Scalar mixing at the outer edge of high density turbulent jets
Final Report Abstract
In the scope of the project period, the effects of variable material properties on the mixing processes and the TNTI in multiple flow configurations were investigated. Even though the presence of variable density and viscosity are the rule rather than the exception in real world applications, this aspect has been neglected in past studies of this kind. Early in the project, it was found that the proposed experimental methods were not suitable to investigate the flow configurations with sufficiently high density ratios with the necessary resolution. Consequently, to answer the research questions, an extensive database of high fidelity DNS was established over the course of the project period. This database far exceeds the originally proposed cases to offset the lack of experimental data. The cases were designed to be useful beyond the immediate scope of the project. The isolated effect of variable material properties on the TNTI and small scale mixing was investigated utilizing the DNS of mixing layers with constant material properties and the material properties of SF6 and N2 . The individual contributions of the transport mechanism of the enstrophy close to the TNTI differ from a configuration with homogeneous case. Furthermore, noteworthy differences in the statistics of important quantities were observed between the high density interface and low density interface. At the low density interface, it was found that the baroclinic term acts as an additional source term for the enstrophy which in turn leads to a significantly higher propagation speed of the TNTI. The influence of non-unity density ratios on the spatial transition of turbulent flows was investigated in a series of spatially evolving jets with fictitious material properties. The scaling of the penetration depth and other important quantities with the square root of the density ratio observed in experiments was also present in the DNS cases with significantly higher density ratios. Independent of the density ratio or the stream wise position, an ensthrophy threshold of ωt = 0.1∆U (x)/δω (x) proved a suitable value for the detection of the TNTI. The intermittency factor γ(y ∗ ) displayed the self-similar shape observed in homogeneous jets. However, for the lower density jet, it was found that the intermittent regions are located progressively closer to the jet with increasing distance to the nozzle in stream-wise direction. Using ∆U (x) and δω (x) in the normalization of important quantities conditioned on the distance to the TNTI, a collapse of the profiles could be achieved for the individual cases. However, further simulations are needed to fully understand and model the impact of the density ratio as well as the influence Reynolds number on these conditional means. In conclusion, the originally posed research questions could be answered and the significant influence of the density ratios on the mixing processes at the edge of jet flows was demonstrated. However, for the proposed composite PDF model for LES, these effect on the conditional means must be understood more properly. Due to the unfortunate lack of additional Reynolds numbers and experimental parameter variations, this remains a task for a future research project. The results and insight generated in the project period strongly suggest that the consideration of the observed effects of variable density in LES and RANS models will greatly improve the predictable capabilities of simulations of engineering applications.
Publications
- “Influences of Varying Density Ratios and Schmid Numbers on the Turbulent/Non-Turbulent Interface”. 16. European Turbulence Conference. 2017
D. Denker, K. Kleinheinz, A. Attili, F. Hennig, J. Boschung, and H. Pitsch
- “The turbulent/non-turbulent interface in turbulent mixing layers”. Euromech Symposium 590: Turbulent/Nonturbulent interfaces, July 3rd-5th, London, United Kingdom. 2017
A. Attili, D. Denker, K. Kleinheinz, F. Bisetti, and H. Pitsch
- “Dissipation Element Analysis of Turbulent Non-Premixed Jet Flames”. VAVIDEN Workshop (Variable Viscosity and Density Flows Workshop), June 27th - 28th, Rouen, France. 2019
D. Denker, A. Attili, M. Bode, M. Gauding, and H. Pitsch
- “Gradient Trajectory Analysis of the Burning Rate in Turbulent Premixed Jet Flames”. Combustion Science and Technology 0.0 (2020), pp. 1–19
Dominik Denker, Antonio Attili, Konstantin Kleinheinz, and Heinz Pitsch
(See online at https://doi.org/10.1080/00102202.2020.1811242)
