AIAA 95-0649

EXPERIMENTAL INVESTIGATION OF VORTEX FLOW CONTROL USING JUNCTURE FILLETS ON A CROPPED DOUBLE-DELTA WING

S. Hebbar, M. Platzer, and A. Khozam Naval Postgraduate School

Monterey, California

33rd Aerospace Sciences Meeting and Exhibit

January 9-12,1995 I Reno, NV For permlsslon to copy or ropubllsh, contact the American lnstltute of Aoronautlcs and Astronautlcs 370 L'Enfant Promenade, S.W., Washington, D.C. 20024

EXPERIMENTAL INVESTIGATION OF VOkmX FLOW CONTROL USING JUNCTURE FILLETS ON A CROPPED DOUBLEDELTA WING

Sheshagiri K. Hebbar', Max F. Plaaer", and Abdullah AI Khozam"' Naval Postgraduate School, Monterey, California

Abstract

A flow visualization study of the vortical flow over a cropped double-delta wing model with sharp leading edges and its three derivatives with small geometric modifications (fillets) at the strakdwing junction was conducted in the Naval Postgraduate School ("S) water tunnel using the dye-injection technique. The fillets increased the wing area of the baseline model by 1%. The main focus of this study was to evaluate the effect of fdets on vortex core trajectories, interactions and breakdown on the leeward surface at high angles of attack (AOAs) with zero sideslip angle. Comparison of test results for different fillet shapes indicates delay in both vortex interaction and breakdown at high AOAs, particularly for the diamond-fillet model. The vortex trajectory and breakdown data for the diamond-fillet model clearly imply lift augmentation, thus supporting the concept of flow control using fillets.

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1. Introduction

The growing interest in high-AOA maneuvering has refocussed the attention of the research community on delta planforms with emphasis on double-delta wing plan-

*Adjunct Professor, Dept of Aeronautics and AS~TOMU~~CS. Associate Fellow AIAA.

Astronautics. Associate Fellow AIM. **Professor, Dept of Aeronautics and

***Graduate Student. Currently, Lieutenant, d Kuwait Air Force.

T h i s paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

forms and their derivatives. The flow over a double-delta wing is very similar to that over a delta wing, but much more complicated. At low AOAs, the interaction of the strake vortex and the wing vortex leads to their coiling-up (intertwmn ' g). At high AOAs, the strake vortex bursts even before the interaction and triggers the bursting of the wing vortex. Recent studies on double-delta wings have generally focussed attention on static conditions of the model, with emphasis on understanding the complicated phenomena of vortex i n t k t i o n and bursting. A brief review of wind tunneVwatex tunnel studies of double-delta wings appeam in Ref. [l].

Since the vortical flow dominates the lift at high AOAs, there is a resurgence of research effort to control flow through vortex manipulation to enhance aircraft maneuverability. Both pneumatic and mechanical devices are being investigated as candidates for vortex manipulation. A number of studies have been reported in the literature that deal with vortex management for control purposes. A recent numerical study by Kern [2] on vortex flow control through small geometry modifications (fillets) at the strakelwing junction of a cropped double-delta wing suggested the use of fillets for lift enhancement and roll- control augmentation at high AOAs. His numerical results also revealed vortex-sheet tearing on the Wing based on which he proposed a tom wing-vortex hypothesis. Clearly, experimental studies are needed to investigate the resulting vortex structure and establish whether small deployable fillets

could provide substantial control over the vortex core trajectory and bursting, particularly during dynamic maneuvering at high AOAs.

c An experimental program has been

underway at N P S to better understand the physics of vortical flows over sharpedged double-delta wings, with and without juncture fillets during both static and dynamic (maneuvering) conditions at high AOAs. The program consists of extensive flow visualization studies in the NF'S water tunnel facility using dyeinjection. The models include a 76'/40° cropped double- delta wing baseline model and its three derivatives with small geometry modification (fillets) at the junction of the strake and wing leading edges. The model and fillet dimensions are identical to the ones used by Kern [2]. The results of the high-AOA investigation at zero sideslip have been reported in a recent M.S. thesis mf. 31. The data from the first series of experiments relate to the vortex breakdown (bursting) characteristics on the leeward surface at zero sideslip and support the concept of flow control by fillets for both static and dynamic conditions pef . 41. The analysis of the data from the second series of experiments focuses on the vortex core trajectories and their interaction for the static condition and sheds interesting light on the resulting vortex structure and the effect of fillets. Some selected data from this series are discussed in this paper. Reference [3] contains full details of the investigation including flow visualization photos, tabulated data, vortex trajectory plots and vortex burst plots.

2. Exueriment

Water Tunnel Facility

The N P S water tunnel is a closed- circuit facility (Fig. 1) whose key design features are high flow quality, horizontal orientation, and continuous operation. The test section is 15 inches wide, 20 inches high, and 60 inches long. Water velocities of up to 1 Wsec with a turbulence intensity of < 1% are possible in the test section. The dye-supply system consists of six pressurized color dyes using water soluble food coloring and is provided with individually routed lines from the dye reservoir to the model support system attached to the top of the tunnel. The model is sting-mounted upside down in the test section. The model support system utilizes a C-strut to vary the pitch travel up to 50" between the liits of -10" and 1 lo", and a turntable to provide yaw variations up to f 20". The model attitude control system consists of two servo motors which provide independent control of model pitch and yaw with two rates.

L/

Double-Delta Wine Models

The baseline double-delta wing model and its three derivatives used in the investigation are shown in Fig. 2. The models were const~~cted of 0.25" - thick plexiglass. with a 76" weep for the strake and a 40" sweep for the wing, both with sharp, bevelled leading edges and flat top surfaces. The geometry modification (fillet) at the intersection (juncture or kink) of the strake and wing leading edges on each derivative model increased the planfom area of the baseliie model by 1%. The upper surf= of the models had grid l i e s marked for easy identification of vortex core trajectory and burst location. The location of the dye-tube tip and the dye-injection rate were crucial to obtain a good flow visualization of the vortical flowfeld. Satisfactory dye injection for vortex

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visualization purposes was achieved by locating small brass tubes flat on the model bottom surface with their tips very close to the apexkink, and adjusting the injection rate properly.

Exuerimental Promam and Test Conditions

The goal of the investigation was to evaluate the effect of frllets on vortex core trajectories, interactions, and breakdown (bursting) on the leeward surface of sharp- edged, double-delta wings at high AOAs with zero sideslip. The experimental program consisted of flow visualization of the models on the upper surface at high AOAs with zero sideslip angle. Both still

of meas$ng the vortex core locations in x- direction (along the model centerline from the apex), y-direction (outboard from the model centerline) and z-direction (perpendicular to, and away from the surface), and plotting the vortex core trajectories at different AOAs. The last point measured on the trajectory usually represented the core location close to vortex breakdown. The measurements were made on both sides of the model centerline using the apex 8s a reference point and average values obtained. The vortex core locations were visually selected from the videotapelphotographs with the utmost care and consistency and nondimensionalii using the centerline chord length.

picture photography and-videotape recording were used for documentation of the dye-flow

Nevdeless, there may be some d e w - of imprecision in the data due to the difficulty

visualization of the model in both topview and sideview. The freestream velocity (Ud in the tunnel was maintained at a nominal value of 0.25 ft/sec, corresponding to a Reynolds number of 24000/ft (18750 based

d on centerlie chord).

3. Results and Discussion

A large amount of flow visualization data has been collected with the double-delta wing models. All the data have been documented and discussed in Ref. [3]. The data pertahhg to the breakdown (bursting) location of the strake vortex have been reported in Ref. [4]. Some selected results pertaining to vortex core trajectories and their interaction for the static condition at zero sideslip are presented below. Typical photographs are included to highlight the flow phenomena.

Data Reduction and Data Ouality

Data reduction essentially consisted

&reading core locations because of the inherently unsteady nature of the vortex bursting that would make the vortex core unsteady, particularly at high AOAs.

Visualization of Vortical Flow Field

Figures 3-5 are typical flow visualization photographs showing the strake vortex core on the diamond-fillet model at AOAs of lo", 20", and 30°, respectively. Note that the nonvisualization of the fillet vortices was deliberate and intended to facilitate identification of strake vortex core location. The vortical flowfield develops over the upper surface of the wing as the AOA is increased from 0". At IO" AOA (Fig. 3), the strake vortex core is already well developed and is seen to burst just around the trailing edge. With further increase in AOA, the vortex core lengths shorten, the burst locations move upstream and f d l y approach the apex. Figure 6 is a typical flow visualization photograph showing both strake and fillet vortices on the

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upper surface of a linear-fillet model at 10" AOA. The trajectories of the vortex cores and their interaction can be clearly seen in this photograph.

The vortex trajectory plots and vortex burst location plots were constructed from the flow visualization data to facilitate quantitative assessment of the effect of different fillet shapes on the vortical flow field. The following discussion is based on vortex trajectory plots.

Vortex Interaction

The vortex interaction phenomenon can be studied with the help of combined trajectory plots. Figures 7-12 represent the interaction of the strake and fillet vortex cores for the basehe model. Figures 7-9 show the interaction in the spanwise- direction as a function of the streamwise location (dc) at AOAs of loo, 20°, and 30", respectively. At 10" AOA both the strake and the wing vortex cores are already well developed over the model and coiled-up around each other (Fig. 7). The strake vortex core is nearly conical over the strake portion of the model but is drawn outboard by the wing vortex. At the same time, the wing vortex is drawn away from the wing leading edge by the strake vortex. The wing vortex originates at x/c = 52.3%. The interaction or coiling-up between the strake vortex core and the wing vortex core starts at 64% of the centerline chord and 1 1% outboard from the model centerline. The vortex cores do not merge into a single vortex. The strake vortex bursts at x/c = 93% and the wing vortex bursts in the wake close to the trailing edge. As the AOA is increased further, the interaction occurs earlier and inboard and the burst locations move forward. At 20" AOA (Fig. 8), no

vortex interaction is possible as the strake vortex core bursts at 66% of the centerline chord just ahead of the wing vortex. At 30" AOA (Fig. 9), the strake vortex core bursts at 37% of the center line chord, way ahead of the origin of the wing vortex. Figures 10- 12 show the interaction in the normal direction as a function of the streamwise location (dc) at AOAs of lo", 20°, and 30°, respectively. As the AOA increases, the interaction of the two vortices moves upward &om the surface.

The combined trajectory plots for the filleted models exhibit similar but stronger and more complex interaction features because of the formation of two or more vortices in the fillet region. Compared to the baseline model, the interaction is seen to persist over a larger range of AOA. At 20" AOA of the diamond-fillet model, the strake vortex m e interacts at x/c = 52.5% and y/c = 8.5% with the beginning-of-the-fillet

12% with the endsf-the-fillet vortex core (Fig. 13). However, at 30" AOA, there is no interaction at all as the fillet vortices are already burst right at their origin and therefore there is only the strake vortex core present (Fig. 14) with its burst location ahead of the fillet location. The vortex interaction does not exhibit any tendency toward vortex merging.

vortex core, and at x/c = 65% and y/c = 'W'

To summarize, the combined trajectory plots highlight the mutual induction effect of strake and wing vortex cores on each other. The strake vortex induces an upward and inboard movement of the wing vortex, and conversely, the wing vortex induces a downward and outboard movement of the strake vortex. The interaction of the two vortex cores leads to their intemhinin g (that is coiling around each

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other). With increasing AOA, the interaction moves upstream until the AOA is large enough to cause the bursting of the strake vortex even before the interaction and subsequent destabilization of the wing vortex. These observations are in general agreement with those reported in Refs. [ 1,5,6, and 71. Over the range of the present investigation, it is seen that the AOA range for coiling-up of the vortices is quite narrow (approximately 10" - 207. Even at higher AOAs, the vortex cores maintain their separate identity without merging into a single vortex. This observation is in direct contrast to that reported in Refs. [5 and 81.

Vortex Core Traiectories

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Figures 15-18 illustrate the effect of AOA on individual vortex core trajectories for the baseline model. The non- dimensional x-y and x-z plots are shown in Figs. 15 and 16 for the strake vortex trajectory and in Figs. 17 and 18 for the

~ Wing vortex trajectory. The plots in Figs. 15and 16 clearly indicate that, as the AOA increases, the srake vortex core length decreases (implying upstream movement of the burst point), the vortex core moves inboard toward the centerline chord, and away from the surface. The wing vortex core length does decrease with the AOA (Figs. 17 and 18) but the trajectory movements in the spanwise and normal directions are clearly not monotonic because of strong interaction, particularly at low AOAs. Similar trends have been observed in the trajectory plots for the filleted models.

Figures 19-24 show the effect of fdlet shape on strake vortex core trajectories. Figures 19-21 represent the strake vortex core location in the y-direction for AOAs of IO", 20", and 30°, respectively. At 10"-AOA

the strake vortex core of the baseline model is closer to the model centerline than those of the filleted models. As the AOA increases from IO" to 30" the vortex cores move inboard for all the models. Compared to the baseline model, the rate of inward movement of the vortex core for the filleted models is slower. Figures 22-24 represent the strake vortex core location in the z- direction for AOAs of lo", 20", and 30", respectively. These plots highlight the fact that at any given AOA, the vortex core trajectory of the baseline model remains farthest away from the surface whereas that for the diamond-fillet model remains closest to the surface. It is also noticed from these trajectory plots that at any given AOA, the strake vortex core of the diamond-fillet model exhibits a more favorable burst response (Le. delayed bursting). Thus, the strake vortex of the diamond-fillet model has a longer core length that remains closer to the surface but farther out from the centexline chord. This implies lift augmentation.

Vortex Tearing

The vortex structure that exists on a double-delta wing downstream of the wing juncture has been the subject of several studies. Several researchers have suggested that the wing vortex is double-branched, meaning that there are two shear layers feeding the vortex [Ref. 81. One of the layers originates at the Wing leading edge and the other is connected to the strake vortex (see Ref. 6 for a schematic). The concept of this latter shear layer - called connecting layer - has been found useful in modeling vortex interaction. The numerical simulation of Kern 123 has revealed an unusual wing vortex structure midway down the leading edge of the wing suggesting a

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second wing vortex. He attributes it to the phenomenon of vortex tearing as a result of the interaction of the LEX and wing vortices, in particular the inboard displacement of the wing vortex. Extending the connecting sheet concept, he has proposed a new vortex structure with a tom wing vortex downstream of the juncture (see Fig. 8 of Ref. 2 for a schematic).

However, available experimental evidence to the presence of a tom wing vortex is very limited and restricted to double-delta wings with moderately swept leading edges in high speed flows. In general, the phenomenon of vortex tearing has not been vimahed in low speed experiments on double-delta wings with highly swept leading edges. It is argued that the particular combination of parameters including the moderate LEX sweep, the moderate wing sweep, and the flow conditions in the numerical simulation of Kern (Mach No. = 0.2, Reynolds No. = 1 million) might have contributed to the formation of a tom wing vortex downstream of the juncture. In an attempt to verify the presence of a tom wing vortex in the present investigation, dye was injected midway down the wing juncture and the dye-filament line was observed carefully to identify if it exhibited any characteristics of avortex core. Based on the flow visualization, it was concluded that for the test conditions (Mach No. ~ i : 0, Reynolds No. sz 0.02 million) there was no visual evidence to support the presence of any tom wing vortex downstream of the juncture. Note that the model configuration was identical to the one used by Kern.

4. Conclusions

A water-tunnel flow visualization

study was conducted to investigate the effect of juncture fillets on the vortical flow over a cropped, double-delta wing model with sharp leading edges. The study focussed on vortex interaction, vortex core trajectories, and vortex breakdown on the leeward surface at high AOAs with zero sideslip angle. The following conclusions are based on the results of the experimental investigation:

The mutual induction effect of strake and wing/filiet vortex cores on each other leads to their inmtwiniu g (i.e., coiling around each other). With increasing AOA, the interaction point moves upstream, inboard, and upward but only until the AOA is large enough to cause the bursting of the strake vortex even before the interaction could begin. The vortex interaction is limited to low AOAs in the range 10" to 20"; vortex merging does not take place at

The flow visualization does not show any tom wing vortex downstream of the juncture. The presence of fillets causes delay in both vortex interaction and vortex bFeakdown at high AOAs. Compared to other models, the strake vortex trajeatory of the diamond-fillet model indicates longer core length, remains closest to the surface and farthest from the centerlime at high AOAs. These features imply lift augmentation and support the concept of flow control using fillets.

higher AOAs. u

Acknowledgements

This work which formed the M.S. thesis of one of the authm (a) was supported by the Naval Air Warfare Center

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Aircraft Division, Warminster, PA. The technical discussions with Steve Kern are

4 acknowledged. The authors sincerely thank Ron M a k e r for fabricating the models. 5.

References

1. Hebbar, S.K., Platzer, M.F., and Li, F.H., "A Visualization Study of the Vortical Flow over a Double-Delta Wing in Dynamic Motion," AIAA Paper 93-3425, Aug. 1993.

using Fillets on a Double-Delta Wing," Journal of Aircraft, Vol. 30, No.6, Nov.-Dec. 1993, p.818. Also, AIAA Paper 92-041 1.

3. Alkhozam, A.M., "Interaction, Bursting, and Control of Vortices of a Cropped Double-Delta Wing at High Angle of Attack," M.S. Thesis, Naval Postgraduate School, Monterey, California, March 1994. Hebbar, S.K., Platzer, M.F., and Alkhozam, A.M., "Investigation into the Effects of Juncture Fillets on the

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2. Kern, S.B., "Vortex Flow Control 7.

8.

4. d

Vortical Flow over a Cropped, Double-Delta Wing," AIAA Paper

Thompson, D.H., "visualization of Vortex Flows around Wings with Highly Swept Leading Edges," 9th Australasian Fluid Mechanics Conference, Auckland, Dec. 1986. Olsen, P. and Nelson, R, "Vortex Interaction over Double-Delta Wings at High Angles of Attack," AIAA Paper 89-2191, July-Aug. 1989. Graves, T.V., Nelson, R.C., Schwimley, S.L., and Ely, W.L., "Aerodynamic performance of Strake

Angle-of-Attack Technology Vol. I,

May 1992. Verhaagen, N.G., "An Experimental Investigation of the Vortex Flow over Delta and Double-Delta Wings at Low Speed," Delft University of Technology, Report LR-372, Sept. 1983.

94-0626, Jan. 1994.

wing Configurations," In: High-

NASA CP-3 149, Part 1, p ~ . 173-204,

v Figure 1 . Model Support System af the NPS Water Tunnel

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d 9.5.'

a) Baseline Model (No Fillet)

9.9,-4

b) Parabolic-Fillet Model

I1 1- 9.5" I

c) Linear-Fillet Model

Fig. 2 The Double-Delta W i n g Models

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'-'

Fig.

Fig.

3 Strake vortex core, dianmnd-fillet d e l . AOA =

4 Strake vortex core, d i m n d - f i l l e t d e l , AOA =

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Fig, 5 Strake vortex core, diamnd-fillet mdel , AOA = 30'

Fig. 6 Strake and f i l l e t vortex cores, linear-fillet mdel , AOA = 10'.

10

0.20 0 z

- @ - S t r a k e vortex -E- Wing vortex

0.20

E O

I u - " 2 0.15

x 0 - g 0.10 I E 0.10

D >

0.05 0.05

0.00 0.00

0.0 0.2 0.4 0.6 0.8 x/c

0.0 0.2 0.4 0.6 0.8 XIG

Fig. 7 Vortex core location YlC tor baseline model Fig. 8 Vortex core location Y/C for baseline model AOA -20 Dog.

AOA - 10 Deg.

0.10 0 ;j 0.20

0 r - -m- Wing vortex

- 0 " x * 0

L

0.06

L

> m 0 2 0.10 7 0.04

0.05 0.02

d L *J B1-m iL-7-T- 0.0 0.2 0.4 0.6 0.8

x/c

0.00 0.00 - ~ ~ ~ - 0 .o 0.2 0.4 0.6 0 .8

XIC Fig.10 Vortex core location Z/C for baseline model

AOA -10 De& Fig. 9 Vortex core location Y/C for baseline model 4 AOA - 30 Deg.

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+Streke vortex +Wlng vortox

0 N

-I . L : 0.06

0.02 1 0.00 - I I I I I I 1 I r

0.0 0.2 0.4 0.6 0.1) x/c

Fig. 11 Vortex core location 2/C for barollns model ADA 20 Dog.

0.12

0.10

t

0.00 .-

0 .o 0.2 0.4 0.6 0.8 x/c

Fig. 12 Vortex core location Z/C for baseline mods1 AOA - 30 Deg.

0.0 0.2 0.4 0.6 0 .E X P

0.0 0.1 0.2 0.3 0.4 x/c

Fig. 14 Vortex core location Y/C for diamond-fillel model Fig. 13 Vortex core location Y/C for diamond-fillet model at AOA - 30 Dog. at AOA - 20 Deg.

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0.05

0.20 - 0 2 E 0

(.

- - Z 0.15 - : - 0 - " x a - g 0.10 - 5

> rn c -

0.05 -

0.00

-E+ AOA-1ODeg -D- A 0 A - 16 De 4 A O A - 2 0 D e g -0- AOA-22.5Deg + AOA-30Deg

Jp

0.8

l

0.0 0.2 0.4 0.6 x/c

Fig.15 Vortex core location Y/C for baseline model

c 0 . - 2 0.06 - u 0 - 0

" x 0

: 0.05 -

- : 0.04 1 > a r !! 0.03 - - v)

0.02 -

0.01 -- I 0.00 -

0.0 0.2 0.4 0.6 0.8 XIC

Fig. 16 Vortex core location Z/C for baseline model d

d.00 I I I I I I

0.50 0.55 0.60 0.65 0.70 0.75

Fig. 17 Vortex core location Y/C for basellne model x /c

0.15 - 1 0 hl

c

- 0.13

L " 0 - a

0.10 u X a - L = 0.08

E - c

0.05

0.03

0.00

Fig.

0.50 0.55 0.60 0.65 0.70 0.75 x/c

18 Vortex core location Z/C for baseline model

13

0 2 0.20 c 0

m ” 0

- - -

0 Fo.20 - c 0

* ” 0

0

“ I

- - -

t 0.15 0 u I m 0 >

;; 0.15 - - 0 0.10 2 z - 01

” ;j

,” 0.10 - 2-

*

.- v) - ./

0.00 --

0.05

0.00

+-baseline model -n- l lnear - f l l le t model -c-parabolic-fillat model

0.0 0.2 0.4 0.6 0.8 0.4 0.6 0.8 XIC W

0.0 0.2 x/c

Fig. 20 Vortex core locatlon Y/C for dllferant models Fig. 19 Vortex core locallon Y/C for different modsla AOA - 20 Dag. AQA - 10 Deg.

- m - l l n e ~ r - f l l l ~ t model -o-paroboIIc-tl l lel model + dlrmond- l l l la l model

0.00 .-rl-- 0.0 0.1 0.2 0.3

x/c

Fig. 21 Vortex core location Y/C for different models AOA - 30 Dag.

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-a - l inear - l l l le t modal +parabollc-l l l let model + diamond-fll let model

N

c 0

m U

c/

2 0 . 0 8 - 0

0 u x

c

E 0 . 0 6 - 0 z. 0 r

- 2 z0.04 -

0.02 -

0.00 -1- 0.0 0.2 0.4 0.6 0.8

XJC

Fig. 22 Vortex core location Z/C for different models AOA -10 Deg.

-0- Ii ne a r . f I I1 e t mod e I -o-parabollc-flllel model +dlamond-lIl lst model

0 2 5 D -

0.08 - 0

D u

Z 0.06 .- .. 0 > 0 x m 2 m 0.04 1

0.02

0.00 + 0.0 0.2 0.4 0.6 0.8

x/c

Flg. 23 Vortex core locatlon Z/C for different models AOA - 20 Deg.

+bssellne model +llnenr-fll let model

g 0.08 + p ~ ~ r a b o l l c - f I l l e t model -+-dlamond-flllet model

N' 0

c 0 - I 0

0

0 U

- L

g 0.06 - L

0 > o

!? 0.04 - z - ul

0.00

0 .o 0.1 0.2 0.3 x/c

Fig. 24 Vortex core location ZlC for different models AOA-30 Deg.

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