Questions about initial perturbations in LES & Vertical wind and temperature profiles

[Ryu_Kor]

Hello,

I am conducting research on urban micro-meteorology.

Someone asked me to explain the initial perturbation in the LES model.

Most of it seems to be related to pressure perturbation.

1. Is there any documentation that explains the initial perturbations for u', v', and w'?

2. When I set the Obukhov length, z_0, and wind speed at 2m as input data, I would like to draw the following vertical profile for temperature and wind speed. This picture is on p.220 of the user guide. How can I create the next picture?

Any advice would be greatly appreciated.

Thank you.

[dr_jfloyd]

1. Search the guide for NOISE

2. All input files for plots in the User's and Verfication Guide are in the Examples folder where you installed fds.  According to the text these plots are generated from the wind_example_nn.fds input files. 

[낭랑나그네교희씨]

Thank you for your advice.

From your advice, initial perturbation and noise are related, and what I understand from the user guide and other materials is as follows. Am I understanding this correctly?

1. When using the LES model in FDS with respect to initial perturbations, “noise” refers to small random fluctuations or perturbations added to the simulation to create the initial state of the turbulent flow. It is used to generate turbulent flow at the start of the simulation, and plays an important role in the development and evolution of turbulence.
2. In FDS, the flow field is initialized with a small amount of “noise” to prevent symmetrical flow that occurs when the boundary and initial conditions are symmetrical. At this time, the noise speed (speed of random noise vector) was run at the default value of 0.005m/s.
3. The purpose of using noise velocity is to simulate the energy cascade in which energy is transferred from Large Eddy to Small Eddy, which is a basic characteristic of turbulent flow.

------------------------------------------------------------------------------------------------------------------------------------------------------

I understand the figure was created using fds 'wind_example_nn.fds' installed via text.

However, I would like to know how the picture was drawn as a result of modeling 'wind_example_nn.fds'.

Do I need to use &DEVC POINTS? When I set &DEVC points=50 in the fds I created, the following 'CHID_line.csv' file was created. The contents are as follows, and all values were 0.

This is because I want to draw the vertical temperature and wind speed from my fds file as shown in the example picture.

2024년 1월 22일 (월) 오후 8:01, dr_jfloyd <drjf...@gmail.com>님이 작성:

--
You received this message because you are subscribed to the Google Groups "FDS and Smokeview Discussions" group.
To unsubscribe from this group and stop receiving emails from it, send an email to fds-smv+u...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/fds-smv/f2d2d3ce-4563-4bf7-b429-bc4da6cc4ccbn%40googlegroups.com.

[dr_jfloyd]

If your line.csv file is all zero, then you likely have an error in your inputs. We would need an input file to do anything more.

[Ryu_Kor]

Hello,
I tested to create identical temperature and wind speed profiles on page 220(userguide).

The value was extracted vertically using wind_example_32.fds, which is provided by default, and was created as shown in the following figure.

First, it is a little strange that the same values appear at regular altitude intervals in the line.csv file.

Also, the bottom layer of the picture I drew is different from the part suggested in the user guide, but do you know how such a value can be obtained?

Thank you

2024년 1월 23일 화요일 오후 9시 37분 4초 UTC+9에 dr_jfloyd님이 작성:

[Randy McDermott]

Please attach your input file.  It looks like you only have 6 vertical cells, maybe on 1 mesh.  You need a grid cell at every DEVC location you want to output.

To view this discussion on the web visit https://groups.google.com/d/msgid/fds-smv/c06a0c0e-b0cc-46a3-9c17-cad584fae973n%40googlegroups.com.

[Kevin McGrattan]

Those images from the FDS User's Guide showing M-O vertical profiles are not created by FDS. They are made by a Matlab script to demonstrate what a typical profile looks like. Your FDS output shows the same values because each of your devices is in the same grid cell. If you want to see the profile in detail, use more grid cells in the vertical direction.

[Kevin]

This thread is now here