Time-averaged Velocity Distributions Research Articles

Experimental Research on Dynamic Characteristics of Synthetic Jet Induced by SJA with HCPCS Membrane

A new half cymbal style piezoelectric-metal composite structure (HCPCS) membrane is proposed and utilized for piezoelectric synthetic jet actuator (PSJA) to increase its output jet velocity. PSJA specimens with both plane and HCPCS membranes were manufactured. An experimental system with hot-wire anemometer was setup to test the jet velocity. The frequency responses of time-averaged jet velocity were measured for both HCPCS diaphragms and plane membranes. The time-averaged velocity distributions were collected along the exit centerline. Experiments indicate that the HCPCS structure brings a promotion of vibration displacement distortion of diaphragm and stronger driving capability, and that the mean jet velocities synthesized from the optimal HCPCS PSJAs are higher than the optimal plane membrane one. Hilbert-Huang Transform was employed to analyze the instantaneous energy distribution characteristics of the jet. The analysis was done for signals along the centerline. The results show that HHT is an effective method for fluid field analysis.

Large Eddy Simulation of open channel flows with non-submerged vegetation

Results of several Large Eddy Simulations (LES) of open channel flows with non-submerged vegetation are presented in this article. It is shown that the vegetation can make the flow structure in the mainstream direction uniform for both supercritical and subcritical flows. For subcritical flows, the LES results of the ensemble-average of time-averaged velocity distributions at four vertical sections around a single plant are in good agreement with measurements. The velocity sees double peaks at the upper and lower positions of flows. For supercritical flows, the ensemble-average velocities see some discrepancy between LES and measurement results. Some secondary flow eddies appear near the single plant, and they just locate in the positions of the double peaks in stream-wise velocity profiles. It is also found that the vegetation drag coefficient deceases as the Froude number increases.

Design of the rotor blades of a mini hydraulic bulb-turbine

The rotor blades of a mini hydraulic turbine were designed using a quasi-three-dimensional method. The meridional flow is computed by a streamline curvature method and the blade-to-blade flow by a singularity method. The rotor blade sections are the NACA 66 (MOD) with a = 0.8 meanline. The camber and the stagger angle of the blade sections are adjusted to fulfil the prescribed angular momentum distributions, at the rotor inlet and outlet sections, and zero-incidence flow angle, at the blade leading edge. Turbine head and efficiency versus flow rate curves were obtained for different rotor blade stagger angles at constant rotational speed. Radial distributions of time-averaged velocity and pressure, measured at the rotor exit section with a five-hole probe, are also presented. The design and the experimental results are compared with three-dimensional inviscid flow numerical results computed by the FLUENT code. The domain is discretized by unstructured meshes with a maximum of 2.5 × 10 6 elements. The numerical results show good agreement with the design values and the experimental results, validating the design hypothesis of small radial velocity in the flow through the rotor.

Modeling of drag reduction in turbulent channel flow with hydrophobic walls by FVM method and weakly-compressible flow equations

In this paper the effects of hydrophobic wall on skin-friction drag in the channel flow are investigated through large eddy simulation on the basis of weakly-compressible flow equations with the MacCormack’s scheme on collocated mesh in the FVM framework. The slip length model is adopted to describe the behavior of the slip velocities in the streamwise and spanwise directions at the interface between the hydrophobic wall and turbulent channel flow. Simulation results are presented by analyzing flow behaviors over hydrophobic wall with the Smagorinky subgrid-scale model and a dynamic model on computational meshes of different resolutions. Comparison and analysis are made on the distributions of time-averaged velocity, velocity fluctuations, Reynolds stress as well as the skin-friction drag. Excellent agreement between the present study and previous results demonstrates the accuracy of the simple classical second-order scheme in representing turbulent vertox near hydrophobic wall. In addition, the relation of drag reduction efficiency versus time-averaged slip velocity is established. It is also found that the decrease of velocity gradient in the close wall region is responsible for the drag reduction. Considering its advantages of high calculation precision and efficiency, the present method has good prospect in its application to practical projects.

Study on the Fundamental Flow Characteristics of Synthetic Jets (1st Report : Behavior of Free Synthetic Jets)

Jet flows have been applied in a number of fields in order to control flow separation. Over the last decade, several studies have been carried out on the production of synthetic jets. However, there are still a large number of unknown quantities, including details of the structure and the formation mechanism of such jets. The present study attempts to clarify some of the fundamental flow characteristics of free synthetic jets, based on experiments and numerical simulations. Experimental velocity measurements and flow visualizations are performed using the hot-wire anemometer and the smoke wire method, respectively. It is found that both the temporal change in flow pattern and time-averaged velocity distribution at the centerline depend on K=ReU/S2 (the ratio of the Reynolds number to the square of the Stokes number). The unsteady downstream flow characteristics are discussed, in addition to the relation between the formative point of the synthetic jet and the value of K. Furthermore, the flow pattern and the unsteady flow characteristics of the synthetic jet are compared with those of a continuous jet.

Measurement of shear rates in a laboratory tumbling mill

A methodology for characterising shear rate profiles in tumbling mills based on in situ trajectory fields from Positron Emission Particle Tracking (PEPT) experiments is presented. Experiments were carried out in a laboratory scale mill fitted with lifter bars, pulp lifters and a discharge grate, and run with radioactively labelled particles within re-circulating slurry environments. Using time-averaged velocity distributions of small PEPT tracers, a model for tangential velocity profiles along radial scan-lines is proposed from which the shear rates are derived.

Zonal Embedded Grids for LES of Self-Sustaining Structures of Wall Turbulence

In this study, a zonal grid refinement method based on the solution of weakly compressible flow equations with large eddy simulation in the finite volume method (FVM) and dynamic subgrid-scale model framework was used to simulate the turbulence structures of a plane-wall flow with Reynolds number Re = U 0 δ/ ν = 10 000. Comparison and analysis were conducted on the distributions of time-averaged velocity and velocity fluctuations, as well as on the actual near-wall structures, to verify the accuracy of the turbulence features that were simulated using the zonal grid refinement method. The results show that the zonal embedded grid is a very efficient way to illustrate the turbulence structures of the near-wall region. The present method is easy to implement and extend to small Mach number flow modeling on a collocated irregular FVM mesh system.

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

Yaw Angle Effect on Flow Structure over the Nonslender Diamond Wing

Effects of residue background events in direct dark matter detection experiments on the reconstruction of the velocity distribution function of halo WIMPs

In our earlier work on the development of a model-independent data analysis method for reconstructing the (moments of the) time-averaged one-dimensional velocity distribution function of Weakly Interacting Massive Particles (WIMPs) by using measured recoil energies from direct Dark Matter detection experiments directly, it was assumed that the analyzed data sets are background-free, i.e., all events are WIMP signals. In this article, as a more realistic study, we take into account a fraction of possible residue background events, which pass all discrimination criteria and then mix with other real WIMP-induced events in our data sets. Our simulations show that, for the reconstruction of the one-dimensional WIMP velocity distribution, the maximal acceptable fraction of residue background events in the analyzed data set(s) of \U0001d4aa(500) total events is ∼ 10%–20%. For a WIMP mass of 50 GeV with a negligible uncertainty and 20% residue background events, the deviation of the reconstructed velocity distribution would in principle be ∼ 7.5% with a statistical uncertainty of ∼ 18% ( ∼ 19% for a background-free data set).

Dynamic analysis of bubble-driven liquid flows using time-resolved particle image velocimetry and proper orthogonal decomposition techniques

AbstractAn experimental study to evaluate dynamic structures of flow motion and turbulence characteristics in bubble-driven water flow in a rectangular tank with a varying flow rate of compressed air is conducted. Liquid flow fields are measured by time-resolved particle image velocimetry (PIV) with fluorescent tracer particles to eliminate diffused reflections, and by an image intensifier to acquire enhanced clean particle images. By proper orthogonal decomposition (POD) analysis, the energy distributions of spatial and temporal modes are acquired. Time-averaged velocity and turbulent kinetic energy distributions are varied with the air flow rates. With increasing Reynolds number, bubble-induced turbulent motion becomes dominant rather than the recirculating flow near the side wall. Detailed spatial structures and the unsteady behavior of dominant dynamic modes associated with turbulent kinetic energy distributions are addressed.Graphical AbstractGraphical Abstract text[IMAGE]

Review of Experimental Studies on Swirling Flow in the Circular Tube using PIV Technique

The study of swirling flow is of technical and scientific interest because it has an internal recirculation field, and its tangential velocity is related to the curvature of streamline. The fluid flow for tubes and elbow of heat exchangers has been studied largely through experiments and numerical methods, but studies about swirling flow have been insufficient. Using the particle image velocimetry(PTV) method, this study found the time averaged velocity distribution with swirl and without swirl along longitude sections and the results appear to be physically reasonable. In addition, streamwise mean velocity distribution was compares with that of other. Furthermore, other experimental investigation was performed to study the characteristics of turbulent water flow in a horizontal circular tube by using liquid crystal. 2D PIV technique is employed for velocity measurement and liquid crystal is used for heat transfer experiments in water. Temperature visualization was made quantitatively by calibrating the colour of the liquid crystal versus temperature using various approaches.

Characteristics of Loose Rough Boundary Streams at Near-Threshold

Laboratory experimental results on the turbulent flow characteristics, measured by an acoustic Doppler velocimeter, over loose rough boundaries at the near-threshold of motion are reported. The primary endeavor was to investigate the response of the turbulent flow field, having zero-pressure gradient, to the uniform gravel beds at the near-threshold of motion with reasonably wide range of relative roughness ε∕h (0.02<ε∕h<0.15; where ε=Nikuradse’s equivalent roughness and h=flow depth). Five uniform gravel sizes, ranging from 4.1to14.25mm, were used in the experiments. The vertical distributions of time-averaged velocity, turbulence intensities and the Reynolds stress were measured and analyzed. The variation of the mixing-length is considerably linear with the depth within the inner layer, whose thickness is 0.23 times the boundary layer thickness δ, and the von Kármán constant is obtained as 0.35. The boundary shear stresses were determined by three independent methods, such as using the logarithmic law (the Clauser method), Reynolds stress profiles and bed slope. It is observed that in the inner layer, the logarithmic law of wall for the time-averaged streamwise velocity holds with the von Kármán constant 0.35 and a constant of integration 7.8; whereas in the outer layer, the law of the wake defines the velocity profiles with average value of the Coles’ wake parameter 0.11. Alternatively, velocity profiles based on 1∕nth power law are also put forward, where n is expressed as a function of ε∕δ. The equation of the friction factor f is obtained from the depth averaging of the velocity profile; and the collapse of the estimated f and the measured data (experimental and field) is reasonable. The turbulence intensities being non-isotropic can be defined by an exponential law; whereas the Reynolds stress varies almost linearly with the depth. The values of the Shields parameter for the threshold of gravel motion obtained experimentally correspond closely with the curve obtained from the modified model of Dey considering the present logarithmic law of velocity. For all the aforementioned parameters, thorough statistical and error analyses of the experimental data are done to ascertain the adequacy of the proposed equations.

複断面蛇行水路の水平渦構造に関する実験的研究

It is very important to reveal the horizontal eddy strucutre in meandering compound open-channel flows for predicting the transport properties of suspended sediments in rivers in flood. However, flow-visualization techniques such as PIV system, have not sufficiently been applied to such flows in previous studies, and thus, there is almost no detailed information on coherent turbulence such as the horizontal eddies in meandering compound open-channel flows. Therefore, in this study, we conducted PIV measurements with a high-speed CMOS camera. The time-averaged velocity distributions and coherent turbulence structure were investigated and considered on the basis of the present measured data.

321 フラッタを用いた噴流加振に関する研究(2)(OS3-5 噴流,後流,せん断流の構造・普遍的特性(噴流3),OS3噴流,後流,せん断流の構造・普遍的特性)

In the present paper, a new type of oscillatory device is proposed. This device is excited by one-degree-of-freedom flutter which does not occur in the uniform flow. As for the characteristics of oscillation, the dependence of pitching amplitude and Strouhal number(St) was measured vs. Reynolds number(Re) on L/c. As for the characteristics of mixing, the time-averaged velocity distributions were measured, and the effect of mixing enhancement was evaluated from the extension of the half-velocity width and the decay of maximum mean velocity. Oscillation characteristics shows that St was almost constant in Re>1.0×10^5. Pitching amplitude was constant when flow velocity is the critical flutter speed. Mixing characteristics shows, the half-velocity width and the decay of maximum mean velocity were found to be greater for the oscillating flat plate than for the without flat plate.

Numerical Simulation of Flow Field in High Reynolds Number Baffled Stirred Tank (Re=45000) and Experimental Verification

Unsteady flow in a baffled stirred tank of which Reynolds number based on the tip velocity and the radius of tank is 45000 has been simulated by solving numerically Navier-Stokes and continuity equations. LES model were used for sub-grid scale flows. Calculated results show unsteady flow fields in the whole region of the tank during about 16 complete impeller revolutions. As the results we present the occurrence and the disruption of large vortices, and the generation of many small eddies which are nearly homogeneous. The calculated visualized flow field, time-averaged velocity and turbulent intensity distributions are fairly in good agreement with the experimental data. It indicates that the present simulation procedure and results are applicable to describe the flow field in the real high Reynolds number baffled stirred tank. The calculated results also show the role of the baffle plate which blocks the tangential flow and induces the complicated flows that increase the stirring effect. The present results elucidate the unsteady flow mechanism in a high Reynolds number baffled stirred tank.

A213 マイクロフローメータの流体特性とPIV計測

A micro flow meter for measuring small liquid flow rate is fabricated by using 3-D microstereolithography. The flow meter consists of an impeller, 500μm in radius, and a casing. The micro system is installed in a PDMS square channel, 500μm in width, through which the liquid to be measured flows. The flow rate is measured by counting the rate of rotations of the impeller. The flow characteristics are studied by visualization and measurement based on PIV technique. It is shown that there is a good linearity between the flow rate and the rate of rotations counted in the range of 5000-25000μL/min. The dynamic response of the present flow meter is evaluated to be about 0.25s, which is short enough to make measurement of unsteady flows. Time-averaged velocity distribution inside the micro flow meter is obtained.

Measurement of LBE flow velocity profile by UDVP

Measurements of liquid metal lead–bismuth eutectic (LBE), flow velocity profile were realized in the spallation neutron source target model by the ultrasonic Doppler velocity profiler (UVDP) technique. So far, it has not been done well, because both of poor wetting property of LBE with stainless steels and poor performance of supersonic probes at high temperatures. The measurement was made for a return flow in the target model, which has coaxially arranged annular and tube channels, in the JAEA Lead Bismuth Loop-2 (JLBL-2). The surface treatment of LBE container was examined. It was found that the solder coating was effective to enhance an intensity of reflected ultrasonic wave. This treatment has been applied to the LBE loop, which was operated up to 150 °C. The electro magnetic pump generates LBE flow and the flow rate was measured by the electro magnetic flow meter. By changing the flow rate of LBE, velocity profiles in the target were measured. It was confirmed that the maximum velocity in the time-averaged velocity distribution on the target axis was proportional to the flow rate measured by the electro magnetic flow meter.

Experimental Validation of the Addition Principle for Pulsating Flow in Close-Coupled Catalyst Manifolds

Designing an exhaust manifold with close-coupled catalyst (CCC) relies heavily on time-consuming transient computional fluid dynamics. The current paper provides experimental validation of the addition principle for pulsating flow in CCC manifolds. The addition principle states that the time-averaged catalyst velocity distribution in pulsating flow equals a linear combination of velocity distributions obtained for steady flow through each of the exhaust runners. A charged motored engine flow rig provides cold pulsating flow in the exhaust manifold featuring blow down and displacement phases, typical of fired engine conditions. Oscillating hot-wire anemometry is used to measure the bidirectional velocity, with a maximum measurable negative velocity of −1m∕s. In part load and zero load conditions, instantaneous reverse flow occurs following the blow-down phase. The two-stage nature of the exhaust stroke combined with strong Helmholtz resonances results in strong fluctuations of the time-resolved mean catalyst velocity. The validity of the addition principle is quantified based on the shape and magnitude similarity between steady and pulsating flow distributions. Appropriate nondimensional groups are used to characterize the flow and quantify the similarity. Statistical significances are provided for the addition principle’s validity. The addition principle is valid when the nondimensional scavenging number S exceeds a critical value Scrit, corresponding to cases of low engine speed and/or high flow rate. This study suggests that the CCC manifold efficiency with respect to catalyst flow uniformity could be quantified using a single scalar parameter, i.e., Scrit. The results from the current study are discussed with respect to previously reported results. The combined results are in good agreement and provide a thorough statistically founded experimental validation of the addition principle, based on a broad applicability range.

Vortex generation in the cross-flow around a cylinder attached to an end-wall

A cylinder attached to an end-wall normal to its axis is a common feature of many practical flow systems, e.g. in turbo-machinery or when a bridge is supported by a pillar from the bed of a river. In this situation, the nominally two-dimensional boundary layer flow incident upon the cylinder develops strong three-dimensional features and a very pronounced vortex structure may arise in the upstream flow close to the wall. For the appropriate Reynolds number range, the upstream vortical structure is nominally steady and is commonly referred to as the “horseshoe vortex system”. In contrast, the flow downstream is unsteady and periodic over a wide range of Reynolds numbers and vortices aligned with the cylinder axis are shed at a regular frequency into the wake. The generation of both these vortex systems requires energy to be extracted from the incident flow with the result that the drag force on the cylinder is increased. This paper concentrates on the upstream region of the cylinder and discusses an investigation in which two-component Particle Image Velocimetry (PIV) has been used to visualise the flow behaviour for a circular cylinder on a plane end-wall. The use of PIV has enabled two orthogonal velocity components to be measured in planes defined by the upstream flow direction and the axis of the cylinder. The third (out-of-plane) velocity component was then calculated by integrating the continuity equation. Subsequently, the velocity field information has been manipulated and converted into time-averaged information. Discussion of the measured results confirms that colour displays are an invaluable aid to understanding this complex fluid flow situation since they reveal substantially more information than grey-scale plots of the same data. In particular, the source of the horseshoe vortex system can be identified when colour plots of the time-averaged velocity and vorticity distributions are obtained. A limited amount of information on the unsteady vortex structures appearing in the end-wall region upstream of the cylinder is also presented. Finally, the experimental findings are discussed in relation to the results of previous workers.

C109 異なる砂丘形状周りの流れの可視化

The study of the formation and migration of sand dunes has a long history, but a sound understanding of the turbulent mechanisms behind dune has proved elusive. In this paper, in order to reveal the detailed vortical structures of different dunes, PIV measurements were performed in a circulating water channel at a constant Reynolds number of 2809. Instantaneous and time-averaged velocity distribution in the side and top planes were obtained and the complex reverse flows and eddies behind different dunes were examined.