CFD-02_Winglet_CFD_Analysis_KCT

8/10/2019 CFD-02_Winglet_CFD_Analysis_KCT

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Simulate to innovate 2

techniques employing inverse design in the 1989-1991 period. Improvements of the winglet airfoil were

performed with both MSES 7, 8 and X FOIL10 codes. The tip of the winglet suffered some redesign in order

to displace the vortices outward more efficiently. The final winglet configuration was very effective, allowing

Embraer to significantly increase the range of the aircraft, which was conceived to fly at a subsonic condition

with a high lift coefficient. In the sequence, a new winglet configuration was designed for the Legacy

Business Jet, which cruises at Mach number of 0.80 and higher lift coefficient. Finally, the Embraer 170/175

and the larger 195 were designed with winglets. Both transonic wind tunnel testing at DNW in the

Netherlands and at TsAGI facilities in Russia showed significant drag reductions provided by some winglets

Configurations under investigation.

Trailing edge vortices

A vortex in general terms is the rotational motion of fluid, often comprising a strong region of low-pressure inits core. Wake vortices are generated whenever an aircraft produces lift. The principal structure of the wakedownstream from a wing comprises a trailing vortex pair resulting of the merge of all vortices shed from thewing trailing edge with the tip vortex. Each vortex of the pair is formed in an the vicinity of the wingtipbecause the tip vortex attracts the remaining weaker vortices. This structure may change if flaps aredeflected (Fig.1). In this case, two vortex pairs will be observed.

Fig. 1 Trailing-vortex structure for a wing with flaps deflected

The vortex wake produced by aircraft is more complicated than had been thought and may produceunforeseen turbulence in the air. Such flow structures play an important role in flight safety, since they caninduce large rolling moments on other neighboring aircraft, and are known to cause loss of maneuverabilitycontrol and fatalities. In rare instances a wake encounter could cause in-flight structural damage ofcatastrophic proportions. However, the usual hazard is associated with induced rolling moments, which canexceed the roll-control authority of the encountering aircraft. In flight experiments, aircraft have been

intentionally flown directly up trailing vortex cores of larger aircraft. It was shown that the capability of anaircraft to counteract the roll imposed by the wake vortex primarily depends on the wingspan and counter-control responsiveness of the encountering aircraft. Continuing growth of air traffic has made "wake vortex"one of the most challenging technical issues in modern civil aviation. The requirement for reduced


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separation distances on densely flown approach routes is closely linked to the hazard caused by wake-generating aircraft and its safety impact on following aircraft. Great efforts have been made in recent years

to increase the knowledge base of aircraft-generated wakes. In the light of a new class of high capacityairliners to enter service in the next decade, research must intensify even more to better understand wakephysics, so that vortex-related hazards can be quantified and means for hazard reduction implemented. Amajor role to achieve this goal is seen in utilizing modern visualization techniques that became available inrecent years. The strength of the wake vortex is, governed by the weight, speed, and shape of the wing ofthe generating aircraft. However, as the basic factor is weight, the vortex strength increases proportionatelywith increase in aircraft operating weight. Peak vortex tangential speeds up to almost 100 meters persecond have been recorded. A lifetime of several minutes and a length of 30 km behind large planes havebeen recorded and are widely, known, though the vortex energy has reached a very low level. Even biggeraircraft can be damaged by wake turbulence. This was the case for a MD-11 airliner during a VFR approachat Runway 24R of an unspecified U.S. airport The airplane was flying 5.6 km behind a Boeing 747 that waslanding at Runway 25L. The parallel runways were 168 m apart and staggered; with the threshold ofRunway 24L located 1,312m beyond of threshold of Runway 24R. The MD-11 was 31 m above the ground

when it rolled left, then right and developed a high sink rate. The captain initiated a go around, but theairplane contacted the runway and bounced-back into the air. The captain discontinued the go-around andlanded the airplane on the runway. The MD-11s aft lower fuselage and aft pressure bulkhead weresubstantially damaged. The accident was ascribed to improper planning by the MD-11 pilot-in-command.The U.S. Aeronautical Information Manual recommends that when an airplane is following a larger airplaneon parallel approaches to runways closer than 763m, the trailing airplane should remain at or above theothers airplane flight path, to avoid the others airplane wake turbulence. Evidently, the energy necessary togenerate the vortex structures and wing downwash is driven out from the power plant. In other words, alarge amount of drag is generated, called drag due-to-lift or induced drag. Induced drag represents 30-40percent of the total drag of a transport airplane at cruise condition so it has a big impact on fuelconsumption. The induced drag is directly proportional to square of the lift coefficient. Therefore, takeoff,climbing, long-range cruise, holding are phases of flight where the induced drag is high because the liftcoefficient is also high. Airbus undertook a special effort to keep the A380 wake vortex no stronger than the

747 so other aircraft wouldn't require extra in trail separation from it. Engineers reviewed NASA, Europeanand Russian TsAGI studies. They noticed that the two-engine Airbus A330 and four-engine A340 havedifferent vortex patterns even though they have the same wing, owing to changes in location of the flaps andengines. They also observed that the A320 and A321 have different patterns, apparently because onemodel has single-slotted and the other, double-slotted flaps. Airbus reports the location of the flaps, aileronsand engines on the A380 was adjusted to minimize the wake vortex, and it is estimated to be a few percentstronger than the 747-400's.

Winglets Performance Characteristics

Winglets belong to the class of wingtip devices aimed to reduce induced drag. Selection of the wingtipdevice depends on the specific situation and the airplane model. In the case of winglets, the reduction of theinduced drag is accomplished by acting like a small sail whose lift component generates a traction force,

draining energy from the tip vortices. The wingtip might be considered a dead zone regarding to theaerodynamic efficiency, because it generates lots of drag and no significant lift. The winglet contributes toaccelerate the airflow at the tip in such a way that it generates lift and improves the wing loading distribution,which is related to the induced drag Fitting winglets can provide improvements in the performance ofaerodynamic efficiency for a range of lift and Mach Number conditions by decreasing the trailing vortexdrag by amounts that more than compensate for any increases in the profile and wave drag contributions.In addition to their effects on trailing vortex drag. Another potential benefit of winglets is that they reduce thestrength of wingtip vortices, which trail behind the plane. When other aircraft pass through these vortices,the turbulent air can cause loss of control, possibly resulting in an accident. Winglets produce an especiallygood performance boosts for jets by reducing drag and that reduction could translate into marginally highercruise speed. Figure 2 shows reduction in drag of the Gulfstream III aircraft using winglet.


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Fig. 2 Drag reduction provided by winglets for the Gulfstream III business jet

Process Methodology

Modeling ONERA M-4 With and Without Winglets using CATIA V5

Using CATIA V5,ONERA M-4 aircraft model is created with following dimensions:

DIMENSIONS OF THE MODEL (shown in Fig.3)

Fuselage length = 511 mm

Fuselage diameter = 60 mm

Wing Span = 474 mm (Without Winglet)

Wing Sweepback angle = 30 deg

Horizontal tail span = 170 mm

H.T sweepback angle = 37.5 deg

Dihedral angle = 3 deg


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Fig. 3 - Front view of ONERA M4 model with winglet

The generative shape design module of CATIA V5is used to construct the both with and without winglet

models. Computational domain is also created using CATIA V5.

The dimensions of the computationaldomain as follows:

Far field Top = 2.5L (L = Fuselage length)Far field Bottom = 2.5LFar field side = 2.5LUpstream = 2.5LDownstream = 5L

Fig. 4 -

ONERA M4 model with computational domain from CATIA


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Meshing And Preprocessing In Acuconsole

The meshing/Discretization are carried out in AcuConsole

for both with and without winglet cases.Unstructured tetra elements are selected for these computations. As per the boundary layer calculation,boundary layer thickness is calculated as 10.4 mm and Reynolds number as 7, 23,670 (for the Aircraftspeed 20m/s). 12 layers are used inside the boundary layer for both the cases. The mesh details of thewithout winglet case as follows:

Element type : Unstructured Tetra elementsTotal elements : 2757733Total nodes : 491323The following Fig. 5 shows the elements around the aircraft from AcuConsole

Fig. 5 - Mesh from AcuSolve of without winglet case

Mesh details of the with winglet case as follows:Element type : Unstructured Tetra elementsTotal elements : 4107555Total nodes : 731088The following Fig. 6 shows the elements around the aircraft from AcuConsole


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Fig 6. Mesh from AcuSolve of with winglet case

After meshing the boundary conditions selection is done in AcuConsole. The selected boundary conditions for both the casesare as follows (from Table 1):

Table 1 - Boundary conditionsComponent Boundary type

Inflow Velocity Boundary (20m/s)Outflow Pressure Outlet

Aircraft WallFar field Slip

Symmetry Symmetry option

Turbulence model selected for both the simulations is "spalart allmaras" with convergence factor of0.0001. Totally 900 time steps are solved for without winglet simulation and 1000 time steps are solved forwith winglet simulation in AcuSolve. Fig. 7 and 8 shows the time step history of the respective cases.


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Fig. 7- Time step history from AcuProbe of without winglet simulation

Fig. 8 - Time step history from AcuProbe of with winglet simulation

Results And Discussion

Post processing of the results is carried out in HyperView. Contours of Pressure, Velocity and Vorticity ofboth the simulations are generated. Fig. 9 and 10 shows the comparison of pressure contours of both thesimulations.


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Fig. 9 - Pressure plot from HyperView of without winglet simulationFig. 10 - Pressure plot from HyperView of with winglet simulation


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Fig. 11 and 12 shows Velocity contour of both the simulations

Fig. 11 - Velocity plot from HyperView of without winglet simulation

Fig. 12 - Velocity plot from HyperView of with winglet simulation


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Fig. 13 and 14 shows Vorticity contour of both the simulations. It shows that the Vorticity strength is low inthe region of winglet in the with winglet simulation case. This gives a clear picture of tip vortices elimination

due to winglet, which leads to reduction in Induced drag.

Fig. 13 - Vorticity plot from HyperView of without winglet simulation

Fig 14. Vorticity plot from HyperView of with winglet simulation

Conclusion

This project proposes that winglet will significantly yield a better performance of an aircraft and reduce thefuel consumption. The present work showed that winglets are becoming an important and commoninstrument to achieve performance improvements in the present highly competitive aircraft market. Variousaspects affecting the design of winglets were described and highlighted, ranging from airfoil characteristicsto the shape and importance of correctly designing the transition surface between the wing and the winglet.For this task, CFD tools proved to be mandatory for an efficient design of winglets for transonic aircraft

configurations. The present simulations of with and without winglet cases promises that winglet installationreduces the tip vortices, which leads to the reduction in induced drag.

Benefits

CFD simulations are now a day's used as a vital analysis tool in many applications to reduce the number ofexperiments, which leads to savings in both cost and effort. AcuSolve is one of the leading CFD solver givesthis benefit.


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Challenges

Meshing process and the pre processing in AcuSolve is user friendly and saves time and cost rapidly.

Future PlansCFD simulation of different AOA's and speeds are planned to be carried in AcuSolve to get clear view aboutthe winglet effects. As well wind tunnel testing of the model is also planned to carry out for validationpurpose.

ACKNOWLEDGEMENTWe thank our management for providing the better computing facility to solve this huge mesh count problemwith in short period of time. We thank Department of Aerospace Engg. , MIT for the modeling support. Wethank Altair customer support for their extensive support.

REFERENCES:BOOKS

1. Anderson, J.D., (1985), Introduction To Flight, 2nd

Edition, Mc-Graw Hill co., New York

JOURNALS

2. Considerations about Winglet Design Bento S. de Mattos Antonini P. Macedo Durval H. da Silva Filho Empresa Brasileira deAeronautica S.A. Av. Brigadeiro Faria Lima, 217012.227-901 Sao Jose dos campos SP Brazil

3. CFD Analysis of Winglets at Low Subsonic Flow M. A Azlin, C.F Mat Taib, S. Kasolang and F.H Muhammad4. Altair user manuals.

CFD-02_Winglet_CFD_Analysis_KCT

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