Particle Tracking Velocimetry Measurements Research Articles

Investigation of Multidimensional Fractionation in Microchannels Combining a Numerical DEM-LBM Approach with Optical Measurements

The fractionation in microchannels is a promising approach for the delivery of microparticles in narrow property distributions. The underlying mechanisms of the channels are however often not completely understood and are therefore subject to current research. These investigations are done using different numerical and experimental methods. In this work, we present and evaluate our method of combining a numerical Discrete Element Method (DEM)-Lattice Boltzmann Method (LBM) approach with experimental long-exposure fluorescence microscopy, micro-Particle Image Velocimetry (µPIV) and Astigmatism Particle Tracking Velocimetry (APTV) measurements. The suitability of the single approaches and their synergies are evaluated using the exemplary investigation of multidimensional fractionation in different channel geometries. It shows that both, numerical and experimental method are well suited to evaluate particle dynamics in microchannels. As they furthermore show strengths canceling out weaknesses of the respective other method, the combined method is very well suited for the comprehensive analysis of particle dynamics in microchannels.

In vitro investigation of the blood flow downstream of a 3D-printed aortic valve

The hemodynamics in the aorta as well as the durability of aortic valve prostheses vary greatly between different types of devices. Although placement and sizing of surgical aortic valve prostheses are excellent, the valve geometry of common devices cannot be customized to fit the patient’s anatomy perfectly. Similarly, transcatheter aortic valve implantation (TAVI) devices are not customizable and may be orientated unfavorably during implantation. Imperfect fit of an aortic valve prosthesis may result in suboptimal performance and in some cases the need for additional surgery. Leveraging the advent of precision, multi-material 3D-printing, a bioinspired silicone aortic valve was developed. The manufacturing technique makes it fully customizable and significantly cheaper to develop and produce than common prostheses. In this study, we assess the hemodynamic performance of such a 3D-printed aortic valve and compare it to two TAVI devices as well as to a severely stenosed valve. We investigate the blood flow distal to the valve in an anatomically accurate, compliant aorta model via three-dimensional particle tracking velocimetry measurements. Our results demonstrate that the 3D-printed aortic valve induces flow patterns and topology compatible with the TAVI valves and showing similarity to healthy aortic blood flow. Compared to the stenosis, the 3D-printed aortic valve reduces turbulent kinetic energy levels and irreversible energy losses by over 75%, reaching values compatible with healthy subjects and conventional TAVIs. Our study substantiates that the 3D-printed heart valve displays a hemodynamic performance similar to established devices and underscores its potential for driving innovation towards patient specific valve prostheses.

Four-dimensional particle tracking velocimetry measurements of unsteady three-dimensional vortex onset and progression for 5415 straight ahead, static drift, and pure sway

Large-scale towed system four-dimensional particle tracking velocimetry (4DPTV) implementation and measurements are presented for towing tank tests for a naval combatant 5415 ship model for straight ahead, static drift β = 10°, and pure sway βmax = 10° conditions. The results, for the first time, provide instantaneous volumetric flow field data around the ship model including near the hull surface, and a complete description of the sonar dome 3D unsteady vortex separation onset and progression due to the 4DPTV larger measurement volume and higher data rates in comparison with previous results using tomographic particle image velocimetry (TPIV); thereby, providing the essential data required for the assessment of current hybrid Reynolds-averaged Navier–Stokes/large eddy simulation turbulence modeling capabilities and guidance for the necessary modeling improvements. The 4DPTV system is summarized and compared with the previous TPIV, including the camera calibration procedures, trigger systems, and synchronization with the sway motion. Analysis procedures, including data reduction procedures and the 3D vortex core detection and voxel labeling techniques, are described. The identification of the complex vortex separations and breakdown is aided using complementary detached eddy simulations. The vortex–vortex interaction process is visualized from instantaneous volumetric datasets. The pros and cons of the 4DPTV vs TPIV, the comparison between the second- and third-generation vortex visualization technique, and the statistical convergence error analysis of sonar dome port vortex for β = 10° are discussed. Plans for increased spatial resolution of the 4DPTV system and additional data reduction techniques for detailed turbulence analysis are also discussed.

Three-Dimensional Quantitative Flow Visualization Around a Thrust Reverser

Volumetric particle tracking velocimetry measurements are performed in a low-speed wind tunnel to study the flow around a 1:12-scale aircraft model with jet engines operating with thrust reversers. The engine jet and freestream flow velocity are varied to yield a jet to freestream velocity ratio of ranging from 1.5 to 6. Measurements at such scale () require the use of strongly scattering helium-filled soap bubbles as flow tracers, which are introduced in both the jet and the freestream flow. The tracer’s three-dimensional motion is determined using an array of cameras and a Lagrangian particle tracking algorithm. The mean velocity field reveals the jet inner structure as well as its interaction with the freestream, the ground board, the nacelle, the fuselage, and the horizontal and vertical tails. The experiments allow detection of exhaust reingestion as well as the aerodynamic interference with control surfaces at the tail segments in a single measurement volume. The results are in good agreement with conventional temperature rake measurements while adding details of the flow topology and of the large-scale unsteady flow fluctuations. Finally, the jet reversal characteristics with varying freestreams and nozzle pressure ratios are assessed, demonstrating the feasibility and versatility of volumetric velocimetry measurements for industrial aerodynamics.

Boundary layer measurements of the flow along a streamwise-oriented circular cylinder using particle tracking velocimetry

Particle tracking velocimetry (PTV) measurements of an axisymmetric turbulent boundary layer (ATBL) around a long circular cylinder in axial flow were carried out in a large-scale water tunnel at different velocities and at different measurement positions along the cylinder. A relatively thick ATBL developed with ratios of δ/a between 4.6 and 8.1 and a/δν between 313 and 1027, where δ is the boundary layer thickness, δν the viscous length scale and a the cylinder radius. At ratios of δ/a above unity and a/δν above 250, transverse curvature effects are known to influence the boundary layer in the outer region. This was observed in the measured mean velocity profiles as the profiles fall below the planar logarithmic law of the wall. Shape factors close to unity with a rather full velocity profile were determined that explain the relatively high skin friction coefficient known to be present for such ATBL flows. Additionally, measurements of the turbulence intensity and Reynolds stress profiles are presented. The non-intrusive PTV measurement approach allowed to gather near-wall velocity data down to a wall-distance below 10 wall-units. The present study provides valuable information regarding ATBL flows along streamwise-oriented cylinders in the flow regime indicated above by the ranges of δ/a and a/δν. In particular, it fills a gap in the literature as far as experimentally obtained higher-order statistical quantities are concerned, such as root-mean-square velocity fluctuations and Reynolds stresses.

Experimental study on flow and turbulence characteristics of jet impinging on cylinder using three-dimensional Lagrangian particle tracking velocimetry

When a round jet impinges on a convex cylindrical surface, complex three-dimensional (3D) flow structures occur, accompanied by the Coanda effect. To characterize the flow and turbulence properties of the general system, ensemble averages of 3D Lagrangian particle tracking velocimetry measurements were taken. The radial bin-averaging method was used in post-processing the tracked particles and corresponding instantaneous velocity vectors to generate appropriate ensemble-averaged statistics. Two impinging angles were selected, and at a fixed Reynolds number, the ensemble-averaged volumetric velocity field and turbulent stress tensor components were measured. The flow and turbulence characteristics of the impinging jet on the cylinder were notably different based on the impinging angle, especially in the downstream region. Surprisingly, the attached wall jet with a half-elliptic shape was abruptly thickened in the wall-normal direction, similar to the axis switching phenomenon observed in elliptic jets in the case of oblique impingement. In the jet-impinging region, the flow spread in all directions with high mean vorticity values. With the development of a 3D curved wall jet, both the Coanda effect and centrifugal force played a significant role in the flow behavior. A notable feature of the self-preserving region was the similarity of mean velocity profiles with scaling by the maximum velocity and the jet half-width for both impinging angle cases. Local isotropy of turbulent normal stresses was observed in this region, supporting the existence of self-preservation in the 3D curved wall jet. The volumetric ensemble-averaged Reynolds stress tensor revealed strong inhomogeneous turbulence in the boundary layer region and the curvature effect on the Reynolds shear stress in the free shear layer.

On the role of transverse motion in pseudo-steady gravity currents

Flow in the body of gravity currents is typically assumed to be statistically two-dimensional, and cross-stream flow is often neglected (Simpson 1997; Meiburg et al. 2015). Here, we assess the validity of such assumptions using Shake-the-Box particle tracking velocimetry measurements of experimental gravity current flows. The resulting instantaneous, volumetric, whole-field velocity measurements indicate that cross-stream and vertical velocities (and velocity fluctuations) are equivalent in magnitude and thus are key to energy distribution and dissipation within the flow. Further, the presented data highlight the limitations of basing conclusions regarding body structure on a single cross-stream plane (particularly if that plane is central). Spectral analysis and dynamic mode decomposition of the fully three-dimensional, volumetric velocity data suggests internal waves within the current body that are associated with coherent three-dimensional motions in higher Reynolds number flows. Additionally, a potential critical layer at the height of the downstream velocity maximum is identified.

Particle tracking velocimetry study of the nonequilibrium characteristics of a fluidized bed

AbstractThe nonequilibrium characteristic of fluidization is closely related to the mesoscale structures. To help uncover this relationship, we performed a particle‐level experiment in a bubbling fluidized bed making particle tracking velocimetry (PTV) measurements. We found a large Knudsen number not only in bubbles or over bubble interfaces, but also in the dense emulsion where various banded structures exist, indicating a pervasive violation of the local equilibrium. Analysis of these banded structures identified stretching, compression, or shearing between particles. Non‐Gaussian velocity distributions were found throughout the bed, including in the dense emulsion with a small Knudsen number. In future work, we expect that the nonequilibrium characteristic will act as a local criterion for the assessment of various models of bubbling fluidized beds.

Development and assessment of a particle tracking velocimetry (PTV) measurement technique for the experimental investigation of oil drops behaviour in dispersed oil–water two-phase flow within a centrifugal pump impeller

The objective of the current work is to present the development of a Particle Tracking Velocimetry (PTV) algorithm for the analysis of oil drops behaviour in two-phase oil–water dispersions within a centrifugal pump impeller. The drop tracking was realized through high-speed camera images in a transparent pump prototype, which enabled the visualization of oil drops dispersed in water in all the impeller channels. The PTV algorithm is based on deep-learning techniques for image processing. The drops are detected by a combined U-Net and Convolutional Neural Network (CNN) method, with the former generating a binary image and the latter detecting valid oil drop contours. After detection, the Labelled Object Velocimetry (LOV) is adopted to calculate the instantaneous oil drop velocity. A synthetic image generator based on a Generative Adversarial Network (GAN) is then developed to assess the results from the U-Net, CNN, and LOV models. Additional validation studies are performed using the results from Perissinotto et al. (2019a). The results reveal that the presented deep-learning PTV algorithm is robust and provides consistent and reliable data for the dispersed oil phase in two-phase oil–water flows.

Non-Invasive Laser-Cross Calibration For Volumetric Flow Measurements In Closed Channels

The calibration of a multi-camera system is a crucial step of a 3D particle tracking velocimetry (3D-PTV) measurement and other optical velocimetry techniques, which are based on multi-camera setups. Conventional calibration methods for multi-camera systems are based on recording hardware targets in the cameras’ fields of view. In measurement volumes that are difficult to access, the insertion of such hardware targets is associated with an increased technical or mechanical effort or is eventually not possible. This work presents a calibration method without the use of a hardware target. Instead, crossing laser beams are introduced into the volume, which create unique calibration points due to their beam paths. The underlying algorithm for detecting the laser beams and the optimization of the calibration is explained. The influence of the beam diameter and positioning errors on the calibration quality are examined by using synthetic data, which is created in Blender (Free and Open 3D Creation Software under GNU GPL). The new laser cross calibration is compared to the calibration with a two-plane target by synthetic data and experimental data from a 3D-PTV measurement in a difficult-to-access volume in a transparent manifold. For the comparison, the synthetic data and the therefore known absolute positions of the lasercross allow to calculate the reconstruction error in addition to the widely used reprojection error. The ransac algorithm is used for the detection of the beam paths from the raw images. The ransac algorithm is well suited for this application, as it is stable against parasitic reflections while its subpixel accuracy is sufficient for this application with small beam diameters. The non-invasive calibration techniques have different advantages over the usage of a hardware target. Due to the in-situ calibration it is not necessary to move the camera setup, which can otherwise lead to errors in the alignment of the cameras to each other. The installing and deinstalling process of the calibration plate is not needed, which reduces the effort for the construction and further reduces errors. Hardware-targets need to be homogenously illuminated, which is not necessary with "self-illuminating" calibration targets like the laser cross. When having opposing cameras, hardware-targets need to be two-faced with front- and back-side being perfectly parallel and the calibration points need to be in defined geometric relation. Deviations can introduce systematic errors. With a non-invasive method like the laser cross there is no obstruction by the calibration plate itself.

Spatial detection and hierarchy analysis of large-scale particle clusters in wall-bounded turbulence

Large-scale inertial particle clustering in a turbulent boundary layer (TBL) was investigated experimentally by two-colour particle-image velocimetry and particle-tracking velocimetry measurements in the wall-parallel plane of the log layer. The particle clusters were detected by Voronoi tessellation and a new application of a technique based on the spatial wavelet transform. Anisotropic wavelets spanning the range of measurable, large-scale particle clusters were applied to experimentally measured particle fields, and significance testing was used to identify hierarchies of particle clusters. Unlike the Voronoi tessellation, which primarily identified small-scale clusters, the wavelet technique also found a significant amount of anisotropic, large-scale clusters on the scale of the large-scale motions (LSMs) of the momentum field. The large-scale clusters were shown to be composed of a hierarchy of smaller clusters of varying degrees of isotropy. In addition to the cluster size distribution, the particle velocities within the clusters were studied as a function of cluster size. Larger clusters exhibited faster particle convection speeds, consistent with observations of the convection speeds of LSMs, thus providing additional evidence for the hypothesis that particles in the TBL accumulate on and are transported by large-scale, attached coherent structures.

Uncertainty of PIV/PTV based Eulerian pressure estimation using velocity uncertainty

This work introduces a method to estimate the uncertainty of the pressure fields reconstructed from particle image velocimetry / particle tracking velocimetry (PIV/PTV) measurements by propagating the instantaneous velocity vector uncertainty through the pressure reconstruction. The uncertainty propagations through the calculation and integration of pressure gradients are modelled as linear transformations. The autocorrelation coefficient was modelled and incorporated in the uncertainty estimation to reproduce the effect of the autocorrelation of velocity errors on the reconstructed pressure’s accuracy. The method was first tested on synthetic velocity fields contaminated with varying levels of artificial noise correlated in space, time, or between components. The error analysis shows that the proposed method could predict the spatiotemporal variations of the pressure errors. The estimated pressure uncertainty also captures the effects of the velocity noise level, the autocorrelation, and the different pressure-gradient integration methods, with more than 80% accuracy in most test cases. The method was applied to an experimental vortex ring flow with planar PIV and a laminar pipe flow with volumetric PTV. The error analysis shows that the obtained pressure uncertainty possessed similar spatial and statistical distributions as the pressure errors. The results also indicate that the performance of the proposed uncertainty estimation method depends on the accuracy of the velocity uncertainty. The proposed uncertainty estimation method exhibits reliability in obtaining the local and instantaneous pressure uncertainty from the PIV/PTV measurements.

Investigation of the flow structure in three-dimensional waves on falling liquid films using light field camera

Fluid velocity measurement in liquid films is a challenging task due to the small film thickness and large transverse gradients of velocity. While this problem was solved for the case of two-dimensional waves by several research groups, measurements in three-dimensional waves impose additional difficulties that we managed to overcome in the course of this study. We used a single light field camera for simultaneous particle tracking velocimetry and laser-induced fluorescence film thickness measurements. In this way, obtained velocity fields were related to the wavy interface shape. The applicability of the proposed methodology was demonstrated in the test experiments, which were carried out for two-dimensional regular waves. Measurements in three-dimensional waves were performed for the fluids with different viscosities and surface tensions but with close values of the Kapitsa number. In all investigated cases, the flow structure in different parts of waves is similar. Comparison of the experimentally obtained velocity fields with calculations by weighted residual integral boundary layer model shows qualitative agreement.

In vitro estimation of the energy loss through turbulence in a porcine aortic valve stenosis model and silicone ascending aorta phantom using backlight particle tracking velocimetry

Abstract Introduction Patients suffering from low-flow, low-gradient aortic stenosis present a decreased stroke volume due to decreased contraction or relaxation function of the left ventricle. As a low stroke volume tends to cause a low transvalvular flow, transvalvular pressure gradient (TVPG) and effective orifice area, the clinician cannot rely on those parameters with confidence for the evaluation of aortic stenosis severity. Hence new diagnostic parameters have to be developed. Energy loss through turbulence associated with aortic stenosis represented the wasted left ventricle work. Currently, echocardiographic measurement of the turbulence intensity is not validated for clinical evaluations of aortic stenosis. Methods Two porcine aortic valves were harvested and inserted in a flow loop that replicates the pulsatile flow of the heart. A stiffening of the valves was achieved by treating them with formaldehyde. The stiffening was externally confirmed by a custom-made force-displacement device quantifying the rigidity of the leaflet yielding two stiffness grades per valve. Each valve was tested under three different mean flow rates (1, 2.5, and 4 l/min) at each of the two stiffness grades. Moreover the pressure in the left ventricle chamber and in the aortic chamber was recorded to calculate the TVPG. Particle tracking velocimetry measurements into the transparent silicone ascending aorta phantom allowed the computation of the turbulent kinetic energy (TKE), to evaluate the energy loss due to turbulence. Results We could confirm the enhanced rigidity of the valve leaflets with our custom device (data not shown) and measure a consistent increase in TVPG across all mean flow rates between the two stiffness grades. Moreover, an explicit increase of the TKE in the aortic phantom could be measured after the stiffening process (73.1% under 1 l/min, and 43% under both 2.5 and 4 l/min). In addition, a good correlation (R = 0.86) between the mean TVPG and the TKE was found. Conclusions This project demonstrated the possibility of quantifying the energy loss attributed to turbulence for porcine valves in vitro for native and added stiffness grade. This project lays the foundation for the development of a new diagnostic tool for the assessment of stenosis severity in patients with low-flow, low-gradient aortic stenosis in cardiac imaging tool such as echocardiography. Funding Acknowledgement Type of funding sources: None. TVPG and its correlation with TKEIntensity graphs of the TKE

Aeroelastic Characterization of a Flexible Wing Using Particle Tracking Velocimetry Measurements

The aerodynamic, elastic, and inertial components in Collar’s triangle of forces acting on a flexible wing with span width and chord are determined based on integrated optical measurements of the structural and aerodynamic response to steady and unsteady periodic inflow conditions at a chord-based Reynolds number of . The measurement device is a coaxial volumetric velocimeter mounted on a robotic arm, which is used to perform optical measurements of fiducial markers on the wing surface, and helium-filled soap bubbles, which are used as flow tracers. The optical measurements of the structural markers and the flow tracers are both processed with the Lagrangian particle tracking algorithm Shake-the-Box. Subsequently, physical models are used to determine the inertial and elastic forces of the aeroelastic interaction from the marker tracking results, and to determine the unsteady aerodynamic lift force from the flow velocity fields. The results of this integrated aeroelastic characterization approach are in physical agreement with each other according to the equilibrium of forces in Collar’s triangle and good agreement with external reference measurements.

Intermittency effects in single- and multi-phase flow and anomalous transport in heterogeneous porous media

Multi-phase flow and transport in porous media is prevalent in a wide range of challenging fluid mechanics problems related to sustainability, energy, and the environment. Accurate prediction of the displacement and interaction of such flows is vital in addressing these problems. In particular it is critical to understand the small- or pore-scale flow and its spatial and temporal evolution, which can impact behaviors at system scales in a nontrivial manner. Intermittency is a phenomenon currently observed in numerical and experimental studies of single-phase flow (Anna et al., n.d.; Morales et al., n.d.), but the case of multi-phase flow has yet to receive much study due to challenges faced in both simulations and experiments. The underlying physics of spreading, mixing, and interfacial processes must be understood for accurate predictions of transport in multi-phase flow systems. Therefore, a comprehensive understanding of multi-phase flow at these very small scales is necessary in the development of accurate system-scale prediction models. We present results from a coordinated numerical and experimental study of intermittency effects over a range of viscous and inertial flow regimes in single- and multi-phase flows in 2D heterogeneous micromodels to quantify Lagrangian flow statistics to better inform pore-scale models. The applicability of different modeling frameworks such as the correlated-continuous time random walk is tested by studying statistics of particle trajectories obtained by particle tracking velocimetry (PTV) measurements and Lattice Boltzmann simulations from single- and multi-phase flows. The results make particular note of the influence of the pore Reynolds number (Re) and inertial effects on intermittency, and compare these effects in the two flow regimes.

Dense flow field interpolations from PTV data in the presence of generic solid boundaries

Three-dimensional flow measurements by Particle Tracking Velocimetry (PTV) provide scattered flow information, that often needs to be interpolated onto a regular grid. Therefore, the use of experimental data assimilation approaches such as VIC+ (Schneiders and Scarano, 2016) were proposed to enhance the instantaneously available spatial resolution limits beyond that of the PTV measurements. Nevertheless, there exists no prior attempt to perform the data assimilation when the flow is in direct contact with physical objects. Thus, in order to handle generic solid body intrusions within the flow fields of VIC+ application, the utilization of Arbitrary Lagrangian-Eulerian and immersed boundary treatment approaches of the computational fluid-structure interaction (FSI) frameworks are proposed. The introduced variants over the standard VIC+ are assessed with a high fidelity numerical test case of flow over periodic hills. The accuracy superiority of the flow field reconstructions with the proposed approaches are denoted especially in close proximity of the interaction surface. An experimental application of the introduced methods is demonstrated to compute the pressure distribution over an unsteadily moving elastic membrane surface, revealing the time-resolved interaction between the flow structures and the membrane deformations.

3D-PTV flow measurements of Newtonian and non-Newtonian fluid blending in a batch reactor in the transitional regime

Lagrangian trajectories obtained through 3D Particle Tracking Velocimetry (3D-PTV) measurements have been used to visualize the flow field of Newtonian and non-Newtonian fluids in a flat-bottomed vessel. The vessel, of diameter T=180 mm, was equipped with a 6-blade Rushton turbine of diameter D=T/3 and four baffles of width b=T/10. The experiments were carried out in the transitional flow regime (73≤Re≤1,257). The velocities and Lagrangian accelerations in the flows have been calculated from the time-resolved tracer coordinates. Non-Newtonian fluids exhibited a smaller impeller flow number compared to Newtonian fluids. The distributions of shear rate have been obtained via interpolation of the Lagrangian velocity data in a 3D Eulerian grid. In the impeller region, the mean shear rate was, to a first approximation, proportional to the impeller rotational speed, although a more detailed analysis revealed influences of both rheology and Reynolds number. The mean Lagrangian acceleration scaled with the mean shear rate raised to the power of 1.8.

Full-Span Topology of Trailing-Edge Separation at Different Angles of Attack

The velocity field over a two-dimensional wing at Reynolds number of 672,000 was investigated to characterize the three-dimensional topology and evolution of the separate flow with variation of the angle of attack. Planar particle image velocimetry over the full span of the wing demonstrated that with increasing angle of attack, isolated pockets of backflow, which appeared near the trailing edge, merged and formed an asymmetric stall cell at angle of attack of 9.7°. The asymmetry was mainly associated with the dissimilar boundary layers developed on the wind tunnel walls at the spanwise ends of the wing. Secondary structures were also observed between the stall cell and the spanwise end. The stall cell topology was characterized using large-scale three-dimensional particle tracking velocimetry measurements using helium-filled soap bubbles. The results showed that the separation bubble had a small wall-normal height with two wall-normal counter-rotating vortices extended up to the edge of the separation bubble. In addition, the investigations demonstrated that vortex generators can induce a symmetric stall cell by removing the secondary structures and isolating the stall cell from the flows at the spanwise ends.

Ratio-cut background removal method and its application in near-wall PTV measurement of a turbulent boundary layer

Optical contamination due to wall reflection creates limitations for near-wall velocity field measurement via either particle image velocimetry (PIV) or particle tracking velocimetry (PTV). In this paper, a simple image pre-processing method, i.e. the ratio cut method, is proposed to deal with this problem. It is based on the ratio between the grayscale intensities of tracer particles and those of the laser-illuminated background, on which a direct minimum cut is applied on the basis of a non-dimensional threshold for background removal. To evaluate its performance in near-wall measurement, this ratio cut method, along with two other typical pre-processing methods, i.e. the minimum removal method and the proper orthogonal decomposition (POD) filtering method, are applied to particle images in the near-wall region of turbulent boundary layers over an opaque roughness wall (ORW), whose characteristic roughness height is small enough to be regarded as hydraulically smooth, but still gives rise to severe wall reflection. Results for a case involving a transparent smooth wall, which suffers less from wall reflection issues, and direct numerical simulation (DNS) data at a similar Reynolds number are employed as reference baselines for performance evaluation. The examination of pre-processed particle images, as well as the probability density function (PDF) of grayscale intensities, indicates that the ratio cut method is capable of eliminating time-dependent flare, reducing noise level, and retaining low-intensity particles in the ORW case. These features are almost completely absent in both the minimum removal method and the POD filtering method. In addition, PTV-obtained velocity statistics for an ORW, pre-processed by the ratio cut method, including data relating to fluctuating intensity and the PDF distribution of fluctuating velocity, are shown to be more consistent with those relating to baseline cases than data obtained by either of the the other two methods used for comparison. Moreover, evidence is also provided regarding the superiority and robustness of this approach, in terms of estimating the mean skin friction from the near-all mean velocity profile.