جستجوی مقالات مرتبط با کلیدواژه « piv » در نشریات گروه « مکانیک »

In this study, experimental and numerical flow analysis was performed on three different blade profiles with a chord length of 165 mm using passive flow control method. The first of the airfoil is the standard NACA 0018 profile. The second airfoil type has a NACA 0018 profile with a gap in the suction surface. The last airfoil is the NACA 0018 profile which is 66% of the trailing edge cut from the chord length. All airfoil profiles were analyzed at the Reynolds number, Re=2x104, and angles of attack α=0o, 5o, 10o, 12o and 14o in both experiment and numerical studies. The experiments were carried out using the Particle Image Velocimetry (PIV) method in a closed-loop open water channel, and the time-averaged velocity vectors, streamlines, and vorticity contours of the flow field were obtained. Subsequently, numerical analyses were performed using the ANSYS Fluent package program, one of the Computational Fluid Dynamics (CFD) programs used frequently in the literature. The streamlines and pressure contours of the airfoil profiles have been compared visually at the same Re and different angles of attack. In addition, according to the angle of attack of the airfoil profiles, lift coefficient CL, drag coefficient CD, and the ratio of lift coefficient to drag coefficient CL/CD graphs were presented. It has been shown that the gap on the airfoil at high attack angles caused changes in lift (up to 0.7) and drag (up to 0.15). These features can allow these models to be used for different purposes in the aerodynamics field.

Particle image velocimetry is used to study the variation around single- and double-hemisphere rough elements with different streamwise spacings immersed in the boundary layer. Instantaneous velocity field information in the streamwise–normal and streamwise–spanwise directions is collected at a Reynolds number of 1800. 3R, 5R, and 7R are determined as the rough element spacing used in the double-hemisphere rough element experiment, representing the smaller, medium transition, and larger rough element spacing, respectively. The average velocity, Reynolds shear stress, shedding frequency, and proper orthogonal decomposition results of the flow field around the rough elements under various working conditions were compared. The downstream hemisphere will encroach on the streamline following the upstream hemisphere, and changing the spacing means changing the position of the encroached area. When the spacing is smaller, the streamline reattachment is destroyed, and the momentum and mass exchange between the two-hemisphere rough elements decreases. The double-hemisphere rough element is a slender, blunt, rough element. At the medium transition spacing, the shear layer and vortex structure shed from the upstream hemisphere are over the downstream hemisphere, and the double-hemisphere rough elements will cause disturbance in a larger wall-normal range. The streamline has been reattached at the larger streamwise spacing, and the interaction between the two hemispheres is the weakest. Here, the double-hemisphere rough element will form the recirculation zone, recirculation arch vortex, and periodic hairpin vortex.

In this study, the flow field of a new secondary-grooved cylinder is determined by using particle image velocimetry (PIV) to understand the wake characteristics at different depths of the secondary grooved cylinders. In order to analyze the wake characteristic behind the secondary grooved cylinder with different depths, time-averaged streamlines, time-averaged velocity, RMS velocity, Reynolds stress, turbulent kinetic energy, and instantaneous flow structures are employed. The recirculation zone for secondary grooved cylinders with different depths decreases when compared to the smooth cylinder; the peak magnitudes of flow velocity fluctuation intensity and transverse velocity fluctuation intensity for secondary grooved cylinders with different depths are reduced and the locations appear delayed. Furthermore, in contrast to the smooth cylinder, the secondary grooved cylinders with different depths' Reynolds stress and turbulent kinetic energy are increased by the wake flow, and the transient large-scale vortex is split into several smaller-scale vortices behind the secondary grooved cylinders. The results obtained for the above flow structure are more significant when h/D = 0.05 for the secondary grooved cylinder.

Surface structure is used to interfere with the turbulent boundary layer in the groove drag reduction, which is important to the endurance and stability of high-speed and ultrahigh-speed aircraft. The size of the groove structure directly affects the flow in the turbulent boundary layer and changes the drag reduction effect. The drag reduction characteristics of bionic triangular (V-groove) riblets were studied through Particle Image Velocimetry (PIV) experiment and Finite Volume Method (FVM) simulation. Triangular riblets with adjacent height ratios (AHR) of 1.00, 1.74, and 3.02 were considered in this research, and the influence of these groove structures on the flow characteristics of turbulence near the wall is compared with those of the smooth plates. The distribution of time-averaged velocity, turbulence intensity, and coherent structures of turbulent boundary layer on the riblet surface is analyzed to document the effects of the geometric parameters of various groove structures on drag reduction rates. Results showed that the best drag reduction is obtained using the V-groove riblets with adjacent height ratio of 1:1 under the low free-stream velocity. The results can be used as a reference for further optimization of drag reduction structures with surface grooves.

A simple model consisting of a mirror-housing and its cylindrical foot is applied to represent the automobile side-view mirror that causes unwanted aerodynamic noise and wind drag during high-speed driving. An additional slot is made on the solid foot to modify the flow around the mirror and thus reduce the side wall pressure fluctuation and aerodynamic drag. Flow fields and wall pressure fluctuations of these side-view mirror models have been investigated experimentally in a wind tunnel. The airflow rate through the slot varies with the changing of the slot area. Wall surface pressure sensors, particle image velocimetry (PIV), and six-component balance were applied to measure the acoustic and flow characteristics. The results demonstrated that, with the increase of slot airflow rate to 30%, the side wall pressure fluctuations were reduced by 5.1 dB and the drag coefficient decreased by 10.2%. The PIV measurements showed that the vortex cluster center behind the mirror was moved upward from the wall surface due to the slot airflow injection into the wake. The turbulent kinetic energy in the side-view mirror wake near the wall decreased with the increment of the airflow rate, reducing the side wall pressure fluctuations and thereby suppressing the noise generation.

Particle imaging velocimetry (PIV) was used to study the near-field variation of a pyramid rough element in clear water and a liquid–solid boundary layer (thickness: 60 mm). Particles with an average diameter of 355 µm and Stokes number of 4.3 were injected into a 1:1000 mass ratio (solid particles: water) liquid–solid two-phase solution. Experiments were conducted to collect instantaneous velocity field information in the streamwise–normal direction and streamwise–spanwise direction at a Reynolds number of 8350. Then, the average velocity field and turbulence intensity of the rough element wake under single-phase and two-phase conditions were compared, and the morphology and periodicity of the shedding structure were analyzed by using proper orthogonal decomposition (POD) combined with the power spectral density function (PSD). Particles were shown to have no significant impact on the recirculation area in the streamwise–spanwise plane but did result in a reduction of the recirculation zone in the streamwise–normal plane and a 0.2h closer location of the streamline's origin to the obstacle. Along with the weakening of the upwash structure, the particle phase diminishes the velocity gradient along the span direction and turbulence intensity. Structural shedding at the top of the pyramid and near the wall occurred simultaneously, and the same shedding period was maintained. Particularly, in the first two POD modes, the energy of the shedding structure near the wall was higher than that at the obstacle tip, with a maximum energy differential of approximately 6%. The Strouhal number of the shedding structure decreased by particles from 0.217 to 0.209. The concentration distribution and degree of dispersion in the particle-laden flow illustrate different results, with lower statistics in the wake flow field.

Selective catalytic reduction (SCR) is an emission control method that reduces the NOx emission using urea sprays as ammonia precursors for exhaust after-treatment systems. The urea injection system is an essential component of the SCR systems. A comprehensive SCR modeling approach is required to design compact after-treatment systems that meet the NOx emission legislation level. In this study, the characteristics of urea spray injectors of the SCR system were investigated using computational fluid dynamics (CFD) and the particulate image velocity (PIV) technique. A validation strategy was developed to model the urea spray evaporation, liquid/wall contact, and formation of solid urea deposits. The sheet atomization model was modified to improve the performance of the CFD model. While the Rosin-rammler method predicted the results of 10% according to the experimental results, the proposed tabular method decreased the difference by 3%. In addition, 500 parcels were determined as an optimum number of parcels for urea spray according to the sensitivity study. Therefore, the validation methodology was proposed to predict more consistent results for urea spray modeling and the formation of solid urea deposits.

This study presents the empirical comparison between the wing root and wingtip corrugation patterns of dragonfly wing in the newly-built wind-tunnel at the IAUN. The main objective of the research is to investigate the effect of wingtip and wing root corrugations on aerodynamic forces and the flow physics around the cross-sections at Re=10000 and the angle of attack of 0° to 30°. For this aim, two cross-sections are extracted from wing root (first cross-section) and wingtip (second cross-section). The first cross-section has corrugations with higher density than the second cross-section. The comparison of lift coefficients obtained from pressure distribution and force measurement indicates an acceptable agreement between the results. Also, Particle Image Velocity (PIV) technique is used to measure the velocity field. The results show that all corrugation patterns do not have positive effects on the aerodynamic forces. The second cross-section can generate considerable aerodynamic forces compared to the first cross-section. At α=25°, the lift coefficient generated by the second cross-section is 90% and 25% higher than that of the first cross-section and the flat plate, respectively. Based on results, corrugations in the wing root's vicinity have a crucial role in the solidity of insect wings; however, corrugations in the wing tip's vicinity play a vital role in generating adequate aerodynamic forces. The comparison conducted in the current research reveals the second cross-section is an appropriate replacement for the flat plate in MAVs due to generating more essential forces for flight.

Renal arteries are the arteries that supply blood to the kidneys. Renal arteries are the arteries that supply blood to the kidneys. Renal artery aneurysm (RAA) is the second most common visceral aneurysm to occur, which accounts for 22% of the visceral aneurysm. In general population, RAA rate of occurrence was only 0.1%. However, due to the extensive used of angiography technique, RAA has been discovered more frequently. Some claimed that the previous rate of incidence should be higher now because of the capability of angiography. The rupture of this aneurysm could result in haemorrhage, kidney lost and mortality. The size of the renal artery which is different compared to other types of arteries such as the abdominal aorta could produce different flow condition when the artery is inflicted with RAA condition. Thus, a thorough analysis is desired as RAA studies are very limited compared to other aneurysm conditions. In this study, the efficiency of the stent porosity was investigated in treating the RAA. Fluid-structure interaction (FSI) simulations and particle image velocimetry (PIV) experiments were the approaches taken to investigate the flow patterns of the blood when the stent of different porosities was placed in the aneurysm entrance. The effect of wall shear stress (WSS), the deformation of the artery and von Mises stress were also observed in determining the possibility of aneurysm rupture. The study found that the placement of stent of different porosities succeeds in providing an obstruction to the blood from circulating inside the aneurysm sac. This in turns reduced the WSS experienced by the aneurysm sac up a significant value of 96%. This reduction is crucial in order to prevent the aneurysm from rupture. Moreover, the placement of the stent provided support to the renal artery and preventing it from experiencing buckling failure. The maximum deformation of the artery reduced by 42% with stent was placed in the renal artery. In fact, the von Mises stress decreased below the threshold limit of 0.5 MPa with the presence of the stent. In addition, the study found that the stent of porosity 80% has a similar impact to the stent of lower porosity in the case of RAA at main renal artery.

This paper presents an experimental study of the effect of a directionally porous wing tip on the tip vortex using particle image velocimetry (PIV) on a half wing model with NACA 653218 as its airfoil section. Four different configurations of the directionally porous wing tip are tested. The vortex generated by the wing tips are examined at four different measuring planes downstream perpendicular to the flow axis. The flow field over the porous wing tip surface along the streamwise direction is obtained as well to understand the effects of the porosity on the flow which in the end affects the vortex downstream. Furthermore, the aerodynamic performance of all different configurations is compared to study their effects on the aerodynamic coefficients of the wing. The results show a high reduction in vorticity, up to 90%; tangential velocity reduction up to 67% and a significant reduction in vortex circulation in the near-far field. Effect on the lift to drag ratio is up to 20 %.

This paper reveals the results of a study of vortex air core formation (Rankine vortex) when a rotated liquid (water) column in a cylindrical vessel is drained through two ports located at equal eccentricity (e) at the vessel base (diameter, 𝑑1and 𝑑2) simultaneously; 𝑑1is fixed whereas 𝑑2 is varied. Just before draining, a rotation (n rpm) is provided to the liquid column in controlled conditions. As draining progresses, when the liquid level reaches certain height called critical height (ℎ𝑐 ), initially a surface dip forms which further develops in to a vortex extending down till the drain port. Results show that critical height increases as the fluid rotation rate increases at the lowest eccentricity. But, at higher eccentricities, ℎ𝑐 , exhibits more or less an increasingdecreasing trend in most of the cases studied. Critical height is observed to be minimum for the largest value of 𝑑2 (equal to 𝑑1) irrespective of the values of the speed of fluid rotation, liquid initial height and port eccentricity. To particularly note, at the highest eccentricity, vortex formation is found to be completely suppressed for all values of port diameter (𝑑2) and initial fluid rotation (n) as indicated by the near-zero critical height values. The tangential velocity measurements using Particle Image Velocimetry are also reported. PIV results obtained for certain cases with induced fluid rotation (normal draining and faster draining) correlate well with the changes in the efflux (axial) velocity (deduced analytically) in these cases studied. The tangential velocity along radial direction obtained (PIV) also indicated the type of vortex formed in normal and faster draining cases. Video visualization of vortex formation carried out reveals that, vortex air core switching takes place between the drain ports maintaining an arched or curvilinear surface profile apart from demonstrating the nature of outlet flow discharge. All the vortex air core formation studies so far carried out were invariably with single drain port except the preliminary novel study by the same author group and the present study is a detailed extension of that novel study.

In the present research, a possible generation mechanism of low-frequency buffeting phenomenon based on a 1:15 open jet automotive wind tunnel was investigated. Evolution of vortex structures and pressure field in the plenum chamber have been visualized and analyzed by Large-eddy simulation (LES). It is shown that the low frequency pressure fluctuation is caused by the large-scale structures and their interaction. Multiple proper orthogonal decomposition was adopted to analyze the flow field in the plenum chamber. The characteristic frequencies of the vortex-rings after pairing is the same as the dominant resonance of buffeting at this wind velocity.

Renal artery aneurysm (RAA) is a condition that affects approximately 0.1% of the general population. The rate of incidence is minimal compared to other type of aneurysm but a high number of ruptures have been reported in pregnancy, especially at the third trimester. The concerning issue is that the maternal mortality rate stretches up to 50% and the fetal mortality rate approaching 85% with a universal loss of the affected kidney. This study aimed at investigating the effectiveness of stent in treatment of RAA using the fluid-structure interaction (FSI) approach. The flow pattern, wall shear stress (WSS), deformation and von Mises stress experienced are compared between RAA model without stent and with Abbott RX Herculink stent. A simple PIV experiment, observing the flow profile was conducted as a validation steps in ensuring the simulation results are reliable and accurate. The findings show that the simulation and PIV data are in good agreement in terms of the flow profile. The presence of stent managed to reduce the blood flow maximum velocity down to 46% and minimized the circulation of blood in the aneurysm dome. As for the WSS, the used of stent succeeded in decreasing the WSS experienced by the wall of aneurysm by 71% and below the baseline level of WSS that could induced rupture. The deformation of RAA and maximum von Mises stress reduced by 58% and 73% respectively when stent is used. In addition, the maximum von Mises stress after the stent placement is lower than the threshold value for the ultimate tensile strength of the tissue. This study concluded that the stent placement is effective in reducing the risk of aneurysm rupture in renal artery it can be one of the baseline for the further study regarding the RAA.

To avoid the complexity of the edge definition by the half width, a new approach to defining the leeward edge of the planar jet in crossflow is introduced in this paper. Particle Image Velocimetry (PIV) experiments were performed to measure different flow regimes within the single jet and the dual jets configurations in crossflow. Based on the experimental data acquired, a series of velocity profiles were extracted from the flow field. In each profile, a velocity threshold was given to distinguish the regions sheltered and the regions not sheltered by the planar jet. The boundary of these regions was accordingly recognized as the leeward edge. Furthermore, fitting of the edge was carried out using a second order polynomial so as to enable a mathematical expression of the leeward edge. An application of the proposed approach towards the flow induced noise reduction using a planar jet is also discussed in this paper. In addition, the PIV frame assembly algorithm used in this study is reported.

In studies of self-aspirating impellers found that gas bubbles are not broken down by the impeller blades. Breakup of bubbles is caused by the eddies generated by the blades. Therefore, to describe how the liquid flow near the blades is an important research issue for this type of impellers. Using the PIV method average velocity fields in the axial-radial plane between baffles in the stirred tank were defined for seven different positions of blades of a self-aspirating disk impeller in relation to that plane. It was found that in the small space in blade vicinity, big changes in fluid circulation were observed depending on the position of the blade relative to the baffle. In front of blade the liquid from bottom and from over impeller is directed radially towards the wall of tank and the average axial velocity is zero. Behind the blade the cavern (cavity) is formed, understood as a space of reduced pressure. Underpressure causes suction effect which directs the liquid inside the cavern. In just a few millimeters from the blade tip average axial and radial velocities are equal to zero. In this region the tangential component of velocity is dominant.

Effect of constrained flow is investigated experimentally for a flapping foil power-generator. The flow structures around and in the near wake of a flat plate placed between two side walls are captured via PIV technique with simultaneous direct force measurements in uniform flow at Re = 10 000. The rectangular flat plate oscillates with periodic non-sinusoidal pitching and plunging motions about its 0.44 chord position with stroke reversal times (  TR) of 0.1 (rapid reversal) to 0.5 (sinusoidal reversal), phase angles of  = 90° and 110°, plunge amplitude of 1.05 chords and pitch amplitude of 73° at a constant reduced frequency of k = 0.8. The non-dimensional distances between the side walls and the oscillating flat plate are dw = 0.1, 0.5 and 1.0. Airfoil rotation speed dictates the strength, evolution and timing of shedding of leading and trailing edge vortices; as the stroke reversal time is decreased, earlier shedding of stronger vortices are observed. Increasing the phase angle between the pitching and plunging motions decreases the power generation efficiency for all cases. The highest power extraction coefficient is acquired for the non-sinusoidal case of  TR = 0.4 in free flow. Optimum choice of side-wall distance improves power generation of flapping foils compared to free flow performance, up to 6.52% increase in efficiency is observed for the non-sinusoidal case  TR = 0.4 with dw = 0.5, with remarkable enhancements for the sinusoidal case; 27.85% increase is observed with dw = 0.5 and 43.50% increase with dw = 1.0 where both cases outperform the highest power generation efficiency of the finite flat plate with non-sinusoidal flapping motion.