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The following example describes wind tunnel experiments conducted by the Environmental Wind Tunnel Laboratory (EWTL) at the University of Hamburg as a validation case in Part 9.4 of the WTG-Merkblatt M3. We are going to use the measured velocity fields and roughness data of the Michel City model (Case BL3-3) to validate numerical CFD simulations in complex urban structures. The example can belong to Group 2, according to Figure 2.2 in WTG-Merkblatt-M3, based on investigating average wind velocity value:
- G2: Absolute values with medium accuracy requirements: The area of application can include parameters or preliminary studies when later investigations with higher accuracy are planned (e.g., wind tunnel examination of class G3).
- R2: Solitary: all relevant wind directions with sufficiently fine directional resolution.
- Z2: Statistical mean values and standard deviations: provided they involve stationary flow processes for which a statistical verification of fluctuations with a peak factor is sufficient.
- S1: Static effects: They are sufficient to represent the structural model with the necessary mechanical detail, but without mass and damping properties.
Description
The investigation focuses on an idealized but geometrically detailed city model placed in an atmospheric boundary layer flow. The wind tunnel measurements were performed in the WOTAN facility, featuring a test section 18 m long, 4 m wide, and 2.75-3.25 m high. The corresponding roughness field was characterized by a roughness length of z0=1.53 m and a profile exponent α=0.27 representing "very rough" terrain conditions. A total of 1,838 measurement points were recorded for several roof configurations. The time-dependent horizontal velocity components u and v, including mean values, variances, correlations, and spectra, were obtained with a 2D Laser-Doppler Anemometer (LDA) at 500-600 Hz. Measurement points were distributed in vertical and horizontal profiles, in street canyons, and at defined repeatability locations. The Michel City dataset serves as a reference validation case (C5) according to VDI Guideline 3783 Part 9 [1]. For validation, in addition to the hit ratio, a relative deviation D=0.25 and an absolute deviation W=0.08 are applied to account for repeatability and measurement uncertainty. This dataset has been verified and adopted by several institutions (e.g., KalWin [2]) for CFD validation and model comparison purposes.
[1] VDI Richtlinie 3783 Blatt 9: Umweltmeteorologie - Prognostische mikroskalige Windfeldmodelle -
Evaluierung für Gebäude- und Hindernisumströmung, 2017
[2] KalWin, “Validation of flow field prediction within a realistic but idealized urban environment with
OpenFOAM”, Report KalWin Engineering GbR, 2022,
http://www.kalwin-engineering.com/wp-content/uploads/2022/10/KalWin-Report-ValidationOkt2022.pdf