Universität Stuttgart

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Author: search.filters.author.Weigand, BernhardSubject: search.filters.subject.670

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    ItemOpen Access
    The effect of patterned micro-structure on the apparent contact angle and three-dimensional contact line
    (2021) Foltyn, Patrick; Restle, Ferdinand; Wissmann, Markus; Hengsbach, Stefan; Weigand, Bernhard
    The measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accordingly, not only one value for the apparent contact angle can be provided, but a contact angle distribution or an interval has to be given to characterize the wetting behavior. For measuring the apparent contact angle distribution on regularly structured surfaces, namely micrometric pillars and grooves, an experimental approach is presented and the results are provided. A short introduction into the manufacturing process of such structured surfaces, which is a combination of Direct LASER Writing (DLW) lithography, electroforming and hot embossing shows the high quality standard of the used surfaces.
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    Transport of turbulence across permeable interface in a turbulent channel flow : interface-resolved direct numerical simulation
    (2020) Chu, Xu; Wang, Wenkang; Yang, Guang; Terzis, Alexandros; Helmig, Rainer; Weigand, Bernhard
    Turbulence transportation across permeable interfaces is investigated using direct numerical simulation, and the connection between the turbulent surface flow and the pore flow is explored. The porous media domain is constructed with an in-line arranged circular cylinder array. The effects of Reynolds number and porosity are also investigated by comparing cases with two Reynolds numbers (Re≈3000,6000) and two porosities (φ=0.5,0.8). It was found that the change of porosity leads to the variation of flow motions near the interface region, which further affect turbulence transportation below the interface. The turbulent kinetic energy (TKE) budget shows that turbulent diffusion and pressure transportation work as energy sink and source alternatively, which suggests a possible route for turbulence transferring into porous region. Further analysis on the spectral TKE budget reveals the role of modes of different wavelengths. A major finding is that mean convection not only affects the distribution of TKE in spatial space, but also in scale space. The permeability of the wall also have an major impact on the occurrence ratio between blow and suction events as well as their corresponding flow structures, which can be related to the change of the Kármán constant of the mean velocity profile.
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    ItemOpen Access
    Experimental investigation of a complex system of impinging jets using infrared thermography
    (2022) Schweikert, Julia; Weigand, Bernhard
    A central task in aviation technology is the development of efficient cooling techniques for thermal highly loaded engine components. For an optimal design of the cooling mechanisms, the heat transfer characteristics have to be known and need to be describable. As a cooling concept for low-pressure turbine casings, complex systems of impinging jets are used in order to reduce blade tip clearances during the flight mission. In order to improve established theoretical model approaches, this paper presents a novel method for the experimental investigation of such a complex system with 200 impinging jets using infrared thermography. The presented experimental method uses a thin electrically heated chrome-aluminum foil as target plate. Modeling the transient effects inside the foil, small structures and high gradients in the heat transfer coefficient can be reproduced with good accuracy. Experimental results of the local heat transfer characteristics are reported for jet Reynolds numbers of Re=2000…6000. The influence of the jet-to-jet distance and the jet Reynolds number on the Nusselt numbers are quantified with Nu∼(S/D)-0.47 and Nu∼Re0.7. The results indicate a dependency of the flow regime for the relatively low jet Reynolds numbers, as it is known from literature.