Comparison of dns of compressible and incompressible turbulent droplet-laden heated channel flow with phase transition

In this paper a turbulent channel flow with dispersed droplets is examined. The dispersed phase is allowed to have phase transition, which leads to heat and mass transfer between the phases, and correspondingly modulates turbulent flow properties. As a point of reference we examine the flow of water droplets in air, containing also the vapor of water. The key element of this study concerns the treatment of the carrier phase as either a compressible or an incompressible fluid. We compare simulation results obtained with a pseudo-spectral discretization for the incompressible flow to those obtained with a finite volume approach for the compressible flow. The compressible formulation is not tailored for low Mach flow and we need to resort to a Mach number that is artificially high for simulation feasibility. We discuss differences in fluid flow, heat- and mass transfer and dispersed droplet properties. The main conclusion is that both formulations give a good general correspondence. Flow properties such as velocity fields agree very closely, while heat transfer as characterized by the Nusselt number differs by around 25%. Droplet sizes are shown to be slightly larger, particularly in the center of the channel, in case the compressible formulation is chosen. A low-Mach compressible formulation is required for a fully quantitative comparison

The effect of phase transitions on the droplet size distribution in homogeneous isotropic turbulence

We investigate the dynamics of an ensemble of discrete aerosol droplets in a homogeneous, isotropic turbulent flow. Our focus is on the stationary distribution of droplet sizes that develops as a result of evaporation and condensation effects. For this purpose we simulate turbulence in a domain with periodic boundary conditions using pseudo-spectral discretization. We solve in addition equations for the temperature and for a scalar field, which represents the background humidity against which the size of the droplets evolves. We apply large-scale forcing of the velocity field to reach a statistically steady state. The droplets are transported by the turbulent field while exchanging heat and mass with the evolving temperature and humidity fields. In this Euler-Lagrange framework, we assume the droplets volume fraction to be sufficiently low to allow one-way coupling of the droplets and turbulence dynamics. The motion of the droplets is time-accurately tracked. The Stokes drag force is included in the equation of motion of the individual droplets. The responsiveness of the droplets to small turbulent scales is directly related to the size of the individual spherical droplets. We perform direct numerical simulation to ultimately obtain the probability density function of the evolving radius of the droplets at different points in time with characteristic heat and mass transfer parameters. We determine the gradual convergence of the distribution function to its statistically stationary state for forced homogeneous, isotropic turbulence

Effects of acoustic neurostimulation in healthy adults on symptoms of depression, anxiety, stress and sleep quality: a randomized clinical study

Aim: To evaluate the application of an acoustic neurostimulation program with binaural beats and isochronic tones isolated or in association, and its effects on sleep, depression, anxiety, and stress in healthy workers. Methods: A randomized, single-blind, parallel-group clinical trial, using acoustic neurostimulation with binaural beats, isochronic tones, or a combination of these in the 10 Hz range (alpha) performed with daily 20-minute sessions for 21 days. Changes in brainwave patterns were assessed by electroencephalogram (EEG). Psycho-emotional state was assessed with the Depression, Anxiety and Stress Scale 21 Items (DASS-21), and sleep quality with the Pittsburgh Sleep Quality Index (PSQI). In addition, salivary cortisol levels were evaluated as a biomarker of stress. Results: The data revealed distinct patterns of brainwave modulation via brainwave entrainment (BWE) techniques. Binaural beats and isochronic tones, alone and in combination, effectively increased alpha brainwaves in the temporoparietal region. However, when assessing theta brainwave frequencies in the same region, only binaural beats showed a significant effect. Furthermore, in the prefrontal cortex, an elevation in beta waves was exclusively observed with the use of binaural beats. These findings underscore the specificity of BWE techniques on different brainwave frequencies and regions. The study demonstrated marked improvements in several symptoms related to stress, depression, anxiety, assessed by psychometry with DASS-21 and related to sleep quality assessed by the PSQI. Conclusions: These results indicate that 10 Hz acoustic neurostimulation in the alpha range, whether through binaural beats, isochronic tones, or a combination of both, can significantly influence brainwave patterns and intensity. Notably, participants exhibited decrease in symptoms of stress, depression, and anxiety, coupled with improved sleep quality. These data suggest that alpha acoustic neurostimulation holds promise as an effective intervention for bolstering mood, mental health, and overall emotional well-being [Brazilian Clinical Trials Registry (ReBec, ensaiosclinicos.gov.br) identifier: RBR-10yj42dj]