Effect of tip vortices on flow over NACA4412 aerofoil with different aspect ratios

Effect of tip vortices on flow and laminar separation bubble over NACA4412 aerofoil at low Reynolds numbers and different angles of attack was investigated in detail by performing force and flow visualization via smoke wire technique. Experiments have been done at Reynolds number of 50000 and the wing model of aspect-ratio was 1 and 3, respectively. From the experimental results, the flow visualization results showed that tip vortices effect on the laminar separation bubble and the bubble reduces over the wing with low aspect ratio as the angle of attack increased. Moreover, it was noticed that stall angles decreased as aspect-ratio increased at the same Reynolds number.

Recently, elliptical and rectangular wing model was thought to be hopefully for unmanned aerial vehicles (UAV), micro air vehicles (MAV) and other applications, since they were noticed that they could maximize the wing area for certain wing dimension.Thus, it can be said that is necessary to understand flow over the low aspectratio wings with these given wing models.
In these days, one study related to effects of aspectratio on the aerodynamic characteristics of wings at low Reynolds number has been presented by M. Mizoguchi and H. Itoh [12].They firstly concluded that influences of aspect-ratio were not as important as they thought as aspect-ratio was 3 or more.But this situation was different at low aspect-ratios wings.Tip vortices had significantly important influences on aerodynamic characteristics of airfoil.Effect of tip vortices increased while aspect-ratio of wing decreased.Maximum lift coefficient and stall angle considerably increased as aspect-ratio was less or equal to 1. Okamoto and Azuma [13] investigated the effects of low Reynolds number and aspect ratios when Reynolds number was around 10000.Their results showed that stall angle increased significantly when aspect-ratio was less than or equal to 1. Torres and Mueller [14] studied low aspect-ratio wings and their characteristics at Reynolds number in the style of 100000.Their studies showed same results for low aspect-ratio wings.Since, aspect ratio reduced to 1 and accordingly maximum lift coefficient and stall angles increased considerably.Other numerical experimental results [15][16][17][18][19] have demonstrated that low aspect-ratio wing performance is affected by both linear and nonlinear sources of lift.
In this report, the effect of the aspect ratio on the aerodynamic characteristics is investigated experimentally.After all tests, results are compared and discussed with literature studies if these experimental results are in coherence.

Experimental Tests
First of all, NACA4412 airfoil model was produced via 3D printer and then it was rubbed to obtain a smoother surface of wing as shown in Figure 1.

Aerodynamic Force Coefficient for NACA4412 airfoil model
In this study, NACA4412 airfoil model was produced with the aspect ratio of 1 and 3 were chosen at Reynolds number of 50000 and at the angles of attack from 0ࡈ to 20ࡈ .Computer-controlled automatic angle changing force measurement system was used.Drag and lift forces were determined with strain-gauge which was connected to system.Then, these drag and lift forces were transformed into drag coefficient (C D ) and lift coefficient (C L ), respectively.Variations of drag and lift coefficients with angle of attack were given at Reynolds number of 50000 as shown in Figure 2. As seen from the figures, the stall angle is 38ࡈ and C L,max is 1.35 when the aspect-ratio is 1.Yet, stall angle is 14ࡈ and C L,max is 1.25 as the aspect-ratio has been increased as 3 at same conditions.The cause of this situation is that the effects of tip vortices over the wing are seen more at low aspect ratio.Thus, stall angle at the aspect-ratio of 1 is bigger than value of AR = 3.In addition to the results, it can be seen that stall at Reynolds number of 50000 for AR = 3 is sharper than stall for AR = 1.That is, it can be said that abrupt stall is seen at aspect ratio of 3 whereas mild stall is seen at AR = 1.

Flow Visualization Device
The smoke-wire test for flow visualization technique is employed in this study.In smoke-wire flow visualization, different smoke-wire tests are applied for visualization with AR = 1 and AR = 3.As shown in Figure 3, z / c was used as a term for location of smoke-wire.It can clearly be seen that this z / c term is located of the outer portion of wing if it is negative sign.For AR = 1, tip-vortices moved horizontally directed towards to middle of wing with increasing of angle of attack whereas they formed at the tip of wing at low angle of attack and continued to move towards to wake region of wing.Yet for AR = 3, movement of tip-vortices was limited as angle of attack increased.

02027-p.3 3 Conclusion
The aerodynamic characteristics and corresponding flow structures happened by separated shear layer and tipvortices over the wing upper surface at Reynolds number of 50000 for AR = 1 and AR = 3 have been experimentally investigated by using flow visualization technique like smoke-wire test and the force measurement tests.For low aspect-ratio wings, tipvortices have a considerable on characteristics of NACA4412 wing model.As mentioned earlier, value of maximum lift coefficient (C L,max ) is 1.35 and stall angle is 38ࡈ for AR=1 at Reynolds number of 50000.On the other hand, C L,max is 1.25 and stall angle 14ࡈ when aspect-ratio has been increased from 1 to 3. It can be noticed that the effect of tip-vortices increases when the aspect-ratio decreases.The results in regards to the sample of flow visualization with separation laminar shear layer, flow reattached property and generation of tip-vortices on the upper region of wing can be demonstrated by the corresponding flow structure showed at figures in the present report.This type of flow structures moves from tip of wing to middle of the wing when angle of attack increases.Yet, they will vanish and they will be replaced by huge portion of wake region after a while.

Figure 2 .
Figure 2. Aerodynamic force coefficient distributions for different aspect ratios at Re = 50000.

Figure 3 .
Figure 3. Different positions of smoke depending on the wire.Flow visualization with smoke-wire test was presented for AR = 1 and AR = 3 at Re = 50000 as shown at Figure4and Figure5.Moreover, flow visualization with smoke-wire test was also presented as top view of wing for AR = 1 and AR = 3 at Re = 50000 as shown in Figure6and Figure7, respectively.

Figure 7 .
Figure 7. Flow visualization with smoke-wire test at Reynolds number of 50000 for AR = 3 and the top of view a) 4ࡈ , b) 16ࡈ .In Figure (4a) and Figure (5a), flow over the wing model was in a normal way and was smooth.Tip-vortices became more dominant when angle of attack increased from 0ࡈ to 24ࡈ .In this type of flow regions for AR =1 and AR = 3, laminar separation bubbles were affected by tipvortices.Tip-vortices over the wing model behaved like additional force which helped to attach for flow and separated flow at the middle of wing formed clinging flow at the tip of wing.Vortices coming from tips affected the majority of wing surface and instabilities increased.Also, rotational flow at wake region improved flows by doing vacuum effect with pressure drop.For AR = 1, tip-vortices moved horizontally directed towards to middle of wing with increasing of angle of attack whereas they formed at the tip of wing at low angle of attack and continued to move towards to wake region of wing.Yet for AR = 3, movement of tip-vortices was limited as angle of attack increased.