Particle Image Velocity Measurements Research Articles

Physical and Numerical Study on Right Side and Front Side Gas Blowing at Walls in a Single‐Strand Tundish

The compromise between inclusions removal by small bubbles and proper agitation of fluid flow in tundish by gas blown is challenging. The single‐strand tundish without flow control devices (bare tundish) is used to study the effects of side‐wall gas blowing on the flow field. The water model, particle image velocity measurement and computational fluid dynamics simulation are used to study bubble behavior, fluid flow field, and tracer transport. A large horse shoe vortex and a short circuit flow at the bottom are formed in the bare tundish. Gas blowing at the outlet‐side wall of the tundish creates multiple vortices in the right‐side region, with blowing rate and position greatly influencing the surface flow within the tundish. Gas blowing at the front‐side wall creates a large spiral vortex, agitating the fluid in the tundish and improving the flow field. Increasing the blowing rate strengthens the vortex in the right region, but raising the blowing position affects surface flow and creates a counterclockwise flow toward the shroud. In all schemes, gas blowing at a low position on the front‐side wall with a small gas flowrate is the optimized scheme which shows the smallest dead volume and enhances surface flow.

The data assimilation of volumetric velocity and temperature data to extract boundary conditions for high-fidelity simulation of the indoor air

The contemporary challenge of the air conditioning sector is to continuously improve the provided comfort conditions while reducing the energy consumption of the devices. This necessity can only be satisfied by improving research and design methods. Therefore, on the one hand, researchers worldwide develop and utilize experimental methods for more accurate measurements. On the other hand, researchers develop numerical methods to investigate design iterations effectively and quickly.The main idea of this study is to create a novel data assimilation method to utilize all the advantages of experimental and numerical methods to develop a high-fidelity investigation method of an indoor air model.In the present study, the three-dimensional velocity distribution at the split air conditioning indoor unit's outlet section was measured by using the Stereo Particle Image Velocity Measurement method. The temperature distribution at the outlet section of the split air conditioning indoor unit was also determined using the Meshed Infrared Thermography method. The data from these experimental measurements were assimilated to generate a highly realistic boundary condition of the steady-state and time-dependent Computational Fluid Dynamics (CFD) models. This is achieved for the first time in the literature.The results of these analyses compared with conventional CFD and the aforementioned experimental methods. The study's results show that the novel data assimilation approach (3.13% average error) is significantly more accurate than the traditional average boundary condition approach (13.19% average error).

Experimental and Computational Fluid Dynamics Study of the Flow Field of a Model Pump Turbine

The pumped storage unit has a hump area and S characteristics during operation. The operating conditions of the unit are complex, and it is necessary to switch between working conditions. The unit frequently crosses the hump area and the S characteristic area, thus hindering the unit to connect to the grid. Severe vibration affects the safe and stable operation of a power station. With the joint efforts of the industry, the impact of the hump and S-shaped problems on project commissioning has been fundamentally resolved, but the mechanism of inducing hump and S-shaped problems has not yet been well established. Therefore, in this study, the particle image velocity measurement method based on a full-characteristic test was adopted. According to the operating characteristics of the unit, 32 operating points with four guide-vane opening degrees were selected to test the flow pattern in the vaneless space. After subtracting the average flow velocity, the vortex image of the leafless area was obtained, which provided a reference for the design of the runner of the water pump turbine.

Investigation of heat treatment process using Particle Image Velocimetry

Martensitic hardening is a technology widely used in mechanical engineering practice to increase the hardness and abrasion resistance of ferrous alloys. To achieve martensitic hardening, the workpiece must be cooled the workpiece very quickly, which can be achieved through using various refrigerants. The quality and the properties of these coolants have a great influence on the success of the process, so their investigation is essential. Throughout the investigation method – according to ISO 9950 standard –various aqueous suspensions used as refrigerants are studied by placing a heated cylindrical specimen in a refrigerated container (which will be described later) and by recording the temperature change over time. The flows generated in the tank during the heat treatment influence the nature of the heat transfer, thus the hardening process itself. The task is to map these flow conditions to give a comprehensive description of the qualification process in question. With the advancement of measurement technologies, there are more and more possibilities to determine the velocity of flowing media, even at several points simultaneously. Using the available PIV (Particle Image Velocity) system, the two-dimensional vector field of the flow can be determined on a complete plane surface. In the present research, the applicability of seeding particles required for the PIV measurement technique is also investigated as part of the mentioned project, establishing further research possibilities. The classification of eight different seeding particles is presented. As a final result, seeding particles suitable in the current arrangement for proper flow display are listed.

Analysis and Modeling of Particle Velocities in Premixed Abrasive Water Jets

The premixed abrasive jet possesses a strong strike ability and is widely used in oil and gas exploitation, machining, rust removal, and other fields. The superstrong, forceful impact of the premixed abrasive jet is mainly provided by high-speed abrasive groups. Hence, the abrasive velocity is the basis of this research, by applying the distribution law of abrasive impact force. In this paper, the particle velocity of the premixed abrasive jet is analyzed theoretically, and the corresponding particle velocity model is established. The real-time contrast interpolation method is employed to solve the problem of the variable drag coefficient. Factors such as the nozzle structure, average abrasive diameter, abrasive density, and jet flow are utilized to determine the abrasive velocity of the nozzle outlet. The numerical solution for the abrasive velocity is obtained by dividing the high-pressure pipe and nozzle into several sections, along the axis. Finally, the calculated particle velocity is compared with the particle image velocity measurement (PIV), to verify the correctness of the established model. These results demonstrate that the model calculation is in effective agreement with the experimental results. The deviation between the theoretical value and the experimental mean is 0.18 m/s. The standard deviation of the experimental results is 3.81-4.22 m/s, while the average error is less than 4%.

Spectral analysis modal methods (SAMMs) using non-time-resolved PIV

We present spectral analysis modal methods (SAMMs) to perform POD in the frequency domain using non-time-resolved particle image velocity (PIV) data combined with unsteady surface pressure measurements. In particular, time-resolved unsteady surface pressure measurements are synchronized with non-time-resolved planar PIV measurements acquired at 15 Hz in a Mach 0.6 cavity flow. Leveraging the spectral linear stochastic estimation (LSE) method of Tinney et al. (Exp Fluids 41:763–775, 2006), we first estimate the cross-correlations between the velocity field and the unsteady pressure sensors via sequential time shifts, followed by a Fast Fourier transform to obtain the pressure–velocity cross spectral density matrix. This leads to a linear multiple-input/multiple-output (MIMO) model that determines the optimal transfer functions between the input cavity wall pressure and the output velocity field. Two variants of SAMMs are developed and applied. The first, termed “SAMM-SPOD”, combines the MIMO model with the SPOD algorithm of Towne et al. (J Fluid Mech, https://doi.org/10.1017/jfm.2018.283, 2018). The second, called “SAMM-RR”, adds independent sources and uses a sorted eigendecomposition of the input pressure cross-spectral matrix to enable an efficient reduced-rank eigendecomposition of the velocity cross-spectral matrix. In both cases, the resulting rank-1 POD eigenvalues associated with the Rossiter frequencies exhibit very good agreement with those obtained using independent time-resolved PIV measurements. The results demonstrate that SAMMs provide a methodology to perform space-time POD without requiring a high-speed PIV system, while avoiding potential pitfalls associated with traditional time-domain LSE.

Wake topology and dynamics over a slender body at a high incidence and their relation to structural loading

The flow over a slender cylindrical body with a hemisphere end was studied experimentally using a combination of force balance and time-resolved particle image velocity measurements. The investigation was performed at a subcritical Reynolds number (Re = 11 000) over a range of high incidence angles from 30° to 90°. The results show that significant cross-flow loading occurs for a range of incidence angles from 50° to 70°, with maximum mean and fluctuating loads taking place at 60°. Within this range of incidence angles, the loading has a bimodal nature, with intermittent switching between two states associated with the positive and negative cross-flow loading direction. The analysis of simultaneous force and wake measurements reveals that the two loading regimes are produced by two distinct wake topologies defined by strongly asymmetric vortex dynamics near the tip of the model. The results provide insight into salient features of the wake development and vortex dynamics and show that transient changes in the cross-flow force direction progress through a consistent change in the wake structure between two bounding quasi-steady states.

Macro-micro interaction mechanism between FRPC pile and sand under horizontal bidirectional load

FRPC pile is a new type of composite pile formed by combining FRP material with traditional material. It has good physical and chemical erosion resistance, light weight, high strength and other advantages. Based on the particle image velocity measurement (PIV) technique, this paper studies the interaction between sand soil and FRPC pile by applying bidirectional horizontal load on the FRPC model pile embedded in sand soil. In the process of loading the FRPC model pile, a CCD high-speed industrial camera is used to take gray image of sand deformation around the test observation surface pile. DH3816 Strain Collecting Instrument was used to collect pile side strain, and image processing software PIVview2C was used to get displacement cloud picture. By analyzing the distribution of total displacement, horizontal displacement and vertical displacement of sandy soil under different loads, the micro mechanism of FRPC pile and sandy soil is clarified. The distribution of pile side moment was studied, and the macro mechanism of FRPC pile and sandy soil is clarified.

Dynamics of 2D, V-shaped Bluff-body Stabilized Turbulent Premixed Flames near and Away from Blowoff with Different Gaseous Fuels

ABSTRACT V-shaped premixed flames of propane, methane, and ethylene-air mixtures were stabilized on a triangular-shaped flame holder under three different free stream turbulence levels; 4%, 18%, and 34% rms at a nominal burner exit velocity of 10 m/s. Flames were studied at strongly burning and near blowoff conditions to observe the changes in the overall flame dynamics. Flame behavior at different upstream flow turbulence conditions was visualized by simultaneously capturing the end and side views of flame chemiluminescence using high speed (4 kHz) imaging. A gradual shift from symmetric to asymmetric flame structures was observed with reduction of equivalence ratio approaching blowoff. Particle image velocity measurements confirmed the gradual switch of vortex shedding from symmetric varicose mode to the asymmetric sinuous mode as flame blowoff is approached by reduction of the mixture equivalence ratio. It is found that average flame front position crosses the shear layer from its outer location at strongly burning conditions to within the shear layers bounding the recirculation zone as flame blowoff is approached. Transition of flame behavior from “varicose” to “sinuous” oscillations is documented by way of a symmetry index as a function of equivalence ratio and turbulence intensity for the three fuels studied.

Experimental Investigation of Rotors Hovering above Inclined Surfaces

Particle image velocity measurements were taken to document the wake generated by a rotor operating in ground effect above inclined surfaces. In particular, the current work focused on the average wake structure and axial velocity distribution through the rotor. A two-bladed rotor was operated at a height of one rotor radius above a ground plane that was inclined at angles from 0° to 30°. Rotor performance measurements were also taken, using a six-axis load cell, to examine the effect ground plane angle had on the thrust produced and power required. The wake structure was found to be very sensitive to ground plane angle causing the radial distribution of axial velocity through the rotor to increase inboard and decrease outboard with increasing ground plane angle. As compared to the level ground case, the peak figure of merit of the rotor decreased by approximately 10% for a ground plane inclination angle of 30°.

Control of flow structures around a cylinder in deep water employing a passive control element

This study aimed to experimentally investigate the swirling flow characteristic in the periphery of a cylinder located vertically in a deep water channel using a particle image velocity measurement method (PIV). In addition to the PIV method, paint visualization experiments were performed to observe the structures of vortical flow qualitatively. Based on the cylinder diameter, Reynolds numbers were changed incrementally as 2.5 × 103, 5 × 103, and 7.5 × 103, while the length of the control element, L = 180 mm was kept constant. The flexible splitter plate made of a plastic material was easily deformed under the influence of vortical flow. In the horizontal plane, taken from the middleheight of the cylinder, instantaneous flow data was measured first. Along these measurements, the distributions and magnitude of the velocity vectors, 〈V〉, vorticity concentrations, 〈ω〉, patterns of streamlines, 〈ψ〉, root mean square of velocity component such as 〈urms〉 and 〈vrms〉, correlations of shear stresses, 〈u′u′〉, 〈v′v′〉 and 〈u′v′〉, turbulent kinetic energy, TKE, and spectral analysis is conducted to define the frequency, f of vortex formations. The presence of a flexible flow control element located along the central axis line of the cylinder significantly affects the vortex formations. The existence of the flexible flow control element can significantly influence the wake flow region. Furthermore, many of the value of flow characteristics decreased as mentioned with respect to the bare case.

Analysis of Flow and Wall Deformation in a Stenotic Flexible Channel Containing a Soft Core, Simulating Atherosclerotic Arteries

A novel eccentric stenotic channel was manufactured to provide an in vivo physical environment simulating an atherosclerotic artery. It was composed of an elastic wall containing a soft semi-cylindrical volume that simulated a lipid core. Wall strain and fluid shear stress distributions were computed using a finite element method incorporating fluid structure interaction. The particle image velocity measurement method was extended to measure wall deformation and wall deformation and flow velocity were simultaneously measured to validate the computational method. The maximum deformation was found to occur in the distal shoulder, while the maximum flow wall shear stress was skewed to the proximal shoulder of the lipid core. Measured maximum wall deformations and flow velocities agreed with the computational results within a 10% margin of error. The wall shear stress on the stenotic wall varied from 0 to 1.2 Pa. The maximum shear strain in the longitudinal plane and the normal strain in the transverse plane for the Core 1 model (Young’s modulus 50 kPa) were 3.8% and 0.7%, respectively. The maximum shear and normal wall strains for the Core 2 model (Young’s modulus 200 kPa) were 34% and 14% lower. These wall stress and strain values corresponded with those found in human atherosclerotic arteries. The developed channel could provide spatial and temporal variations as well as magnitudes of wall shear stress and strain observed in atherosclerotic arteries.

Combining magnetic forces for contactless manipulation of fluids in microelectrode-microfluidic systems

A novel method to drive and manipulate fluid in a contactless way in a microelectrode-microfluidic system is demonstrated by combining the Lorentz and magnetic field gradient forces. The method is based on the redox-reaction [Fe(CN)6]3−/[Fe(CN)6]4− performed in a magnetic field oriented perpendicular to the ionic current that crosses the gap between two arrays of oppositely polarized microelectrodes, generating a magnetohydrodynamic flow. Additionally, a movable magnetized CoFe micro-strip is placed at different positions beneath the gap. In this region, the magnetic flux density is changed locally and a strong magnetic field gradient is formed. The redox-reaction changes the magnetic susceptibility of the electrolyte near the electrodes, and the resulting magnetic field gradient exerts a force on the fluid, which leads to a deflection of the Lorentz force-driven main flow. Particle Image Velocity measurements and numerical simulations demonstrate that by combining the two magnetic forces, the flow is not only redirected, but also a local change of concentration of paramagnetic species is realized.

Particle Image Velocity Measurement and Mesh-Free Method Modeling Study of Forced Hydraulic Jumps

AbstractThe current study aims at examining the internal flow field and the corresponding phenomena in the developing stage of forced hydraulic jumps. Experiments with a range of Froude numbers wer...

Study on law of negative corona discharge in microparticle-air two-phase flow media

To study the basic law of negative corona discharge in solid particle-air two-phase flow, corona discharge experiments in a needle-plate electrode system at different voltage levels and different wind speed were carried out in the wind tunnel. In this paper, the change law of average current and current waveform were analyzed, and the observed phenomena were systematically explained from the perspectives of airflow, particle charging, and particle motion with the help of PIV (particle image velocity) measurements and ultraviolet observations.

PIV measurements in a turbulent wall jet over a backward-facing step in a three-dimensional, non-confined channel

A study of a turbulent wall jet over a backward-facing step is especially of interest because it shows a rich phenomenon flow and a mechanism to alter the flow characteristics downstream of the step. However, studies on this flow configuration are rare. In this paper, we considered this flow configuration in a non-confined channel as the specific engineering applications of electrical rotating machines and alternator that can be found in modern wind generators of the power production industry and automobile engines. The turbulent wall jet over a backward-facing step in a non-confined wind tunnel had the jet Reynolds number of 24,100 and the step Reynolds number of 11,900. Particle image velocity (PIV) and stereoscopic PIV measurements were performed along the central plane and several cross-stream planes. Numerical simulation of the test configuration was conducted by solving the three-dimensional Reynolds Averaged Navier–Stokes (RANS) equations with the second-order closure Reynolds stress model (RSM). The mean flow fields and second-order statistical moments from the RSM simulation were compared to results that were obtained through the PIV and stereo-PIV experiments. The mean reattachment length obtained from the current configuration was much shorter than those from the backward-facing step in the plane channel. The stereo-PIV measurements in the cross-stream planes revealed a high three-dimensionality of the flow, a high population of streamwise vortice in the upper region, near the side walls and the corners formed by the side walls and the bottom wall. The obtained results also confirmed the presence of the wall-jet formation on the bottom wall.

Self-excited oscillations of two opposing planar air jets

This paper investigates the self-excited oscillations of two co-planar air jets impinging on each other, referred to, hereafter, by opposing planar jets. The study is performed for a single jet thickness of h = 2 mm and a span-wise aspect ratio of 50. The effects of flow velocity of the jets and the distance between them are examined. The opposing planar jets are found to produce strong acoustic tones over a wide range of jet Mach number and separation distance. Phase-locked averaged particle image velocity measurements showed that the tone is generated by out-of-phase flapping oscillations of the planar jets. These oscillations are sustained by two effects. The first is the high stagnation pressure at the impingement region, which deflects the jets laterally in opposite directions away from their common centerline, while the second effect is the cross stream oscillating velocity in the entrainment regions around the jet exits, which forces the jets back towards their common centerline.

High spatial range velocity measurements in a high Reynolds number turbulent boundary layer

Here, we detail and analyse a multi-resolution particle image velocity measurement that resolves the wide range of scales prevalent in a zero pressure gradient turbulent boundary layer at high Reynolds numbers (up to Reτ ≈ 20 000). A unique configuration is utilised, where an array of eight high resolution cameras at two magnification levels are used simultaneously to obtain a large field of view, while still resolving the smaller scales prevalent in the flow. Additionally, a highly magnified field of view targeted at the near wall region is employed to capture the viscous sublayer and logarithmic region, with a spatial resolution of a few viscous length scales. Flow statistics from these measurements show good agreement with prior, well resolved hot-wire anemometry measurements. Analysis shows that the instantaneous wall shear stress can be reliably computed, which is historically known to be challenging in boundary layers. A statistical assessment of the wall shear stress shows good agreement with existing correlations, prior experimental and direct numerical simulation data, extending this view to much higher Reynolds numbers. Furthermore, conditional analysis using multiple magnification levels is detailed, to study near-wall events associated with high skin friction fluctuations and their associated overlaying structures in the log region. Results definitively show that the passage of very large-scale positive (or negative) velocity fluctuations are associated with increased (or reduced) small-scale variance in wall shear stress fluctuations.

Optimization of Lobed Perforated Panel Diffuser: Numerical Study of Orifice Geometry

Passive control of jet flows in order to enhance mixing and entrainment is of wide applicative interest. Our purpose is to develop new air diffusers for Heating Ventilating Air Conditioning systems, by using lobed diffusers, in order to ameliorate the users’ thermal comfort. For a lobed perforated panel as a diffuser (Meslem et al., 2010), the jet induction optimization will be made by means of optimizing elementary orifice geometry and by arrangement of the neighboring orifices on the perforated panel. An experimental and a numerical investigation using different turbulence models has been carried out over the flow field of turbulent twin cross-shaped jet, in a recent study (Meslem et al., 2011). In comparison to Particle Image Velocity measurements, it was shown that among the investigated viscous models, the SST k-w turbulence model is able to reproduce reasonably well ambient air induction, jets interaction and global expansion of the studied jets when the flow is resolved by numerical simulation through the lobed diffuser. In this study the SST k-w turbulence model is used for the numerical simulations of parallel cross-shaped jets, based on the previous experimental validation. This work is dedicated to the lobe shape effect on flow dynamics and induction performance of the twin cross-shaped jet. It was shown that the lobes with rounded peaks are more advantageous than the right angled lobes.

Investigation of thermal convection in water columns using particle image velocimetry

Particle image velocity measurements were applied on thermally driven convection at low Rayleigh numbers. In a model experiment using a water column heated from bottom and cooled from above, the velocity field was studied at different vertical temperature gradients. In the testing facility with high aspect ratio (about 19) representing a 1-m-long column with 5 cm diameter, occurrence of free convection was verified for destabilizing temperature gradients of 0.1–2 K/m. The PIV results revealed that significant flow exists already at low vertical temperature gradients. The velocity of the stable large-scale circulations increased linearly with temperature gradient (<1 K/m) from 8 × 10−5 to 1 × 10−3 m/s. At higher temperature gradients (1–2 K/m), a transition from quasi-stationary into time-dependent flow was observed, where convection cells changed position, number, and form temporarily. The motivation of this research was to gain more insight into density-driven convection in boreholes and groundwater monitoring wells.