that the aspect ratio does not affect thenormalized maximum impact pressure significantly. Theimpulses for the three aspect ratios are shown in Fig. 10. Itis found that the impulse, after normalization, is 0.29 forthe aspect ratio 1.5:1, 0.23 for the aspect ratio 2.0:1, and0.26 for the aspect ratio 2.5:1.

3.2. Bird striking a fan blade

In these simulations, the bird impacts the fan blade at aheight of 85% radius of the blade from the hub. Due to theflexibility of the fan blade, the impact duration will belonger than the nominal impact time T0 ¼ L/w0. Forexample, for the hemispherical-ended cylindrical bird, itsimpact with the fan blade is completed at normalized time


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T ¼ 1.34. The time histories of the impact process atdifferent phases are shown in Fig. 11. The deformation ofthe blade when the impact force finally vanishes is alsoshown in Fig. 11(f). Nevertheless, both the bird and theblade will continue deforming even after the impact forcevanishes. However, in the present investigation, only themajor bird–blade coupled effects are of interest, and weterminate the simulation when the impact force vanishes.

3.2.1. Effects of bird configuration

The impact forces between the hemispherical-endedcylindrical bird and the fan blade are shown in Fig. 12.



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Fig. 12. Impact force variations for a hemispherical-ended cylindrical bird (UD ¼ 2.0) striking a flexible fan blade.

Fig. 13. Effect of artificial bird configurations on normalized impact pressure when striking a flexible fan blade.


Since the geometry of the blade is curved, the impact forcesare resolved into the X, Y, and Z directions. However, itcan be seen from this figure that the impact force in the Z-direction is dominant, because the initial velocity of thebird is in the Z-direction. Henceforth, only the results inthe Z-direction will be shown in the following results.There are multiple peaks in the impact force evolution,which is due to the coupling between the bird and theflexible blade. The blade, upon being impacted by the bird,accelerates in the same direction as the bird’s initialvelocity, i.e., the Z-direction. When this happens, thefrontal elements of the bird which are in close contact withthe blade would expand and their high pressures wouldthus decrease. Therefore, the total impact force would alsodecrease dramatically. Consequently, with the loweredimpact force from the bird, the bending of the blade woulddecelerate, and as a result the contact between the bird andthe blade becomes intense once more. The impact forcebetween the bird and blade would increase again. The


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whole procedure will repeat several times before the impactforce eventually vanishes.The normalized impact pressures between the birds and

fan blade are shown in Fig. 13 for all the three birdconfigurations. The dominant peaks are 1.75 at T ¼ 0.094,1.35 at T ¼ 0.189, and 0.77 at T ¼ 0.32 for the straight-ended cylindrical bird, and 1.62 at T ¼ 0.06, 1.78 atT ¼ 0.16, and 1.46 at T ¼ 0.45 for the hemispherical-endedcylindrical bird. For the ellipsoidal bird, the peaks are 0.79at T ¼ 0.06, 1.53 at T ¼ 0.16, and 1.34 at T ¼ 0.39. FromFig. 13, we can observe that the normalized impactpressure variation profiles for the three bird configurationsare quite different from the preceding rigid impact cases.This is especially so for the straight-ended cylindrical bird,which no longer possesses the highest peak impact pressurevalue among the three bird configurations. As there is anattack angle of 601 between the bird trajectory and theblade, as shown in Fig. 4(b), the initial contact areabetween the straight-ended cylindrical bird and the blade is



ARTICLE IN PRESSS.A. Meguid et al. / International Journal of Impact Engineering ] (]]]]) ]]]–]]]10

not as large as the rigid panel case. Thus, the initial contactforce between the bird and target is also significantlyreduced. More importantly, the blade is not rigid butflexible, and the maximum impact force between thestraight-ended cylindrical bird and blade is found to beonly 47% of the rigid impact case. Similar conclusions havebeen reached by Shimamura et al. [10] that the target’sflexibility strongly influences the impact force between thebird and target. The impact force between the hemisphe-rical-ended cylindrical bird and blade is about 68% of theformer rigid impact case, and correspondingly 76% for theellipsoidal bird model. Comparing the three geometries, itis noted that the ellipsoidal bird model has a maximumimpact force, which is lower than the other two configura-tions by about 15%.

Fig. 14. Effect of artificial bird configurations on norm

Fig. 15. Effect of aspect ratios of artificial birds on normal


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It is also interesting to note that all the three types ofbird configurations have three significant peaks within theirvariations of the impact forces. This is mainly because thebird mass and density are similar for the three configura-tions. By varying the mechanical properties of either thebird or the blade, including their masses, densities and/orother properties, it can be expected that the number ofsignificant peaks for the impact force may change as well.The normalized impulse between the bird and blade is

shown in Fig. 14. The momentum of the bird is transmittedto the fan blade over the duration of the impact process.However, only 24.4%, 37.5%, and 40.2% of the initialmomentum of the bird are finally transmitted to the bladein the Z-direction, for the straight-ended cylinder, hemi-spherical-ended cylinder, and ellipsoidal bird, respectively.

alized impulse when striking a flexible fan blade.

ized impact pressure when striking a flexible fan blade.



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Fig. 16. Effect of aspect ratios of artificial birds on normalized impulse when striking a flexible fan blade.


The present simulation has been successful in suppressingthe stairway profiles of the impulse accumulation, whichwas frequently observed in earlier results, such as Stoll andBrockman [25], and Langrand et al. [12].

3.2.2. Effects of bird aspect ratio

Fig. 15 illustrates the evolution of the normalized impactpressure between the fan blade and hemispherical-endedcylindrical birds with three different aspect ratios of 1.5:1,2.0:1, and 2.5:1. Since their configurations are similar, theirevolution histories are also found to be moderatelycomparable. For all of the cases considered, there arethree significant peaks within their impact profiles, and allof them reach their respective maximum values at thesecond peak, at magnitudes of 1.66 for aspect ratio 1.5:1,1.78 for aspect ratio 2.0:1, and 1.70 for aspect ratio 2.5:1.The relative differences are less than 7%. Next, thenormalized impulse variations for the three aspect ratiosare shown in Fig. 16. It can be seen that the impulseevolutions are quite similar as well. Within the normalizedtime T ¼ 0–0.6, the impulse increases in an almost linearmanner. The rate of increase in the impulse plateaus levelsoff after T ¼ 0.6. When the impact force reaches zero, wenote that 34%, 37%, and 32% of the initial momentum ofthe bird has been transmitted to the fan blade, for the threerespective aspect ratios. Thus, we can draw from this studythat the aspect ratio does not have significant effects on thenormalized impact pressure and impulse results.

4. Conclusions

Bird strikes pose one of the most dangerous threats tothe safety of modern aircrafts. Bird strike tests are costlybut necessary. However, it is also highly desirable todevelop accurate predictive numerical tools to assess thebird impact resistance of aeroengine components. In thisrespect, the geometrical modeling of the artificial bird isvery important, though a widely accepted standardized


doi:10.1016/j.ijimpeng.2007.04.008

bird model has not been realized yet. To address this, threedifferent frequently used configurations and aspect ratios,representing diverse biometric bird species, have beenexamined in the present investigation. It is found from thisstudy that the initial contact area between the bird andtarget in the early phase of the impact event would have asignificant effect on the peak impact force value. For thebird model impacting a rigid target, the maximum impactforce for the case of a straight-ended cylindrical bird isabout 43% higher than that of a hemispherical-endedcylindrical bird, which in turn is 30% higher than that ofan ellipsoidal bird. On the other hand, the impact forceprofile is also found to be highly dependent on thedeformation of the fan blade. Due to the flexibility of thecurved fan blade, the maximum impact forces from thestraight-ended cylinder, hemispherical-ended cylinder, andellipsoidal bird models impacting the presently studied fanblade would be respectively reduced by 53%, 32%, and24%, when compared with their corresponding rigidimpacts. The present numerical simulations also revealthat the length-to-diameter aspect ratio of the bird strikingboth a rigid panel and a flexible fan blade has littleinfluence on the results, especially the normalized impactpressure and impulse.

Acknowledgements

The authors would like to thanks DSO laboratories,Singapore for their kind supports to the present investiga-tion.

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[2] Allan JR. The costs of birdstrikes and birdstrike prevention. In:

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