| 1. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
-
Breakup of small aggregates in bounded and unbounded turbulent flows
- 2020
-
Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
-
Konferensbidrag (refereegranskat)abstract
- Breakup of small tracer-like aggregates is studied by means of numerical simulations in four different flows, namely homogeneous isotropic turbulence, smooth stochastic flow, turbulent channel flow, and developing boundary layer flow. Aggregate breakup occurs when the local hydrodynamic stress σ ∼ ε1/2, where ε is the local energy dissipation, overcomes a given threshold value σcr [or equivalently εcr ∼ σcr2 ] characteristic for a given type of aggregates. Following the aggregate trajectory upon release and detecting the first occurrence of local energy dissipation exceeding the predefined threshold allows for estimating the breakup rate as a function of εcr. Results show that the breakup rate decreases with increasing threshold. For small values of the threshold, this decrease assumes consistent scaling among the different flows which is explained by universal small scale flow properties.
|
|
| 2. |
- Bäbler, Matthäus, et al.
(författare)
-
Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
- 2015
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 766
-
Tidskriftsartikel (refereegranskat)abstract
- Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.
|
|
| 3. |
- Habibi Khorasani, Seyed Morteza
(författare)
-
Turbulent flows over permissive boundaries and porous walls
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates how in wall-bounded turbulent flows changes to the wall can induce changes in the flow. To this end, we investigate the use of wall boundary conditions meant to mimic the effect of textured surfaces. We also study the effect of porous walls on their overlying bulk turbulent flow and the consequences of these effects for heat transfer.Textured surfaces can alter near-wall turbulence in subtle to dramatic ways. Vanishingly small surface textures of the order of the viscous sublayer thickness cause the displacement of the near-wall turbulence-generating flow structures by either restricting or permitting transpiration from taking place close to the surface. The former leads to drag reduction and the latter to its increase. As the textures increase in size and become comparable to the turbulence scales, they alter the near-wall dynamics and cause structural changes to occur in the flow. These effects are emulated using slip and transpiration boundary conditions called the Transpiration-Resistance model (TRM). Its utility in acting as an effective model for wall roughness is assessed. It captures the effect of vanishingly small roughness well, and to a limited extent larger roughness which protrude into the buffer layer. The TRM also helps to shed light on which near-wall structures play an essential role in the near-wall cycle of turbulence.Porous walls permit the exchange of mass, momentum and energy with the overlying turbulence. Their permeable structure quickly causes the turbulence to depart from its canonical smooth-wall-like structure, and induce a Kelvin-Helmholtz-like instability which leads to the emergence of spanwise rollers. These rollers efficiently redistribute momentum and turbulent kinetic energy into the porous wall. This pronounced interaction between the wall and bulk flow regions is detrimental for drag but beneficial for the transport of heat. The potential of porous walls for enhancing heat transfer exceeds that of other passive wall structures such as roughness.As an elementary study of the type of fluid-fluid-solid interactions which can take place on the microscale within porous media, the stability of a cylinder-wrapping corner film is investigated. It is shown that linear stability analysis (LSA) can predict the number of primary droplets when the film breaks up. The film morphology, however, exhibits complexities which cannot be predicted using LSA. A disjoining-pressure model (DPM) demonstrates that smaller secondary droplets may emerge during the film breakup process. Additionally, Volume of Fluid (VoF) simulations show that two initially emerging primary droplets may eventually coalesce into one, highlighting the non-linear mechanisms involved in the film morphology evolution.
|
|
| 4. |
- Wang, Jietuo, et al.
(författare)
-
Modelling the direct virus exposure risk associated with respiratory events
- 2022
-
Ingår i: Journal of the Royal Society Interface. - : The Royal Society. - 1742-5689 .- 1742-5662. ; 19:186, s. 20210819-20210819
-
Tidskriftsartikel (refereegranskat)abstract
- The outbreak of the COVID-19 pandemic highlighted the importance of accurately modelling the pathogen transmission via droplets and aerosols emitted while speaking, coughing and sneezing. In this work, we present an effective model for assessing the direct contagion risk associated with these pathogen-laden droplets. In particular, using the most recent studies on multi-phase flow physics, we develop an effective yet simple framework capable of predicting the infection risk associated with different respiratory activities in different ambient conditions. We start by describing the math- ematical framework and benchmarking the model predictions against well-assessed literature results. Then, we provide a systematic assessment of the effects of physical distancing and face coverings on the direct infection risk. The present results indicate that the risk of infection is vastly impacted by the ambient conditions and the type of respiratory activity, suggesting the non-existence of a universal safe distance. Meanwhile, wearing face masks provides excellent protection, effectively limiting the transmission of pathogens even at short physical distances, i.e. 1 m.
|
|
