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-   -   Low Speed Airfoil (http://www.cfd-online.com/Forums/fluent/35284-low-speed-airfoil.html)

Mancusi December 15, 2004 05:01

Low Speed Airfoil
 
I can't get make any sense of the drag,lift and momentum coef's that I get when modelling a low speed airfoil. I want to model the DAE11 airfoil at 10m/s.

I've created a circular mesh with diameter 20 meters in the middle is the airfoil with a chord of 1 meter. I have a very fine boundary layer and a very fine mesh around the airfoil. I set the circular edge to be a pressure farfield (which I can change to a velocity inlet in FLUENT afterwards) and the upper and lower parts of the airfoil as one wall. The airfoil is almost completely laminar so I should get good result running it in laminar solver.

The pressure graph over the airfoil matches in shape and so do the pressure fields near the airfoil when I dsiplay them, just like you would epext them to be over an airfoil only the coef's dont make any sense.

I've run my problem with different turbulence model to and applied all the obvious settings.

Can anybody please help me I'm stuck.

Mark December 15, 2004 08:11

Re: Low Speed Airfoil
 
Hi

The force coefficients are calculated using the reference values (Area, density & velocity) set in the report menu. Have you set these?

Mark

mateus December 15, 2004 10:02

Re: Low Speed Airfoil
 
Hi!

Also it is common that pressure and lift coeficient are in good agreement wit exp. data. On the other hand the drag coef. is usually overpredicted. the reason lies in bad simulation in boundary layer flow. It usually helps a bit to use low-Re model bu the discrepancies will remain.

mateus

Mancusi December 16, 2004 11:04

Re: Low Speed Airfoil
 
I use these settings

FLUENT Version: 2d, dp, coupled imp, S-A (2d, double precision, coupled implicit, Spalart-Allmaras) Release: 6.1.22 Title:

Models ------

Model Settings

--------------------------------------------------------------

Space 2D

Time Steady

Viscous Spalart-Allmaras turbulence model

Production Option Vorticity

Heat Transfer Enabled

Solidification and Melting Disabled

Radiation None

Species Transport Disabled

Coupled Dispersed Phase Disabled

Pollutants Disabled

Soot Disabled

Boundary Conditions -------------------

Zones

name id type

------------------------------------------

fluid 2 fluid

farfield 3 pressure-far-field

airfoil 4 wall

default-interior 6 interior

Boundary Conditions

fluid

Condition Value

---------------------------------------------------------------

Material Name air

Specify source terms? no

Source Terms ()

Specify fixed values? no

Fixed Values ()

Motion Type 0

X-Velocity Of Zone 0

Y-Velocity Of Zone 0

Rotation speed 0

X-Origin of Rotation-Axis 0

Y-Origin of Rotation-Axis 0

Deactivated Thread no

Laminar zone? no

Set Turbulent Viscosity to zero within laminar zone? yes

Porous zone? no

X-Component of Direction-1 Vector 1

Y-Component of Direction-1 Vector 0

Direction-1 Viscous Resistance 0

Direction-2 Viscous Resistance 0

Direction-1 Inertial Resistance 0

Direction-2 Inertial Resistance 0

C0 Coefficient for Power-Law 0

C1 Coefficient for Power-Law 0

Porosity 1

Solid Material Name aluminum

farfield

Condition Value

---------------------------------------------

Gauge Pressure 101325

Mach Number 0.028999999

Temperature 300

X-Component of Flow Direction 1

Y-Component of Flow Direction 0

Turbulence Specification Method 0

Modified Turbulent Viscosity 0.001

Turbulence Intensity 0.1

Turbulence Length Scale 1

Hydraulic Diameter 1

Turbulent Viscosity Ratio 10

airfoil

Condition Value

-------------------------------------------------------------

Wall Thickness 0

Heat Generation Rate 0

Material Name aluminum

Thermal BC Type 1

Temperature 300

Heat Flux 0

Convective Heat Transfer Coefficient 0

Free Stream Temperature 300

Wall Motion 0

Shear Boundary Condition 0

Define wall motion relative to adjacent cell zone? yes

Apply a rotational velocity to this wall? no

Velocity Magnitude 0

X-Component of Wall Translation 1

Y-Component of Wall Translation 0

Define wall velocity components? no

X-Component of Wall Translation 0

Y-Component of Wall Translation 0

External Emissivity 1

External Radiation Temperature 300

Wall Roughness Height 0

Wall Roughness Constant 0.5

Rotation Speed 0

X-Position of Rotation-Axis Origin 0

Y-Position of Rotation-Axis Origin 0

X-component of shear stress 0

Y-component of shear stress 0

Surface tension gradient 0

default-interior

Condition Value

-----------------

Solver Controls ---------------

Equations

Equation Solved

-------------------------------------

Flow yes

Modified Turbulent Viscosity yes

Numerics

Numeric Enabled

---------------------------------------

Absolute Velocity Formulation yes

Relaxation

Variable Relaxation Factor

------------------------------------------------

Modified Turbulent Viscosity 0.80000001

Turbulent Viscosity 1

Solid 1

Linear Solver

Solver Termination Residual Reduction

Variable Type Criterion Tolerance

--------------------------------------------------------------------------

Flow F-Cycle 0.1

Modified Turbulent Viscosity Flexible 0.1 0.7

Discretization Scheme

Variable Scheme

--------------------------------------------------

Flow Second Order Upwind

Modified Turbulent Viscosity Second Order Upwind

Time Marching

Parameter Value

-------------------------

Solver Implicit

Courant Number 5

Solution Limits

Quantity Limit

---------------------------------------

Minimum Absolute Pressure 1

Maximum Absolute Pressure 5000000

Minimum Temperature 1

Maximum Temperature 5000

Maximum Turb. Viscosity Ratio 100000

Material Properties -------------------

Material: air (fluid)

Property Units Method Value(s)

--------------------------------------------------------------------

Density kg/m3 ideal-gas #f

Cp (Specific Heat) j/kg-k constant 1006.43

Thermal Conductivity w/m-k constant 0.0242

Viscosity kg/m-s constant 1.7894001e-05

Molecular Weight kg/kgmol constant 28.966

L-J Characteristic Length angstrom constant 3.711

L-J Energy Parameter k constant 78.6

Thermal Expansion Coefficient 1/k constant 0

Degrees of Freedom constant 0

Material: aluminum (solid)

Property Units Method Value(s)

---------------------------------------------------

Density kg/m3 constant 2719

Cp (Specific Heat) j/kg-k constant 871

Thermal Conductivity w/m-k constant 202.4

Mancusi December 16, 2004 11:15

Re: Low Speed Airfoil
 
How do I set it to use low-Re

Charles December 17, 2004 02:40

Re: Low Speed Airfoil
 
Are you aware that Fluent's Lift and drag coefficients are given in body axes, not wind axes? So if you are changing the angle of the incoming flow, the calculated lift and drag forces are actually normal and axial forces. Also, by default, moments are taken about 0,0,0 , but you can set this.

Mancusi December 17, 2004 10:45

Re: Low Speed Airfoil
 
I am aware of this and do compensate for it but I run at 0 degrees so it cant be a problem

wcchan2 April 3, 2014 07:11

Hai Charles, here to ask if we are not changing the angle of the incoming flow(constant flow), but change the angle of object (yaw), the Fluent's Lift and drag coefficients given are in body axes or wind axes?

Thanks,Regards wcchan2.


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