SUN Hai－sheng，JIANG Yu－biao，LIU Zhi－tao，SHI Zhe－yu

（1.School of Aeronautics，Northwestern Polytechnical University，Xi'an 710072，China；2.China Aerodynamics Research ＆Development Center，Mianyang Sichuan 621000，China）

0 Introduction

Delta wing is considered as the first choice for modern fighter because of its good drag characteristics at high speed and good lift characteristics at high angle of attack（HAoA）at low speed.To harmonize the conflict between subsonic，transonic and supersonic flight condition and improve the fighter's post and stall maneuverability and supersonic cruise capability，strake－wing is the most popular arrangement for fighter design［1］.A lot of researches on the aerodynamic characteristics and flow mechanism of double－delta wing have been made at home and abroad，indicating that the flowfield around doubledelta wing is very complicated at HAoA and contains strong vortex motion and interaction［2－4］.

In this paper，the aerodynamic characteristics of an 80°／65°double－delta wing have been introduced at first.Secondly，several typical wing sections are chosen for pressure and flow structure measurement to study the double－delta wing flow mechanism at HAoA［5－6］.

1 Test contents

The tests were carried out in theΦ3.2mlow speed wind tunnel at CARDC.

Force test：velocity is 40m／s，Re is 1.3×106，angle of attack is 0°～60°，sideslip angle is 0°.

Pressure test：velocity is 35m／s，Re is 0.55×106，angle of attack is 0°～60°，sideslip angle is 0°.

Smoke test：velocity is 15m／s，Re is 0.49×106，angle of attack is 0°～44°，sideslip angle is 0°.

PIV test：velocity is 15m／s，Re is 0.49×106，angle of attack is 0°～44°，sideslip angle is 0°.

2 Test model and facilities

2.1 Wind tunnel

Ф3.2mwind tunnel is a single return wind tunnel.The test section is 5mlong with a round cross section of 3.2mdiameter.Its maximum velocity is 115m／s and bias angles are｜Δα｜，｜Δβ｜≤0.5°.

2.2 Test model

The model is made of aluminium alloy.The sweep angle is 80°for front wing and 65°for rear wing，and all the edges have 10°rake angle.Fig.1 shows the model sketch of pressure measurement test.The model size of the force measurement，smoke and PIV test is twice as large as the pressure measurement.

Pressure measurement points are distributed on the upper wing surface of the right side.The four sections are perpendicular to the model longitudinal axis with x／Cr＝61%，70%，77.5%and 83.7%respectively（xis the distance from the section to the model forefront）.The distance between neighboring points in one section is 10mm.The sequence number of pressure measurement point starts from wing root.Figure 2gives the distribution of pressure measurement points.

The PIV measurement cross sections are located at D＝240mm（x／Cr＝77.5%）and D＝320mm（x／Cr＝70%），where Dis the distance from cross section to trailing edge.The viewfield of PIV measurement section is 400mm×600mm，which can cover most of upper flowfield of the right wing.

Fig.1 Model sketch圖1 實驗模型

Fig.2 Pressure points圖2 壓力測量點

Fig.3 PIV sections圖3 PIV測量剖面位置

2.3 Model support equipment

The model sting support system was used.Its main capacity parameters are presented in Table 1.

Table 1 Parameters of sting support system表1 支撐系統(tǒng)性能參數

Fig.4 Sting support system圖4 模型支撐系統(tǒng)

2.4 Measurement equipment

（a）Balance and scan valve

An internal six－component strain gauge beam balance TG0401Awas used for the force test.Its performance is listed in Table 2.

Table 2 Balance performance parameters表2 天平性能參數

DSM3400electronic scan valve system was used for pressure measurement，its main performance is as follows：

· Number of channels：1280

· Precision：±0.08%FS

· Measurement range：7000Pa

（b）Flow visualization test equipment

The single tube smoke－generator system was used in smoke flow test.This system is composed of smoke pipe，heater，nozzle，pump and smoke flow pipe.

The PIV system consists of four primary parts：

Laser system：The Vlite－500Nd：YAG doubleimpulse laser's maximal energy is 500mJ，the impulse width is 6～8ns，and the interval of impulse can be adjusted.

Recording system：The RedLake ES11000 Charge Coupled Device（CCD）'s resolution is 4008 pixel×2672pixel.

Synchronization system：The MicroPulse725 synchronizer can control the trigger schedule of the laser and the recording.

Glycol was used as the smoke agent for both smoke and PIV test，because its good following behaviour can visualize the flowfield well.It is volatilizable and will not pollute the wind tunnel.

3 Results and discussion

3.1 Force measurement results

Figure 5gives the aerodynamic characteristics at high AoA.The lift－curve slope drops slightly aroundα＝26°.The lift coefficient reaches maximum atα＝30°，and the maximum value is 1.313.In the range ofα＝30°toα＝37°，lift coefficient changes little.When the AoA exceeds 37°，the lift coefficient drops rapidly.

Fig.5 Aerodynamic characteristics at HAoA圖5 大迎角氣動特性

3.2 Pressure measurement results

Figure 6gives the pressure characteristics at two pressure measurement cross sections.The fol lowing discussion focuses on the cross section of x／Cr＝83.7%.Whenα＜5°，the pressure coefficient changes little with different spanwise location，and the flowfield is mostly attached flow.Atα＝10°，the pressure coefficients near the root of the rearwing are evidently larger than that near the leadingedge of the rear－wing，indicating that leading－edge vortex comes into being.For its suction force，there is a comparatively large negative pressure area near the leading－edge.With the AoA increasing，negative pressure area and the absolute value of the pres－sure coefficients both increase.Whenα＜25°，this phenomenon is approximately linear，and the strength of leading－edge vortex increases continuously.The pressure curve ofα＝25°intersects with the pressure curve ofα＝30°.The pressure coefficients near the wing root increase as the AoA increases，but the pressure coefficients near the leading－edge decrease as the AoA increases.It indicates that the strength of the leading－edge vortex becomes weak.This behaviour accords to the lift characteristics，which shows a downward turn atα＝26°.

Fig.6 Pressure characteristics at HAoA （the number located on xaxis is the sequence number of pressure point）圖6 大迎角壓力特性（x軸上的數字為測壓點序號）

In the cross section x／Cr＝83.7%，in the range ofα＝30°～38°，the｜Cp｜near leading－edge decreases slowly with the increasing of AoA，and then decreases rapidly as the AoA increases from 38°to 40°.It indicates that the vortex break down at the trailing edge.When AoA exceeds 40°，the pressure peak disappears，and the value of pressure coefficients remain constant，which indicates that the vortex break down totally.This phenomenon is consistent with the stall characteristic at HAoA.Figure 6（b）shows that the suction peak is near the 7th and 8th pressure measure points，at where the vortex core locates.

In the cross section x／Cr＝70%，the pressure characteristics is similar to that of cross section x／Cr＝83.7%.At the same AoA the pressure coeffi－cient of the preceding point is larger than that of the the following point.

3.3 Smoke flow test results

Observations from Figure 7are presented as follows：vortex is not visiable atα＝5°；evident flow rolling appears whenαgets to 10°；evident leading edge vortex appears atα＝25°，and front wing vortex links up with rear wing vortex at connect area of front ＆rear delta wings；phenomenon of breaking of leading edge vortex appears at downstream of trailing edge underα＝27°；breaking point of leading edge vortex moves forward to near the trailing edge whenαgets to 30°；breaking point of leading edge vortex moves forward to about x／Cr＝83.7%atα＝32°；breaking point of leading edge vortex moves forward to about x／Cr＝70%atα＝34°；afterwards the breaking point moves forward with the increasing of AoA，and leading edge vortex disapears completely when AoA gets to 40°.

Spanwise position of vortex core moves little with the changing of AoA.

Above results of smoke flow test have good correlation with results of force and pressure test.

3.4 PIV Test

Flow results of PIV test in cross section 1at distance D＝320mm （x／Cr＝70%）in front of trailing edge are presented in Figure 8.

Within AoA range of 25°～34°，while the AoA getting higher，center point of rotational flow gets higher but spanwise position moves little，and area of high vorticity gets bigger but the maximum vorticitdecreases.Maximumvorticitdrosuickl when AoA increases from 32°to 34°，this contributes to that breaking point of leading edge vortex has moved beyond cross section 1（reminding former results of smoke flow test），in other words，vortex has already broken down at cross section 1.Within AoA range of 34°～40°，maximum vorticity continues to decrease with increase of angle of attack.One unsTable limit cycle appears in the flow field at angle of attack of 36°；the limit cycle still remains at angle of attack of 38°but the area of limit cycle decreases；anditdisaearswhenanleofattacketsto40°and flow field behaves as a weaker rotational flow，reminding that leading edge vortex of rear wing has broken completely form result of smoke flow test.

Flow results of PIV test in cross section 1at distance D＝240mm （x／Cr＝77.5%）in front of trailing edge are presented in Figure 9.It can be seen that at the same angle of attack，maximum vorticity of cross section 2is less than that of cross section 1.Flow field structure of cross section 2change similar to that of cross section 1.Maximum vorticity drops quickly when angle of attack increases from 32°to 34°，contributing to the break of leading edge vortex.One unsTable limit cycle appears in the flow field at angle of attack of 36°，and limit cycle disappears when angle of attack gets to 38°.

Fig.9 Flow structure above right wing（cross section of D＝240mm）圖9 右側機翼流場結構（D＝240mm剖面處）

4 Conclusions

（1）Investigations on the high AoA aerodynamics and flow field structures of an 80°／65°doubledelta wing were made by the use of combined methods of force，pressure，smoke and PIV measurements.The test results demonstrate good correlation between these measurements.

（2）Lift coefficient of the double－delta wing reaches its maximum atα＝30°.Betweenα＝30°～37°the lift coefficient changes little，then it drops quickly beyondα＝37°.

（3）Single vortex appears on the double－delta wing，and front wing vortex links up with rear wing vortex at connection area of front ＆rear delta wings.Leading edge vortex starts to break when angle of attack exceeds 30°.Suction peak starts to disappear and pressure coefficient drops quickly when angle of attack increases from 38°to 40°.When angle of attack exceeds 40°，suction peak disappears completely while pressure coefficient changes little and vortex breaks down completely.

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