Airflow measurement - again...

[Mazda Ebrahimi]

Stephen Dubovsky wrote:
> 
>   First of all, thanks for all of the replies/ideas.  Seems to have
> generated quite a stir.  As for the problems mentioned:
>   1) I had not thought of the cars acceleration affecting the sensor...  It
> could be conteracted like in a MAF sensor by using 2 sensors, one in the
> airflow, and one not, and use the differential forces to null the 'common
> mode' acceleration.
>   2) someone mentioned that a flap type meter does not measure density,
> which I disagree with.  (Moving in a vacuum produces less drag - the old
> physics class trick of dropping a feather and a hammer...) Drag is a
> function of density as some kind person dug up the equation:
> Drag=Coeff_drag*.5*air_density*air_velocity^2*projected_area
>   3) humidity.  I agree humidity affects the density also and would create
> measurement errors, but this also affects the MAF (which doesn't REALLY
> measure MAF).  I brought up this point a while ago and I believe Todd
> Knighton of Promotive Engineering (I hope I got this right;) says from 0-99%
> RH the density changes <2%.  I have not rerun the numbers (haven't had a
> chance to find some of my materials handbooks) to verify this, but Ill take
> someones word for now (he seems to know alot about other stuff, so I dont
> doubt him).
> 
>   As for LDA or other types of doppler anemometry (sp?), this also seems
> quite interesting.  Will have to look into it...
> 
> Thanks, SMD
> --
> Stephen Dubovsky
> dubovsky at vt.edu
> 
> 95 Yamaha FZR600
> 83 Porsche 911SC
> 84 Jeep Cherokee


I just finished reading 60 messages.  WOW!
I like this idea alot.  I agree that using an identical sphere outside the flow should 
negate the acceleration of the car effects.  About mass flow measurement, 

  mass flow = density * average air_velocity * projected_area

where as the drag equation is

Drag=Coeff_drag*.5*air_density*air_velocity^2*projected_area

Notice the air velocity^2 term.  If we were to rewrite the drag equation in terms of 
mass flow we would get
 
Drag = Coeff_drag*.5* mass_flow * air_velocity

It's late at night, so I might be making a mistake, but I think the above is right.  If 
that's the case, it means your measurement is mass flow * velocity, which means if we 
had the same mass flow rate at a slower velocity (may be due to a lower temp.), then our 
 reading would be lower.  Right?

Best Regards, Mazda