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Procedia Engineering00 (2014) 000–000

www.elsevier.com/locate/procedia

* Corresponding author. Tel.: +8801819070344.

E-mail address:shafnehal@gmail.com

1877-7058 © 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the Department of Mechanical Engineering, Bangladesh University of Engineering and

Technology (BUET).

10th International Conference on Mechanical Engineering, ICME 2013

Numerical analysis of role of bumpy surface to control the flow separation of an airfoil

Syed Hasib Akhter Faruqui a,*, Md. Abdullah Al Bari a, Md Emran a & Ahsan Ferdaus a

a Department of Mechanical Engineering, Khulna University of Engineering & Technology, Bangladesh

Abstract

The efficiency of mission varying aircrafts can be altered by either flow control method or adaptive wing technology. Here we

have researched the control of the flow separation of airfoil by providing partial bumpy on the upper surface of the geometry. Flow

of low speed is considered. For the analysis purpose NACA 4315 was chosen. Two models were generated (i) regular airfoil using

NACA 4315 profile (ii) providing bumpy surface on the NACA 4315 surface on the trailing edge at 80%C. Numerical approach is

undertaken to observe the flow separation on the aerofoil. From the observation it was noted that by using the bumpy surface on

the upper surface of the aerofoil the flow separation was delayed. Flow separation occurs at 9 degree angle of attack in the smooth

surface whereas in bumpy surface it occurs at 15 degree angle of attack. Thus, indicating the increase of lift force and control of

flow separation of an airfoil due to bumpy surface.

© 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the Department of Mechanical Engineering, Bangladesh University of

Engineering and Technology (BUET).

Keywords: CFD; Flow separation control; bumpy surface; airfoil; aerodynamics ;

1. Introduction

Due to viscosity when a real fluid passes over a solid boundary a layer of fluid adjacent to the boundary

adheres to it. At the boundary surface there is literally no relative velocity between the boundary surface and the

adjacent fluid layer. Pressure gradient over the surface in different regions of the surface due to flow phenomena is

seen. Pressure gradient affects the boundary thickness. If the pressure gradient is zero than the boundary layer

continues to grow in thickness. Momentum in the boundary layer decreases with the adverse pressure gradient and the

boundary shear stress decrease. After a certain length the flow starts to separate from the surface. Controlling fluid

flow separation is of importance in case of aerodynamics. Fluid flow separation can be controlled by various ways

2 Syed Hasib Akhter Faruqui et al / Procedia Engineering 00 (2014) 000–000 ;

such as co-flow jet system, solid wall, providing bumpy surface/ surface roughness etc. The proposed method of flow

control here is the implication of bumpy surface [1]. The selected airfoil profile is NACA 4315, a relative thick airfoil.

2. Mathematical Formulation

To describe the transport of a conserved quantity or Properties like mass, energy, momentum are conserved

under their respective appropriate conditions Continuity equation is used. Continuity Equation-

[(𝛿𝑢/𝛿𝑡) + (𝛿𝑢/ 𝛿𝑥) + (𝛿𝑢/𝛿𝑦) = 0

To describe the motion of fluid substances Navier Stroke equation is used. These equations arise from applying

Newton's second law to motion. The Navier–Stokes equations in their full and simplified forms help with the design

of aircraft and cars. The Navier- Stroke equation-

𝜌 (𝛿𝑣/𝛿𝑡 + 𝑣. 𝛻𝑣) = − 𝛻𝑝 + 𝜇 𝛻2𝑣 + 𝑓

Writing the equations externally:

X-momentum: 𝜌 (𝛿𝑢

𝛿𝑡+ 𝑢

𝛿𝑢

𝛿𝑡+ 𝑣

𝛿𝑢

𝛿𝑡+ 𝑤

𝛿𝑢

𝛿𝑡) = −

𝛿𝑝

𝛿𝑥 + 𝜇 (

𝛿2𝑢

𝛿𝑥2 +𝛿2𝑢

𝛿𝑦2 +𝛿2𝑢

𝛿𝑧2 ) + 𝜌𝑔𝑥

Y-momentum: 𝜌 (𝛿𝑢

𝛿𝑡+ 𝑢

𝛿𝑢

𝛿𝑡+ 𝑣

𝛿𝑢

𝛿𝑡+ 𝑤

𝛿𝑢

𝛿𝑡) = −

𝛿𝑝

𝛿𝑥 + 𝜇 (

𝛿2𝑢

𝛿𝑥2 +𝛿2𝑢

𝛿𝑦2 +𝛿2𝑢

𝛿𝑧2 ) + 𝜌𝑔𝑥

The Navier-Stroke equation assumes that the fluid being studied is a continuum (not composed of particles).

2.1. Two equation turbulence models

Two equation turbulence models are one of the most common type of turbulence models. By definition, two

equation models include two extra transport equations to represent the turbulent properties of the flow. One of the

transported variables is the turbulent kinetic energy, K & the second transported variable is the turbulence dissipation

ε. The second variable can be thought of as the variable that determines the scale of turbulence (length scale or time

scale), whereas the first variable K, determines the energy of the turbulence. This model is chosen as it has shown to

be useful for free share layer flows with relatively small pressure gradients. The kinetic energy k and rate of dissipation

ε are obtained from the turbulence kinetic energy equation written as

(𝛿𝑘

𝛿𝑡+ 𝛻(𝜌𝑢𝑘)) = 𝛻 ([𝜇𝑙𝑎𝑚 +

𝜌𝑣𝑡

𝜎𝑘] 𝑔𝑟𝑎𝑑 𝑘) + 𝜌𝑣𝑡𝐺 − 𝜌ε

And the dissipation equation

(𝛿ε

𝛿𝑡+ 𝛻(𝜌𝑢𝑘)) = 𝛻 ([𝜇𝑙𝑎𝑚 +

𝜌𝑣𝑡

𝜎𝑘] 𝑔𝑟𝑎𝑑 ε) + 𝐶1ε𝜌𝑣𝑡𝐺

ε

k− 𝐶2ε𝜌

ε2

k

Where, 𝜇𝑡 = 𝐶2ε𝜌ε2

k

Production of K, 𝑃𝑘 = 𝜇𝑡𝑆2

𝑃𝑘 = −𝜌𝑢′𝑖𝑢′𝑗𝛿𝑢𝑗

𝛿𝑥𝑖

Where is the modulus of the mean rate-of-strain tensor, defined as,

𝑆 ≡ √2𝑆𝑖𝑗𝑆𝑖𝑗

Syed Hasib Akhter Faruqui et al / Procedia Engineering 00 (2014) 000–000 3

Model Constants attained empherically

𝐶1ε = 1.44 𝐶2ε = 1.92 𝜎𝑘 = 1.0 𝜎ε = 1.3 𝐶𝜇 = .009

For the walls assumed as symmetry having no surface roughness and mass transfer we get

u+ = = f1(y+)

3. Grid Generation

All of the computations were performed using a C-grid as shown in Fig. 3. The top and bottom far field

boundaries are six chord lengths from the airfoil; the upstream and downstream boundaries are five and seven chord

lengths away, respectively. Maximum height of the bumpy surface is 6.35 unit i.e. 2.5% of total chord length. [3]

4. Model:

Figure-1: Regular surface model Figure-2: Bumpy Surface sample model

Figure-3: Grid Used for computation

There were two model considerations (a) regular model surface & (b) partial bumpy surface. Maximum

height of the bumpy surface is 6.35 unit i.e. 2.5% of total chord length.

4 Syed Hasib Akhter Faruqui et al / Procedia Engineering 00 (2014) 000–000 ;

5. Results and Discussion

The Numerical results of surface pressure distributions are shown in figures 4 to 10 for regular and bumpy

surface model for different angle of attack (AOA). As shown in graph no flow separation occurs for both model

(regular and bumpy) at zero degree AOA. As the AOA is increased from 0° to 12°, flow separation starts to occur at

70% to 75% of the chord length from the leading edge. Due to flow separation, the value of the pressure coefficient

becomes almost zero. As the AOA is increased from 12° to 14° flow separation on the upper surface is clearly visible.

In case of regular surface the flow separation occurs at 9 degree which is clear from figure 6 and 7. In case of bumpy

surface it' It is shown that the bumpy has no effect at 20° AOA

Figure-4: Co-efficient of pressure vs. distance at 0° AOA

Figure-5: Co-efficient of pressure vs. distance at 4° AOA

Figure-6: Co-efficient of pressure vs. distance at 8° AOA

Figure-7: Co-efficient of pressure vs. distance at 12° AOA

Figure-8: Co-efficient of pressure vs. distance at 14° AOA Figure-9: Co-efficient of pressure vs. distance at 16° AOA

1, 0

-1.5

-1

-0.5

0

0.5

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssu

re C

o-e

ffic

ien

t

x/cPressure Co-efficient of Upper Surface

Upper Surface of the regular airfoil

Upper Surface of the bumpy airfoil

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssure

Co

-eff

icie

nt

x/cPressure Co-efficient of Upper Surface

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssu

re C

o-e

ffic

ien

t

x/cPressure Co-efficient of Upper Surface

-2

-1

0

1

2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssure

Co

-eff

icie

nt

x/c

Pressure Co-efficient of Upper Surface

-2

-1

0

1

2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssu

re C

o-e

ffic

ien

t

x/c

Pressure Co-efficient of Upper Surface

-3

-2

-1

0

1

2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssu

re C

o-e

ffic

ien

t

x/c

Pressure Co-efficient of Upper Surface

Syed Hasib Akhter Faruqui et al / Procedia Engineering 00 (2014) 000–000 5

Figure-10: Co-efficient of pressure vs. distance at 20° AOA

(a) (b)

(c) (d)

Figure-11: Pressure Distribution on the bumpy surface for (a) AOA=12 (b) AOA=14 (c) AOA=16 (d) AOA=20 degree.

-4

-3

-2

-1

0

1

2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pre

ssu

re C

o-e

ffic

ien

t

x/c

Pressure Co-efficient of Upper Surface

6 Syed Hasib Akhter Faruqui et al / Procedia Engineering 00 (2014) 000–000 ;

6. Conclusion

From the numerical Investigation of the following airfoil we come to the decision that a higher angel of attack

can be attained by using the bumpy surface over the upper surface of the body at 80% camber for NACA 4315 profile

and thus higher proficiency is obtained.

References

[1] Md Abdullah Al Bari et al Role of partially bumpy surface to control the flow separation of an airfoil. ARPN journal of Engineering

and Applied Science. Vol 7 No-5

[2] Mueller T. and De Laurier J. 2003. Aerodynamics of Small Vehicles. Annual Rev. of Fluid Mech. 35: 89-111.

[3] Stanewsky E. 2001. Adaptive Wing and Flow Control Technology. Progress in Aerospace Sciences. 37: 583-667.

[4] Lissaman P. 1983. Low-Reynolds-Number Airfoils. Annual Rev. of Fluid Mech. 115: 223-239.

[5] Carmichael B. 1981. Low Reynolds Number Airfoil Survey. NASA CR–165803.

[6] Jacob J. 1998. On the Fluid Dynamics of Adaptive Airfoils. Proc. ASME International Mechanical.

[7] Engineering an Exposition, ASME, Anaheim, CA. Gad-El-Hak M. 2001. Flow Control: The Future. Journal of Aircraft. 38: 402-418.