1,355,004 research outputs found
Backward energy transfer and subgrid modeling approaches in wall-turbulence
No full textWe report here results from a Large Eddy Simulation (LES) of a turbulent channel flow at a friction Reynolds number Reτ = 550 performed with a new subgrid modeling approach proposed by the same authors in Cimarelli et al., Phys. Fluids, 26, 055103 (2014), [1]. This subgrid scale model aims at reproducing the double feature of energy sink and source of the small scales of wall flows which become relevant when large filter lengths are adopted. Here we report a further analysis of the model by considering the instantaneous behavior of events of backward and forward energy transfer
Backward energy transfer and subgrid modeling approaches in wall-turbulence
No full textWe report here results from a Large Eddy Simulation (LES) of a turbulent channel flow at a friction Reynolds number Reτ = 550 performed with a new subgrid modeling approach proposed by the same authors in Cimarelli et al., Phys. Fluids, 26, 055103 (2014), [1]. This subgrid scale model aims at reproducing the double feature of energy sink and source of the small scales of wall flows which become relevant when large filter lengths are adopted. Here we report a further analysis of the model by considering the instantaneous behavior of events of backward and forward energy transfer
Dynamic tensorial eddy viscosity model: Effects of compressibility and of complex geometry
No full textA previous paper by Cimarelli et al. ["General formalism for a reduced description and modelling of momentum and energy transfer in turbulence,"J. Fluid Mech. 866, 865-896 (2019)] has shown that every decomposition of turbulent stresses is naturally approximated by a general form of tensorial eddy viscosity based on velocity increments. The generality of the formalism is such that it can also be used to give a reduced description of subgrid scalar fluxes. In the same work, this peculiar property of turbulent stresses and fluxes has been dynamically exploited to produce tensorial eddy viscosity models based on the second-order inertial properties of the grid element. The basic idea is that the anisotropic structure of the computational element directly impacts, although implicitly, the large resolved and small unresolved scale decomposition. In the present work, this new class of turbulence models is extended to compressible turbulence. A posteriori analysis of flow solutions in a compressible turbulent channel shows very promising results. The quality of the modeling approach is further assessed by addressing complex flow geometries, where the use of unstructured grids is demanded as in real world problems. Also in this case, a posteriori analysis of flow solutions in a periodic hill turbulent flow shows very good behavior. Overall, the generality of the formalism is found to allow for an accurate description of subgrid quantities in compressible conditions and in complex flows, independent of the discretization technique. Hence, we believe that the present class of turbulence closures is very promising for the applications typical of industry and geophysics
Assessment of the turbulent energy paths from the origin to dissipation in wall-turbulence
No full textThe present study is devoted to the description of the energy fluxes from production to dissipation in the augmented space (3-dimensional space of scales plus wall-distance) of wall-turbulent flows. As already shown in Cimarelli et al. (2010), an interesting behavior of the energy fluxes comes out from this analysis consisting of spiral-like paths in the combined physical/scale space where the controversial reverse energy cascade plays a central role. The observed behaviour conflicts with the classical notion of the Richardson/Kolmogorov energy cascade and may have strong repercussions on both theoretical and modeling approaches to wall-turbulence. Two dynamical processes are identified as driving mechanisms for the fluxes, one in the near wall region and a second one further away from the wall. The former, stronger one is related to the dynamics involved in the near-wall cycle. The second suggests an outer self-sustaining mechanism. Here we extend these results to larger Reynolds number using LES data of a turbulent channel flow at Re τ = 970 confirming the presence of an outer regeneration cycle which seems to be composed by systems of attached eddies
A novel high-fidelity two-way coupling model for fluid-structure interaction in wind energy
No full textTo increase the attractiveness of wind energy, wind turbines are continuously scaling up, with diameters now exceeding 200 m. If on the one hand, this trend guarantees an increased power production, on the other hand, it imposes harsher aerodynamical and structural requirements – on the blades in particular – that are difficult to characterise. In particular, the significant size of the state-of-the-art wind turbines suggests a more relevant Fluid-Structure Interaction (FSI) that could alter dramatically the operating life of the full machine. Given the difficulties and the costs of measuring the phenomena occurring at significant scales, researchers advocate the development of high-fidelity numerical models exploiting Computational Fluid and Structural Dynamics (CFD-CSD models). For this reason, in this work we present a novel FSI model for wind turbines combining our Large Eddy Simulation (LES) fluid solver with a modal beam-like structural solver. In the first part of the work, we present the details of our FSI methodology, and we analyse the effects of different coupling conditions. A loose algorithm couples the Actuator Line Model (ALM), which represents the blades in the fluid domain by means of body forces, with the structural model, which represents the flexural and torsional deformations. For a reference utility-scale wind turbine, we compare the results of three sets of simulations. Firstly, we consider one-way coupled simulations where only the fluid solver provides the structural solver with the aerodynamic loads; then, we consider two-way coupled simulations where
the structural feedback to the fluid solver is made of the out-of-plane and in-plane bending deformation velocities only; finally, we add to the feedback also the torsional deformation.
However, to accurately reproduce the airloads, one should notice that the blades in particular are subjected to many relevant sources of unsteadiness, e.g. tower shadowing, yawed and waked conditions, environmental effects. Therefore, researchers have questioned the use of steady aerodynamics in the numerical fluid and aeroelastic models used in wind energy that do not have the sufficient resolution to solve the flow close to the blade, arguing that the use of tabulated airfoil coefficients could neglect effects that alter the estimation of the turbine behaviour. Different unsteady aerodynamics models have been proposed to account for these effects but have been mainly implemented in low-fidelity engineering models, which lack the complete capability of describing the multiscale and multi-physics phenomena characterising the wind turbine. For this reason, in the second part of the work, a 2D unsteady aerodynamics model is implemented in the sectional estimation of the airloads of the Actuator Line Model. At each section of the blade, a semi-empirical Beddoes-Leishman model includes the effects of additional noncirculatory terms, unsteady trailing edge separation and dynamic stall in the dynamic evaluation of the aerodynamic coefficients of the airfoils, used to determine the ALM body forces. Different inflow conditions and aeroelastic behaviours are examined with the aim of examining the effects of the model, and thus of providing a deeper insight into the unsteady characterisation of large wind turbines by means of a high-fidelity CFD-CSD model
Analysis of the Kolmogorov equation for filtered wall-turbulent flows
No full textThe analysis of the energy transfer mechanisms in a filtered wall-turbulent flow is traditionally accomplished via the turbulent kinetic energy balance, as in H ̈artel et al. (Phys. Fluids, vol. 6, 1994, p. 3130) or via the analysis of the energy spectra, as in Domaradzki et al. (Phys. Fluids, vol. 6, 1994, p. 1583). However, a generalized Kolmogorov equation for channel flow has recently been proven successful in accounting for both spatial fluxes and energy transfer across the scales in a single framework by Marati, Casciola & Piva (J. Fluid Mech., vol. 521, 2004, p. 191). In this context, the same machinery is applied for the first time to a filtered velocity field. The results will show what effects the subgrid scales have on the resolved motion in both physical and scale space, singling out the prominent role of the filter scale compared to the cross-over scale between production-dominated scales and inertial range, lc, and the reverse energy cascade region ΩB. Finally, we will briefly discuss how the filtered Kolmogorov equation can be used as a new tool for the assessment of large eddy simulation (LES) models. Classical purely dissipative eddy viscosity models will be analysed via an a priori procedur
Anisotropic dynamics and sub-grid energy transfer in wall-turbulence
No full textPurpose of the present work is the analysis of the generalized Kolmogorov equation applied to the direct numerical simulation data of a turbulent channel flow. The multi-dimensional description of the anisotropic behavior of turbulent energy production, transport, and dissipation is shown to be relevant for the understanding and modeling of the wall-turbulent physics with special care to the phenomenon of reverse energy flux. These results are proven instrumental also for the correct computation of wall-turbulence when a large eddy simulation approach is considered. The capability of a filtered velocity field to correctly reproduce the wall-turbulent dynamics at different ranges of scales and wall-distances as a function of the filter length will be assessed via filtered direct numerical simulation (DNS) and large eddy simulation data. The possibility of new modeling approaches is also highlighted
The CeCl3 Lewis Acid Promoter in the Stereoselective Construction of Carbon-Carbon Double Bonds
No full textThe presence of a C-C double bond in polyfunctionalized organic molecules is a crucial requirement
for the control of its biologically activity.(1) The importance of having a site in the molecule that is
able to generate geometrical isomerization of a carbon-carbon double bond stimulated the
development of new olefination methodologies. In particular, some efforts focused on the ability of
Lewis acids to provide a cheap alternative for the synthesis of molecules with C-C double bond in a
highly stereoselective fashion.
For several years, we have been investigating CeCl3 promoted organic reactions. This Lewis acid has
been found to efficiently promote carbon-carbon (2) and carbon-heteroatom bond formation
reactions.(3) In addition to being green in nature (4), CeCl3 has been widely used for both inter- and
intramolecular reactions for the synthesis of organic molecules with significant biological
importance.
Regarding the total synthesis of biologically active small molecules containing a carbon-carbon
double bond, we saw the possibility to employ CeCl3 in the stereoselective construction of 2,3-
dihydropyridones 1,(5) and 1,2-dihydroquinolines 2.(6)
The additional advantage of using CeCl3 in a reaction includes its selectivity and tolerance in the
presence of other functional groups. For instance, it can be used during the functionalization of
molecules at late stage involving complex molecules or undesirable use of protecting groups.
Introduction of C-C double bonds, which are known to increase the activity in macrolides against
bacterial RNA polymerase, is currently in progress in our laboratory.
References: 1. Shen, X.; Nguyen, T. T.; Koh, M. J.; Xu, D.; Speed, A. W.; Schrock, R. R.; Hoveyda, A. H. Nature 2017,
541, 380-385. 2. Bartoli, G.; Marcolin, M.; Sambri, L.; Marcantoni, E. Chem. Rev. 2010, 110, 6104-6143. 3. Properzi, R.
Marcantoni, E. Chem. Soc. Rev. 2014, 43, 779-791. 4. Cimarelli, C.; Di Nicola, M.; Diomedi, S.; Giovannini, R.;
Hanprecht, D.; Properzi, R.; Sorana, F.; Marcantoni, E. Org. Biomol. Chem. 2015, 13, 11687-11695. 5. Bordi, S.;
Cimarelli, C.; Lupidi, G.; Marsili, L.; Piermattei, P.; Marcantoni, E. J. Org. Chem. 2017, in preparation. 6. Cimarelli, C.;
Bordi, S.; Piermattei, P.; Pellei, M.; Del Bello, F.; Marcantoni, E. Tetrahedron 2017, submitted
Turbulent production and subgrid dynamics in wall flows
No full textThe Kolmogorov equation generalized to wall-turbulence has been recently proven to give a detailed description of the multi-dimensional features of such flows[1]. As emerging from this approach, the small scales of wall turbulence are found to drive the quasi-coherent motion at large scales through a reverse energy transfer. At the base of this phenomenology is the focusing of production of turbulent fluctuations at small scales. These observations may have strong repercussion on both theoretical and modeling approaches to wall-turbulence. Here, we aim at using the Kolmogorov equation not only for the study of the mechanisms altering the energy transfer but also for modeling purpose
A priori analysis and benchmarking of the flow around a rectangular cylinder
No full textThe flow around bluff bodies is recognized to be a rich topic due to its huge number of applications in natural and engineering sciences. Of particular interest is the case of blunt bodies where a reattachment of the separated boundary layer before the definitive separation in the wake occurs. One of the main feature of this type of flows is the combined presence of small scales due to the occurrence of self-sustained turbulent motions and large scales due to classical vortex shedding. The complete understanding of these multiple interacting phenomena would help for a correct prediction and control of relevant features for engineering applications such as wind loads on buildings and vehicles, vibrations and acoustic insulation, heat transfer efficiency and entrainment. Archetypal of these kind of flows is the flow around a rectangular cylinder. Many studies have been carried out in the past. The general aim is the understanding of the main mechanisms behind the two unstediness of the flow, the shedding of vortices at the leading-edge shear layer and the low-frequency flapping mode of the separation bubble, see e.g Cherry et al (J Fluid Mech, 144:13–46, 1984, [1]), Kiya and Sasaki (J Fluid Mech, 154:463–491, 1985[2]), Nakamura et al (J Fluid Mech, 222:437–447, 1991, citeNakamura)
- …
