ACU-T: 5300 Ship Hull Static

This tutorial provides the instructions for setting up, solving and viewing results for a simulation of flow around a static ship hull. In this simulation, a wave hits the static ship hull and the flow around the ship is simulated. This tutorial is designed to introduce you to a number of modelling concepts necessary to perform Free-Surface simulations.

The basic steps in any CFD simulation are shown in ACU-T: 2000 Turbulent Flow in a Mixing Elbow. The following additional capabilities of AcuSolve are introduced in this tutorial:
  • Use of a User Defined Function (UDF) for the gravity wave generation
  • Mesh extrusions
  • Periodic boundary conditions
  • Use of Surface Manager to apply surface attributes
  • Free surface
  • Guide surface
  • Use of hydrostatic pressure for boundary conditions
  • Arbitrary mesh motion using ALE (Arbitrary Lagrangian-Eulerian) method
In this tutorial you will do the following:
  • Analyze the problem
  • Start AcuConsole and create a simulation database
  • Set general problem parameters
  • Set solution strategy parameters
  • Import the geometry for the Wigley-hull
  • Create a volume group and apply volume parameters
  • Create surface groups and apply surface parameters
  • Set global meshing parameters
  • Set mesh extrusion and periodic boundary conditions
  • Generate the mesh
  • Run AcuSolve
  • Monitor the solution with AcuProbe
  • Post-processing the nodal output with AcuFieldView

Prerequisites

You should have already run through the introductory tutorial, ACU-T: 2000 Turbulent Flow in a Mixing Elbow. It is assumed that you have some familiarity with AcuConsole, AcuSolve, and AcuFieldView. You will also need access to a licensed version of AcuSolve.

Prior to running through this tutorial, copy AcuConsole_tutorial_inputs.zip from <Altair_installation_directory>\hwcfdsolvers\acusolve\win64\model_files\tutorials\AcuSolve to a local directory. Extract wigley_hull.x_t and wave.c from AcuConsole_tutorial_inputs.zip.

The color of objects shown in the modeling window in this tutorial and those displayed on your screen may differ. The default color scheme in AcuConsole is "random," in which colors are randomly assigned to groups as they are created. In addition, this tutorial was developed on Windows. If you are running this tutorial on a different operating system, you may notice a slight difference between the images displayed on your screen and the images shown in the tutorial.

Analyze the Problem

The problem to be addressed in this tutorial is shown in Figure 1. It is a mid-section of a Wigley Ship model. Wigley hulls have been widely used as test cases for evaluating hydrodynamic behavior of ships. The present tutorial demonstrates the simulation of gravity waves hitting a static Wigley hull (a hypothetical situation of a ship anchored in sea). Since the motion considered in this tutorial is perpendicular to the length of the ship, an analysis of a 2D section of the ship hull would be appropriate with lesser computation time without compromising on accuracy. The mid-section dimensions of the Wigley hull is a function of total ship length and the model used in this tutorial is the mid-section of Wigley hull whose ship length is 1 m.


Figure 1. Schematic of a Ship Hull

Generation of Surface Gravity Waves

Gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the ocean, which gives rise to wind waves [1]. The mechanism of surface gravity waves is that a fluctuation causes water to rise above the equilibrium surface level, gravity pulls it back down because water is heavier than air, inertia acquired during the falling movement causes water to penetrate below its level of equilibrium and a bouncing motion results. The oscillation is similar to that of a spring that has been stretched and released. The ‘spring’ action in a surface water wave is the gravity, hence the name of surface gravity wave [2]. In the present simulation, wind-generated gravity waves on the free surface of the sea are generated using a UDF (User-Defined Function).


Figure 2. Gravity Waves

Figure 2 depicts parameters that define a simple, progressive gravity wave. This wave can be modeled in the form of the sinusoidal wave profile, shown below.

(1) u = U 0 + U cosh k ( D − z ) sinh ( k D ) cos ( ω t − k x )

where

u is the horizontal particle velocity of wave

U 0 is the speed of the wave

U is the velocity amplitude of disturbance

k is wave number = 2 π / λ

λ is the wave length of the wave

D is the depth of the water

ω is the frequency of the wave = 2 π T

T is the time period of the wave

t is the time

In the present simulation we use the following values for the variables of above equation:

U = 0.1256 m/s

T = 1.0 sec

k = 12.566 m-1

U 0 = 0.01 m/s

D = 0.5 m

In the present tutorial, at the inlet you will generate the wave for 2 seconds and simulate the motion of the wave for 5 seconds. A UDF (wave.c) written in C language is used for this purpose. For the details of the functions used in the wave.c, refer to the AcuSolve User-Defined Functions Manual.

Two Dimensional Simulations in AcuSolve

AcuSolve does not support a CFD analysis on a 2D surface mesh. However a 2D analysis can be simulated on a volume mesh by having a single element extrusion along the perpendicular direction of 2D surface of interest and then having identical boundary conditions of symmetry or slip on both sides of extrusion. This way you can ensure that the solution does not vary along the thickness (extrusion), which is essentially a 2D representation of the problem. The present tutorial uses this approach.

Define the Simulation Parameters

Start AcuConsole and Create the Simulation Database

In this tutorial, you will begin by creating a database, populating the geometry-independent settings, loading the geometry, creating groups, setting group attributes, adding geometry components to groups, and assigning mesh controls and boundary conditions to the groups. Next you will generate a mesh and run AcuSolve to solve for the number of time steps specified. Finally, you will visualize some characteristics of the results using AcuFieldView.

  1. Start AcuConsole from the Windows Start menu by clicking Start > All Programs > Altair <version> > AcuConsole.
  2. Click the File menu, then click New to open the New data base dialog.
    Note: You can also open the New data base dialog by clicking on the toolbar.
  3. Browse to the location that you would like to use as your working directory.
    This directory is where all files related to the simulation will be stored. The AcuConsole database file (.acs) is stored in this directory. Once the mesh and solution are created, additional files and directories will be created within this directory.
  4. Create a new directory in this location. Name it Ship_hull_static and open this directory.
  5. Enter Ship_hull_static as the file name for the database, or choose any name of your preference.
    Note: In order for other applications to be able to read the files written by AcuConsole, the database path and name should not include spaces.
  6. Click Save to create the database.

Set General Simulation Parameters

In next steps you will set attributes that apply globally to the simulation. To simplify this task, you will use the BAS filter in the Data Tree Manager. This filter reduces the number of items shown in the Data Tree to make navigation of the entries easier.

The general attributes that you will set for this tutorial are for turbulent flow, transient analysis, and mesh type as arbitrary mesh movement (ALE).

  1. Click BAS in the Data Tree Manager to switch to basic view in the Data Tree.


    Figure 3.
  2. Expand the Global Data Tree item.
  3. Double-click Problem Description to open the Problem Description detail panel.
  4. Enter AcuSolve Tutorial as the Title for this case.
  5. Enter Ship hull motion as the Sub title for this case.
  6. Click the Analysis type drop down menu and select Transient.
  7. Change the Turbulence equation to Spalart Allmaras.
  8. Set the Mesh type to Arbitrary Mesh Movement (ALE).


    Figure 4.

Set Solution Strategy Attributes

  1. Double-click Auto Solution Strategy to open the Auto Solution Strategy detail panel.
  2. Ensure the Analysis type is set to Transient.
  3. Set the Max time steps to 100.
  4. Set the Initial time increment to 0.05 seconds.
  5. Change the Max stagger iterations to 4.
    Stagger iterations define how many iterations will be performed within each time step. Changing the maximum stagger iterations to 4 means that AcuSolve will perform a maximum of four iterations at every time step whether convergence is achieved or not. Setting the minimum stagger iterations to 0 indicates that there is no minimum number of iterations within a time step. In this case, AcuSolve will proceed to the next time step when it has either reached the desired convergence tolerance or the maximum number of stagger iterations within the step.
  6. Check that the Relaxation factor is set to 0.0.
    When solving transient solutions, the relaxation factor should be set to zero. A non-zero relaxation factor causes incremental updates of the solution, which will impact the time accuracy of the solution for transient cases.


    Figure 5.

Set Material Model Parameters

AcuConsole has three pre-defined materials, Air, Aluminum and Water, with standard parameters defined. In the next steps you will check the material properties of the predefined Water to match the desired properties for this problem.


Figure 6.
  1. Double-click Material Model in the Data Tree to expand it.


    Figure 7.
  2. Double-click Water in the Data Tree to open the Water detail panel.

    The material type water air is Fluid. Fluid is the default material type for any new material created in AcuConsole.

  3. Click the Density tab. The density of water is 1000.0 kg/m3.
  4. Click the Viscosity tab. The viscosity of water is 0.001 kg/m – sec.
  5. Save the database to create a backup of your settings. This can be achieved with any of the following methods.
    • Click the File menu, then click Save.
    • Click on the toolbar.
    • Click Ctrl+S.
    Note: Changes made in AcuConsole are saved into the database file (.acs) as they are made. A save operation copies the database to a backup file, which can be used to reload the database from that saved state in the event that you do not want to commit future changes.

Import the Geometry and Define the Model

Import the Geometry

You will import the geometry in the next part of this tutorial. You will need to know the location of wigley_hull.x_t in order to complete these steps. This file contains information about the geometry in Parasolid ASCII format.
  1. Click File > Import.
  2. Browse to the directory containing wigley_hull.x_t.
  3. Change the file name filter to Parasolid File (*.x_t *.xmt *X_T …).
  4. Select wigley_hull.x_t and click Open to open the Import Geometry dialog.


    Figure 8.

    For this tutorial, the default values for the Import Geometry dialog are used to load the geometry. If you have previously used AcuConsole, be sure that any settings that you might have altered are manually changed to match the default values shown in the figure. With the default settings, volumes from the CAD model are added to a default volume group. Surfaces from the CAD model are added to a default surface group. You will work with groups later in this tutorial to create new groups, set flow parameters, add geometric components, and set meshing parameters.

  5. Click Ok to complete the geometry import.


    Figure 9.
  6. Rotate the visualization to view the entire model.

Set Body Force

As discussed in the section Analyze the Problem, gravity is the important aspect of the simulation. In AcuConsole it is defined as the Body Force of standard Gravity (g = 9.81 m/s2) along the Z-axis is applied to the model.
  1. In the Data Tree, double-click Body Force to expand it.
  2. Double-click Gravity to open the detail panel.
    The Medium for Gravity is Fluid. The Gravity defined here is applicable only on material models whose material type is Fluid.
  3. Next to the Gravity field, click Open Array.
  4. In the X-components and Y-components fields, enter 0.
  5. In the Z-components field, enter 9.81 m/s2
  6. Click OK to complete the definition of Gravity.


    Figure 10.
    Note: The definition of Gravity here will have no effect on the simulation unless it assigned to a Volume in the model.

Apply Volume Attributes

When the geometry was imported into AcuConsole, all volumes were placed into the "default" volume container.

In the next steps you will rename the default volume group container, set the material for that group and set mesh motion for the fluid volume.

  1. Minimize Global in the Data Tree Manager and expand the Model tree item by clicking .
  2. Expand the Volumes tree item.
  3. Expand Volumes. Toggle the display of the default volume container by clicking and next to the volume name.
    Note: You may not see any change when toggling the display if Surfaces are being displayed, as surfaces and volumes may overlap.
  4. Create a new volume group for the solid ship hull.
    1. Right-click on Volumes.
    2. Click New.
  5. Rename the new volume group to Guide_Vol_Ship.
  6. Add the ship hull component in the geometry to this group.
    1. Right-click Guide_Vol_Ship.
    2. Click Add to.
    3. Click the ship hull portion of the geometry in the modeling window.
      If you rotate the view by Ctrl+ left-clicking, you can see that only the ship volume is highlighted.


      Figure 11.
    4. Click Done to add the selected volume to the solid volume group.
  7. Set the medium for the volume to None.
    The material model of the ship hull is inappropriate to the present simulation. Hence, the medium is set to None.
    Note: The element sets of the solid volume are necessary in the pre-processing stage (AcuPrep) of the simulation for the evaluation of normal directions for the guide surfaces. However the element sets are not necessary during the solver module because the only interaction between the fluid and solid is at the guide surface. The use of None for the Medium of this volume ensures that no elements of this volume are carried over to the solver, thus saving the computational time.
    1. Expand the Guide_Vol_Ship volume group in the tree.
    2. Double-click Element Set to open the Element Set detail panel.
    3. Change the Medium to None.
    4. Check that the Mesh motion is set to None.

When the geometry was loaded into AcuConsole, all geometry volumes were placed in the default volume group container. In the previous steps, you selected a geometry volume to be added to Guide_Vol_Ship container that you created. At this point, all that is left in the default volume group is the fluid volume. Rather than create a new container, add the fluid volume in the geometry to it, and then delete the default volume container, you can rename the container and modify the attributes for this group.

  1. In the Data Tree, right-click on default and rename it to Fluid.
  2. Set up the Fluid volume element set.
    1. Expand the Fluid volume group in the tree.
    2. Double click Element Set under Fluid to open it in the detail panel.
    3. Ensure that the Medium for the volume is set to Fluid. If not, change it to Fluid.
    4. Change the Material model to Water.
    5. Change the Body force to Gravity.

Create Surface Groups and Apply Surface Parameters

Surface groups are containers used for storing information about a surface, including solution and meshing parameters, and the corresponding surface in the geometry that the parameters will apply to.

In the next steps you will define surface groups, assign the appropriate settings for the different characteristics of the problem, and add surfaces to the group containers.

In the process of setting up a simulation, you need to move into different panels for setting up the boundary conditions, mesh parameters, and so on, which can sometimes be cumbersome, especially for models with too many surfaces. To make it easier, less error prone, and to save time, two new dialogs are provided in AcuConsole. Use the Volume Manager and Surface Manager to verify and provide the information for all surface or volume entities at once. In this section some features of Surface Manager are exploited.

  1. Turn-off display for Volumes by right-clicking Volumes and selecting Display off .
  2. Right-click Surfaces in the Data Tree and select Surface Manager.
  3. In the Surface Manager dialog, click New eight times to create eight new surface groups.
  4. Turn off the display for all surfaces except for the default surface.
  5. Rename the default surface to Hull_guide.
  6. Rename Surface 1 through Surface 8 and set the Simple BC Type and Simple BC Active options according to the figure below.


    Figure 12.
  7. Assign the surface for Outlet.
    1. In the Surface Manager, next to Outlet, click Add to.
    2. Select the surface of the Fluid with maximum X-coordinate.
    3. Click Done.


    Figure 13.
  8. Similarly, add the following surface to the appropriate surface groups.
    • Bottom: Surface with maximum Z-coordinate (Bottom surface of water)
    • Top: Two surfaces at minimum Z-coordinate (Top surfaces of water)
    • Side1: Surface with maximum Y-Coordinate
    • Side2: Surface with minimum Y-Coordinate
    • Inlet: Surface with minimum X-Coordinate
    • No_Bc: 5 surfaces of the Guide_Vol_Ship that are not in contact with Fluid as shown below. Surfaces shown in gray belong to the No_Bc surface set.


      Figure 14.
  9. Assign the surface for Guide_surf.
    Guide_surf is the surface that belongs to the Guide_Vol_ship and is in contact with Fluid.
    1. In the Surface Manager, next to Guide_surf, click Add to.
    2. Select all of the surfaces that belong to the Guide_vol_ship and are in contact with Fluid.


      Figure 15.
  10. Assign the surface for Hull_guide.
    Hull_guide is the surface that belongs to the fluid and is in contact with the Guide_Vol_Ship. At this point, all the surfaces present in the default Surface set are eligible for Hull_Guide.
    1. In the Surface Manager, next to Hull_guide, click Add to.
    2. Select all of the surfaces present in the default surface set.


      Figure 16.
  11. Clear the empty surface sets.
    1. In the Data Tree, right-click Surfaces and select Purge.
  12. Make sure that all the surfaces in Hull_guide belong to Fluid Volume and all surfaces in Guide_surf belong to Guide_Vol_ship Volume.
    1. Under Surfaces, right-click Hull_guide and select Info.
    2. In the Information Window, check that Parent Volume is Fluid.
    3. Under Surfaces, right-click Guide_surf and select info.
    4. In the Information Window, check that Parent Volume is Guide_Vol_ship for all the surfaces in Guide_surf.
  13. Close the Surface Manager.

Side1 and Side2

The present simulation is a 2D representation of a ship in water. Hence it is appropriate to set the Side1 and Side2 with the slip boundary condition to simulate that effect.

  1. In the Data Tree, expand Side1.
  2. Double-click Simple Boundary Condition to open the Simple Boundary Condition detail panel.
  3. Check that the Type is Slip.
  4. Change the Mesh displacement BC type to Slip.
  5. Repeat the settings for the surface Side2.

AcuConsole has a special feature named Propagate that can speed up the process of simulation setup and save time. This feature copies the attributes (it may be Simple Boundary Condition, Surface Output, Surface Mesh Attributes, Element Set, Volume Mesh Attributes, and so on) set for one surface set or volume set to another surface set or volume set. For example, in a simulation model if there are 10 surface sets with simple boundary condition set to Slip, then you can use this feature. You need to manually set the boundary condition for one surface and use the Propagate feature for all other surfaces.

  1. Under Side1, right-click Simple Boundary Condition and select Propagate.
  2. Select the surface Side2 and click Propagate.

Top

The Top surface is the top surface of water which is in contact with air and hence Free Surface is the appropriate boundary condition.
Note: As the name suggest, the free surface is a surface of the fluid which is not constrained by any physical boundary. This type of the boundary condition imposes normal component of mesh velocity to the flow velocity at this surface.
  1. In the Data Tree, expand Top.
  2. Double-click Simple Boundary Condition to open the Simple Boundary Condition detail panel.
  3. Check that the Type is Free Surface.
  4. Check that the Surface tension model is set to None.
    Note: Surface tension model is the user-given model of surface tension defined under Global > Surface Tension Model. Since the surface tension is not modelled in this simulation, this parameter is set to None.
  5. Check that the Contact angle model is set to None.
    Note: Contact angle model is the user-given model of contact angle defined under Global > Contact Angle Model. The contact angle model is used in conjunction with the surface tension model. Since the surface tension is not modelled in this simulation, this parameter is set to None.
  6. Check that the Pressure is set to 0.
  7. Check that the Pressure loss factor is set to 0.
    Note: Use of Pressure loss factor (k) would add the following term to the pressure term

    1 2 sgn ( u ⋅ n ) k ρ ( u ⋅ n ) 2

    where

    sgn ( u . n ) = -1 for the inflow

    sgn ( u . n ) =1 for the outflow

    k = Pressure loss factor

    ρ = density of fluid

    u = velocity of fluid

    n = outward pointing normal of the surface

    The higher the value of pressure loss factor, stiffer the free surface behaves, that is, lesser the displacement of the free surface.

Bottom

  1. In the Data Tree, expand Bottom.
  2. Double-click Simple Boundary Condition to open the Simple Boundary Condition detail panel.
  3. Check that the Type is Slip.
    A simple boundary condition type Slip imposes zero nodal boundary condition on velocity normal to the given surface.
  4. Change the Mesh displacement BC type to Fixed.
    The Bottom surface is a stationary surface and a Mesh displacement BC type Fixed imposes zero mesh displacement with respect to the surface.

Outlet

  1. In the Data Tree, expand Outlet.
  2. Double-click Simple Boundary Condition to open the Simple Boundary Condition detail panel.
  3. Check that the Type is Outflow.
  4. Use the default values for Pressure and Pressure loss factor.
  5. Change Hydrostatic pressure to On.
    The pressure is not constant throughout the Outlet surface. The pressure at this surface varies along Z- axis because of gravity (Hydrostatic pressure variation). In general, when setting Hydrostatic pressure to On, the pressure varies as given below

    ρ ( z − z 0 ) ⋅ g

    where

    ρ = density of fluid

    z = coordinate vector of the point on the surface

    z0 = coordinate vector where the hydrostatic pressure is zero. Z0 defined below using Hydrostatic pressure origin

    g = gravity vector

  6. Next to Hydrostatic pressure origin, click Open Array to define the pressure origin.
  7. Provide the coordinates of origin (0, 0, 0) in the Array Editor.
    Note: Hydrostatic pressure will be zero on Free surface (that is, the Top surface). The point (0, 0, 0) is on the Top surface. In particular, any point on the Top surface can be chosen as Hydrostatic pressure origin.

During the simulation there will be certain time points (particularly when trough is formed at the outlet surface) at which the flow enters the domain through certain portion of outlet surface, which is called back flow. Back flow may lead to instability temperature, turbulence variables. Enabling Back flow conditions allows nodal boundary conditions to be specified for these variables only on nodes where there is flow re-entering the domain. Assuming the outlet is sufficiently far away from ship hull, eddy viscosity value can set as that of the Inlet, for example, 1e-05

  1. Set Back flow conditions to On.
  2. Set the Eddy viscosity back flow type to Value.
  3. Set Eddy viscosity to 1e-05.
  4. Change the Mesh displacement BC type to Slip.
    Setting the Mesh displacement BC type to Slip will allow the nodes on this surface (Outlet) to move freely along the surface.

No_Bc

The No_Bc surface set contains the surfaces of Guide_Vol_Ship which do not participate in actual simulation. Hence it is appropriate to disable the Boundary condition for this surface.

  1. In the Data Tree, expand No_Bc.
  2. Uncheck the Simple Boundary Condition check box.

Guide_surf

This surface belonging to the Guide_Vol_Ship will remain stationary in the present simulation and provide as a guide for the fluid around. Hence we define it as Guide Surface with no mesh motion.

  1. In the Data Tree, expand Guide_surf.
  2. Uncheck the Simple Boundary Condition check box.
    Note: This ensures that boundary condition for the Guide_surf surface is not defined by using Simple Boundary Conditions. The boundary condition will be defined as a Guide surface using the following steps.
  3. Click ALL in the Data Tree Manager.
  4. Under the Guide_Surf surface, check the box next to Guide Surface.
    The Guide Surface type will ensure that Guide_Surf surface can be used to guide mesh nodes of Hull_guide surface. Refer to the Hull_guide surface attributes defined later in this tutorial.
  5. Make sure that Mesh motion is set to None.
    This ensures that the Guide_surf surface is static and has no motion.

Hull_guide

  1. In the Data Tree, expand Hull_guide.
  2. Double-click Simple Boundary Condition to open the Simple Boundary Condition detail panel.
  3. Check that the Type is Wall.
  4. Change Wall velocity type to Match Mesh Velocity.
  5. Change the Mesh displacement BC type to Guide_Surface.
  6. Change Guide_surface from None to Guide_surf.
    This allows the mesh nodes of Hull_guide surface to slip along Guide_surf surface.

Inlet

At the Inlet, you will provide the horizontal velocity of the gravity waves given by Equation 1. This boundary condition at the inlet will be defined using Nodal Boundary conditions with UDF.

  1. In the Data Tree, expand Inlet.
  2. Double-click Simple Boundary Condition to open the Simple Boundary Condition detail panel.
  3. Check that the Type is Inflow.
  4. Change the Mesh displacement BC type to Slip.
    Note: Setting the Mesh displacement BC type to Slip will allow the nodes on the Inlet surface to move freely along the surface.
  5. Check that the Turbulence input type is set to Direct.
  6. Change the Eddy viscosity to 1e-5.
    Note: X, Y, Z velocities in the Simple Boundary Condition detail panel can be left to default values of zero, because these values will be overwritten with Nodal Boundary conditions below. In case of conflict of inputs, Nodal Boundary conditions have higher precedence than Simple Boundary conditions.
  7. Change the filter to ALL in the Data Tree Manager
  8. Under Inlet, expand Advanced Options then expand Nodal Boundary Conditions.
  9. Check the box adjacent to X-Velocity.
  10. Change Type to User Function.
  11. For User function name, enter usrWaveHorizontal.
  12. Next to User function values, click Open Array.
  13. In the Array Editor, click Add five times.
  14. Enter the values as shown in the figure below:


    Figure 17.

    The values provided above are the ones described in the section Analyze the Problem. The user values should be provided in the same order as shown above, because these values will be passed on to the UDF script which refers these values in that specific order.

  15. Click OK.

Define the Periodic Boundaries

The present simulation is a 2D representation of infinite ship model. So the solution should be periodic on the surfaces Side1 and Side2. The following steps define the periodic boundary conditions.
Note: The following steps will only ensure that the mesh is periodic. The definition of periodic boundary conditions for particular variables has to be made separately.
  1. In the Data Tree, under Model, right click on Periodics and select New.
  2. Rename Periodics 1 to Side1-Side2.
  3. Right click on Side1-Side2 and select Define.
  4. In the Periodics BC dialog, select the surfaces Side1 and Side2, respectively.
  5. Verify that Type is Translational.
  6. Set the Y-Offset to -0.01 m (which is the distance between Side1 and Side2).


    Figure 18.
    Transformation information should be provided so that the Side 1 surface after transformation matches the Side 2 surface. For the present case, Side 1 should be translated along (-Y) axis for a distance of 0.01m. Hence the Type as Translational and the Y-Offset as -0.01m.
  7. Click OK.

Check Periodic Boundary Conditions

The following steps define the periodic boundary conditions for various variables for Side1 and Side2.

  1. Click BAS in the Data Tree Manager to switch to basic view in the Data Tree.
  2. Under Model, expand Periodics and expand Side1-Side2.
  3. Make sure that the Periodic Boundary Condition box is checked On.
  4. Double-click Periodic Boundary Condition and verify that Type is set to Periodic and Active Type is set to Always.


    Figure 19.

Set Nodal Output Frequency

The Nodal Output Frequency determines at what frequency or time interval the solution results would be stored to be used for post processing within AcuFieldView.

  1. In the Data Tree, under Global, double-click Output.
  2. Double-click Nodal Output to open the detail panel.
  3. Enter 2 as the Time step frequency.
    This value indicates that AcuSolve should write results every 2 time steps.
  4. Turn Output initial condition on.
    This indicates that AcuSolve writes initial conditions.
  5. Save the database.

Compile the UDF

A UDF in the form of C language (wave.c) is provided with the tutorial. This C program should be compiled using the following steps.

  1. To compile the UDF on Windows:
    1. Start AcuConsole Command Prompt from the Windows Start menu by clicking Start > All Programs > Altair Hyperworks <version> > AcuSolve > AcuSolve Cmd Prompt.
    2. Use the cd command to change the directory to the current working directory.
    3. Enter the command acuMakeDll –src wave.c.
  2. To compile the UDF on Linux:
    1. In the terminal, use the cd command to change the directory to the current working directory.
    2. Enter the command acuMakeLib -src wave.c.
A set of files necessary for the use of UDF are created.

Assign Mesh Controls

Set Global Mesh Parameters

Now that the simulation characteristics have been set for the whole problem, the next step is to generate the mesh.

Global mesh attributes are the meshing parameters applied to the model as a whole without reference to a specific geometric volume, surface, edge, or point. Local mesh attributes are used to create mesh generation controls for specific geometry components of the model.

In the next steps you will set the global mesh attributes.

  1. Click MSH in the Data Tree Manager to filter the settings in the Data Tree to show only the controls related to meshing.
  2. Expand the Global Data Tree item.
  3. Double-click Global Mesh Attributes to open the Global Mesh Attributes detail panel.
  4. Change the Mesh size type to Absolute.
  5. Enter 0.01 m for the Absolute mesh size.


    Figure 20.

Define Mesh Extrusion

The present simulation is equivalent to a 2D representation of the model. In AcuSolve, 2D models are simulated by having just one element across the thickness. In this case, there has to be only one element between Side1 and Side2. This can achieved with mesh extrusion process. Since you are using identical boundary conditions on Side1 and Side2, using one element between them will ensure that there is no variation in nodal values across thickness and hence ensuring the simulation of a 2D model. In the following steps you will define the process of extrusion of the mesh from Side1 to Side2.

  1. Expand the Model Data Tree item and right-click Mesh Extrusions.
  2. Select New from the context menu to create a new entity, Mesh Extrusion 1.
  3. Rename Mesh Extrusion 1 as Side1-Side2.
  4. Right-click Side1-Side2 and select Define from the context menu.
  5. In the Mesh Extrusion dialog, make the following settings.
    1. Use the drop down arrows to select the surfaces for Side 1 and Side 2 as Side1 and Side2, respectively.
    2. Ensure that the Extrusion type is set to Number of layers.
    3. Set Number of layers equal to 1.
    4. Set Extrusion options to All tets.
    Use the following figure for reference.


  6. Click OK to close the dialog.

Generate the Mesh

In the next steps you will generate the mesh that will be used when computing a solution for the problem.

  1. Click on the toolbar to open the Launch AcuMeshSim dialog.
    For this case, the default settings will be used.
  2. Click Ok to begin meshing.

    During meshing an AcuTail window opens. Meshing progress is reported in this window. A summary of the meshing process indicates that the mesh has been generated.



    Figure 21.
    Note: The actual number of nodes and elements, and memory usage may vary slightly from machine to machine.
  3. Visualize the mesh in the modeling window. Turn on the display of surfaces, and set the display type to solid and wire.


    Figure 22.

Split Nodes

At this point, the Hull_guide surface has all nodes that are attached to Fluid. A duplicate set of nodes has to be created, so that one set of the nodes follow the Fluid motion and another set stays attached to the surface Guide_surf. The following steps illustrate the process of splitting the nodes.

In the Data Tree, under Surfaces, right-click Hull_guide and select Mesh Op. > Split internal faces.
There is an increase in the number of nodes.

Compute the Solution and Review the Results

Run AcuSolve

In the next steps you will launch AcuSolve to compute the solution for this case.

  1. Click on the toolbar to open the Launch AcuSolve dialog.
    For this case, the default values will be used.

    AcuSolve will run using four processors (if available, higher number of processors may be specified) and AcuConsole will generate AcuSolve input files and will launch AcuSolve. AcuSolve will calculate the transient solution for this problem.

  2. Click Ok to start the solution process.

    While computing the solution, an AcuTail window opens. Solution progress is reported in this window. A summary of the solution process indicates that the run has been completed.

    The information provided in the summary is based on the number of processors used by AcuSolve. If you use a different number of processors than indicated in this tutorial, the summary for your run may be slightly different than the summary shown.



    Figure 23.
  3. Close the AcuTail window and save the database to create a backup of your settings.

Monitor the Solution with AcuProbe

AcuProbe can be used to monitor various variables over solution time. In the present simulation it is worthwhile to monitor the forces on the ship hull.

  1. Open AcuProbe by clicking on the toolbar.
  2. In the Data Tree on the left, expand Surface Output > Hull_guide > Forces and Moments.
  3. Right-click on x_wall_shear_stress and select Plot.
    Note: You might need to click on the toolbar in order to properly display the plot.
  4. Similarly, right-click on z_wall_shear_stress and click on Plot.


    Figure 24.
  5. You can also save the plots as an image.
    1. From the AcuProbe dialog, click File > Save.
    2. Enter a name for the image and click Save.
  6. The time series data of the variables can also be exported as a text file for further post-processing.
    1. Right-click on the variable that you want to export and click Export.
    2. Enter a File name and choose .txt for the Save as type.
    3. Click Save.

View Results with AcuFieldView

Now that a solution has been calculated, you are ready to view the flow field using AcuFieldView. AcuFieldView is a third-party post-processing tool that is tightly integrated to AcuSolve. AcuFieldView can be started directly from AcuConsole, or it can be started from the Start menu, or from a command line. In this tutorial you will start AcuFieldView from AcuConsole after the solution is calculated by AcuSolve.

In the following steps you will start AcuFieldView and create an animation of the ship hull motion with the contours of z-mesh displacement.

Start AcuFieldView

  1. Click on the AcuConsole toolbar to open the Launch AcuFieldView dialog.
  2. Click Ok to start AcuFieldView.
    When you start AcuFieldView from AcuConsole, the results from the last time step of the solution that were written to disk will be loaded for post-processing.

View Water Displacement Around the Ship

These steps are provided with the assumption that you are able to manipulate the view in AcuFieldView to have a white background, perspective turned off, outlines turned off, and the viewing direction set to +z. If you are unfamiliar with basic AcuFieldView operations, refer to Manipulate the Model View in AcuFieldView.

  1. Orient the geometry so you can see the Top surface clearly, as shown in the figure below.


    Figure 25.
  2. In the Boundary Surface dialog, uncheck Show Mesh.
  3. Select z-mesh-displacement as the scalar function and click Calculate.
  4. In the Legend tab, click Show Legend.
  5. Change the color to black.
    Note: You can move the legend using Shift + left-click.
  6. From the Colormap tab, turn on Local.
  7. Click Tools > Flipbook Build Mode and click OK .
  8. Click Tools > Transient Data and move the slider all the way back to reflect the first time step data on the boundary surfaces.
  9. Click Build to build the animation.
  10. Once the animation is built, click Frame Rate and change it to 0.1.
  11. Save the animation as Z-Displacement.

Summary

In this tutorial, you worked through a basic workflow to set up a static ship-hull simulation with surface gravity waves. Once the case was set up, you generated a mesh and obtained a solution using AcuSolve. Results were post-processed in AcuFieldView to allow you to create an animation of the free surface movement with time. New features introduced in this tutorial include:
  • User Defined Function (UDF) for surface gravity wave generation
  • Mesh extrusions and periodic boundary conditions
  • ALE based mesh motion approach
  • Use of the Surface Manager to apply surface attributes
  • Use of hydrostatic pressure as a boundary condition
  • Free surface and Guide surface capabilities in AcuSolve