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Wind speed inside a street canyon

Posted: Tue Aug 04, 2020 11:37 am
by kac116
Hi ENVI-met team,

I need to validate my ENVI-met results for my research. I'm using CODASC validation data (wind tunnel experiment data) because this validation data has been used in other research with ENVI-met. However, in my case, I don't get a good validation.

I'm modelling an isolated street canyon of 180m length, and 36m width (H/W=2) with 2 buildings of 18m height and 18m width. I used simple forcing and constant weather conditions during the day because I want to simulate similar wind tunnel conditions. Wind speed at reference height (10m) was set to 3m/s and the wind direction was set to 90 degrees. My model area has 4 traffic lines of NO2 with an emission rate of 2.12 ug/m*s.

I got an acceptable result when I compare the normalised vertical velocity (Uref) against wind tunnel experiment. However, inaccurate results are obtained when I compare normalised NO2 concentrations against the wind tunnel data. I think this is because there isn't good ventilation inside the canyon. The wind velocity was set to 3m/s, but inside the canyon is below 0.71m/s (as you can see in a Figure attached).

I have tried different configuration, smaller and taller building, wider and narrower buildings, different wind speed and direction, different pollutants and I always get very low pollutant concentrations inside the street canyon. I'm sure that this is not correct. Moreover, a small difference, less than 1ug/m3, I got between leeward and windward side. I would expect higher concentration in the leeward side as many research informs.

It's probably that I'm setting something wrong, but I can't find what it's. Could you give me some advice? Also, Why do I get almost equal concentrations inside the canyon? Why don't I see the canyon vortex and corner eddies commonly seen on other models?

I look forward to hearing from you.
Best
Carina

Re: Wind speed inside a street canyon

Posted: Tue Aug 04, 2020 3:24 pm
by Tim
Hi Carina,

It is hard to tell why you got those results. Could you give some more information, e.g. attach more screenshots of the model area and/or attach SIMX, INX files as zip?

Best regards,
Tim

Re: Wind speed inside a street canyon

Posted: Wed Aug 05, 2020 9:46 am
by kac116
Hi Tim,
Thank you for your reply. I attached the figure of my model area.
I didn't use nesting grid, instead, I increased the border area. The grid dimensions are 192 x 254 x 54 with a resolution of 1m.

Thank you once more for your help in this matter.
Best

Re: Wind speed inside a street canyon

Posted: Wed Aug 05, 2020 10:26 am
by Tim
Hi Carina,

Wind flow looks reasonable to me. The sources are located between both buildings. Since wind speed is very low because of the buildings blocking the flow, they are not transported away and accumulate there. How many hours did you simulate? Are there differences in results during daytime hours?

Best regards,
Tim

Re: Wind speed inside a street canyon

Posted: Wed Aug 05, 2020 2:43 pm
by kac116
Thank you very much for your reply.
I simulated for 2 hours, starting at 2 am. I decided this because the wind tunnel experiment doesn't have the influence of weather or sunlight. I haven't tried at different times during the day because I have constant weather conditions like temperature and humidity, so I thought the simulation would be the same. Anyway, I'll simulate at different times of the day to see what happens.

Thank you for your time and help.
Best wishes