How do I use grid-overlay boundaries to prescribe fully developed flow profiles at inlets?
The art of successful simulation often boils down to choosing a reasonable computational domain and meaningful boundary conditions. Modeling a domain that is too large may increase the runtime unnecessarily. Setting boundary conditions to be as close to the correct physical situation as possible is also desirable.
Grid-overlay restarts help users design simulations, which accomplish both of these goals.
Grid-overlay restarts allow users to restart a simulation and specify boundaries that overlay the original solution as overlay boundary conditions. Boundaries selected as grid overlay pick up their boundary conditions from the previous solution. These conditions—pressures, temperatures, velocities, etc—are held constant for all time during the restart computation. Therefore, to be a meaningful boundary condition the flow in the previous simulation should be steady.
A good use of grid overlay restart might be a situation where fully developed pipe flow enters a domain. We'd prefer not to model the flow in the pipe, since it adds to the runtime. Instead, we would probably simulate the pipe flow until it is steady and then restart the simulation with only a portion of the pipe in the model. The boundary conditions for the overlaid boundary would retain the fully developed profile in the pipe.
How can I compare results from different simulations?
One of the goals of simulation is to compare results obtained when particular parameters are changed. An easy way to compare results of different simulations is to use the plot overlay option available in the FLOW-PLOT and PLTFSI display utilities. Users running on UNIX and LINUX platforms will have to use PLTFSI.
In either case the first step is to open a flsgrf results file from Results, Custom. It is fairly straightforward to compare time history plots, 1D and 2D spatial plots from the same results file, since they can be directly overlaid. To overlay the two plots using PLTFSI, enter D n+m where n and m represent the plot numbers. In FLOW-PLOT you must change the option in the Control Panel from “single” to “overlay,” then click on the plots to be overlaid.
Comparing plots from two or more different flsgrf files requires an extra step. Let's say you'd like to compare the temperature history at a particular point in the domain. The simplest method is to first open an flsgrf file containing one set of results to be compared. In FLOW-PLOT, choose Files, Create, then open another flsgrf results file containing the other set of data using option 8 in PLTFSI or the Files, Open(Add) option in the Control Panel of FLOW-PLOT. This creates a combined plot file from which you can select members to be overlaid.
An alternative that is sometimes useful is to save the plots to be compared from one flsgrf results file to a temporary file. Then open the second results file and add to it the temporary file. This creates a smaller file to deal with. Of course, you could then select the plots to be compared in the combined file and save them to another temporary file. Continuing in this way it is possible to build up a file containing selected results from many separate flsgrf files.
A useful, but often overlooked feature in PLTFSI is the ability to specify the colors of individual plot lines. For example, you could set the line color for plot number 2 to red and plot number 3 to violet. To set plot attributes, enter option 10 (select plotting options), and then option 7 (select plot line attributes).