Peter Lammers

Interpretation of the mechanism associated with turbulent drag reduction in terms of anisotropy invariants

A central goal of flow control is to minimize the energy consumption in turbulent flows and nowadays the best results in terms of drag reduction are obtained with the addition of long-chain polymers. This has been found to be associated with increased anisotropy of turbulence in the near-wall region. Other drag reduction mechanisms are analysed in this respect and it is shown that close to the wall highly anisotropic states of turbulence are commonly found. These findings are supported by results of direct numerical simulations which display high drag reduction effects of over 30% when only a few points inside the viscous sublayer are forced towards high anisotropy.

Direct Simulation with the Lattice Boltzmann Code BEST of Developed Turbulence in Channel Flows

In spite of the dramatic increase of the performance of recent supercomputers the direct numerical simulation (DNS) of turbulent flows is still an expensive venture in view of the high memory and CPU time requirements. Today, the DNS is limited to very low Reynolds number and simple flow geometries which are far away from technical relevance. Therefore, there is an increasing demand in the development of numerical schemes to simulate fluid flows resolving the turbulent scales in order to exploit existing and future supercomputers more efficiently. In that context, the lattice Boltzmann method have challenged the traditionally used DNS method based on FV or pseu-dospektral . The potential of these LBM have been clearly shown in various publications [4]. The goal of the present paper is to specify the advantages of the LBM to DNS more quantitatively and to make use of the LBM to investigate new phenomena related to wall bounded turbulence.

Statistics and Intermittency of Developed Channel Flows: a Grand Challenge in Turbulence Modeling and Simulation

Studying and modeling turbulence in wall-bounded flows is important in many engineering fields, such as transportation, power generation or chemical engineering. Despite its long history, it remains disputable even in its basic aspects and even if only simple flow types are considered. Focusing on the best studied flow type, which has also direct applications, we argue that not only its theoretical description, but also its experimental measurement and numerical simulation are objectively limited in range and precision, and that it is necessary to bridge gaps between parameter ranges that are covered by different approaches. Currently, this can only be achieved by expanding the range of numerical simulations, a grand challenge even for the most powerful computational resources just becoming available. The required setup and desired output of such simulations are specified, along with estimates of the computing effort on the NEC SX-8 supercomputer at HLRS.

The role of turbulent dissipation for flow control of near-wall turbulence

One of the major goals of flow control is the reduction of energy consumption in wall bounded flows by minimizing the viscous drag. In the present work it is shown that the turbulent dissipation needs to be minimized in order to obtain energy savings. Analytical considerations lead to the conclusion that this can be achieved by forcing near-wall turbulence to an axisymmetric state. To confirm this finding direct numerical simulations were carried out in which the boundary conditions were such that near-wall fluctuations were forced towards axisymmetry. Additionally, a surface structure that minimizes turbulent dissipation at the wall was designed and tested experimentally. Both, numerical and experimental investigations yield significant drag reduction.

Hydraulic Model of the Skin Friction Reduction with Surface Grooves

The reduction of skin friction in turbulent flows holds considerable promise for energy savings. The present work shows how and why skin friction and the dissipation are interrelated in turbulent channel flows. A hydraulic model formulation is presented for the skin friction reduction that can be obtained with a surface structure recently proposed for flow control. The model predictions are validated with results from direct numerical simulations.