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Two series of visualization experiments were performed in the boundary layer of Rayleigh—Bénard convection with the goal to get more information on the behavior and characteristics of streaks elongated in the streamwise direction observed in the boundary layers. These streamwise streaks are a new type of coherent structures in the Rayleigh—Bénard convection.

A canonical turbulent boundary layer on a large flat plate has been investigated for Reynolds numbers 4 × 10 ≤ ≤ 5.5 × 10. This experimental investigation includes hot-wire measurements of the mean and fluctuating velocity profiles and of two-point velocity correlations in the wall-normal direction. Complementary Particle Image Velocimetry (PIV) measurements in a plane given by the streamwise and the wall-normal direction provide additional information about the instantaneous velocity field.

The comparison between these two measuring techniques is of special importance since we are not aware of published PIV measurements at comparable Reynolds numbers. In addition, the validity of Taylor's hypothesis towards the high Reynolds-number range was investigated.

The project is part of the “High Reynolds Number Turbulence” group of the “Initiative of Turbulence” and also provides experimental data for other projects.

Lie group analysis is used to derive (exponential laws) for ZPG turbulent boundary layer flow. A new scaling group was found in the two-point correlation equations. DNS of such a flow was performed at = 2240 using a spectral method with up to 160 million grid points. The results of the numerical simulations are compared with the new scaling laws and good agreement is achieved.

The measurement of thermal fluctuations is important for the investigation of the transport features of passive and active scalars in fluids. As an addition to the established cold-wire technique we present a thermal sensor based on a miniaturized coaxial thermocouple. The advantage of such a sensor is first of all its size. The active area extends only a few hundreds of square nanometers sitting at the tip of a thin glass rod of less than a micrometer in diameter. The preferred field of application of this sensor are all measurement situations which require a high spatial resolution of temperature measurements for example within the boundary layer [1]–[7]. The sensors coaxial setup results from its fabrication as a micropipette and has the advantage of an intrinsic shielding against external distortions. The glass micropipettes contain a core of platinum and are coated with gold and are fabricated similar to the ones in [8]. Because of its chemically inert coating, these sensors are applicable for detecting temperature fluctuations in a large variety of liquids and gases. The fabrication and characterization of these sensors is presented here.

Two series of visualization experiments were performed in the boundary layer of Rayleigh—Bénard convection with the goal to get more information on the behavior and characteristics of streaks elongated in the streamwise direction observed in the boundary layers. These streamwise streaks are a new type of coherent structures in the Rayleigh—Bénard convection.

The statistics and scaling properties of the velocity field in turbulent Rayleigh-Bénard convection in water has been measured using both laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) techniques. It is found that results from both techniques for the mean velocity and all the statistical quantities examined agree with each other. The measurements reveal that the pdfs for the velocity are non-Gaussian in the cell center but more close to Gaussian near the cell boundaries. In addition, the Reynolds shear stress is found to have different signs near the sidewall and near the plates of the cell, suggesting that different mechanisms are responsible for driving the mean flow at different locations of the cell. Moreover, our results confirm a prediction of a recent model by Grossmann & Lohse, in which flow geometries are classified according to the shape of the container.

Two series of visualization experiments were performed in the boundary layer of Rayleigh—Bénard convection with the goal to get more information on the behavior and characteristics of streaks elongated in the streamwise direction observed in the boundary layers. These streamwise streaks are a new type of coherent structures in the Rayleigh—Bénard convection.

The generation of mean flows in horizontally periodic Rayleigh-Bénard convection is studied using direct numerical simulations (DNS) with Rayleigh numbers up to 10. We present the spatial and temporal characteristics of this flow component and its dependence on the parameters of the problem.

A short review of numerical simulation approaches for transitional and turbulent shear flows is presented. Some results using large-eddy simulation (LES) are for canonical turbulent and transitional flows obtained with different subgrid-scale (SGS) models such as a variant of the approximate deconvolution (ADM) and high-pass-filtered (HPF) eddy-viscosity model. Special focus is the LES of transition in incompressible flow.

The temporal evolution of passive lines in turbulence of an incompressible viscous fluid is simulated numerically for the Taylor-length Reynolds umber up to 252. The passive lines elongate in average exponentially in time. The mean exponential stretching rate obeys the Kolmogorov scaling law, i.e. = 0.17/, being the Kolmogorov time if it is estimated with chopped lines of a fixed length of (), being the energy-containing-eddy length. However, the mean stretching rate estimated with natural (or unchopped) passive lines increases with Reynolds number more rapidly than at the rate of the Kolmogorov scaling law.