Assessment of CFD Work in India

Gopal R. ShevareDepartment of Aerospace Engineering, IIT Bombay & Zeus Numerix Private Limited

ABSTRACT

CFD Software is off the shelf commodity. Availability of teraflop computers (the old bottleneckin CFD) does not exist anymore and teraflop computers can be bought, built or hired. It looksas if CFD has managed to come to the centre stage. Naturally, assessment of its strengths andweakness will be useful for planning, adopting and deploying this powerful technology for aerospaceprojects. The paper classifies CFD in three separate activities: Teaching/R&D in institutes,developing and deploying CFD software in industry and CFD as an automated process to beused by aerospace designer in future. India has made some progress in the first two. It is arguedthat improvements in CFD alone are not enough in future. The design process must change totake advantage of virtual/numerical simulations, CFD being one the many simulation technologies.

Keywords: Computational Fluid Dynamics, Aerospace CFD, Assessment of CFD

1. INTRODUCTION

Computational fluid dynamics (CFD) has made great strides in the last decade. AerospaceCFD seems to have made an impact in the 1970s when a supersonic pocket and a shockwave embedded in subsonic flow were automatically captured. The next important milestone was reached in 1980s, when CFD could simulate heat transfer rates in hypersonicflows for reentry vehicles. In India, the first step in CFD was taken in 1981, when aninternational workshop in CFD was held in Trivandrum. The second step and probably themost important development of CFD happened when Aeronautical Development Agency(ADA) took a lead and challenged a scattered aerospace CFD community to work togethercohesively towards analysing Light Combat Configuration (LCA) configurations. Sincethen educational institutes, research labs and design offices have made investment inunderstanding CFD and its usage in aerospace engineering analysis.

There have been discussions on facets, especially, scientific facets of CFD in variousforums helping aerospace community chart its healthy and robust growth [1]. Presently,there are many civil and military aerospace projects on the drawing board and hence it istimely that we assess our capability in developing and deploying CFD as a technology.

SAROD 2009 – 102

Symposium on Applied Aerodynamics and Design of Aerospace Vehicles (SAROD 2009)December 10-12, 2009, Bengaluru, India

Assessment of CFD Work in India 15

2. AEROSPACE CFD

There is a dilemma of accessibility of highly matured CFD technology but not the usefulnessof CFD technology. One hand, most of our organizations have suites of CFD softwarehaving different maturity levels in algorithms and tools. They use CFD based on the levelof uncertainty they place in CFD predictions. On the other hand, many phenomena cannotbe modeled effectively in CFD. Turbulence model with and without compressibility effects,transition and relaminariation, massively separated flows, aerothermodynamics, non-equilibrium flows, multi-phase flows are some examples. Thus ability to model multi-scale physics must be the first metric to assess our CFD.

Designers are at most specialists of flow physics among things; they are not theexperts in numerical methods. In fact designers need a quick tool to help them in theirglobal thinking and it is undisputed that CFD is quicker and cost effective tool comparedto wind tunnel testing. Naturally to make CFD as one of such tool, we need to develop aninfrastructure / systems that do require designers to worry about availability to worryabout such diverse things as no. of CPUs, types and quality grids, tuning parameters to beused in turbulence models, etc. Our ability to create CFD hardware/software systemwhich enables the designers to deploy CFD properly, carefree and yet very is the secondimportant metric.

The last and equally important requirement of CFD technology is its ability to co-existin a suite of complex multi-disciplinary analysis and design tools. Faster turnaround timeand an ability to conceptualize a non-conventional design is the order of the day. Flappingwings, cm-sized engines, zero RCS signatures are possible and have a meaning if and onlyif, CFD co-exists with combustion, analysis of composite materials, electromagnetic analysisof dielectric materials, controls, etc. The ability to design and develop CFD software so thatit co-exists with CAD on one hand and numerous simulations tools with correct an automaticdata transfer between them is the third important metric. Admittedly, this requirement isnot specific to CFD alone.

Indian CFD is assessed here in terms of the above three metrics.

3. CFD ALGORITHMS AND MINDS-ON TRAINING IN CFD

The seeds of CFD, like any subject, are sowed in educational institutes. Undergraduates /postgraduates get initiated to applied mathematics, fluid mechanics, gas dynamics, heattransfer and advanced topics such as turbulence, stability, etc. It is here that young CFDengineers get to know the interplay between various competing phenomena and how tomodel them. There is an emphasis on concepts rather than problem solving ability. Thetraining is therefore conceptual, mentally challenging or “minds on” rather than “hands-on” which demands solving well defined problems rather mechanically. By nature, thisneeds to be an individual oriented work seeking original contributions to the science. Beinga non-profit activity, it needs to intrinsically driven by interest of faculty and funded bygovernment.

For CFD to survive and thrive, it is essential that basics science and numerical methodsare covered in great details. The activity must produce researchers who understand basic

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physics and chemistry of the flow, though processes which leads to novel algorithms forproducing behaviour of fluids. It is natural that the activity is funded by Govt. as thetraining is broad based. This CFD activity is lively in the country, but its spread is limited.Industry and R&D laboratories are getting ready manpower from these institutions, thoughthe number seems to be too small against the requirement.

The manpower being trained is mainly in the area of what can be called as mainstreamCFD. There is hardly any attempt modeling physics in some novel way. Particle basedCFD remains unexplored. Large eddy simulations do not find place in national conferences,transition studies are rare. Multi-phase flows are not even attempted. CFD effort has notbeen diversified. It evolved mostly in experimenting with algorithms for convective termsin Navier-Stokes algorithm. Serious code development has suffered. This needs to bechanged. This may change with increase in post graduate intake. It is well-known that theproductivity in this kind of activity has large dispersion and hence it is difficult define thegoals and even more difficult to realize them.

AR&DB had an important role to play in shaping this activity. It has funded large noof projects in the past and it has been funding CFD activity aggressively. A large numberof CFD scientist / engineers got their grinding through the funds provided by AR&DB.Recently funded CFD centres at IITs and IISc is a timely step in this direction. However,it would be better that mandate to the centres is (a) training of CFD manpower and (b)carrying out basic research in algorithm in CFD and CFD related areas.

Ref [2] states that CFD has been used for high-speed cruise wing design andpropulsion / airframe integration. Not only this, it states that CFD has been used foranalysis / design of flap support fairings, identifying locations air-data sensors,environmental control system (ECS) inlet and exhaust ports, cabin pressurization andoutflow valves were position with CFD. There is a claim that CFD can provide insight tohigh-lift concepts and hence used for assess plan form effects. But on the other hand, Ref[3] shows that RANS simulation is failing in predicting of stall of 2D airfoil and RANS/LESis with compact differencing provides the answer. See Fig.1

Figure1: Lift curve of NACA 64A006 airfoil [4]

Assessment of CFD Work in India 17

Fig.2 shows pressure distribution on triangular wing [3, 4]. The author has usedapprox. 8 million cells but feels that unless the turbulence model is improved the pressuredistribution may not match.

This uncertainty where CFD can be used and where it should not be used needsimmediate attention. One of the ways is to carry out verification & validation forum. Thiswill lead to (a) awareness of scatter in CFD results, (b) limits on the usefulness of models,(c) direction for CFD R&D.

Non-dimensional Span wise distance

Figure2: Cp variation along span wise direction [4].

European effort QNET-CFD and Drag Prediction workshops (DPW) by Boeing andNASA [5] in US are worth emulating. The modest aim could be to (a) collate known problemswhere CFD does not work, existing knowledge on the industrial application of CFD and tomake these available to European industry, (b) conduct workshops to show to improve theaccuracy of CFD. Luckily, there has been initiative in this direction already. Symposiumon promotion of Indigenous CFD in Engineering Services (SPICES) needs support formaerospace community.

4. TECHNOLOGY DEVELOPMENT AND HANDS-ON TRAINING IN CFD

The speed is the pivotal in usage of CFD. It can create paradigm shift in the design andanalysis because it is fast. The speed with which CFD can be used is therefore an importantmetric in the assessment of CFD. Though understanding of CFD is essential, it is possiblethat repetitive procedures are made automatic. Work which is time consuming, but essentialfor CFD simulations is laid out as sequence of procedures so that novice user finds simulationintuitative and regular user, easy. This is typically done through the usage of software(CFD software) where each individual step processes the data as intended by the type ofCFD simulation required to be carried out. At every step, the user will have plethora ofoptions to choose from software’s graphics user interface but he must know the precise

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options. This activity is supposed to provide “Hand-on” training. The emphasis is on theunderstanding what is to be done rather than why it is required to be done. It is for thesereasons, that the user is rarely a CFD scientist or CFD engineer; instead users of CFDsoftware are aerodynamicist. There are two options to access CFD software: (a) acquirecommercial software, (b) Build CFD software.

Acquiring commercial CFD software option is faster and easier, if not cheaper. Thereare many aerospace organizations in the country that find this as a better option. Thoughthe organizations may have capability of building CFD software, they do not venture inthis activity as they have mandate of designing systems meeting product delivery withschedule and budget following prevailing design practices, though some daredeviltechnology managers take up the risk of developing the tools along with the products. Itis thus satisfying that many aerospace organizations using commercial CFD tools for theiranalysis and design activity. Due to lack of in-depth knowledge, industrial users ofcommercial software tend to use CFD for applications where it may not be applicable. Itis for this reason that advanced large aerospace organizations develop their own code asthe developers constantly talk to the designers and advice them to use it correctly.

Unfortunately, building CFD software requires many challenging developments otherthan algorithmic development of solving Navier-Stokes equations. CAD repair tools andCAD repair is the first challenge. It is well-known that CAD repair for military aircraftgeometry can be the most daunting task, especially when it is weaponised. Automatic oralmost automatic CAD repair is an order of the day. The second challenge is the pre-processor. This is not unique to CFD. It is a universal challenge for solving problems inengineering physics posed using partial differential equations. The challenge getscompounded in CFD because it needs to model volume not modeled by solids in CAD. Infact this problem can be so complex that many software vendors prefer to get pre-processordeveloped by a sub-vendor. The third challenge happens to be the usage of high performancecomputers in solving and complex fluid flow. Present simulations use several tens ofmillions of meshes in routine CFD. If the configuration has to analysed for a range of Machnos. and angles of attack, several hundreds computer runs will be required. It will bepossible to generate CFD data in the flight envelop within months if and only if thousandsof cores (CPUs) are simultaneously used. Last but not the least, exploring the large fielddata generated in CFD simulations requires post-processing tools capable of extractingmillions of cells and displaying their field properties for interpretation of the designer.

As an alternative to full blast CFD software, it should be possible for a design officeto craft customized set of tools for their own individual needs. A typical set of tools for anaircraft design office may consist of a surface modeler (not solid modeler with IGES file I/O), structured multi-block surface and volume mesh generator (no other types of meshes),density based parallelised finite volume solver with SA as turbulence model (no otherturbulence models) and a post-processor compatible with each other through CGNS. Thetools do not need GUI, in fact GUI is not essential. If CFD can be made scriptable, forstandard configurations of the organisation, designer becomes an order of time more efficient.It was satisfying that efforts in this directions were made, but implementation did not eventake off.

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CFD is used in many engineering disciplines in addition to aerospace applications.This has made providing CFD services and also developing CFD software as a businessproposition. There are a large number of industrial houses boasting of providing CFDsolutions to aerospace and non-aerospace sectors. There a couple of groups planning todevelop commercial CFD products. This innovation in Indian CFD market has just begun.Around 2005, the market saw what is referred to as “fluid phase” and now there aremultiple business models in CFD products and CFD services. Indian CFD business is in atransitional phase in which a viable business model is likely to emerge. It is likely that inthe next three to four years only incremental product innovation will take place, but numberof industrial organizations offering services aerospace may continue to grow.

5. CFD AS A PROCESS

The only way CFD can deliver value to the organization is that it must affect its product.To affect the product, it must become an integral part of the engineering process for thedesign, manufacture, and support of the product. Otherwise, CFD is just an add-on; it mayhave some value but its effect is limited. Presently, it an add-on tool; it is not an integralpart of the design process.

This phase is the most difficult phase as diverse team of design engineers, managers,consultants and developers, need to contribute. They require far more faith in each otherscapability than in all other phases. If this phase is successful, there will be visible returnson the investment. Without investments in this activity, the enormous pay-off can nothappen.

A key component of CFD is the CAD system. Design is embedded in CAD system;designer accepts design in CAD systems; in fact they own the design only if it is in his CADsystem. The reality is designer is the customer for CFD researcher / developer / serviceprovider. This is because managing data has become far more complex task than generatingdata through numerical simulations or through experiments. In electronics industryprocesses have become CAE driven as the diversity is far less. It can happen in aerospaceengineering in near future. The designer needs to accept the demand his own customer. Heshould be prepared to get his designs rejected and hence CFD must CFD should be readyto accept rejection of his best tools, if designer does not need them because it could bedifficult, inappropriate or costly, etc. CFD developers must understand that use of CFDrequires a pull from the designers. It is an eye opener to note that 30% of some NASAconferences discuss “how to develop systems CFD systems which are easier to use andmore reliable”.

However, designers need to understand that there is sufficient evidence to show thatusage cost incurred in the usage of CFD makes economic sense. Continuous innovation isthe backbone of aerospace engineering. At every stage they will face a dilemma of acceptinginferior design vs. increasing the cost of project. It need not always in favour of acceptinginferior design.

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6. CONCLUSION

CFD has grown in India as a science and also as technology. There are groups andorganizations in the country utilizing their resources to do research, produce CFD productsand provide services. The evolution will certainly throw up researchers, software productsand organizations providing services. The quality will depend on the patronage receivedfrom technology mangers. The first activity will need constant support from the government.The last activity does not need and this activity has just begun. The fate of second activityremains uncertain as it does not guarantee returns on investment, at the same time it is notconsidered as teaching & research activity which Govt. can fund. Absence of analysis toolsfor multi-physics, and interplay between the tools and optimizers will decide the inroadsCFD make in Indian industry.

REFERENCES

[1} S. S. Desai “Relative roles of computational fluid dynamics and wind tunnel testing in thedevelopment of aircraft, CURRENT SCIENCE, VOL. 84, NO. 1, 10 JANUARY 2003.

[2] Forrester T. Johnson, Edward N. Tinoco, N. Jong Yu, “Thirty Years of Development andApplication of CFD at Boeing Commercial Airplanes, Seattle” AIAA 2003-6919.

[3} Kozo Fujii, “Progress and future prospects of CFD in aerospace—Wind tunnel and beyond”Progress in Aerospace Sciences 41 (2005) 455–470.

[4] James M. Luckring, Reynolds number, Compressibility, and Leading-edge bluntness effectson Delta-wing Aerodynamics”, 24th International Congress of the Aeronautical Sciences”.

[5] John C. Vassberg, Mark A. DeHaan, Melissa Rivers, Richard A. Wahls, “Development ofa Common Research Model for Applied CFD Validation Studies” AIAA 2008-6919.