Periodic Flow Patterns Research Articles

Dependence of the aeroelastic stability of a slender U‐beam on the realized flow pattern

AbstractWe investigate the aeroelastic behaviour of a slender U‐beam in cross‐flow. Two distinct, time periodic flow patterns are observed in simulations of the flow around this beam. Its aeroelastic properties depend on the realized flow pattern, especially so for low reduced velocities: One flow pattern leads to pronounced torsional vortex‐induced vibrations. No such vibrations could be observed under the other flow pattern. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A comprehensible low-order model for wall turbulence dynamics

Streamwise vortices play an important role in the sustainment of wall turbulence. They are associated with regions with strong Reynolds shear stress production. In turbulent plane Couette flow at low Reynolds numbers, these streamwise vortices fill the whole gap between the plates. Using a low-order model obtained from the Navier-Stokes equations through a two-step Galerkin projection, the dynamics of these streamwise vortices is shown to be similar to the dynamics observed in the near-wall region of turbulent boundary layers [M. Lagha, J. Kim, J. D. Eldredge, and X. Zhong, “Near-wall dynamics of compressible boundary layer,” Phys. Fluids 23, 065109 (2011)]. A spanwise vortex filling the whole gap between the plates and with vorticity opposite in sign to that of the base flow, is tilted in the streamwise direction by the spanwise shear of the streaks. The resultant vortex has a crescent shape and its legs are two streamwise vortices. They regenerate the streaks by the lift-up effect. Through its ability to generate periodic spatio-temporal flow patterns, this model is shown to provide an ideal tool for studying the different mechanisms at work.

An accurate quantification of the flow structure along the acoustic signal cycle in a forced two-phase jet

This paper provides an experimental study of an acoustically forced two-phase air jet generated by a convergent nozzle. The used particles are transparent glass spheres with diameters between 2 and 50 μm (which gives Stokes number of order 1) and the selected forcing frequency (f =400 Hz) induces a powerful nearly periodic flow pattern. Measurements were done by a two-colour Phase-Doppler Anemometer. The experimental setup is computer-controlled to provide an accurate control with a high long-term stability. Measurements cover the whole forcing signal cycle. Raw measurements were carefully post-processed to avoid bias induced by the forcing and the instrument setup, as well as obtain right mean values of the dispersed flow. The effect of the forcing and the particle load allows authors to establish the effect of the acoustic forcing and the particle load on the jet.

Global two-dimensional stability of the staggered cavity flow with an HOC approach

In this paper, we have carried out a global stability analysis of the flow inside a two sided staggered lid-driven cavity for both the parallel and anti-parallel motions of the lids. A fourth order spatially accurate and second order temporally accurate compact scheme is used to discretize the two-dimensional Navier–Stokes equations governing the flow as well as the perturbed equations leading to the generalized eigenvalue problem. Estimates of the critical parameters are found for several grids and extensive grid convergence study is performed to accurately identify them. Computation is performed for a wide range of Reynolds numbers (Re) on either side of the critical values in the range 50≤Re≤12,000. For flows below the critical Reynolds number Rec, our numerical results are compared with established steady-state results and an excellent agreement is obtained in all the cases. For Reynolds numbers above Rec, phase plane and spectral density analysis confirmed the existence of periodic and chaotic flow patterns.

Experimental and numerical studies on periodic convection flow and heat transfer in a lid-driven arc-shape cavity

This paper presents experimental and numerical studies on periodic convection flow and heat transfer in a lid-driven arc-shape cavity with temperature differential. Three cases were considered: Gr=2×105, 5×105 and 1.2×106 at Re=100 (Gr = Grashof number; Re = Reynolds number). The mathematical model was proposed in our previous study. The current study performs an experiment to validate this model, to corroborate the existence of the periodic flow, and to more deeply probe the internal flow and temperature characteristics. The experimental setup primarily comprised an arc-shape cavity, a moving lid, a thermo-system, a smoke generator and an image acquisition system. The periodic convection flow in the cavity was visualized using kerosene smoke. The numerical and experimental results consistently reveal that the periodic flow pattern was observed in the case with Gr=5×105, whereas the steady-state flow pattern took place in the other two cases (Gr=2×105 and Gr=1.2×106). The numerical simulation produced reasonable and satisfactory agreement with the experiment for the periodic flow pattern and period. The difference between the predicted and measured periods is less than 5%. The transport properties, such as average kinetic energy, overall Nusselt number, stream function, phase space trajectory, local kinetic energy, velocity history and temperature distribution, were further analyzed and discussed in this paper. The proposed numerical simulation not only confirms the experimental observation, but also enhances the understanding of periodic convection in an arc-shape cavity subjected to a moving lid and temperature differential.

Mixing Enhancement in Gas-Stirred Melts by Rotating Magnetic Fields

A model experiment of a submerged gas injection system in a cylindrical vessel under the influence of a rotating magnetic field and its effect on liquid metal mixing is presented. Argon gas is injected through a nozzle into a column of the eutectic alloy GaInSn, which is liquid at room temperature. Without a magnetic field, the bubble plume in the center region of the cylindrical vessel produces a recirculation zone with high fluid velocities near the free surface, while the fluid velocities in the bottom region are rather low. Our measurements revealed the potential of rotating magnetic fields to control both the amplitude of the meridional flow and the bubble distribution and to provide an effective mixing in the whole fluid volume. Various periodic flow patterns were observed in a certain parameter range with respect to variations of the magnetic field strength and the gas flow rate.

3D simulation of laminar flow and heat transfer in V-baffled square channel

The article presents a numerical investigation on laminar flow and heat transfer characteristics in a three-dimensional isothermal wall square-channel fitted with inline 45° V-shaped baffles on two opposite walls. The computations based on the finite volume method with the SIMPLE algorithm have been conducted for the airflow in terms of Reynolds numbers ranging from 200 to 2000. The inline V-baffles with its V-tip pointing downstream and the attack angle (or half V-apex angle) of 45° relative to the flow direction are mounted repeatedly on the lower and upper walls. The baffled channel flow shows a fully developed periodic flow and heat transfer profile for BR = 0.2 at x/ D≈ 8 downstream of the inlet. Influences of different baffle height ratios ( BR) and pitch ratios, ( PR) on thermal behaviors for a fully developed periodic condition are investigated. It is apparent that the longitudinal counter-rotating vortex flows created by the V-baffle can induce impingement/attachment flows over the walls resulting in greater increase in heat transfer over the test channel. Apart from speeding up the fully developed periodic flow pattern, the rise of the BR leads to the increase in Nu/Nu 0 and f/f 0 values while that of the PR provides an opposite trend. The V-baffle performs better than the angled baffle at a similar condition. The V-baffle with BR = 0.2 and PR = 1.5 yields the maximum thermal performance of about 3.8 whereas the Nu/Nu 0 is some 14 times above the smooth channel at higher Re.

2차원 원심 압축기의 깃 없는 디퓨저에서의 불안정 유동

This study investigated on details of flow characteristics in a vaneless diffuser of a compressor with 2-dimensional impeller at various flow rates. Experiment for a low speed compressor model in a water reservoir was performed to analyze the flow field in the vaneless diffuser and volute casing, which was done by PIV measurement. It was also focused on the periodic flow patterns occurring at low flow rate near unstable operating region of the compressor. At low flow rate condition, the flow visualization clearly shows that the flow energy from impeller is highly accumulated at the compressor exit by the blockage effect of a flow damper and consequently the reverse flow occurs in the diffuser.

MicroPrep: Chip‐based dielectrophoretic purification of mitochondria

We have developed a microfluidic system--microPrep--for subcellular fractionation of cell homogenates based on dielectrophoretic sorting. Separation of mitochondria isolated from a human lymphoblastoid cell line was monitored by fluorescence microscopy and further characterized by western blot analysis. Robust high throughput and continuous long-term operation for up to 60 h of the microPrep chip system with complex biological samples became feasible as a result of a comprehensive set of technical measures: (i) coating of the inner surfaces of the chip with BSA, (ii) application of mechanical actuators to induce periodic flow patterns, (iii) efficient cooling of the device to ensure integrity of organelle, (iv) a wide channel to provide for high fluidic throughput, and (v) integration of a serial arrangement of 10 dielectrophoretic deflector units to enable separation of samples with a high particle load without clogging. Hence, microPrep yields tens of micrograms of enriched and purified mitochondria within hours. Western blots of mitochondria fractions showed that contaminating endoplasmatic reticulum was reduced by a factor 6 when compared with samples prepared by state of the art centrifugation.

Natural convective flow and heat transfer of supercritical CO 2 in a rectangular circulation loop

Fluid dynamics and heat transfer of supercritical CO 2 natural convection are important for nuclear engineering and new energy system design etc. In this paper, in order to study the flow and heat transfer behavior of supercritical CO 2 natural circulation system, a computational simulation on a closed natural circulation loop (NCL) model has been carried out. The fluid temperature in the loop varies between 298.15 K and 323.15 K, which is across the CO 2 critical temperature, and the density is found to be in the range of 250–800 kg/m 3. The results show a small temperature difference of 25 °C between heating and cooling sources can induce a mass flow with the Reynolds number up to 6 × 10 4 using supercritical CO 2 fluid. A periodic reversal flow pattern is found and presented in this paper. Enhanced heat transfer phenomenon is also found for the supercritical CO 2 natural convective flow. The mechanisms to this enhancement and the heating effect on the flow are also discussed in detail in the present study.

Bifurcation analysis in a vortex flow generated by an oscillatory magnetic obstacle

A numerical simulation and a theoretical model of the two-dimensional flow produced by the harmonic oscillation of a localized magnetic field (magnetic obstacle) in a quiescent viscous, electrically conducting fluid are presented. Nonuniform Lorentz forces produced by induced currents interacting with the oscillating magnetic field create periodic laminar flow patterns that can be characterized by three parameters: the oscillation Reynolds number, Reomega, the Hartmann number, Ha, and the dimensionless amplitude of the magnetic obstacle oscillation, D. The analysis is restricted to oscillations of small amplitude and Ha=100. The resulting flow patterns are described and interpreted in terms of position and evolution of the critical points of the instantaneous streamlines. It is found that in most of the cycle, the flow is dominated by a pair of counter rotating vortices that switch their direction of rotation twice per cycle. The transformation of the flow field present in the first part of the cycle into the pattern displayed in the second half occurs via the generation of hyperbolic and elliptic critical points. The numerical solution of the flow indicates that for low frequencies (v.e. Reomega=1), two elliptic and two hyperbolic points are generated, while for high frequencies (v.e. Reomega=100), a more complex topology involving four elliptic and two hyperbolic points appear. The bifurcation map for critical points of the instantaneous streamline is obtained numerically and a theoretical model based on a local analysis that predicts most of the qualitative properties calculated numerically is proposed.

Pressure drop across micro-pin heat sinks under unstable boiling conditions

Flow boiling was investigated under unstable boiling conditions in three different micro-pin fin heat sinks using water and R-123 as working fluids. Once boiling was initiated severe temperature fluctuations were recorded for all the tested (three) micro-pin fin heat sinks. Flow images and fast-Fourier transform (FFT) of pressure signals during flow boiling were used to explain experimental results. The boiling instability mechanisms behind unstable boiling were discussed for both water and R-123. Accordingly, no significant pressure fluctuations with respect to time averaged pressure drop were evident for the tested micro-pin fin heat sinks before and after flow boiling instability initiates. However, a step change in the pressure signals were recorded with the inception of unstable boiling, and a sharp increase in the magnitude peaks of the FFT profiles was observed in the device operated with R-123, while there was no significant change in the FFT profiles in the devices operated with water. According to complementary flow visualization studies, the oscillation frequency of the periodic flow patterns for the device operated with R-123 was higher ( f∼80 Hz) than that of the devices operated with water ( f∼20 Hz).

Numerical study of the effects of lid oscillation on the periodic flow pattern and convection heat transfer in a triangular cavity

This study is concerned with a periodic flow pattern with mixed convection in a triangular cavity caused by the effects of lid oscillation and buoyancy. The dimensionless stream function–vorticity formulation is adopted, and a curvilinear grid method for solving the stream function–vorticity equations in irregular geometries is used. Attention is in particular focused on the flow behaviour under the interaction between the frequency of the oscillation of the lid velocity and the frequency of the natural periodic flow. Meanwhile, numerical predictions of the thermal characteristics represented by local and average Nusselt numbers on the walls as well as the transient flow patterns are also provided. Results show that the frequency of oscillation of lid velocity and the natural periodic flow frequency both appear to be major frequencies in the frequency spectrum of the cavity flow, for cases with a dimensionless lid oscillation frequency less than 0.5. When the dimensionless frequency of the oscillation of the lid velocity is equal to or higher than 0.5, the flow and thermal fields are completely locked-on to the lid oscillation, and the natural periodic flow frequency is no longer visible in the spectrum.

A Behavior of the Diffuser Rotating Stall in a Low Specific Speed Mixed-Flow Pump

The flow instability in a low specific speed mixed-flow pump, having a positive slope of head-flow characteristics was investigated. Based on the static pressure measurements, it was found that a rotating stall in the vaned diffuser occurs at about 65% flow rate of best efficiency point (BEP). A dynamic Particle Image Velocimetry (DPIV) measurement and the numerical simulations were conducted in order to investigate the flow fields. As a result, the diffuser rotating stall was simulated even by Computational Fluid Dynamics (CFD) and the calculated periodic flow patterns agree well with the measured ones by DPIV. It is clarified that a periodical large scaled backflow, generated at the leading edge of the suction surface of the diffuser vane, causes the instability. Furthermore, the growth of the strong vortex at the leading edge of the diffuser vane induces the strong backflow from the diffuser outlet to the inlet. The scale of one stall cell is covered over four-passages in total thirteen vane-passages.

Numerical simulation of periodic mixed convective heat transfer in a rectangular cavity with a vibrating lid

Periodic behavior of the mixed convective flow in a rectangular cavity with a vibrating lid is investigated numerically in this study. The periodic flow patterns and heat transfer characteristics found are discussed with attention being focused on the interaction between the frequency of the lid velocity vibration and the frequency of the natural periodic flow. Several practical cases are investigated to evaluate the effects of the frequency of vibration of lid velocity ( W) at Re = 100 and Gr = 5 × 10 5. The frequency of the natural periodic flow can be most clearly seen when the lid is moved at a constant-velocity (without vibration). As the vibration is imposed on the lid with the dimensionless frequency of the lid velocity varied between 0.1 and 0.3, the combined effects of the natural frequency and the lid vibration frequency on the transient variations in the flow and the thermal characteristics have been observed. When the dimensionless frequency of lid velocity vibration is increased to be equal to or higher than 0.4, the natural periodic flow frequency disappears, and the lid vibration frequency dominates the flow field, which means that the periodic flow field has locked-on to the lid vibration.

J0203-2-6 バイパス型ネーザルCPAP素子の開発(医療診断のためのマルチスケール血流動態計測と解析(2))

As an artificial ventilation to assist breathing of a premature baby who is able to breathe spontaneously, the nasal-CPAP has been used in order to assist the baby in its clinic. For the improvement of the treatment, the authors developed a bypass type nasal-CPAP device to attain the smaller load for the premature baby rather than the existing nasal-CPAP device. In the present report, we analyzed a fundamental flow mechanism in respiratory phase for the proposed device experimentally, and its periodic flow pattern and the relation between the flow pattern and pressure variation was examined with numerical simulation.

Transient flow patterns in a microfluidic chip with a complicated microstructure

AbstractThe transient flow patterns of the boiling flow in a microfluidic chip with a complicated microstructure were studied at low mass fluxes and high heat fluxes. The periodic flow pattern in the timescale of milliseconds and the stratified flow pattern were observed. For a specific separated zone, the liquid film thickness was increased along the flow direction and the dry‐out always occurred earlier at the microchannel upstream rather than downstream. However, for different microchannel zones, the dry‐out took place earlier in the downstream zone. It was determined that the low liquid Froude number was responsible for the formation of the stratified flow. The large boiling number resulted in a large shear stress at the vapor–liquid interface, leading to the accumulation of the liquid in the microchannel downstream, causing the increased liquid film thickness along the flow direction. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(4): 224–231, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20201

Uniform Mems Chip Temperatures in the Nucleate Boiling Heat Transfer Region by Selecting Suitable, Medium Boiling Number Range

The not only lower but also uniform MEMS chip temperatures can be reached by selecting suitable boiling number range that ensures the nucleate boiling heat transfer. In this article, boiling heat transfer experiments in 10 silicon triangular microchannels with the hydraulic diameter of 155.4 μm were performed using acetone as the working fluid, having the inlet liquid temperatures of 24–40°C, mass fluxes of 96–360 kg/m2s, heat fluxes of 140–420 kW/m2 , and exit vapor mass qualities of 0.28–0.70. The above data range correspond to the boiling number from 1.574 × 10−3 to 3.219 × 10−3 and ensure the perfect nucleate boiling heat transfer region, providing a very uniform chip temperature distribution in both streamline and transverse directions. The boiling heat transfer coefficients determined by the infrared radiator image system were found to be dependent on the heat fluxes only, not dependent on the mass fluxes and the vapor mass qualities covering the above data range. The high-speed flow visualization shows that the periodic flow patterns take place inside the microchannel in the time scale of milliseconds, consisting of liquid refilling stage, bubble nucleation, growth and coalescence stage, and transient liquid film evaporation stage in a full cycle. The paired or triplet bubble nucleation sites can occur in the microchannel corners anywhere along the flow direction, accounting for the nucleate boiling heat transfer mode. The periodic boiling process is similar to a series of bubble nucleation, growth, and departure followed by the liquid refilling in a single cavity for the pool boiling situation. The chip temperature difference across the whole two-phase area is found to be small in a couple of degrees, providing a better thermal management scheme for the high heat flux electronic components. Chen's [1] widely accepted correlation for macrochannels and Bao et al.'s [2] correlation obtained in a copper capillary tube with the inside diameter of 1.95 mm using R11 and HCFC123 as working fluids can predict the present experimental data with accepted accuracy. Other correlations fail to predict the correct heat transfer coefficient trends. New heat transfer correlations are also recommended.

“Vortex formation mechanisms in the wake behind a sphere for 200 < Re < 380”

Direct numerical simulation and visualization of three-dimensional separated flows of a homogeneous incompressible viscous fluid are used to comprehensively describe different mechanisms of vortex formation behind a sphere at moderate Reynolds numbers (200 ≤ Re ≤ 380). For 200 270 it is a chain of vortex loops. The three unsteady periodic flow patterns corresponding to the 270 < Re ≤ 290, 290 < Re ≤ 320, and 320 < Re ≤ 380 ranges are characterized by different vortex formation mechanisms. Direct numerical simulation is based on the Meranzh (SMIF) method of splitting in physical factors with an explicit hybrid finite-difference scheme which possesses the following properties: secondorder approximation in the spatial variables, minimal scheme viscosity and dispersion, and monotonicity. Two different vortex identification techniques are used for visualizing the vortex structures within the wake.

Periodic flow patterns of the magnetic fluid in microchannel

In this study, of interests are the periodic flow patterns of the oil-based magnetic fluid in microchannels. A microfluidic chip is made of poly-dimethylsiloxane (PDMS) and contains cross-shape microchannels. The microchannels are 1000 μm in width and 200 μm in depth. A syringe pump was used to drive the fluids. Periodic flow patterns were seen and the slugs of magnetic fluid and DI water were generated. The operating factors discussed in the present work are the flow rates and the magnetic field. The frequency of generation of the slugs increases with increase in the flow rates. Besides, by settling the permanent magnet around the microchannel, the periods of the slug generation are changed. Different positions of the magnet lead to different periods for generating the slugs. By adjusting operating conditions, to control the frequency and the volume of the slugs is practical.