Material Properties for accurate MRT calculation (UTCI component)

sineadn · September 12, 2023, 10:56am

Hi all,
I have a HB problem- Outdoor thermal comfort related.

I’ve been working on comparing MRT field measurements with the outputs from the HB UTCI component but am having trouble getting results close to my measurements. I THINK because I don’t have the correct material properties set for the model’s ground/floor so I’m wondering if anyone can help my finetune the material properties? - The ground is a kind of pebble dash roofing tile which i’ve currently set based off a mix of the properties from Solid rock & Dry Dust because Solid Rock was giving me close LW MRT and Dry Dust close SW MRT. But that isnt really working out
Images of experimental set up and HB model below. (Basically the roof tiles are the roof of the ground zone with an ‘orphan’ shade connected to the model as the textile)

Material Properties:

Here are the current results with red as the simulated values and blue as the measured values. First pic is: MRT, longwave MRT and MRT shortwave delta

Second pic is: air temp under the shade, and ground surface temp

And here are the direct and diffuse rad values for the same period if anyone is wondering about that weird peak mid morning. Wouldn’t mind an explanation for that too?

Also the EPW values have been replaced with local weather data collected on the analysis day including air temp, rh, wind speed and direction and radiation values.

Thank you!

ale.bog · September 21, 2023, 2:53am

Hi @sineadn, sounds like you’re well across what you’re doing, but I ask a primitive question - where did you get material properties from? Solar and visaul absorbtance in particular. It loooks a bit high at the first glance.

We did a study comparing reflectance parameters and it’s very critical for MRT results, more than surface temperature outcomes. May check out the discussion here MRT with UTCI (HB)_Materials impact - grasshopper / honeybee - Ladybug Tools | Forum

Is the grid set up at the same height with the site measurements?

As an idea, could cloud cover create the spike in the air temp observations?

sineadn · October 9, 2023, 11:04pm

Hi! Thanks for the response.

It was a good pointer to go back to the grid - the height wasn’t the problem as i’m looking at surface temp and used an infrared sensor to measure that (ground filled the entire field of view). BUT i did realise the resolution of the grid made a big difference to the results i was getting, and once the division of the grid was close to the field of view of the sensor I had better results. This is using the Solid Rock ground material with the shade material set to 0.1 transmissivity and 0.71 reflection.
These are the Solid Rock Properties, but i set the thickness to 0.1 as that approximates the tile thickness.

And these are the results>

As you can see lw MRT is pretty spot on but still issues with swMRT possibly because of the reflectivity/of the ground material. But when i increased the visible and solar absorptance to 0.65 the ground temp increases along with the lwMRT and the swMRT while getting closer values to those measured also gets some weird peaks

Would you have any thoughts on this?
Thanks!