@phdthesis{9728,
  abstract     = {Most real-world flows are multiphase, yet we know little about them compared to their single-phase counterparts. Multiphase flows are more difficult to investigate as their dynamics occur in large parameter space and involve complex phenomena such as preferential concentration, turbulence modulation, non-Newtonian rheology, etc. Over the last few decades, experiments in particle-laden flows have taken a back seat in favour of ever-improving computational resources. However, computers are still not powerful enough to simulate a real-world fluid with millions of finite-size particles. Experiments are essential not only because they offer a reliable way to investigate real-world multiphase flows but also because they serve to validate numerical studies and steer the research in a relevant direction. In this work, we have experimentally investigated particle-laden flows in pipes, and in particular, examined the effect of particles on the laminar-turbulent transition and the drag scaling in turbulent flows.

For particle-laden pipe flows, an earlier study [Matas et al., 2003] reported how the sub-critical (i.e., hysteretic) transition that occurs via localised turbulent structures called puffs is affected by the addition of particles. In this study, in addition to this known transition, we found a super-critical transition to a globally fluctuating state with increasing particle concentration. At the same time, the Newtonian-type transition via puffs is delayed to larger Reynolds numbers. At an even higher concentration, only the globally fluctuating state is found. The dynamics of particle-laden flows are hence determined by two competing instabilities that give rise to three flow regimes: Newtonian-type turbulence at low, a particle-induced globally fluctuating state at high, and a coexistence state at intermediate concentrations.

The effect of particles on turbulent drag is ambiguous, with studies reporting drag reduction, no net change, and even drag increase. The ambiguity arises because, in addition to particle concentration, particle shape, size, and density also affect the net drag. Even similar particles might affect the flow dissimilarly in different Reynolds number and concentration ranges. In the present study, we explored a wide range of both Reynolds number and concentration, using spherical as well as cylindrical particles. We found that the spherical particles do not reduce drag while the cylindrical particles are drag-reducing within a specific Reynolds number interval. The interval strongly depends on the particle concentration and the relative size of the pipe and particles. Within this interval, the magnitude of drag reduction reaches a maximum. These drag reduction maxima appear to fall onto a distinct power-law curve irrespective of the pipe diameter and particle concentration, and this curve can be considered as the maximum drag reduction asymptote for a given fibre shape. Such an asymptote is well known for polymeric flows but had not been identified for particle-laden flows prior to this work.},
  author       = {Agrawal, Nishchal},
  issn         = {2663-337X},
  keywords     = {Drag Reduction, Transition to Turbulence, Multiphase Flows, particle Laden Flows, Complex Flows, Experiments, Fluid Dynamics},
  pages        = {118},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Transition to turbulence and drag reduction in particle-laden pipe flows}},
  doi          = {10.15479/at:ista:9728},
  year         = {2021},
}

@article{22433,
  abstract     = {Process-based hydrological models have a long history dating back to the 1960s. Criticized by some as over-parameterized, overly complex, and difficult to use, a more nuanced view is that these tools are necessary in many situations and, in a certain class of problems, they are the most appropriate type of hydrological model. This is especially the case in situations where knowledge of flow paths or distributed state variables and/or preservation of physical constraints is important. Examples of this include: spatiotemporal variability of soil moisture, groundwater flow and runoff generation, sediment and contaminant transport, or when feedbacks among various Earth’s system processes or understanding the impacts of climate non-stationarity are of primary concern. These are situations where process-based models excel and other models are unverifiable. This article presents this pragmatic view in the context of existing literature to justify the approach where applicable and necessary. We review how improvements in data availability, computational resources and algorithms have made detailed hydrological simulations a reality. Avenues for the future of process-based hydrological models are presented suggesting their use as virtual laboratories, for design purposes, and with a powerful treatment of uncertainty.},
  author       = {Fatichi, Simone and Vivoni, Enrique R. and Ogden, Fred L. and Ivanov, Valeriy Y. and Mirus, Benjamin and Gochis, David and Downer, Charles W. and Camporese, Matteo and Davison, Jason H. and Ebel, Brian and Jones, Norm and Kim, Jongho and Mascaro, Giuseppe and Niswonger, Richard and Restrepo, Pedro and Rigon, Riccardo and Shen, Chaopeng and Sulis, Mauro and Tarboton, David},
  issn         = {1879-2707},
  journal      = {Journal of Hydrology},
  keywords     = {Modeling, Interdisciplinary, Watershed processes, Virtual experiments, Change assessments, Natural and built environment},
  pages        = {45--60},
  publisher    = {Elsevier},
  title        = {{An overview of current applications, challenges, and future trends in distributed process-based models in hydrology}},
  doi          = {10.1016/j.jhydrol.2016.03.026},
  volume       = {537},
  year         = {2016},
}

@article{22476,
  abstract     = {Projections of the future carbon and water cycles rely on knowledge on how forests will respond to rising atmospheric CO2. Experiments with elevated CO2 are logistically challenging and carbon pools and fluxes are difficult to measure and upscale due to their spatiotemporal heterogeneity. Therefore, it is important to combine the knowledge derived from experimental results with modeling. Here, we systematically compare data from a free air CO2 enrichment (FACE) experiment in a mature deciduous forest in Switzerland with realizations from an ecohydrological model (Tethys–Chloris). We test whether a mechanistic ecohydrological model is able to simulate physiological plant responses under ambient and elevated CO2 concentration. We overcome measurement limitations by quantifying differences in response to ambient and elevated CO2 over ten years. The reliability of model realizations is demonstrated by comparing simulations with field observations of stomatal conductance, sap flow, leaf and fruit litter, and stem growth. The model successfully captures the observed CO2-induced difference in stomatal conductance and transpiration and its sensitivity to atmospheric demand, as well as qualitative changes in soil moisture. The simulated differences between CO2 scenarios generally fall within the uncertainty of experimental observations, both for the carbon and water balance. Simulated total evapotranspiration is 2.8% (18 mm yr−1) lower and soil moisture 1.2% higher in the CO2-enriched scenario. Latent and sensible heat are modified by ca. 1 W m−2. Net primary production is simulated to increase by 19.8% and allocation to stem growth is 53 gC yr−1 m−2 higher in the elevated CO2 scenario, which represents the limit of the detection threshold of the experiment. Results show that while ecohydrological models can be used to reliably simulate multi-year energy, water, and carbon fluxes at the stand level, testing carbon allocation remains critical with current accuracy of field measurements. Uncertainties due to the simplified carbon allocation scheme are shown to be more significant for carbon than for energy and water fluxes. Generally, we conclude that for this type of forest, differences in annual energy and water fluxes induced by elevated CO2 are likely to be less than 10%.},
  author       = {Fatichi, Simone and Leuzinger, Sebastian},
  issn         = {1873-2240},
  journal      = {Agricultural and Forest Meteorology},
  keywords     = {Ecohydrological modeling, Plant water relations, FACE experiments, Forest productivity, CO2 enrichment, Swiss Canopy Crane},
  pages        = {144--157},
  publisher    = {Elsevier},
  title        = {{Reconciling observations with modeling: The fate of water and carbon allocation in a mature deciduous forest exposed to elevated CO2}},
  doi          = {10.1016/j.agrformet.2013.02.005},
  volume       = {174-175},
  year         = {2013},
}