Re: [EnergyPlus_Support] Mosquito net

[David Lorenzetti <DMLorenzetti@xxxxxxx>]

Manlio,

Assuming the mosquito net is at the perimeter of the building, then
the wind speed's effect on the airflow model should already be
nonlinear in the way you describe.  Therefore you don't necessarily
need to worry about the discharge coefficient being a linear
parameter.

All my comments below are from the perspective of somebody who has
used multizone airflow models extensively.  My experience with
EnergyPlus is fairly small.  However, I do know that the
"AirflowNetwork" model implements an early version of the multizone
code I now primarily work on.  Therefore I am going to assume that
AirflowNetwork has the functionality I'll describe below.

Pressure-based multizone models account for wind by converting the
wind to a pressure on the building facade.  The equation is basically

P = Cp * rho * V^2 / 2

where {P} is the increase in pressure, compared to the hydrostatic
pressure on the building surface.  I talk a little bit more about this
wind pressure equation below, but for now, let's focus on what it
means for the airflow calculations.

In the discharge coefficient model, the flow through the building
facade (and indeed from zone to zone) is something like

Q = Cd * A * dP^n

where
** Q is the flow (generally described as a volume flow rate, but could
be a mass flow rate)
** Cd is the discharge coefficient you're asking about
** A is the area of the orifice
** dP is the pressure drop across the flow element
** n is some exponent, usually between 0.5 and 1.  The standard
practice in multizone modeling is to take n=0.65.  Possibly for
mosquito nets there's a good case to make it different.  I'm not sure
and would have to look at the literature before I'd make a definitive
statement.

The key to your question is that, for a flow path across the building
envelope, the pressure drop {dP} reflects the wind pressure.  It's not
completely set by the wind pressure, but wind pressure is in there
(along with the effects of temperatures and height and mechanical
services-- that's why it's a network calculation).

So let's say that, to a first approximation, {dP} is exactly the wind
pressure.  Then

Q = Cd * B * V^(2n)

where
** B = (A * Cp^n * rho^2 / 2^n) is all the terms I don't want to look
at right now.

So you see that the flow is not linear in the wind speed, even though
it's linear in the discharge coefficient.

Since you seem to have a sense of the shape of the wind-speed / flow
curve you want to reproduce, you may be able to choose an appropriate
value of {n} using the relation Q proportional to V^(2n) shown above.
However, I don't know whether the AirflowNetwork model in EnergyPlus
gives you the ability to change the flow exponent.

Another thing to think about is the flow area.  For mosquito nets, you
may just say that the flow area is the cross-sectional area of the
opening the net covers.

There are a couple other caveats to this modeling approach.  I know
there is some rule of thumb in the airflow literature that says if a
facade is more than something like 60% open, the wind pressure
relationship breaks down.  If you have actual data, then you may be
able to ignore this concern.

And of course the mosquito net doesn't really define a "zone" in the
usual sense.  In particular, treating the netting as a wall will
destroy any momentum effects of wind.  That is, you'll lose the real
wind-like effects on driving flow through the actual walls, windows,
and doors of the building.  This is one reason that Edwin Lee's
suggestion to use CFD is a good idea, if you have the expertise to do
so.  Also, any contaminant transport calculations related to that
netted-off zone will likely be flat-out wrong.

Finally, if you have a mosquito net inside a room, say just
surrounding a bed, then you should not bother trying to model the net
at all.  In the model, the wind speed won't have any effect inside a
room.  Furthermore, multizone models are not able to predict flows at
the room level (again, for this you would want CFD).  I assume that
you understand this, since your question made clear you're interested
in wind effects.

Back to the wind speed / pressure equation.  There is some strangeness
in there that usually takes people a little while to appreciate when
they first start using these models, and so I'll just warn you about
two of the things that you have to come to grips with.

Most of the modeling complexity is bound up in that Cp term.  It can
be positive or negative, depending on the wind direction (for example,
the upwind side of the building will have positive Cps, downwind
negative.  The AirflowNetwork model should give you some way to set
the Cp values based on wind direction.  There are tools out there for
estimating Cp, and there are some standard curves (search for "wind
pressure coefficient" online).  But estimating Cp curves (Cp versus
wind direction) is a bit of an art-- expect to not really know what
the "correct" value is.

Another bit of complexity is bound up in estimating V, the wind speed.
 Generally it is assumed to have some kind of exponential or power-law
variation, running from zero at ground level, up to some free-stream
velocity above the building.  And generally there are correlations
built in that adjust the wind speed measured at some remote location,
like an airport, to the free-stream conditions near the building.  So
again there is a bit of art here.  The main problem you see in the
multizone modeling fora is people saying "My wind speed is 3 m/s, and
so the pressure effect should be 2 Pa, but I'm only getting 1 Pa wind
effect", and what's happening is the wind speed calculated at the flow
path is less than 3 m/s.

If you want to talk about just the multizone modeling aspect of this,
feel free to contact me off the EnergyPlus discussion group.  I don't
have a lot of free time, but yours is an important and interesting
problem.

-Dave


> I'm involved in building energy simulation in Tropical Countries, mostly East African Counrties, where mosquitos are malaria vectors, so they must be stopped by using mosquito nets. Since natural ventilation is a very important issue to evaluate in order to ensure indoor comfort condition, how could i take into account for airflow reductions due to mosquito nets?
> I modeled the building using AirflowNetwork, but i couldn't find the solution. I supposed to change the Discharge Coefficient, but is not so correct because airflow reductions are not linearly proportional to the wind speed. In fact they will vary according a curve that has an higher gradient at low wind speeds. Are there any modules under development  in Energyplus for this issue?
>
> Thanks in advance
> Manlio Mazzon


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