Visualization of the Ludford column. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 721 (2013), S. 438-453
When a liquid metal flows around a truncated cylinder in the presence of a magnetic field which is parallel to the xis of the cylinder, a stagnant region develops above the cylinder. We call this region a Ludford column. The Ludford column represents the magnetohydrodynamics (MHD) analogue of the well-known Taylor columns in rotating flows. Whereas Taylor columns can be easily visualized using dye, the visualization of Ludford columns has remained elusive up to now because liquid metals are opaque. We demonstrate that this fundamental limitation of experimental MHD can be overcome by using a superconducting 5 T magnet. This facility permits us to perform MHD experiments in which the opaque liquid metals are replaced with a transparent electrolyte while maintaining the key MHD effects. We report results of a series of flow experiments in which an aqueous solution of sulphuric acid flows around a bar with square cross-section (which for simplicity shall be referred to as a cylinder). We vary the Reynolds number in the range 5 < Re < 100 and the Hartmann number in the range 0 < Ha < 14. The experimental procedure involves flow sualizations using tracer particles as well as velocity measurements using particle image velocimetry (PIV). Our experiments provide direct access to the Ludford column for the first time and reveal the spatial structure of this basic feature of MHD flows.
http://dx.doi.org/10.1017/jfm.2013.76
On the transition from cellular to wavelike patterns during solutal Marangoni convection. - In: European physical journal special topics, ISSN 1951-6401, Bd. 219 (2013), 1, S. 121-130
We study characteristic convection patterns emerging during the mass transfer of acetic acid from a glycerol-water layer to a superposed acetone layer by means of experiments and numerical simulations. The patterns form as a result of the stationary Marangoni instability. The initial phase of the pattern evolution is studied using high-resolution simulations. They show hierarchically ordered cellular structures which closely resemble experimental observations. In the later stages presently accessible to the experiments, the cells are locally replaced by relaxation oscillation waves. The emergence of these structures is favored when the experiment is performed in narrow cuvettes.
http://dx.doi.org/10.1140/epjst/e2013-01787-1
Patterned turbulence in liquid metal flow: computational reconstruction of the Hartmann experiment. - In: Physical review letters, ISSN 1079-7114, Bd. 110 (2013), 8, 084501, insges. 5 S.
We present results of a numerical analysis of Hartmann's historical experiments on flows of mercury in pipes and ducts under the influence of magnetic fields. The computed critical parameters for the laminarturbulent transition as well as the friction coefficients are in excellent agreement with Hartmann's data. The simulations provide a first detailed view of the flow structures that are experimentally inaccessible. Novel flow regimes with localized turbulent spots near the sidewalls parallel to the magnetic field and otherwise laminar flow are discovered. We finally suggest how these predictions can be tested in a transparent fluid using optical flow measurement.
https://doi.org/10.1103/PhysRevLett.110.084501
Viscous instability of a sheared liquid-gas interface: dependence on fluid properties and basic velocity profile. - In: Physics of fluids, ISSN 1089-7666, Bd. 25 (2013), 3, 032103, insges. 37 S.
In the framework of linear stability theory, we analyze how a liquid-gas mixing layer is affected by several parameters: viscosity ratio, density ratio, and several length scales. These scales reflect the presence of a velocity defect induced by the wake behind the splitter plate and the presence of boundary layers which develop ahead of the plate trailing edge. Incorporating such effects, we compute the various temporal and spatial instability modes and identify their driving instability mechanism based on their Reynolds number dependence, spatial structure, and energy budget. It is examined how the velocity defect modifies the temporal and the spatial stability properties. In addition, the transition from convective to absolute instability occurs at lower velocity contrast between gas and liquid free streams when a defect is present. This transition is also promoted by surface tension. Compared to inviscid stability computations, our spatial stability analysis displays a better agreement with measured growth rates obtained in two recent air-water experiments.
https://doi.org/10.1063/1.4792311
Thermal boundary layers in turbulent Rayleigh-Bénard convection at aspect ratios between 1 and 9. - In: New journal of physics, ISSN 1367-2630, Bd. 15 (2013), 013040, insges. 23 S.
We report highly resolved temperature measurements in turbulent Rayleigh-Bénard 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 0. These transitions might be associated with a change in the global flow pattern from a single-roll mode at 0 = 1 toward a double- or a multi-roll mode pattern at higher aspect ratios.
https://doi.org/10.1088/1367-2630/15/1/013040
Properties of large-scale flow structures in an isothermal ventilated room. - In: Building and environment, ISSN 0360-1323, Bd. 59 (2013), S. 563-574
In the present work experimental and numerical investigations of the large-scale structures of isothermal air flow in a highly simplified model room are reported and discussed. We compare the measured velocity distribution with direct numerical simulations (DNSs) for the reference Reynolds number Reref 2.4 104, which is based on the maximum inlet velocity and the height of the room. This comparison shows a high similarity concerning the flow structures. Furthermore, we investigate the dependence of the spatial and temporal behavior of flow structures on the Reynolds number from measurements covering a Reynolds number range of 1.0 104 Re 7.0 104. The major finding is that there is a coherent oscillation of the large-scale flow structures, which depends on the Reynolds number. Our findings show that the frequencies of the oscillations are in a good agreement with an empirical model, which describes the auto-oscillation of two colliding planar jets with respect to the Reynolds number and the geometry relations of the inlets. Moreover, the present work indicates that the chosen flow geometry is well suited as a simplified model for problems of room ventilation and can serve as a base for forthcoming studies of non-isothermal cases.
http://dx.doi.org/10.1016/j.buildenv.2012.10.011
Solar thermally driven cooling systems: some investigation results and perspectives. - In: Energy conversion and management, ISSN 0196-8904, Bd. 65 (2013), S. 663-669
http://dx.doi.org/10.1016/j.enconman.2011.09.022
Thermodynamic analysis of a pulse tube engine. - In: Energy conversion and management, ISSN 0196-8904, Bd. 65 (2013), S. 810-818
The pulse tube engine is an innovative simple heat engine based on the pulse tube process used in cryogenic cooling applications. The working principle involves the conversion of applied heat energy into mechanical power, thereby enabling it to be used for electrical power generation. Furthermore, this device offers an opportunity for its wide use in energy harvesting and waste heat recovery. A numerical model has been developed to study the thermodynamic cycle and thereby help to design an experimental engine. Using the object-oriented modeling language Modelica, the engine was divided into components on which the conservation equations for mass, momentum and energy were applied. These components were linked via exchanged mass and enthalpy. The resulting differential equations for the thermodynamic properties were integrated numerically. The model was validated using the measured performance of a pulse tube engine. The transient behavior of the pulse tube engine's underlying thermodynamic properties could be evaluated and studied under different operating conditions. The model was used to explore the pulse tube engine process and investigate the influence of design parameters.
http://dx.doi.org/10.1016/j.enconman.2012.03.013
New method for designing an effective finned heat exchanger. - In: Applied thermal engineering, ISSN 1359-4311, Bd. 51 (2013), 1/2, S. 539-550
This paper presents a computational analysis of the heat transfer and pressure drop in a finned tube heat exchanger. The main objectives are to develop a method for designing an effective finned heat exchanger through changing the arrangement of the fins and tubes so that the tubes can be bent in a zigzag shape. Thus, they present an angle of less than 90 to the vertical fins. Consequently, the fin block forms a zigzag shape. The construction and the dimensions of the developed alternative heat exchanger are comprehensively presented. This new design is characterized by smaller dimensions of the heat exchanger and allows an increase of the heat transfer on the surface of the fins. Furthermore, a slight increase in the fan and pump power can result, which is very small compared with the recovered heat transfer. However, an effective model is analyzed by means of the computational fluid dynamics (CFD), FLUENT code, which is used to solve the equation for the heat transfer and pressure drop. The major results of the work show an increase in the heat transfer value of 59.13% in comparison to the existing old model. This increase is also accompanied by an increase in the ratio between the total power consumption and the amount of heat transfer obtained of 3.84% compared to 0.8% in the old model.
https://doi.org/10.1016/j.applthermaleng.2012.09.042
Investigation of flow structure in electromagnetically driven liquid metal confined in an annulus by Ultrasonic Doppler Velocimetry - ISUD8 2012. - In: Proceedings of the 8th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering, (2012), S. 83
Electromagnetic driving of the fluids as the driving mechanism in electromagnetic pumps is of great interest in industry. On the other hand the fluid flow in annulus has its own place in investigating the channel flow structure in the sense that we can use it as a model for a very long channel. It means that we can have the same flow structure in a confined area without wasting too much energy and effort to build up a long channel. Although the combination of electromagnetic force with toroidal geometry sounds efficient and practical, but lots of questions will arise when we do combine them. These questions are mainly about the formation of Hartmann layer and its role on transition to turbulence. During the last decades several numerical and experimental investigations were aimed to determine the instability criteria of this problem. In the previous experiments the potential drop across the annular channel has been used as a measure to show the transition to turbulence. Since these voltage drops can't give us any information about the structure of the flow, we decided to spot the possibility of using UDV to detect this structure. A rectangular channel with 10mm x 67mm cross section and an inner radious of 40mm is used in this investigation. The channel is filled up with eutectic alloy GaInSn. Two insulating Hartmann walls and two conducting side walls which are 10mm apart (in the radial direction) are the special boundary conditions in our case. Side walls are parallel to the magneitc field. Our facilities let us test 500mT as maximum magnetic field and 2000A as the maximum driving current. An ultrasounic 8 MHz sensor with the aid of DOP2000 is used for this investigation.Here we will reperesnts the results of our experiments and compare them with the theoretical predictions of the reference literatures. This will help understanding the role of Hartman layer on transition to turbulence.