Quasi-steady Flow Research Articles

Analyzing the inspiratory airflow unsteadiness in the respiratory tract using dynamic mode decomposition method

To enhance the understanding of airflow characteristics in the human respiratory tract, the inspiratory airflow field was simulated under both tidal and quasi-steady inspiratory flow rates at the mouth inlet using the large eddy simulation method. Special attention was paid on analyzing the inspiratory airflow unsteadiness using the dynamics mode decomposition (DMD) method based on the vorticity field and comparing it with the proper orthogonal decomposition (POD) method. The following novel findings were obtained. (1) Power spectral density indicates that the inspiratory airflow is highly turbulent in the pharynx–larynx region. The vorticity field in the upper airway is more affected by inspiratory patterns compared to turbulence fluctuations. (2) The DMD results indicate that the shear flow in the pharynx–larynx region is mainly caused by flow under low-frequency modes, while the disturbances of the jet flow are caused by flow under multiple frequency modes. Steady-state inspiratory pattern demonstrates the decay characteristics different from the tidal inspiratory pattern. (3) Compared to the POD method, which may contain multiple frequency components, the DMD decomposition yields modes with a single frequency, enabling a more accurate capture of the frequency and decay characteristics of the respiratory flow under each mode. In conclusion, this study demonstrates that the DMD method is more suitable for studying the respiratory airflow unsteadiness and further confirms the necessity of adopting clinically measured inspiratory data to investigate airflow unsteadiness.

Energy harvesting from transverse galloping using a two-degree-of-freedom flow energy harvester

Abstract A two degree-of-freedom (2-DOF) galloping piezoelectric energy harvesting system where both oscillators are excited by the incoming flow is studied in this paper. A coupled lumped-parameter model is developed to simulate the electro-fluid-structural coupling behaviour assuming a quasi-steady aerodynamic flow field. The differential equations governing the dynamics of the lumped system are converted into nonlinear algebraic equations employing the harmonic balance method (HBM) and then solved using Newton’s method. The approximate analytical solutions are compared to the solutions obtained by numerical integration, and the results agree, both qualitatively and quantitatively. The solutions were subsequently used to investigate the effects of the bluff body dimension, mechanical, electrical, aerodynamic, and electromechanical parameters on the performance of the harvester and to illustrate how to optimise some of these parameters to reduce the cut-in wind speed and maximise the power output of the energy harvester. It will be shown that the energy harvesting performance could be significantly improved by introducing piezoelectric transducers on both oscillators and including both masses in the incoming flow. A comparative study between the proposed 2-DOF flow energy harvesting system, a single-degree-of-freedom (SDOF) galloping piezoelectric energy harvester (GPEH), and other configurations of 2-DOF GPEH shows that the present 2-DOF GPEH generates the largest power peak.

Experimental investigation on aerodynamic noise of a small-scale multi-blade centrifugal fan

In this study, the aerodynamic noise of a small-scale centrifugal fan was experimentally investigated using acoustic testing techniques and time-resolved stereoscopic particle image velocimetry (SPIV). During the acoustic experiments, both far-field noise and near-field pressure fluctuations of the test fan were measured. The overall far-field noise towards the fan inlet side was found to be higher than that of the back side. The pressure fluctuations on the fan upper casing exceeded those on the side wall due to the uncontracted volute tongue, indicating pronounced flow-to-wall interactions. Moreover, based on a simultaneous measurement, the coherence between the near-field pressure fluctuations and far-field noise highlighted the significant contributions of impeller rotation to noise radiation. SPIV measurements uncovered the time-averaged and transient flow fields at the fan's inlet and outlet. The time-averaged results demonstrated the concentrated inlet flow and outlet flow separation, leading to high flow unsteadiness. Transient flow fields displayed an asymmetric jet-wake region characterized by both quasi-steady flow and rotational flow behaviours. The instantaneous flow results were analyzed using the dynamic mode decomposition (DMD) method, which clearly recognized the jet-wake patterns with frequencies corresponding to the rotational frequency. The observed consistency in frequency characteristics among noise, pressure fluctuations, and unsteady flow affirms that flow dynamics are crucial to the primary noise mechanisms.

Transition from decaying to decayless kink oscillations of solar coronal loops

ABSTRACT The transition of an impulsively excited kink oscillation of a solar coronal loop to an oscillation with a stationary amplitude, i.e. the damping pattern, is determined using the low-dimensional self-oscillation model. In the model, the decayless kink oscillations are sustained by the interaction of the oscillating loop with an external quasi-steady flow. The analytical solution is based on the assumption that the combined effect of the effective dissipation, for example, by resonant absorption, and interaction with an external flow, is weak. The effect is characterized by a dimensionless coupling parameter. The damping pattern is found to depend upon the initial amplitude and the coupling parameter. The approximate expression shows a good agreement with a numerical solution of the self-oscillation equation. The plausibility of the established damping pattern is demonstrated by an observational example. Notably, the damping pattern is not exponential, and the characteristic decay time is different from the time determined by the traditionally used exponential damping fit. Implications of this finding for seismology of the solar coronal plasmas are discussed. In particular, it is suggested that a very rapid, in less than the oscillation period, decay of the oscillation to the stationary level, achieved for larger values of the coupling parameter, can explain the relative rareness of the kink oscillation events.

Wind pattern clustering of high frequent field measurements for dynamic wind farm flow control

In this work, we investigate a method to derive characteristic dynamic flow field behavior from field measurements. We further explore how these changes impact the performance of a wind farm flow control strategy. For a long time, hourly to 10-min averaged data has been the predominant form to store meteorological quantities such as wind speeds and wind directions. With the decreasing cost of digital storage and improvements in measurement technology, the assimilation of higher frequent data has become more feasible. We use one of these open-source datasets provided by the KNMI to explore what characteristic flow behavior is described in the high-frequency recordings of a Wind-LiDAR located in the North-Sea. To this end we employ a K-Means algorithm to cluster 10-min time series of wind direction changes sampled at 20 s. Our study finds that the majority of wind direction changes within this time window can be described by five main clusters with clock- and counterclockwise changes of the wind direction in the range of ±4 deg. Subsequently we investigate the implications for quasi-steady wind farm flow control. We employ look-up table yaw-steering control next to baseline control in selected cases in a turbulent Large Eddy Simulation to verify the predictions made by a dynamic parametric engineering wake model. We find good agreement between both simulation environments and use the engineering model to investigate all wind directions in 2 deg resolution. The results show that the identified wind direction changes can have a significant negative impact on the power generated by a 10 turbine wind farm. The study also shows that the fixed yaw-steering set-points are still favorable over baseline operation for wind direction changes in the range of ±1.6 deg, but can act detrimental for larger changes.

3D Origami-Inspired Electrochemical Paper-Based Analytical Devices with Flow-Through Electrodes and Quasi-Steady Flow

3D Origami-Inspired Electrochemical Paper-Based Analytical Devices with Flow-Through Electrodes and Quasi-Steady Flow

A new one-dimensional model of free-piston Stirling engine based on quasi-steady flow approximation and simi-implicit numerical method

Free-piston Stirling engine is a widely used device for thermal-to-electric conversion especially in aerospace applications. A dynamic model for free-piston Stirling engine is established in this paper for the purpose of modeling the free-piston Stirling engine more accurately and understanding its operation characteristics. This paper proposes a new one-dimension numerical model with quasi-steady flow approximation for free-piston Stirling engine with linear alternator, which can be categorized as third-order theoretical model for Stirling engine. The corresponding simi-implicit discretizationmethod is presented. In order to accurately obtain the thermal efficiency of the Stirling machine, various energy loss mechanisms are taken into consideration. A beta type free-piston Stirling engine, Component Test Power Convertor, is studied to demonstrate the correctness and accuracy of the numerical model established in this paper. The dynamic steady-state anticipated operation condition is simulated first. Then, the sensitivity analysis is carried out in which the effect of load resistance, heater temperature and cooler temperature, and charged pressure on the system operation parameter is studied. The predicted system parameters including frequency, displacer and piston amplitude, output power, and alternator output voltage and current agrees well with the designed value. Under the dynamic steady operation condition, the simulated indicated power is 15.51 kW and the electric power is 13.12 kW. The relative deviation between the predicted electric power and the measured output electric power of 12.78 kW is 2.66 %. The corresponding indicated efficiency and thermal-to-electric efficiency is 25.06 % and 21.20 % respectively. The amplitude of displacer and piston is 13.96 mm and 13.69 mm for this condition. Compared with the experimental data at different piston amplitude, the derivation of predicted engine parameters is within 10 %. The comparison with experimental results proves the accuracy of the proposed model in this paper. Compared with the previous model based on the adiabatic thermodynamic model, the third-order model presented in this paper shows higher accuracy on system parameter prediction for free-piston Stirling engine.

Numerical implementation and verification of the Zwart-Gerber-Belamri cavitation model with the consideration of thermodynamic effect

In the world of pumps, turbopumps, and industrial applications, the absorption and release of latent heat in high-temperature zones play a pivotal role in the cavitation process. This phenomenon exerts a profound influence on system performance, especially when handling hot and cryogenic fluids, which are particularly susceptible to thermodynamic influences. The pressing need arises for a robust computational approach to address the intricacies of thermodynamic effects on cryogenic cavitation. The study at hand introduces a modification to the Zwart-Gerber-Belamri cavitation model, incorporating thermodynamic considerations to simulate quasi-steady cavitating flow around a NACA0015 hydrofoil. The SST k-ε turbulence model and homogeneous mass transfer cavitation model are employed to account for thermal effects, while the Clausius-Clapeyron equation is utilized to adjust the saturated vapor pressure within the cavitation model. Comparing the results with experimental data from Cervone et al. [1], especially in the thermal domain, reveals congruence in the estimated pressure and temperature drop (ΔT) within the cavity under varying free stream temperature conditions. Notably, thermodynamic effects exert a significant influence on cavitation dynamics during the phase-change process, potentially hindering or delaying cavitation in hot and cryogenic fluids. This enhanced cavitation model offers a marginally improved prediction of cavitation in water.

Influence of rotor blade flexibility on the near-wake behavior of the NREL 5 MW wind turbine

Abstract. High-fidelity computational fluid dynamics (CFD) simulations of the National Renewable Energy Laboratory (NREL) 5 MW wind turbine rotor are performed, comparing the aerodynamic behavior of flexible and rigid blades with respect to local blade quantities as well as the wake properties. The main focus has been set on rotational periodic quantities of blade loading and fluid velocity magnitudes in relation with the blade tip vortex trajectories describing the development of those quantities in the near wake. The results show that the turbine loading in a quasi-steady flow field is mainly influenced by blade deflections due to gravitation. Deforming blades change the aerodynamic behavior, which in turn influences the surrounding flow field, leading to non-uniform wake characteristics with respect to speed and shape.

Development and Validation of an Improved Quasisteady Flow Model with Additional Parasitic Loss Effects for Stirling Engines

This paper presents the development and validation of an improved quasisteady flow model (iQSFM) that applies comprehensive parasitic losses to the quasisteady flow model (QSFM) considering an oscillating flow, which is the actual type of flow occurring in a regenerator. Validation of iQSFM was evaluated by comparing it with a QSFM based on the experimental results of a RE-1000 regenerator. Compared to QSFM, iQSFM improved the prediction accuracy by reducing the indicated power error from 66.7% to 24.9% and the efficiency error from 35.3% to 9.4%. In addition, the prediction accuracy of iQSFM was compared when the oscillating flow and the steady flow correlation were applied to a regenerator. When iQSFM applied an oscillating flow correlation to the regenerator, it predicted the experimental results of RE-1000 slightly more accurately than in a steady flow correlation. Finally, the engine performance and parasitic losses were analyzed through a parameter study of RE-1000 using iQSFM. Through this, it was confirmed with iQSFM that the RE-1000 is designed to maximize the engine performance by minimizing the parasitic losses.

Examining the Quasi-Steady Airflow Assumption in Irregular Vocal Fold Vibration

The quasi-steady flow assumption (QSFA) is commonly used in the field of biomechanics of phonation. It approximates time-varying glottal flow with steady flow solutions based on frozen glottal shapes, ignoring unsteady flow behaviors and vocal fold motion. This study examined the limitations of QSFA in human phonation using numerical methods by considering factors of phonation frequency, air inertance in the vocal tract, and irregular glottal shapes. Two sets of irregular glottal shapes were examined through dynamic, pseudo-static, and quasi-steady simulations. The differences between dynamic and quasi-steady/pseudo-static simulations were measured for glottal flow rate, glottal wall pressure, and sound spectrum to evaluate the validity of QSFA. The results show that errors in glottal flow rate and wall pressure predicted by QSFA were small at 100 Hz but significant at 500 Hz due to growing flow unsteadiness. Air inertia in the vocal tract worsened predictions when interacting with unsteady glottal flow. Flow unsteadiness also influenced the harmonic energy ratio, which is perceptually important. The effects of glottal shape and glottal wall motion on the validity of QSFA were found to be insignificant.

Laboratory investigation of nominally two-dimensional anabatic flow on symmetric double slopes

We investigated the dynamics of highly turbulent thermally driven anabatic (upslope) flow on a physical model inside a large water tank using particle image velocimetry (PIV) and a thermocouple grid. The results showed that the flow exhibited pronounced variations in velocity and temperature and, importantly, could not be accurately modeled as a two-dimensional quasi-steady flow. Five significant findings are presented to underscore the three-dimensional nature of the flow, namely, the B-shaped mean velocity profiles, B-shaped turbulent flux profiles, synthetic streaks that revealed particles flowing perpendicular to the laser sheet, average vorticity maps revealing helical structure splitting, and identified vortices shooting away from the boundary toward the apex plume. Collectively, these findings offer novel insights into the flow behavior patterns of thermally driven complex terrain flows, which influence local weather and microclimates and are responsible for scalar transport, e.g., pollution.

Self-sustained azimuthal aeroacoustic modes. Part 1. Symmetry breaking of the mean flow by spinning waves

In this paper, we study the aeroacoustic instability which occurs in a deep axisymmetric cavity in a turbulent pipe flow. This phenomenon is the axisymmetric counterpart of the classical whistling of a rectangular deep cavity subject to a grazing flow. The whistling of such axisymmetric cavity originates from the interaction of the coherent fluctuations of the vorticity at the cavity's opening with one of its trapped azimuthal or radial acoustic modes. We focus here on the situation involving the first pure azimuthal mode, which is trapped in the cavity. As a consequence of the rotational symmetry of the configuration, azimuthal modes are actually pairs of degenerate eigenmodes, or almost degenerate in the presence of small asymmetries. Therefore, the aeroacoustic instabilities exhibit more complex mechanisms than in the case of a rectangular deep cavity. In particular, we show that self-sustained spinning modes induce a symmetry breaking of the mean flow and we will elucidate the details of this phenomenon. To that end, simultaneous acoustic and time-resolved stereoscopic particle image velocimetry (PIV) measurements are performed. They reveal that when large-amplitude aeroacoustic waves spin around the cavity, a quasi-steady mean flow starts whirling slowly in the opposite direction to the wave propagation. A linear perturbation analysis around an axisymmetric mean flow confirms the experimental observations: although the incoming pipe flow is not swirling, the hydrodynamic component of the aeroacoustic wave induces such whirling motion of the mean flow because of the forcing from the steady part of the coherent Reynolds stress tensor.

Polarisation of decayless kink oscillations of solar coronal loops

Decayless kink oscillations of plasma loops in the solar corona may contain an answer to the enigmatic problem of solar and stellar coronal heating. The polarisation of the oscillations gives us a unique information about their excitation mechanisms and energy supply. However, unambiguous determination of the polarisation has remained elusive. Here, we show simultaneous detection of a 4-min decayless kink oscillation from two non-parallel lines-of-sights, separated by about 104∘, provided by unique combination of the High Resolution Imager on Solar Orbiter and the Atmospheric Imaging Assembly on Solar Dynamics Observatory. The observations reveal a horizontal or weakly oblique linear polarisation of the oscillation. This conclusion is based on the comparison of observational results with forward modelling of the observational manifestation of various kinds of polarisation of kink oscillations. The revealed polarisation favours the sustainability of these oscillations by quasi-steady flows which may hence supply the energy for coronal heating.

Effects of a spherical slip cavity filled with micropolar fluid on a spherical viscous droplet

An analytical study for the quasisteady flow caused by a spherical viscous fluid droplet translating at a concentric position in a second immiscible micropolar fluid within a spherical cavity with a slip surface is presented. Attention is focused on the case when one of the two fluid phases has a microstructure nature (micropolar fluid). The droplet translates along a diameter connecting their centres under the conditions of low Reynolds numbers. To solve the Stokes equations for the velocity fields inside and outside the droplet, general solutions are obtained in terms of spherical coordinates based on the concentric position. For the case of the viscous droplet within a micropolar fluid, a boundary condition related to vorticity with microrotation is used. The wall correction factors acted on the viscous droplet for the different cases are represented through graphs for various values of relative viscosity, radii ratio of droplet and cavity, and the parameter that connects the vorticity with microrotation. The wall-corrected drag force is found to be a monotonically increasing function of the ratio of drop-to-cavity radii. The present work is important because of its applications in various natural, industrial, and biological processes, such as raindrop formation, the study of blood flow, liquid–liquid extraction, the prediction of atmospheric conditions, sedimentation phenomena, and the rheology of emulsions.

Simulation of laminar to transitional wakes past cylinders with a discontinuous Galerkin inviscid shallow water model

Laminar to transitional wakes occur in slow, quasi-steady flows past cylinders at low cylinder Reynolds numbers (R ed ≤ 250). Inviscid numerical solvers of the depth-averaged shallow water equations (SWE) introduce numerical dissipation that, depending on Red , may imitate the mechanisms of viscous turbulent models. However, the numerical dissipation rate in a second-order finite volume (FV2) SWE solver is so large at a practical resolution that this can instead hide these mechanisms. The extra numerical complexity of the second-order discontinuous Galerkin (DG2) SWE solver results in a lower dissipation rate, making it a potential alternative to the FV2 solver to reproduce cylinder wakes. This paper compares the DG2 and FV2 solvers, initially for wake formation behind one cylinder. The findings confirm that DG2 can reproduce the expected wake formations, which FV2 fails to capture, even at a 10-fold finer resolution. It is further demonstrated that DG2 is capable of reproducing key features of the flow fields observed in a laboratory random cylinder array.

Optimizing the Electrolyte Mixing in Industrial Tanks of Vanadium Redox Flow Batteries

Redox flow batteries (RFB) are an emerging technology for large-scale grid energy storage. Different approaches have been adopted to investigate the operation of RFBs, such as experimental testing and mathematical modelling of the electrochemical cells [1]. However, the effect of the flow and mixing in the electrolyte tanks on the performance of RFBs has been largely overlooked and only recently has started to receive some attention [2]. In a recent work, the authors used numerical simulations to show how the fluid dynamics of the electrolytes within small lab-scale tanks can lead to different flow behaviors depending on whether forced or natural convection is dominant. The irregular transient mixing of the electrolyte in the tanks modifies both the cell potential and battery capacity with respect to those predicted under the widely used perfect mixing assumption (i.e., continuous stirred-tank reactor) [3]. However, this first attempt relied on several simplifying assumptions, such as two-dimensional geometries under small-cell (i.e., laminar) flow conditions, and thus provided only a qualitatively understanding of the flow in the tanks.This work exploits the idea of using CFD simulations to investigate the effect of operating conditions on the electrolyte flow in the tanks and its impact on battery response, focusing on high capacity industrial tanks with high (i.e., turbulent) flow rates and less restrictive simplifying assumptions. Our numerical simulations show how turbulence improves mixing, but also produces quasi-steady flows where a turbulent jet crosses the tank leaving dead electrolyte regions. Different tank solutions are presented aiming to improve the mixing and increase the capacity of the battery, playing both with the tank geometry and the operating conditions. Results on the charging and discharging operations, cell potential and electrolyte mixing index are presented and discussed. Acknowledgments This work has been partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación Projects PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033, and by Grant IND2019/AMB-17273 of the Comunidad de Madrid.

Control-Oriented Reduced-Order Modeling of Conversion Efficiency in Dual-Layer Washcoat Catalysts With Accumulation and Oxidation Functions

Abstract This work proposes a model for predicting conversion efficiency in multifunctional catalysts with dual-layer washcoat. The mass transfer is more relevant in these devices than in single-layer washcoats due to additional transport steps between the catalytic layers. In addition, the different reaction mechanisms between layers make the concentration of the chemical species differ in each layer. To deal with this boundary while considering the need for real-time computation, a reduced-order explicit solver for the convective diffusive reactive transport is presented for the case of dual-layer washcoats. Assuming one-dimensional quasi-steady flow, the solution procedure consisted of substituting the diffusive interfacial fluxes in the bulk gas and washcoat conservation equations by expressions that depend explicitly on the average concentration in the gas phase. The solution was then applied to model the performance of dual-layer oxidation catalysts with reductant accumulation in one washcoat layer, such as diesel oxidation catalyst (DOC) and ammonia slip catalyst (ASC) systems, during driving cycles. First, the response of these catalysts was analyzed by comparing them against experimental data and considering additional parameters provided by the model. Next, the importance of the mass transfer limitations was discussed to complete the analysis. The proposed model was compared with a simplified solver where the mass transfer steps were omitted, thus deteriorating the prediction capabilities in some driving cycle phases. Finally, a sensitivity study was performed to assess the impact of the mesh size on the prediction capabilities and computational requirements.

Three-dimensional simulations of fluid flows in oscillating lid-driven cavities using lattice Boltzmann method

Abstract We utilize the lattice Boltzmann method to conduct three-dimensional simulations of incompressible flows in oscillating cubic lid-driven cavities. Our investigation focuses on examining the impact of Reynolds number and oscillating frequency on the flow field. Notably, we observe that the flow field can be adequately approximated as two-dimensional within the low and intermediate Reynolds number range, but this approximation is no longer valid for high Reynolds numbers. Additionally, we find that high Reynolds numbers correspond to transient flow fields, while low and moderate Reynolds numbers exhibit quasi-steady periodic flow fields. Our study holds significant relevance for industrial processing applications, where the Reynolds numbers and oscillating frequencies can be optimized to achieve a desired flow field.

Interaction of a moving shock wave with a turbulent boundary layer

In the present study, the influence of a uniformly moving impinging shock on the resulting shock wave–turbulent boundary layer interaction is numerically investigated. The relative Mach number of the shock front travelling above a flat-plate model varied between 0 and 2.3, while the quasi-steady inflow conditions remained constant with a Mach number of 3. To quantitatively evaluate the effect of shock travelling speed, a well-known scaling method for interaction length in quasi-steady flows was applied as a reference after significant improvements in modelling the effects of Reynolds number and wall temperature using new and existing data. Moreover, previously obtained experimental results for a limited range of travelling speeds were employed to validate the obtained numerical results. Three ranges of shock travelling speeds with distinctly different properties were extracted and quantitatively described using a developed correlation-based approach built on the extended quasi-stationary scaling law. In the first range, the scaled interaction length reaches its maximum for the given interaction strength and can be directly described by the scaling law obtained for quasi-stationary interactions. In the second travelling-speed range, the dependence of the interaction length on the interaction strength is explicitly influenced by the shock movement. With increasing shock travelling speed, the scaled interaction length here decreases significantly faster than in the quasi-stationary reference case. The end of this speed range is reached when the absolute shock front speed has caught up with the maximum speed of sound in the interaction zone, and thus the interaction length has fallen to zero. This travelling-speed limit signifies the transition to the third range, where upstream influence is no longer possible.