Large-Eddy Simulation of Tracer Gas Dispersion Events with Focus on Peak Concentration in the Mock Urban Setting Test Experiment MUST

The Mock Urban Setting Test (MUST) experiment in the dessert of Utah/USA represent an urban roughness geometry by placing 120 shipping containers ordinary arranged in an array. This field experiment is conducted to provide comprehensive data for model evaluation in a mock urban geometry. An extensive set of meteorological data is sampled by high-resolution measurements. Additional puff tracer gas experiments with high temporal resolution are performed. This near full scale experiment was used for micro-scale flow simulations in an urban environment with the All Scale Atmospheric Model (ASAM).

The used building (container)-resolved resolution is able to capture dynamic flow structures like specific recirculation regions or eddy detachment. ASAM results are compared with the measurement data. The dynamic fluctuating behaviour of the wind velocity components is reproduced by the model with peak magnitudes and their temporal occurrence. Satisfying agreement is found between tracer gas dispersion field measurements and the model results by capturing the fluctuating concentration magnitude and in some extend the mean values.

For this level of agreement to experimental data, the model ASAM was enriched with modifications. To get close-to-reality results turbulent inflow boundary conditions are required for spatially inhomogeneous turbulent flows. Periodic boundary conditions, which are commonly used in LES flow configurations, reach their limits in complex geometry. Recent studies showed a large impact of varying inflow conditions on pollutant dispersion simulations through urban geometry.

With a synthetic turbulent inflow generation method it is possible to perturb mean profiles with ordinary turbulence characteristics at the outer boundaries. To modify a mean given flow, turbulent fluctuations are generated by superposition of sinusoidal and cosinesoidal modes depending on inflow spanwise location and time. The meteorological conditions like wind speed/direction and turbulent kinetic energy from the measurement are used to generate the inflow boundary conditions with the new implemented approach. The added turbulent scales depend thereby on the chosen frequencies and their associated amplitudes in the superposition ansatz. Due to the implementation of this turbulent inflow method, the model ASAM has the ability to reproduce a given wind field with information from its mean wind speed and their fluctuation energy spectrum.

Furthermore the model ASAM is enriched with new eddy viscosity based dynamic Smagorinsky subgrid-scale models. Therefore the model is more physically based to study atmospheric flow configurations at several atmospheric scales with main focus to urban scale flow with building-resolved resolution.