Thoughts on bouancy driven natural ventilation

Replies to This Discussion

Permalink Reply by Chris Mackey on January 4, 2015 at 3:19pm

Hi Youngjae,

I saw this post a while ago and I am sorry for responding so late.  Natural ventilation is definitely something that I am going to need for my thesis work and my part time job so I have begun implementing some basic natural ventilation stuff over the last couple of weeks.  Coincidentally, right as you made this post that included the orifice equation with Qstack, I was implementing a component that allows you to use this equation in E+.  The component (and the corresponding equation) is mostly meant for cases where you have  zones with windows that are NOT connected by an air wall (or a larger airflow network).

To use the more general natural ventilation lingo, this is for cases where you want to look at single-sided or cross ventilation within individual zones.

Over the next week, I will be implementing a component to add a chimney to a zone, which I think will serve many of your purposes that you seem to be suggesting in this post.

Eventually, we will try to implement the complete Airflow Network (AFN), which will allow you to model multi-zone airflow but this is really a huge task and is likely months away.

Let me know if you get the chance to try the new Nat Vent component on the github and I will let you know once I implement the chimney.

-Chris

Permalink Reply by yj on January 10, 2015 at 9:32am

Hi Chris, Thank you for responding. 

After this post, I did some searching and I was able to obtain an education license for simulation cfd 2015 and another cfd software called hyperworks. 

I first looked at autodesk simulation cfd 2015 and was optimistic because it had an export plugin from revit, which i use anyway for material takeoffs and etc, but found that it did not take solar radiation into account. This was a downer because I have heard that solar radiation could effect indoor airflow - convection - as much as 50 percent at a time. 

Then I searched again and found that Hyperworks, a software by altair technology can be coupled with a radiation software. So I went through the trouble of obtaining an educational license of Hyperworks. However, though some email exchange I have found that the coupling is a one-way. The radiation analysis software was used, I think, for understanding the solar loading for a SOM project called church of light. 

The support guy said : "Unfortunately our coupling with Hyperworks is really a one way coupling.  We can accept H coefficients from their software in RadTherm, but they will not read in our wall temps.  That said, it still can be a useful coupling in the sense that you can run the analysis in Hyperworks, send H coefficients to RadTherm, and run the analysis to better understand radiation and conduction.  Most importantly, that analysis can be done for longer transient analysis, but will require much less compute time and resources."

Not only did I not understand what he means by the H coefficients, my wanting to get a CFD understanding coupled with solar radiation was again, unsatisfied. In the mean while I had to finish a presentation so I haven't had the time to try to get some result on the natural ventilation. I would probably need to look into how their solutions work before I can understand if their software would "do the job"

Thank you for letting me know about your work on this. I downloaded the Honeybee_Set EP Natural Ventilation component and made sure that it is allowed, but it does not show up in grasshopper.

You pointed out that "The component (and the corresponding equation) is mostly meant for cases where you have  zones with windows that are NOT connected by an air wall (or a larger airflow network)." I wondered if you are suggesting it would be a code violation for zones to be connected by an air wall for fire safety reasons. It would be a violation I guess, like not putting an fiber insulation or some kind of smoke stop between Spandrel panels and the edge of a floor plate would be a code violation for a typical office building.

There is a project by kevin daly architects where you can see a section drawing with what seems like a cfd analysis (could be an illustration)  

it was my initial visualization/simulation goals were  for a facade design I am working on

1) an average air velocity across a zone at noon, for example, if a passive design strategy like this was used. for this I am guessing cfd is not entirely necessary. probably means that it could be used earlier in a design process, too. This would be more about user comfort.

2) at a later phase, like in detailing facade components, if airflow is indeed as expected for a zone that is connected to an air wall / chimney like feature (and to see if there is a proper mixing of air)

3) and a projection of energy savings, of course.

After seeing a video of simulation cfd I was optimistic, but like I said sim cfd does not take account of solar loading. I think I would probably go ahead start with one zone with sim cfd first, try three zones stacked on top of each other, then try hyperworks and try to factor in solar radiation.

For analyzing multiple zones on different levels, being able to add a chimney would be especially useful, I think. Having said that, I don't have a lot of experience of using honeybee except for the daylight component so it would take some time for me to understand the components. 

I hope some of the information here is useful for you. after all, both sim cfd and hyperworks are commercial softwares and somewhat different than the e plus project you are working on, I guess but still trying to address a similar problem.

so.. in cased you missed it I was asking I downloaded the Honeybee_Set EP Natural Ventilation component and made sure that it is allowed and placed in the user object foler, but it does not show up in grasshopper. what could be the reason?

Permalink Reply by yj on January 10, 2015 at 9:36am

just realized that with both sim cfd and hyperworks, this thing called "wall temperature" keeps popping up. must be important for cfd :)

so if I am in a detailing phase, it would be important for me to be able to do a radiation analysis and get a wall temperature separately for faces of the facade components facing the sun. are there any components in either honeybee or ladybug that gives wall temperature, at any chance?

Permalink Reply by Mostapha Sadeghipour Roudsari on January 12, 2015 at 3:16pm

"are there any components in either honeybee or ladybug that gives wall temperature, at any chance?"

You can use energyplus to get surface temperature for surfaces.

Permalink Reply by Chris Mackey on January 12, 2015 at 10:00pm

Hi Youngjae,

To respond quickly to your struggles with CFD, I would encourage you first (if you are going to use CFD) to use a package that is validated like Phoenics or Fluent (I believe none of the Autodesk tools are validated yet).  Unfortunately, all of the validated packages tend to cost a great deal of money with the exception of the CFD engine OpenFOAM, which is open source but currently runs exclusively on Linux.

Nearly all CFD software will be "one way" as you describe it since CFD simulations often take a long time to converge (sometimes days or even weeks depending on the size of the model).  As such, they are really only used for steady-state calculations in most standard practice.

Ultimately, for any non-steady-state analysis (or for an analysis for a whole year), you want to be using some type of quick formula like the orifice equation that you posted at the start of this discussion.  You can use a CFD simulation to ensure that your coefficients for this formula are correct and "calibrated" with a CFD study that you know is trustworthy.

As inputs to your initial CFD simulation, you will need to put in either temperature values for surfaces or heat flux values for surfaces.  You can get both of these out of Honeybee simulations that you run with an initial guess of airflow.  Just request surfaceEnergyAnalysis and surfaceTempAnalysis on the Write EP Result Parameters component and, after the simulation, use the Read EP Surface Result to get get the surface results into GH.  Use the Color Surfaces by EP Result component to visualize surface temperature differences.

Once you get CFD air flow values for a given temperature difference between your zone and outside air, you can use the orifice equation and solve for your discharge coefficient (Cd).  You can then plug this discharge coefficient into your HB energy model and get a sense of airflow through the chimney for the whole year (see attached file for an example).  The attached file includes the Nat Vent component (now re-named the Air Flow component).  Try restarting GH if the component did not appear the first time.  Here are some screenshots of the chimney example file:

I hope that this helps and good luck!

-Chris

Attachments:

• ChimneyExample File.gh, 478 KB

Permalink Reply by yj on January 19, 2015 at 11:28am

Hi again Chris and Mostapha, I make quite a few posts in many different forums and subforums but I have to say you guys - Chris and Mostapha - are among the most helpful and prompt. amazing, thank you. I wish there is a website where people vote for the most helpful and informative forum on the web.

I am not sure how I can get a hold of fluent or phonics but thank you for explaining a good practice for using CFD i.e. to confirm the coefficients for a formula.

Its beginning to take a shape on my head how to approach the topic. I am under the impression what you are referring to as a steady state is distinct from a transient analysis, which looks at the beginning and end of a system behavior. So I understand as a steady state analysis as an analysis about a kind of a "running" mode, at least in this context. so it makes sense that there would need to be a guess about an initial air flow.

I just tried your definition and I can see the Nat Vat component and it is running without problem. However I got an error message "1. no eio file was found adjacaent to the .csv _resultFileAddress.results cannot be read back into grasshopper without this file."  from the honeybee read EP result component. 

so I double clicked the unnamed.idf file, and EP-launch popped up. 

and I clicked simulate button, and now in the folder there is an eio file. 

now I run the Honeybee_run_simulation component again and this time the read EP component works without error, and as a result I do get an operative temperature for zone graph, though mine looks different, and based in san fransisco Intl AP.

Anyways, I do get a result, I just did not know that running EP-launch from the unnamed.idf file was a part of the workflow so if anyone is trying this chimneyExample File and getting an error about a missing eio file, just double click on the idf file created by the Run_Energy_Simulation component.

About what you said Chris, " Once you get CFD air flow values for a given temperature difference between your zone and outside air, you can use the orifice equation and solve for your discharge coefficient (Cd).  You can then plug this discharge coefficient into your HB energy model and get a sense of airflow through the chimney for the whole year"

I have to try this workflow since I am preparing a CFD analysis. My only question right now is, Chris, which one of "operative temperature" and " meanRadTemperature" output from Read EP result component equivalent to temperature values for surfaces or heat flux values for surfaces ?

Thank you again! hope to share a result of coupling a CFD and Honeybee-ladybug soon, hopefully by the end of this month. might be next month because I have a different presentation at the end of this month.

Attachments:

• Screenshot 2015-01-19 12.55.25.png, 344 KB
• Screenshot 2015-01-19 13.08.59.png, 344 KB
• Screenshot 2015-01-19 13.16.47.png, 360 KB

Permalink Reply by Chris Mackey on February 3, 2015 at 3:27pm

Hi Youngjae,

Sorry for getting back so late.  The difference between steady state and transient calculations is the basis of a lot of important building science debates these days.  Notably, its responsible for a lot of the present practice's emphasis on the benefits of insulation over thermal mass.

Along this line of thinking, I would strongly recommend that you use surface flux values as inputs to your CFD simulation if you can (instead of surface temperature).  While EnergyPlus tends to view large surfaces as being at the same temperature, this is usually far from reality and it can actually lead to misleading understandings of airflow patterns since flux includes a time element of Joules/second (or Watts) instead of temperature, which is time independent.

I attached a GH file showing how to bring in surface flux data and color Zone surfaces based on these results.  You should pick an hour of the simulation that you are interested in CFD modeling and use these individual surface flux values and assign them to the surfaces in your CFD model.  Let me know if that is unclear.

Running EP-launch is not supposed to be a part of the workflow.  The reason why the simulation did not automatically run after you set "_runEnergyPlus" to True on the run simulation component is that you currently have EnergyPlus 8.2 installed and Honeybee only works with EnergyPlus 8.1 right now. Normally, we would keep everything up-to-date with the DoE but there was a particularly massive change that happened in the shift form 8.1 to 8.2, which is that all of the original FORTRAN code of E+ was translated over to C.  In the long run, this will be great since many more people know C than FORTRAN.  However, for the release of 8.2, there are a lot of bugs and certain processes that are slower than usual and so our plan is to wait for the next release to upgrade.  So, long story short, just install EnergyPlus 8.1.

Stay awesome,

-Chris

Attachments:

• ChimneyExampleFile2.gh, 507 KB