Results Of PIV Measurements Research Articles

Role of Partial Flexibility on Flow Evolution and Aerodynamic Power Efficiency over a Turbine Blade Airfoil

In this study, the aerodynamic performance of a cambered wind turbine airfoil with a partially flexible membrane material on its suction surface was examined experimentally across various angles of attack and Reynolds numbers. It encompassed physical explanation at the pre/post-stall regions. The results of particle image velocimetry revealed that the laminar separation bubble was diminished or even suppressed when a local flexible membrane material was employed on the suction surface of the wind turbine blade close to the leading edge. The results of the deformation measurement indicated that the membrane had a range of flow modes. This showed that the distribution of aerodynamic fluctuations due to the presence of LSB-induced vortices was reduced. This also led to a narrower wake region occurring. Aerodynamic performance improved and aerodynamic vibration significantly lowered, particularly at the post-stall zone, according to the results of the aerodynamic force measurement. In addition to the lift force, the drag force was enormously reduced, corroborating and matching well with the results of PIV and deformation measurements. Consequently, significant benefits for a turbine blade were notably observed, including aerodynamic performance enhancement, increased aerodynamic power efficiency, and reduced aerodynamic vibration.

PIV Measurements of Turbulent Airflow in a Channel with Trapezoidal Ribs on One Wall

This paper presents the results of PIV measurements of turbulent airflow in a channel with a single ribbed wall. The rough elements had the form of evenly spaced trapezoids. The ratio of the height of a single rib to the height of the measurement channel was 0.1. The ratio of the distance between adjacent ribs to the height of a single rib (i.e., the w/k ratio) was 5, which suggests a transitional region within the airflow structure between the d- and k- types. The measurements were carried out for the inlet velocity, at which the Reynolds number (Re) was equal to 32,500. The distance to the first rib, at which the turbulent velocity profile is fully developed, was determined. The distribution of the average velocity components, the velocity fluctuation components, turbulent shear stress, as well as the turbulence kinetic energy between two adjacent ribs was presented. The analysis of average streamlines indicated the existence of two vortex structures between the ribs.

High optical quality models for flow visualization and PIV measurement

Visualization of internal flow or PIV measurements in areas of real pipe elements is a method commonly used in flow studies. For these applications, transparent models with very complex internal geometries are required. Key features of the model are the high-quality optical access, which ideally allows to obtain image of the whole measured area without deformation and with noise suppression. The technology of 3d printing in combination with casting allows the production of almost any type of transparent models of piping elements. Models produced using this technique have almost ideal optical properties. By using index-matching fluid, optical distortions as well as reflections and refractions of light on the inner edges of the model are suppressed. The paper presents the results of visualization and 2D PIV measurements in the model of the globe valve with adjustable valve plug. The valve’s inner geometry was rapid prototyped using a water-soluble material and casted with clear silicone Sylgard 184 under vacuum. The model exhibits excellent optical properties for PIV measurements, especially in the vicinity of walls.

Inner aerodynamics of the improved four-vortex furnace model

The internal aerodynamics of a perspective vortex furnace chamber of a pulverized coal boiler with a diagonal arrangement of burners is studied using the non-contact optical method of flow diagnostics: Particle Image Velocimetry - PIV. The results of PIV measurements, characterizing the complex spatial structure of a swirled flow in an isothermal laboratory model of the furnace device, are presented. The velocity distribution in the vortex chamber volume is obtained, and the flow structure in the form of four conjugate closed vortices with curved axes is visualized.

Investigation on Oxygenation Performance and Numerical Simulation of Swirling Aerator

The oxygenation performance of the swirling aerator was evaluated by an air-water system oxygenation experiment. The flow pattern analysis on the oxygenation process of the swirling aerator was carried by PIV measurement and numerical simulation. The test results showed that the oxygen transfer coefficient (KLa) increased with the increase of intake air volume, and the maximum value of KLa (20°C) was 0.721L/min under 8m3/h condition. PIV measurement results showed that the number of tiny bubbles in the water increased with the increase of intake air volume and the bubbles form vortex in the pool with the water flow, which enhanced the mass transfer effect. The maximum number of tiny bubbles appeared at 8m3/h. The results of numerical simulation showed that due to the difference in speed, obvious vortex phenomenon occurs. With the increase of intake air volume, the intensity and quantity of vortex increase, the air-water mixing was more sufficient, and the air volume fraction was higher.So the oxygenation performance was stronger, which was consistent with the PIV measurement and oxygenation test results.

Considerable drag reduction and fuel saving of a tractor–trailer using additive aerodynamic devices

The drag reduction of commercial vehicles is the principal challenge to improving fuel saving and decreasing air pollution. Accordingly, several flow control devices, including gap fairing, cab roof fairing, boat tail, and side skirt were introduced to reduce the aerodynamic drag exerted on heavy vehicles. Although such devices have exhibited good drag reduction performance, their aerodynamic performance can be improved further. Moreover, a comprehensive and systematic research is required, including drag force measurement, flow field analysis and real-scale proving ground test, to evaluate the fuel consumption of heavy vehicles. In this study, the drag reduction effect of the aero full package (AFP) of a tractor–trailer, which integrated gap fairing, flap-type side skirt and lower inclined air deflector boat tail, was investigated through wind tunnel experiments and proving ground test. On the basis of the results of drag and PIV measurement, the integrated AFP was found to significantly modify the flow structure around the vehicle, thereby leading to a 26.5% reduction in drag coefficient compared with the reference tractor-trailer model (CD = 0.693). Furthermore, the proving ground test using real tractor–trailer verified that the integrated AFP provides 13.4% fuel saving. The present results will provide practical information for improving the aerodynamic performance and fuel efficiency of heavy vehicles.

Effect of boost pressure on the in-cylinder tumble- motion of GDI engine under steady-state conditions using Stereoscopic-PIV

This paper experimentally investigates the effect of boost pressure on the in-cylinder flow field under steadystate conditions using stereoscopic particle image velocimetry (Stereo-PIV) through increasing the pressure difference across the intake valves. The FEV steady-state flow bench was modified to provide an optical access into the cylinder region. The stereoscopic PIV measurements were carried out at various pressure differences viz., 300, 450, and 600 mmH2O across the intake valves of Gasoline Direct Injection (GDI) head for the mid cylinder vertical tumble-plane. Ensemble average velocity vectors were used to characterize the tumble flow structure and for the calculation of tumble ratio and average turbulent kinetic energy. Moreover, the Proper Orthogonal Decomposition (POD) technique was conducted on the PIV measured velocity vector maps to identify the most energetic structures generated at different valve lifts and pressure differences. The results of stereoscopic PIV measurements showed that the overall in-cylinder flow structures were mainly dependent on the valve lift irrespective of the applied pressure difference. However, the level of the turbulence kinetic energy increased as the boost pressure increased. © 2018, Isfahan University of Technology.

Improved version of LeMoS hybrid model for ambiguous grid densities

Application of the LeMoS hybrid (LH) URANS/LES method for the wake parameters prediction is considered. The wake fraction coefficient is calculated for inland ship model M1926 under shallow water conditions and compared to results of PIV measurements. It was shown that due to lack of the resolved turbulence at the interface between LES and RANS zones the artificial grid induced separations can occur. In order to overcome this drawback, a shielding function is introduced into LH model. The new version of the model is compared to the original one, RANS k−ω SST and SST-IDDES models. It is demonstrated that the proposed modification is robust and capable of wake prediction with satisfactory accuracy.

Evolution of Rayleigh streaming flow velocity components in a resonant waveguide at high acoustic levels

The interaction between an acoustic wave and a solid wall generates a mean steady flow called Rayleigh streaming, generally assumed to be second order in a Mach number expansion. This flow is well known in the case of a stationary plane wave at low amplitude: it has a half-wavelength spatial periodicity and the maximum axial streaming velocity is a quadratic function of the acoustic velocity amplitude at the antinode. For higher acoustic levels, additional streaming cells have been observed. In the present study, results of LDV and PIV measurements are compared to direct numerical simulations. The evolution of axial and radial velocity components for both acoustic and streaming flows is studied from low to high acoustic amplitudes. Two streaming flow regimes are pointed out, the axial streaming dependency upon acoustics going from quadratic to linear. The hypothesis of the radial streaming velocity being of second order in a Mach number expansion is shown to be invalid at high amplitudes. The change of regime occurs when the radial streaming velocity amplitude becomes larger than the radial acoustic velocity amplitude, high levels being therefore characterized by nonlinear interaction of the different velocity components.

유동분배판에 의한 원통-다관형 열교환기의 성능 특성에 관한 수치해석적 연구(II): 전열특성

The flow pattern inside the inlet chamber of the tube side is one of the key parameters influencing on the performances of the shell-and-tube type of heat exchangers(STHE). In order to improve the flow distribution, the baffle shaped as the porous plate is installed in the inlet chambers. In the present study, numerical simulation has been performed to investigate the flow features of the tube side of the STHE in sense of the hydraulic performances. The flow fields have been analysed by the three-dimensional Navier-Stokes solvers with the proper turbulent models. Computational domain is ranged in the whole of the tube side of the STHE. The numerical results showed that the presence of the baffles improves the redistribution of the flow injecting to the tube bundels. The good agreements of the numerical results with the experimental results of PIV measurements have been shown for the validation of the numerical methods adopted in the present papers.

유동분배판에 의한 원통 다관형 열교환기의 성능 특성에 관한 수치해석적 연구(I): 유동특성

The flow pattern inside the inlet chamber of the tube side is one of the key parameters influencing on the performances of the shell-and-tube type of heat exchangers(STHE). In order to improve the flow distribution, the baffle shaped as the porous plate is installed in the inlet chambers. In the present study, numerical simulation has been performed to investigate the flow features of the tube side of the STHE in sense of the hydraulic performances. The flow fields have been analysed by the three-dimensional Navier-Stokes solvers with the proper turbulent models. Computational domain is ranged in the whole of the tube side of the STHE. The numerical results showed that the presence of the baffles improves the redistribution of the flow injecting to the tube bundels. The good agreements of the numerical results with the experimental results of PIV measurements have been shown for the validation of the numerical methods adopted in the present papers.

Electrohydrodynamic secondary flow and particle collection efficiency in spike-plate multi-electrode electrostatic precipitator

In this work the results of electrohydrodynamic (EHD) secondary flow measurements in a spike-plate type electrostatic precipitator (ESP) are presented. In the investigated ESP two-sided and one-sided spike electrodes were used as a discharge electrodes. The results of 2D PIV measurements showed that flow pattern obtained for a different spike tips positions in respect to the primary flow direction significantly changing the flow structures in the ESP duct. The submicron dust particles collection measurements in this ESP were also performed. The obtained results showed that the collection efficiency of submicron dust particles depends on the generated EHD secondary flow.

Wake Vortex Control Using Modified Flaps

The present paper is on the experimental investigation of a potentially effective method to alleviate the strength of aircraft wake vortex. The research work was carried out in a water-towing tank which equipped with a 2-D PIV system. By using modified flaps attached to the baseline-airfoil, the Rayleigh-Ludwieg instability of the wake vortex was activated, resulting in a premature dissipation of the wake vortex. The experimental results of PIV measurement exhibited that the circulation of the wake vortex was reduced to over 50% for the optimal case at 45 wingspans downstream with suitable settings, which proved the effectiveness of the modified-flap treatment in alleviating the wake vortex under labor conditions.

Validation of a lattice Boltzmann method implementation for a 3D transient fluid flow in an intracranial aneurysm geometry

The unsteady fluid flow inside the complex geometry of an intracranial aneurysm during a cardiac cycle was simulated. The results obtained by several versions of a lattice Boltzmann based code were compared with the results of PIV and LDA measurements and with those of a commercial finite volume solver. The best version from the tested lattice Boltzmann codes was chosen for further comparisons and grid convergence measurement. It was shown that the lattice Boltzmann method is capable of producing results with an accuracy comparable to the finite volume solver in a 3D unsteady flow and also that the results are in good agreement with the experimental results. It was also shown that using a GPU implementation of the lattice Boltzmann method it is possible to significantly reduce the run-time length compared to the CPU only implementation of the lattice Boltzmann method.

Evaluation of Artificial Caudal Fin for Fish Robot with Two Joints by Using Three-Dimensional Fluid-Structure Simulation

A fish robot with image sensors is useful to research for underwater creatures such as fish. However, the propulsion velocity of a fish robot is very slow compared with live fish. It is necessary to swim at a speed several times faster than the speed of the current robots for various usages. Therefore, we are searching for the method of making the robot swim fast. The simulation before making the robot is important. We have made the computational simulation program of three-dimensional fluid-structure analysis. The flow around the caudal fin can be examined by analyzing the fin as an elastic body. We compared the results of numerical analysis with the results of PIV measurement. Both were agreed well. Because the performance of a fish robot with two joints is better than that of a fish robot with one joint, we searched for an excellent fin for the fish robot with two joints by using CFD. We confirmed that the swimming performance of a fish robot becomes very good when the caudal fin is rigid except for the root of the fin which is comparatively flexible.

Passive control of flow structures around a circular cylinder by using screen

The results of PIV measurements are presented in this paper for the water flow downstream of a circular cylinder surrounded with a screen (meshed outer control element) which had a streamlined shape. The diameter of cylinder, D was 50mm and the Reynolds number based on the cylinder diameter was 5200. The characteristic length of the control element, L was tested for different cases so that the values of L/D were 2, 2.4, 2.8 and 3.2 in the experiments. It was noted that a forced reattachment of the shear layers separated from the cylinder was achieved by setting up the screen around the cylinder. As a consequence, the formation of vortical flow pattern was suppressed and turbulence statistics of the flow such as the intensity of turbulence, Reynolds shear stress, and turbulent kinetic energy were drastically diminished in comparison to the bare cylinder case. It was also found that the variable parameter of L/D did not dominantly influence the characteristics of the distribution of turbulence statistics along streamwise and transverse directions, however, the turbulence level decreased slightly for a smaller case of L/D.

G404 流量配分制御を行う円環型流体素子内流れのPIV計測(GS4 閉空間内の流れ,一般セッション)

Pressure loss and flow distribution control performance of an annular fluidic diverter is reported. The annular fluidic diverter is intended to be used for air flow control in an air blast fuel nozzle for jet engine combustor. A liquid flow test was conducted. PIV measurement inside the fluidic diverter with index matching method was performed. The flow distribution was calculated from the results of PIV measurement. The results shows that increasing the pressure loss at downstream of fluidic diverter, the flow distribution control capability became poor.

Similarity Solutions of Jet Development Mixing Layers Using Algebraic and 1-Equation Turbulence Models

Mixing layers are formed between two parallel fluid streams having different velocities. One of the velocities may be zero, as is the usual case of the mixing layer that surrounds, immediately downstream from the nozzle, the core of a developing jet issuing into stagnant surroundings. Earlier – but so far not properly published – experimental evidence shows a remarkably weak effect of transversal curvature, making the present solution applicable with acceptable precision to description of developing round jets. This paper presents solutions of a planar mixing layer by a similarity transformation, which reduces the problem to solving ordinary differential equations. Two solutions are investigated: one based on an algebraic model and the other using the 1-equation model of turbulence. They are compared with recent results of PIV measurements of a developing jet.

Stereoscopic PIV measurements of flow in and around axial flow fan

To analyze the complex three-dimensional flow structure of an axial flow fan and determine the validity of its application, PIV is used to provide detailed space and time resolved experimental data for understanding and control of flow field. The high resolution stereoscopic PIV system was successfully employed in this study for the investigation of flow structure around the axial flow fan. Using the once-per-revolution signal from the rotor, image fields were captured at a fixed position of the blades and hence provides the ability to do phase-averaging. The three-dimensional instantaneous velocity fields, phase-averaged velocity fields, instantaneous and mean vorticity distributions of the stereoscopic PIV measurement results were represented at typical planes of the flow field. Phase-averaged velocity fields were calculated based on 200 frames of the instantaneous stereoscopic PIV measurement results. From the velocity distribution, the vorticity and turbulent intensity distribution, which are known to be major factors of fan noise, were calculated and its diffusion was discussed as they travel downstream. From the reconstructed three-dimensional velocity iso-surface at 8 cross planes of the outlet flow fields, the three-dimensional features can be seen clearly.

PIVによるマイクロチャネル内電気浸透流の可視化計測

The micro PIV system has been developed as a new diagnostic tool for microscale flow and applied to the measurement of microchannel flow in order to investigate the properties of electroosmotic flow. The field of view of the PIV system is 580 μm×580μm and velocity vector fields can be measured at 9.3μm vector-to-vector intervals using two-frame cross-correlation method with 32×32 pixels interrogation spot. In this study, two different types of microchips, which are developed for genetic analyses, are employed : PDMS (polymethylmethacrylate) microchip and PMMA (polydimethylsiloxane) microchip. The size of flow channel of each chip is 100 μm×30 μm. The electroosmotic flow shows flat velocity profiles independently of electric field intensity except near the side wall. The, effects of ζ-potential of tracer particles and channels on electroosmotic flow have been evaluated based on the results of ζ-potential measurements and PIV measurements.