Amplification factors wind analysis

[st_george]

I am trying to use MicroFlo for analysis of Amplification factors, due to wind impact on my buildings surroundings.

The amplification factor is defined as the ratio of the local wind speed to the wind speed that would occur at the same position without the buildings present.


Here is an example of the amplification factor analysis in some other CFD application:



Is this possible to be done in MicroFlo?

Thanks.

[liamh]

Plotting of amplication factors is not possible in Microflo but as amplification factor is proportional to air speed you could:

1) manually work out the amplication factors
2) capture image of a filled contour plot of the air speed
3) photoshop out the air speed text in the captured image (other image manipulation software is available)
4) photoshop in the amplication factor values

This would be a pain if you have lots of images but for a few images it isn't a big job. Actually with these image manipulation programs there is often a way to batch process files so if you keep the scale the same in all images it may be a quick job for multiple images.

[st_george]

thank you for the reply Liam.

But I am not sure I did understand you sorry.
Can you be a bit more specific?

[liamh]

Which bit don't you understand? 1) How to work out the amplification factor? 2) Or how to manipulate the images in photoshop to give amplification factor rather than air speed?

If it is (1) then.....

Page 5 of the "Macroflo Calculations Methods.pdf" document (from the VE help menu) will give you a calculation method to work out the wind velocity at a particular height above ground level (using the ASHRAE method). Use the formula for an "open" terrain if amplification factor is defined as the wind speed without any buildings.

Or you could just redefine your amplification factor to be the

local wind speed / wind speed at 10m on a open terrain
i.e.
the CFD prediction / the wind speed you set in settings in a Microflo simulation

and then you don't have to take into account the height of the plane that you've plotted your contour plot across. Which means you don't need to use the AHSRAE formula for the wind velocity profile.

[st_george]

to be quite honest I am a MicroFlo newbie. I used to use the SolidWorks Flow Simulation by now, but it seems the MicroFlo is better when it comes to usage of wind on buildings.

What I still do not understand is the procedure (these steps you mentioned).

The amplification factor is defined as the ratio of the local wind speed to the wind speed that would occur at the same position without the buildings present.

So I need to separately do the wind velocity plots plans - the ones with the buildings, and the ones without the builids, something like this for example:


velocity plot plan without the buildings (on the left) and the one with the buildings (on the right)-two red squares:


And now what? How can I "manually work out the amplication factors"? By pasting these two plans in Photoshop one over the other and then setting the transaprency of the second one:



?

I mean, I do not understand the procedure.

Sorry for disturbance.

[liamh]

I assumed the amplification factor was the ratio:
1) local wind speed with all buildings / local wind speed without any buildings (i.e. open space)
and not
2) local wind speed with buildings plus proposed development / local wind speed with buildings but without proposed development

My suggested method could work with (1) above but not (2). The images in your last post suggests you are asking for (2). My method wouldn't work for (2) so I won't go to the bother in explaining further.

To do (2) in another CFD code would be difficult as you would have to read in the velocity field of one simulation into a second simulation to perform the ratio calculation in order to display the images. If or how this could be done would depend on the CFD code, or separate post-processor program, used.

Can you find out
1) the exact meaning of amplification factor
2) what software was used to produce the image of amplification factor
and I may be able to help further.

[st_george]

Now I finally understand you, this is what you were saying, right:





And your second definition is correct:

[quote=amplification factor is]2) local wind speed with buildings plus proposed development / local wind speed with buildings but without proposed development[/quote]

So basically the amplification factor is the ratio of the:
local wind speed on certain area, after the changes has been made on it (say adding a building, two buildinsg, three, bench, three, anything)
and a local wind speed on the same area before those changes have been made.

So the area does not necessary need to be empty before the changes have been made.

I think ANSYS was used for getting the amplification factor image. I am not sure, but I think it is ANSYS. I am using SolidWorks for my calculations though.

[liamh]

Because the denominator in method (1) is a constant for a particular height above ground level you could have just plotted an image of wind speed, erased the wind speed text, and the added text to give the amplification factor.

The denominator in method (2) is a function of position in x, y and z so it makes the image plotting of amplification factor more difficult as you need your plotting software (the CFD program or a separate post-processor) to have access to the field functions of velocity for both geometries.

The easiest way to get around this is, rather than plot an image, produce a table of data for amplification factor for a number of known locations around the site.

[st_george]

I understand.

But I did not understand this part:

The easiest way to get around this is, rather than plot an image, produce a table of data for amplification factor for a number of known locations around the site.

How do you think this is possible to be done?

[liamh]

I mean:

1) pick say 10 different points around the site
2) do a sim without the proposed development and find the air speed at these 10 locations
3) do a sim with the proposed development and find the air speed at the 10 locations
4) divide the 10 numbers from (3) by the 10 numbers from (2)
5) in your report put the 10 numbers from (4) into a table

If you use Microflo you can get the program dump the data at 10 locations by setting up 10 monitoring locations in the residual monitoring window. Then Microflo will write out 10 files, 1 for each monitoring point. The first 3 columns give the x-velocity, the y-velocity and the z-velocity.

So air speed is
(x-velocity^2 + y-velocity^2 + Z-velocity^2)^0.5

Use the numbers at the end of the files when the solution has converged. The first line in the files is iteration 1 and the last line of the files is from the last iteration before the simulation stops.

The monitoring point data is saved into the directory

project_dir\CFD\cell

Each monitoring point gets its own file. The x, y and z location of the monitoring point is used in the filename. But there is no header in the files to help tell you the location. Sometimes the filenames make it obvious which monitoring point they are from but unfortunately sometimes it is very confusing.

There is a work-around though. By adding each monitoring point during a different stage of the simulation will mean each file will look different when opened. E.g. you can start off the simulation with one monitoring point, then stop it at 10 iterations to add a second monitoring point, before resuming the simulation. Then the file for the second monitoring point will have no values for the first 10 lines of the file.