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maartje
Posts: 4
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 Posted: Mon Jan 09, 2012 10:38 am Post subject: velocity profile |
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Dear Sir,
I'm trying to get DesignBuilder CFD approved for Green Mark assessments, however, the Singapore Building & Construction Authority has a couple of queries. It would be great if you could help me answer these.
1. More detailed technical description than the one online, and any test cases done for Green Mark assessment if available.
2. Hardware Requirement; i.e. max number of cells allowed (if any), memory requirement per 1 million cells, 64 bit compatible etc etc.
3. Other validation studies besides the ones online, to determine the numerical accuracy of this CFD software.
4. Can the user prescribe a formula to represent the inbound velocity profile?
5. How does the Cartesian grid handle challenging complex geometries?
7. Is there any development of the anisotropy turbulence model, e.g. Reynolds Stress Turbulence Model, apart from the available k-epsilon turbulence model with constant eddy viscosity.
8. It is mentioned that DesignBuilder is steady state CFD simulation, and that false time step is generally more effective than relaxation factors. Can it be further elaborated? Any further reference/published article to substantiate the argument?
9. Any treatment on density variation with temperature?
Your help is greatly appreciated. I'm meeting the BCA this Thursday to try to convince them. If anybody from Designbuilder is available in SIngapore this THursday, or some other time, to join a meeting with the BCA that would be great.
Best regards,
Maartje |
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Steve Potter
Posts: 572
Location: newcastle |
| Posted: Tue Jan 10, 2012 10:03 am Post subject: |
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Dear Maartje,
In answer to your questions:
| Quote: | | 1. More detailed technical description than the one online, and any test cases done for Green Mark assessment if available. |
You should be able to find more technical information on the CFD module using the Support->Program Help option on the website. If you select this option and search for CFD and then select the second option in the search reslts (Computational Fluid Dynamics) this should provide links to various topics concerning DB CFD. Unfortunately, we do not have any test cases for Green Mark assessment.
| Quote: | | 2. Hardware Requirement; i.e. max number of cells allowed (if any), memory requirement per 1 million cells, 64 bit compatible etc etc. |
You will be able to obtain hardware recommendations from the Knowledgebase on the website (under Hardware and OS). There are no fixed limits to grid size, just that imposed by the available machine memory. When a CFD project is created, the software automatically checks to see if there is adequate physical memory on the machine to allow the required allocation. The memory required is around 125 MB per 1 million cells. We do not currently offer a 64-bit version of DB but we will be making this option available in due course.
| Quote: | | 3. Other validation studies besides the ones online, to determine the numerical accuracy of this CFD software. |
As far as we know, the only independent validation of DB CFD that has been conducted to date is that carried out by Northumbria University here in the UK. The findings of this study may be downloaded from this website using the Downloads->Documents->Designbuilder CFD validation against Phoenics option.
| Quote: | | 4. Can the user prescribe a formula to represent the inbound velocity profile? |
Currently, there is only one wind profile used for external analysis. The profile is one recommended by ASHRAE.
| Quote: | | 5. How does the Cartesian grid handle challenging complex geometries? |
DB CFD uses a rectilinear cartesian grid and is consequently best suited to spaces exhibiting a significant degree of orthogonality. However, the software is designed to handle non-orthogonal geometry using a grid 'blocking' procedure whereby the faces of non-orthogonal objects are represented using a step-wise profile. The accuracy of this representation is of course partially dependent on the grid resolution used, i.e. the finer the grid, the better the representation.
| Quote: | | 7. Is there any development of the anisotropy turbulence model, e.g. Reynolds Stress Turbulence Model, apart from the available k-epsilon turbulence model with constant eddy viscosity. |
The only true turbulence model currently available in DB CFD is the standard k-epsilon model (with wall functions). This model has been adopted because it is generally considered the most widely used and tested and found to be the most generally applicable. We do however have plans to introduce additional turbulence models in due course.
| Quote: | | 8. It is mentioned that DesignBuilder is steady state CFD simulation, and that false time step is generally more effective than relaxation factors. Can it be further elaborated? Any further reference/published article to substantiate the argument? |
Although DB CFD is a steady-state solution, the solver has been formulated in the form of a dynamic solver because in practice the transient term has been found to be a very effective inertial relaxation factor. Instead of real time steps, the solver uses false time steps, the duration of which may be changed to affect the convergence of the solution. Conventional relaxation factors are available but have been found to be much less effective.
| Quote: | | 9. Any treatment on density variation with temperature? |
DB CFD assumes incompressible flow which is acceptable for most building air flow situations but means that it cannot strictly be applied to fire situations where the flow cannot be assumed to be of constant density.
Hope this is of help.
Regards,
Steve |
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