Numerical Study for Optimum The stall Angle by Ansys Program

Numerical Study for Optimum The stall Angle

by Ansys Program

Qasim Nazim Zakat1 , Zainul-aabideen Abdul-hakeem Ali2 , Nawfel Muhammed Baqer Muhsin3 ,

Hyder H. abd Balla4

1,2B. Sc ,Technical College of Najaf, Al-Furat Al-Awsat Technical University, Iraq. *3Technical College of Najaf, Al-Furat Al-Awsat Technical University, Iraq,([email protected]).

4Technical Institute of Najaf ,Al-Furat Al-Awast Technical University, Iraq.

ABSTRACT

This project aims to optimum the stall angle (which is the maximum angle possible that after it

the wing/airfoil would experience stall) by making improvements or adding equipment to the

structure of the wing, in this research we used an airfoil with section model NACA 0012 because

this airfoil is symmetric in shape and there are other researches we used to reveal this project.

There are several improvements to the wings, but not all of them are used to increase the stall

angle. There are improvements or additions are used to increase the coefficient of lift and reducing

the coefficient of drag like the flap but its important disadvantage that it is reducing the stall angle

(important to note that the flap has many types and from them there are ones that increase the stall

angle). In this project, we will study the effect of these three improvements: Backward facing step

in the upper airfoil side (KFm – 2) .,Backward facing step in the lower airfoil side (KFm – 1) .,

Suction system with various sucking .All these improvements have their way to increase the stall

angle either by making vortex on the airfoil surface or by converting the turbulent airflow that

happens when the stall is happening. All this will be discussed in later chapters of this project.

Keywords: Airfoil, stall, vortex

I. INTRODUCTION

The airfoil shape was originally designed to

produce lift, regardless of the separation,

because the effort was made to make the

airplane fly up. What encountered them was

the stall, whenever the airflow around an

airfoil separates, stall is said to have

started[1-4]. There are two types of stall –

figure (1-1), a and b – occurs over an airfoil:

gradual and abrupt.The first type of stall is

characterized by gradual stall followed by a

shallow drop-off of the lift coefficient, this

type frequently occurs on airfoils frequently

occurs on airfoils with moderate or thick

sections.The second type of stall is

characterized by an abrupt drop-off of the

section lift coefficient. It is often

associated[5-7] with thin airfoil sections.If

we want to understand the stall, we have to

understand the separation, but first we have

to know what the boundary layer is.The

boundary layer is a layer of fluid close to a

rigid body due to the Coandă effect, which

makes the fluid attached to the rigid body

when flowing over it.The separation occurs

when the angle of attack increases[8-11],

which makes the stagnation point moving to

the lower side of the airfoil that make the

maximum thickness is the front of the airfoil

and the slope is so sharp that the Coandă

effect will not occur, and this makes the

separation point go upward until it reaches

Journal of Xi'an University of Architecture & Technology

Volume XII, Issue III, 2020

Issn No : 1006-7930

Page No: 2647

mailto:[email protected]

https://www.researchgate.net/institution/Al-Furat_Al-Awsat_Technical_University/department/Technical_Institute_of_Najaf

the frontal curve of the airfoil[12-18].

Boundary Layer Control For The

Prevention Of Separation

Various methods of flow-separation control

are used on aircraft and in other engineering

applications[19,20]. These methods be either

active or passive and we mean by that the

following.

The active method is require the expenditure

of additional power from the propulsion

units.

Some active techniques:-

1- Boundary-layer suction, figure(1-2)

2- tangential blower, figure(1-3)

The passive method is not requiring any

additional power; however, they usually lead

to increased drag at cruise when they are not

required.

Some passive techniques: -

1- Vortex generators

2- Wing fence

3- Saw-tooth leading edge

4- Leading-edge strake

5- Forward facing step (KFm family)

The Purpose From This Project

The purpose from this project is to study the

methods that make the separation latent and

the stall angle is increased, and to make a

good reference to the students ahead for

doing an advanced project for every single

method, other purpose is to show which

method is the best and more efficient in

doing this job of increasing the stall angle by

simulating each method in the same air and

flight conditions, and by using the same

airfoil NACA 0012. This project also

provide a cl-alpha for every method in every

positive angle of attack until after the stall

angle to make sure of the angle we presume,

that it is the stall angle indeed.

Figure (1-1), a

Figure (1-1), b

Figure (1-2), Separation



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Figure (1-3), Suction Method

Figure(1-4),a blown trailing-edge flap

Figure (1-5), passive techniques


(Backward facing step - upper side)


(Backward facing step - lower side)

II. PREVIOUS RESEARCHES

Preface

Taking into perspective that the methods

that we will discuss is relatively new.

Therefore, the researchers are not so many.

At this chapter, we will show a brief of these

researches, what is there importance, what is

there connection to our project and what is

the goal from this project.



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Researches Concerning Backward Facing

Step

The idea of making a backward facing step

on the surface of the airfoil was originally

made by Kline and Fogleman in 1972 which

they get from it a patent to their own glider

which they improve in it the lift force and

latent the separation phenomenon.In the year

1974, Top-flight laps followed by santich did

multiple of experiments on wings similar to

figure (2-1). At these experiments the

models was containing step not only in the

lower side like Kline and Fogleman's model

(KFm for short) but in the upper side as well.

The laminar flow in these types of models is

attaching closely to the outer radius of the

vortex that extend and shrink depending on

the pressure at the time.In the middle of the

1990's, NASA showed a concern again to the

(KFm), when Fertis presents an experimental

study on (New Airfoil Design Concept with

Improved Aerodynamics Characteristic).

The study gives a new design to the airfoil

section NACA 23012 and by the new design,

he proves that the airfoil with the backward

facing step has improved lift coefficient and

stall angle than the original NACA 23012.

The researcher did a flight tests to a

prototype with (2.134m) in length and

having a wing with the improved airfoil in a

speed of (12.2 – 79.25 m/s) and this was for

knowing the actual aerodynamic

performance and found the that it is

improved extremely with increasing the

flight speed.In the year 2006, the researcher

(Laith Ayad Salman) provides his

experimental and theoretical study (The flow

behavior and its effect on the aerodynamic

properties of the airfoil sections), in the

experimental part, he used four models of

steps, two in the upper side and two in the

lower side. He did the experiment on a wind

tunnel has a smoke blower to see the

vortices. The results came promising; the

four models gave a noticeable increase in the

lift coefficient and in the stall angle than the

original airfoil NACA 0015. In the

theoretical part, he used (Ansys 5.4), to

know the behavior of the flow on the surface

of the backward facing step and its effect on

the aerodynamic performance for the same

airfoil section, the results came with a

positive effect on the performance by

improved flow profile and a good increase in

the stall angle than the original stall angle

which is 12 deg.

Researches Concerning Boundary Layer

Suction

The earliest work on the boundary layer

suction was carried out in Germany and by

Ludwig prandtl and his papers on the

boundary layer theory during the late 1930s

by using wind tunnels. Research on suction-

type LFC continued up to the 1960s in Great

Britain and the United States, but owing to

the practical difficulties and to the relatively

low price of aviation fuel, boundary layer

suction researches was discontinued in the

late 1960's. More recently, with the

advanced technologies, the researchers are

back to this concept.In the year 2010, H.J.B.

van de Wal presented a thesis with the name

(Design of a Wing with Boundary Layer

Suction). In this thesis, he presented a new

design of an airfoil BW 10–144 to be his

wing cross section with a flap and aileron

22% of the cord and the suction holes

beginning in 75% of the cord. Thesis was

theoretical and he used (XFOIL) program.

He noticed a good improvement with

aerodynamic properties and a raise in the

stall angle and the maximum lift coefficient.

This Project And Its Relation To The

Previous Researches

After taking the knowledge from reading

previous researches, it is obvious that all the

researches concentrated on one method of

improving the aerodynamic performance and

increasing the stall. While in this research,

our goal is to study different methods and

more important is to compare between them

to see which one of them is the more

efficient by simulating the original airfoil

section NACA 0012 in (ANSYS 16)

program and exert the values of the

coefficient of lift and drag. After that, we

took four models of modifications to the



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original airfoil and they are:-

1- Airfoil with backward facing step of

(Lu=0.5C, Du=0.5T) in the upper

side.

2- Airfoil with backward facing step of

(Lu=0.5C, Du=0.5T) in the lower

side

3- Airfoil with suction system (capacity

of suction is 0.03U∞).

4- Airfoil with suction system (capacity

of suction is 0.1U∞).

We did a simulation on these models to

make a reliable chart of cl – α; therefore, the

simulation was to the models from there αcl=0

until after the stall angle with two degrees to

clarify the stall angle of each modified

model. In this project, we succeeded to

make a limit on the suction capacity needed

to make the airfoil overcome the regular stall

angle.

Figure(2-1),Vortex on backward facing

step airfoil

Figure(2-2), suction on H.J.B. van

de Wal's model

AIRFOIL’S NEW METHODS

Preface

In this chapter we will talk about the new

methods we studied and there effect on the

stall angle and take into consideration the

environmental conditions would be the same

for all of them, for us to compare between

them.

Upper Backward Facing Step (KFm- 2)

This modification is considerably improved

by air drones amateurs who made a whole

family from it. This modification is done by

taking a cut from the upper side of the airfoil

with known length and thickness and the

limitation of it is tacking into consideration.

From [1] we found that the best cut which

gives a relatively high stall angle is Lu= 0.5

C and Du=0.5 T. The way of this method

making improvements on the airfoil is by

making a circulation beneath the step,

making the stream to deflect back from its

separating state. Another way of

understanding this is by knowing that the

circulation pressure is less than the stream

pressure (like making a vacuum pressure)

which make the separating stream pulled

down again making it laminar. From figures

3-1 and 3-2 we will distinguish the

difference between the flow with and

without the backward facing step, notice that

the angle in the two conditions is 12 degrees,

which is the original stall angle.



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Figure 3-1 , the original 0012

Figure 3-2, 0012 with UBFS

The disadvantage that this modification

brings is the space. We know that most the

civil airplanes has their fuel thanks in the

wings, and the new shape of the wing will

make it difficult to design fuel tanks taking

into consideration the movement of the fuel

in the tank and even making tanks enlarge

the same amount of the original tanks.

Lower Backward Facing Step(KFm -1)

This modification is similar in shape with

the upper backward facing step, but has

difference with how it makes the stall angle

increase. From [1] we found that the best cut

which gives a relatively high stall angle is

Lu= 0.5 C and Du=0.5 T just like the upper

backward facing step. This method increase

the stall angle by generating a pressure

difference in the lower side of the airfoil

which make the lower side even less

pressure than the upper side generating more

lift hence letting the airfoil stand for a longer

time and a greater stall angle.

Figure 3-3, circulation in the

lower side of 0012 airfoil.

The disadvantage of this method an

addition to the space of the fuel tanks is that

the circulations developed in the lower side

tends to make the airfoil to rotate on its

center of gravity in the anticlockwise

direction, which is the opposite direction of

the moment generated by the air free stream

and its coefficient of moment Cm ,and this

will make a noticeable vibration in the wing

would break the structure apart. And also the

disadvantage of the both previous methods is

that they are passive techniques.

Airfoil With Suction System

This modification is the hardest to study

because the lack of a previous studies and

information about it. This method is

consisting of making small holes in outer

structure of the airfoil (wing), and these



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holes is connected to tubes leading to a

system generates a vacuum pressure sucking

from the air stream. This method helps

remaking the turbulent flow be a laminar

flow, by proper suction method, the air flow

will reattach with the surface again

preventing the separation from happening.

In this method, we studied two suction

velocities to show the difference if the

suction velocity changed, and they are, 3

percent of the air stream velocity and 10

percent of the air stream velocity. The results

were tremendously different in the stall

angle and the lift produced.

The advantage of this method over the

previous ones is that this method is an active

technique.

Figure 3-4, 3% suction.

Figure 3-5, 10% suction.

However, there are disadvantages for this

method, which is It will take a noticeable

space from the wing interior and it will add

to the weight of the airplane furthermore, the

minimum suction velocity needed for this

method to work is very high. The last

disadvantage limit the use of this method to

the low speed airplanes (like the private

airplanes).

III. ANALYSIS BY USING

ANSYS WORCKBENCH

Ansys develops and markets finite element

analysis software used to simulate

engineering problems. The software creates

simulated computer models of structures,

electronics, or machine components to

simulate strength, toughness, elasticity,

temperature distribution, electromagnetism,

fluid flow, and other attributes. Ansys is

used to determine how a product will

function with different specifications,

without building test products or conducting

crash tests. For example, Ansys software

may simulate how a bridge will hold up after

years of traffic. In this chapter, we will walk

through the procedure of the analysis and the

specifications using ansys workbench (fluid

flow [fluent]).

Specifications

Here we will talk about the specifications

we took and why. First, we have to say that

all the analysis was considering

aerodynamics and not the structure.

Therefore, there is no mentioning about the

type of the material or the increase/decrease

in deflection occurs during every method

used. The analysis is two-dimensional

because we considered the modifications that

can be analyzed in 2D and not be affected if

we neglect the depth of it.

To make sure that the results we give is

eligible; we build up our base case according

to the data we take from [2] for the

coefficient of lift to NACA 0012.



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We used a method called C-mesh method

(Figure 4-1)

figure 4-1, c-mesh method

This method is common in aerospace

analysis because it makes a good approach to

the real wind tunnel test and by making the

walls far enough from the airfoil. The

dimensions of the geometry is as following,

the radius of the curve has to be 12.5 of the

cord and the distance from the trailing edge

to the outlet has to be 12.5 of cord. The

advantage of this method is to make the

mesh in a more organized way, which make

the iteration in the coming steps faster with

the same number of elements.The number of

elements in the analysis is 400000 elements

with minimum size of 0.0017 m, and with

the use of refinement and sizing tools to

make the elements smaller at the walls of the

airfoil. Now we'll talk about the

environmental specifications and we made it

non-dimensional as much as we can. The

pressure and temperature used is the ambient

that is 101.3 KPa and 15 C respectively, the

air mach number is 0.258, the Reynolds

number is 6e+6

From figure 4-2 we can see that the inlet

include all the walls except for one which is

the outlet

figure 4-2, inlet, outlet and airfoil

The reason why we make this is to make it

easier to change the angle of attack by

changing the flow angle instead of changing

the angle of the airfoil itself every time,

changing the airfoil every time make us

change the geometry and thus generating the

mesh every time and that's a time consuming

procedure. Instead, we can change the flow

angle hence the angle of attack by changing

the velocity specification method from

normal to components and insert the x and y

components. (See figure 4-3 a and b)

figure 4-3 a, zero angle of attack



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figure 4-3 b, 2 degrees angle of attack

The solver type we chose is pressure-based

type because we have subsonic compressible

flow, we know that from the mach number

and we are using air as a material [3]. It’s a

steady flow and the velocity formulation to

be absolute.

From [4] and [5] we'll know that the

best flow model for our case is SST k- 𝜔,

and it's governed by the equations:-

(4.1)

(4.2)

Note that the procedure for all the methods is

almost the same.

Finally, we ran the calculation for the

iteration, which took between 2300 to 3000

iterations and a time of 47 to 61 minutes per

convergence.

figure 4-4, design modular window

figure 4-5, mesh window



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figure 4-6, setup window

figure 4-7, solution window

RESULTS and DISCUSION

Preface

In this chapter, we'll show the charts of the

coefficient of lift to make the stall angle

clear for each method and then compare

them to make a conclusion of which method

is best and why.

First, we have to make clear how to get

the lift coefficient from fluent. In the

solution window we'll go to the reports and

then select the forces and select the right

direction vector as illustrated in the figure 5-

1, from this figure we will know that the

direction of the lift is always perpendicular

to the flow direction, we have to keep that in

mind because we are altering the flow NOT

the airfoil.

figure 5-1, lift and drag direction

figure 5-2, showing lift coefficient

5-2 Results

The base case we got very accurate results,

comparing it to the data we take from [2].

We saw that the error was not exceeding



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0.1% and figure 5-3 illustrate that.

figure5-3,theoretical and experimental

results

Upper Backward Facing Step

The KFm-2 shows good improvements in lift

and angle wise. It showed an increase in the

stall angle by the third, rising up from 12

degrees to 18 degrees and a maximum lift

coefficient increase by 45%, rising up from

1.03 to 1.489.

figure5-4,experimental and kfm-2

Lower Backward Facing Step

The KFm-1 has good improvements too

but not like KFm-2. It shows an increase in

the stalling angle by 17%, rising up from 12

degrees to 14 degrees and a maximum lift

coefficient increase by 40%, rising up from

1.03 to 1.447.

figure5-5,experimental and kfm-1

Figure5-6,KFm-2 and KFm-1

Suction

3% suction had its share from

improvements with 17% increase of stall

angle, rising up from 12 to 14 degrees and a

maximum lift coefficient increase by 24%,

rising up from 1.03 to 1.28.

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30

CO

FF. O

F LI

FT

ANGLE

tyeoretical experimental

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30

CO

FF. O

F LI

FT

ANGLE

experimental upper

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30

CO

FF. O

F LI

FT

ANGLE

experimental lowar

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30

CO

FF. O

F LE

FT

ANGLE

upper lowar



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figure5-7,experimental and 3% suction

However, the 10% suction showed better

characteristics than the 3% suction with

improvements in both the lift coefficient and

the stall angle. It showed an increase by the

third in terms of stall angle, rising up from

12 to 16 degrees. The 10% suction have a

very good rise in terms of the maximum lift,

which is by 61% from 1.03 to 1.661.

figure5-8,experimental and 10%suction

figure5-9, 3% suction and 10% suction

Comparison and Conclusion

From 5-2 we will see that if we consider

just the stall angle, then the best method is

the upper backward facing step, which has a

stall angle of 18 degrees. However, if we

consider the maximum lift produced, then

we have to consider using the 10% suction

which has a maximum lift of 1.661. It

actually depends on what we care the most.

figure5-10, comparison

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30

CO

FF. O

F LE

FT

ANGLE

suction 3 % experimental

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30

CO

FF. O

F LI

FT

ANGLE

suction 10 % experimental

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30

CO

FF. O

F LI

FT

ANGLE

suction 3 % suction 10 %

-0.5

0

0.5

1

1.5

2

0 10 20 30

CO

FF. O

F LI

FT

ANGLE

suction 3 % suction 10 %

tyeoretical experimental

upper lowar



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In our opinion, we prefer the upper

backward facing step for the reasons below:-

1. The method is passive which means it

doesn't add further installments into the

airplane.

2. The method has a fair maximum lift of

1.489.

3. The method has the best stall angle

among the other methods mentioned

earlier which is 18 degrees.

The disadvantages this modification brings is

the decreasing of the space of the tanks and

this can be overcome by adding extra fuel

tank in the fuselage with pump system

connecting with it. The other disadvantage is

the decrease in the strength of the structure

by cutting a fair amount of it, and this also

can overcome by using tougher materials or

increasing the rips and the stringers.

Conclusions

We don't have recommendations

considering the methods. The only

recommendation we want to give is to

encourage one of the groups that will come

after us to make a project that is the same as

ours but experimentally by using the wind

tunnel we have in the aerodynamics lab and

compare the results they will get, wishing for

them all the best.

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Numerical Study for Optimum The stall Angle by Ansys Program

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