Prof. Dr. Alexandra von Kameke
Department of Mechanical Engineering and Production
Hamburg University of Applied Sciences
Berliner Tor 21
20099 Hamburg, Germany
Email: Prof. Dr. Alexandra von Kameke
Phone: +49 40 428 75 - 8624
- Reaction-Diffusion-Advection Systems
- Faraday Flow
- Vorticity generation
- Global and local mixing dynamics and statistics
- Turbulent inter-scale kinetic energy transfer
- Pipe turbulence
- Reaction front spreading
We are glad to welcome our new employee Raffaele Colombi (M.Sc. in Aeronautical Engineering, Linköping Sweden). He will be concerned with the DFG-Project:
"Generation of energy and vorticity production by surface waves through two-dimensional turbulence effects"
We study energy condensation in quasi two-dimensional turbulence that is driven by surface waves. This physical mechanism is investigated with regard to its potential for energy production.
In two-dimensional turbulence the net energy is transferred from small scales to large scales. Energy condensation develops when large scale friction is low and energy piles up at large scales. In this way, energy condensation produces large ordered flow structures from disordered small scale forcing that drives the two-dimensional turbulence. It was shown only recently that two-dimensional turbulence can also be driven by surface waves [von Kameke et al. 2011].
However, it is unclear if two-dimensional turbulence and energy condensation can also be driven by more naturally occurring unordered forcing as for instance provided by oceanic surface waves. Further, it is not yet fully understood how non-breaking surface waves generate horizontal vorticity, and if the waves have to possess certain properties, i.e., if they need to be standing, non-linear or monochromatic [Francois et al. 2014, Filatov et al. 2016]. Additionally, the necessary boundary conditions for energy condensation are vague and need clarification. And, it needs to be addressed if the process of energy condensation is stable to the introduction of further sources of drag, i.e., when a turbine is plugged into the fluid flow in order to retrieve energy.
Here, these open points are to be investigated using a Faraday experiment [von Kameke et al. 2010, von Kameke et al. 2011, von Kameke et al. 2013]. The generation of vorticity by the surface waves and the influence of the boundary- and forcing- conditions on energy condensation will be studied as well as the velocity statistics. To this end the full unsteady three-dimensional velocity field at the water surface and below the water surface needs to be recorded which has not been investigated so far. The latest optical methods will be used, such as time-resolved high speed planar particle image velocimetry and time-resolved three-dimensional particle image velocimetry and particle tracking. The complete velocity data allows to doubtlessly verify, if the flow obtained in each case is two-dimensional and, if energy condensation takes place. Two-dimensionality is analyzed on the basis of energy and enstrophy spectra and spectral fluxes, calculated with the aid of a novel filtering method [Eyink, 1995, von Kameke et al. 2011, von Kameke et al. 2013]. Moreover, existing three- dimensional flow structures will be identified and characterized. The forcing, exerted by the surface waves on the fluid-particles, and the resulting vorticity generation will be quantified by measuring the fluid surface elevation simultaneously to the PIV measurements and the subsequent usage of Lagrangian methods [von Kameke et al. 2011, von Kameke et al. 2013, LaCasce 2008] that allow to correlate both movements. The objective of this study is to uncover a new effective mechanism to retrieve renewable energy and will broaden insight into surface wave physics and two-dimensional turbulence.
Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 395843083
|Title: Novel Evaluation Method to Determine the Local Mixing Time Distribution in Stirred Tank Reactors|
|Written by: Fitschen, J.; Hofmann, S.; Wutz, J.; Kameke, A. v.; Hoffmann M.; Wucherpfennig T.; Schlüter, M.|
|in: Chemical Engineering Science: X May 2021|
|Volume: 10 Number:|
|DOI: https://doi.org/10.1016/j.cesx.2021.100098 [Titel anhand dieser DOI in Citavi-Pr|
Abstract: Stirred tank reactors are frequently used for mixing as well as heat- and mass transfer processes in chemical and biochemical engineering due to their robust operation and extensive experiences in the past. However, for cell culture processes like mammalian cell expression systems, special requirements have to be met to ensure optimal cell growth and product quality. One of the most important requirements to ensure ideal transport processes is a proper mixing performance, characterized typically by the global mixing time or the dimensionless global mixing time .As an evaluation method for mixing time determination, the time is usually determined until a tracer signal (e.g. conductivity) has reached a constant value after a peak has been introduced (e.g. by adding a salt). A disadvantage of this method is, that the position of tracer feeding as well as the position of the probe significantly influences the detected mixing time. Further on, the global mixing time does not provide any information about the spatial and temporal ”history” of the mixing process to identify areas that are mixed poorly or areas that form stable compartments. To overcome this disadvantage, a novel image analysis will be presented in this study for the detailed characterization of mixing processes by taking into account the history of mixing. The method based on the experimental determination of the local mixing time distribution by using a multi-color change caused by a pH-change in a bromothymol blue solution. A 3 L transparent stirred tank reactor is used for the benchmark experiment. To demonstrate the suitability of the new characterization method for the validation of numerical simulations, a calculation with a commercial Lattice-Boltzmann approach (M-Star CFD) has been performed additionally and evaluated regarding mixing time distributions. The exemplary application of image analysis to a numerical mixing time simulation shows good agreement with the corresponding experiment. On the one hand, this shows that the method can also be interesting for numerical work, especially for experimental validation, and on the other hand, this allows much deeper insights into the mixing behavior compared to conventional mixing criteria. For example the new method enables the characterization of mixing on different scales as well as the identification of micor- and macroscopic flow structures. The strong influence of the acid to base ratio on mixing time experiments becomes clearly visible with the new method.