[Seminar] Dr. Hyeyum Shin

Supporting Urban Air Mobility Operations with Building-Resolving Simulations: Weather Sensitivities of Fine-Scale Urban Flows

The recent emergence of unmanned aerial system (UAS) and urban air mobility (UAM), the new frontiers in air transportation operating in the atmospheric boundary layer (ABL), is expanding the scope of aviation turbulence forecasting system from ensemble-mean to eddy-resolving forecasts (Muñoz-Esparza et al. 2018). Despite its tangible benefits, operational turbulence- resolving forecasting at scales of 10-100 m is not quite feasible yet due to its high computational costs, and resolving urban landscapes at scales of 1 m for UAM applications poses additional challenges. This study investigates the impact of urban landscapes on low-level turbulence and associated turbulence hazards to UAM operations, based on ideal- and real-case building- resolving simulations using the GPU-accelerated FastEddy® large-eddy simulation (LES) model (Sauer and Muñoz-Esparza 2020) with explicit representation of building effects by an immersed body force method (Muñoz-Esparza et al. 2020).

We first explore the interactions between urban landscapes and ABLs, using idealized building- resolving LESs of the flow around isolated cuboid buildings to variations in the incoming turbulence arising from changes in ABL stability. 21 building-resolving simulations are performed under varying ABL stability conditions, from weakly stable to convective ABLs, and for different building sizes (H), resulting in LABL /H ≈ 0.1 – 10, where LABL is the integral length scale of the incoming ABL turbulence. The building-induced flow features observed in the canonical neutral ABL simulation, e.g., the upstream horseshoe vortex and the downstream arch vortex, gradually weaken with increasing surface-driven convective instability due to the enhancement of background turbulent mixing. By considering the ABL turbulence scale and building size altogether, it is shown that the building impact on the surrounding flow decreases with increasing LABL /H, as coherent turbulent structures (e.g., near-surface streaks, convective rolls and open cells) in the ABL become more dominant over building-induced flow response for LABL /H > 1.

We further extend our analysis on the interactions between urban landscapes and ABLs by performing building-resolving simulations over the realistic urban topography of downtown Dallas, TX, USA. The realistic three-dimensional building information is generated from Dallas Lidar dataset and building footprints, provided by the Texas Natural Resource Information System and the City of Dallas GIS Department, respectively. The weather dataset, which provides the ABL forcing conditions to building-resolving LESs, is generated from a series of 48-hour weather forecasts centered at downtown Dallas using the Weather Research and Forecasting model. Typical weather scenarios of the downtown Dallas region at 06, 09, 12, 15 and 18 LT for January and July are examined to investigate how local weather patterns that vary seasonally and diurnally modify low-level wind and turbulence in the urban environment, and impact UAM operations including turbulence hazards during landings and takeoffs.