Boundaries

Fluid Body

For this example we will define the hull boundary conditions by using the Fluid Body element set.  The properties of this set are shown in Figure 52 and will be applied to the hull surface. Note that only the wetted surface of the hull must be defined as Fluid Body.

Figure 52. BOUNDARIES. Fluid Body window

It is important to remark that in general in free surface problems (NOT in the present example), a control in the stern of bodies (in the transpiration problem) has to be carried out. This control will be applied in those points of the floating line of the body where the angle between the normal and the velocity is greater that SternC Angle. The recommended values for SternC Angle vary from 45º to 70º (see Reference Manual for further information).

Free surface

The free surface properties will be assigned as shown in Figure 53 to the top surface of the volume. All the data will keep its default value, with the exception of the Length option (characteristic length of the analysis usually taken as the length between perpendiculars of the ship), which will set to the ship length value (6.0 m).

Figure 53. BOUNDARIES. Free Surface window.

Initial data

In the Initial Data window (Data>Initial Data), the field Initial Velocity X has to be set to 2.424 (this will set the velocity of the analysis to 2.424m/s, by means of the Velocity Field condition applied before).

Problem data

Once defined the boundary conditions you have to specify the data of the problem. This is done trough the Problem Data window (Data>Problem Data>PROBLEMmenu option).

It is important to remark that the recommended value of the Time Increment for free surface analyses is dt < 0.05·L/V or dt < h/V (being L a characteristic length, V the mean velocity, and h an average of the element size).

In our case the only data that have to be changed are listed next:

Problem page (see Figure 54):

Mark Solve FluidFlow to solve the RANSE problem.

Mark Solve Transpitation in Fluid to solve the free surface problem.

Analysis page (see Figure 54):

Set Number of Steps to 600.

Set Time Increment to 0.125 ( »0.05·L/V).

Set Max Iterations to 1 (since only the steady state is of interest).

Set Initial Steps to 50 ( > 5% Number of Steps).

Set Start Up Control to Time.

Results page (see Figure 55):

Set Output Step to 50.

Other page (see Figure 55):

Mark Give Pressure Origin and set Pressure Origin OZ to 0.0.

Mark Y Plane Symmetry in Fluid and set Y Plane Symmetry Position to 0.0 to define the symmetry plane.

 

Figure 54. Problem Data window.
Problem and Analysis pages.

Note that it is strongly recommended to define Initial Steps» 5-10% Number of Steps in problems with Free Surface.

 

Figure 55. Problem Data window.
Results and Other pages.

Modules data

In Modules Data window (Data > Modules Data) it is possible to select the turbulence model to be used in the problem. For this problem selected turbulence model is K Energy Two Layers as shown in

Figure 56. Modules Data window.
Turbulence model definition.