aerodynamic effect of 3d pattern on airfoil effect of 3d pattern on airfoil xiao yu wang, sooyoung...

AERODYNAMIC EFFECT OF 3D PATTERN ON AIRFOIL

Xiao Yu Wang, Sooyoung Lee, Pilkee Kim and Jongwon SeokSchool of Mechanical Engineering, College of Engineering, Chung-Ang University, Seoul, Republic of Korea

E-mail: [email protected]

ICETI-2014 J1031_SCINo. 15-CSME-31, E.I.C. Accession 3806

ABSTRACTIt is known from recent observations that the textured surface plays a role in reducing the drag force andincreasing the lift force of a moving body. Comparing the numerical simulation between smooth surfaceand textured surface in this study, we also observe that the textured surface reduces the drag coefficient andincreases the lift coefficient of the surface. As for the two simulation models performed in this study, we usethe modified NACA0018 model for the basic airfoil configurations. After the simulation using Fluent, theresults about the two models are mutually compared, and we found that the textured airfoils can decreasethe drag coefficient and increase the lift coefficient dramatically. We also found that there exists an optimalangle, at which both the drag coefficient decrement and the lift coefficient become maximum. The finalgoal of this study is to design the airfoil with the reduced drag coefficient and the improved aerodynamicefficiency

Keywords: textured surface; aerodynamic; drag coefficient; lift coefficient; separation region.

EFFET AÉRODYNAMIQUE D’UN MOTIF EN 3D SUR UNE SURFACE PORTANTE

RÉSUMÉIl est bien connu d’après de récentes observations qu’une surface texturée joue un rôle dans la réduction dela force de poussée et l’augmentation de la force de portance d’un corps en mouvement. En comparant lasimulation numérique entre une surface lisse et une surface texturée dans cet article, nous observons aussique la surface texturée réduit le coefficient de poussée et augmente le coefficient de portance de la surface.Quant aux deux modèles de simulation réalisés dans cette étude, nous avons utilisé le modèle NACA0018pour les configurations de base du profil aérodynamique. Après la simulation, utilisant FLUENT, les résultatsdes deux modèles sont comparés mutuellement, et nous trouvons que le profil texturé peut diminuer lecoefficient de trainée et augmenter le coefficient de portance de façon considérable. Nous avons trouvéégalement qu’il y a un angle optimal auquel le coefficient de trainée et le coefficient de portance sont aumaximum. Le but ultime de cette étude est la conception d’un profil aérodynamique avec le coefficient detrainée réduit et une efficacité améliorée.

Mots-clés : surface texturée; aérodynamique; coefficient de trainée; coefficient de portance; région de sé-paration.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015 537

1. INTRODUCTION

It has become an international hot spot in recent years to research for drag reduction of non-smooth surfaceapplied to the aerodynamic engineering technology. Therefore, to improve the working efficiency of the air-foil, different types of surface modifications are often attempted. A vortex generator is the most commonlyused airfoil surface modification [1–5]. It helps to reduce the pressure drag at high angle of attack, and alsomake overall improvement of an airplane’s performance [6].

It has plenty of important practical applications for the unsteady aerodynamic phenomena on airfoils be-low Reynolds number of 5×105 [1], such as in micro-air vehicle, wind driven generator, draught fan, rud-der, airplanes and so on [1, 7]. As to the characteristic of operation under low Reynolds number, researchersincreasingly pay attention to study the incompressible flow in recent years [8, 9]. In 2003, Mueller andDeLaurier [10] pointed out most researches mainly concentrated upon the characteristic of boundary layeron suction surface, for example: laminar separation, transition, turbulent reattachment, bubble bursting, etc.It has significant influence on aerodynamic performance. Moreover, the controls of unsteady flow separa-tion as well as the lift and drag characteristics for flow around airfoils are also important objectives in theaerodynamic area. The vortex generations will affect the performance of devices such as: producing aerody-namic noise increasing drag force, causing the stall of aerodynamic force and even damaging the structureof airfoils [11–13]. Up to now, many experiments involving the analysis of quantitative value and theoreticalstudy are being done to realize the fundamental principle of fluid dynamics for conventional airfoils underdifferent angles of attack and different airfoil profiles. For example, a two-dimensional NACA0024 airfoilequipped with a leading edge rotating cylinder was investigated experimentally in 2000 by Al-Garni et al.[14]. In 2004, under the Reynolds number of 5×105 and angle of attack 18, the jet was used in experimentto control the air flow on surface of NACA0012 airfoil by Huang et al. [15]. In 2005, the investigation aboutthe lift force of a wing pitching in dynamic stall for a comfort control vessel was done by Hoo et al. [16].

Also, the active and passive control were considered to find an effective method such as: control trailingedge vortex shedding of half-ellipse shaped bodies. Do et al. [17], Liu et al. [18], and Tang and Dowell[19] controlled the aerodynamic flow of an airfoil with different modifications of the trailing edge. It provedthat the thin extended trailing edge can enhance lift force while the zero-lift drag has no significant increase.Kuya et al. [20] and Shan et al. [21] both used the vortex generator to control the separation of airfoiloverflow in their numerical analysis. The passive vortex generators get rid of the separation partially byreconnecting the separated shear layer to the airfoil over a significant extent.

When airfoil attains an angle of attack, wake’s formation starts due to boundary layer separation. Appli-cation of dimples to the aircraft wing also works in the same manner as the vortex generators. They createa turbulence boundary layer which delays the separation and reduces the wake and the pressure drag. In asmall angle of attack, the airfoil begins to produce small flow separation. As the angle of attack increases, theseparation area is also increased and the lift (drag) force is reduced (increased). Generally, a larger loadedthick wing is beneficial from the structural strength viewpoint. However, it would result in the flow sepa-ration deterioration on the aerodynamic performance of such a wing. Use of dimple can cause an increasein the overall lift and reduction in drag of the airfoil as observed by Srivastav [22]. Shan et al. [21] foundthat the flow control with an active vortex generator is more efective and the separation zone is not visiblein the averaged results Lin et al. [7] found that the flow control on various wavy surfaced airfoils causes thechange of lift and drag coefficients They investigated the effects of dimple that affect the separation area andthe change of drag and lift coefficients of the airfoil.

The surface texturing technique is used to reduce the friction or drag force on the surface. A typicalrepresentative example of inward dimple for drag reduction is the golf ball. A golf ball, which is patternedwith inward dimples, is known to receive the drag force only about a half of that of a smooth ball. Whena golf ball is flying, some small vortices are generated near the dimples, because the suction of these small

538 Transactions of the Canadian Society for Mechanical Engineering, Vol. 39, No. 3, 2015

Fig. 1. Modified NACA0018 profiles (a) Model 1 and (b) Model 2.

vortices causes the delay of the separation point of boundary layer. Furthermore, the vortex zone formedbehind the golf ball becomes much smaller than that behind the smooth ball, and the drag force formed bythe pressure difference tends to be greatly reduced. At the same time, the lift force of the golf ball tendsto increase. Choi et al. [23] found through an experimental research for the golf that the inward dimplecauses an instability of the airflow shear layer, and the resulting instability can make the airflow of the localseparation with great momentum to adhere to the golf ball surface again, which, in turn, causes to possess anability to overcome strong inverse pressure gradient and delay the flow separation, and therefore, reduce thedrag force. Lienhart et al. [24] performed an experiment and numerical analysis on the smooth plate withinward dimple, then they found that inward dimple did not have any notable drag reduction and pressureloss effects.

Riblet drag reduction is one of the most hotspot problems for the research area on the non-smooth surfacedrag reduction. So far, several researchers have carried out a large number of experimental studies onthe plate and airplane wings with riblet surface for drag reduction and flow control. These experimentsdemonstrated the effectiveness of the non-smooth surface on the drag reduction and flow controllability.The surface friction force occupies about 40–50% of its total drag force when the airplane is in flight. Evenif the degree of drag reduction effect is small, the reduction of surface friction can bring considerable amountof energy savings. Viswanath [25] studied the application of the riblet for the drag reduction on an airplane.Caram and Ahmed [26] measured by experiments the wake region of turbulence flow for NACA0012 airfoilat 0 angle of attack, and found that the drag reduction rate was 13.3% when the riblet height = 0.152 mm,and the drag reduction was 2.7 and 7.3% when the riblet height = 0.076 and 0.023 mm, respectively.

In this study, we attempt to verify the aerodynamic effect of the textured surfaces, analyze the modifiedNACA 0018 model with plain and textured surfaces. The airfoil configurations are modeled using Solid-Works 2014TM and the drag and lift coefficients are computed using a CFD program, FluentTM. After thesimulation, the drag and lift coefficients of the two models are compared. The Spalart–Allmaras turbu-lent model is employed in the present 3D airfoil analysis. The drag and lift coefficients are calculated bychanging the angle of attack.

2. NUMERICAL SIMULATION ON DRAG REDUCTION OF NON-SMOOTH SURFACE

2.1. The Physical ModelThis paper mainly treats the NACA0018 model. Model 1 and Model 2 are, respectively, plain and texturedmodels. As shown in Fig. 1, the chord length is 16 cm for both models. In Fig. 2, 25 dimples in totalare composed on the surface of Model 2 with 2.5 cm in center-to-center separation. The dimple is semi-ellipsoidal and the size is determined after performing preliminary 2D simulations with the lengths of theminor and major axes of 3.75 and 5 mm, respectively, and the size of the dimple is also determined after the2D simulations.


Fig. 2. Configuration of dimples in Model 2 (top view).

Table 1. Concrete terms of symbols in the governing equation.Setting φ Γφ Sφ

Continuity equation 1 0 0Momentum equation ui µ −∂ p/∂xi +ρgi

In this study, we change the angle of attack of Models 1 and 2 to examine the changes of the drag and liftcoefficients.

2.2. The Governing Equation with a Turbulence Model and Computational ConditionsThe governing continuity and momentum equations can be expressed as

∂ (ρφ)

∂ t+div(ρuφ) = div(Γφ gradφ)+Sφ , (1)

where u is the velocity vector composed of the x,y and z directional velocities of ux,uy and uz, respectively.The symbol φ is the quantity representing either 1 or ui,Γφ is the generalized diffusion coefficient and Sφ isgeneral source term. In Eq. (1), the values of φ ,Γφ and Sφ are expressed in Table 1.

In this study, the Spalart–Allimaras turbulent model is employed, which is a relatively simple ‘one-equation’ model and can solve a transport equation with the kinematic eddy (turbulent) viscosity. Fur-thermore, this model is designed specifically for aerospace applications involving wall-bounded flows andis preferred for boundary layers subjected to adverse pressure gradients.

As shown in Fig. 3, the left side of the calculation domain is the velocity inlet conditions and the rightside is the pressure outlet conditions. In this model, the Reynolds number is 320,000, which is higher thanthe critical Reynolds number (i.e., 4000). The compressible fluid is air, and its density is 1.2 kg/m3 and itsviscosity is 1.2×10−5 Pa-s. The velocity of air is set to be 20 m/s, and the hydraulic radius is 0.01 mm andthe initial turbulent intensity is 3.28%.

3. RESULTS AND DISCUSSIONS

Figure 4a shows the variations of the drag coefficients (Cd) of Models 1 and 2 simulated with different angleof attack, α . In this figure, Cd of Model 1 is larger than that of Model 2 when α is greater than 10◦. When α

is smaller than 10◦, Cd of Model 1 is smaller than that of Model 2. Figure 4b shows the decrement of Cd ofModel 2 with respect to the Cd of Model 1. When α is 20◦, the decrement of Cd becomes the largest valueof 20.5%.


Fig. 3. 3D airfoil calculation domain.

Fig. 4. (a) Drag coefficient versus angle of attack and (b) Drag coefficient decrement of Model 2 with respect toModel 1 versus angle of attack.

Fig. 5. (a) Lift coefficient versus angle of attack and (b) Lift coefficient increment of Model 2 with respect to Model 1versus angle of attack.


Fig. 6. Location for the observation of separation area (hatched area).

Figure 5a shows the variations of the lift coefficient (Cl) of Model 1 and 2 simulated with different angleof attack, α . In this figure, Cl of Model 2 is larger than Model 1 when α is greater than 10◦. When α issmaller than 10◦, the Cl of Model 2 is smaller than Model 1. Figure 5b shows the increment of Cl of Model 2with respect to Cl of Model 1. When α is 20◦, the increment of Cl becomes the largest value of 34.19%.Based on the simulation results, the lift coefficient of Model 1 shows the rising trend before the angle ofattack increases to 10◦, but it begins to decrease when the angle of attack further increases. However, forModel 2, the lift coefficient maintains the rising trend before angle of attack increases to 15◦, but it beginsto decrease when the angle of attack further increases.

For the airfoil structure given above, when the attack angle is increased, the airfoil surface happens to pro-duce a separation area. The generation of the separation area is one of the reasons for the drag force increasewhen the airfoil works under a large attack angle, which causes a reduction on the working efficiency of theairfoil. To make the airfoil work more effective in this study, the dimple surface was designed as depicted inFig. 6.

Figure 7 shows the separation regions of Models 1 and 2 with respect to angle of attack. In case when thedrag and lift coefficients of Model 2 are less than those of Model 1, i.e., when the angle of attack is less than10◦, no visible difference is observed in the separation region. However, as the angle of attack increases, theseparation area gradually increases, which is observed in both Model 1 and Model 2.

With the identical angle of attack, the separation area of Model 2 is remarkably reduced compared tothat of Model 1. In addition, the spot where the wake is generated for Model 2 tends to be pushed fartherbackward compared to that of Model 1. Due to this separated flow, the drag and lift coefficients tend to bedecreased and increased, respectively. As the separation area becomes smaller, the smaller drag coefficientand the larger lift coefficient can be induced.

4. CONCLUSIONS

In this paper, we studied the variations of the drag and lift coefficients for a patterned airfoil structure. Themodified NACA 0018 airfoil model was used for both Model 1 (plain surface model) and Model 2 (texturedsurface model). The dimple was textured on the upper surface of the airfoil and the total number of dimplewas set to be 25. The computer simulation was conducted using a CFD program, FluentTM.

1. As for the drag coefficient, the higher value for Model 2 could be obtained than Model 1 when theangle of attack is small. However, as the angle of attack increases, Model 2 shows smaller dragcoefficient values than Model 1. Furthermore, when the angle of attack reaches 20◦ the decrement ofdrag coefficient of Model 2 becomes maximum (20.5%).


Fig. 7. Comparisons of separation regions of Models 1 and 2 with respect to the angle of attack.


2. As for the lift coefficient, the trend appears similar to that of the drag coefficient. When the angle ofattack is small, Model 1 shows higher value than Model 2.However, as the angle of attack increases,Model 2 shows higher lift coefficient values than Model 1. Also, similar to the drag coefficient case,when the angle of attack reaches 20◦, the increment of lift coefficient of Model 2 becomes maximum(34.19%).

ACKNOWLEDGEMENT

This work was supported by the Industrial Strategic Technology Development Program (No. 10039982,Development of next generation multi-functional machining systems for eco/bio components), funded bythe Ministry of Trade, Industry and Energy (MI, Korea).

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aerodynamic effect of 3d pattern on airfoil effect of 3d pattern on airfoil xiao yu wang, sooyoung...

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