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Units in CFD and FEA plus Free Flow Calculator Utility

During a recent Google search, I stumbled across this interesting NASA paper which determined the root cause for the loss of the Mars Climate Orbiter in 1999 was a mismatch of units. A subroutine coded for thruster data in metric units was instead fed English units, which resulted in a trajectory error.

Even with advancements in modern CFD and FEA software interfaces, the burden is still on the user to get the units right.  Back in the early days of Algor (now Autodesk Simulation Mechanical), we were required to select a system of units and perform all necessary conversions manually on the side; a tedious and error-prone process.  Today, the user can easily toggle between various units while setting up the simulation.  Although it is a major improvement in convenience, the user still needs to associate the right numerical value with the proper units at the point of entry.

For example, while working at CFdesign (now Autodesk Simulation CFD), a colleague asked me to review a conjugate heat transfer model which was not heating up as expected.  At first glance, the setup looked solid, but then I noticed that the film coefficients for natural convection were entered as 5 W/mm^2-K instead of 5 W/m^2-K; a difference of 6 orders of magnitude!  This mistake is easier to make that you might think; these 2 units just happen to be next to each other in the pull-down menu.

As CFD consultants, we are constantly dealing with unit conversions and doing quick hand calculations on flow rates and other parameters.  While there are many good online calculators and unit conversion sites, we thought we would share the free VolFlowCalc utility that we developed and use internally.  VolFlowCalc  not only permits the mixing of units, it will also lets the user set the volumetric flow rate, velocity or area as the unknown value to be calculated.



However, VolFlowCalc has the same limitation as CFD and FEA codes.  The user must associate the correct number with the proper units before hitting the Calculate button.  So slow down and double-check those units; that is one of the first places I look when results seem suspect.

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