Jens Honore Walther

Theoretical models of capillary flow typically rely on macroscopic governing parameters, such as the dynamic contact angle and effective viscosity. However, as pore dimensions approach the nanoscale, the curvature dependence of surface tension may become relevant, affecting theoretical predictions of capillary flow. The length scales and magnitude of this effect remain unresolved. In this work, we employ all-atom molecular dynamics simulations to investigate spontaneous water imbibition in parallel-plate silica nanochannels with heights ranging from 4.5 to 10.5 nm. The simulation data are used to extract dynamic contact angles, velocity profiles, inertial regimes, and the mean curvature of the menisci, enabling a direct evaluation of the Lucas–Washburn–Bosanquet (LWB) equation. For the 10.5 nm channel, incorporating the dynamic contact angle model into the LWB equation yields excellent agreement with the capillary filling simulation. However, as the channel height decreases, a systematic departure between the theoretical model and the simulation results is observed. To address this, the curvature dependence of surface tension is incorporated using an upper-bound value for the Tolman length. This correction introduces an increase in the effective surface tension, partially compensating for the discrepancies observed in narrower channels. The resulting surface tension enhancement ranges from 11% at 4.5 nm to 4% at 10.5 nm, suggesting that curvature effects become negligible at larger nanoscale confinements. These results indicate that the curvature dependence of surface tension contributes to deviations from classical capillary models at the nanoscale, but does not fully account for them.

Two-phase turbines offer the potential to significantly enhance the performance of power generation and refrigeration systems. However, their development has been hindered by comparatively lower efficiencies resulting from additional loss mechanisms absent in single-phase turbines. In this context, computational fluid dynamics (CFD) emerges as a crucial tool to predict the performance of two-phase turbines and guide the design process toward higher efficiency. To date, most multiphase CFD studies on turbomachinery have focused on condensation in the final stages of steam turbines, and on cavitation in hydraulic pumps and turbines. These applications, however, are not representative of the conditions in two-phase turbines, where a liquid-dominated mixture undergoes a large expansion ratio, leading to a significant increase in the gas phase volume fraction throughout the entire flow. Recognizing the lack of an established modeling approach, this paper aims to identify a suitable modeling methodology for two-phase turbines. Our evaluation is centered around two models: the mixture model and the barotropic model. The validity and accuracy of these two modeling approaches are assessed using existing experimental data from a single-stage impulse turbine operating with several mixtures of water and nitrogen as working fluid. The results indicate that both the mixture and barotropic models are consistent and accurately predict the nozzle mass flowrate, yet, both models systematically overpredict the nozzle exit velocity and rotor torque. Adding correction terms for windage and unsteady pumping losses significantly improves the torque predictions, bringing them within the uncertainty range of the experimental data. In addition, refining the models to account for the effect of slip presents a promising avenue to enhance the prediction of nozzle exit velocity and overall performance of two-phase turbines.

The present paper reports on wind tunnel tests and analyses carried out to investigate the effect of the static angle of attack on the aerodynamic stability of a twin-box bridge deck section. It is found that the critical wind speed for onset of flutter increases with increasing positive static angles (nose-up) and that this effect relates mainly to a decrease in the loss of aerodynamic stiffness. A simplified flutter analysis, linking the slope of the static moment coefficient to the increase of flutter stability for increasing positive angles of attack. It is concluded that it is desirable to design twin-box bridge deck sections to have a positive moment coefficient at zero angle of attack and a positive, decreasing moment slope for increasing nose-up angles. With these requirements fulfilled, the present study show that the critical wind speeds for onset of flutter increase with increasing angles, and ensures that the elastically supported deck will always meet the wind at ever increasing angles for increasing wind speeds.

Understanding the effects of phenomena occurring at electrically charged interfaces, such as charge inversion (CI), is crucial for enabling electroosmosis as an efficient transport mechanism in nanodevices. Here, we employ molecular dynamics (MD) simulations to systematically analyze the effect of CI on the electrokinetic transport of multivalent ionic solutions confined in amorphous silica nanochannels. We employ mixtures of monovalent and multivalent counterions while fixing the total ionic concentration to establish correlations between observed phenomena and the amount of multivalent ionic species in the electrolyte solution. The results show that the development of CI is related to a decrease in the mobility of the fluid layers adjacent to the charged surface. In addition, we observe that interfacial overcharging disrupts the water molecular orientation in the fluid layers adjacent to the channel walls. From the non-equilibrium MD simulations of electro-osmotic flow, we disclose the influence of phenomena related to the presence of CI. In particular, flow reversal occurs in scenarios involving CI due to increased local viscosity and a higher concentration of coions within the hydrodynamically mobile and electrokinetically active region of the charged interface. We also find that the magnitude of the wall zeta (ζ) potential displays a monotonic increase with the development of CI in the system. Moreover, we explain why positioning the wall ζ potential at an imaginary (slip) plane, which separates the hydrodynamically mobile and immobile fluid, is misleading.