Computational Study onEppler 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. bands describing all kinds of flow regimes. Below are the bands of interest

Very Low Reynolds No. : This is fractional Reynolds no. where flow is completely viscous. The practical considerations are falling rates of smoke, dust, fog, pollen particles. This is outside our interest

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 flow regime for most insects. A curved surface plate is found superior to flat 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 lift coefficients 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 the adverse pressure gradient and separates.

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

Turbulent flow has higher energy as compared to laminar flow and hence 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 in angle of attack up to stalling angle and then again decrease without stopping the flow, the coefficients of lift and drag do not follow the same path and generates a loop called Hysterisis loop.

Hysterisis loop is caused by the separation bubble. Study of hysterisis loop is important in determining stall

characteristics of the airfoil.

Low Reynolds No. Airfoil

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

Camber and thickness of the airfoil affects the aerodynamic efficiency of the airfoil.

For low Reynolds no., airfoil is desired to have low thickness, high camber and small leading edge radius which is in contrast with the high 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 is

controlled 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 addition

coefficient.

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

Cl

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

Cl

XFLR5

Numerical Fluent

Muller et. al.

Comparison of Cl with & without tripping for 46,000 Reynolds No.

Comparison of Cl with & without tripping for 87,000 Reynolds No.

Comparison of Cl with & without 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

Cp

α=-4 degα=-3 degα=-2 degα=0 degα=1 degα=2 degα=3 degα=5 degα=6 degα=7 degα=8 degα=9 degα=10 degα=11 degα=12 degα=13 degα=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

Cp

α=-4 degα=-3 degα=-2 degα=-1 degα=-0 degα=1 degα=2 degα=3 degα=4 degα=5 degα=7 degα=8 degα=9 degα=10 degα=11 degα=12 degα=13 degα=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 also in 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.