Christian Resagk

Thermal boundary layers in turbulent Rayleigh–Bénard convection at aspect ratios between 1 and 9

We report highly resolved temperature measurements in turbulent Rayleigh–Benard convection in air at a fixed Prandtl number Pr = 0.7. Extending our previous work (du Puits et al 2007 J. Fluid Mech. 572 231–54), we carried out measurements at various aspect ratios while keeping the Rayleigh number constant. We demonstrate that the temperature field inside the convective boundary layers of both horizontal plates is virtually independent on the global flow pattern accompanying the variation in the aspect ratio. Thanks to technical upgrades of the experimental facility as well as a significant improvement of the accuracy and reliability of our temperature measurement — and unlike in our previous work — we find that the measured profiles of the time-averaged temperature field neither follow a clear power-law trend nor fit a linear or a logarithmic scaling over a significant fraction of the boundary-layer thickness. Analyzing the temperature data simultaneously acquired at both horizontal plates, various transitions in the cross-correlation and the auto-correlation function of the temperature signals are observed while varying the aspect ratio Γ. These transitions might be associated with a change in the global flow pattern from a single-roll mode at Γ = 1 toward a double- or a multi-roll mode pattern at higher aspect ratios.

A universal noncontact flowmeter for liquids

Lorentz force velocimetry (LFV) is a noncontact electromagnetic flow measurement technique for liquid metals that is currently used in fundamental research and metallurgy. Up to now, the application of LFV was limited to the narrow class of liquids whose electrical conductivity is of the order 106 S/m. Here, we demonstrate that LFV can be applied to liquids with conductivities up to six orders of magnitude smaller than in liquid metals. We further argue that this range can be extended to 10−3 S/m under industrial and to 10−6 S/m under laboratory conditions making LFV applicable to most liquids of practical interest.

Flow rate measurement of weakly conducting fluids using Lorentz force velocimetry

We present a novel application of a contactless flow measurement system and validate its feasibility on an electrolyte pipe flow. The device relies on the technique of Lorentz force velocimetry (LFV). LFV operates without any contact to either the fluid or the surrounding pipe walls. This is advantageous if the fluid under consideration is hot and aggressive, like a glass melt for example. Glass melts, however, have a very low electrical conductivity, resulting in Lorentz forces in the micronewton range. In order to resolve these tiny forces, we developed a measurement system based on the principle of deflection. Experiments on an electrolyte flow with an electrical conductivity of less than 20 S m −1 prove to be successful and to agree well with numerical simulations, and therefore show for the first time the applicability of LFV for fluids of such low conductivities.

Measurements of the instantaneous local heat flux in turbulent Rayleigh–Bénard convection

We present measurements of the instantaneous local heat flux in highly turbulent Rayleigh–Benard (RB) convection in air (Pr=0.7) for aspect ratios in the range 1.13≤Γ≤9.00 and for Rayleigh numbers in the range 1.3×109≤Ra≤9.6×1011. The measurements have been carried out simultaneously at the surfaces of the heating and the cooling plate using a commercial sensor whose diameter is 360 times smaller than the diameter of the RB facility. We find that for all investigated values of Ra and Γ the time-averaged local heat flux at the centres of the heating and cooling plates is significantly higher than the global heat flux obtained in previous measurements. In particular, for the smallest investigated aspect ratio, Γ=1.13, the scaling exponents of the local heat fluxes as functions of the Rayleigh number are found to be considerably below those of the global heat flux obtained in experiments with cryogenic helium at Γ=1 by Niemela and Sreenivasan (2003 J. Fluid Mech. 481 355–84). Our measurements indicate that the spatial distribution of the heat flux at the heating and cooling plates is strongly nonuniform and that this nonuniformity decreases with increasing Ra and Γ. Our investigations of the time dependence of the local heat fluxes show that these quantities undergo fluctuations up to ±15% of their time-averaged values. Our work suggests that local heat flux measurements at different positions along heating and cooling plates are useful for a deeper understanding of the scaling properties of the (global) Nusselt number in RB convection.

Flow field detection using acoustic travel time tomography

Acoustic travel time tomography uses the dependence of sound speed on temperature and flow properties along the propagation path to measure these parameters. An algorithm is introduced that is capable of resolving the two-dimensional flow field within a specified measuring area. Different flow fields have been simulated to explore the reconstruction properties of the algorithm. Furthermore, an experiment within the Barrel of Ilmenau has been carried out, which demonstrates that the acoustic tomographic method is able to detect flow fields in a closed convection chamber. The simulations reveal that the vector tomographic algorithm can resolve the flow field characteristics speed and direction. Furthermore it is demonstrated that the distribution of sound sources and receivers has to be homogeneous around the measuring site. For the reconstruction of flow distributions, this requirement is even more important compared to temperature reconstructions, since information along inclined sound ray paths within each grid cell are needed to reconstruct the flow vector properly.

Interface reconstruction between two conducting fluids applying genetic algorithms

In magnetic fluid dynamics, there appears the problem of reconstruction of free boundary between conducting fluids, e.g., in aluminum electrolysis cells. We have investigated how the interface between two fluids of different conductivity of a highly simplified model of an aluminum electrolysis cell could be reconstructed by means of external magnetic field measurements using a simple genetic algorithm.

Extended Three Dimensional Particle Tracking Velocimetry for Large Enclosures

An extended three-dimensional particle tracking velocimetry (3D PTV) technique for the determination of trajectories, velocity vectors and flow pattern within a large cylindrical observation volume was developed and applied in a Rayleigh-Benard (RB) cell. The 3D PTV system consists of four CMOS cameras, two flash lamps, an image recording and data processing system. Helium filled soap bubbles and latex balloons have been used as tracer particles. The spatial resolution and accuracy have been investigated in a test cell with the dimensions of 4×3×4m 3 by means of a given trajectory of a small glass sphere. A first application of the developed 3D PTV system in the large RB cell with a diameter of 7.15m and a height of 3.5m showed different characteristic flow patterns of the mean flow outside the boundary layers. The proposed technique has a wide range of potential application for the analysis of indoor air flows.

Magnetic field tomography on two electrically conducting fluids

A cylindrical cell containing GaInSn alloy and aqueous solution of KOH, a highly simplified aluminium reduction cell, is constructed. A direct current is applied to the cylindrical cell to generate the magnetic field. In a range of amplitude and frequency of vertical vibration, a stable, non-axisymmetric wave pattern is produced, and the displacement of the oscillating interface is measured by a digital camcorder. The perturbation of the magnetic field caused by the non-axisymmetric interface is measured by fluxgate sensors and processed by fast Fourier transforms. The measurements are consistent with forward calculations, and have been exploited to reconstruct the deformed interface by solving an inverse problem so as to develop magnetic field tomography to reconstruct an unknown interface between two electrically conducting fluids.

Surface current reconstruction using magnetic field tomography

The identification of characteristics or shape reconstruction of boundary surfaces are inverse problems arising in industrial applications, e.g., in magnetic fluid dynamics. We have investigated the identification of the interface between two conducting fluids using multichannel magnetic field measurements (magnetic field tomography). We considered a highly simplified model of an aluminum electrolysis cell consisting of a cylinder containing two compartments with different conductivities. Numerical simulations using the finite element method (FEM) have been evaluated. Additionally, magnetic field measurements taken from an experimental setup of this cylindrical object have been used as a reference for comparison. We propose a new test problem for the evaluation of inverse methods which consists in the estimation of the interface shape characteristics based on the current density distribution in the vicinity of the deformed interface using magnetic field measurements.

Towards metering tap water by Lorentz force velocimetry

In this paper, we present enhanced flow rate measurement by applying the contactless Lorentz Force Velocimetry (LFV) technique. Particularly, we show that the LFV is a feasible technique for metering the flow rate of salt water in a rectangular channel. The measurements of the Lorentz forces as a function of the flow rate are presented for different electrical conductivities of the salt water. The smallest value of conductivity is achieved at 0.06 Sm−1, which corresponds to the typical value of tap water. In comparison with previous results, the performance of LFV is improved by approximately 2 orders of magnitude by means of a high-precision differential force measurement setup. Furthermore, the sensitivity curve and the calibration factor of the flowmeter are provided based on extensive measurements for the flow velocities ranging from 0.2 to 2.5 ms−1 and conductivities ranging from 0.06 to 10 Sm−1.