Cfd-tut Profil h105

A CFD GUIDLINE TO SIMULATE THE FLOW AROUND AN AIRFOIL

2 Profile data 2.1 downloading the the profile coordinates .

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1 Introduction .

1.1 Objective .

The main goal of this exercise is to give the cfd-newcomer the opportunity to simulate in an almost autonomous way the flow around an airfoil with cfd. To make so a minimum of indications will be imparted.

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with your favourite browser go to: http://www.nasg.com/afdb/list-polar-e.phtml check polar availability browse the list for your favourite profile e.g. Speer H105 load profile coordinates Search -> Airfoil -> “H105” Search v “speer H105(...” S h o w A i r f o i l ContourData Save as .dat or .txt file Prepare coordinates for Gambit  open the .dat file into Excel v Delimited Start import at row 2 (where coord. begin),  Ne xt  v Space delimited, N e x t   Finish if column A is empty, remove it fill column C with 0 be sure that the trail edge point appear only one time (suppress redundant point where necessary) save as Text (Tab delimit ed) Ye s

1.2 Procedure .

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look for profile coordinates and polar curve import the profile coordinates into Gambit  draw the 2D profile in its surrounding flow ow,, and mesh this computational domain simulate the incompressible, 2D and turbulent airflow around the airfoil with Fluent  investigate pressure and velocity fields, flow trajectories, lift, drag and moment coef ficients.

2.2 Read pro profile coordinates into Gambit  File->Import->Vertex Data->Browse....Accept/Accept File->Import->Vertex Geom->Edge->Create Edge/NURBS

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Proceed with several nurbs (6 to 8) containing a measured amount of vertex. Check that the nurb nurbss are all “healthy” and connected.

3 Geometry .

Geom->Face->Form Face/Wirefram Face/Wireframee

assemble all Nurbs into an airfoil-profile face .

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rotate the profile to the desired angle of attack: -3°, -1°, 1°, 2°, 3°, 4°, 6°, 8°, 10°, 12° Create the computational domain with 2 rectangular faces

4 Mesh .

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Determine the thickness of the first cells near the profile wall. For external flow, let: 9 ! L Y P " ---------- !  y + ; with 30 #  y + # 500  Re L and

velocity inlet outer box airfoil

inner box

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1.15 kg·m-3; %

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1.73 ! 10 – 5 kg·m-1·s-1

c& =

55.56 m1·s-1

for this exercise, a coarse value of 400 should be suf ficient. Cell length should not exceed 2 to 3 times Y P Create a boundary layer all around the profile with about 10 layers and a growth factor of about 1.1 to 1.15

pressure outlet edges to be connected

wall (symmetry)

The dimensions of the computational domain should be at least 3 airfoil lengths in front of the airfoil, and 5 lengths behind. The displacement of the airfoil (thickness) should be not greater than 1-1.5% of the total cross sectional area. (This is not applicable if  the domain boundaries represent the walls of a real wind-tunnel. In this case the simulation should take into account the related wall effects). In order to control the volume mesh near the airfoil, an “inner” box may be helpful. This box should extend about half a airfoil length in front, and to t he sides, and about an airfoil length in t he wake. This “inner-box” is not mandatory. .

proceed by faces substraction Be sure that the two boxes remain connected. If necessary: Geometry/Edge/connect and select the 4 outer edges of the i nner box together with the 4 inner edges of the outer box and connect them

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Choose the right solver (Fluent 5/6) .

Create a fixed size function (Tools/Size Function) attached to the inner box with the airfoil edges as sources start size = 2 to 3 timesY P ; end size = 50 Y P , growth factor 1.1

Strouhal Number St  ( 0.25 characteristic length l = profile thickness, The Frequency gives the number of periodical variations per second. Each variation should be resolved by at least 30 time steps, although 50 would be better. This gives the time-step for the calculation. The correctness of the time-step is verified if the continuity residual drops about 2 orders of magnitude from the beginning of the time-step within 20 iterations. To obtain a meaningful solution for time averaging a periodical behaviour of  the flow field has to appear. Therefore at least 10 (20 would be better) periods have to be calculated. Besides the transient parameters, the model set-up is the same as for steady-state simulations.

7.4 Calculate .

Drag c d , Lift cl and Moments-factors c M  ; compare with wind tunnel measurements or third party simulation. Moments are measured at a quarter length of the airfoil, so set the Moment center to  x = 0.25 m and  y = 0 .

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Aerodynamic forces due to pressure and wall friction

7 Post-Processing 7.1 check  y + .

on the airfoil profile

7.2 Grid independence .

By good cfd-practice, you should ensure that solution is grid-independent and use grid adaption to modify the grid or create additional meshes for the gridindependence study. Anyhow, to save time, we will bypass this important step!

7.3 Visualise and analyse .

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pressure field velocity field turbulence kinetic energy k , turbulence dissipation rates ' , vortricity ) velocity vectors passlines Checked to see that the solution makes sense based on engineering judgment. If  flow features do not seem reasonable, you should reconsider your physical models and boundary conditions. Reconsider the choice of the boundaries location (or the domain). An inadequate choice of domain (especially the outlet boundary) can significantly impact solution accuracy.

7.5 Estimate .

the boundary layer thickness and compare it with the one of the flat plate

8 Transmitting your results until 25.04.2006 .

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Use Moodle copy in there your following files .jou .trn .msh .cas .dat and some nice plots your made