Flow Amplitude Research Articles

Gust encounters of rigid wings: Taming the parameter space

Recent efforts to study discrete large-amplitude gust encounters have often focused on one of three canonical problems: transverse, vortex, or streamwise gust encounters. A selection of this work is highlighted, concentrating on two-dimensional problems where the length scale of the gust flow is similar to the wing chord, and the amplitude of the gust flow is similar to the freestream. Current and outstanding questions are outlined for future work, including the role of low-order models and the limitations of using canonical problems to represent real-world gust encounters.

The Impact of Wind on Flow and Sediment Transport over Intertidal Flats

Sediment transport over intertidal flats is driven by a combination of waves, tides, and wind-driven flow. In this study we aimed at identifying and quantifying the interactions between these processes. A five week long dataset consisting of flow velocities, waves, water depths, suspended sediment concentrations, and bed level changes was collected at two locations across a tidal flat in the Wadden Sea (The Netherlands). A momentum balance was evaluated, based on field data, for windy and non-windy conditions. The results show that wind speed and direction have large impacts on the net flow, and that even moderate wind can reverse the tidal flow. A simple analytical tide–wind interaction model shows that the wind-induced reversal can be predicted as a function of tidal flow amplitude and wind forcing. Asymmetries in sediment transport are not only related to the tide–wind interaction, but also to the intratidal asymmetries in sediment concentration. These asymmetries are influenced by wind-induced circulation interacting with the large scale topography. An analysis of the shear stresses induced by waves and currents revealed the relative contributions of local processes (resuspension) and large-scale processes (advection) at different tidal flat elevations.

Influence of Unsteady Flow Induced by a Large-Scale Hydropower Station on the Water Level Fluctuation of Multi-Approach Channels: A Case Study of the Three Gorges Project, China

Unsteady flow induced by hydropower stations exerts a significant impact on the water level in multi-approach channels, which directly threatens the safe passage of ships. In this study, a one-dimensional and a two-dimensional hydrodynamic model are adopted to simulate the water level fluctuations at the entrance of multi-approach channels and the lower lock head of a ship lift with consideration of initial water surface elevation, base flow, flow amplitude, regulation time, and locations of hydropower stations, unfavorable conditions are successfully identified; and the fluctuations at the approach channel entrance and the lower lock head of a ship lift under single-peak and double-peak regulating mode are analyzed considering the flow regulating of the Gezhouba Hydropower Station (GHS), thus, the water level oscillation process in the multi-approach channels is presented. Results show that the largest wave amplitude in the multi-approach channels manifests under unfavorable conditions including lower initial water surface elevation, smaller base flow, larger flow variation, and shorter regulation time; and water level fluctuation in the multi-approach channel is primarily induced by flow amplitude and net flow between the Three Gorges Hydropower Station (TGHS) and the GHS, with consideration of the counter-regulation process of the GHS. This research contributes to providing a reference for a similar large-scale cascade hydropower station regarding regulation and control of navigation conditions.

Stress-driven spin-down of a viscous fluid within a spherical shell

Abstract

Flapping of heavy inverted flags: a fluid-elastic instability

Abstract

Magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux

It has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core-mantle boundary (Bloxham, 1986). In this process intense horizontal field lines are concentrated below the outer boundary, introducing small radial length scales and consequently strong radial diffusion. Here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies. Our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell. In our models mean magnetic boundary layer thickness ranges ≈200 − 400 km and decreases with increasing magnetic Reynolds number. Extrapolation or interpolation to Earth's core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of ≈220 and ≈260 − 330 km, respectively. We find magnetic boundary layers associated with the azimuthal field at the equatorial region, whereas magnetic boundary layers associated with the meridional field are found at mid latitudes. Negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below Africa and the Pacific, while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the Americas and East Asia. Furthermore, we find a tendency of the azimuthal field to low latitudes of the Northern Hemisphere. Our results suggest that the local diffusion time is on the order of several kyr and the local magnetic Reynolds number is on the order of ≈10, both much smaller than classical estimates.

Drag enhancement in a dusty Kolmogorov flow

Particles suspended in a fluid exert feedback forces that can significantly impact the flow, altering the turbulent drag and velocity fluctuations. We study flow modulation induced by particles heavier than the carrier fluid in the framework of an Eulerian two-way coupled model, where particles are represented by a continuum density transported by a compressible velocity field, exchanging momentum with the fluid phase. We implement the model in direct numerical simulations of the turbulent Kolmogorov flow, a simplified setting allowing for studying the momentum balance and the turbulent drag in the absence of boundaries. We show that the amplitude of the mean flow and the turbulence intensity are reduced by increasing particle mass loading with the consequent enhancement of the friction coefficient. Surprisingly, turbulence suppression is stronger for particles of smaller inertia. We understand such a result by mapping the equations for dusty flow, in the limit of vanishing inertia, to a Newtonian flow with an effective forcing reduced by the increase in fluid density due to the presence of particles. We also discuss the negative feedback produced by turbophoresis which mitigates the effects of particles, especially with larger inertia, on the turbulent flow.

Performance Characteristics of Positive Expiratory Pressure Devices.

Positive expiratory pressure (PEP) therapy imposes expiratory flow resistance to increase airway diameter and enhance mucus clearance. PEP is achieved several ways. Oscillatory PEP devices (OPEP) generate repeated occlusions that are known to reduce mucus viscosity. There are many marketed devices, but comparative performance is mostly unreported. The purpose of this study was to evaluate performance characteristics of many PEP/OPEP devices. For OPEP devices, we defined an optimal performance metric by creating an oscillation index that combines the OPEP performance characteristics. PEP devices (TheraPEP, EzPAP, VersaPAP, Resistex, AccuPEP, AccuPAP, and Threshold PEP) and OPEP devices (Acapella DH, Acapella DM, Acapella Choice, ShurClear, Aerobika, VibraPEP, vPEP, and PocketPEP with and without the Oxyjet attachment) were tested by adjusting simulated expiratory flow from 5 L/min to 30 L/min in increments of 5 L/min using a standard flow meter. All devices showed varying performance characteristics. As expiratory flow increased, mean PEP increased for most devices. The TheraPEP showed a mean PEP of 13 cm H2O across all settings. For OPEP devices, there was a major difference between pressure and flow waveforms. The Acapella DH, ShurClear, and Aerobika showed the highest flow amplitude, flow frequency, and oscillation index. PEP devices behaved similarly and as expected, with increased pressure with increased flow (flow resistors) or flow independence (threshold resistors). There was much greater variation in the performance of the OPEP devices. A higher oscillation index indicates better mechanical performance characteristics. Many devices have similar characteristics. However, the devices with the highest oscillation index have the highest flow amplitude and frequency, which may indicate better clinical performance.

Identification of pulsating flow effects with CNT nanoparticles on the performance enhancements of thermoelectric generator (TEG) module in renewable energy applications

In this study, performance assessment of a thermoelectric generator module located in between two channels where carbon-nanotube/water nanofluid streams flow is studied with combined effects of nanoparticle inclusion and flow pulsations. The finite element method is used to solve the 3D unsteady coupled fluid flow, heat transfer and electric field equations. Different pertinent parameters effects such as Reynolds number (between 250 and 1000), nanoparticle volume fraction (between 0 and 0.04), pulsating flow frequency (Strouhal number between 0.01 and 0.1) and amplitude (between 0.25 and 0.95) on the power generation are examined. It is observed that flow pulsation changes the dynamic features of thermoelectric power generated in the device. Higher power values are obtained when Reynolds number, flow pulsation amplitude and nanoparticle solid volume fraction rise. However, the effect is reverse for higher pulsation frequencies. Including nano sized particles further enhances the performance of the device with flow pulsation. It is also observed that 24.4 % enhancement in the power are achieved for nanofluid with flow pulsation when lowest and highest pulsation amplitudes are compared. At lowest pulsation frequency and highest amplitude 14.2 % enhancement in power is obtained for water as compared to steady flow case while this amount rises to 31 % for carbon nanotube nanofluid at the highest solid volume fraction. System identification method is used to obtain dynamic lower order model of the system for different pulsation amplitudes to predict the time dependent power generated in the thermoelectric generator device. • TEG module performance with flow pulsation and nanofluid is examined. • Higher amplitudes, ϕ, Re number and lower frequency resulted in higher powers. • When amplitudes are varied, 24.4% power enhancement is obtained with pulsation. • As compared to steady case, 31% enhancement is seen with nanofluid and pulsation. • System identification is used to construct dynamic model of the TEG module.

MHD mixed convective radiative flow of Eyring‐Powell fluid over an oscillatory stretching sheet using bivariate spectral method on overlapping grids

AbstractThe bivariate spectral quasilinearization method (BSQLM) on overlapping grids is presented and applied in the analysis of unsteady magnetohydrodynamic mixed convection flow of Eyring‐Powell fluid over an oscillatory stretching sheet embedded in a non‐Darcy porous medium with nonlinear radiative heat flux and variable thermophysical properties. The fluid properties, namely the fluid viscosity, thermal conductivity, and mass diffusivity, are assumed to vary with temperature. It is assumed that the first‐order chemical reaction with heat generation/absorption takes place in the flow. The flow domain is subject to uniform transverse magnetic field perpendicular to the stretching surface. The transformed flow equations are solved numerically using BSQLM on overlapping grids. The convergence properties and accuracy of the method are assessed. The proposed method is computationally efficient, and it gives stable and highly accurate results after few iterations and using few grid points in each subinterval. The improved accuracy rests upon the use of the overlapping grid, which produces sparse coefficient matrices that are easy to invert and have small condition numbers. The effects of physical parameters on the flow fields, local skin friction, the Nusselt number, and the Sherwood number are exhibited through graphs and tables. Amongst other findings, we found that the amplitude of the fluid flow along with flow characteristics may efficiently improve through the utilization of variable fluid viscosity. Heat and mass transportation processes enhance with the inclusion of nonlinear radiative heat flux, temperature‐dependent thermal conductivity, and mass diffusion coefficient, whereas they diminish with the increase in the local inertia coefficient. The current flow analysis can be useful in various engineering applications including paper production, polymer solution, glass blowing, extrusion of thermal system manufacturing process, and heat transportation enhancement.

Higher order symmetric cumulants

We present the generalization of recently introduced observables for the studies of correlated fluctuations of different anisotropic flow amplitudes, dubbed Symmetric Cumulants. We introduce a new set of higher order observables and outline a unique way how the genuine multi-harmonic correlations can be extracted from multi-particle azimuthal correlators. We argue that correlations among flow amplitudes can be studied reliably with the general mathematical formalism of cumulants only if that formalism is applied directly on the flow amplitudes. We have tested all the desired properties of new observables with the carefully designed Toy Monte Carlo studies. By using the realistic iEBE-VISHNU model, we have demonstrated that their measurements are feasible and we have provided the predictions for the centrality dependence in Pb--Pb collisions at LHC energies. A separate study was performed for their values in the coordinate space. The new observables contain information which is inaccessible to individual flow amplitudes and correlated fluctuations of only two flow amplitudes, and therefore they provide further and independent constraints for the initial conditions and the properties of Quark-Gluon Plasma in high-energy nuclear collisions.

Microcirculatory Flux and Pulsatility in Arterial Leg Ulcers is Increased by Intermittent Neuromuscular Electrostimulation of the Common Peroneal Nerve

Neuromuscular electrical stimulator (NMES) devices increase blood flow to the lower limb by a process of intermittent muscular contraction initiated by a transdermal stimulus to the common peroneal nerve. However, its effects on localized microvascular blood supply to lower limb wounds are unknown. This study is a single-center open label study measuring the effect of neuromuscular stimulation of the common peroneal nerve on the microvascular blood flow within the wound bed of arterial leg ulcers. Eights patients with ischemic lower limb wounds had an NMES (geko™) applied to the common peroneal nerve. Baseline and intervention analysis of blood flow to the wound bed and edge was performed using Laser Speckle Contrast Imaging. Mean flow (flux) and pulse amplitude (pulsatility) were measured. Stimulation of the common peroneal nerve with the NMES resulted in a significantly increased flux and pulsatility in both the wound bed and the wound edge in all 8 patients. Neuromuscular electrical stimulation immediately increases microcirculatory blood flow to the wound bed and edge in patients with ischemic lower limb wounds. These data may provide mechanistic insight into the clinical efficacy of NMES in healing wounds. www.clinicaltrials.gov NCT03186560.

Numerical modelling of the oscillatory flow effect around submarine pipeliness

This work describes the dynamics of an underwater pipeline subjected to an incident flow composed of a uniform flow and a sinusoidal component. The motion equations of the pipeline and the fluid flow around the pipeline are solved simultaneously with a numerical model that considers in-line oscillations (one degree of freedom) and a second numerical model that additionally includes cross flow (two degrees of freedom). The amplitude response and drag forces on the pipe are compared for both models considering parameters such as the difference between the excitation and natural frequencies of pipelines and the relative value between the sinusoidal and uniform components of the incident flow. Important differences in numerical predictions of both models are observed when the excitation frequency is greater than the natural frequency of the system and when the amplitude of the oscillatory component of the incident flow is greater than the amplitude of the uniform flow.

Energy loss and developing length during reciprocating flow in a pipe with a free-end

Direct numerical simulations of reciprocating pipe flow in a straight pipe with a free-end are presented. The range of amplitudes and frequencies studied span the laminar regime and the beginning of transition toward a conditionally turbulent flow. Two primary results are reported: the measurement of the flow development length and the loss of energy, both due to the presence of the free-end. Two regimes of flow are identified with distinct length scales. For low frequencies, the development length scales with the pipe diameter D. However, for higher frequencies, the development length scales with the Stokes layer thickness δ=2ν/ω. The energy loss is studied by calculating the viscous dissipation function, indicating where energy is lost, and allowing the energy lost due to the presence of the free-end to be isolated. While strong vortices are formed and convected from the exit, most of the energy they dissipate is lost within a few pipe diameters of the exit. It is shown that these trends continue even as the amplitude and frequency are increased so that the flow begins to transition from a laminar toward a turbulent state. Two modes of instability are observed in the Stokes layers near the free-end, one short wavelength mode with a wavelength set by the Stokes layer thickness and the other long wavelength mode with a wavelength set by the amplitude of the oscillatory flow. These modes are related to those observed in the fully developed oscillatory flow.

Multi-scale multi-mode nonlinear interaction in tokamak plasma turbulence with moderate small-scale shear flow

Effects of moderate small-scale shear flow, e.g., which may be created by the trapped electron mode, on electromagnetic (EM) ion-scale turbulence in tokamak plasmas are numerically investigated via a self-consistent Landau-fluid model. A modeling analysis is carried out in slab geometry to reveal the underlying mechanism of the multi-scale multi-mode nonlinear interaction. Results show that while a Kelvin–Helmholtz (KH) instability with long wavelengths may be excited by the shear flows to dominate the multi-scale EM fluctuation, shorter wavelength ion temperature gradient (ITG) modes experience multiple quasi-steady (QS) stages with enhanced fluctuation level through different driving and saturation mechanisms. One mechanism is the secondary ITG instability due to the decrease in flow stabilization modified by the zonal flow. Meanwhile, the other one is the modulational interaction between the EM ITG and KH modes through the nonlinear mode coupling. Moreover, the synergism of these two mechanisms may sustain the final QS state near the marginal KH instability threshold. Complex linear and nonlinear interactions among multiple modes and external flow, as well as self-generated zonal flow, result in a weak dependence of the final saturation level of the dominant EM ITG mode on the small-scale flow amplitude. The turbulent heat transport is visibly suppressed by weaker shear flow, but is almost not affected by stronger shear flows. The underlying mechanism is elaborated.

P1094PULSE AMPLITUDE OF THE TOE MEASURED WITH A PORTABLE LASER DOPPLER FLOW METER IS USEFUL FOR SCREENING OF DIALYSIS PATIENTS FOR PERIPHERAL ARTERIAL DISEASE

Abstract Background and Aims Peripheral arterial disease (PAD) occurs at a high frequency in hemodialysis (HD) patients and is known to be associated with poor outcomes. Measurement of the ankle-brachial blood pressure index (ABI) or toe brachial index (TBI) has been used for screening, diagnosis and severity determination of PAD. However, the procedures for measurement of these indices are complicated and the measurements cannot be performed during dialysis sessions. The purpose of the present study was to clarify the ability of the toe blood flow and pulse amplitude measured with a portable laser Doppler flow meter (LDF) to detect PAD in dialysis patients, in comparison with the diagnostic abilities of ABI and TBI. Methods Fourteen patients on maintenance HD participated in this study. The mean age was 65.2 years old and the mean duration of HD was 9.1 years. We measured the ABI and TBI before HD, and the blood flow and pulse amplitude on the ventral side of the first toe with a portable LDF (pocket LDF®, JMS Co., Ltd., Tokyo, Japan) during the HD session on the same day. The correlations between the blood flow / pulse amplitude in the toe and the ABI / TBI were investigated. The sensitivity and specificity of the blood flow and pulse amplitude in the toe for PAD detection were determined by a receiver operating characteristic curve analysis. Statistical analysis was performed using the Fisher’s exact test. A p value of <5% was considered to indicate a significant difference. Results and Discussion No significant correlation was found between the blood flow and the pulse amplitude in the toe (p = 0.454). The blood flow was not correlated with either the ABI or the TBI (p = 0.286, p = 0.115, respectively). However, the pulse amplitude showed significant correlations with both the ABI and TBI (p < 0.001, p < 0.05). When we used the general reference cutoff values for ABI and TBI of 0.9 and 0.6, respectively, the sensitivity and specificity of ABI for the detection of PAD were 0.75 and 0.75, respectively, and those of TBI were 1.0 and 0.75, respectively. On the other hand, the sensitivity and specificity of blood flow in the toe were 0.50 and 1.0, respectively, when the cutoff value was set at 23.8 mL/min. The sensitivity and specificity of the pulse amplitude were 0.75 and 1.0, respectively, when the cutoff value was set at 7.6 mL/min. These results indicate that the pulse amplitude in the toe showed a high diagnostic ability, like ABI and TBI, for the detection of PAD. Although our results in this study were obtained from a small sample size and further investigation is warranted, the pulse amplitude measured with the portable LDF was correlated well with both the ABI and TBI, and its sensitivity and specificity for the detection of PAD were also high, indicating the high diagnostic ability of the pulse amplitude for PAD. As compared to the measurement of ABI or TBI, the pulse amplitude in the toe can be easily measured with a portable LDF, even during a dialysis session. Thus, we believe that the use of a portable LDF might be of great benefit as a screening tool for PAD. Conclusion The pulse amplitude in the toe measured with a portable LDF may have the potential to become a novel easily measurable index in the screening of dialysis patients for PAD.

Radial-velocity variations due to meridional flows in the Sun and solar-type stars: impact on exoplanet detectability

Context. Stellar variability due to magnetic activity and flows at different spatial scales strongly impacts radial velocities. This variability is seen as oscillations, granulation, supergranulation, and meridional flows. The effect of this latter process is currently poorly known but could affect exoplanet detectability. Aims. We aim to quantify the amplitude of the meridional flow integrated over the disc and its temporal variability, first for the Sun, as seen with different inclinations, and then for other solar-type stars. We then want to compare these amplitudes with low-mass exoplanetary amplitudes in radial velocity. Methods. We used long time series (covering two 11-yr cycles) of solar latitudinal meridional circulation to reconstruct its integrated contribution and study its properties. We then used scaling laws from hydrodynamical simulations relating the amplitude of the meridional flow variability with stellar mass and rotation rate to estimate the typical amplitude expected for other solar-type stars. Results. We find typical rms of the order of 0.5–0.7 m s−1 (edge-on) and 1.2–1.7 m s−1 (pole-on) for the Sun (peak-to-peak amplitudes are typically 1–1.4 m s−1 and 2.3–3.3 m s−1 resp.), with a minimal jitter for an inclination of 45–55°. This signal is significant compared to other stellar activity contributions and is much larger than the radial-velocity signal of the Earth. The variability is strongly related to the activity cycle, with maximum flows during the descending phase of the cycle, and possible variability on timescales lower than the cycle period. Extension to other solar-type stars shows that the variability due to meridional flows is dominated by the amplitude of the cycle of those stars (compared with mass and rotation rate), and that the peak-to-peak amplitudes can reach 4 m s−1 for the most variable stars when seen pole-on. The meridional flow contribution sometimes represents a high fraction of the convective blueshift inhibition signal, especially for quiet, low-mass stars. For fast-rotating stars, the presence of multi-cellular patterns should significantly decrease the meridional flow contribution to the radial-velocity signal. Conclusions. Our study shows that these meridional flows could be critical for exoplanet detection. Low inclinations are more impacted than edge-on configurations, but these latter still exhibit significant variability. Meridional flows also degrade the correlation between radial velocities due to convective blueshift inhibition and chromospheric activity indicators. This will make the correction from this signal challenging for stars with no multi-cellular patterns, such as the Sun for example, although there may be some configurations for which the line shape variations may be used if the precision is sufficient.

Effect of the electron redistribution on the nonlinear saturation of Alfvén eigenmodes and the excitation of zonal flows

Numerical simulations of Alfvén modes driven by energetic particles are performed with the gyrokinetic (GK) global particle-in-cell code ORB5. A reversed shear equilibrium magnetic field is adopted. A simplified configuration with circular flux surfaces and large aspect ratio is considered. The nonlinear saturation of beta-induced Alfvén eigenmodes (BAE) is investigated. The roles of the wave–particle nonlinearity of the different species, i.e. thermal ions, electrons and energetic ions are described, in particular for their role in the saturation of the BAE and the generation of zonal flows. The nonlinear redistribution of the electron population is found to be important in increasing the BAE saturation level and the zonal flow amplitude.

Dynamic response of a compressible binary fluid mixture

In a binary fluid subject to periodic forcing the compressibility of the lower viscosity phase generates resonances in the system dynamics. In a zero-mean flow the flow amplitude could be optimized by driving the system at certain frequencies with possible increases of over an order of magnitude. When compressibility is irrelevant single fluid universal behavior can be recovered. In both situations the results may apply to a variety of microfluidic systems, such as devices with drops, fluids with trapped air bubbles, and mixing devices, since these are often subject to periodic forces.

0600 Automated Oximetric Versus Standard Sleep Polygraphy Scoring

Abstract Introduction Agreement between automated standard respiratory event scoring and a novel, validated, and patented oximetry-based algorithm was assessed. Methods The standard apnea-hypopnea index (AHI) was derived by adding apneas (flow drop >= 90% for 10 to 30 s) and hypopneas (flow drop >= 30% for 10 to 60 s with oxygen saturation (SpO2) dropping >=3%). The novel oxygen index (ODI4) was derived by scoring events where SpO2 dropped in each of three successive samples and cumulatively by >= 4%. Agreement was assessed by Bland-Altman analysis Results AHI versus ODI4 and Bland-Altman plots showed a high prevalence of AHI > ODI4 when AHI was< 30/h. Negative difference outliers were frequent when mean index difference was > 30/h. There was a bias of 2.83/h in the difference between AHI and ODI4 with upper and lower confidence limits of 22.0/h and -16.3/h. Conclusion Standard respiratory event scoring overestimates respiratory disturbance compared to a novel oximetric index. Standard automated scoring frequently over scores events when basal flow amplitude is low. Support None.