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The effect of the rotation on the turbulent mixing of two miscible fluids of small contrasting density, induced by Faraday instability, is investigated using direct numerical simulations. We quantify the irreversible mixing, which depicts the conversion of the available potential energy (APE) to the background potential energy (BPE) through the irreversible mixing rate M. We demonstrate that at lower forcing amplitudes, the turbulent kinetic energy (t.k.e.) increases with an increase in the Coriolis frequency f till (f / ω) 2 < 0.25 , where ω is the forcing frequency, during the sub-harmonic instability phase. This enhancement of t.k.e. is attributed to the excitement of more unstable modes. The irreversible mixing sustains for an extended period with increasing (f / ω) 2 till 0.25 owing to the prolonged sub-harmonic instability phase and eventually ceases with instability saturation. When (f / ω) 2 > 0.25 , the Coriolis force significantly delays the onset of the sub-harmonic instabilities. The strong rotational effects result in lower turbulence because the bulk of the APE expends to BPE, decreasing APE that converts back to t.k.e. reservoir for (f / ω) 2 > 0.25. Therefore, in the subsequent oscillation, the t.k.e. available to contribute to the external energy input from periodic forcing is small. Since the instability never saturates for (f / ω) 2 > 0.25 , the conversion of APE to BPE via M continues, and we find prolonged irreversible mixing. At higher forcing amplitudes, the instability delaying effect of rotation is negligible, and the turbulence is less intense and short-lived. Therefore, the irreversible mixing phenomenon also ends quickly for (f / ω) 2 < 0.25. However, when (f / ω) 2 > 0.25 , a continuous irreversible mixing is observed. We also examine the mixing efficiency in terms of M and find that the mixing is efficient at lower forcing amplitudes and rotation rates of (f / ω) 2 > 0.25 because the major portion of APE expends to BPE. [ABSTRACT FROM AUTHOR]

The flow characteristics and heat transfer during nasal breathing in the complete human upper airway were investigated through the respiratory cycle using transient numerical simulations. We postulate that the complete airway from the nasal cavity to the trachea most accurately represents dynamic airflow patterns during inhalation and exhalation as they are likely to be affected by downstream anatomical structures. A 3D model was constructed from a healthy adult computed tomography scan. Computational fluid dynamics simulations were performed with Ansys Fluent software [ANSYS Fluent, R1 User's Guide (ANSYS, Inc., 2020)] using the stress-blended eddy simulation turbulence model looking at airflow patterns, velocity, mucosal temperature, and humidity (H2O fraction). One and a half breathing cycles were simulated for a total of 5.65 s, where the first inhalation cycle was discarded to avoid start-up effects. The results demonstrated that airway geometry structures, including the turbinates, the soft palate, and the glottic region, affect the flow patterns differently during inspiration and expiration. It also demonstrated phenomena not seen in steady flow simulations or in those without the lower respiratory tract geometry, including the nasopharyngeal temperature imprint during inhalation, the nasopharyngeal jet during exhalation, and the flow structures of the larynx and laryngeal jet. The inclusion of the exhalation phase demonstrates airflow preconditioning before inhalation, which we postulate contributes to achieving alveolar conditions. Alveolar temperature and humidity conditions are not achieved by the nasal cavity alone, and we demonstrate the contribution of the nasopharynx and larynx to air conditioning. Including the complete airway with realistic anatomy and using transient airflow modeling provided new insights into the physiology of the respiratory cycle. [ABSTRACT FROM AUTHOR]

Airflow through the nasal cavity exhibits a wide variety of fluid dynamic behaviors due to the intricacy of the nasal geometry. The flow is naturally unsteady and perhaps turbulent, despite Computational Fluid Dynamics (CFD) in the literature being assumed as having a steady laminar flow. Time-dependent simulations can be used to generate detailed data with the potential to uncover new flow behavior, although they are more computationally intensive than steady-state simulations. Furthermore, verification of CFD results has relied on a reported pressure drop (e.g., nasal resistance) across the nasal airway although the geometries used are different. This study investigated the unsteady nature of inhalation at flow rates of 10 l/min, 15 l/min, 20 l/min, and 30 l/min. A scale resolving CFD simulation using a hybrid Reynolds-averaged Navier–Stokes--large eddy simulation model was used and compared with experimental measurements of the pressure distribution and the overall pressure drop in the nasal cavity. The experimental results indicated a large pressure drop across the nasal valve and across the nasopharynx, with the latter attributed to a narrow cross-sectional area. At a flowrate of 30 l/min, the CFD simulations showed that the anterior half of the nasal cavity displayed dominantly laminar but disturbed flow behavior in the form of velocity fluctuations. The posterior half of the nasal cavity displayed turbulent activity, characterized by erratic fluctuating velocities, which was enhanced by the wider cross-sectional areas in the coronal plane. At 15 l/min, the flow field was laminar dominant with very little disturbance, confirming a steady-state laminar flow assumption is viable at this flow rate. [ABSTRACT FROM AUTHOR]