Particle Image Velocimetry Technology Research Articles

Application of PIV Technique in Teaching Experiment of Solitary Wave Flow Field Measurement

Particle Image Velocimetry (PIV), as an advanced experimental technique of fluid mechanics, has been widely used in many fields such as experimental fluid mechanics and practical engineering applications. The solitary wave is a kind of advanced wave that has only one peak or trough and only appears in shallow water. This kind of wave is quasi-particle, and its theory is used to study the water depth of wave breaking, which has a guiding effect on the design of offshore structures. To let students understand PIV technology visually, and at the same time, learn the transmission and evolution process of solitary waves, this paper puts forward a PIV measurement platform for the fine flow field of solitary waves, which measures the velocity vector distribution, velocity scalar distribution and vorticity distribution of the flow field in the transmission, climbing deformation and crushing process of the two-dimensional solitary wave. Through this experiment, students have a more direct impression of the fluid phenomenon of transmission and deformation of solidarity waves and understand the principle and use of PIV technology. This experiment deepens students’ understanding of the knowledge, stimulates students’ interest in learning, cultivates their practical ability, improves their innovation ability, and lays a good foundation for future scientific research and exploration.

Experimental investigation of turbulent flow under different Reynolds numbers and blockage ratios in a heated rectangular channel

Particle Image Velocimetry (PIV) technology is used in current paper to investigated turbulent flow of central blocked accidents in a heated rectangular channel. Proper Orthogonal Decomposition (POD) method and Power Spectral Density (PSD) method are also used to expose the turbulent characteristics in the wake. The velocity field at different Reynolds numbers and blockage ratios is analyzed. During the blockage experiments, the movement of eddies are captured in detail in the instantaneous flow field. The vortex gradually decreases with the increase of Reynolds number, but gradually increases with the increase of blockage ratio. The dominant vortex structures in the first two POD modes gradually become sharper at higher blockage ratio. Subsequently, the peak frequencies of the instantaneous flow field are the same as those of POD modes at various blockage ratios.

Health Monitoring System with Hybrid Laser Sensor Networks and Cloud Computing for Subway Tunnels

The settlement and deformation monitoring for existing tunnels would encounter the difficulties in sensor installation and health evaluation in real time. Thus, it is necessary to develop effective methods for settlement and deformation measurements in the existing tunnels. This study proposed a more practical tunnel deformation monitoring theory and developed remote wireless tunnel deformation real-time monitoring and early warning system based on infrared laser ranging technology and cable sensing technology. The system could monitor the settlement and deformation of the tunnel in real time from a long distance and transmit the deformation data wirelessly in real time; then, three-dimensional space model of tunnel deformation was established, which intuitively and comprehensively reflected the actual deformation of the tunnel and intelligently identified the deformation. Finally, based on the practice of deformation control during tunnel excavation at Xiaohongshu Station of Wuhan Metro Line 8 of China, the measurement results of this new method were compared with the measurement results of particle image velocimetry technology (PIV), the reliability of the new tunnel deformation monitoring method was proved, the settlement error of each monitoring point was found to be less than 1 mm, and the deformation trend was consistent with the actual deformation. The results of this research could provide useful guidance for tunnel deformation monitoring.

Evolution and fluidization behaviors of wet agglomerates based on formation-fragmentation competition mechanism

When the liquid is injected into a gas-solid fluidized bed, particles are easily wetted and prone to form liquid bridges among them, further leading to particle agglomeration. Wet particle agglomeration induced by liquid bridge is dynamic and unstable, and its evolution behavior affects the stable operation of fluidized bed reactor. Therefore, based on the formation-fragmentation competition mechanism, a wet agglomeration evolution model is proposed to predict the evolution characteristics of wet agglomerates under different conditions. Furthermore, a modified drag force model which attributed the effect of wet agglomerates into the drag force was established and applied to the CFD simulation of a fluidized bed with wet agglomerates under the liquid spray-evaporation equilibrium state. Meanwhile, the particle image velocimetry (PIV) technology is applied in this work, and it was found that the vertical solid velocity simulated by CFD is in good agreement with the PIV measurement results.

On the mass exchange mode controlled by high-speed compressive mainstream/cavity interaction in a laboratory dual-mode scramjet combustor

Stable and efficient mass exchange is crucial to the combustion stability of a dual-mode scramjet. In subsonic mode, the progression of mass exchange between the cavity fluid and the mainstream, which is controlled by the cavity shear layer, is different from the scramjet mode due to the weak compressibility effects. However, mass exchange mode studies in the subsonic mode are rare. In this study, experimental research has been conducted to obtain the flow structure of Mach 0.3 and 0.5 inflow over a rectangular cavity and reveal the properties of mass exchange using particle image velocimetry technology. The results reveal the weak compressibility effects and geometry effects on the mass exchange by analyzing the growth rate and turbulence characteristics of the cavity shear layer. The mass exchange between the cavity fluid and mainstream is determined based on the vertical velocity along the cavity lip line. When the flow Mach number increased from 0.3 to 0.5, the growth rate of the cavity shear layer decreases due to the weak compressibility effects. Also, the growth rate of the cavity shear layer is smaller than that of the compressible free shear layer due to the effects of recirculation within the cavity. For length-to-depth (L/D) ratios ranging from 0.8 to 1.2, the cavity shear-layer growth rate decreases with increasing L/D. However, at an L/D of 1.5, the shear-layer growth rate increases, which changes the mass exchange mode. Finally, the effects of increasing the Mach number (Mach = 0.3 versus 0.5) show that the L/D ratio at the “crossover” point of the mass exchange mode transition is reduced (L/D = 1.5 versus 1.2) due to the weak compressibility at a higher Mach number.

Multiple parameters and target optimization of splitter blades for axial spiral blade blood pump using computational fluid mechanics, neural networks, and particle image velocimetry experiment.

The blood pump is an implantable device with strict performance requirements. Any effective structural improvement will help to improve the treatment of patients. However, the research of blood pump structure improvement is a complex optimization problem with multiple parameters and objectives. This study takes the splitter blade as the object of structural improvement. Computational fluid mechanics and neural networks are combined in research and optimization. And hydraulic experiments and micro particle image velocimetry technology were used. In the optimization study, the number of blades, axial length and circumferential offset are optimization parameters, and hydraulic performance and hemolytic prediction index are optimization targets. The study analyzes the influence of each parameter on performance and completes the optimization of the parameters. In the results, the optimal parameters of number of blades, axial length ratio, and circumferential offset are 2.6° and 0.41°, respectively. Under optimized parameters, hydraulic performance can be significantly improved. And the results of hemolysis prediction and micro particle image velocimetry experiments reflect that there is no increase in the risk of hemolytic damage. The results of this study provide a method and ideas for improving the structure of the axial spiral blade blood pump. The established optimization method can be effectively applied to the design and research of axial spiral blade blood pumps with complex, high precision, and multiple parameters and targets.

Effect of submerged entry nozzle wall surface morphologies on boundary layer structure and alumina inclusions transport

During the Al-killed steel continuous casting process, the molten steel corrosion and the accumulation of alumina inclusion deposits affect the submerged entry nozzle (SEN) wall surface, including the surface morphologies of the smooth wall, porous refractory wall, and clogged wall. The SEN wall surface morphology affects the boundary layer structure and alumina inclusions transport. In this study, a physical modeling method was adopted, and the surface morphologies simulation was realized by filling up the natural porous refractory material and inserting the real clog material in the polymethyl methacrylate SEN model. The velocity in the boundary layer was measured using the particle image velocimetry (PIV) technology, and the alumina inclusions transport path in the boundary layer was calculated by MATLAB software. The MATLAB codes combined the velocity data from the PIV measurement results and the inclusion transport equation. The four-quadrant analysis showed that sweep and ejection events existed in the boundary layer. The fluctuations of the velocity and the turbulent kinetic energy in the normal direction were increased in the porous refractory and the clogged wall boundary layer when the sweep and ejection events existed. The transport of the alumina inclusions with a diameter of 1–15 μm was affected by the ejection and the sweep events. The alumina inclusions moved toward the boundary in the sweep event. During the sweep event, the transport path of alumina inclusions with 1 μm diameter was close to the boundary; the alumina inclusions were more easily attached to the boundary. The alumina inclusions escaped from the boundary in the ejection event. In the porous refractory and the clogged walls, the alumina inclusion transport path in the normal direction was increased. When the SEN wall’s morphologies changed from smooth wall to porous refractory wall and clogged wall, the sweep event area proportion increased from 10.17% to 39.77%, and the ejection event area proportion decreased from 32.96% to 9.24%. Moreover, the sweep event’s probability increased from 25.83% to 28.24% when the morphologies of the SEN wall changed from smooth wall to porous refractory wall and clogged wall, which will increase the alumina inclusion deposition rate in the porous refractory wall and the clogged wall boundary.

Experimental study on bubble sweep-down characteristics of research vessels

The issue of bubble sweep-down is of great importance within the field of marine research, as the phenomenon greatly affects the measurement accuracy of research vessels. To explore the characteristics of bubble sweep-down, an experimental study into this phenomenon in relation to research vessels was carried out. With the help of particle image velocimetry technology, the microbubble trajectory on the X-Y and X-Z planes was obtained by observing bubble sweep-down. It was found that the evolution trajectory of the X-Z plane had an approximately linear distribution in the experimental measurement range. By comparing the microbubble trajectory with the bow streamline obtained by the tuft method, it was found that the distribution of the streamline in the bow cannot be regarded as the trajectory of the microbubble, but the influence of bubble buoyancy and the tension of the water should be considered. Through further analysis, it was found that the thickness and width of the microbubble cloud cluster change greatly when the cluster is close to the bottom of the ship, and the change of the microbubble cloud cluster under high-speed conditions is much larger than that under low-speed conditions because is easier for the microbubble cloud to diverge and deform at high speed. The change in the microbubble cloud area under high-speed condition was lower than that under low-speed conditions, which was probably caused by the extrusion effect of flow fields around the bow on microbubble cloud clusters at high speed.

Instability characteristics of a co-rotating wingtip vortex pair based on bi-global linear stability analysis

The Stereo Particle Image Velocimetry (SPIV) technology is applied to measure the wingtip vortices generated by the up-down symmetrical split winglet. Then, the temporal bi-global Linear Stability Analysis (bi-global LSA) is performed on this nearly equal-strength co-rotating vortex pair, which is composed of an upper vortex (vortex-u) and a down vortex (vortex-d). The results show that the instability eigenvalue spectrum illustrated by (ωr, ωi) contains two types of branches: discrete branch and continuous branch. The discrete branch contains the primary branches of vortex-u and vortex-d, the secondary branch of vortex-d and coupled branch, of which all of the eigenvalues are located in the unstable half-plane of ωi > 0, indicating that the wingtip vortex pair is temporally unstable. By contrast, the eigenvalues of the continuous branch are concentrated on the half-plane of ωi < 0 and the perturbation modes correspond to the freestream perturbation. In the primary branches of vortex-u and vortex-d, Mode Pu and Mode Pd are the primary perturbation modes, which exhibit the structures enclosed with azimuthal wavenumber m and radial wavenumber n, respectively. Besides, the results of stability curves for vortex-u and vortex-d demonstrate that the instability growth rates of vortex-u are larger than those of vortex-d, and the perturbation energy of Mode Pu is also larger than that of Mode Pd. Moreover, the perturbation energy of Mode Pu is up to 0.02650 and accounts for 33.56% percent in the corresponding branch, thereby indicating that the instability development of wingtip vortex is dominated by Mode Pu. By further investigating the topological structures of Mode Pu and Mode Pd with streamwise wavenumbers, the most unstable perturbation mode with a large azimuthal wavenumber of m = 5–6 is identified, which imposes on the entire core region of vortex-u. This large azimuthal wavenumber perturbation mode can suggest the potential physical-based flow control strategy by manipulating it.

Experimental Investigation on Flow Field Characteristics of Impinging-Film Cooling

This paper describes an experimental investigation on flow field characteristics of impinging-film cooling. Particle Image Velocimetry (PIV) technology has been applied to observe the effect of blowing ratio ( 0.04 ≤ M ≤ 0.3 ), temperature ratio ( 0.73 ≤ T u ≤ 0.91 ), jet-to-plate pitch ( 1.6 ≤ Z n ≤ 3.2 ), and spacing of impinging holes ( 1.94 ≤ Y n ≤ 3.5 ) on the flow field patterns in an impinging-film cooling test rig under atmospheric pressure. Experiment results show that the near-wall entrained vortex at the downstream of the slit moves downstream of the test rig as the blowing ratio increases, which increases the effective protection length of the film. While the vortex at the end of the inducting slab is stronger, this will increase the mixing in the shear layer. The radial size of the near-wall entrained vortex tends to decrease as the temperature ratio increases at the low blow ratio, and the entrainment effect on the downstream of the slit becomes smaller, causing the separation zone to decrease. Increasing the jet-to-plate pitch, the size of the near-wall entrained vortex increases, and the thickness of the film layer increases, this strengthens the separation effect of the near-wall airflow from the wall surface. The larger the spacing of the impinging holes, the more uneven the velocity distribution of the film.

Research on PIV Image Matching Algorithm of Turbulent Velocity Field Based on Circular Projection

Based on the complexity and particularity of the images processed by the particle image velocimetry technology and the non-rigid motion mode of the fluid, a particle image matching algorithm with low complexity, high accuracy, anti-deformation, anti-rotation, high robustness to illumination and wide measuring range of velocity is proposed by combining the nature of the rotation invariance of circle and the theory of image warping. The algorithm uses transformation of circular projection instead of FFT to ensure that the information of interpretation window and the search area has sufficient similarity; the complexity of the algorithm proposed is greatly reduced by setting the quarter circle and establishing relative coordinate system; the size of the interpretation window can be changed with the characteristics of the flow field by setting the threshold of the number of particles and the threshold of the window size, which improves the accuracy and applicability of the algorithm; it makes the algorithm suitable for turbulent flow fields with high Reynolds number and large deformation by warping the image iteratively. Experiments have proved that the algorithm is suitable for matching images of turbulent flow which with rotational transformation, the matching rate is over 99%, and the complexity of the algorithm is the same order of magnitude as the cross-correlation algorithm based on fast Fourier transform.

Experimental research on flow field of high head pump turbine based on PIV test

Pump turbine operating conditions are complex, mainly covering turbine mode and pump mode. Pump turbines have various instability problems during operation, such as S-shaped, pump hump, pressure pulsation and cavitation. PIV (Particle Image Velocimetry) is a very effective test technique for the internal flow field observation of pump turbines. In this paper, the S-shaped, pump hump and pressure pulsation of the pump turbine are tested by PIV technology. The experimental observations show that the internal flow of the pump turbine is extremely complicated. For example, the full quadrant flow characteristics of the S-shaped working condition are observed. It is found that there is a phenomenon of single vortex and double vortex coexisting in the Vane-less area between guide vane and impeller blade of the pump turbine. Through the experimental research in this paper, it can provide useful help for understanding the vortex separation on different quadrant working conditions inside the pump turbine.

RESEARCH ON THE INFLUENCE OF FREE SURFACE ON THE FLOW CHARACTERISTICS OF SUBMARINE WAKE

The flow characteristics around the submarine will affect the maneuverability of the submarine, especially when the submarine is near the water surface, the existence of the free surface will increase the complexity of the wake field of the submarine. In order to explore the influence mechanism of free surface on the flow characteristics of submarine wake when the submarine near the free surface, the research on the flow characteristics of submarine wake is carried out with the help of large-scale underwater three-dimensional particle image velocimetry technology. Firstly, the accuracy of the test method in this study is verified by the standard model experiment of DTMB in USA; Then, the wake field of the submarine is measured by the model test, and the axial velocity and pulsating velocity of the propeller disk under different submergence depths and different speeds are obtained. At the same time, the wave and the velocity field of the longitudinal section which can not be measured by the model test are supplemented by numerical simulation, and the influence mechanism of the free surface on the flow characteristics of the wake field is expounded from the wave point of view. The results show that: When the submarine near the water surface, with the increase of Fr, the upper isopleth of the axial velocity at the propeller disk tends to be flattened; and compared with the 4D submergence depths, the local maximum value of pulsating velocity appears in the 1.5D submergence depths, and the structure of pulsating velocity moves down as a whole. When the free surface exists, the velocity of flow field between the hull and the free surface increases obviously, especially in the area of the propeller disk. With the increase of Fr, the height of the free surface of the propeller disk decreases gradually, which leads to a larger fluid velocity and the squeeze phenomenon of wake field on propeller disk.

Numerical simulation of aerodynamic noise and noise reduction of range hood

The hybrid CAA (Computational Aeroacoustics) method is used to numerical study on the aerodynamic noise of a certain range hood. The CFD numerical calculation method was used to calculate the unsteady flow field, and the velocity and pressure distribution of the range hood under unsteady conditions were obtained. The particle imaging velocimetry (PIV) technology experiment was carried out by set up an internal flow field test bench, to record the flow status of the flow field in the measurement area by a CCD camera. Also, applied with FLUENT and Actran acoustic software, and based on the results of unsteady flow field calculation of the range hood as well as the Lighthill sound analogy method, the internal flow field information of the range hood is converted into the sound field information, and the noise of the range hood can be numerically simulated. The results showed that the range hood noise mainly concentrated on low and middle frequencies and characterized by discrete noise. The noise of the range hood Having distinct feature of direction at the blade passing frequency, and the noise is mainly transmitted to the outside through the main air inlet and secondary air inlet of the centrifugal fan in the range hood. Installing noise reduction structures at the main and secondary air inlet can effectively reduce noise.

Effect of water content on near-pile silt deformation during pile driving using PIV technology

Piles are widely used in structural foundations of engineering projects. However, the deformation of the soil around the pile caused by driving process has an adverse effect on adjacent existing underground buildings. Many previous studies have addressed related problems in sand and saturated clay. Nevertheless, the failure mechanism of pile driving in unsaturated soil remains scarcely reported, and this issue needs to be studied. In this study, a modeling test system based on particle image velocimetry (PIV) was developed for studying deformation characteristics of pile driving in unsaturated silt with different water contents. Meanwhile, a series of direct shear tests and soil-water characteristic curve (SWCC) tests also were conducted. The test results show that the displacement field shows an apparent squeezing effect under the pile end. The installation pressure and displacement field characteristics are sensitive to the water content. The installation pressure is the largest and the total displacement field is the smallest, for specimens compacted at water content of 11.5%. These observations can be reasonably interpreted according to the relevant unsaturated silt theory derived from SWCC tests and direct shear tests. The variation characteristics of the soil displacement field reflect the macroscopic mechanical properties of the soil around the pile.

Effect of Nozzle Clogging on the Fluid Flow Pattern in a Billet Mold with Particle Image Velocimetry Technology

The flow pattern of molten steel in a mold of continuous casting affects the temperature distribution and initially solidifies the shell thickness homogeneity, inclusion transport, and liquid slag entrapment. Alumina inclusions in Al-killed steel tend to adhere to the inner wall of the nozzle, which changes the nozzle geometry structures and affects the molten steel flow pattern in the mold. In this study, two different clog shapes—the hemispherical clog and tube clog—were identified based on the clogged straight-nozzle dissection results. The hydraulic straight-nozzle casting model with different clog situations was designed to investigate the effect of the clog status on the fluid flow in the mold using particle image velocimetry (PIV) technology. The results show that a tail stream with a length of 80 mm forms near the nozzle wall in the fluid flow direction when the hemispherical clog sticks to the nozzle and the jet stream deflects to the clogged side after the jet stream and tail stream mix. However, when the hemispherical clog sticks to the nozzle outlet, the jet stream deflects to the nonclogged side. The increase in the tube clog thickness causes an increase in the jet stream velocity magnitude near the nozzle outlet.

Study on the flow field and improved separation performance of a three-stage cone water-only cyclone

In this study, computational fluid dynamics (CFD) numerical simulations were applied to comparatively evaluate the flow field of a three-stage cone water-only cyclone (TWOC) with that of a single-stage cone water-only cyclone (SWOC). The reliability of numerical simulations was validated using particle image velocimetry (PIV) technology. According to the analyses of the influences of the flow field characteristics on the separation process, the reasons for the improved separation performance of the TWOC were reasonably explained. The differences in the flow fields inside of the two cyclones only exist in their cone components. In the TWOC cone, the tangential velocity and the axial velocity are both smaller than those of the SWOC, the near-wall fluid is disturbed and two secondary eddies are formed. The secondary eddies increase the looseness of the coal particles and prompt the radial separation of the coal particles. In the TWOC, low-ash coal particles below the theoretical separation density (δ50) are sufficiently separated into the overflow products throughout the downward motion. Most of the coal particles reaching the cone bottom are high-ash coal particles above δ50 that are basically discharged through the underflow pipe. Even if some coal particles are forced into the internal spiral flow, these particles will be re-separated by the secondary eddy.

Particle image velocimetry measurement of flow fields in a mixed-flow pump with non-uniform tip clearance

In order to better understand the flow characteristics of mixed-flow pump with non-uniform tip clearance, a test rig is established and the flow fields of various cross section in impeller are studied based on particle image velocimetry technology. The velocity vector distributions of three cross sections, e.g., impeller blade inlet, middle and outlet, were measured. The influences of flow rate and eccentricity on the flow fields of mixed-flow pump with non-uniform tip clearance were analyzed, and the relative velocity distribution of different positions at the same cross section is quantitatively discussed. The results show that the external characteristics have been greatly affected by the eccentricity of impeller. There is a significant negative correlation between eccentricity and efficiency. The maximum head and efficiency drop of eccentric impeller reach 11% and 20% separately. With the increase in eccentricity, the secondary flow phenomenon increases, such as backflow and vortex flow in impeller flow passage. Vortex structure is easily formed in the end wall region, which will give rise to backflow and even blocks the flow passage near impeller hub. The energy loss increases continuously in this process which is the main reason for the significant drop in efficiency. The research results are instructive for revealing the flow characteristics of flow fields of mixed-flow pump with non-uniform tip clearance.

Physical simulation of flow field and construction process of horizontal salt cavern for natural gas storage

Underground salt caverns provide appropriate space for storing natural gas. However, salt caverns have to be constructed in bedded salt formations in China. The traditional construction method of salt cavern encounters serious problems in such formations. Instead, horizontal salt caverns are proposed as a possible alternative for gas storage. Horizontal caverns can be created by “multi-step retreating” (MSR) method. Two laboratory tests were conducted to understand the shape development and flow field of such cavern. In one test, we measured fluid velocity distribution in a simulated salt cavern environment by particle image velocimetry technology. In another test, the construction process of a salt cavern was simulated through MSR method. Results show that the flow field in horizontal cavern can be divided into five regions. The flow characteristics and rock salt dissolution properties in these regions are different. A semi-cylindrical cavern with similar cross section shape along the horizontal direction is obtained through the physical experiment. This study provides important guidance for analyzing the flow field and construction process of the horizontal salt cavern.

Experimental investigation on the structures and induced drag of wingtip vortices for different wingtip configurations

As an effective method to reduce induced drag and the risk of wake encounter, the winglet has been an essential device and developed into diverse configurations. However, the structures and induced drag, as well as wandering features of the wingtip vortices ( WTVs) generated by these diverse winglet configurations are not well understood. Thus, the WTVs generated by four typical wingtip configurations, namely the rectangular wing with blended/raked/split winglet and without winglet (Model BL/ RA/ SP/NO for short), are investigated in this paper using particle image velocimetry technology. Comparing with an isolated primary wingtip vortex generated by Model NO, multiple vortices are twisted coherently after installing the winglets. In addition, the circulation evolution of WTVs demonstrates that the circulation for Model SP is the largest, while Model RA is the smallest. By tracking the instantaneous vortex center, the vortex wandering behavior is observed. The growth rate of wandering amplitude along with the streamwise location from the quickest to the slowest corresponds to Model SP, Model NO, Model BL, Model RA in sequence, implying that the WTVs generated by model SP exhibit the quickest mitigation. Considering that the induced drag scales as the lift to power 2, the induced drag and lift are estimated based on the wake integration method, and then the form factor λ, defined by [Formula: see text], is calculated to evaluate the aerodynamic performance. Comparing with the result of Model NO, the form factor decreases by 7.99%, 4.80%, and 2.60% for Model RA, Model BL, Model SP, respectively. In sum, Model RA and BL have a smaller induced drag coefficient but decay slower; while Model SP has a larger induced drag coefficient but decays quicker. An important implication of these results is that reducing the strength of WTVs and increasing the growth rate of vortex wandering amplitude can be the mutual requirements for designing new winglets.