Numerical Simulation of Aircraft Ditching of aGeneric Transport Aircraft: Contribution to

Accuracy and Efficiency

Maria Inês Costa Cadilha

Instituto Superior Técnico, University of Lisbon

Supervisors: Dipl.-Ing. Martin Siemann, Prof. Afzal Suleman

November 2014

Abstract—Planned water impact should be covered for the certification of an aircraft operating over sea. In that case, themanufacturer should show compliance to the specific regulations relating to ditching. Most certification testing is currentlyexperimentally based, due to the interdisciplinary character of the physical problem and complex hydrodynamic effectsarising from the high forward velocity of the impacting body. However, test campaigns are expensive and time-consuming,and thus the industry is now turning its attentions to numerical tools to assist in crashworthy structural design.Of key importance for ditching investigations is the ability for the numerical method to accurately capture the highdeformations of the fluid domain in such events. To that aim, the present thesis focuses on the classical finite-element (FE)method and the meshless Smoothed Particle Hydrodynamic (SPH) techniques, for the modeling of the water and aircraftstructure. Contact is facilitated as both formulations are of Lagrangian nature. Recently, the hybrid FE-SPH explicit codeVPS (formerly known as PAM-CRASH) from ESI-Group was extended by various features to simulate hydrodynamiceffects. These innovations have been validated for guided ditching tests in which representative plates simulate ditchingconditions. The present research aims to transpose the acquired knowledge to the problematic of full-scale, fixed wingaircraft involving high-forward velocity.Within the framework of this task, a computational pre-processing tool is developed for automated model generation,and a comparison of different modeling approaches is performed in terms of quality of the results and computationalrequirements. Water modeling techniques are investigated with the objective of reducing computational effort. For theaircraft, different modeling schemes are considered, ranging from simple rigid body approaches to flexible FE models. Aseparation stress feature is tested to simulate suction forces between fuselage and water domain. Finally, shared memoryprocessing and multi-model coupling computational schemes are considered.Key words: Smoothed Particle Hydrodynamics, Fluid-Structure Interaction, Fixed-Wing Aircraft Ditching, Water Impact

I. INTRODUCTION

On January 15, 2009, US Airways Flight 1549 broughtaircraft ditching into public attention. The airplane wasstruck by a flock of geese soon after takeoff from NewYork’s La Guardia airport, losing thrust in both engines.The pilot told his passengers to brace for a hard impactand then set the plane down safely on the Hudson River.All 155 occupants safely evacuated the plane. More thanthe "Miracle on the Hundson", as it became known, itwas a testament to the skill of the crew, and a proof ofthe robustness of the airframe of the Airbus A320.

In fact, since many airports are close to water andmany flights are operating partly overseas, impacting onwater is a possibility that could be encountered by mostaircraft. Even tough this is (fortunately) a very rare event,regulations address ditching in the certification of anaircraft operating over the sea. The manufacturer shouldshow compliance to the specific airworthiness regulations,which can be roughly summarized as in the following:the aircraft should be able to land on water as safely aspossible and to float long enough in order to enable thepassengers to evacuate.

The demonstration of compliance to ditching require-ments may be realized by either testing or comparing toaircraft of similar design, of which the ditching behaviorhas already been certified. The use of sub-scale aircraftmodels, launched in a water tank, is currently morecommon to investigate various ditching conditions. Thekinematics of the aircraft impacting on the water stronglydepends on its configuration at the moment of the event.It is, therefore, required to thoroughly investigate theinfluence of position of flaps, location of the center ofgravity, weight and/or eventual damage on the structurefor the forced landing on water. External factors likea rough sea may have to be taken into account. Astest campaigns are time-consuming and very expensive,the use of simulation tools capable of predicting theresponse of aeronautical components, or the entire aircraft,facilitates the development and the evaluation of newdesigns.

Numerical modeling provides a platform to evaluatenew crashworthy concepts prior to expensive testingprograms, and the global research focus is on developingefficient simulation tools, particularly using explicit FiniteElement (FE) modeling methods. The challenge withsimulating the ditching case is the high deformation ofthe water domain, which needs to be modeled effectivelyto predict the structural response. Standard FE models areoptimum for structural representation, but grid based nu-merical methods suffer from difficulties which limit theirapplications to the water domain in ditching simulationsinvolving forward velocity. One alternative which hasrecently been a major research focus is the use of meshfreemethods for the water, which overcome the problem ofrestrictive mesh distortion. Exemplary is Smoothed Parti-cle Hydrodynamics (SPH), a meshfree particle methodbased on Lagrangian formulation, which has been widelyapplied to different areas of engineering and namely tothe study of aircraft water landing. One other significantadvantage of SPH is the simplified contact between FEstructural aircraft models and the water media for FSI,as both the structure and the water are modeled usingLagrangian formulations. Investigations using this methodshowed promising results, with the drawback of increasedcomputational expense when compared to the standard

FE formulation.Within the course of the European commission’s

SMAES project, initiated in 2011, water impact wasinvestigated using the FE-SPH method with the goalto permit cost effective design and entry-into-serviceof aircraft able to protect its occupants during a waterlanding. The results were validated with data originatingfrom a guided ditching campaign, in which representativeplates were experimentally investigated under realisticditching conditions. Within this research, the hybrid SPH-FE explicit code VPS (formerly known as PAM-CRASH)from ESI-Group was extended by various features forimprovement of water modeling and reproduction of FSIphenomena. The main objective of this thesis is to com-bine the state-of-the-art modeling techniques developedin SMAES with the aircraft modeling knowhow of theDLR Institute of Structures of Design, applying themto full-scale aircraft ditching simulations. The researchalso aims to explore advantages and limitations of thedifferent modeling options currently available.

Within the framework of this task, a computationalpre-processing tool was developed for automated modelgeneration, and a comparison of different modeling ap-proaches was performed in terms of quality of the resultsand computational efficiency. Accuracy and efficiencyare forefront in order to provide an improvement in thepredictability of loads occurring during ditching.

II. COUPLED FINITE ELEMENT-SMOOTHED PARTICLE

HYDRODYNAMIC FORMULATION

The structural models in the current investigation arecomposed of rigid and flexible classic finite elements. Thefluid domain is modeled using both weakly compressibleSPH method, in the vicinity of impact where largerdeformations are expected, and FE hydrodynamic volumesfor the regions where less disturbance occurs.

Smoothed Particle Hydrodynamics was initially de-veloped in 1977, by Lucy [1] and Gingold [2], for thesimulation of astrophysics problems. Since then, manyresearchers have conducted investigations on the thismethod, improving it to the state of a very powerfulformulation for problems governed by the Navier-Stokesequations. There are two main steps in obtaining an SPHformulation. The first step is to represent a function

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and/or its derivatives in continuous form as integralrepresentation, and this step is usually termed as kernelapproximation. The second step is usually referred to asparticle approximation. In this step, the computationaldomain is first discretized by set of of particles rep-resenting the initial settings of the problem. During thesimulation, field variables (e.g. density, pressure, velocity)on a particle are approximated by a summation of thevalues over the nearest neighbor particles [3]. A schemeis presented in Figure 1.

Figure 1: SPH particle approximation in problem domainΩ with a surface S. W is the smoothing function that isused to approximate the field variables at particle i usingaveraged summation over particles j within the supportdomain with a cutoff distance of κhi [3].

The Wendland kernel function is used in the presentresearch with a radius of 2h influence of twice the smooth-ing length h. The numerical stability is treated by usingthe standard Monaghan-Gingold artificial viscosity term[5]. The system of Navier-Skotes differential equationsis closed by adding an equation of state (EOS) relatingpressure to density. The frequently used Tait EOS (alsoreferred as Murnaghan law) [6] with reference pressure p0,speed of sound c0, ratio of current over initial mass densityρ/ρ0 and adiabatic exponent of the fluid γ reduces thecomputation time by artificially increasing compressibility,and is therefore used in the present thesis:

p = p0 +c2

0ρ0

γ

[(ρ

ρ0

)γ

−1]

(1)

Coupling of the SPH water particles and the aircraftFE model is achieved by using a node-to-segment penaltycontact formulation.

Previous publications highlight the advantage of mod-eling the water domain closer to impact with SPHelements due to high deformations perceived, and touse inexpensive classic hydrodynamic FE volumes inthe areas of less disturbance. However, computationaltimes remained excessive and suction forces could notbe modeled using SPH for the water domain. Simulatingaircraft ditching required a list of improvements over thestate of the art to enhance the accuracy and efficiencyof existing numerical tools. Within the EC-FP7 SMAESproject, the German Aerospace Center, Airbus Militaryand ESI-Group teamed up to simulate the hydrodynamiceffects with the hybrid FE-SPH code VPS from ESI-Group. To this end, the SPH module within VPS has beenextended by various features including a separation stressoption to mimic suction, periodic boundary conditionsand particle regularization methods. Validation has beenperformed in studies using both sub-scaled models andguided plates impacting in water. The next logical stepwas to transfer this knowledge into full-scale aircraftditching simulations, to further understand the potentialof applying this methodology in numerical tools to assistin design and certification.

III. AUTOMATED MODEL GENERATION TOOL FOR

NUMERICAL INVESTIGATIONS

Finite element analysis can be separated in three distinctphases: pre-processing, solution and post-processing. Thepre-processing phase consists of developing an appro-priate geometry, finite element mesh, designate suitablematerial properties, and apply boundary conditions asrestraints and loads. When dealing with hybrid FE-SPHanalysis, this also includes appropriate particle distributionand contact definition. Preparation of large models canthus be very time consuming and consistency betweendifferent modeling approaches is easily compromised dueto the large number of parameters involved.

The VPS explicit solver requires a specific format ofinput data organized in so-called input cards. The setupof the model can be performed by using the windows-based graphical interface Visual-Crash PAM and exportingthe resulting geometry files, or by directly typing inthese cards into a text file from scratch. This latter

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option requires good knowledge of solver commandsand corresponding keywords.

The automation of this input generation was performedby developing an intuitive pre-processing script dedicatedto modeling of ditching scenarios. The tool will automat-ically generate all the necessary VPS cards (e.g. nodalpoints, materials, function curves, contact definitions) forthe model specified by the user. This includes the setupof several aircraft and fluid domain configurations. Ifspecified, the tool will directly feed the input files to thesolver and start the referred simulation, and all what isleft to the user is the post-processing of the results.

The code was developed predominantly in Pythonprogramming language (Version 2.7.2), making use of thecommercial software ANSYS (Version 14.5) to performcertain geometry meshing tasks. The VPS input generatedis specified for version 2010, due to restrictions imposedby the development version available.

The user is allowed to choose between a graphicinterface or a fully automated mode in the tool. Inputis made through water parameter definition and desiredaircraft geometry. Several different modeling approachesfor the aircraft are allowed, and are described in thefollowing paragraphs.

General and Detailed Finite Element Models

(GFEM/DFEM) generation is entirely performed by anincorporated pre-existing in-house tool, AC-CRASH. Thisfuselage modeling tool was developed to automaticallygenerate a global FE model using elastic beam elementsfor preliminary sizing purposes (GFEM). To be also usedin crash simulations this model was extended in a way thatcertain regions where high plastic deformations or failureis expected may be modeled by the use of shell elementsand a finer mesh (DFEM). Those local refinements arenamed detailed regions. By this approach the advantagesof both discretization methods are unified: the globalaircraft stiffness is mapped correctly by cheap beamelements and the areas of interest can be computed usingshell elements, thus benefiting from their advantages overbeam elements (see Figure 2).

Rigid Body Models (DMM): The RBM is a simplified,undeformable version of the detailed FE model presentedbefore. It is created by constraining the shell elements of

Figure 2: DFEM with local refinement in the area firstimpacting the water.

the fuselage skin into a single rigid entity, and deletingall the remaining structural items (frames, stringers,passenger seats). Total mass, center of gravity androtational inertia are transferred to the simplified geometry.The entire transformation process is automated by thedeveloped pre-processing tool, through mesh file analysis.The main advantage to this approach is the considerablereduction in computational expense of the model.

Dynamic Master Models do not allow for skin de-formation, but present an alternative to compute energypropagation through the fuselage. Aircraft simplified beamFE models, also known as stick models, are commonlyused in civil aircraft design. Accurate prediction ofbending and twisting deformations of the aircraft structuredepends on extracting the stiffness properties of the mainstructure and applying it to a set of beam elements. Themodel considered in the present thesis (DMM) takes fromthis condensed beam concept and adapts it to be utilizedin the context of ditching simulations. This is done byconnecting a set of rigid shell elements to the beamstructure, in order to represent the contact surface withthe water domain. Stiffness distribution is computed withrecurse to another external tool, which computes localstiffness properties for each section in between frames,directly from the GFEM or DFEM model (see Figure 3).

The correct mass distribution, originating from nodalmass data, is applied after the geometry generation iscompleted. The point mass distributions to be appliedto the different aircraft models are defined in separatetext files. The developed toolbox offers several examplesof these distributions but the user is free to modified

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Figure 3: Example of Dynamic Master Model

them in order to test different mass load cases. The toolalso provides a beam model of the wing and empennagestructures. This segment can be left out, attached toany of the models as a rigid structure or attributed avariable stiffness distribution. Its purpose is to evaluatethe influence of the flexibility and mass of these structureson the fuselage only, and consequently no contact betweenthese parts and the water is included.

Water generation follows. Concerning water features,one can mention the application of translating periodicboundaries, which allows moving the SPH domain witha velocity linked to the COG of the aircraft (see Figure4), or the active box, where particle location is checkedat each time step and particles outside the domain ofinterest (further from the impact area) are deactivated.For the latter option, an algorithm was implemented tolinearly vary the length of the active region throughoutthe simulation.

Figure 4: Example of Periodic Boundary Conditions

Coupling between FE and SPH elements for thewater domain and in between structure and water wasalso automated, and the separation stress feature canbe activated to portray suction forces. Finally, differentmemory processing schemes are allowed, such as sharedmemory processing or Multi-Model Coupling (MMC).The latter allows for the structure and water to beseparated and computed with different time steps, andrequires a specific data layout to be implemented. The pre-processing tool successively achieves the required input

file architecture for all models, so long as combinationbetween approach is allowed for the VPS solver.

IV. COMPARISON OF MODELING APPROACHES

In order to provide a test to the capabilities ofthe developed pre-processing tool, and the potential ofrecently implemented features in the VPS explicit solver,several modeling options were investigated.

The aircraft used in the simulations performed repre-sents a generic single aisle, 37.5m long airplane with acapacity to hold up to 150 passengers. Maximum fuselageheight reaches 4.1 m and main wing is defined with aspan of around 34 m and mean sweep back angle of 25degrees. Vertical tail extends to 9.5 m height (from lowestfuselage point). Wings are modeled as representative beamelements and no engines nor landing gear are includedin the geometry.

Mass distribution originates from available mass pointdata representative of a 50% payload and 50% fuel uponimpact, totaling at around 72500 kg. All simulations areperformed considering null yaw and roll angles, and anangle of attack of 8 degrees on approach. Initial horizontaland descent velocities are fixed as vx = 65m/s and vz =

−1.5m/s, respectively.In order to portray realistic conditions, gravity is

applied to both structure and water and a simple linear liftmodel is applied to the aircraft model. The latter consistsof linearly decreasing the lift force from balance (equalto gravity force) to zero during the first second of impact.Air is not modeled.

It is important to highlight that no experimental datais available for comparison, and as such the presentedresults should not be seen as an accurate representationof the aircraft dynamics upon ditching. The objective isto compare the different modeling approaches in termsof consistency, accuracy and time efficiency.

Investigations Concerning Water Modeling

In this first part of the research, the RBM approach isused for all simulations for time efficiency. The classicapproach using a full pool with FE and SPH elements(see Figure 5) was used as a reference, and the influenceof different parameters was studied.

In the reference analysis, splash formation and differentphases over impact could be observed. The aircraft first

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Figure 5: RB aircraft model and full length FE-SPHoption for water basin.

impacts the water domain and then bounces, landing afew meters ahead on a second impact. The disturbancecaused by the impacting body is plausible, and splasheffects can be reproduced due to the nature of the SPHmethod applied in the central region of the water domain.The classical Lagrangian finite elements placed in theouter region of the basin still deform, but not enough asto cause a dramatic decrease in critical time step.

Cross-section of the pool should be enough as toprovide sufficient impulse and remove boundary interfer-ence. The investigations performed show that the FEvolumes used around the particles for extension arecomputationally cheap and as so cross-section can beincreased with no excessive CPU expense. The contactbetween particles and FE volumes is also investigated,testing different particle element to FE size ratios. Apenalty contact is also tested for contact computation,as oppose to the tied connection typically used for thispurpose. Results show that even though no significantdifferences are observed for the different referred ratiosand contact algorithms, further investigations should beperformed on the subject. One other important aspectis the long CPU times, due to the required extensivelength of the SPH domain. Translating periodic boundariesaddress this issue.

For the translating periodic boundary conditions, it wasfound that the translating SPH domain on its own doesnot provide time improvements as the reduction in lengthis overcome by the necessary increase in cross-section.The problem was addressed to the ESI-Group, whichdeveloped an algorithm to allow for a U-section of FEhydrodynamic volumes to be used in coupling with thetranslating particle domain. The algorithm locates the FE

volumes outside the region adjacent to the particles ateach time step, and sets accelerations of these elementsand velocities to zero. As a result, the FE which are not incontact with the particles do not collapse under their ownweight. This option reveals to be effective in producingconstant results and achieves a considerable reduction(54 %) in CPU time.

The active box via nodes option was also consideredtogether with the FE volume U-section mentioned. It wasshown that the ability to vary the active domain over theperiod of simulation brings further improvements in termsof computational time (65% overall reduction in CPUtime), and results are practically the same in comparisonthe full-length approach (see Figures 6 and 7).

Figure 6: Adaptive Active Box over simulation (from leftto right, t = 0ms, t = 450ms and t = 900ms). In the waterbasin: hydrodynamic FE volumes (light blue), activatedparticles (dark blue), deactivated particles (green)

Figure 7: Comparison of COG acceleration historiesfor full-length pool and adaptive active box options(CFC180).

Another important observation was the effect of particlerefinement in the acceleration history of the aircraft.Different particle sizes were tested, maintaining theparticle length to FE size ratio. It is observed that abig discrepancy arises between larger and smaller particle

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Figure 8: Mesh Refinement, Active box constant size,with time step constraint (CFC180)

modeling. The effect of time step reduction in contactcomputation is considered. It is proven that by artificiallyreducing critical time step, results show convergence.Results for 100 mm and 70 mm particles with and withoutartificial time step constraint are shown in 8, for 300 msof simulation.

The accelerations showed that when restraining thecritical time step to the same maximum value, the resultsare much closer between simulations. Other than this,the results showed the initially expected reduction ofoscillations with smaller particle size. This is a veryrelevant observation, since it indicates that time step is acritical factor to the accuracy of the results, other thanparticle refinement or modeling approach for the waterbasin. One possible explanation could be that due to theviolent impact of the large structure, the critical timestep defined by the individual elements is maybe not lowenough to accurately portray the contact between aircraftand water. Nonetheless, this is an unsure conclusion, as itshould not be the case. Further investigations are requiredon the subject as not enough simulations were performedin order to understand the relation between impact forceand necessary time step for contact.

Investigations Concerning Aircraft Modeling

analysis is performed by using the same input filesand alternating within the four modeling possibilitiesallowed by the tool, from simple rigid body to fullydetailed finite element models. The different models arefirst compared in terms of global mass and inertias toensure consistency. It is proven that even though the

developed pre-processing tool used different methods formass distribution in each of the models, global propertiesare successfully consistent. Modeling approaches aretested to simpler to more detailed: first the RBM, followedby the DMM and GFEM schemes, and finally the DFEM.It is shown that in this particular case using flexiblebeams for wing and empennage structures does not havea significant influence in kinematics.

The geometry tested contains a total number of 87frames located length wise in the fuselage, and as sothe DMM approach tested contains the same number ofindependent "rings". The dynamic master model correctlyportrays structural deformation through the fuselagestructure, but for the tested case this parameter does notseem to significantly influence acceleration time histories.In Figure 9 the model after 120 ms of simulation isdisplayed. Due to its small magnitude, deformation isamplified by a factor of 200 in the referred image. Itcould be observed that the energy arising from the impactis propagated by the elastic elements present, resultingin a quasi-sinusoidal dislocation of the skin rings acrossthe fuselage.

Figure 9: Deformation of DMM at t = 120ms. Structuraldeformation is amplified in all directions by a factor of200. Beam elements and water basin are hidden for claritypurposes.

The GFEM approach is tested with coarser and finermeshes, and it is observed that a fine mesh is necessary inthe impact region to accurately portray skin deformationand eventual panel failure. To this aim, the DFEMoption was considered. Overall conclusion was that skindeformation and panel failure appear to be a decisivefactor in the contact force and acceleration histories, asresults from the latter approach present a considerabledifference to those using rigid elements for the fuselageskin (see Figures 10 and 11). Time increase is alsoremarkable: the latter approach took up to 12 times the

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necessary CPU effort of the rigid body model.

Figure 10: Displacement in Z-direction of DFEM refine-ment region, at t=390ms.

Figure 11: Contact force time histories in Z-directionsfor RBM, DMM and DFEM models (CFC180).

RBM DMM GFEMComputational time [h] 6.8 18.5 82.9

Table I: Global finite element investigations: computa-tional times of rigid and flexible wing options.

In terms of the consistency of the results, severalobservations were made. First, previous investigations(for the water basement particle refinement) showedthat considerable differences in critical time step of thesimulations may have an effect on the contact betweenthe water particles and the FE fuselage structure. TheRBM presents a critical time step of around one order ofmagnitude higher than the DFEM, due to element sizeand elasticity. It is therefore considered that the significantdecrease in time step, visible when comparing the threeapproaches with rigid wing in 11, may be a factor forthe disparity of the results.

Another factor for the different results between mod-eling approaches was the considerable deformation andconsequent failure of the skin elements portrayed bythe DFEM. Even though one could visualize structuraldeformation throughout the fuselage in the DMM, theresults of the DFEM indicate that skin deformation isof high significance for fluid-structure interaction duringthe first impact. However, aircraft pitch angle evolutionshows the same tendency over the three results, althoughwith a less smooth behavior for the DFEM.

It is therefore advised to weigh in accuracy and effi-ciency costs, as the several days necessary for the solutionof the DFEM may be impractical for an analyst. If globalpitch kinematics is to be evaluated, the RBM approachgives a reasonable first approximation with considerabletime benefits. For harder impact conditions (such as higherdownward velocity upon impact), the DMM approachmay be considered as overall structural deformation isportrayed. On the other hand, if fuselage skin rupture isanalyzed or accurate portray of accelerations is required,it is advised to opt for the DFEM approach. Nonetheless,the lack of experimental data for validation of the aircraftstructural models in the present investigations should berecalled, and as such previous conclusions may needfurther analysis. Still, the comprehensive methodologyfollowed is valid for any model, and the automated pre-processing tool facilitates the testing of all options incoping with the best approach for the ditching analysisin question.

Investigations Concerning Separation Stress Feature

In some ditching experimental test cases, suction forcesarise in the rear part of the fuselage, caused by anacceleration of the water flow around the impactedarea, leading to a drop of the water pressure in theregion. The referred forces are located with at a fairlylarge distance from the structure’s center of gravity, andtherefore even if the force magnitude is not elevatedthe momentum produced is of significance. In order toassess the importance of the mentioned effect in thepresent full-scale ditching simulations, investigations in[8] are taken as reference. In the latter research, a standardtransportation aircraft geometry (of similar length as tothe one used in the present investigations) is tested with

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the separation stress feature available in the VPS FE-SPHmodule. The model used in the simulations is scaled-down by a ratio of 1:8 applying Froude scaling rules, anda rigid body approach is used. Separation stress SPST R

and separation thickness factor SPT HK are investigateduntil suction is realistically portrayed. As models aresimilar in shape, Froude rules were employed to theseparation feature results in order to apply them to thepresent research.

In the reference investigation, the aircraft model re-mained attached to the water surface during the first350 ms of impact. Similar results were expected for thefull-scale case now tested. However, the behavior of theaircraft is not as predicted. At t = 300 ms, the aircraft wasno longer in contact with the water surface, similarly towhat had been observed in previous investigations werethe separation stress featured was not used. The maindifference is a layer of particles that remain attachedto the structure, contrary to the objective of simulatingsuction forces. It was thought that the disparity arosefrom the lack of scaling of the particle size, which wasfairly coarser in the reference simulations. To this aim,SPST R and SPT HK were increased to compensate forthis effect. However, no improved results are obtained.Finally, the same was tested with a larger particle size.The results were presented in Figure 12 in comparisonwith the same simulation without the separation stressfeature.

Figure 12: Separation stress investigations: simulationwithout separation stress feature activated (left) and withSPST RS = 1E3GPa, SPT HK = 1.75 (right) for 300 mmparticle spacing.

It could be observed that in the latter case, theapplication of the separation stress feature resulted ina longer attachment of the aircraft to the pool. It seemsas if the relation between individual particle size andmass of the impactor plays an important role for this tobe observed. Further investigations should be conductedon the subject as larger particle size may lead to lessaccurate results in terms of pressure distribution. However,the main objective behind this investigation was to proveif suction forces could be portrayed for full-scale aircraftmodels impacting on water recurring to the FE-SPHmethod. Despite initial difficulties in parameter scaling,it is thought that the results prove that this statement istrue.

Investigations Concerning Computational Aspects

In the previous benchmarking sections, computationalefficiency has been address by the modeling options pointof view. A shared memory processing (SMP) schemewas used in the previous investigations, running thesimulations in one computational node over eight cores.However, computational configuration may have an evenlarger effect in CPU requirements. For the referred reason,the present section aimed to explore the capabilities ofthe Multi Model Coupling (MMC) option. MMC allowsfor the simulation domain to be separated in aircraftand water, and for each of the components to run onits own time step. Contact is then achieved over super-cycles, where data from both parts is interchanged. Theprospects of this feature are promising, especially forthe more detailed aircraft modeling approaches, wherethe time step of the structure is considerably lower thanthe one of the water particles. Another feature is thepossibility of assigning a different number of CPUs foreach part.

The MMC parameters were previously studied in [9] forthe vertical impact of a fuselage section on water, usinga computational cluster assembly. These investigationsconcluded that optimum results were achieved whenattributing the same number of nodes to both structure andwater, and that TSR ∆twater/∆tstruct. should be kept under30 to ensure correct contact computation. Taking theseregards as a reference, the MMC option was tested in thethesis for the present model with the GFEM approach.

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One important notice is that to date the MMC optionwas only available for the commercial version of theVPS solver. Features such as active box via nodes wereexclusively available in the development version usedin previous simulations. This means that both, until themoment of the present research, could be combined.

The methodology followed for the MMC simulationswas similar to the one performed in [9]. One computa-tional node is used and 4 cores were attributed to eachthe structure and the water. First, no time step ratio wasartificially imposed, to verify the natural TSR of the fullmodel. In a second step, TSR was artificially constrainedby imposing the corresponding value in the water sectionof the simulation. In the present case, ∆twater/∆tstruct. = 30and a more conservative ∆twater/∆tstruct. = 5 were tested.Finally, after the simulations were completed, the contactforce history was checked for ensure consistency. Thisdata is plotted in Figure 13.

Figure 13: MMC investigations: GFEM model using SMPand MMC with time step ratios TSR=30 and TSR=5.

SMP MMC, TSR=30 MMC, TSR=5Comp. time [h] 132.5 24.4 (-81.5%) 49.8 (-62.4%)

Table II: Computational times for SMP (No MMC) andMMC computational schemes with TSR=30 and TSR=5.

It is in fact very encouraging to observe that even withfairly low TSR, time savings overcome 60%. Althoughthis comes with a penalty in accuracy, the remarkabledecrease in computational expense should be takeninto consideration. It was concluded that as computertechnology evolves, memory processing schemes shouldnot be disregarded as it was obvious that this could leadto a significant reduction in time spent analyzing moredetailed aircraft models.

V. CONCLUSIONS

Summarizing, the contribution of the present workfor the state of the art in FE-SPH ditching simulationscan be separated into two main achievements. First, aparametrized pre-processing tool was developed for gen-eration of complete ditching models, including advancedmodeling options concerning aircraft, water, contact andmemory processing schemes. The developed tool is nowavailable for comprehensive studies to be efficientlyperformed in the future, and possibly for the ditching loadcase to be incorporated in larger computational chains.In second place, the state-of-the-art features developedduring the SMAES project for VPS concerning waterand FSI modeling, the pre-existing in-house tools of theDLR dedicated to structural modeling, and an adaptedcondensed beam approach (DMM) implemented by theauthor have been successfully applied to the case of full-scale fixed-wing aircraft ditching simulations, with veryencouraging results.

REFERENCES

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[3] Liu, M. B. and Liu, G. R. (2010). "Smoothed Particle Hydro-dynamics (SPH): an Overview and Recent Developments". InArchives of computational methods in engineering, 2010, vol. 17,no 1, pp. 25-76.

[4] Monaghan, J. J. (2012). "Smoothed Particle Hydrodynamics andIts Diverse Applications". In Annual Review of Fluid Mechanics,44(1), pp. 323-346.

[5] Monaghan, J. J. (1992). "Smoothed particle hydrodynamics". InAnnual review of astronomy and astrophysics, 30, pp. 543-574.

[6] Tait, P. G. (1965). "Report on some of the physical propertiesof fresh water and of sea water". Johnson Reprint Corporation.

[7] Schwinn, D., Scherer, J., Kohlgrüber, D., and Harbig, K. (June,2013). "Development of a Multidisciplinary Process Chain for thePreliminary Design of Aircraft Structures". In NAFEMS WorldCongress 2013.

[8] Benítez, L. M., Montañés, H. C., Siemann, M., and Kohlgrueber,D. (November, 2012). "Ditching Numerical Simulations : RecentSteps in Industrial Applications". In Aerospace Structural ImpactDynamics International Conference, pp. 12-40.

[9] Hyra, P. (September, 2012). "Investigation of Multiscale Model-ing/Multi Model Coupling - Numerical simulations of aircraftstructures impacting on water". DLR-Interner Bericht.

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