# Use of the Continuative Mesh Boundary Condition

When there is a need to have fluid flow out one or more of the boundaries of a computational domain, the question arises: what is the right boundary condition? Several mesh boundary conditions can serve as outflow boundaries in FLOW-3D, including specified pressure, continuative and outflow.

The simplest and most commonly used outflow condition is the continuative boundary. A continuative boundary condition consists of zero normal derivatives at the boundary for all flow quantities. The zero-derivative condition is intended to represent a smooth continuation of the flow through the boundary. Therefore, using the continuative boundary condition is most suitable when the flow upstream from the boundary exhibits little variation in space (at least in the direction normal to the boundary) and time. This condition usually requires the continuative boundaries to be placed far downstream from any disturbances in the flow like constrictions or submerged bodies.

It must be stressed that the continuative boundary condition has little physical basis. Rather, it is a mathematical statement that may or may not provide the desired flow behavior. In particular, if flow is observed to enter the computational region across such a boundary, then the computations may be wrong because nothing has been specified about flow conditions existing outside the boundary. The entering flow is forced to do so from the condition of rest. This practice helps to reduce inflow and often results in a reasonable approximation of a smooth outflow condition.

Among other things, the zero-derivative condition at continuative boundaries implies that there is no acceleration or deceleration of the flow as it crosses the boundary. Therefore, the continuative boundary condition is not consistent with a significant non-zero force component normal to the boundary (for example, gravity).

The only input variable that can be defined at a continuative mesh boundary is fluid height, which can be used in free-surface type flows. However, its effect on the flow is quite limited since the flow near the boundary is not computed from the conservation equations, but rather set from the zero-derivative condition.

In short, a continuative boundary employs a very simple algorithm, but the results must be viewed with a good degree of suspicion. As a general rule, a more physically meaningful boundary condition, such as a specified pressure condition, should be used at outflow boundaries whenever possible.

## Having Troubles with FLEXlm on Windows XP?

When Service Pack 2 for Windows XP is installed on a PC acting as a FLEXlm license server, client PCs will not be able to acquire a license because the Windows firewall will block the required ports. The firewall must be configured manually to open the required ports, or to add the license server applications to the Windows Firewall Exceptions List, which will allow Windows to open the required ports automatically. Here's the soltion:

1. Stop the FLEXlm license server
a. ;At the top of the FLOW-3D interface, click Utilities / lmtools
b. In the Start/Stop/Reread tab, click the Stop Server button
2. Click on Start in the lower left corner of your desktop and select Control Panel | Security Center.
3. Select Windows Firewall.
4. On the General tab, ensure that the "On" radio button is selected and the "Don't allow exceptions" is NOT checked.
5. Click on the Exceptions tab.
6. Click on Add Programs, and browse to C:\FLOW3D\LICENSES (or the "LICENSES" directory where FLOW-3D was installed).
7. Select "lmgrd.exe" and then click Open, then OK.
8. Again, click on Add Programs, and browse to C:\FLOW3D\LICENSES (or the "LICENSES" directory where FLOW-3D was installed).
9. Select "f3dtknux.exe" and then click Open, then OK.
10. Start the FLEXlm license server.
a. At the top of the FLOW-3D interface, click Utilities / lmtools
b. In the Start/Stop/Reread tab, click the Start Server button

Clients should now be able to check out a license.