Jörg Stiller

A method to estimate the planar, instantaneous body force distribution from velocity field measurements

We present a simple method to derive a planar, instantaneous body force distribution from a given two-dimensional velocity field without knowledge of the pressure field, under the specific restriction that the body force is dominated by one component only. Spatial integration then completely recovers this component. Particle image velocimetry and direct numerical simulations of a wall jet induced by a known body force were conducted to validate the method, demonstrating a good agreement of the original and reconstructed force fields.

Large-Eddy Simulation of Inhomogeneous Canopy Flows Using High Resolution Terrestrial Laser Scanning Data

The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation (LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 190 m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30 m) along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing, where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure. Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing.

Transitional and weakly turbulent flow in a rotating magnetic field

The early stage of turbulent flow driven by a rotating magnetic field is studied via direct numerical simulations and electric potential measurements for the case of a cylindrical geometry. The numerical results show that the undisturbed flow remains stable up to the linear stability limit (Tac), whereas small perturbations may initiate a nonlinear transition at subcritical Taylor numbers. The observed instabilities occur randomly as isolated pairs of Taylor-Gortler vortices, which grow from spots to long tubes until they are dissipated in the lid boundary layers. At 7.5Tac, the flow is governed by large-scale three-dimensional fluctuations and may be characterized as weakly turbulent. Taylor-Gortler vortices provide the major turbulence mechanism, apart from oscillations of the rotation axis. As the vortices tend to align with the azimuthal direction, they result in a locally two-dimensional turbulence pattern.

Large-Eddy Simulation Study of the Effects on Flow of a Heterogeneous Forest at Sub-Tree Resolution

The effect of three-dimensional plant heterogeneity on flow past a clearing is investigated by means of large-eddy simulation. A detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately

A fast spectral element solver combining static condensation and multigrid techniques

328\,\mathrm {m}

STABILIZED DISCONTINUOUS GALERKIN METHODS FOR FLOW–SOUND INTERACTION

328m length and

A Parallel PSPG Finite Element Method for Direct Simulation of Incompressible Flow

172\,\mathrm {m}