External Flow Velocity Research Articles

音場浮遊液滴の温度変化が内外部流動と対流熱伝達に及ぼす影響

The acoustic levitation is one of the levitation techniques to control a sample in sound pressure. The acoustic levitation induces the internal and external flow of the levitated droplet. It is considered that these flows affect heat and mass transfer of the levitated droplet. The purpose of this study is to investigate the relationship between convective heat transfer and flow behavior of the levitated droplet. The convective heat transfer between ambient air and the droplet is calculated by an evolution of the surface area and temperature. The convective heat transfer increases as the temperature of water droplet increases. The external flow structure is measured by PIV. The non-heated water droplet has a toroidal vortex below a levitated droplet. As the temperature of water droplet increases, the external flow structure changes from that of non-heated droplet. The larger the diameter and temperature of droplet become, the larger external flow velocity near the surface of the levitated droplet becomes. It is suggested that the increase of external flow velocity enhances the convective heat transfer and the convective heat transfer is more enhanced by the increase of temperature.

Analysis of local frequency response of flow to actuation: Application to the dielectric barrier discharge plasma actuator

The present study provides a methodology to derive the local frequency response of flow under actuation, in terms of the magnitude of actuator induced perturbations. The method is applied to a dielectric barrier discharge (DBD) plasma actuator but can be extended to other kinds of pulsed actuation. The actuator body force term is introduced in the Navier-Stokes equations, from which the flow is locally approximated with a linear-time-invariant system. The proposed semi-phenomenological model includes the effect of both viscosity and external flow velocity, providing a system response in the frequency domain. A validity criterium is additionally devised for the estimation of the threshold frequency below which the developed approach can be applied. Analytical results are compared with experimental data for a typical DBD plasma actuator operating in quiescent flow and in a laminar boundary layer. Good agreement is obtained between analytical and experimental results for cases below the model validity threshold frequency. Results demonstrate an efficient and simple approach towards prediction of the response of a convective flow to pulsed actuation.

A bioinspired touching sensor for amphibious mobile robots

An amphibious mobile robot relies on effective sensing ability to adapt itself in complicated amphibious environments. In this paper, we present a multifunctional whisker-like touching sensor with low energy consumption, inspired by amphibious animals. The sensor comprises a leverage system and a two-dimensional position tracing system, transforming the moving position of biowhisker to a changing laser spot coordinates. On land, the sensor driven by a motor is able to track the movement of biowhisker directly, telling the change of contact position, to sense nearby objects and explore their surface by touching. In underwater environment, the sensor can obtain in real-time external flow direction and velocity by passive impulsion. Testing results showed that our prototype can sense flow or drag force direction in 360° exactly, and tell flow velocity under 1 m/s, it can also recognize line or arc edges of obstacle correctly by touching.

Energy harvesting prospects in turbulent boundary layers by using piezoelectric transduction

Thin flexible cantilever beams with patches of piezoelectric materials or surrogate beams with attached strain gages have been electromechanically characterized for energy harvesting inside turbulent boundary layers. A turbulent boundary layer carries mechanical energy distributed over a range of temporal and spatial scales and their interaction with the immersed piezoelectric beams results in a strain field which generates the electrical charge. This energy harvesting method can be used for developing self-powered electronic devices such as flow sensors. In the present experimental work, various energy harvesters were placed in the boundary layers of a large scale wind tunnel with Reθ between 2000 and 7500. The orientation of the beam relatively to the incoming flow and the wall was found to be critical parameters affecting the energy output. “Power maps” are presented for various roll and pitch angles as well as external flow velocities. The role of large instantaneous turbulent boundary layer structures in this rather complex fluid–structure interaction is discussed in interpreting the electrical output results. The forces acting on the vibrating beams have been measured dynamically and a theory has been developed which incorporates the effects of mean local velocity, turbulence intensity, the relative size of the beam׳s length to the integral length scale of turbulence, the structural properties of the beam and the electrical properties of the active piezoelectric layer to provide reasonable estimates of the mean electrical power output.

Direct numerical simulation of spatially developing turbulent boundary layers with opposition control

Opposition control of spatially developing turbulent boundary layers for skin friction drag reduction is studied by direct numerical simulations. The boundary layer extends 800θ0 in the streamwise (x) direction, with θ0 denoting the momentum thickness at the flow inlet. The Reynolds number, based on the external flow velocity and the momentum thickness, ranges from 300 to 860. Opposition control applied in different streamwise ranges, i.e. and as well as the uncontrolled case, are simulated. Statistical results and instantaneous flow fields are presented, with special attention paid to the spatial evolution properties of the boundary layer flow with control and the underlying mechanism. It is observed that a long spatial transient region after the control start and a long recovery region after the control end are present in the streamwise direction. A maximum drag reduction rate of about 22% is obtained as the transient region is passed, and an overshoot in the local skin friction coefficient (Cf) occurs in the recovery region. A new identity is derived for dynamical decomposition of Cf. Reduction of Cf by opposition control and overshoot of Cf in the recovery region are explained by quantifying the contributions from the viscous shear stress term, the Reynolds shear stress term, the mean convection term and other terms.

Examples of the Re‐number effect on the transitional flat plate boundary layers

AbstractDimensionless mean flow characteristics and the location of the transitional region were investigated of flat plate boundary layers (smooth surface or covered by sandpaper 60 grit) at the external flow velocities about 5 m/s, 10 m/s and 14 m/s and with turbulence levels either natural (less than 0.003) or gained by a grid across the flow (FST level 0.03 at the leading edge). Preliminary comparison of the received results with those previously published [1] is presented. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of internal carotid artery stenting on superior thyroid artery Doppler flow.

Patients with carotid disease are frequently referred for carotid artery stenting based on the results of carotid duplex studies. During carotid artery stenting, the stent is usually extended into the common carotid artery, thereby crossing the external carotid artery. Previous studies have shown conflicting results regarding internal carotid stenting and external carotid artery flow velocities, but the effect of stenting on ipsilateral superior thyroid artery velocities has not been defined. This study examined the effect of internal carotid angioplasty and stenting on the ipsilateral superior thyroid artery Doppler-derived flow parameters. We prospectively studied preinterventional and postinterventional duplex scans obtained from 41 patients (mean age ± SD, 64 ± 10 years) who underwent carotid artery stenting. The Doppler-defined preprocedural peak systolic velocity (PSV) end-diastolic velocity (EDV), resistive index (RI), and pulsatility index (PI) in the ipsilateral external carotid and superior thyroid arteries were compared with postprocedural values. Among patients with stenting, the preprocedural PSV, EDV, RI, and PI in the ipsilateral superior thyroid artery were 30 ± 11 cm/s, 13 ± 6 cm/s, 0.62 ± 0.11, and 1.04 ± 0.28,respectively; after stenting, they were 36 ± 8 cm/s, 14 ± 9 cm/s, 0.71 ± 0.07, and 1.11 ± 0.19. The preprocedural PSV, EDV, RI, and PI in the ipsilateral external carotid artery were 79 ± 24 cm/s, 17 ± 7 cm/s, 0.77 ± 0.26, and 1.27 ± 0.22; after stenting, they were 94 ± 31 cm/s, 20 ± 6 cm/s, 0.80 ± 0.4, and 1.25 ± 0.31. Despite a slight increase in superior thyroid and external carotid artery flow, there was no statistically significant change from before to after stenting. This study showed no differences in blood velocity profiles in the ipsilateral superior thyroid and external carotid arteries after stenting.

Effect of external flow velocity on momentum transfer of dielectric barrier discharge plasma actuators

An experimental study is performed towards identifying cross-talk effects between DBD plasma actuators and external flow. An actuator is positioned in a boundary layer operated in a range of free stream velocities from 0 to 60 m/s, and tested both in counter-flow and co-flow forcing configurations. Electrical measurements are used for estimating the power consumption and the discharge formation is visualized using a CCD camera. The actuator's force is measured using a sensitive load cell. Results show the power consumption is constant for different flow velocities and actuator configurations. The plasma light emission is constant for co-flow forcing but shows a trend of increasing intensity with counter-flow forcing for increasing free stream velocities. The measured force is constant for free stream velocities larger than 20 m/s, with same magnitude and opposite direction for the counter-flow and co-flow configurations. In quiescent conditions, the measured force is smaller due to the change in wall shear force by the induced wall-jet. An analytical model is presented to estimate the influence of external flow on the actuator force. It is based on conservation of momentum through the ion-neutral collisional process while including the contribution of the wall shear force. Satisfactory agreement is found between the prediction of the model and experimental data at different external flow velocities.

Nanosecond-pulsed plasma actuation in quiescent air and laminar boundary layer

An experimental investigation of the working principles of a nanosecond-pulsed dielectric barrier discharge (ns-DBD) plasma actuator has been conducted. Special emphasis is given on the thermal effects accompanying the rapid deposition of energy associated with this kind of actuation. A ns-DBD plasma actuator has been operated in quiescent air conditions as well as in a flat plate laminar boundary layer, with external flow velocity of 5 and 10 m s−1. Schlieren imaging and particle image velocimetry have been used to characterize the actuation. Additionally, the back-current shunt technique has been used for current measurements, from which energy input (per pulse) is calculated. Cases of 10-, 20- and 50-pulse bursts are tested. Schlieren imaging in still air conditions shows the formation of a high-temperature region in the vicinity of the discharge volume. The spatial extent of the visible ‘hot spot’ depends upon the number of pulses within the burst, following a power law. Schlieren imaging of the span-wise effect of the plasma actuator reveals weak compression waves originating from the loci of discharge filaments. The thermal ‘hot spots’ exhibit significant three-dimensionality. Particle image velocimetry is used to measure the velocity field resulting from the ns-DBDs acting on a laminar boundary layer. The disturbance leads to formation of a Tollmien–Schlichting wave train, with spectral content in good agreement with linear stability theory. It is observed that the group length of the wave train is proportional to the number of pulses within the burst.

Extinction limits of spreading flames over wires in microgravity

Tests with flames spreading over wires in microgravity were performed at external opposed flow conditions from 60 to 200mm/s to examine the influence of flow velocity on the extinction limit. In the experiments, low density polyethylene insulated Nickel–chrome and Copper wire samples were used. The experiments were conducted both in normal gravity and microgravity attained by parabolic flights. The experimental results are discussed based on non-dimensional numbers developed by Takahashi et al. This study added the heat conduction through the wire insulation to the inner core to their model, which is considered as a heat loss term arising due to the presence of the inner core. Further the effect of curvature of the sample is taken into consideration. From the results of the experiments in normal gravity, there was a low oxygen limit for Copper wire at around 125mm/s. However there was no low oxygen limit for Nickel–chrome wire; the limits of the oxygen concentration decreased monotonically with decreases in the external flow velocity. The non-dimensional number implies that the decrease in inner core temperature around the preheat zone occurring with Copper wire plays a role in the change in low oxygen limits. Further, in the external flow velocity range investigated here, there was no low oxygen limit in microgravity with the Copper wire. This might arise due to the changes in the flame shapes by gravity. The heat input from flame to the wire is supposed to increase in microgravity because the wire is surrounded by flame due to inhibited natural convection. This increased heat input might help the increase of the inner core temperature around the preheat zone. For this reason, there is a possibility that the low oxygen limit with Copper wire in microgravity exists at lower flow velocity than that in normal gravity.

Design of a Single-Plate Electrowetting-on-Dielectric Device

Due to its simple structure, low consumption of energy but strong driving forces, Electrowetting on Dielectric (EWOD) is used most frequently in digital microfluidics for manipulation and control of droplets. In this paper, the internal mechanism of EWOD is explained though establishing the geometric model of unipolar board structure digital microfluidic chip. The digital simulation software COMSOL Multiphysics is applied to analyze the coupling fields. The results show that external flow velocity of micro-droplet is greater than the internal velocity. Based on the theoretic analysis, surface micromachining technologies are employed to fabricate the single-plate EWOD chip. Finally, an experiment platform is set up to test this chip. Experimental results show that 2μL droplet can be driven in velocity of 10cm/s and two droplets can be merged successfully. It will possibly provide an effective solution to the manipulation of droplets.

Limiting conditions for flame spread in fire resistant fabrics

Fire resistant (FR) fabrics are used for astronauts, firefighter and racecar driver suits. However, their fire resistant characteristics depend on the environmental conditions and require study. Particularly important is the response of these fabrics to varied environments and radiant heat from a source such as an adjacent fire. In this work, experiments were conducted to study the effect of oxygen concentration, external radiant flux and oxidizer flow velocity on the concurrent flame spread over two FR fabrics: Nomex HT90-40 and a Nomex/Nylon/Cotton fabric blend. Results show that for a given fabric the minimum oxygen concentration for concurrent flame spread depends strongly on the magnitude of the external radiant flux. At increased oxygen concentrations the external radiant flux required for flame spread decreases. Oxidizer flow velocity influences the external radiant flux only when the convective heat flux from the flame has similar values to the external radiant flux. The results of this work provide further understanding of the flammability characteristics of fire resistant fabrics in environments similar to those of future spacecrafts.

Numerical Simulation of Multi-Physical Fields Coupling and Design of a Digital Microfluidics Chip

Due to its simple structure, low consumption of energy but strong driving forces, Electrowetting on Dielectric (EWOD) is used most frequently in digital microfluidics for manipulation and control of droplets. In this paper, the internal mechanism of EWOD is explained though establishing the geometric model of the unipolar board structure digital microfluidic chip. And the boundary conditions of equations are determined. Three coupling physical fields: electric field, flow field and temperature field in the digital microfluidic chip are simulated and analyzed. With the electric field equation coupled, Navier-Stokes equations and energy equation of the temperature control, the numerical simulation of the chip is conducted. The results show that the internal flow of micro-droplets is counterclockwise and swirling flow. The external flow velocity of micro-droplet is greater than the internal velocity. In addition, micro-droplets near the electrode applied temperature are higher than the internal temperature. Surface micromachining technologies are employed to fabricate the chip. Experimental results show that the droplet can be driven in a velocity of 25cm/s. It will possibly provide an effective solution to the manipulation of droplets.

Homogenization of boundary layer of pseudo-plastic fluid in the presence of rapidly oscillating external forces

In this paper, we study the homogenization problem for equations of magnetohydrodynamic boundary layer of pseudo-plastic fluid. It is assumed that the external flow velocity and the external magnetic field are described by oscillating functions and the frequency depends on a small parameter. In von Mises variables and in Cartesian variables, we construct the homogenized problem, establish strong convergence of solutions in a special norm, and estimate the rate of this convergence. We show that in von Mises variables the convergence rates in different norms are of different orders of smallness.

Numerical predictions of the broadband turbulent noise for axial flow cooling fans

A statistical model was established in this article for predicting the broadband turbulent noise of axial flow fans. First, an integral formula was deduced based on the Ffowcs Williams-Hawkings (FW-H) equations for the purpose of calculating the sound power spectrum of aeroacoustic sources with low rotating speed. The auto-correlation spectrum of the fluctuating pressure on the fan blades was modeled by a two-section exponential function. The characteristic aerodynamic parameters, such as boundary layer thickness, external flow velocity, wall shear stress, and average static pressure gradient on blade surfaces, were obtained by solving the steady Reynolds-averaged Navier-Stokes equations and used to normalize the auto-correlation spectrum. Then, the unknown coefficients in the exponential function were determined by fitting the experimental noise spectra for two typical and commercially available axial flow cooling fans. Finally, under the conditions of retaining the similar aerodynamic performances of the two baseline fans, a new fan design was developed and optimized using this method, of which the overall noise level was 2 dB lower than the quieter of the two baseline fans. The predicted noise spectra for all three fans agreed very well with the experimental data.

Filtration at the microfluidic level: enrichment of nanoparticles by tunable filters

We present an electrohydrodynamic device for filtration of nanometre-sized particlesfrom suspensions. A high-frequency electric field is locally generated through theaction of mutually parallel microelectrodes integrated into a microfluidic channel.Due to the mechanism of ohmic heating, a thermal gradient arises above theseelectrodes. In conjunction with temperature-sensitive properties of the fluid, an eddyflow behaviour emerges in the laminar environment. This acts as an adjustablefilter. For quantification of the filtration efficiency, we tested a wide range ofparticle concentrations at different electric field strengths and overall externalflow velocities. Particles with a diameter of 200 nm were retained in this mannerat rates of up to 100%. Numerical simulations of a model taking into accountthe hydrodynamic as well as electric conditions, but no interactions between thepoint-shaped particles, yield results that are similar to the experiment in both the flowtrajectories and the particle accumulation. Our easy technique could becomea valuable tool that complements conventional filtration methods for handlingnanometre-scaled particles in medicine and biotechnology, e.g. bacteria and viruses.

Experimental Research on Lift up and Drag Reduction Effect of Streamwise Travelling Wave Wall

Travelling wave wall used for Micro Air Vehicle flow control is studied to find out its drag reduction effect. Aerodynamic characteristics of the airfoil with and without a streamwise travelling wave on its surface are calculated. The interrelationship is also analyzed among lift, drag and ratio of motion phase speed to the external flow velocity. The flexible airfoil that can generate streamwise travelling waves is realized via electromagnetic coil actuation. Thirty-six coils are placed streamwisely under heat shrink film and magnets producing magnetic field are placed under coils. Every single coil is controlled by current to move vertically in harmonic motion with 30 degrees phase difference between adjacent ones. So heat shrink film is actuated, and streamwise travelling wave is generated. Meanwhile, a prototype is designed and tested in wind tunnel. The experimental aerodynamic data proves that streamwise traveling wave airfoil can increase lift and reduce air drag.

Lifted flame structure of coannular jet flames in a triple port burner

In this study, we investigated the combustion field in a triple port burner. The coannular burner consists of three concentric tubes, where air flows in both inner (central) and outer tubes, and fuel flows in the annulus between these air tubes. In order to investigate the combustion characteristics in the triple port burner, the liftoff height and concentrations of soot and NOx were experimentally examined. A numerical simulation was also conducted to discuss the flame structure in detail. Since two non-premixed flames are formed in the boundaries of fuel and air, there are four flame configurations, consisting of attached flames, inner attached/outer lifted flames, inner lifted/outer attached flames, and twin lifted flames. By increasing the external air flow velocity, unique flame behaviors are observed, including (1) inner flame re-attachment and (2) flip-flop between inner and outer flames. When the flame is lifted, the maximum soot concentration and NOx emission are decreased. Based on the flame index, the attached flame is mainly a diffusion flame. When the inner or outer flame is lifted, the so-called triple flame structure is observed. Interestingly, in the case of twin lifted flames, two rich premixed flames are merged each other.

A cell for studying discharge in gas flow with rotating electrodes

A cell with rotating electrodes for studying discharge in gas flow is described. It is demonstrated that, using a reasonable combination of the values of external longitudinal magnetic field, flow velocity, and electrode rotation rate, it is possible under certain conditions to control the delay of gas breakdown and the duration of a short volume discharge preceding the discharge contraction stage.

Homotopy Solution for Non-Similarity Boundary-Layer Flow near a Stagnation Point

In this paper, non-similarity boundary-layer flow of a Newtonian fluid near an asymmetric plane stagnation point with a dimensionless external flow velocity ue = x/(x+1) is studied. The original boundary-layer equations are transferred into a nonlinear partial differential equation (PDE) with variable coefficients. An analytic technique for strongly nonlinear equations, namely the homotopy analysis method (HAM), is applied to replace the nonlinear PDE by an infinite number of linear ordinary differential equations (ODEs) with constant coefficients. An artificial parameter, called the convergence-control parameter, is introduced to ensure the convergence of solution series. Accurate analytical approximations of skin friction and boundary-layer thickness are obtained, and the effect of the external flow velocity on the non-similarity flows is investigated. This approach has general meanings and can be applied to many other non-similarity boundary-layer flows.