In this paper, a systematic discussion on the generation of synthetic turbulence using spectral methods is presented. After a brief introduction which reviews existing methodologies, the role played ...by the fulfilment of the divergence-free condition and Taylor assumption is investigated. Special attention is given to the case in which such random fields are applied as inflow condition for Computational Fluid Dynamics simulations. Subsequently, a new methodology of general applicability for the generation of synthetic turbulence is proposed. The strength of the new approach lies in its generality and conceptual simplicity. The obtained random field fulfils the divergence-free condition as well as Taylor assumption and it is approximately characterised by preselected spectral content in each spatial direction, so also providing direct control over all turbulence integral scales. Synthetic turbulent fields characterised by different spectral content are generated confirming the soundness of the proposed approach and showing its ability to target strongly anisotropic fields. Finally, some remarks on the generation of inhomogeneous fields, obtained by combination of homogeneous ones, are provided so generalising the proposed procedure.
Liquid–polymer contact electrification between sliding water drops and the surface of polytetrafluoroethylene (PTFE) was studied as a function of the pH and ionic strength of the drop as well as ...ambient relative humidity (RH). The PTFE surface was characterized by using SEM, water-contact-angle measurements, FTIR spectroscopy, XPS, and Raman spectroscopy. The charge acquired by the drops was calculated by detecting the transient voltage induced on a specifically designed capacitive sensor. It is shown that water drops become positively charged at pH > pH zch (pH zch being the zero charge point of the polymer) while they become negatively charged for pH < pH zch . The addition of non-hydrolysable salts (NaCl or CaCl 2 ) to water decreases the electrical charge induced in the drop. The charge also decreases with increasing RH. These results suggest proton or hydroxyl transfer from the liquid to the hydrophobic polymer surface. A proposed thermodynamic model for the ion transfer process allows explaining the observed effects of RH, pH and ionic strength.
Neutron diffraction data with hydrogen isotope substitution on aqueous solutions of NaCl and KCl at concentrations ranging from high dilution to near-saturation are analyzed using the Empirical ...Potential Structure Refinement technique. Information on both the ion hydration shells and the microscopic structure of the solvent is extracted. Apart from obvious effects due to the different radii of the three ions investigated, it is found that water molecules in the hydration shell of K+ are orientationally more disordered than those hydrating a Na+ ion and are inclined to orient their dipole moments tangentially to the hydration sphere. Cl- ions form instead hydrogen-bonded bridges with water molecules and are readily accommodated into the H-bond network of water. The results are used to show that concepts such as structure maker/breaker, largely based on thermodynamic data, are not helpful in understanding how these ions interact with water at the molecular level.
Here, a mathematical model is presented, which accounts for the dependence of the surface electrical charge density (σ) on pH and the concentration of added salts (C s), generated when a water drop ...rolls or slides on the surface of a hydrophobic polymer, a process known as liquid–polymer contact electrification (LPCE). The same model was successfully applied to fit the isotherms of ξ-potential as a function of pH, reported in the literature by other authors for water–poly(tetrafluoroethylene) (PTFE) interfaces. Hence, the dependence of σ and ξ on pH was described using the same concept: acid–base equilibria at the water–polymer interface. Equilibrium constants were estimated by fitting experimental isotherms. The experimental results and the model are consistent with a number of 10–100 acid–base sites/μm2. The model predicts the increase of |σ| and |ξ| with pH in the range of 2–10 and the existence of a zero-charge point at pHzcp ≅ 3 for PTFE (independent of C s). Excellent fits were obtained with K a/K b ∼ 9 × 107, where K a and K b are the respective acid and base equilibrium constants. On the other hand, the observed decrease in |σ| and |ξ| with C s at fixed pH is quantitatively described by introducing an activity factor associated with the quenching of water activity by the salt ions at the polymer–water interface, with quenching constant K q. Additionally, the quenching predicts a decrease in |σ| and |ξ| at extreme pH, where I > (1/K q) (I: ionic strength), in agreement with literature reports.
It is well known that the generation of appropriate unsteady boundary conditions represents an important component of successful Large Eddy Simulations in turbulent flows. In particular, when ...Computational Wind Engineering applications are considered, a recurrent problem consists in imposing turbulent fluctuations characterized by given spectra and length scales at the inlet boundary. In the present contribution, firstly, currently available techniques for the generation of synthetic turbulence are revised with a focus on their mathematical formulation. Then, two new approaches for the generation of homogeneous turbulence are proposed. The first one can be seen as a correction over existing techniques which allows to control the obtained length scales. The second method, conceived to generate anisotropic turbulence characterized by arbitrary harmonic content in both time and space, is designed to be computationally efficient, to guarantee the divergence-free condition and to ensure a good approximation of the resulting turbulence integral scales. Finally, the procedure is validated by synthesizing a homogeneous turbulent field characterized by time and length scales typical of the atmospheric boundary layer showing good results.
•Traditional methods used for the generation of synthetic turbulence are reviewed.•The mechanisms embedded in the DSRFG method are discussed in detail.•Two new approaches to the generation of homogeneous divergence-free velocity fields are proposed.•An anisotropic field characterized by prescribed spectra and length scales is successfully synthesized.
•Wind loads on a low-rise building are evaluated by means of LES.•The incoming flow turbulence is synthetically generated by means of the MDSRFG method.•LES results are compared in terms of pressures ...to experiments for three angles of attack.•A comparison between experimental and numerical results in terms of design forces is provided.
The correct and safe design of structures subjected to the wind actions requires a realistic estimate of the wind effects on their resisting systems. In this context, the present paper proposes a complete numerical study that, starting from Large Eddy Simulation of the turbulent flow around a low-rise building, arrives to the assessment of the wind loading effects, that is the evaluation of design forces in all structural elements. Since it is well known that a realistic representation of the turbulent features found in the lower part of the atmospheric boundary layer is required in order to obtain accurate predictions of the structural response, firstly, the incoming flow turbulence is synthetically generated by means of the Modified Discretizing and Synthesizing Random Flow Generator technique. Then, the obtained synthetic fluctuation field is used as inflow condition for the subsequent Large Eddy Simulations taking into consideration different angles of attack. Results in terms of pressure distributions statistics are analyzed and systematically compared to experimental data. Finally, starting from both simulated and experimental pressure fields, dynamic structural analyses are performed and results directly compared in terms of design forces in the structural elements.
Biosimilars are increasingly available for the treatment of many serious disorders, however some concerns persist about switching a patient to a biosimilar whose condition is stable while on the ...reference biologic. Randomized controlled studies and extension studies with a switch treatment period (STP) to or from a biosimilar and its reference biologic were identified from publicly available information maintained by the U.S. Food and Drug Administration (FDA). These findings were augmented with data from peer reviewed publications containing information not captured in FDA reviews. Forty-four STPs were identified from 31 unique studies for 21 different biosimilars. Data were extracted and synthesized following PRISMA guidelines. Meta-analysis was conducted to estimate the overall risk difference across studies. A total of 5,252 patients who were switched to or from a biosimilar and its reference biologic were identified. Safety data including deaths, serious adverse events, and treatment discontinuation showed an overall risk difference (95% CI) of -0.00 (-0.00, 0.00), 0.00 (-0.01, 0.01), -0.00 (-0.01, 0.00) across STPs, respectively. Immunogenicity data showed similar incidence of anti-drug antibodies and neutralizing antibodies in patients within a STP who were switched to or from a biosimilar to its reference biologic and patients who were not switched. Immune related adverse events such as anaphylaxis, hypersensitivity reactions, and injections site reactions were similar in switched and non-switched patients. This first systematic review using statistical methods to address the risk of switching patients between reference biologics and biosimilars finds no difference in the safety profiles or immunogenicity rates in patients who were switched and those who remained on a reference biologic or a biosimilar.
PDF
