Amplitude Of Flow Oscillations Research Articles

Numerical study on flow instability in parallel helical tubes under ocean conditions

The superimposed coupling of flow oscillations generated by transient external forces under ocean conditions and flow oscillations in parallel helical tubes during two-phase flow instability may lead to more complex flow oscillation phenomena in parallel helical tubes. However, there is still a lack of understanding regarding the effect of ocean conditions on the two-phase flow instability within parallel helical tubes. The two-phase flow instability in parallel helical tubes under ocean conditions is numerically investigated in the present study. The effects of ocean conditions on the flow behavior and instability threshold in parallel helical tubes are studied with mass flux 200 kg/m2s and inlet subcooling within 5–70 °C. The results show that the mass flow rate decreases as the inclination angle increases. A larger rolling amplitude or shorter period increases the flow and temperature oscillation amplitude. Two characteristic frequencies are found in the natural circulation flow instability under the rolling motion. As the rolling amplitude increases, the compound flow oscillation amplitude is larger and more chaotic with a certain phase shift. The effect of stationary inclining on the flow instability threshold is rather significant than rolling effect. The results of the present study can serve as a significant reference for the operation and design of the helical-coiled once-through steam generator under ocean conditions.

Numerical investigation of the response of turbulent swirl non-premixed flames to air flow oscillations

The response of swirl non-premixed flames to air flow oscillations is studied using Large-Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model, focusing on the physical mechanisms leading to the heat release rate oscillations observed in a parallel experimental study. Cases relatively close to blow-off and characterized by different amplitude of the flow oscillations are considered. Numerical results are in good agreement with the experiment in terms of both mean flame shape and heat release rate response. Simulations show that the oscillation of the air flow leads to an axial movement and fragmentation of the flame that are more pronounced with increasing amplitude of the forcing. The flame response is characterized by fluctuations of the flame area, time-varying local extinction and lift-off from the fuel injection point. LES-CMC, due to the inherent capability to capture burning state transitions, predicts properly the flame transfer function as a function of the amplitude of the air flow oscillations. This suggests that the response mechanism for this flame is not only due to time-varying flame area, but also local extinction and re-ignition. This study demonstrates that LES-CMC is a useful tool for the analysis of the response of flames of technical interest to large velocity oscillations and for the prediction of the flame transfer function in conditions close to blow-off.

Perivascular pumping in the mouse brain: Improved boundary conditions reconcile theory, simulation, and experiment

Cerebrospinal fluid (CSF) flows through the perivascular spaces (PVSs) surrounding cerebral arteries. Revealing the mechanisms driving that flow could bring improved understanding of brain waste transport and insights for disorders including Alzheimer’s disease and stroke. In vivo velocity measurements of CSF in surface PVSs in mice have been used to argue that flow is driven primarily by the pulsatile motion of artery walls — perivascular pumping. However, fluid dynamics theory and simulation have predicted that perivascular pumping produces flows differing from in vivo observations starkly, particularly in the phase and relative amplitude of flow oscillation. We show that coupling theoretical and simulated flows to more realistic end boundary conditions, using resistance and compliance values measured in mice instead of using periodic boundaries, results in velocities that match observations more closely in phase and relative amplitude of oscillation, while preserving the existing agreement in mean flow speed. This quantitative agreement among theory, simulation, and in vivo measurement further supports the idea that perivascular pumping is an important CSF driver in physiological conditions.

Passive solute separation in AC electroosmosis including surface charge-coupled hydrodynamic slip effects.

Separation of electrically neutral, mutually noninteracting passive solutes via AC electroosmotic slit channel flows is investigated for general asymmetric wall surface zeta potentials and apparent hydrodynamic slip lengths. We consider the nontrivial coupling between the surface potentials (or charge densities) and the apparent slip lengths, and focus our attention on the occurrence of a so called "crossover phenomenon" for separating out the slow diffusers when both slow and fast diffusers are present. Results show that regardless of the potential-slip coupling, wider bandwidths become available for crossover phenomenon to occur when the electroosmotic velocity gradient (magnitude) is greater. Contrarily, plug-like velocity profiles inhibit crossover phenomenon, and the potential-slip parametric combinations leading to such profiles can be easily identified by the conditions for minimal transport enhancement reported in recent literature. When separating out the slow diffuser or crossover phenomenon is desired, we recommend incorporating significant asymmetry in the surface potential and apparent slip boundary conditions such that the operating frequency and flow oscillation amplitude may be lowered to more practical values. Our results also agree with and strengthen the physical picture for explaining crossover phenomenon in macroscopic pressure-driven oscillatory flows.

Flow Instability Investigation at Supercritical Pressures in Parallel Channels by Computational Fluid Dynamics Adopting Low- and High-Power Boundaries

Abstract Supercritical water-cooled reactor (SCWR), which is considered as the logical extension of existing light water reactors (LWRs) (pressurized water reactor and boiling water reactor (BWR)), has the potential of increasing the efficiency of power generation to 45% compared to 33% of that of LWRs. But without the challenges of heat transfer and hydrodynamics, and reactor core design materials due to supercritical flow instability which is associated with sharp variation in fluid properties near the vicinity of the pseudo-critical temperature. Supercritical flow instability therefore needs to be addressed ahead of the deployment and operation of SCWR in the near future. The main purpose of this study is to carry out flow instability analysis in parallel channels with supercritical water. The study also aims at examining the capability of using three-dimensional (3D) simulation of turbulent flow in arbitrary regions computational continuum mechanics C++ based code (3D STAR-CCM+ CFD code) to predict flow oscillation amplitude and periods, and instability power boundaries at low-power boundary (LPB) and at high-power boundary (HPB). Parameters considered in the investigation include mass flowrate, system pressure, and gravity. Two different threshold power instability boundaries were obtained from the study. These instability power boundaries include lower threshold where stability of the parallel channel system decreases with increasing coolant inlet temperature, and upper threshold where stability of the parallel channel system increases with increasing coolant inlet temperature. From the results of the investigation, it can be found that: (1) for LPB at 23 MPa, only lower threshold was obtained as flow instability power boundary; and for HPB at 23 MPa, both lower and upper thresholds were obtained as flow instability power boundaries. The numerical findings quite well agree with the experimental findings at 23 MPa for both LPB and HPB; (2) only lower threshold was obtained as flow instability power boundary at both 23 MPa and 25 MPa for LPB. For HPB, both lower and upper thresholds were obtained as flow instability power boundaries at both 23 MPa and 25 MPa; (3) only lower threshold was obtained as flow instability power boundary for the parallel channel system with or without gravity influence for LPB. For HPB, both lower and upper threshold flow instability power boundaries were obtained for the parallel channel system with gravity influence, but only lower threshold flow instability power boundary was obtained for system without gravity influence; (4) only lower threshold was obtained as flow instability power boundary at system mass flowrates of 125 kg/h and 145 kg/h for LPB. For HPB, both lower and upper threshold flow instability power boundaries were obtained for system mass flowrate of 125 kg/h, but only lower threshold flow instability power boundary was obtained for system mass flowrate of 145 kg/h. For both LPB and HPB, the numerical findings agree quite well with the experimental results for a system operated at 125 kg/h and 145 kg/h; (5) the investigated parameters such as mass flowrate, pressure, and gravity have significant effects on amplitude of mass flow oscillation, but have little effects on the period of mass flow oscillation for both LPB and HPB. Results from the numerical simulation were compared with the results from the experiment for both LPB and HPB. The numerical amplitude results obtained were far less than the amplitude results obtained from the experiment. But there was no significant difference between the oscillation periods obtained from both the numerical simulation and experiment. (6) Flow instability studies including predicting flow oscillation amplitude and periods, and instability power boundaries could be carried out using 3D STAR-CCM+ CFD code. The effects of heating structures on flow instability results have not been considered in this study. Previous studies have shown that including heating structures in geometrical models for numerical studies may have effects on flow instability results. More experimental studies are needed for validation of similar numerical studies carried out at supercritical pressures using various numerical tools.

Comparison of 6 Oscillatory Positive Expiratory Pressure Devices During Active Expiratory Flow.

Air-flow oscillations generated by exhaling through oscillatory positive expiratory pressure (OPEP) devices favor airway clearance. Variations in mechanical properties between different devices may influence therapeutic efficacy. The objective of this study was to assess mechanical properties in vitro and to compare the performance of 6 OPEP devices at different resistance levels under active expiratory flow patterns. 4 gravity-dependent OPEP devices (ie, Flutter, Gelomuc, Pari O-PEP, Shaker Medic Plus) and 2 gravity-independent OPEP devices (ie, Acapella Choice and Aerobika) were each tested at low, medium, and high resistance settings. All devices were independently connected to a pulmonary waveform generator that reproduced active exhalation flows. Expiratory flow-volume curves were retrieved from 4 subjects with different stages of obstruction severity and were scaled according to either peak expiratory flow (4, 6, and 8 L/s) or volumes (2, 3 and 4 L), thus amounting to 24 active exhalations. Resulting waveforms were divided into 4 parts and the 2 middle parts were used to extract the following mechanical data: positive expiratory pressure (PEP), maximum expiratory pressure (Ppeak), oscillation frequency, and flow oscillation amplitude. The percentage of tests achieving oscillation frequencies ≥ 12 Hz and PEP ≥ 10 cm H2O was calculated for each device. Mechanistic effects of the Acapella, Aerobika, and Shaker devices were not comparable. The Flutter, Gelomuc, and Pari devices behaved similarly and achieved more tests with optimum oscillation frequency and PEP values than the other devices. These 3 devices also produced the highest oscillation amplitudes at the low-resistance level, whereas the Aerobika elicited higher and consistent oscillation amplitudes at medium and high resistance settings. Operational parameters differed between and within devices, yet the Flutter, Gelomuc, and Pari devices were similar in many aspects. Therapeutic efficacy may depend on the selected OPEP device and set resistance.

Energy-saving management of liquid’s transportation in pipelines

AnnotationTransportation of coolants, oil and oil products are associated with the flow of liquid medium through pipelines of circular cross-section. When the fluids move through the pipes, pressure losses arise on the friction of the flow against the pipe wall. Reduction of wall friction, and accordingly energy loss for pumping liquid can be achieved by pulsating flow in the pipeline. The pulsating mode has acceleration phases, which corresponds to a more complete velocity and deceleration profile, which is characterized by less filled velocity profile, in comparison with a stationary flow. The mathematical model is based on the theory of the boundary layer, using the Prandtl hypothesis on the length of the mixing path. As a result of the numerical experiment, qualitative and quantitative information on the effect of harmonic oscillations of the fluid flow on the hydrodynamic flow characteristics was obtained. The influence of frequency factors and the amplitude of flow oscillations is shown, as well as one of the main characteristics determining the economic effect of the process that is pressure loss on friction along the length of the channel. A method for creating harmonic and nonharmonic oscillations is proposed.

Theoretical investigation of effects of flow oscillations on ultrasound Doppler velocity measurements

Effects of flow oscillations on spectrum of Ultrasound Doppler Velocimetry (UDV) signals were investigated theoretically and numerically. A laminar pipe flow with a superimposed oscillating component was considered. Negative impact of oscillation on the ultrasound signal hence on the flow images was observed in the form of spreading of spectral ultrasound signal energy around mean component, leading to image artifacts. Both analytical and numerical results revealed the strong effect of a group of parameters including Doppler frequency, flow oscillation amplitude and frequency. Exceeding a particular value of the group, 1.45, resulted in artifacts in the flow images. Revealing the mechanisms involved in the deteriorations associated with the flow oscillations is potentially useful in UDV studies involving random flow fluctuations such as turbulence.

Analysis and active control of pressure-drop flow instabilities in boiling microchannel systems

Pressure-drop oscillations are one of the most severe dynamic instabilities for boiling flow especially in microchannel systems. This paper presents a systematic framework for the transient analysis and active control of microchannel flow oscillations at a system-level view. To quantify the upstream compressibility and the associated oscillatory transients in an experimental microchannel boiling system, a lumped oscillator model is derived from the momentum balance equation, and both analytical and numerical nonlinear parameter identification methods are proposed. The predictions from the flow oscillation model agree well with the experimental pressure-drop observations across a flow meter and a microchannel heat sink. Based on the identified nonlinear oscillator model, a virtual state observer is designed to estimate the mass flow acceleration from the mass flowrate measurement in the boiling channel. Then a family of state and dynamic output-feedback active flow controllers is developed and evaluated for the dynamic pressure-drop instability suppression. Further analysis and simulations show the flow oscillation amplitude can be regulated to whatever level is desired under certain conditions.

Numerical investigations on power ramping procedure for a natural circulation boiling water reactor

Startup of a natural circulation boiling water reactor (NCBWR) is studied numerically, using a thermal-hydraulic system code RELAP5. A number of numerical experiments are carried out using various power ramps, and a suitable heat-up rate is identified to pressurize the reactor to the desired operating conditions in a reasonable time without considerable void generation in the core. It is observed that the occurrence of flashing in the riser section is unavoidable. Although flashing helps in steam production, the amplitude of flow oscillations induced by flashing is the event of concern, as in the case of the pressure tube type NCBWR studied here. Therefore, the feasibility of a complete single-phase startup is also examined and found not attractive. A new startup procedure, which completely bypasses the unstable two-phase region, is conceptualized, and the method to take the system to the operating condition without encountering flow oscillations is numerically investigated.

Grouping and trapping of evaporating droplets in an oscillating gas flow

A new approach to the analysis of droplet grouping in an oscillating gas flow is suggested. This is based on the investigation of droplet trajectories in the frame of reference moving with the phase velocity of the wave. Although the equations involved are relatively simple, the analysis shows distinctive characteristics of grouping and non-grouping cases. In the case of grouping, droplet trajectories converge to the points for which the ratio of flow velocity in this frame of reference and the amplitude of flow oscillations is less than 1, and the cosine of the arc sine of this ratio is positive. In the case of non-grouping, droplet trajectories in this frame of reference oscillate around the translational velocity close to the velocity of flow in the same frame of reference. The effect of droplet size on the grouping pattern is investigated. It has been pointed out that for the smaller droplets much more stable grouping is observed. The effect of droplet evaporation is studied in the limiting case when the contribution of the heat-up period can be ignored. It is shown that evaporation can lead to droplet grouping even in the case when the non-evaporating droplets are not grouped. This is related to the reduction in droplet diameter during the evaporation process. Coupling between gas and droplets is shown to decrease the grouping tendency. A qualitative agreement between predictions of the model and in-house experimental observations referring to Diesel engine sprays has been demonstrated.

Stability of an oscillatory shear flow in a differentially heated vertical channel

The linear stability of an oscillatory shear flow in a differentially heated vertical channel was investigated numerically for Re = 1000 and Pr = 0.7. The Galerkin method is used to solve the disturbance momentum and energy equations. The results show that the least stable disturbance could be three-dimensional for higher flow oscillation frequency and larger flow oscillation amplitude, while it is two-dimensional in the isothermal oscillatory and heated steady channel flows. The flow oscillation acts to stabilize the flow at moderate and high oscillation frequencies, where the degree of stabilization increases with the oscillation amplitude; but, it acts to destabilize the flow and the amount of destabilization increases with the oscillation amplitude at low oscillation frequency. It is shown from the balance of disturbance kinetic energy budget that shear production is responsible for the flow instability. For the 2-D wave initiated instability, almost all the shear production is generated during a very short time interval at low oscillation frequency, while it is generated during most of the time of a cycle for the 3-D disturbance.

Laminar convective heat transfer from a circular cylinder exposed to a low frequency zero-mean velocity oscillating flow

Heat transfer characteristics of a circular cylinder exposed to a slowly oscillating flow with zero-mean velocity were investigated. The flow oscillation amplitude and frequency were changed in the range where the flow remains laminar and fluid particle travels back and forth over much larger distance compared to the cylinder diameter. The time- and space-averaged Nusselt number was measured by transient method, while two-dimensional numerical simulation was conducted to discuss the instantaneous flow and thermal fields around the cylinder. It was found that the time- and space-averaged Nusselt number can be correlated with the oscillating Reynolds number and Richardson number. Unique heat transfer characteristics under oscillating flow condition can be seen at the phases when the cross-sectional mean velocity is small or increasing from small value. During such period, heat transfer can be enhanced due to the local fluid motion induced by the vortices around the cylinder, which once moved away but returned back by the reversed flow. This heat transfer enhancement, however, is countered by the local warming effect of the hot vortices clinging around the cylinder at such phases.

Combustion of premixed fuel and air downstream of a plane sudden-expansion

Experiments have been performed to quantify the isothermal and combusting flows downstream of a plane sudden-expansion. The detailed measurements correspond to an area expansion ratio of 2.86 and a Reynolds number of 20000, and the combusting flows comprised premixed methane and air over a range of equivalence ratios with emphasis on values of 0.72 and 0.92 which gave rise to smooth and rough combustion, respectively. The results show that the extent of asymmetry of the isothermal flows was reduced by coupling the pressures between the two recirculation regions, and by imposing oscillations at the half-wave or full-wave frequency of the duct, and by combustion. Periodic variations of flame shape, velocities, acceleration, and temperature were observed in sympathy with the dominant pressure oscillation of rough combustion, and the length of the recirculation zones varied from less than 0.5 to 3 step heights. Rich and lean limits were established for combustion within the duct and, whereas the flame blew off at the lean limit, it detached from the expansion at the rich limit and stabilised on the flange at the duct exit. Within these limits, there were ranges of equivalence ratios over which the flame stabilised on one of the two steps with incomplete combustion. The imposition of oscillations narrowed the range of equivalence ratios over which the flame could be stabilised but reduced the equivalence ratio of the lean limit at which the flame could be stabilised on both steps and the effect increased with amplitude and was greatest when the frequency of the imposed oscillations corresponded to that of the half-wave in the duct. An increase in the amplitude of flow oscillations, natural or imposed, caused the concentrations of NO x measured at the duct exit to decrease. Active control of flows with high amplitude of oscillations produced the expected reductions, but not over the entire measured range of equivalence ratio, and the imposition of pressure oscillations at the second harmonic of the half-wave frequency had a greater effect and over a wider range.

Quantitation of thyroid hormone effect on skin perfusion by laser Doppler flowmetry.

Clinical observations have long suggested that skin perfusion (SP), among other factors, depends on thyroid status. However, quantitative data concerning this relationship are rather sparse. This study characterizes SP by means of laser Doppler flowmetry (LDF) in various thyroid dysfunctional states. The data reveal that mean capillary flow velocity, capillary pulse wave amplitude, and capillary flow oscillation amplitude, but not capillary flow oscillation frequency, are increased in hyperthyroid patients and decreased in hypothyroid patients. Regaining the euthyroid state is accompanied by normalization of LDF parameters only in hyperthyroid patients. It is concluded that LDF is useful in monitoring the thyroid effect on SP. However, it may not be used as a biological marker for the thyroid status of a given individual.

Initiation and suppression of combustion instabilities by active control

An active instability control (AIC) system is used to suppress low-frequency combustion instabilities in a non-premixed multiple-flame combustor and to study their initiation. The AIC device is based on a feedback control loop: The signal provided by a microphone is filtered, processed and sent to driver units plugged on the air duct. It allows the suppression of all unstable modes of the combustor and also provides a new and powerful method to study the instability initiation. Starting from a controlled flame with AIC, the control system is switched off and the growth of the instability is analysed through high speed Schlieren cinematography and sound pressure and reaction rate measurements. Nonlinear spectral analysis is used to extract spectral information during the oscillation growth from the initiation to the limit cycle. Results reveal that five different phases take place between stable combustion and fully established instability. The source of the first flame movements is the flow excitation by longitudinal acoustic waves. When the flow oscillation amplitude is high enough to allow interactions between flame sheets, combustion becomes periodically enhanced. This coupling leads to a fast instability growth until a limit cycle is reached.

STUDY ON THE FALL VELOCITY OF PARTICLES IN OSCILLATING FLUIDS

The simulation of particle settlement in oscillating fluids, which motion is followed by the equation of motion derived by Tchen, is performed, and the results of simulation are verified by the experimental data obtained by Ho. It is revealed that the factors governing retardation of fall velocity are particle Reynolds number, Strouhal number and the ratio of the amplitude of flow oscillation to a particle diameter. Futhermore, when the suspended load in the channel flow is analysed by the simulation model, the criterion of the reasonable calculating time step is proposed based on the results of this study.

Critical heat flux of forced convection boiling in an oscillating acceleration field—II. Contribution of flow oscillation

Critical heat flux (CHF) with forced convection generally decreases under an oscillating acceleration condition. Contribution of flow oscillation on the decrease in CHF was investigated experimentally and theoretically. The experiments were performed with a Freon-113 boiling loop. The results showed that, in the high exit quality region deteriotation of CHF could almost wholly be attributed to the variation of inlet flow rate induced by motion. The amplitude of flow oscillation for a given acceleration variation could be predicted from a transfer function derived with linearization technique. Prediction of the transient CHF on the basis of “local-conditions hypothesis” gave conservative values.

Steady and pulsatile flow distribution in a multiple branching network with physiological applications

Flow rate distribution in steady and pulsatile flow is investigated in a multiple branching network including six successive generations in the same plane. In this model, the geometry dependence of flow rate distribution has already been pointed out in steady flow (previous study) by observing the occurrence of non-uniform flow rate distribution at terminal orifices despite the symmetrical dichotomy, identical distance and cross-sectional profile of the 64 parallel pathways. In the present study, we point out two additional properties of the steady flow rate distribution. 1. (i) The flow rate distribution is not markedly sensitive to a change in viscosity and therefore Reynolds number does not appear to be a determinant factor to modify the flow distribution. 2. (ii) The effect of a branch obstruction on flow rate distribution is limited in space and its extension remains the same for the different Reynolds numbers tested. These properties also characterize the interdependence between the model geometry and the flow distribution in steady state. The results obtained in pulsatile flow show that the flow distribution remains similar to steady state as long as the parameter λ = Q p Q s ( Q p = amplitude of flow oscillation, Q s = steady component of the total flow) is less than 1. When λ is greater than 1, the flow distribution may become uniform, which means that contrary to the steady flow case, the velocity profiles become rapidly symmetrical downstream from the bifurcations. Physical explanations for this are proposed after considering the theoretical problem of pulsatile developed flow in a straight tube. In this case, the parameter λ again plays a crucial role in the velocity distribution.

The effect of oscillation on flat plate heat transfer

The time-mean heat transfer of the incompressible laminar boundary layer on a flat plate under the influence of oscillation is studied analytically. Flow oscillation amplitude outside the boundary layer is assumed constant along the surface and the viscous dissipation effect is considered. First, the small velocity–amplitude case is treated and the approximate formulae are obtained in the extreme cases when the frequency is low and high. Next, the finite velocity–amplitude case is treated under the condition of high frequency and it is found that the formulae obtained for the small amplitude and high frequency case are also valid. These results show that, when the oscillation is of high frequency, the time-mean heat flux to the wall can be several times as large as that without oscillation. This is due wholly to the viscous dissipation effect combined with oscillation.