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# Using the Gas Dynamic Calculator in Flow

Wednesday November 5, 2014 at 11:40am
Do you need to know how long a tank will take to evacuate or what the Reynolds Number is for a particular flow condition? If so, and you have SOLIDWORKS Flow Simulation, then why not use the 'Gas Dynamic Calculator' to do the work for you?

SOLIDWORKS Flow Simulation is a superb tool for analysing fluids and helping you design products that require you to understand fluid flows, pressures and temperatures.

However, very few users are aware that there is a useful 'Gas Dynamic Calculator' hidden away within Flow. It allows you to calculate a range of fluid parameters where there is a known mathematical equation to calculate the fluid dynamic or thermodynamic value. You can even create your own bespoke formulae and save them for future use if you wish. Examples of calculations are dimensionless gas constants (like Reynolds, Knudsen, Stanton, Nusselt, Prandtl etc.); flow rates; pressure coefficients and even the time to evacuate a tank of pressurised fluid. The beauty is that for a range of common calculations the formula is available to be added from a library saving you the trouble of building it from scratch with the possibility (or likelihood in my case) that you get it wrong!

You can access this tool from the Flow drop down menu under Tools > Calculator.

This opens up a table much like a Excel spread sheet. It will initially be empty but you can start adding formulae by clicking on the fx icon at the top of the sheet. This opens up the library and you can browse to a host of common equations. Below I have shown the ones for the dimensionless parameters ...

You can tick the preview box and then see the formula being created. Below is the formula for a tank evacuation through a single opening ...

I would rather Flow create this for me than attempt to build it in Excel!

To prove it works, I have created a Calculator sheet for calculating the Reynolds Number of water flowing at 10m/s over a cylinder of 100 mm diameter and also the results of the tank evacuation. Both can be on the same sheet ...

For the tank evacuation I entered 25 time steps for the calculations. This shows that the air (initially under 10 bar of pressure) would take 74 secs to evacuate through a 50 mm dia opening.

Note that for a tank evacuation you must go to Edit > Run numeric calculation to see the results.

There are other little useful tools too. You can right mouse button click on a parameter name (say 'Dynamic Viscosity') and select 'Import from Engineering DB ...' This opens the database and allows you to select from the library of fluids.

Once you have run the calculations you can export the results by using the 'Report' function. This outputs the data to Excel. You can then produce graphs or manipulate the data in other ways. Below is the Calculator results for my tank evacuation with a graph to show how the pressure drops from 10 bar to 1 bar over 74 secs.

Whilst the average user may not need to do these calculations, more experienced users will find this tool helpful, especially when checking results and anticipating values for parameters like transient run times.

By Andy Fulcher

Technical Manager

Solid Solutions Management Ltd

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