Phase-averaged Velocity Profiles Research Articles

Influence of dispersed phase flow-rate pulsations on the liquid–liquid parallel flow in a T-junction microchannel

Active flow control in microfluidic devices to broaden the range of desired conditions is a significant practical concern. In this study, we experimentally investigate liquid–liquid parallel flow under the dispersed phase flow-rate pulsations in a T-junction microchannel. Sinusoidal flow rate disturbances with variations in amplitude and period were applied to dispersed phases of different viscosities. The study involves flow visualization and velocity field measurements using the micro-PIV technique for both free and excited flow conditions. The analysis of flow pattern maps, velocity fields, velocity gradients, and phase-averaged velocity profiles in a T-junction region and far downstream provided insights into the evolution of disturbances, which is different in low- and high-viscosity liquids. In the less viscous liquid, transverse waves amplify longitudinal ones due to relaxation of the liquid–liquid interface, while in the high-viscosity dispersed phase, longitudinal waves are damped significantly, with the transverse wave amplitude remaining almost constant. As a result, we revealed two distinct mechanisms of disturbance wave propagation and the loss of parallel flow stability, which are both dependent on the Ohnesorge number of the dispersed phase. Destabilization of the parallel flow led to the formation of plugs with a narrow length distribution. Single-mode, double-mode, triple-mode, and multi-mode plug formation regimes were identified. The results of the study can potentially be used to extend the range of segmented flow regimes and generate plugs of a desired length.

Experimental Investigation of Air Bubble Curtain Effects on Water Wave Field

This paper studies the effects of an air bubble curtain on surface water waves. Water particle velocities and free surface elevations were measured simultaneously at two cross-shore locations downstream of the air bubble curtain. Measurements were carried out for regular waves using different air bubble curtain configurations. Free surface elevations were measured using resistive gauges and the instantaneous velocities were acquired using an Acoustic Doppler Velocimeter (ADV). The characteristics of the free surface elevation time series, velocity field and turbulence are analyzed and discussed. The free surface elevation was found to be attenuated by the air bubble curtains. The phase averaged velocity profiles also depict the effect of the air bubbles in the flow field by generating milder longitudinal velocities (u) and by increasing the transverse component of the velocity (w). The increase in the turbulence intensity and the different energy spectrum produced by the air bubble curtain is also observed. The experimental results indicate that the thickness of the air bubble curtain and the total air flow rate affects the wave field.

Hybrid RANS/LES Turbulence Model Applied to a Transitional Unsteady Boundary Layer on Wind Turbine Airfoil

A hybrid Reynolds-averaged Navier Stokes/large-eddy simulation (RANS/LES) turbulence model integrated with a transition formulation is developed and tested on a surrogate model problem through a joint experimental and computational fluid dynamic approach. The model problem consists of a circular cylinder for generating coherent unsteadiness and a downstream airfoil in the cylinder wake. The cylinder flow is subcritical, with a Reynolds number of 64,000 based upon the cylinder diameter. The quantitative dynamics of vortex shedding and Reynolds stresses in the cylinder near wake are well captured, owing to the turbulence-resolving large eddy simulation mode that was activated in the wake. The hybrid model switched between RANS and LES modes outside the boundary layers, as expected. According to the experimental and simulation results, the airfoil encountered local flow angle variations up to ±50°. Further analysis through a phase-averaging technique found phase lags in the airfoil boundary layer along the chordwise locations, and both the phase-averaged and mean velocity profiles collapsed into the Law-of-the-wall in the range of 0 < y + < 50 . The features of high blade-loading fluctuations due to unsteadiness and transitional boundary layers are of interest in the aerodynamic studies of full-scale wind turbine blades, making the current model problem a comprehensive benchmark case for future model development and validation.

A hybrid LES-RANS study on square cylinder unsteady heat transfer

Flow passing a heated square cylinder is investigated using a hybrid LES-RANS approach on unstructured grids at a moderate Reynolds number of 22,050. The implicit sub-grid scale model is apply to LES, and two turbulence models, the Spalart-Allmaras model and the SST k-ω model, are employed for near-wall RANS. Combined with LES, both models present good predictions of the time- and phase-averaged velocity profiles on a 4-million-cell grid. Results of the LES-SST approach agree better with the experimental data especially at locations close to the cylinder surface and this leads to improved surface convective heat transfer compared to LES-SA. Grid convergence study shows that grid resolution in the near-wall region and on the cylinder surfaces is important in resolving the unsteady convective heat transfer. Results of velocity field and surface heat transfer from the fine grid with 8 million cells compare favourably with the experimental data and show significant improvement over that of the medium and coarse grids. Analysis of turbulent statistics is performed by means of energy spectra and anisotropy invariants of the Reynolds stress tensor. Proper orthogonal decomposition (POD) is used to identify the vortex shedding phases. It is shown that the POD based phase-averaging produces more accurate velocity profiles than the conventional pressure-signal based method.

Turbulent Boundary Layer Synthetic Jet Interaction: Slot Inclination Effects

The present paper deals with synthetic jet actuators. The synthetic jet actuator is a low power, highly compact fluidic system and efficient to control a boundary layer. In this study, the development of synthetic jet is first analysed and characterized then its interaction with boundary layer along a flat plate is studied. The designed actuator is used in a wind tunnel. Different inclinations of the jet are investigated for a fixed forcing frequency (60 Hz). Both hot wire anemometer and the Particle Image Velocimetry (PIV) are used. First, the synthetic jet normal to the wall without cross flow is characterised. The inclination effect is then examined. The same situations are studied in presence of cross flow. For the normal jet, the PIV results show a decrease of the mean wall velocity. The phase-averaged velocity profiles revealed that spanwise large-scale vortices are generated downstream of the slot and persist farther downstream. The inclination of the synthetic jet has an important effect on the expansion of the jet. It has been observed that the interaction with the wall modify strongly this expansion. The modification induced by the jet angle variation will be analysed in details in this paper. We will focus on the wall behaviour.

Studies of plug formation in microchannel liquid–liquid flows using advanced particle image velocimetry techniques

Two complementary micro Particle Image Velocimetry (μPIV) techniques have been developed in this work to study plug formation at a microchannel inlet during the flow of two immiscible liquids. Experiments were conducted for different fluid flow rate combinations in a T-junction, where all branches had internal diameters equal to 200μm. The dispersed phase was a water/glycerol solution and was injected from the side branch of the junction, while the continuous phase was silicon oil and was injected along the main channel axis. In the two-colour μPIV technique two laser wavelengths are used to illuminate two different tracer particles, one in each fluid, and phase averaged velocity profiles can be obtained in both phases simultaneously. In the high speed bright field μPIV technique, a backlight illuminates the test section, where the dispersed phase plug is seeded with tracer particles. This approach allows velocity profiles of the forming dispersed plugs to be followed in time.Non-dimensional plug lengths were found to vary linearly with the aqueous to organic phase flow rate ratio, in agreement with a well-known scaling correlation. The flowrate ratio also affected the velocity profiles within the forming plugs. In particular, for a ratio equal to one, a vortex appears at the tip of the plug in the early stages of plug formation. The interface curvature at the rear of the forming plug changes sign at the later stages of plug formation and accelerates the thinning of the meniscus leading to plug breakage. The spatially resolved velocity fields obtained in both phases with the two-colour PIV show that the continuous phase resists the flow of the dispersed phase into the main channel at the rear of the plug meniscus and causes the change in the interface curvature. This change of interface curvature was accompanied by an increase in vorticity inside the dispersed phase during plug formation.

J054033 壁面近傍に設置された三角柱周りの乱流解析

Numerical analysis has been performed for turbulent flow with heat transfer in a rectangular duct with triangular cylinder placed near a wall by using an algebraic Reynolds stress model and the turbulent Prandtl number assumed to be constant in temperature field. Calculated results are compared with the experiment in order to confirm the validity of the presented model. It has been found that the calculation can reproduce reasonably the phase-averaged velocity profiles including vorticity. And calculation also predicts that the vortex affects the flow near the top and bottom walls. Adding to this, although the presented method underestimates Nusselt number in the region behind the triangular cylinder, two characteristic peaks of Nusselt number profile are predicted as well as the experiment. The compared results with the experimental and numerical data suggest that an algebraic Reynolds stress model has the potentiality to reproduce the turbulent flow with vortex.

Separation control on high lift low-pressure turbine airfoils using pulsed vortex generator jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under low (0.6%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25000 and 50000. Jet pulsing frequency and duty cycle were varied. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was achieved. At sufficiently high pulsing frequencies, separation control was possible with low jet velocities and 10% duty cycle. At lower frequencies, a 50% duty cycle helped by separating the disturbances associated with the jets turning on and turning off, thereby doubling the frequency of separation control events above the pulsing frequency. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can re-separate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

Separation Control on a Very High Lift Low Pressure Turbine Airfoil Using Pulsed Vortex Generator Jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Jet pulsing frequency, duty cycle, and blowing ratio were all varied. Computational results from a large eddy simulation of one case showed reattachment in agreement with the experiment. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was possible even at the lowest Reynolds number. Pulsed VGJs were more effective than steady jets. At sufficiently high pulsing frequencies, separation control was possible even with low jet velocities and low duty cycles. At lower frequencies, higher jet velocity was required, particularly at low Reynolds numbers. Effective separation control resulted in an increase in lift and a reduction in total pressure losses. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can reseparate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

Uncertainty Quantification Integrated to CFD Modeling of Synthetic Jet Actuators

The Point Collocation Non-Intrusive Polynomial Chaos (NIPC) method has been applied to two stochastic synthetic jet actuator problems used as test cases in the CFDVAL2004 workshop to demonstrate the integration of computationally efficient uncertainty quantification to the high-fidelity CFD modeling of synthetic jet actuators. In Case1 where the synthetic jet is issued into quiescent air, the NIPC method is used to quantify the uncertainty in the long-time averaged u and v-velocities at several locations in the flow field, due to the uniformly distributed uncertainty introduced in the amplitude and frequency of the oscillation of the piezo-electric membrane. Fifth order NIPC expansions were used to obtain the uncertainty information, which showed that the variation in the v-velocity is high in the region directly above the jet slot and the variation in the u-velocity is maximum in the region immediately adjacent to the slot. Even with a ten percent variation in the amplitude and frequency, the long-time averaged u and v velocity profiles could not match the experimental measurements at y=0.1mm above the slot indicating that the discrepancy may be due to other uncertainty sources in CFD or measurement errors. In Case 2 which includes a cross flow, the free stream velocity is treated as an uncertain input variable. Fifth degree NIPC expansions were employed to quantify the uncertainty in phase averaged velocity profiles as well as long-time averaged wall pressure and skin friction coefficient distributions. The results of Case 2 show that the uncertainty in phase averaged velocity profiles gets larger when approaching the main stream. The size of a separation bubble observed in this case remains relatively insensitive to the uncertain free stream velocity within the tolerance range considered.

Application of a Novel Moving-Grid Methodology to Model the Interaction of a Synthetic Jet with a Turbulent Boundary Layer

ABSTRACT This article presents a computational model that employs a novel moving-grid methodology to investigate the interaction of an isolated synthetic jet in a crossflow. This moving-grid methodology can be said to be novel because this scheme conserves space automatically. In the current study, numerical simulation was performed to investigate the transient behavior of a single, two-dimensional synthetic jet that interacts with a turbulent boundary layer. Unsteady, Reynolds-averaged Navier-Stokes equations were solved numerically by a finite-volume method. Results on several phase-averaged velocity profiles agree with the trend of data obtained from experiments set up by NASA Langley Research Center.

Experimental investigation of oscillatory air flow in a bronchial tube model with HFOV mode

Mechanical ventilations for artificial respiration have been developed to improve the medical treatment of the patients showing respiratory disorder. In various types of ventilation, High Frequency Oscillatory Ventilation (HFOV) is one of the most effective techniques of medical care for pulmonary disease patients, especially, infants or premature infants. HFOV is a ventilation technique with high breathing rate in comparison with the normal breathing rate. Some successful studies have focused on the effect of treatment using HFOV. However, the mechanism of gas exchange in bronchial tube under the medical treatment by HFOV has not been clarified. In this study, the oscillatory flow in a micro-channel model of bronchial with single bifurcation in HFOV mode has been investigated experimentally with micro Particle Image Velocimetry (micro PIV) technique. The phase averaged velocity profiles changing with the driving frequency of HFOV have been investigated.

Unsteady Simulation of Synthetic Jet in a Crossflow

The purpose is to address some issues concerning the computation of the interaction of a synthetic jet with a turbulent boundary layer. The main question arising is, What accuracy can be expected according to the grid resolution and the turbulence model? With a view of comparison, large-eddy simulation (LES) and unsteady Reynolds-averaged Navier-Stokes (URANS) simulations on three grids are presented. The computational configuration corresponds to the case number 2 of the CFD Validation Workshop of Synthetic Jets (CFDVAL2004) held in March 2004 at the NASA Langley Research Center. The incoming flow is a canonical turbulent boundary layer with a momentum-thickness-based Reynolds number Re θ equal to 4275. The synthetic jet has a Strouhal number based on the jet diameter and the jet velocity of 0.019. The ratio of the jet maximum velocity to the crossflow one is 1.45. The ratio of the jet diameter to the boundary-layer thickness is one-third. The whole cavity is computed, and an alternating blowing/suction condition is imposed on its lower surface to model the diaphragm displacement. The effects of mesh size, time step, turbulence modeling, and boundary conditions have been carefully documented. The variation of these numerical parameters has only a moderate influence on the computation/experiment agreement, which remains very satisfactory in all cases. For example, even the URANS computation on the coarsest grid with only 220,000 cells is able to reproduce the synthetic jet dynamics and provides good results for the mean and phase-averaged velocity profiles. Nevertheless, in this case turbulence stresses are not computed with sufficient accuracy, and a LES with realistic inflow boundary conditions on a fine mesh is necessary to recover the right stresses behavior.

Reduced frequency effects on the near-wake of an oscillating elliptic airfoil

An experimental study was carried out to investigate the reduced frequency effect on the near-wake of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched around the center of the chord between -5° and +25° angles of attack at an airspeed of 3.4 m/s. The chord Reynolds number and reduced frequencies were 3.3 X104, and 0.1, 0.7, respectively. Phase-averaged axial velocity and turbulent intensity profiles are presented to show the reduced frequency effects on the near-wake behind the airfoil oscillating in pitch. Axial velocity defects in the near-wake region have a tendency to increase in response to a reduced frequency during pitch up motion, whereas it tends to decrease during pitch down motion at a positive angle of attack. Turbulent intensity at positive angles of attack during the pitch up motion decreased in response to a reduced frequency, whereas turbulent intensity during the pitch down motion varies considerably with downstream stations. Although the true instantaneous angle of attack compensated for a phase-lag is large, the wake thickness of an oscillating airfoil is not always large because of laminar or turbulent separation.

Measurement of local forcing on a turbulent boundary layer using PIV

An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. Local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f +=0.44, 0.66 and 0.88) with a fixed forcing amplitude (A +=0.6) were employed at Re θ =690. The effect of three different forcing angles (α=60°, 90° and l20°) was investigated under a fixed forcing frequency (f +=0.088). The PIV results showed that the wall-region velocity decreases on imposition of the local forcing. Inspection of the phase-averaged velocity profiles revealed that spanwise large-scale vortices are generated downstream of the slot and persist farther downstream. The highest reduction in skin friction was achieved at the highest forcing frequency (f +=0.088) and a forcing angle of α=120°. The spatial fraction of the vortices was examined to analyze the skin friction reduction.

A121 アスペクト比の異なる矩形管内の脈動流のPIV計測

To clarify the effect of the aspect ratio on the pulsating flow through the rectangular channel the pulsating flow with a reciprocated 50Hz fluctuation has been investigated experimentally. The experiment was carried out with two different aspect ratios (α??1, 8.33) and the technique of PIV with the recursive direct cross-correlation was used to accomplish velocity field measurements. The PIV measurement was synchronized with the AC power supply to the pulse pump and the phase averaged velocity profiles were measured at 10°phase angle intervals. The experimental results revealed that the ensemble averaged axial velocity profile changes from the parabolic profile to the flat one according to increase of the aspect ratio from 1 to 8.33. It was found that the phase averaged axial velocity profile changes with the phase angle and the inflection points due to the friction of the channel wall appear near the wall in those profiles.

The flapping shear layer formed by flow separation from the forward corner of a square cylinder

The turbulent shear layer and the associated recirculation region on the sidewall formed in flow separation from the forward corner of a square cylinder have been studied with one-component laser-Doppler velocimetry. Because of vortex shedding, the flow is approximately periodic, and is treated as a separated flow undergoing largeamplitude forcing at the shedding frequency. Phase (ensemble)-averaged velocities and turbulence intensities were obtained, and a close relationship in phase and amplitude between phase-averaged turbulence intensities and gradients of phase-averaged velocity is found in much of the flow region. The similarity behaviour of the phase-averaged profiles in the shear layer as well as the streamwise growth of the shear layer are investigated. While phase-averaged velocity profiles collapse well in similarity coordinates, normalized turbulence intensities exhibit systematic deviations from similarity. Shear-layer growth also departs markedly from the linear growth law of unforced plane mixing layers. The effect of the recirculation is suggested as a possible explanation for some of these deviations. Similarities to and differences from steady and forced mixing layers, steady separated flows with recirculation, and unsteady boundary layers are discussed.

Effects of excitation level on the stability of an axisymmetric mixing layer

The effect of various levels of excitation on the stability and development of an axisymmetric mixing layer was studied experimentally. The flow was excited axisymmetrically by a single speaker placed at the base of the plenum chamber. Measurements of mean and phase-averaged velocity profiles were made using an array of hot-wire probes. The measured profiles were compared to eigenfunctions calculated from linear, viscous stability theory. It is shown that the theoretical predictions, based on measured profiles of mean velocity, compare very well with the phase-averaged measurements, even when the local disturbance reaches levels as high as 24% of the jet speed. The cumulative effect of excitation on the mean flow is examined as a function of local Strouhal number as well as excitation level.