Low-frequency Flow Oscillations Research Articles

Study on the Influence of Low-Frequency Flow Oscillation on the Atomization Characteristics of Gas-Centered Coaxial Centrifugal Nozzle

The dynamic atomization characteristics of gas center coaxial centrifugal nozzle under the condition of pressure oscillation in the supply system were studied by applying external excitation through the flow pulsator. Obtaining spray image by high-speed photography while using hydro-mechanical pulsator to get generate of supply system. The results show that the spray width changes periodically with low-frequency pressure oscillation. For self-pulsation sprays, it is challenging to maintain a “Christmas tree” shape with low-frequency oscillation. Pressure oscillation can effectively suppress spray self-pulsation. However, the pressure oscillation amplitude may be much larger than that of the self-pulsation itself. Therefore, the premise of applying this method is that the applied external excitation cannot stimulate or amplify the combustion instability. The gas post-pressure drop determines the amplitude and frequency of the self-pulsation, while the excitation frequency will slightly affect the amplitude and frequency of the self-pulsation.

Assessment of intracranial vascular flow oscillations in Alzheimer’s disease using real time 4D flow MRI

AbstractBackgroundRecent models suggest clearance of soluble metabolites from the brain can be driven by low frequency flow oscillations (LFO) of vascular smooth muscle cells through the intramural periarterial drainage (IPAD) pathway. This work derives LFO from measures of blood flow variance using real time 4D flow MRI in the intracranial arteries of clinically diagnosed Alzheimer’s disease (AD) subjects.MethodsSubjects: Study groups are summarized in Figure 1. The APOE4+,FH+ group consisted of healthy late middle‐aged subjects with both APOE ε4 allele and parental family history of AD dementia positive. MRI: Volumetric, time‐resolved 4D flow MRI data were acquired on a 3.0T system (MR750, GE Healthcare) using a 32‐channel head coil (GE Healthcare). Images were reconstructed to a spatial resolution of 1.38mm isotropic and a temporal resolution of 4.3sec (scan time=7.1min). To evaluate the ability of 4D flow to distinguish physiologic flow changes from noise, healthy volunteers were scanned during a scan with breath‐holds (BH). LFO analysis: Flow waveforms in the internal carotid artery (ICA) and superior sagittal sinus (SSS) were recorded from the dynamic data. To quantify LFO demeaned temporal flow changes and standard deviations were calculated. Global atrophy was estimated as the ratio of CSF volume to brain volume. Group differences were assessed using ANOVA followed by Tukey‐Kramer method for pairwise comparison for adjusted means (P<0.05).ResultsFigure 2 shows an example of flow curves from a young volunteer during free breathing and a scan with BHs. Flow variance increased during the BH scan. The ICA flow variance normalized to the intracranial volume (ICV) and global atrophy, and ICA blood flow ranges were significantly larger in age‐matched controls compared to AD (P=0.025, P=0.024, P=0.004) (Figures 3&4). LFO from demeaned flow profiles are shown in Figure 5 for all subjects. The LFO in age‐matched controls were noticeable larger than in AD subjects. Finally, average flow rates in age‐matched controls were significantly larger than in AD subjects in the ICA (P=0.034) (Figure 6).ConclusionsSignificantly higher LFO, as measured by standard deviations, are measurable in age‐matched controls when compared to AD subjects suggesting decreased vascular compliance and vasomotion in AD.

Investigation of the Low-Frequency Oscillations in the Flowfield About an Airfoil

The present investigation shows that a triad of three vortices is behind the quasi-periodic self-sustained low-frequency flow oscillation phenomenon. Large-eddy simulations are carried out at , , and sixteen angles of attack at near-stall conditions. It is shown that the best description of the underlying mechanism is that of a button whirligig. A globally oscillating flow whirls around the airfoil in the clockwise direction and stores energy in the triad of vortices. When the oscillating flow loses momentum and comes to an equilibrium, the energy stored in the triad of vortices reaches a threshold. Thus, the triad of vortices reverses the direction of rotation of the oscillating flow, gives it a pulse of energy to whirl in the anticlockwise direction, and starts a new disequilibrium. When the oscillating flow rotates in the clockwise direction (in the streamwise direction on the suction surface of the airfoil), it adds momentum to the boundary layer and helps it to remain attached, and vice versa. Consequently, the instantaneous flowfield switches between an attached phase and a separated phase in a periodic manner with some disturbed cycles.

Intracranial vascular flow oscillations in Alzheimer’s disease from 4D flow MRI

Recent modeling and experimental evidence suggests clearance of soluble metabolites from the brain can be driven by low frequency flow oscillations (LFOs) through the intramural periarterial drainage (IPAD) pathway. This study investigates the use of 4D flow MRI to derive LFOs from arterial and venous measures of blood flow. 3D radial 4D flow MRI data were acquired on a 3.0 T scanner and reconstructed using a low-rank constraint to produce time resolved measurements of blood flow. Physical phantom experiments were performed to validate the time resolved 4D flow against a standard 2D phase contrast (PC) approach. To evaluate the ability of 4D flow to distinguish physiologic flow changes from noise, healthy volunteers were scanned during a breath-hold (BH) maneuver and compared against 2D PC measures. Finally, flow measures were performed in intracranial arteries and veins of 112 participants including subjects diagnosed with Alzheimer’s disease (AD) clinical syndrome (n = 23), and healthy controls (n = 89) on whom apolipoprotein ɛ4 positivity (APOE4+) and parental history of AD dementia (FH+) was known. To assess LFOs, flow range, standard deviation, demeaned temporal flow changes, and power spectral density were quantified from the time series. Group differences were assessed using ANOVA followed by Tukey-Kramer method for pairwise comparison for adjusted means (P < 0.05). Significantly lower LFOs as measured from flow variation range and standard deviations were observed in the arteries of AD subjects when compared to age-matched controls (P = 0.005, P = 0.011). Results suggest altered vascular function in AD subjects. 4D flow based spontaneous LFO measures might hold potential for longitudinal studies aimed at predicting cognitive trajectories in AD and study disease mechanisms.

Numerical investigation of the low-frequency flow oscillation over a NACA-0012 aerofoil at the inception of stall

High-fidelity large eddy simulation is carried out for the flow field around a NACA-0012 aerofoil at Reynolds number of [Formula: see text], Mach number of 0.4, and various angles of attack around the onset of stall. The laminar separation bubble is formed on the suction surface of the aerofoil and is constituted by the reattached shear layer. At these conditions, the laminar separation bubble is unstable and switches between a short bubble and an open bubble. The instability of the laminar separation bubble triggers a low-frequency flow oscillation. The aerodynamic coefficients oscillate accordingly at a low frequency. The lift and the drag coefficients compare very well to recent high-accuracy experimental data, and the lift leads the drag by a phase shift of [Formula: see text]. The mean lift coefficient peaks at the angle of attack of [Formula: see text], in total agreement with the experimental data. The spectra of the lift coefficient does not show a significant low-frequency peak at angles of attack lower than or equal the stall angle of attack ([Formula: see text]). At higher angles of attack, the spectra show two low-frequency peaks and the low-frequency flow oscillation is fully developed at the angle of attack of [Formula: see text]. The behaviour of the flow-field and changes in the turbulent kinetic energy over one low-frequency flow oscillation cycle are described qualitatively.

The Impact of Stochastic Perturbations upon the Hydrodynamic Relationship between the Activity of Human Cardiac Ventricles and Low-Frequency Blood Flow Oscillations in the Microcirculatory Bed

Abstract—The experimentally revealed high phase coherence between low-frequency oscillations in the cutaneous blood perfusion signal at the contralateral skin sites indicates the existence of a central mechanism of its regulation. The vascular bed can be such a regulatory mechanism because it is a closed hydrodynamic system. It has been shown in this study using a mathematical model of the human cardiovascular system that the impact of low-intensity stochastic perturbations on the stiffness of the cardiac ventricles leads to the formation of low-frequency oscillations in microvascular blood flow. The results show that there is a relationship between the activity of the heart and low-frequency oscillations of microcirculatory skin blood flow, which is due to the hydrodynamic properties of the vascular bed without the involvement of autonomous control from autonomic nervous system.

High-fidelity numerical simulation of the flow field around a NACA-0012 aerofoil from the laminar separation bubble to a full stall

ABSTRACTIn the present work, large eddy simulations of the flow field around a NACA-0012 aerofoil near stall conditions are performed at a Reynolds number of 5 × 104, Mach number of 0.4, and at various angles of attack. The results show the following: at relatively low angles of attack, the bubble is present and intact; at moderate angles of attack, the laminar separation bubble bursts and generates a global low-frequency flow oscillation; and at relatively high angles of attack, the laminar separation bubble becomes an open bubble that leads the aerofoil into a full stall. Time histories of the aerodynamic coefficients showed that the low-frequency oscillation phenomenon and its associated physics are indeed captured in the simulations. The aerodynamic coefficients compared to previous and recent experimental data with acceptable accuracy. Spectral analysis identified a dominant low-frequency mode featuring the periodic separation and reattachment of the flow field. At angles of attack α ≤ 9.3°, the low-frequency mode featured bubble shedding rather than bubble bursting and reformation. The underlying mechanism behind the quasi-periodic self-sustained low-frequency flow oscillation is discussed in detail.

Dynamics of laminar separation bubble over NACA-0012 airfoil near stall conditions

In this paper, we investigated the dynamics of laminar separation bubble over an airfoil near stall conditions. Large eddy simulation of flow around NACA-0012 airfoil was conducted at an angle of attack α=11.5∘ for a Reynolds number Rec=1.3×105 and Mach number M∞=0.4. A self-sustained low-frequency flow oscillation was observed and its life cycle was described and analyzed. The computed Strouhal number of the oscillation was in good agreement with the experimental data. The flow oscillation was due to quasi-periodic bursting and reforming of the laminar separation bubble. The size of the computed bubble was twice that of the experiment and resulted in 19% difference between the computed and the experimental lift coefficients. The average of the maximum reverse velocity percentage inside the bubble, during attached phase, was 23.4%. The characteristics of the transition in the separated shear layer changed through the low-frequency cycle in which the turbulent kinetic energy was maximum when the bubble is at its shortest length. Dynamic mode decomposition analysis of the computed flow identified two dominant modes; the low-frequency oscillation mode and a high frequency mode representing trailing-edge shedding. The reconstructed flow of the DMD modes produced two distinct events, i.e., (1) fully separated flow with intense shedding at the trailing-edge and (2) a fully attached flow with no apparent vortex shedding at the trailing-edge. The DMD results suggest a critical role of acoustic feedback from the trailing edge in promoting earlier transition and forcing the separated flow to reattach.

Numerical Investigation of the Low-Frequency Flow Oscillation over NACA-0012 Aerofoil

The present study investigates the low-frequency flow oscillation phenomenon near stall conditions for NACA0012 aerofoil at Reynolds number of 9 × 104, Mach number of and angle of attack of 11.0°. The phenomenon is clearly captured in the time histories of aerodynamic coefficients. Statistical and spectral analysis are carried out for time histories of aerodynamic coefficients. Qualitative study is performed to investigate the unsteady behaviour of laminar separation bubble over one low-frequency cycle.

Plasma flow, turbulence and magnetic islands in TJ-II

The effect of magnetic islands on plasma flow and turbulence has been experimentally investigated in ohmically induced magnetic configuration scans at the stellarator TJ-II. This operational mode allows sweeping the radial position of a low order rational surface from the plasma core towards the edge in a controlled way, what reveals effects that are difficult to notice in scans performed on a shot to shot basis. The main diagnostic used in the present work is a two-channel Doppler reflectometer that allows the measurement of the perpendicular rotation velocity of the turbulence and density fluctuations with good spatial and temporal resolution. A characteristic signature of the n/m = 3/2 magnetic island as it crosses the measurement position is clearly detected: the perpendicular flow reverses at the center of the magnetic island and a flow shear develops at the island boundaries. Fluctuations of the perpendicular flow and density have been also measured along the 3/2 magnetic island. An increase in the low frequency flow oscillations is measured at the magnetic island boundaries together with a reduction in the density fluctuation level; the later being more pronounced at the inner island boundary. These observations could explain the link between magnetic islands and transport barriers observed in a number of fusion devices.

Aeroacoustic source analysis in a corrugated flow pipe using low-frequency mitigation

Our study is focused on a phenomenon often encountered in flow carrying pipes, since flow instabilities caused by geometric features may generate acoustic signals and, thereafter, interact with these signals in such a way that powerful pure tones are produced. A modern example is found in the so-called ‘singing risers’, or the gas pipes connecting gas production platforms to the transport network. But the flow generated resonance in a fully corrugated circular pipe may be silenced by the addition of relatively low frequency flow oscillations induced by an acoustic generator. Experiments reported here, aimed at investigating in more detail the coupling between the flow in the pipe, the acoustically generated flow oscillations and the emitted resulting noise, are performed in a specifically designed facility. A rectangular transparent channel using glass walls enables us to use optical techniques to describe in detail the flow field in the corrugation vicinity, in addition to more standard hot-wire anemometry and acoustic pressure measurements with microphones, with and without the acoustically generated low-frequency oscillations.

Wavelet phase coherence analysis of the skin blood flow oscillations in human

The wavelet phase coherence of oscillations in the peripheral blood flow of contralateral skin sites was studied in 20 healthy subjects. Skin perfusion was registered simultaneously on similar regions of the outer sides of the right and left forearms by the laser Doppler flowmetry technique. To estimate the reliability of the obtained wavelet phase coherence values we applied the comparative method using amplitude-adjusted Fourier transform surrogates. High median values (0.63 and 0.59) of the wavelet phase coherence were obtained for the frequency intervals of respiratory (0.145–0.6Hz) and cardiac (0.6–2Hz) rhythms in 18 and 20 participants, respectively. In all the 20 participants we detected high and reliable values (Ме=0.72) of the wavelet phase coherence for skin blood flow oscillations in the myogenic interval (0.052–0.145Hz). Additionally, we demonstrated high wavelet phase coherence in the neurogenic (0.021–0.052Hz) and endothelial (0.0095–0.021Hz) intervals in 8 and 7 participants, respectively. The corresponding medians of the reliable wavelet phase coherence values for these intervals were 0.74 and 0.82. The obtained results suggest that the microvascular blood flow possesses not only the local mechanisms of generating low-frequency blood flow oscillations, but also a central mechanism, which is likely to synchronize low-frequency oscillations throughout the whole cardiovascular system.

Modelling transition due to backward-facing steps using the laminar kinetic energy concept

Boundary layer transition estimation and modelling is essential for the design of many engineering products across many industries. In this paper, the Reynolds-averaged Navier–Stokes are solved in conjunction with three additional transport equations to model and predict boundary layer transition. The transition model (referred to as the kT–kL–ω model) is based on the k–ω framework with an additional transport equation to incorporate the effects low-frequency flow oscillations in the form of a laminar kinetic energy (kL). Firstly, a number of rectifications are made to the original kT–kL–ω framework in order to ensure an appropriate response to the free-stream turbulence level and to improve near wall predictions. Additionally, the model is extended to incorporate the capability to model transition due to surface irregularities in the form of backward-facing steps with maximum non-dimensional step sizes of approximately 1.5 times the local displacement thickness of the boundary layer where the irregularity is located (i.e k/δ∗⪅1.5) at upstream turbulence intensities in the range 0.01<Tu(%)<0.8. A novel function is proposed to incorporate transition sensitivity due to aft-facing steps. This paper details the rationale behind the development of this new function and demonstrates its suitability for transition onset estimation on a flat plate at zero pressure gradient.

Large-Eddy Simulation of Natural Low-Frequency Oscillations of Separating–Reattaching Flow Near Stall Conditions

The present work investigates the natural low-frequency flow oscillation phenomenon using large-eddy simulation. Four simulations were carried out for flow around a NACA 0012 airfoil with Reynolds number of 50,000 and Mach number at angles of attack of 9.25, 9.29, 9.4, and 9.6 deg. The results clearly indicate that natural low-frequency flow oscillation is taking place. The oscillations are self-sustained and caused by periodic bubble forming and bubble bursting on the suction side. The low-frequency oscillation phenomenon was found to exist over a range of angles of attack near stall. Computed maximum reverse flow inside the bubble indicates an absolute instability mechanism. The time- and span-averaged flowfield as well as the instantaneous turbulent flowfield illustrated the dynamics of low-frequency oscillation near stall. The location of transition is shown to move downstream during bubble bursting. Spectral analysis of the surrounding acoustic field reveals that the observed phenomenon affects the circulation around the entire airfoil. Analysis of the flowfield led to the proposition of a mechanism for low-frequency flow oscillations near stall in which the separated shear layers from the leading edge and the trailing edge play a major role in driving the separated flow to reattachment.

Intermittent Bursting of a Laminar Separation Bubble on an Airfoil

Large eddy simulation of flow around a NACA-0012 airfoil at a Reynolds number of 50,000 has been used to study the behavior of a laminar separation bubble near stall. The effects of the subgrid-scale model and explicit filtering were studied for a test case in which direct numerical simulation results were available. It was found that a method incorporating a mixed-time-scale model in addition to explicit filtering gave improved results compared with a method with filtering alone. Statistical results as well as snapshots of the flow below stall exhibit good agreement with the direct numerical simulations. For a configuration near still, the effect of the spanwise domain width was investigated by increasing the spanwise length from 20 to 50% chord. Two-point velocity correlations showed a significant improvement for the wider computational domain, in which the simulation was able to capture a low-frequency flow oscillation, in which intermittent bursting of the bubble was observed. The bubble bursting observed here is more irregular than in experiments at higher Reynolds number. The amplitude and frequency are compared with experimental results and with an unsteady viscous-inviscid interaction method which is shown to be capable of capturing unsteady behavior during stall.

Low-frequency blood flow oscillations in congestive heart failure and after β1-blockade treatment

Laser Doppler flowmetry (LDF) of forearm skin blood flow, combined with iontophoretically-administered acetylcholine and sodium nitroprusside and wavelet spectral analysis, was used for noninvasive evaluation of endothelial function in 17 patients newly diagnosed with New York Heart Association class II–III congestive heart failure (CHF). After 20±10 weeks' treatment with a β1-blocker (Bisoprolol), the measurements were repeated. Measurements were also made on an age- and sex-matched group of healthy controls (HC). In each case data were recorded for 30 min. In HC, the difference in absolute spectral amplitude of LDF oscillations between the two vasodilators manifests in the frequency interval 0.005–0.0095 Hz (p<0.01); this difference is initially absent in patients with CHF, but appears following the β1-blocker treatment (p<0.01). For HC, the difference between the two vasodilators also manifests in normalised spectral amplitude in 0.0095–0.021 Hz (p<0.05). This latter difference is absent in CHF patients and is unchanged by treatment with β1-blockers. It is concluded that there are two oscillatory skin blood flow components associated with endothelial function. Both are reduced in CHF. Activity in the lower frequency interval is restored by β1-blocker treatment, confirming the association between CHF and endothelial dysfunction but suggesting the involvement of two distinct mechanisms.

Cyclic flow oscillations in a system of repeatedly branching channels

The unsteady nature of the flow within a cascade of bifurcating branches was investigated for Reynolds numbers of 900 and 2700 in the mother branch. The cascade consists of three generations of planar symmetric bifurcations with a fixed branching angle and ratio of branch length to width. Chronological flow field measurements were carried out using digital-particle-image-velocimetry (DPIV) to determine the time-dependent velocity distribution within the branches. Though the flow input was steady, low-frequency flow oscillations were found within the branches which were coupled with the dynamics of the separation regions and the cross-talk between the communicating daughter branches. The toggling flip-flop character of the flow was explained by means of a feedback loop in which the cyclic growth, wash-out and re-generation of the separation regions is assumed to be the inner motor for the periodic continuation. These local excitation mechanisms were suggested to be similar to those causing the generation of low-frequency oscillations in shear-layers impinging on edges like in a rectangular cavity. The results demonstrate that unsteady effects and excited oscillations play an additional role in uneven flow partitioning in flow bifurcation cascades, however, they do not favor any flow partitioning in a preferential pathway constant in time

Flow oscillation over an airfoil near stall

The low-frequency flow oscillation on airfoils near stall has been observed on several airfoils over a range of Reynolds numbers. However, the mechanism of selection of the long time scale involved in the oscillation remained far from clearly understood. This provided motivation to continue to pursue the topic. Here, efforts are made to further document the characteristics of the phenomenon and gain additional insight into the flow mechanisms.

Low-Frequency Flow Oscillation over Airfoils near Stall

Low-Frequency Flow Oscillation over Airfoils near Stall

Hot film coronary artery velocity measurements in horses.

Coronary velocity measurements have been carried out in anaesthetized, open-chest horses using a constant-temperature, hot-film anemometer system. L-shaped needle probes inserted by direct vessel puncture have been used to measure velocity profiles in the left common, left anterior descending (LAD), and left circumflex coronary arteries. The flow conditions were characterized by peak Reynolds numbers from approximately 200 to 1500 and values of the unsteadiness parameter from 3 to 10. These measurements indicate that in the left common coronary artery the profile is in general skewed towards the outer wall as would be expected for fully viscous flow in a curved tube. In the left anterior descending and left circumflex coronary arteries just distal to the bifurcation, the skewing was found in general to be away from the flow divider. However, in regions of the LAD and left circumflex 5-6 diameters downstream of the bifurcation, the peak systolic and diastolic profiles were indicative of a more fully developed, Poiseuille type flow with only slight skewing observed. The results of this study indicate that the flow in the coronary system, though in general laminar and disturbance free, is extremely varied in character and may exhibit large amplitude, low frequency flow oscillations. Furthermore, for these vessels which all lie on the surface of the myocardium, large systolic flows were observed to be present, even though the major portion of the volume flow was reserved for the diastolic period.