Quiescent Flow Conditions Research Articles

Synthetic Jet Actuator Techniques in Quiescent Flow Condition: A Review

Synthetic Jet Actuator Techniques in Quiescent Flow Condition: A Review

Enhanced Solutal Marangoni Flow Using Ultrasound-Induced Heating for Rapid Digital Microfluidic Mixing

Digital microfluidics based on sessile droplets has emerged as a promising technology for various applications including biochemical assays, clinical diagnostics, and drug screening. Digital microfluidic platforms provide an isolated microenvironment to prevent cross-contamination and require reduced sample volume. Despite these advantages, the droplet-based technology has the inherent limitation of the quiescent flow conditions at low Reynolds number, which causes mixing samples confined within the droplets to be challenging. Recently, solutal Marangoni flows induced by volatile liquids have been utilized for sessile droplet mixing to address the above-mentioned limitation. The volatile liquid vaporized near a sessile droplet induces a surface tension gradient throughout the droplet interface, leading to vortical flows inside a droplet. This Marangoni flow-based droplet mixing method does not require an external energy source and is easy to operate. However, this passive method requires a comparably long time of a few tens of seconds for complete mixing since it depends on the natural evaporation of the volatile liquid. Here, we propose an improved ultrasound-induced heating method based on a nature-inspired ultrasound-absorbing layer and apply it to enhance solutal Marangoni effect. The heater consists of an interdigital transducer deposited on a piezoelectric substrate and a silver nanowire-polydimethylsiloxane composite as an ultrasound-absorbing layer. When the transducer is electrically actuated, surface acoustic waves are produced and immediately absorbed in the composite layer by viscoelastic wave attenuation. The conversion from acoustic to thermal energy occurs, leading to rapid heating. The heating-mediated enhanced vaporization of a volatile liquid accelerates the solutal Marangoni flows and thus enables mixing high-viscosity droplets, which is unachievable by the passive solutal Marangoni effect. We theoretically and experimentally investigated the enhanced Marangoni flow and confirmed that rapid droplet mixing can be achieved within a few seconds. The proposed heater-embedded sessile droplet mixing platform can be fabricated in small size and easily integrated with other digital microfluidic platforms. Therefore, we expect that the proposed sample mixing method can be utilized for various applications in digital microfluidics and contribute to the advancements in the medical and biochemical fields.

Optical Experiments on Auto-Ignition Modes in a Turbulent Field under Engine-Relevant Conditions

ABSTRACT Thermal efficiency improvement of downsized spark-ignition engines is limited by the knocking issues, which are closely associated with auto-ignition modes. Previous studies are mainly conducted under quiescent or standard flow conditions, but the role of turbulence-chemistry interactions is not fully considered, motivating further fundamental investigations on auto-ignition modes. In this study, an optical experiment using a rapid compression machine that allows for turbulence effect was carried out, and the difference of auto-ignition modes between standard flow and turbulence conditions was comparatively investigated under engine-relevant conditions. The results show that under given target thermodynamic conditions, the overall temperature decrease caused by turbulence and cold-wall interactions reduces the reactivity of end-gas mixtures, inhibiting the transition of auto-ignition modes into developing detonation. Compared with standard flow scenarios, the propagation speed of auto-ignition reaction waves is much lower under turbulence conditions. Meanwhile, multiple secondary auto-ignition kernels occur sequentially in the unburned region, and normal combustion characteristics are observed under low thermodynamic conditions. However, further increases in thermodynamic conditions result in an intensive secondary auto-ignition, manifesting deflagration-to-detonation transition and thereby strong pressure oscillations. Besides, the correlations between knocking intensity, unburned mass fraction, and energy density indicate that the thermo-chemical-turbulent mechanism dominates the auto-ignition processes. The current studies shall give insights into the auto-ignition and turbulent combustion as well as knocking suppression under engine-relevant conditions.

Aerodynamic Performance Improvement of a Wing Model Using an Array of Slotted Synthetic Jets

Abstract This work deals with the improvement in aerodynamic performance of a NACA0025 wing model using an array of slotted synthetic jets (SJs). A novel SJ actuator was designed and located at 30% of the chord from the leading edge (LE). Time-resolved particle image velocimetry (TR-PIV), force balance, static pressure distribution, and hotwire measurements were carried out in a subsonic wind tunnel to assess the performance enhancement due to the slotted SJ array. Initially, the SJ velocity was measured in quiescent flow condition at different actuation frequencies and amplifier voltages. Actuation at 1000 Hz and 200 V resulted in the highest blowing velocity of 10.5 m/s. Experiments were performed at various actuation frequencies, namely, 200, 600, and 1000 Hz. It was observed that actuation at 1000 Hz led to the highest increase in lift coefficient by 35.6% and reduction in average drag coefficient by 33%. TR-PIV measurements showed flow separation with flow reversal in the baseline case. After switching on the SJ array at 1000 Hz, the flow separation was completely eliminated. The momentum transfer from the highenergy primary flow to the retarding boundarylayer flow and actuation of SJ in a particular frequency range was observed to be the mechanisms for the flow separation control. Subsequently, fast Fourier transform (FFT) power spectra of hotwire data were computed from 40% to 80% of the chord. The FFT power spectra showed the successful stabilization of the flow field at the actuation of 1000 Hz.

Parametric investigations of different variables on liquid–solid fluidization in a HydroFloat cell using computational fluid dynamics

In flotation, the fluidization can enhance the coarse particle recovery which is dependent on the quiescent flow condition and is also affected by the liquid–solid interphase drag force. In this study, the liquid and particle flow behaviours are investigated by using the Eulerian-Eulerian two-fluid model combining the kinetic theory of granular flow (KTGF) coupled with the RNG k-ε turbulence model in 3D computational geometry. Syamlal & O’Brien, Gidaspow, Gidaspow+Ganser, and Gidaspow+Haider & Levenspiel drag models are examined. The comparison of predicted results and experimentally obtained data using a HydroFloat (HF) cell showed that the drag models for the irregular particle (Gidaspow+Ganser and Gidaspow+Haider & Levenspiel) have a maximum relative error of 3.05% which is much better than the drag models for the sphere particle (Syamlal & O’Brien and Gidaspow). Also, the effect of restitution coefficient for particle-particle collisions, specularity coefficient for wall-particle collisions and initial solid holdup on the overall bed voidage, axial RMS (root mean square) velocity, granular temperature, and pressure of the solid particle are investigated, while the Gidaspow+Haider & Levenspiel model is used as the drag model. The voidage increases with increasing restitution and specularity coefficients. The particle axial RMS velocity is less significant up to 6.3% of the total HF height and the location of strong circulation is identified. The granular temperature is insignificant for the case of nearly inelastic collision (0.0001) or nearly no-slip condition (0.0001), but superficial liquid velocity affects significantly the granular temperature for both cases. The effect of restitution coefficient and superficial liquid velocity on solid pressure is very significant for the case of the nearly no-slip condition.

Simulation of Liquid and Gas Phase Characteristics of Aerated-Liquid Jets in Quiescent and Cross Flow Conditions

The simulation of the liquid- and gas-phase properties of aerated-liquid jets in various quiescent and cross flow conditions are presented in the study. For simplicity, water is used as the liquid for all test conditions. The effect of various air-to-liquid ratios under super-sonic cross flow conditions are simulated and compared to experimental conditions, which is taken in the supersonic wind tunnel with a dimension of 762 × 152 × 127 mm. An injector with an orifice diameter of 0.5 mm is used both in a non-aerated and aerated injection into a supersonic cross flow prescribed by the momentum flux ratio of the liquid jet to free stream air, q0. The initial conditions of the spray calculation were estimated from internal flow simulation using VOF and X-ray data. The conservation-element and solution-element (CE/SE) method, a novel numerical framework for general conservation law, is applied to simulate the compressible flow. The effect of degree of aeration, breakup, and mixing of the liquid spray are demonstrated. The spray penetration height and average droplet size along with a spray penetration axis are quantitatively compared with data. The shock train flow structures induced by the presence of a liquid jet are further discussed.

Flow-induced density fluctuation assisted nucleation in polyethylene.

The nucleation processes of polyethylene under quiescent and shear flow conditions are comparatively studied with all-atom molecular dynamics simulations. Under both conditions, nucleation is demonstrated to be a two-step process, which, however, proceeds via different intermediate orders. Quiescent nucleation is assisted by local order structures, while flow-induced nucleation is promoted by density fluctuation, which is a coupling effect of conformational and orientational orderings. Flow drives the transformation from flexible chains to conformational ordered segments and circumvents the entropic penalty, which is the most peculiar and rate-limited step in polymer crystallization. This work suggests that the acceleration of the nucleation rate in orders of magnitude by flow is mainly attributed to the different kinetics pathway via conformational/orientational ordering-density fluctuation-nucleation.

Qualitative and quantitative characterization of a synthetic jet mounted on a convex torpedo-like surface under cross-flow

Understanding the fundamental flow physics of a synthetic jet in cross flow is important for a variety of engineering and scientific applications. In Particular, a synthetic jet mounted on a convex torpedo surface at high velocity ratios is of great importance for naval applications. However, despite exhaustive research done in this area, most of the studies have been conducted for synthetic jets on a flat surface and at low velocity ratios (ratio of average jet velocity and the cross-flow velocity). Studies at high velocity ratios particularly on a curved surface has largely not received much attention. Thus, the present study employs flow visualization and the laser doppler velocimetry (LDV) to qualitatively and quantitatively characterize such a synthetic jet under cross-flow conditions. Experiments have been conducted for quiescent flow conditions (i.e. 0 Hz) and at jet actuation frequencies of 1, 2, 4 and 6 Hz, at cross-flow velocities of 7.2, 20 and 32 cm/s. Our study indicates that the behaviour of synthetic jet changes significantly in the presence of cross-flow since it affects the formation, convection, and interaction among vortices, particularly at higher velocity ratios. Flow visualization provides interesting insights into the two major types of vortex rings and the interplay between the vortex structures and the boundary layer. Further, a comparison of the time-averaged velocity variation in the vertical direction reveals the formation of two local peaks at high velocity ratios and high actuation frequency of the jet. While the cross-flow is also found to be significantly affected by the high velocity ratio jet, the low velocity ratio jet in cross flow does not show substantial influence on the cross-flow except in regions close near the wall. The high velocity ratio jet shows the formation of a wake behind the jet region which is believed to be generated because of the blockage of the cross-flow created by the jet. Trailing jet and leading rings follow two different projectiles. Finally, a test-bed of LDV data has been reported in the study, which can prove useful in the validation of computational results of synthetic jets on a curved torpedo surface.

Direct numerical simulation of particle sedimentation in a Bingham fluid

The settling efficiency, and stability with respect to settling, of a dilute suspension of infinite circular cylinders in a quiescent viscoplastic fluid is examined by means of direct numerical simulations with varying solid volume fraction, $\phi$, and yield number, $Y$ . For $Y$ sufficiently large we find higher settling efficiency for increasing $\phi$, similar to what is found in shear-thinning fluids and opposite to what is found in Newtonian fluids. The critical yield number at which the suspension is held stationary in the carrier fluid is found to increase monotonically with $\phi$, while the transition to settling is found to be diffuse: in the same suspension, particle clusters may settle while more isolated particles remain arrested. In this regime, complex flow features are observed in the sedimenting suspension, including the mobilization of lone particles by nearby sedimentation clusters. Understanding this regime, and the transition to a fully arrested state, is relevant to many industrial and natural problems involving the sedimentation of viscoplastic suspensions under quiescent flow conditions.

Survey of the near wake of an axial-flow hydrokinetic turbine in quiescent conditions

Abstract Flow field results are presented for the near-wake of an axial flow hydrokinetic turbine in quiescent flow conditions. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine modeled after the Sandia National Laboratory Reference Model 1 Tidal Current Turbine. All measurements were obtained in the large towing tank facility at the United States Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio corresponding to maximum power production. The turbine is scale independent with respect to lift and very slightly dependent with respect to drag for these conditions ( R e c @ 0.7 R ≈ 4 × 10 5 ). The wake velocity field data was obtained using a two-dimensional particle image velocimetry (PIV) system. PIV ensembles were obtained for phase locked conditions. This paper focuses on characterizing the velocity and the mean flow structure in the near wake. Specifically, the downstream evolution of coherent tip vortices shed by the rotor blades were examined. Vortex aperiodicity was shown to increase with downstream distance. The streamwise spacing between adjacent vortex cores was shown to be constant within a diameter downstream of the rotor. Further downstream, significant vortex filament interaction was observed, including leapfrogging. This interaction is thought to be the primary mechanism for wake breakdown and re-energization.

Impacts of exhalation flow on the microenvironment around the human body under different room temperatures

Exhalation flow and room temperature can have a considerable effect on the microenvironment in the vicinity of human body. In this study, impacts of exhalation flow and room temperature on the microenvironment around a human body were investigated using a numerical simulation. For this purpose, a computational fluid dynamic program was applied to study thermal plume around a sitting human body at different room temperatures of a calm indoor room by considering the exhalation flow. The simulation was supported by some experimental measurements. Six different room temperatures (18 to 28 °C) with two nose exhalation modes (exhalation and non-exhalation) were investigated. Overhead and breathing zone velocities and temperatures were simulated in different scenarios. This study finds out that the exhalation through the nose has a significant impact on both quantitative and qualitative features of the human microenvironment in different room temperatures. At a given temperature, the exhalation through the nose can change the location and size of maximum velocity at the top of the head. In the breathing zone, the effect of exhalation through the nose on velocity and temperature distribution was pronounced for the point close to mouth. Also, the exhalation through the nose strongly influences the thermal boundary layer on the breathing zone while it only minimally influences the convective boundary layer on the breathing zone. Overall results demonstrate that it is important to take the exhalation flow into consideration in all areas, especially at a quiescent flow condition with low temperature.

Computational Fluid Dynamic Analysis of Fluidic Actuator for Active Flow Control Applications

A numerical study is conducted to explore the unsteady nature of fluidic oscillators, which are responsible for generating sweeping jets for effective flow control of large-scale applications. Two- and three-dimensional unsteady Reynolds-averaged Navier–Stokes and three-dimensional improved delayed detached-eddy simulations are employed to simulate and resolve the flow structures in the internal flow passages of a fluidic oscillator and an external flowfield in a quiescent flow condition. The predicted flow generated by an enlarged actuator with an outlet diameter of 25 mm using air as a working fluid is validated against measurements at various supply rates. A comparison of current computational results using two-dimensional and three-dimensional unsteady Reynolds-averaged Navier–Stokes simulations with experiments indicates a reasonable agreement in the jet oscillation frequency. However, a significant discrepancy in the streamwise velocity field is observed between the two-dimensional unsteady Reynolds-averaged Navier–Stokes and experimental data. The three-dimensional unsteady Reynolds-averaged Navier–Stokes simulation also underpredicts the jet width, and consequently overpredicts the extension of the jet in the axial direction downstream of the actuator exit. Excellent agreement in the jet oscillation frequency and internal and external flowfields is obtained at two supply rates between the three-dimensional improved delayed detached-eddy simulation and the measurements. Simulations are also performed for a small-scaled actuator with a hydraulic outlet diameter of 1.3 mm, with water as the working fluid. The predicted oscillation frequency of the jet is compared with the experiments. A scaling analysis is also conducted by analyzing and correlating the flow characteristics and key parameters of the two actuators’ scales.

Path instabilities of light particles rising in a liquid with background rotation

The present paper reports on experimental observations performed on 2–6mm low-density polystyrene particles rising freely in a water-glycerol mixture for the liquid at rest as well as with slow background rotation (axisymmetric solid body rotation around the vertical axis). Particle trajectories were studied in the Reynolds-numbers range of 200<Re<800 at a fixed rotation rate of Ω=0.81/s. Both the trajectory measurements and the wake visualization indicate that there are differences in the wake structure of the particles as well as in the path instabilities compared to those in quiescent fluid. In the swirling flow, the two-threaded wake mode is dominant up to Reynolds numbers of about 500 with typical spiral-type trajectories until vortex shedding commences. In contrast, under quiescent flow conditions vortex shedding already occurs above Re=230, and the particles predominantly move in zig-zagging paths. Thus, the presence of background rotation is able to suppress vortex shedding and to delay the transition. For the co-rotating observer, the radius of the spiral-type path is comparable to that of the transversal motion under quiescent flow conditions. Therefore, the magnitude of lateral lift-forces is not greatly affected by the presence of the swirl.

Selection of Colloidal Silica Grouts with Respect to Gelling and Erosion Behaviour

Cembinder, Eka EXP36, and MEYCO MP320 are three colloidal silica materials that have been proposed for post-excavation grouting of deep tunnels in a radioactive waste repository. In this study, samples of these colloidal silicas were tested for their particle size distribution, gel induction time (tG), gel time (TG), and physical erosion, under mildly saline groundwater flow conditions. In order to achieve a desired gel time range, from 15 to 50 min, it is recommended that the colloidal silica is mixed with a NaCl accelerator at a 5:1 volume ratio. At 20 °C, the concentration range for the NaCl solution should be 1.5 to 1.7 M for MEYCO, 1.23 to 1.38 M for Eka EXP36, and 1.3 to 1.47 M for Cembinder. The physical erosion of the set silicas remained steady during a 10 h flow cell experiment, when grouts were subjected to 0.05 M NaCl at a superficial velocity of 2.2 × 10−5 m/s. For these test conditions, the results show that MEYCO has the highest average erosion rate (0.85 mg/h) of the three grout materials, as well as the greatest variability in this rate. Cembinder performed best with the lowest silica removal rate. Extrapolation of the measured erosion rates suggests that grout fracture dilation would not be significant under natural quiescent groundwater flow conditions, but would be high if there was hydraulic communication between the geosphere and the repository.

Highly time-resolved investigation of the electric wind caused by surface DBD at various ac frequencies

Surface plasma discharges find relevant applications for manipulating laminar or turbulent flows. The control mechanism can be related to time-averaged momentum transfer or be associated to favorable interactions between the fluctuations imposed by the discharge and the natural instabilities developing in the incoming flow. In the present paper, the electric wind produced by a non-thermal surface dielectric barrier discharge is investigated by a non-intrusive measurement technique having a high temporal resolution. The measurements are carried out for an actuator in quiescent flow condition, but also for an actuator placed in the test section of a wind tunnel with external flow speed from 5 to 30 m/s. The results confirm that a surface DBD actuator has a low control authority on the mean characteristics of the velocity profiles developing above the plasma region. However, it is also demonstrated that the plasma actuator imposes periodic fluctuations at the electrical frequency driving the gas ionization, this over the whole range of tested speeds.

Effects of wing flexibility on sound characteristics of a four-wing flapping wing micro air vehicle

The objective of this study was to understand the sound generation of flapping wings. In particular, the structural properties of the wings were varied to assess the effects of wing flexibility on the sound generation of flapping wings. A four-wing, hummingbird-inspired flapping wing micro air vehicle (FMAV) was considered. The half wing span was approximately six centimeters and the flapping frequency was approximately 25 Hz. The sound produced by the FMAV was measured using a 1/2 inches diameter microphone in an acoustic chamber under the quiescent flow condition. Time-averaged aerodynamic forces were measured by the force and torque sensor Nano17 (ATI Industrial Automation). Results revealed the characteristics of flapping-wing sound in terms of directivity, frequency response, and attenuation. Moreover, the degree of wing deformation led to a variation in the frequency response corresponding to the maximum sound pressure level as well as time-averaged aerodynamic force, even when the flapping frequenc...

Characterisation of plasma synthetic jet actuators in quiescent flow

An experimental characterisation study of a large-volume three-electrode plasma synthetic jet actuator (PSJA) is presented. A sequential discharge power supply system is used to activate the PSJA. Phase-locked planar particle image velocimetry (PIV) and time-resolved Schlieren imaging are used to characterise the evolution of the induced flow field in quiescent flow conditions. The effect of orifice diameter is investigated. Results indicate three distinct features of the actuator-induced flow field. These are the initial shock waves, the high speed jet and vortex rings. Two types of shock waves with varied intensities, namely a strong shock wave and a weak shock wave, are issued from the orifice shortly after the ignition of the discharge. Subsequently, the emission of a high speed jet is observed, reaching velocities up to 130 m s−1. Pronounced oscillation of the exit velocity is caused by the periodical behaviour of capacitive discharge, which also led to the formation of vortex ring trains. Orifice diameter has no influence on the jet acceleration stage and the peak exit velocity. However, a large orifice diameter results in a rapid decline of the exit velocity and thus a short jet duration time. Vortex ring propagation velocities are measured at peak values ranging from 55 m s−1–70 m s−1. In the case of 3 mm orifice diameter, trajectory of the vortex ring severely deviates from the actuator axis of symmetry. The development of this asymmetry in the flow field is attributed to asymmetry in the electrode configuration.

On the use of synthetic jet actuator arrays for active flow separation control

Present study highlights the flow control effectiveness of newly designed synthetic–jet–actuator (SJA) arrays placed at 23% and 43% of the chord from the leading edge of a low-speed wing model. First, the characteristics of a single SJA were investigated under the quiescent-flow condition. It was found that the jet velocity attains a peak between 400Hz and 500Hz, which corresponds to the SJA’s Helmholtz resonance frequency. The SJA arrays were then implemented on the wing model and investigated under the cross-flow condition. Both force balance and hot-wire measurements showed that the SJA arrays are able to effectively delay the flow separation and hence improve the aerodynamic performance of the wing model. The highest improvement in the lift coefficient is 27.4% and the average reduction in the drag coefficient is 19.6%. It was also found that the front (upstream) SJA array is more effective than its rear (downstream) counterpart. Tomographic-PIV measurements were also performed to unravel the three-dimensional flow details over the suction surface of the wing model with and without the SJA actuation. The time-averaged results showed that the introduction of synthetic jets substantially changes the flow structures of the separated shear layer, brings the outer high-momentum flow into the near-wall region, and delays the flow separation. In the end, the power consumption of a single SJA array was measured and found to be less than 5W.

Influence of a particulate nucleating agent on the quiescent and flow-induced crystallization of isotactic polypropylene

Flow induced crystallization of commercial isotactic polypropylene (iPP) and its blends with sodium 2,2′-methylene bis-(4,6-di-tert-butylphenyl) phosphate (also known as NA11) is studied by means of in-situ time resolved small-angle X-ray scattering (SAXS). The isothermal crystallization at 145 °C (i.e. well below melting temperature of polymer) is performed after the application of steady shear to probe the anisotropic structure formation. In order to separate the influence of shear rate and shear time on polymer crystallization, four different shear conditions (60 s−1 for 1 s, 30 s−1 for 2 s, 15 s−1 for 4 s and 6 s−1 for 10 s) are applied while maintaining the same imposed strain in the polymer melt. Further the effect of different concentration of nucleating agent on the crystallization kinetics of iPP is examined both under quiescent and shear flow conditions. For instance, under quiescent condition, the crystallization half-time (τ1/2) decreases with the increasing concentration of nucleating agent in the polymer. Under shear flow conditions, our observations are as follows: In the case of neat iPP, τ1/2 decreases significantly at higher shear rates (≥30 s−1). Compared to the neat iPP, for the same concentration of NA11 in the NA11/iPP blends differences in τ1/2 with the increase in applied shear rates are significantly smaller. In other words, the crystallization kinetics is dominated by the amount of nucleating agent in the NA11/iPP blends as opposed to shear rates in the neat iPP. The present study shows that the critical value of shear rate required for chain orientation in the molten polymer is lower in the presence of the nucleating agent compared to neat iPP. The self-nucleation process investigated with the aid of differential scanning calorimetry (DSC), indicates that the nucleating efficiency of NA11 on iPP is around 60%.

Experimental and modeling study of single coal particle combustion in O2/N2 and Oxy-fuel (O2/CO2) atmospheres

Coal particle combustion experiments were performed in a drop tube furnace (DTF) with oxygen concentration from 21% to 100%, in N2 and CO2 mixtures, under quiescent flow conditions. Small particles (75–90μm) of a high-volatile bituminous coal (PSOC-1451) and a lignite coal (DECS-11) are analyzed with particular attention to the particle burnout times and the particle surface temperatures. These experimental measurements are compared with the predictions of a comprehensive model of coal combustion. Combustion of coal particles is a multi-scale process where both chemical and physical phenomena are involved, thus it requires a coupled and accurate description of the kinetics as well as of the heat and mass transport phenomena. Important features of the model are a multistep kinetic scheme of coal volatilization and detailed kinetics of the successive gas-phase reactions and of the heterogeneous reactions of both char oxidation and gasification. The achieved overall agreement between the experimental data and the numerical predictions, in terms of particle temperature and burnout times, highlights the capability of the model to simulate the effect of different operating conditions in the coal combustion processes.