15. SAILBOAT GEOMETRY

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1 SAILBOAT GEOMETRY 15. SAILBOAT GEOMETRY In this tutorial you will import a STEP file that describes the geometry of a sailboat hull. You will split the hull along the symmetry plane, create a flow volume around the hull, apply size functions that allow the mesh to capture the detail of the hull curvature, create first-height/aspect-ratio-based 3-D boundary layers around the hull, and use a special operation to pre-mesh and view the 3-D boundary layers. Finally, you will mesh the volume with prism and tetrahedral elements using an algorithm that will automatically apply meshed size functions on the boundary layer caps. The combined use of aspect-ratio-based 3-D boundary layers and automatically applied meshed size functions on the boundary layer caps is especially useful when meshing geometry for external aerodynamics or other flows around bluff objects, such as that illustrated by this tutorial. The combined features aid in capturing near-wall physics while creating a high-quality unstructured mesh in the flow volume. In this tutorial you will learn how to: Import a STEP file Align volumes Split faces Split volumes and perform Boolean volume subtraction operations Create and apply curvature size functions Mesh faces using a triangular pave meshing scheme Create and view aspect-ratio-based 3-D boundary layers Apply a meshed size function on boundary layer caps Mesh a volume using a tetrahedral meshing scheme Prepare the mesh to be read into FLUENT 5/ Prerequisites This tutorial assumes that you have worked through Tutorials 1, 2, 3, and Fluent, Inc. All rights reserved. 15-1

Problem Description SAILBOAT GEOMETRY 15.2 Problem Description Figure 15-1 shows the geometry to be imported and meshed in this tutorial. This geometry represents a sailboat hull.

2 Problem Description SAILBOAT GEOMETRY 15.2 Problem Description Figure 15-1 shows the geometry to be imported and meshed in this tutorial. This geometry represents a sailboat hull. The purpose of this tutorial is to create a hybrid viscous mesh that can be used to model flow around the hull. Figure 15-1: Sailboat geometry Fluent, Inc. All rights reserved.

3 SAILBOAT GEOMETRY Strategy 15.3 Strategy In this tutorial, you will create a tetrahedral mesh in a flow volume surrounding a sailboat hull. The geometry will be imported as a STEP file consisting of a single volume in the shape of the full sailboat hull. You will create a rectangular brick flow volume and use a Boolean operation to split the hull along the symmetry plane. Then you will apply curvature size functions to the hull side faces and create triangle face meshes on all of the hull surfaces. Finally, you will apply aspect-ratio-based 3-D boundary layers around the hull, use a GAMBIT function that meshes and displays only the boundary layers, and mesh the volume employing size functions that are automatically applied on the boundary layer caps to create a mesh consisting of tetrahedral and wedge elements Fluent, Inc. All rights reserved. 15-3

4 Procedure SAILBOAT GEOMETRY 15.4 Procedure 1. Copy the file path/fluent.inc/gambit2.x/help/tutfiles/sailboat-hull.stp (where 2.x is the GAMBIT version number) from the GAMBIT installation area in the directory path to your working directory. 2. Start GAMBIT using the session identifier Sailboat. Step 1: Select a Solver 1. Choose the solver from the main menu bar: Solver FLUENT 5/6 The choice of a solver dictates the options available in various forms (for example, the boundary types available in the Specify Boundary Types form). For some systems, FLUENT 5/6 is the default solver. The solver currently selected is shown at the top of the GAMBIT GUI Fluent, Inc. All rights reserved.

5 SAILBOAT GEOMETRY Procedure Step 2: Import the STEP File File Import STEP This command sequence opens the Import STEP File form. 1. Click the Browse... button. This action opens the Select File form Fluent, Inc. All rights reserved. 15-5

Procedure SAILBOAT GEOMETRY a) Select sailboat-hull.stp in the Files list. b) Click Accept on the Select File form. 2. Retain the Make Tolerant option.

6 Procedure SAILBOAT GEOMETRY a) Select sailboat-hull.stp in the Files list. b) Click Accept on the Select File form. 2. Retain the Make Tolerant option. The Make tolerant option improves geometric connectivity. 3. Click Accept. The STEP file for the sailboat hull will be read into GAMBIT (see Figure 15-2) Fluent, Inc. All rights reserved.

Procedure SAILBOAT GEOMETRY Step 3: Align the Geometry with the Global Coordinate System The imported geometry is offset slightly from the global coordinate system (c_sys.1).

8 Procedure SAILBOAT GEOMETRY Step 3: Align the Geometry with the Global Coordinate System The imported geometry is offset slightly from the global coordinate system (c_sys.1). In this step, you will create a vertex at the origin of the global coordinate system and use the vertex to align the sailboat geometry with the origin. Doing so will facilitate the volume splitting operation you will use in a later step to create the flow volume. 1. Create a vertex to be used in aligning the geometry. GEOMETRY VERTEX CREATE REAL VERTEX This command sequence opens the Create Real Vertex form. a) Click Apply. GAMBIT creates a vertex at the origin (0, 0, 0) of the global coordinate system (c_sys.1). 2. Align the sailboat hull with the part of the geometry to be split. GEOMETRY VOLUME MOVE/COPY/ALIGN VOLUMES R This command sequence opens the Align Volumes form Fluent, Inc. All rights reserved.

SAILBOAT GEOMETRY Procedure a) Select the hull geometry in the graphics window and accept the selection. GAMBIT activates the Translation Vertex Pair list box in the Align Volumes form.

9 SAILBOAT GEOMETRY Procedure a) Select the hull geometry in the graphics window and accept the selection. GAMBIT activates the Translation Vertex Pair list box in the Align Volumes form. You will now select a vertex on the volume and the vertex with which it will be aligned. b) Select the vertex at the tip of the sailboat bow (see Figure 15-3) Fluent, Inc. All rights reserved. 15-9

10 Procedure SAILBOAT GEOMETRY Start vertex End vertex Figure 15-3: Start and End vertices for the volume alignment c) Select the vertex you created at the origin of the global coordinate system (c_sys.1). d) Click Apply. GAMBIT aligns the geometry with the global coordinate system as shown in Figure Fluent, Inc. All rights reserved.

Procedure SAILBOAT GEOMETRY Step 4: Create the Flow Volume In this step, you will create a brick around the sailboat geometry, move the brick so that one side passes through the hull along the z-x

12 Procedure SAILBOAT GEOMETRY Step 4: Create the Flow Volume In this step, you will create a brick around the sailboat geometry, move the brick so that one side passes through the hull along the z-x plane, and subtract the sailboat geometry volume from the brick, thereby creating the flow volume. 1. Create a brick surrounding the sailboat geometry. GEOMETRY VOLUME CREATE VOLUME This command sequence opens the Create Real Brick form. a) Enter a value of 125 for the Width of the brick. b) Enter 25 for the Depth and 25 for the Height. c) Retain the Direction:Centered option. d) Click Apply. 2. Click the FIT TO WINDOW command button at the top left of the Global Control toolpad to see the brick and sailboat geometry in the graphics window. Figure 15-5 shows the sailboat geometry and flow volume brick Fluent, Inc. All rights reserved.

13 SAILBOAT GEOMETRY Procedure Figure 15-5: Sailboat geometry and flow volume brick 3. Move the brick so that one of its vertical faces passes through the center of the sailboat geometry. GEOMETRY VOLUME MOVE/COPY/ALIGN VOLUMES R This command sequence opens the Move / Copy Volumes form Fluent, Inc. All rights reserved

14 Procedure SAILBOAT GEOMETRY a) Select (Shift-left-click) the brick in the graphics window. b) Retain the Move option under Volumes in the Move / Copy Volumes form. c) Retain the Operation:Translate option. d) Enter (0, -12.5, 0) under Global to move the brick units in the y direction. Note that GAMBIT automatically fills in the values under Local as you enter values under Global. e) Click Apply. 4. Click the FIT TO WINDOW command button at the top left of the Global Control toolpad to see the full brick and sailboat geometry in the graphics window. GAMBIT changes the position of the brick as shown in Figure Fluent, Inc. All rights reserved.

SAILBOAT GEOMETRY Procedure Figure 15-6: Repositioned brick and sailboat geometry 5. Subtract the sailboat geometry from the brick to create the flow volume.

15 SAILBOAT GEOMETRY Procedure Figure 15-6: Repositioned brick and sailboat geometry 5. Subtract the sailboat geometry from the brick to create the flow volume. GEOMETRY VOLUME BOOLEAN OPERATIONS R This command sequence opens the Subtract Real Volumes form. a) Shift-left-click the brick in the graphics window Fluent, Inc. All rights reserved

16 Procedure SAILBOAT GEOMETRY b) Left-click in the list box to the right of Subtract Volumes to accept the selection and make the Subtract Volumes list box active.! Alternatively, you could continue to hold down the Shift key and click the right mouse button in the graphics window to accept the selection of the brick and move the focus to the Subtract Volumes list box. c) Select the sailboat geometry. Selecting the volumes in this order ensures that the sailboat geometry is subtracted from the brick and not vice versa. d) Click Apply. GAMBIT subtracts the sailboat geometry from the brick to create the single flow volume shown in Figure Figure 15-7: Flow volume Fluent, Inc. All rights reserved.

SAILBOAT GEOMETRY Procedure Step 5: Split the Sailboat Hull In this step, you will create a horizontal face and use it to split the sailboat hull face at its water line. 1.

17 SAILBOAT GEOMETRY Procedure Step 5: Split the Sailboat Hull In this step, you will create a horizontal face and use it to split the sailboat hull face at its water line. 1. Create a real rectangular face to use as a split tool. GEOMETRY FACE CREATE FACE This command sequence opens the Create Real Rectangular Face form. a) Enter a value of 60 for the Width of the face. b) Enter a value of 25 for the Height of the face. c) Retain the Direction:XY Centered option. d) Click Apply. 2. Move the new split-tool face to the water-line height on the hull. GEOMETRY FACE MOVE/COPY/ALIGN FACES This command sequence opens the Move / Copy Faces form Fluent, Inc. All rights reserved

Procedure SAILBOAT GEOMETRY a) Select (Shift-left-click) the split-tool face in the graphics window. b) Retain the Move option under Faces in the Move / Copy Faces form.

18 Procedure SAILBOAT GEOMETRY a) Select (Shift-left-click) the split-tool face in the graphics window. b) Retain the Move option under Faces in the Move / Copy Faces form. c) Retain the Operation:Translate option. d) Enter (0, 0, -1.54) under Global to move the face units in the z direction. e) Click Apply. GAMBIT locates the split-tool face as shown in Figure Fluent, Inc. All rights reserved.

19 SAILBOAT GEOMETRY Procedure 3. Split the hull face at the water line. Figure 15-8: Repositioned split-tool face GEOMETRY FACE SPLIT/MERGE/COLLAPSE/SIMPLIFY FACES This command sequence opens the Split Face form Fluent, Inc. All rights reserved

Procedure SAILBOAT GEOMETRY a) Select (Shift-left-click) the sailboat hull side face in

21 SAILBOAT GEOMETRY Procedure Split-tool face Face to be split Figure 15-9: Hull face to be split b) Select the Split with:faces option. c) Click in the lower Faces list box to make it active. d) Select (Shift-left-click) the split-tool face in the graphics window. e) Retain the Connected option. f) Click Apply. GAMBIT splits the hull side face as shown in Figure Fluent, Inc. All rights reserved

SAILBOAT GEOMETRY Procedure Step 6: Specify Size Function Background Tree Depth Now that you have completed creation of the flow volume and hull, you will apply size functions to the hull sides to

23 SAILBOAT GEOMETRY Procedure Step 6: Specify Size Function Background Tree Depth Now that you have completed creation of the flow volume and hull, you will apply size functions to the hull sides to facilitate a good-quality mesh. Before creating and applying the size functions, however, you will modify the values of the GAMBIT default variables that controls the background tree depth. 1. Modify the size-function background grid tree depth default variable. Edit Defaults This command sequence opens the Edit Defaults form Fluent, Inc. All rights reserved

24 Procedure SAILBOAT GEOMETRY a) Select the TOOLS tab. b) Select the SFUNCTION radio button. GAMBIT displays the size-function variables for which defaults can be specified. c) Select BGRID_MAX_TREE_DEPTH in the Variable list. BGRID_MAX_TREE_DEPTH appears in the text entry box at the bottom of the list and its default value (16) appears in the Value text entry box. The BGRID_MAX_TREE_DEPTH default variable controls the degree of refinement to which the size functions resolve the mesh size variation within the attachment geometry. d) Enter a value of 30 in the Value text entry box. e) Click the Modify button. GAMBIT updates the Value of the variable BGRID_MAX_TREE_DEPTH. 2. Close the Edit Defaults form Fluent, Inc. All rights reserved.

SAILBOAT GEOMETRY Procedure Step 7: Apply a Size Function to the Sides of the Hull In this step, you will apply a curvature size function to the sides of the sailboat hull.

25 SAILBOAT GEOMETRY Procedure Step 7: Apply a Size Function to the Sides of the Hull In this step, you will apply a curvature size function to the sides of the sailboat hull. Curvature size functions limit the allowable angle between outward-pointing normals for any two adjacent mesh elements located immediately adjacent to the surface of a source entity. They are especially useful when meshing highly curved surfaces, for which a fixed-size surface mesh might not resolve the curvature adequately. 1. Create and apply a curvature size function to the hull bottom and side faces. TOOLS SIZE FUNCTIONS CREATE SIZE FUNCTION This command sequence opens the Create Size Function form. a) Select the Type:Curvature option. b) On the Source option button, select the Faces option. c) Select the two curved faces at the bottom and side of the hull (see Figure 15-11) Fluent, Inc. All rights reserved

26 Procedure SAILBOAT GEOMETRY Source faces Figure 15-11: Size function source faces d) On the Attachment option button, select the Faces option. e) Click in the Attachment:Faces list box to make it active. f) Select the five faces that comprise the entire hull (see Figure 15-12) Fluent, Inc. All rights reserved.

27 SAILBOAT GEOMETRY Procedure Attachment faces Figure 15-12: Size function attachment faces g) In the Angle text box, enter 10. h) In the Growth rate text box, enter 1.2. i) In the Max. size text box, enter 2. j) Retain the default Min. size value (0.001). k) Click Apply to create and attach the size function Fluent, Inc. All rights reserved

Procedure SAILBOAT GEOMETRY Step 8: Mesh the Hull Faces In this step, you will create triangle meshes on the faces of the hull. 1. Create triangular meshes on the surfaces of the sailboat.

28 Procedure SAILBOAT GEOMETRY Step 8: Mesh the Hull Faces In this step, you will create triangle meshes on the faces of the hull. 1. Create triangular meshes on the surfaces of the sailboat. MESH FACE MESH FACES This command sequence opens the Mesh Faces form. a) In the graphics window, select all five faces that comprise the sailboat hull (see Figure 15-12, above) b) Select the Elements:Tri option. c) Retain the Type:Pave option. d) Under Spacing, retain the Interval size of 1. (NOTE: The size function applied in the previous step will override the Spacing specifications on the Mesh Faces form.) Fluent, Inc. All rights reserved.

29 SAILBOAT GEOMETRY Procedure e) Click the Apply button at the bottom of the form. GAMBIT meshes the faces as shown in Figure Figure 15-13: Triangle paved mesh on sailboat surface faces Fluent, Inc. All rights reserved

Procedure SAILBOAT GEOMETRY Step 9: Set Boundary-Layer Angle Default Value The bow of the sailboat forms a very large angle with the symmetry face, which can lead to complications when attaching 3-D

30 Procedure SAILBOAT GEOMETRY Step 9: Set Boundary-Layer Angle Default Value The bow of the sailboat forms a very large angle with the symmetry face, which can lead to complications when attaching 3-D boundary layers to the hull. You can avoid the complications by changing the default that controls the angle at which advancing boundary layers are allowed to project to adjacent surfaces. Outer edges of advancing boundary layers are projected to adjacent faces only when the interior angle between the faces is less than or equal to the default value. 1. Modify the boundary-layer angle default value. Edit Defaults This command sequence opens the Edit Defaults form Fluent, Inc. All rights reserved.

31 SAILBOAT GEOMETRY Procedure a) Select the MESH tab. b) Select the VERTEX radio button. c) Select MAX_END_DANGLE in the Variable list. MAX_END_DANGLE appears in the space at the bottom of the list and its default value appears in the Value text entry box. d) Enter a value of 170 in the Value text entry box. e) Click the Modify button to the left of MAX_END_DANGLE. The Value of the variable MAX_END_DANGLE is updated in the list. f) Close the Edit Defaults form Fluent, Inc. All rights reserved

Procedure SAILBOAT GEOMETRY Step 10: Apply 3-D Boundary Layers to the Hull In this step, you will attach a first-height/last-aspect-ratio-based boundary layer to the hull.

32 Procedure SAILBOAT GEOMETRY Step 10: Apply 3-D Boundary Layers to the Hull In this step, you will attach a first-height/last-aspect-ratio-based boundary layer to the hull. This type of boundary layer is useful for meshing external aerodynamics such as those that are common in aerospace, marine, turbomachinery, and automotive applications. A boundary layer with a specific cell height in the first layer allows the capture of near-wall turbulence, while a specific aspect ratio in the outermost layer allows the growth of high-quality tetrahedral cells from the outer surface of the boundary layer. 1. Remove the mesh from the display before creating the 3-D boundary layers. Removing the mesh display makes it easier to see the edges and faces of the geometry. The mesh is not deleted, just removed from the graphics window. a) Click the SPECIFY DISPLAY ATTRIBUTES command button at the bottom of the Global Control toolpad to open the Specify Display Attributes form. b) Select the Mesh:Off option near the bottom of the form Fluent, Inc. All rights reserved.

SAILBOAT GEOMETRY Procedure c) Click Apply to turn off the mesh display, and Close the form. 2. Create and apply 3-D boundary layers on the hull faces.

33 SAILBOAT GEOMETRY Procedure c) Click Apply to turn off the mesh display, and Close the form. 2. Create and apply 3-D boundary layers on the hull faces. MESH BOUNDARY LAYER CREATE BOUNDARY LAYER This command sequence opens the Create Boundary Layer form. a) Under Definition, select the Algorithm:Aspect ratio (last) option. b) In the First row text box, enter Fluent, Inc. All rights reserved

34 Procedure SAILBOAT GEOMETRY This defines the height of the first row of elements on the faces. c) Retain the default Rows value (4). GAMBIT automatically sets the Last percent value to 50. d) Retain the default Last percent value (50). e) Select the Internal continuity option. f) Select the Attachment:Faces option and click in the Faces list box to make it active. g) Select all five of the hull faces. h) Click Apply. GAMBIT applies 3-D boundary layers to the sailboat hull as shown in Figure (NOTE: The figure shows only the projection of the boundary layers onto the symmetry face of the flow volume.) Figure 15-14: Projection of 3-D boundary layer onto flow volume face Fluent, Inc. All rights reserved.

35 SAILBOAT GEOMETRY Procedure Step 11: View 3-D Boundary Layers GAMBIT includes a function that automatically meshes and displays 3-D boundary layer regions so that you can view the mesh in such regions before meshing the entire volume. In this step, you will use the function to examine the meshing regions near the hull. 1. Create and view the 3-D boundary layer meshes. MESH BOUNDARY LAYER VIEW 3D BOUNDARY LAYER This command sequence opens the View 3D Boundary Layer form. a) In the graphics window, select any of the faces to which the 3-D boundary layer is attached. b) Click Apply. GAMBIT meshes the 3-D boundary layer region, automatically opens the Examine Mesh form, and displays the entire mesh in the boundary layer region (see Figure 15-15). You can use the Examine Mesh form to analyze the 3-D boundary layer mesh as you would any volume mesh. For example, if you select the Display type:plane option on the Examine Mesh form, you can use the X slider bar to display slices of the mesh aligned with the y-z plane (see Figure 15-16) Fluent, Inc. All rights reserved

37 SAILBOAT GEOMETRY Procedure! When you are finished examining the 3-D boundary layer mesh, you must close the Examine Mesh form before continuing with other GAMBIT operations Fluent, Inc. All rights reserved

Procedure SAILBOAT GEOMETRY Step 12: Mesh the Flow Volume In this step you will mesh the volume using a tetrahedral meshing algorithm and specify that the GAMBIT applies a meshed size function on the

38 Procedure SAILBOAT GEOMETRY Step 12: Mesh the Flow Volume In this step you will mesh the volume using a tetrahedral meshing algorithm and specify that the GAMBIT applies a meshed size function on the boundary layer cap. The meshed size function on the boundary layer cap grows the mesh from the outer surface of the boundary layer into the volume. 1. Mesh the volume with a tetrahedral mesh. MESH VOLUME MESH VOLUMES This command sequence opens the Mesh Volumes form. a) Select the volume in the graphics window. b) Under Scheme, select the Elements:Tet/Hybrid option and the Type:TGrid option. See the GAMBIT Modeling Guide for more information on meshing schemes. c) Select the Meshed S.F. on B.L. cap option. GAMBIT activates the Growth rate and Max. size text boxes Fluent, Inc. All rights reserved.

39 SAILBOAT GEOMETRY Procedure d) Retain the default Growth rate (1.2). e) Retain the default Max. size (2). f) Under Spacing, specify an Interval size of 5. g) Click Apply. GAMBIT meshes the flow volume as shown in Figure Figure 15-17: Mesh of flow volume Fluent, Inc. All rights reserved

40 Procedure SAILBOAT GEOMETRY Step 13: Examine the Mesh 1. Click the EXAMINE MESH command button on the Global Control toolpad. This action opens the Examine Mesh form. a) Click Update at the bottom of the Examine Mesh form Fluent, Inc. All rights reserved.

41 SAILBOAT GEOMETRY Procedure GAMBIT does not automatically update the graphics display when you open the Examine Mesh form or modify its specifications, such as Display Type or Quality Type. To update the graphics display, you must click the Update pushbutton located at the bottom of the form. GAMBIT displays the Update pushbutton label in red lettering whenever the display needs to be updated to reflect the current Examine Mesh specifications. Some Examine Mesh operations automatically update the graphics display. For example, if you select the Display Type:Range option and click one of the histogram bars, GAMBIT automatically updates the display. b) Select the Display Type:Plane option at the top of the form. The Examine Mesh form allows you to view mesh characteristics for various types of 3-D mesh elements. In this case, the volume mesh consists of wedge and tetrahedral elements. GAMBIT automatically selects the 3-D Element wedge display option, but you must manually specify the display of tetrahedral elements, as well, to view all elements in the mesh. c) Click the 3-D Element option tetrahedral button at the top of the form, and use the X and Y slider bars near the bottom of the form to display cutting planes through the mesh in the y-z and z-x directions, respectively. Figure and Figure show y-z and z-x cutting planes through the mesh Fluent, Inc. All rights reserved

tetrahedral elements d) Click the Close button at the bottom of the

42 Procedure SAILBOAT GEOMETRY Figure 15-18: Cutting plane (y-z) showing tetrahedral and wedge elements Figure 15-19: Cutting plane (z-x) showing tetrahedral elements d) Click the Close button at the bottom of the Examine Mesh form to close the form Fluent, Inc. All rights reserved.

b) Click Accept on the Export Mesh File form. The file will be written to your working directory. 2. Save the GAMBIT session and exit GAMBIT.

43 SAILBOAT GEOMETRY Procedure Step 14: Export the Mesh and Save the Session 1. Export a mesh file for the sailboat flow volume. File Export Mesh This command sequence opens the Export Mesh File form. a) Enter the File Name for the file to be exported (Sailboat.msh). b) Click Accept on the Export Mesh File form. The file will be written to your working directory. 2. Save the GAMBIT session and exit GAMBIT. File Exit GAMBIT will ask you whether you wish to save the current session before you exit. a) Click Yes to save the current session and exit GAMBIT Fluent, Inc. All rights reserved

44 Summary SAILBOAT GEOMETRY 15.5 Summary This tutorial illustrated how to import geometry and use GAMBIT curvature size functions to create a mesh for a sailboat hull geometry and how to create, apply, and view 3-D boundary layers. It also illustrated how to automatically apply a meshed size function on a boundary layer cap to grow the mesh from the outer surface of the boundary layer into the volume Fluent, Inc. All rights reserved.

14. AIRPLANE GEOMETRY

14. AIRPLANE GEOMETRY In this tutorial you will import a STEP file that describes the geometry of an airplane body, including the wing and nacelle that houses the engine. You will clean up the geometry