Nikolaus Peller

Turbulent Channel Flow with Periodic Hill Constrictions

This paper presents a Direct Numerical Simulation (DNS) of turbulent channel flow with periodic hill constrictions at Re = 2808. For the DNS, special attention is paid to the grid design by analysis of the Kolmogorov length scale and the wall shear stress in order to be well resolved over the entire numerical domain. Therefore the DNS simulation provides data for a detailed study of physical flow phenomena and near-wall studies. Results for instantaneous and averaged flow fields are presented. An investigation of the near wall behaviour reveals the applicability of explicit wall models for Large Eddy Simulation (LES) in regions with high wall shear stress and a complete failure of conventional wall scaling in separation and reattachment regions where the wall shear stress is small.

Assessment of eddy resolving techniques for the flow over periodically arranged hills up to Re=37,000

The turbulent flow over periodically arranged geometrically two-dimensional hills in a channel at a Reynolds number of Re=37,000 has been considered as benchmark case for various eddy-resolving methods. The aim of this study is to assess various LES models and numerical approaches in a turbulent flow detaching from a curved surface. We compare results of a Cartesian grid solver using the immersed boundary method with various curvilinear approaches ranging from standard eddy-viscosity subgrid-scale models to hybrid LES-RANS models. The results are validated by a recent experiment conducted in a water channel by particle image velocimetry and laser-Doppler anemometry.

Wall Layer Investigations of Channel Flow with Periodic Hill Constrictions

The paper presents near-wall investigations for turbulent attached and separated flow in a channel flow with periodic hill constrictions at Re = 5600. An extended scaling for the streamwise velocity profile is introduced which takes into account wall shear stress and streamwise pressure gradient at the same time. A parameter α is defined representing the relative contributions of wall shear stress or pressure gradient, respectively. By this parameter it is possible to classify certain positions in the flow. Since wall shear stress and pressure gradient are extremely unlikely to be become zero at the same time in the average flow field this scaling can also be applied in regions of separation and reattachment.

Aerodynamics and Acoustic Sources of the Exhaust Jet in a Car Air-Conditioning System

An application of Large-Eddy Simulation to noise prediction is presented. We consider a car air-conditioning jet exiting into the passenger’s compartment. It produces a broadband noise. Within a hybrid approach, we compute acoustic sources using Lighthill’s acoustic analogy. We solve the flow using a Cartesian solver combined with an Immersed Boundary Method, which takes into account the complex exhaust nozzle. This technique results in an efficient solver that allows for highly resolved computations of the flow field. We investigate the influence of grid resolution and boundary approximation order on time averaged velocity fields as well as on acoustic sources. The mean flow results show that the complex geometry jet does not behave like a classical turbulent jet. The numerical investigations show that a smooth representation of the surface together with an adequate grid resolution is required to represent the small vortical structures giving the main contribution to the acoustic quadrupole terms. If one requirement is not met, errors in the strength of the acoustic source terms and their dynamic response can be considerable.

Wall Scaling and Wall Models for Complex Turbulent Flows

The near wall behaviour of complex turbulent flows is investigated by dimensional considerations and a priori investigations. We consider separated incompressible and compressible flows. In incompressible turbulent flows, it is found that the flow in the immediate vicinity of the wall is dependent on two parameters, based on wall friction and local pressure gradient, respectively. In compressible turbulent flows, a third parameter based on the wall heat flux is relevant for the flow profiles in vicinity of the wall. While in the viscous sub-layer, an exact law of the wall can be formulated, the buffer and log-layer is dependent on empirical formulations. The significance of the wall scaling is shown by carefully carried out Direct Numerical Simulations and Large-Eddy Simulations of incompressible and compressible separated flows.