Reducing FLOW-3D Results File Sizes
As the power and capabilities of computer hardware increases, so does the size and complexity of computer simulations. With increasing simulation size (i.e., number of computational cells) come extremely large (many gigabytes) results files. While terabyte-size hard drives are easily attainable, large results files can be troublesome. One reason is that large files are difficult to back up. Compression time increases significantly as file sizes approach gigabyte sizes while the cost of backup becomes prohibitive for uncompressed files. Another reason is that post-processing time increases significantly as file size reaches the gigabyte level. Fortunately, there are some simple things users can do to reduce results file sizes.
Selected Data for Output
Since frequent data outputs are usually necessary to generate animations with smooth motion, users will often set the Spatial Data interval to a small value. The result can be enormous (many gigabytes) files. A better solution is to set the Spatial Data interval to the Finish Time and set the Selected Data interval to 1/100th of the Finish time. The user must then select the variables they wish to have output as Selected Data. This data is selected on the Output tab of the Model building tab as shown in Figure 1.
For a fairly large simulation with approximately 8 million computational cells, the results file size is about 82 GB (using 160 Spatial Data intervals). Using Selected Data (7 output quantities) and 160 Selected Data intervals reduces the results file to 40 GB, which is a 50% reduction. The post-processing time is also reduced by approximately 50%.
Occasionally, users are interested in simulation results within a short time period but at frequent intervals. Time Edits can be used to define time intervals and the frequency of data output within that interval. Time edits must be added manually to the PREPIN file via a text editor.
Time Edit intervals can be specified on the Output tab for all types of data, including History data, Spatial data, and Selected data. To specify a Time Edit interval for any of the quantities, simply click the large button in the group box for the data type.
Let's say a user would like to see Selected Data output at a frequency of 0.1 second between t=1.4 and 1.5 seconds. This would be done by clicking the Selected Data button and then entering the values shown in Figure 2. Note that the value for the interval is entered next to the end time of the interval, not the beginning time.
Runtime compression will compress results files while they are being generated and can result in a 90% reduction in file sizes. While reductions in file sizes can be achieved using runtime compression, there can be a corresponding increase in postprocessing time. If post-processing time is an issue, consider purchasing a high-speed hard drive such as a 10,000 RPM Seagate Raptor. High-speed drives can reduce post-processing time up to 50% over standard 7,200 RPM drives.