Query regarding mechanical flow drop off and zero flow

atwhaley / cfast

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Hi Rick, this is more of a query rather than a problem.

One can set a "Begin Dropoff At:" and "Zero Flow At:" pressure settings for 
when the mechanical ventilation ceases to provide a mass flow of oxygen to the 
compartment.

Does the volumetric flow rate of the mechanical ventilation decrease linearly 
with increasing pressure? 

e.g. If I have the following settings:
initial mechanical flow rate setting of 0.2m^3/s
Begin Dropoff At: 200Pa
Zero Flow At: 300Pa
Current pressure in compartment: 225Pa

Would the mechanical flow rate at that point in time be reduced by 25% to 
0.15m^3/s?

There is no literature in the technical manual about this. The only text on it 
is in the user manual on page 35: 

"Begin Drop Off Pressure (default units: Pa, default value: 200 Pa): The 
description of the fan includes a drop off in flow beginning at a pressure 
specified by the user. Above this pressure drop, the flow gradually drops to 
zero flow"

Thanks,

Sam

Original issue reported on code.google.com by samgreg4...@googlemail.com on 19 Mar 2015 at 2:27

We actually use a hyperbolic tangent to provide a smooth cutoff on both ends. Wikipedia (http://en.wikipedia.org/wiki/Hyperbolic_function) can show you the shape of the function. We just scale it so it goes from 1 at the "Begin Dropoff" value to 0 at the "Zero Flow" value. I'll put something in the upcoming User's Guide for version 7 of the model to better explain this. We also use the same function for the lower oxygen limit.

atwhaley / cfast

Query regarding mechanical flow drop off and zero flow #135