VIEWS: 192 PAGES: 17 CATEGORY: Technology POSTED ON: 10/1/2011
COMPUTATIONAL FLUID DYNAMICS GAMBIT PORTION SUBMITTED TO: MR. GULRAIZ AHMED SUBMITTED BY: ARSLAN SHAFIQ BE-05 (AERO) Table of Contents 1 Introduction: ............................................................................................. 3 1.1 Airfoil: ................................................................................................. 3 1.2 GAMBIT Software:............................................................................... 3 2 Gambit: ..................................................................................................... 4 2.1 Coordinate Format: ............................................................................. 4 2.2 Creating Airfoil Geometry: .................................................................. 4 2.3 Create Farfield Boundary: ................................................................... 5 2.4 Create Faces: ..................................................................................... 6 3 Mesh Geometry in GAMBIT ........................................................................ 7 3.1 Mesh Edges: ........................................................................................ 7 3.2 Boundary Layer Approximation ........................................................... 8 3.2.1 Split edge: .................................................................................... 8 3.2.2 Meshing on airfoil edges: .............................................................. 8 3.2.3 Y+ Approximation......................................................................... 9 4 Specify Boundary Types in GAMBIT ......................................................... 11 4.1 Group Edges: .................................................................................... 11 5 Quality of the Structured Mesh ................................................................ 13 6 Unstructured Mesh: ................................................................................. 14 6.1 Meshing the Edges ............................................................................ 14 6.2 Meshing the Faces............................................................................. 14 6.3 Quality of the Unstructured Mesh ..................................................... 16 7 References .............................................................................................. 17 1 Introduction: In this report we first of select an airfoil and then generate the data points of the airfoil. This file is imported in gambit and then we define a computational domain in it. Boundary layer is then plotted around the airfoil. Then the structured and unstructured meshing is done inside the domain. If the meshing is not fine then it is refined. 1.1 Airfoil: The airfoil which I used is NACA 2415. I generated it using the software profilli. The number of points on the upper and lower surface are equal. Points on each surface are 127. 1.2 GAMBIT Software: Gambit is basically meshing software. In this we will define a computational domain around our system under consideration e.g airfoil. In this computational domain the meshing is generated. Meshing helps in the flow analysis of the prototype model. The numerical techniques are used for evaluation from one grid point to other. This is the first step in building and analyzing a flow model. 2 Gambit: 2.1 Coordinate Format: The format of the vertices of the airfoil are in .dat format. The points generarted by the softwareprofilli are saved in it. The fiirst line represents the number of points on each edge. 2.2 Creating Airfoil Geometry: The .dat file of the vertices of the airfoil is imported in the gambit through ICEM input. 2.3 Create Farfield Boundary: Then the farfield boundary is created. LABEL X Y Z A 1 13.5 0 B 22 13.5 0 C 22 0 0 D 22 -13.5 0 E 1 -13.5 0 F -12.5 0 0 G 1 0 0 After generating the points the edges are made. 2.4 Create Faces: There are basically four faces are drawn namely, rec1, rec2, airfol and circ1. 3 Mesh Geometry in GAMBIT 3.1 Mesh Edges: After making the faces the next step is meshing. All the edges are meshed. First we are doing the structured meshing. Edges Arrow direction First length Interval count GA Upwards .01 70 BC Upwards .01 70 AB Left to right .01 85 CG Left to right .01 85 DE Left to right .01 85 EG Downwards .01 70 CD Downwards .01 70 3.2 Boundary Layer Approximation 3.2.1 Split edge: Next, we will split the top and bottom edges of the airfoil into two edges so that we have better control of the mesh point distribution. Figure of the splitting edges is shown below. We need to do this because a non-uniform grid spacing will be used for x<0.3 and a uniform grid spacing for x>0.3. We will split the top edge into HI and IG and the bottom edge into HJ and GJ. 3.2.2 Meshing on airfoil edges: Before making the boundary layer, there should be meshing done on the airfoil. Edges Arrow direction First length/ Interval count Successive ratio HI From H to I .025 35 HJ From H to J /1 76 IG Left to right .025 35 JG Left to right /1 76 For edges AF and EF, the number of divisions needs to be equal to the number of divisions on the line opposite to it. In order to calculate the boundary layer thickness we have the formula, 0.2 U x BL thickness 0.37 x where U =10m/s and =1.46e-05 and x =1m So the boundary layer equation comes out to be, BL thickness = 0.0252m Now create the boundary layer 0.0252m thick. But before that approximate y+ as below, 3.2.3 Y+ Approximation y+ can be approximated by the formula given below, y y1u* -------- (1) Where u * can be approximated from the following equation, 2 0.2 u* U x U 0.0296 -------- (2) Where for my case, U = 10m/s = 1.46e-05 m2/s And x = 1m So, from eqn (2) we can get the u * which is equal to, u * = 0.4488 m/s Now using eqn (2) to get the value of y+, I have taken the value of y1 to be equal to, y1 = 0.00055 m Then plugging the values in eqn (1), we have, y = 16.9068 The number of rows of the boundary layer is 15. After the making of boundary layer all the faces are meshed. 4 Specify Boundary Types in GAMBIT 4.1 Group Edges: We will create groups of edges and then create boundary entities from these groups. First, we will group AF and EF together. Select Edges and enter farfield1 for Label, which is the name of the group. In the transcript window, you will see the message “Created group: farfield1 group”. Similarly, create the other two far field groups. We should have created a total of three groups: Group Name Edges in Group farfield1 AF,EF farfield2 AB,DE farfield3 BC,CD airfoil HI,IG,HJ,JG 4.2 Define Boundary Types: We have grouped each of the edges into the desired groups; we can assign appropriate boundary types to these groups. Select any edge belonging to the airfoil surface and that will select the airfoil group. Next to name: enter airfoil. Leave the Type as WALL. Similarly, create boundary entities corresponding to farfield1, farfield2 and farfield3 groups. Set the Type to Pressure Far field in each case. 5 Quality of the Structured Mesh The quality of the structured mesh is: The worst element is .370028 and lies in circ1. 6 Unstructured Mesh: 6.1 Meshing the Edges All the edges of the same computational domain are meshed again for the unstructured case. Edges Arrow direction First length/Last Interval count length/ Successive ratio GA Upwards .03 115 BC Upwards /-/1 115 AB Left to right .2 100 CG Left to right .05 150 DE Left to right .3 100 EG Downwards .03 115 CD Downwards /-/1 100 HI From H to I /.025/ 25 HJ From H to J /-/1 40 IG Left to right /.025/ 25 JG Left to right /-/1 40 6.2 Meshing the Faces After meshing the edges, now comes the turn of meshing the faces. But before that the boundary layer of same approximation as that of structured is drawn. Then mesh the faces. 6.3 Quality of the Unstructured Mesh The quality of the structured mesh is: The worst element is .482127 and lies in rec1. 7 References http://courses.cit.cornell.edu/fluent/airfoil/index.htm http://www.pdas.com/ Profilli software http://www.fluent.com/solutions/whatcfd.htm http://www.cfd-online.com/Wiki/Fluent_FAQ#What_is_Gambit.3F