Folded double facade

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Permalink Reply by Salma El Ahmar on April 30, 2015 at 9:01am

Could I also take the chance to ask if it is ok to begin with to assume that the double façade is treated as one thermal zone?

I understand that energy plus assumes that air is well mixed in each thermal zone, yet it could simulate natural ventilation for a 'chimney' for example  as I see in the new Natural Ventilation component..i am trying to optimize the form of the façade to decrease air temperature inside the cavity while increasing airflow inside it as it is intended for a hot climatic context...any comments about that?

Thanks again

Permalink Reply by Chris Mackey on May 2, 2015 at 7:03pm

Hi Salma,

I do not think that the line that you pointed to was an error but I updated your file to the latest version of HB on the github and exploded your complex polysurface into individual faces and the file seems to be running well now (see attached GH file).  I question whether you need to include such detailed geometry to get an accurate result but the components can handle it if you are willing to wait a long time for the simulation to run (it took 21 minutes to run a single month on my fairly high-power machine).

For a single-zone model like this, you really do not need an airflow network and I would just use the simple bouyancy driven orifice equation available in the "Set EP Airflow" component.  Generally, I prefer this simple method in these cases because it is much easier to understand what is going on and what the assumptions are.  They also make clear that the major aspects of your "form" that will increase/decrease your airflow is the area of openings, the vertical height distance between the inlet and outlet, and the minimization of any screens or internal surfaces that can cause friction.  So you don't really need to run a simulation to design a facade with good airflow.  You just need it to make sure that it is working as you intend it.

I can confirm that assuming a double-skin facade as a single zone is a decent assumption in most cases.  However, there are a few things that you have to fix in your model to make this assumption work.  First, you need to use the glazing based on ratio component to assign glazing to your outdoor elements (I imagine that this complex facade is supposed to be glazed).  You can see this in the attached file but you need to separate out your mullion surfaces first.  Next, you need to get rid of all the lighting and internal heat gains of the zone.  You can do this easily with the EP Plenum component in the file.  Lastly, you really need to include the building zone behind the double-skin facade zone since there is probably going to be a lot of heat flow in that direction, especially if this occupied zone behind the facade is conditioned.  This tutorial walks you through setting up a multi-zone model (https://www.youtube.com/playlist?list=PLruLh1AdY-SgW4uDtNSMLeiUmA8Y...).

Finally, I would be wary of DesignBuilder's CFD engine.  It is not validated and I don't think that I would really trust it with such complex geometry.  Using something validated like OpenFOAM, Phoenics, Fluent, etc. would be much safer.

-Chris

Attachments:

• HBpinwheelfoldedfacadeinquiryCWM.gh, 651 KB

Permalink Reply by Salma El Ahmar on May 4, 2015 at 6:31am

Hello Chris,

Thank you for your quick and detailed reply!

-        It works fine now, you are right it doestake a lot of time..took 21 minutes on my laptop to run just a single day! I found that a bit strange because when I ran the same model in Archsim switching off the Airflow Network mode and working on Simple Ventilation (for wind and buoyancy) it took around 2 minutes only to run the simulation for a single day. I don’t understand why the big difference in time since both plugins use Energy Plus?

-        Do you happen to know why Airflow network simulation produce quite different results (temperature and ach) from those obtained from Simple Ventilation (both in Archsim)?

-        I agree that I do not need the Air flow Network now, since at this phase I am trying to optimize the form of the double façade (folds, opening sizes, depth, etc.) to get a balance between temperature, airflow, and daylighting needs in the office rooms. I really think heat gain, airflow, daylight (thermal and visual comfort) all influence each other and if possible should be studied together while designing building skins. The plan was to do this using Galapagos, but I am not so sure now given the long simulation time (adding to it daylight simulation too). Anyway, in a later phase I wanted to know how the airflow in the office rooms would be like…as I understand, that could not be done yet in HB right? that’s why I thought of the Airflow network in Archsim.

-        Thanks for reassuring that the double façade could be treated as a single zone. I am adjusting the model now as you said..that first trial was just to know if it works. I’ll probably use custom openings as some of them will be glazed (on some of the triangular faces, and sizes will depend on incident solar radiation on each face) while others are small air openings (on the mullions and the top horizontal face) to ventilate the cavity and avoid overheating. Also when external air temperature is within comfort range, it could be used for ventilation he office spaces.

-        About CFD in design builder, i double checked again on their website when u mentioned it, and found the report comparing Design Builder’s results to Pheonics: https://edmshk.files.wordpress.com/2014/08/designbuildercfdvalidati...

However I am not sure if this is considered and official validation, and weather it would be valid for complex geometry, not just a room in the form of a cube as in the report. Anyway I don’t think it will work out for me as I tried exporting a gbxml and when I opened it in DesignBuilder it does not understand that this is a zone…also all openings (which I added later to the file) were not exported. I tried exporting your parents' house since I’ve been working on your tutorials (great videos by the way! So much help!) and when I opened it in Design builder, not all rooms were understood as zones (especially the attic). So that rules out DB for me I guess.. I downloaded OpenFoam but turns out to be quite a challenge to use. Lots of learning awaits me!

Sorry for my long reply, and thank you for your help!