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UTCIFoam - Universal Thermal Climate Index Calculator

Overview

UTCIFoam is an OpenFOAM post-processing utility that calculates the Universal Thermal Climate Index (UTCI) from CFD simulation results. It's designed to work seamlessly with buoyantHumiditySimpleFoam and other OpenFOAM solvers.

Features

Version 2.0 Improvements

  1. Sky View Factor (SVF) Calculation

    • Automatic ray-tracing based SVF calculation for outdoor environments
    • Simplified height-based SVF for large meshes
    • Optional SVF field input

  2. Enhanced Mean Radiant Temperature (Tmrt)

    • Improved calculation considering SVF
    • Support for direct Tmrt input
    • Outdoor environment considerations

  3. UTCI Polynomial Approximation

    • Implementation of 6th-order polynomial structure
    • Proper input validation for UTCI ranges
    • Wind speed adjustment to 10m height

  4. Physical Accuracy

    • Validated input ranges for all parameters
    • Proper unit conversions
    • Clear warnings for out-of-range values

Required Fields

  • T: Air temperature [K]
  • U: Velocity field [m/s]
  • thermo:relHum or relHum: Relative humidity [%]
    • buoyantHumiditySimpleFoam writes as thermo:relHum
    • Other solvers may use relHum
  • qr: Radiation flux density [W/m²] (or provide Tmrt directly)

Optional Fields

  • SVF: Sky View Factor [-] (default: calculated or 0.7)
  • Tmrt: Mean radiant temperature [K or °C] (if not provided, calculated from qr)

Usage

After running your CFD simulation (e.g., with buoyantHumiditySimpleFoam):

Basic usage (requires qr field):

UTCIFoam

With EPW weather data (recommended for outdoor):

UTCIFoam -epw weather.epw

With specific date/time:

UTCIFoam -epw weather.epw -month 7 -day 21 -hour 15

For specific CFD time steps:

UTCIFoam -time 1000 -epw weather.epw

EPW Weather Data

When using EPW files, UTCIFoam:

  • Reads location (latitude, longitude, timezone, elevation)
  • Extracts solar radiation data (direct, diffuse)
  • Calculates mean radiant temperature using:
    • Direct normal irradiance (DNI)
    • Diffuse horizontal irradiance (DHI)
    • Sky view factor (SVF) from geometry
    • Ground surface temperature
  • Uses actual weather conditions for the specified date/time

EPW files can be downloaded from:

Output

The utility creates a UTCI field containing the Universal Thermal Climate Index values in °C.

Valid Ranges

UTCI calculations are valid for:

  • Air temperature: -50 to +50°C
  • Wind speed (10m): 0.5 to 30 m/s
  • Vapor pressure: 0 to 50 hPa
  • Tmrt - Ta: -30 to +70°C

Values outside these ranges will be marked as -9999.

Implementation Notes

Sky View Factor Calculation

The SVF is calculated using:

  • Ray tracing for meshes < 100,000 cells (50 rays per cell)
  • Simplified height-based estimation for larger meshes

Mean Radiant Temperature

Tmrt calculation considers:

  • Direct radiation flux (qr)
  • Sky View Factor
  • Human body emissivity (0.97)
  • Stefan-Boltzmann law

Wind Speed Adjustment

Wind speeds are adjusted from measurement height (assumed 1.5m) to standard 10m height using:

v10 = v_h * (10/h)^0.25

Limitations

The current implementation uses example polynomial coefficients. For production use, the full UTCI polynomial coefficients should be obtained from:

  • Bröde et al. (2012) Int J Biometeorol 56:481-494
  • Official UTCI website (www.utci.org)

References

  1. Bröde, P., Fiala, D., Błażejczyk, K., et al. (2012). "Deriving the operational procedure for the Universal Thermal Climate Index (UTCI)". International Journal of Biometeorology, 56(3), 481-494.

  2. Fiala, D., Havenith, G., Bröde, P., et al. (2012). "UTCI-Fiala multi-node model of human heat transfer and temperature regulation". International Journal of Biometeorology, 56(3), 429-441.

  3. Lindberg, F., Holmer, B., & Thorsson, S. (2008). "SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and mean radiant temperature in complex urban settings". International Journal of Biometeorology, 52(7), 697-713.