Initial and boundary conditions

Once the geometry of the control domain is defined, we have to proceed with setting the boundary conditions of the problem (access the Conditions menu as shown in example 1, Figure 10). The conditions to be applied in this example are:

a)      Velocity Field [lines]

b)      Fixed Velocity [lines]

c)      Pressure Field [lines]

a) Velocity Field

This condition is used to impose the velocity on a line equal to its value given in the corresponding Material (Fluid) window (fields VelX_Field, VelY_Field and VelZ_Field).

Any of the above fields can be a time dependant function and thus the Velocity Field condition can be used to specify a variable inflow. In order to do this, the corresponding Fix_Field flag has to be marked.

It is also possible to fix the Velocity (during the run) to the initial value of the function given in the corresponding General Data option. In order to do this, the corresponding Fix_Initial flag has to be marked.

For this example this condition is going to be assigned to the inflow line of the channel. Coincidentally, this happens to be similar as the Fixed Velocity condition used in the cavity flow example, the difference here is that the fluid now enters through this surface, whereas in the previous case it was only tangent to the surface to which the condition was assigned.

The options to be selected for the inflow surface are shown in Figure 46.

Figure 46 Initial Velocity Line condition

Please note that the above condition will not work with the Start_up_control option (if Fix_Field flags are marked). Then this option must be switched off. It can be done through the Other Data window of the Data > Problem Data menu.

b) Fix Velocity [line]

As mentioned in example 1, this condition is used to impose the velocity on a line. For this example this condition is going to be assigned to the step line (see Figure 47). Both velocity components will be set to 0.0 m/s. Then, all the velocity components have to be fixed to the specified value (i.e. mark Fix_X and Fix_Y).

Figure 47 Fixed Velocity Line condition assignment

c) Pressure Field [line]

As mentioned in example 1, in order to solve the problem, the pressure must be fixed at least in one point of the control domain (taken as reference). Here we will apply the Pressure Field condition to the outflow line of the domain (see Figure 48). The value of the dynamic pressure defined in the corresponding Material (Fluid) window (p = p- rgz = 0 in our case) will be then assigned to this line (see Figure 48).

Figure 48 Pressure Field condition