Computational Study on Eppler61Airfoil

8/4/2019 Computational Study on Eppler61Airfoil

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Computational Study on

Eppler 61 Airfoil



Introduction:

The objective of the project is to analyze Eppler 61 airfoil, with use

of CFD.

Two commercial codes, XFLR5 & Fluent, have been used for the

analysis. The data obtained was compared to the experimental data obtained

by Mueller et. al..

To validate that Eppler 61 airfoil has a good performance at low

Reynolds no., and thus can be used in future designs of micro-air

vehicles.



Roadmap

Low Reynolds no. aerodynamics

Low Reynolds no. airfoils

Eppler 61 airfoil

Grid generation and meshing

Result and Discussion

Conclusion



Low Reynolds No. Aerodynamics

A flow with Reynolds no. less than 2,00,000 are termed as low

Reynolds no. flow regime.

The aerodynamics of low Reynolds no. flow regime is

fundamentally different than high Reynolds no. flow regime. In this flow regime airfoil performance is highly affected by laminar

separation, transition and turbulent reattachment.



Reynolds No. Bands

According to Carmichael’s Report there are 12 Reynolds no. bandsdescribing all kinds of flow regimes. Below are the bands of interest

Very Low Reynolds No. : This is fractional Reynolds no. whereflow is completely viscous. The practical considerations are fallingrates of smoke, dust, fog, pollen particles. This is outside ourinterest

Reynolds No. below 150 : This regime is of interest for design of low turbulence reducing screens & smoke streak producing wires

in low turbulence wind tunnel. The flow is laminar & unseperated Reynolds No. between 1000 - 10,000 : Flow is strongly laminar

and it’s difficult to produce turbulent boundary layer. It’s a flowregime for most insects. A curved surface plate is found superior toflat plates or airfoils.



Reynolds No. between 10,000 – 30,000 : This regime is for hand

launched glider models. It seems that we have 100% laminar flow

for some time. The other side of coin is that that it operates at low

lift coefficients around 0.5 or less. Trimming of model to higher liftcoefficients can produce separated laminar layer without

reattachment.

Reynolds No. between 30,000 – 70,000 : This regime is technically

for all model aircraft builders and flyers. Induced drag

considerations call for great care in the choice of airfoil section. 6%thick airfoil can become supercritical. Reattachment of seperated

layer occurs



Reynolds No. between 70,000 – 2,00,000 : This regime is for most

small radio controlled model sailplanes & model power planes.

Extensive laminar flow is easy to obtain. No need of tripping. The

laminar separation bubble is the primary performance robber. Reynolds No. between 2,00,000 – 7,00,000 : This regime is for all

birds, large radio controlled models, RPV, ultra light man carrying

gliders.

Higher Reynolds No. bands : This regime is for all the aviation and

aerospace applications.



Flow Characteristics:

Laminar Separation

Transition

Turbulent Reattachment

Separation Bubble

Hysterisis Loop



Laminar Separation Bubble

Due to the low energy of the laminar flow, it fails to overcome theadverse pressure gradient and separates.

Separated flow mixes with the high energy free stream flow andtransits from laminar to turbulent

Turbulent flow has higher energy as compared to laminar flow andhence the flow reattaches forming the separation bubble.

The phenomena is also referred to as Transitional separation bubble

Separation bubble often has dramatic effect on stall characteristics of an airfoil

The behavior of separation bubble is also a factor in the occurrence

of Hysterisis for some airfoils.





Hysterisis Loop

When we observe the flow over an airfoil with gradual increase inangle of attack up to stalling angle and then again decrease withoutstopping the flow, the coefficients of lift and drag do not follow thesame path and generates a loop called Hysterisis loop.

Hysterisis loop is caused by the separation bubble.

Study of hysterisis loop is important in determining stallcharacteristics of the airfoil.



Low Reynolds No. Airfoil

Came into existence only after the performance of thick airfoils gotdeteriorated at low Reynolds no.

Camber and thickness of the airfoil affects the aerodynamicefficiency of the airfoil.

For low Reynolds no., airfoil is desired to have low thickness, highcamber and small leading edge radius which is in contrast with thehigh Reynolds no. airfoils.



Effect of Camber:



Effect of Camber:



Effect of Thickness:

For low Reynolds no. flow, airfoils having small thickness give high

lift to drag ratio.

Slope of the Cl vs α curve increases.

Small thickness helps in reattachment of the separated flow.



Eppler 61 airfoil: Geometry of the profile:

• Thickness = 5.64%

• Max thickness position = 23.80%

• Camber = 6.69%

• Max camber position = 51%

Characteristics of the profile:

• Thin and highly cambered.

• Delays laminar separation.

• Increases stalling angle.

• Produces lift at zero angle of attack.

• Highly concave underside of the airfoil is the major contribution of lift.





Computational Fluid Dynamics

It is a combined approach of theoretical and practical approach for

study of fluid dynamics.

It’s results are analogous to wind tunnel experiments, except the

algorithms have an error. Here two softwares are used – Fluent and XFLR5



Grid Generation

For simulation in Fluent one needs to generate geometry & grid

using Gambit.

The geometry is created using importing coordinates.

After that the grid generation is done. The grid generated iscontrolled by different ratios by available tools.

Then appropriate boundary conditions are given.







Fluent

The grid from gambit is imported.

The fluent has an ability to give very specific initial conditions.

Velocity conditions are given based on the boundary conditions for

Reynolds no. 46,000 & 87,000 and angle of attack. Other conditions such as density, viscosity, ambient pressure,

viscous model, etc. are specified.



XFLR5

In this software only the coordinates are imported.

It is given initial conditions as Reynolds no. and angle of attack.

Tripping points can also be specified.

This is based on vortex lattice method and viscous additioncoefficient.



Test

For Fluent Reynolds No. 46,000 & 87,000 and for angle of attacks -4

to 12 deg.

For XFLR5 Reynolds No. 46,000, 87,000 & 1,60,000, for angle of

attacks -4 to 16 deg and for with & without tripping.



Comparison of Cl data for 46000 Reynolds

Number

-0.5

0

0.5

1

1.5

2

-5 0 5 10 15 20

angle of attack

C l

XFLR5

Numerical Fluent

Muller et. al.

Result and Discussion:



Comparison of Cl data for 87000 Reynolds Number

-0.5

0

0.5

1

1.5

2

-5 0 5 10 15 20

Angle of attack

C l

XFLR5

Numerical Fluent

Muller et. al.



Comparison of Cl with & without

tripping for 46,000 Reynolds No.




tripping for 87,000 Reynolds No.




tripping for 1,60,000 Reynolds No.



Upper surface Cp distribution for different angles of attacks

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20

x

C p

α=-4 deg

α=-3 deg

α=-2 deg

α=0 deg

α=1 deg

α=2 degα=3 deg

α=5 deg

α=6 deg

α=7 deg

α=8 deg

α=9 degα=10 deg

α=11 deg

α=12 deg

α=13 deg

α=14 deg

For 46,000 Reynolds No.



upper surface cp distribution for different angles

of attack

-5

-4

-3

-2

-1

0

1

2

0.00 0.20 0.40 0.60 0.80 1.00 1.20

x

C

p

α=-4 deg

α=-3 deg

α=-2 deg

α=-1 deg

α=-0 deg

α=1 deg

α=2 deg

α=3 deg

α=4 deg

α=5 deg

α=7 deg

α=8 deg

α=9 deg

α=10 deg

α=11 deg

α=12 deg

α=13 deg

α=14 deg

For 87,000 Reynolds No.



Conclusion:

The data obtained computationally reasonably compares with

Mueller et. al. experimental results. At higher angle of attacks the

values are not matching due to limitations of CFD codes used herein.

The results for XFLR5 overshoots lift values at higher a.o.a. is alsoin accordance to Kellog & Bowman results for five different airfoils.

This analysis can be taken further by developing CFD codes specific

to low Reynolds no. flow regime simulation.

Eppler 61 airfoil performance can be increased using the tripping.

It can thus be deduced that it makes a good application for MAV.

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Computational Study on Eppler61Airfoil