Reversals of coherent structures in turbulent mixed convection. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 904 (2020), A33, S. A33-1-A33-33
Reconfiguration events in turbulent mixed convection, i.e. the superposition of thermal and forced flow contributions, at the two different Richardson numbers and and similar Rayleigh numbers of are investigated with tomographic particle image velocimetry in combination with local temperature measurements. For both cases, the three-dimensional velocity fields reflect diagonally aligned large-scale circulations (LSC) switching their alignment by rotating their axes around a pivot located at the centre of the LSC, while the temperatures perform a translation movement of the structures in agreement with earlier temperature-based investigations. For the high case, the switching process of the observed spontaneous reconfigurations is induced by a reversing thermal flow contribution while the forced flow contribution is constant. Furthermore, it is shown that a secondary roll structure, which drives the reconfiguration process in Rayleigh-Bénard convection, also exists in mixed convection. However, in the latter, the flow reversals are triggered by different structures which accumulate and release their kinetic energy according to a proper orthogonal decomposition analysis. In contrast, for the low case, the structure formation during continuous reconfigurations is governed by a Taylor- or Görtler-type instability. This means that the forced convection substantially affects the reconfiguration mechanism of these structures. Therefore, the reconfigurations cannot be described by a simple superposition of structures associated with the two flow contributions as for the high.
https://doi.org/10.1017/jfm.2020.705
Fluctuations of the wall shear stress vector in a large-scale natural convection cell. - In: AIP Advances, ISSN 2158-3226, Bd. 10 (2020), 7, 075105, insges. 10 S.
https://doi.org/10.1063/5.0006610
Experimental study of moist air flow in the gap between the aircraft's fuselage and its cabin wall. - In: CEAS Aeronautical Journal, ISSN 1869-5590, Bd. 11 (2020), 3, S. 591-607
We carried out an experimental study on the moisture transfer and the heat transport in warm and humid air flows between the cabin lining and the fuselage skin. The measurements were performed in a rectangular gap channel, representing the space between fuselage and cabin wall. Long-term measurements were performed for three configurations: without insulation, with fibreglass blanket and with melamine resin foam blanket. To simulate realistic flight conditions in a laboratory setup, we applied a concept of scaling. This concept is intended to guarantee similitude between the real flight conditions and the laboratory experiment. The results reveal that without insulation, the moisture transfer rate is much higher compared to the configurations with insulation blankets. With insulation, most of the water evaporates during ground conditions and just a small amount is entrapped in the insulation. Without insulation, just a small part of the frozen water evaporates on the ground. When comparing the two insulation blankets, it is found that they both have a similar heat transmittance coefficient. However, the condensation rate of the water and the resulting accumulation of water are significant, higher for the fibreglass blanket.
https://doi.org/10.1007/s13272-020-00440-3
Condensation-induced flow structure modifications in turbulent channel flow investigated in direct numerical simulations. - In: Physics of fluids, ISSN 1089-7666, Bd. 32 (2020), 1, 015115, S. 015115-1-015115-11
The turbulent flow of a fluid carrying trace amounts of a condensable species through a differentially cooled vertical channel geometry is simulated using single-phase direct numerical simulations. The release of latent heat during condensation is modeled by interdependent temperature and vapor concentration source terms governing the relation between the removal of excess vapor from the system and the associated local increase in fluid temperature. A coupling between condensation and turbulence is implemented via solutal and thermal buoyancy. When compared to simulations of an identical system without phase transition modeling, the modifications of the subcooled boundary layer due to the transient and highly localized release of latent heat could be observed. A separate analysis of fluid before and after phase transition events shows a clear increase in post-interaction streak spacing, with the release of latent heat during condensation events opposing the cooling effect of the channel wall and the associated damping of turbulence.
https://doi.org/10.1063/1.5128976
Mean temperature profile and thermal displacement thickness in turbulent Rayleigh-Bénard convection. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 148 (2020), 119021, insges. 7 S.
The shape of the mean temperature profile in turbulent RayleighBénard Convection (RBC) plays an important role in the understanding and prediction of heat transfer. Based on the characteristics of the mean temperature profile, we propose a four-layer structure for each half of a turbulent RBC cell. Layer I, a thin layer adjacent to the bottom or top plate, is the traditional thermal diffusional sub-layer, in which the mean temperature is dominated by linear growth. It is found, empirically, that there exists a layer in which the mean temperature deficit can be approximated by a power-law decay, and this layer is called Layer III. Between Layer I and Layer III there exists a transitional or buffer layer, which is designated Layer II. Outside Layer III is a well-mixed outer layer, Layer IV. In a fully turbulent RBC, Layers I, II, and III occupy a small fraction of the RBC cell, but these layers contain nearly all the temperature increment. Layer IV occupies a large fraction of the RBC cell, but with negligible temperature increment. One parameter that quantifies the shape of the mean temperature is the thermal displacement thickness [delta theta]d. The four-layer model is used to clarify the contribution of different layers to the thermal displacement thickness in a turbulent RBC. It is found that the integral in the calculation of the thermal displacement thickness mainly comes from Layer III.
https://doi.org/10.1016/j.ijheatmasstransfer.2019.119021
Numerical study of the airflow distribution in a passenger car cabin validated with PIV. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 457-467
Towards aerodynamically optimized freight wagons: an experimental study on container designs. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 437-446
Development of artificial neural networks with integrated conditional random fields capable of predicting non-linear dynamics of the flow around cylinders. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 71-79
Reynolds number dependency of the heat and mass transfer in mixed convective duct flow with condensation at a cooled wall. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 523-532
Comparison of two unstable flow states in turbulent mixed convection. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 543-552