Optical Velocimetry Research Articles

Perspective Camera Model With Refraction Correction for Optical Velocimetry Measurements in Complex Geometries.

Camera calibration is among the most challenging aspects of the investigation of fluid flows around complex transparent geometries, due to the optical distortions caused by the refraction of the lines-of-sight at the solid/fluid interfaces. This work presents a camera model which exploits the pinhole-camera approximation and represents the refraction of the lines-of-sight directly via Snell's law. The model is based on the computation of the optical ray distortion in the 3D scene and dewarping of the object points to be projected. The present procedure is shown to offer a faster convergence rate and greater robustness than other similar methods available in the literature. Issues inherent to estimation of the refractive extrinsic and intrinsic parameters are discussed and feasible calibration approaches are proposed. The effects of image noise, volume size of the control point grid and number of cameras on the calibration procedure are analyzed. Finally, an application of the camera model to the 3D optical velocimetry measurements of thermal convection inside a polymethylmethacrylate (PMMA) cylinder immersed in water is presented. A specific calibration procedure is designed for such a challenging experiment where the cylinder interior is not physically accessible and its effectiveness is demonstrated by providing velocity field reconstructions.

High-precision shock equation of state measurements for metallic fluid carbon between 15 and 20 Mbar

Diamond is an efficient ablator material to convert the energy of high-power giant lasers into ablation pressure with applications for High-Energy-Density (HED) science, planetary science, and Inertial Confinement Fusion (ICF) research at the National Ignition Facility (NIF). Unfortunately, current theoretical equation of state models cannot reproduce all the observed experimental data in the multi-megabar regime particularly relevant for HED and ICF research. New experimental data on the behavior of carbon at extreme pressures and temperatures are, therefore, essential to improve our predictive capability to design and analyze dynamic compression experiments for HED or ICF research and build improved equation of state models in the future. Here, we report high-precision laser-driven shock compression measurements on diamond single crystals at the Omega Laser Facility. Using ultrafast Doppler optical Velocimetry Interferometer System for Any Reflector (VISAR) to track the leading shock front and a quartz plate as an in situ reference, we obtain relative pressure-density shock equation-of-state measurements between 15 and 20 Mbar with an impedance-matching procedure. We also report shock-and-release measurements in a spherical geometry at the NIF. The new data provide tight constraints on the compressibility of warm dense carbon along the Hugoniot of full density diamond, allowing us to discriminate between existing theoretical equation-of-state models. We find that both LLNL LEOS 9061 and LANL Sesame 7835 models capture well the shock compressibility in the explored range. LANL Sesame 7835 also reproduces well the observed shock-and-release behavior of diamond near 10–20 Mbar.

Data-driven order reduction and velocity field reconstruction using neural networks: The case of a turbulent boundary layer

We present a data-driven methodology to achieve the identification of coherent structure dynamics and system order reduction of an experimental turbulent boundary layer flow. The flow is characterized using time-resolved optical flow particle image velocimetry, leading to dense velocity fields that can be used both to monitor the overall dynamics of the flow and to define as many local visual sensors as needed. A Proper Orthogonal Decomposition (POD) is first applied to define a reduced-order system. A non-linear mapping between the local upstream sensors (inputs sensors) and the full-field dynamics (POD coefficients) as outputs is sought using an optimal focused time-delay Artificial Neural Network (ANN). The choices of sensors, ANN architecture, and training parameters are shown to play a critical role. It is verified that a shallow ANN, with the proper sensor memory size, can lead to a satisfying full-field dynamics identification, coherent structure reconstruction, and system order reduction of this turbulent flow.

Optical tweezers-based velocimetry: a method to measure microscale unsteady flows

In the study of micro-scale biological flows, velocimetry methods based on passive tracers, such as micro-PIV and micro-PTV, are well established to characterize steady flows. However, these methods become inappropriate for measuring unsteady flows of small amplitude, because, on these scales, the motion of passive tracers cannot be distinguished from Brownian motion. In this study, we use optical tweezers (OTs) in combination with Kalman filtering, to measure unsteady microscopic flows with high temporal accuracy. This method is referred to as optical tweezers-based velocimetry (OTV). The OTV method measures the nanometric displacements of a trapped bead, and predicts the instantaneous velocity of the flow by employing a Kalman filter. We discuss the accuracy of OTV in measuring unsteady flows with 1.5–70 upmum s^{-1} amplitudes and 10–90 Hz frequencies. We quantify how the bead size and the laser power affect the velocimetry accuracy, and specify the optimal choices for the bead size and laser power to measure different unsteady flows. OTV accurately measures unsteady flows with amplitudes as small as 3–6 upmum s^{-1}. We compare the accuracy of OTV and micro-PTV, and characterize the flow regime for which OTV outperforms micro-PTV. We also demonstrate the robustness of OTV by measuring the unsteady flow created by the cilia of green alga Chlamydomonas reinhardtii, and comparing with numerical predictions based on Stokes equations. An open-source implementation of the OTV software in Matlab is available through the 4TU.Centre for Research Data.Graphic abstract

High-resolution velocity measurements in turbulent premixed flames using wavelet-based optical flow velocimetry (wOFV)

Wavelet-based optical flow velocimetry (wOFV) is used to estimate two-dimensional velocity fields from tracer particle images acquired simultaneously with CH PLIF imaging within two highly turbulent premixed methane-air flames. Unlike conventional correlation-based PIV approaches, the current wOFV approach yields a dense (i.e. one vector per pixel) estimation of the velocity field which offers the possibility of greatly enhanced spatial resolution as compared to correlation-based PIV. Results from the wOFV method and correlation-based PIV are directly compared in terms of instantaneous fields, flow statistics, energy spectra, and derived gradient quantities such as vorticity and 2D strain rate. An assessment of the results indicates an increase in spatial resolution and dynamic range of almost one order-of-magnitude when using wOFV as compared to high-resolution PIV. In fact, the use of wOFV permits resolution of the smallest dissipative scales within the premixed flames with turbulent Reynolds numbers of O(104). A particular focus concerns velocity measurements near and within the CH layers, which are excellent markers of the local flame front but are spatially narrow and intermittent. The high-resolution velocity results from the wOFV method provide an opportunity for improved velocity estimations, derivative quantities, and their statistical characterization at and near the flame front.

Characterization of an interstitial current model around a cancer nodule using optical tweezers.

Currently, the process of diagnosis and treatment of a patient with metastatic cancer is highly inefficient due to the complexity of the disease (metastasis is the spread of cancer cells from a primary tumor to secondary tumors at distant sites [1]). However, recent studies have shown that shear stress, caused by natural microfluidic currents, causes cancer cells to break away, spreading them to secondary sites [2] and aggravating the disease. The extent of shear stress on nodules due to microfluidic currents has not been experimentally proven. In the present study, a methodology developed to induce local shear stresses on a cancer nodule model from velocity field measurements is presented. Such methodology is based on the use of the optical tweezers velocimetry technique reported by Eom, et al. [3] and Almendarez, et al. [4]. The methodology consists on using the holographic optical tweezers velocimetry technique (i.e. multiple trapping in one domain), in order to measure, in a discretized way, the flow field at different positions, and approximate through least squares the velocity profiles; with such information, the shear stresses on the surface of the nodule model will be approximated. The methodology contributes to the understanding of metastasis process and other applications, such as: the development of thrombosis, tumor formation, stopping bleeding, etc.

Development of an improved view registration method with application in optical velocimetry

View registration (VR) refers to the process of determining the orientations and locations of the images captured by one or more cameras. It represents a fundamental procedure underpinning a wide spectrum of applications, ranging from optical sensing, object identification, and image processing. Existing VR procedures typically rely on a set of images captured on a fixed calibration target (referred to as FVM, fixed view method, hereafter). This work reports the development of a new view registration procedure, referred to as the variable view method (VVM). The VVM procedure relies on a set of images taken on a calibration target at various orientations and/or distances. As shown in this work, both numerically and experimentally, the VVM can significantly improve the VR accuracy compared to the FVM, by more than 40% in the tests conducted here. Such enhanced accuracy is expected to benefit a range of applications, and a specific example is demonstrated involving three-dimensional velocity measurement based on particle imaging velocimetry.

Polysulfone shock compressed above the decomposition threshold: Velocimetry and modeling of two-wave structures

Polysulfone was shock loaded to pressures of 14.6–26.2 GPa in a series of gas gun-driven plate-impact experiments measuring material response with embedded electro-magnetic particle velocity gauges and optical velocimetry. The embedded electro-magnetic particle velocity gauges did not show a distinct two-wave structure but did show rounding that suggested a reaction but not a distinct separation of the reactants and product waves. In contrast, the transmission experiments fielded with optical velocimetry, with product pressures ranging from 21.0 to 26.2 GPa, showed well-defined two-wave structures due to shock-driven chemical decomposition of the polymer to products at a higher density. Distinct two-wave structures have not previously been published in literature during polymer compression; here, we observed these two-wave structures at both the polymer/lithium fluoride and polymer/polymethylmethacrylate interfaces. Hydrodynamic simulations of the experiments were performed using a newly constructed SESAME equation of state (EOS) for the polymer, a thermochemical EOS for the decomposition products, and an Arrhenius reaction rate model for the kinetics of the reaction. Simulation results also demonstrated two-wave structures but were unable to quantitatively reproduce either the embedded gauge or optical velocimetry data.

Improvements in the accuracy of wavelet-based optical flow velocimetry (wOFV) using an efficient and physically based implementation of velocity regularization

This manuscript details recent improvements in a wavelet-based optical flow velocimetry (wOFV) method that represents a more physically sound implementation and results in increased accuracy of the velocity estimation. A novel regularization scheme is presented that is based on penalization of directional derivatives of the estimated velocity field or more specifically, second-order penalization of the gradients of divergence and curl, which enforces realistic flow structure. The regularization is performed in the wavelet domain with symmetric boundary conditions for the first time using an alternative wavelet transform approach of matrix multiplications. The method for the computation of full two-dimensional wavelet transforms by a single pair of matrix multiplications is described and shown to be significantly more efficient than a lifting implementation or convolution in MATLAB. Velocity fields are estimated from synthetic tracer particle images generated from 2D DNS of isotropic turbulence and from experimental results from a turbulent flow. Results are compared to an advanced correlation-based PIV algorithm and previous advanced optical flow methods. The velocity results estimated with the new regularization scheme are shown to be more accurate and exhibit a significant reduction in non-physical small-scale artifacts compared to previous results. A significant result from the current method is the ability to generate 2D velocity field images that resolve the dissipative scales in high-Reynolds number, turbulent flows.

An Evaluation of Image Velocimetry Techniques under Low Flow Conditions and High Seeding Densities Using Unmanned Aerial Systems

Image velocimetry has proven to be a promising technique for monitoring river flows using remotely operated platforms such as Unmanned Aerial Systems (UAS). However, the application of various image velocimetry algorithms has not been extensively assessed. Therefore, a sensitivity analysis has been conducted on five different image velocimetry algorithms including Large Scale Particle Image Velocimetry (LSPIV), Large-Scale Particle Tracking Velocimetry (LSPTV), Kanade–Lucas Tomasi Image Velocimetry (KLT-IV or KLT), Optical Tracking Velocimetry (OTV) and Surface Structure Image Velocimetry (SSIV), during low river flow conditions (average surface velocities of 0.12–0.14 m s − 1 , Q60) on the River Kolubara, Central Serbia. A DJI Phantom 4 Pro UAS was used to collect two 30-second videos of the surface flow. Artificial seeding material was distributed homogeneously across the rivers surface, to enhance the conditions for image velocimetry techniques. The sensitivity analysis was performed on comparable parameters between the different algorithms, including the particle identification area parameters (such as Interrogation Area (LSPIV, LSPTV and SSIV), Block Size (KLT-IV) and Trajectory Length (OTV)) and the feature extraction rate. Results highlighted that KLT and SSIV were sensitive to changing the feature extraction rate; however, changing the particle identification area did not affect the surface velocity results significantly. OTV and LSPTV, on the other hand, highlighted that changing the particle identification area presented higher variability in the results, while changing the feature extraction rate did not affect the surface velocity outputs. LSPIV proved to be sensitive to changing both the feature extraction rate and the particle identification area. This analysis has led to the conclusions that for surface velocities of approximately 0.12 m s − 1 image velocimetry techniques can provide results comparable to traditional techniques such as ADCPs. However, LSPIV, LSPTV and OTV require additional effort for calibration and selecting the appropriate parameters when compared to KLT-IV and SSIV. Despite the varying levels of sensitivity of each algorithm to changing parameters, all configuration image velocimetry algorithms provided results that were within 0.05 m s − 1 of the ADCP measurements, on average.

PIV measurements on a Kaplan turbine and comparison with scale-adaptive numerical analysis

Kaplan turbines build a mainstay of the hydro power generation as they offer best efficiency even for high flow conditions. A double regulating approach facilitates a high degree of flexibility but yields to be a sophisticated turbine type. Typical phenomena like gap flow and flow separation are challenges in simulation and complicate reliable predictions. To get a better understanding of flow physics elaborate numerical simulations by means of Computational fluid dynamics (CFD) are conducted as well as experimental flow investigation using the non-intrusive optical measurement technique Particle Image Velocimetry (PIV). Measurements are taken at the spiral case inlet, the draft tube cone and the diffuser giving instantaneous or phase-locked time averaged vector fields of flow velocity in a plane. Depending on the optical access it is possible to gather the in-plane velocity components or all three velocity components using Stereoscopic PIV. The obtained measurement results are then taken as reference on the one hand side for a definition of realistic inlet boundary condition and on the other hand side for assessment of the chosen numerical approach for predicting flow phenomena in Kaplan turbines. In this particular case the focus is on the interaction of runner and draft tube. Results of unsteady simulations with standard two equation and advanced hybrid RANS-LES turbulence models are compared to measurement data.

Extinction strain rate suppression of the precessing vortex core in a swirl stabilised combustor and consequences for thermoacoustic oscillations

In the current paper, time resolved high speed optical Particle Image Velocimetry and CH* chemiluminescence measurements were performed, to study self-excited limit cycle combustion instabilities in a swirl stabilized model gas turbine combustor operating at atmospheric pressure with choked propane and air flow supplies. The combustor was operated under a constant Reynolds number (Re=22,000) and four equivalence ratios, namely ϕ=0.50 for operation susceptible to extinction, and ϕ=0.55, ϕ=0.60 and ϕ=0.65 for operation under a thermoacoustically unstable combustion regime, to encounter two limit cycle dynamic states. The period-1 limit cycle was driven by thermoacoustic coupling between the acoustic and the thermal field at a fundamental timescale dictated by an acoustic eigenmode of the combustor. The period-2 limit cycle, further to the fundamental acoustic timescale featured a subharmonic aerodynamic signature in the heat release rate and dynamic pressure spectra caused by the helical coherent structure of a Precessing Vortex Core (PVC). Previous studies have shown that the PVC in the limit cycle regime may be suppressed by the temperature stratification at the inlet of the combustor. A mechanism is suggested to interpret the flame anchoring locations which effectively regulated whether PVC was excited or suppressed. It is showed that the conditions under which the flame attached to the centerbody and suppressed the PVC can be explained by the spatial distribution of the relative ratio of the flow imposed to the mixture extinction strain rate. The PVC was excited due to local extinction by aerodynamic straining at the inlet of the combustor, at the phase angle of maximum dynamic pressure. On increasing the equivalence ratio, the flame became robust to aerodynamic straining and flashed back at the phase angle of maximum dynamic pressure. The PVC was then suppressed due to the relative ratio of the flow imposed to the extinction strain rate, which allowed the establishment of swirl-damping temperature gradients at the combustor inlet. The paper underlines the importance of quantifying the relation between the flow imposed and extinction strain rate, as it largely dictates the eventual combustor limit cycle dynamic state and its resonant frequencies.

Comparison of velocimetry methods for horizontal air jets in a semicircular fluidized bed of Geldart Group D particles

Image-based velocimetry methods have become a widely used method to characterize solids velocity fields in particulate devices; two of the most commonly used methods being particle image velocimetry (PIV) and particle tracking velocimetry (PIV). Often, PIV and PTV are used at different scales or resolutions. Here, both velocimetry methods are applied to same high-speed video dataset. The quantity of interest is the measurement of jet penetration depths of four opposing, horizontal high-speed air jets into a semi-circular particulate bed maintained near minimum fluidization. In addition, a novel method, optical flow velocimetry (OFV), is applied for the first time to particulate flows and compared to the well-known PIV and PTV methods. Results show favorable agreement: within 20% over 120 different measurements. Generally, the PTV measurements fall between PIV and OFV. The grid-resolution used to resolve the velocity field was also studied, finding the PTV and OFV methods relatively insensitive.

Optical coherence tomography velocimetry based on decorrelation estimation of phasor pair ratios (DEPPAIR).

Quantitative velocity estimations in optical coherence tomography requires the estimation of the axial and lateral flow components. Optical coherence tomography measures the depth resolved complex field reflected from a sample. While the axial velocity component can be determined from the Doppler shift or phase shift between a pair of consecutive measurements at the same location, the estimation of the lateral component for in vivo applications is still challenging. One approach to determine lateral velocity is multiple simultaneous measurements at different angles. In another approach the lateral component can be retrieved through repeated measurements at (nearly) the same location by an analysis of the decorrelation over time. In this paper we follow the latter approach. We describe a model for the complex field changes between consecutive measurements and use it to predict the uncertainties for amplitude-based, phase-based and complex algorithms. The uncertainty of the flow estimations follows from a statistical analysis and is determined by the number of available measurements and the applied analysis method. The model is verified in phantom measurements and the dynamic range of velocity estimations is investigated. We demonstrate that phase-based and complex (phasor) based lateral flow estimation methods are superior to amplitude-based algorithms.

The effects of surface roughness on specular diagnostics in shocked experiments.

Many shock experiments, whether impact, laser, or magnetically driven, use reflected optical light from shocked samples to diagnose their material properties. Specifically, optical velocimetry diagnostics, which do not require absolute power measurements, are regularly used to obtain equation-of-state information of materials. However, new diagnostics will be necessary to expand the realm of measured material properties, and many useful diagnostic techniques do require absolute measurements. Thus, it is important to understand what happens at the reflective surface of shock experiments, and the effect scattering has on the light collection of optical probes. To this end, we present results from experiments done to observe the behavior of a reflected beam from a specular coating on an optical window during shock impact. We find that the specular condition of the coating is adversely affected by the shock front, but this can be mitigated by minimizing roughness on the surface preceding the coating.

A field-segmentation-based variational optical flow method for PIV measurements of nonuniform flows

Air flow fields have nonuniform velocity distributions because of the compressibility of air. Current optical flow-based particle image velocimetry (PIV) methods cannot thoroughly solve these distributions because of the global optical flow formulation. In this study, we propose a novel field-segmentation-based variational optical flow (FS-VOF) method for preserving the spatially discontinuous characteristics of nonuniform flow fields. The proposed method is designed based on the level-set segmentation method, in which the particle image is segmented according to the velocity distribution of the fluid flow. Subsequently, a segmented smoothness constraint term built from the assumption that velocity varies continuously in each segmented particle image region and the discontinuous structures of the velocity field are preserved at the boundaries of the segmented regions. In addition, the data term of the proposed method is based on the segmented regions and derived from the physics-based optical flow equation. The proposed method is evaluated over synthetic and experimental images, and the comparison with advanced variational optical flow algorithms demonstrates the effectiveness of our method in preserving the spatially discontinuous characteristics of nonuniform flow fields.

Flow investigation of complex suspension electrodes for battery applications using ultrasound imaging velocimetry

Flow batteries using suspension electrodes are currently investigated due to their potentially high energy densities and individual scalability of capacity and power density. In contrast to batteries with statically contacted electrodes or conventional redox flow batteries, their performance depend significantly on the local flow conditions of the electrode suspension in the electrochemical cell. Hence, it is crucial to understand and model the complex flow characteristics of such solid-liquid suspensions.Detailed ultrasound flow measurements are combined with multidimensional modeling of the bulk rheology and wall interactions for opaque, thixotropic multiphase fluids in a real-world flow setup. The present flow investigations are conducted for zinc-air type of batteries using a gelled alkaline electrolyte with suspended microscopic zinc particles, however the utilized methods could be adapted to other complex suspension flows of interest. Due to the opacity of the suspension, an ultrasound flow mapping technique, the ultrasound imaging velocimetry (UIV), was applied. In a reference measurement using the model fluid glycerol, the UIV is validated by comparing the measured flow field to optical particle image velocimetry (PIV) measurements and a numerical simulation.In contrast to a Newtonian fluid, the gelled suspension with 8 vol.% particle loading exhibits significant time-dependent non-Newtonian flow characteristics with slip at the wall. While initially strongly sheared zones retain a comparatively low viscosity and allow for high velocity gradients, in areas with low initial shearing (e.g. in corners) the fluid structure is able to rebuild, resulting in a solid-like behavior. An in-depth understanding of the development and the extent of such zero-flow zones can be critical for reliable and efficient designs of the relevant fluidic structures. The measurement data of the zinc-electrolyte suspension flow were used as reference to estimate parameters for a computational fluid dynamics model considering the time-dependent, non-Newtonian rheology of the bulk fluid and the apparent slip at the walls. The simulation model is able to predict the main characteristics of the measured two-dimensional flow fields. Despite some uncertainties, the methods prove to be effective for investigating complex suspension flows and they can be applied to improve and optimize the corresponding fluidic structures, either in flow batteries with flowing suspension electrodes or other industrial applications, including the processing of polymers, minerals, cosmetics, food products, pharmaceuticals and ceramics.

A semi-empirical laminar-to-turbulent flame transition model coupled with G equation for early flame kernel development and combustion in spark-ignition engines

It has been reported that early combustion in a spark-ignition engine determines the subsequent combustion. Also, the early combustion has a very strong correlation with cycle-to-cycle variability, which limits engine operating range. As such, accurate modeling of the early flame development is very important in accurate simulation of spark-ignition engine combustion. During the early flame development, the flame kernel, initiated by spark, grows initially at laminar flame speed. As the kernel grows, the flame surface wrinkles due to surface instability and interacts with the flow turbulence as the flame transitions from laminar to turbulent flame. In this study, a semi-empirical model is proposed to simulate the laminar-to-turbulent flame transition process during early spark-ignition combustion. A hyperbolic tangent function was used to emulate the laminar-to-turbulent flame speed transition process. The proposed transition function was evaluated during early flame kernel development for both Reynolds-averaged Navier–Stokes and large eddy simulation models against combustion analysis data from high-speed optical particle image velocimetry. Difference in Reynolds-averaged Navier–Stokes and large eddy simulation transition function was analyzed and discussed.

Investigation of dynamic fragmentation of laser shock-loaded tin at different phases with the integrated diagnostic techniques

We investigated the dynamic fragmentation of laser shock-loaded tin to gain insights on the underlying mechanism of this process. In the experiments, tin samples were shock-loaded by nanosecond laser over sequential pressures ranging from 7 GPa to 43 GPa. Integrated diagnostic techniques, including four-frame optical transverse shadowgraph imaging system, soft recovery, and photonic Doppler velocimetry (PDV), were employed. They provide the shape of the ejecta in the dynamic fragmentation process, the recovered ejecta, as well as the loading parameters. These experimental results were compared with those obtained using one-dimensional Lagrangian hydrodynamics simulation. The crossed results over sequential pressures allowed us to gain better insights on the different dynamic fragmentation processes of spall fracture and micro-spalling, which can help develop reliable models for dynamic fragmentation of triangular-wave shocking metals.

Constriction flow of cellulose laden air-aqueous foam

Foams are encountered in everyday life across wide applications, e.g., in foods and cleaning products. They have also been widely used in different industries in processes such as flotation and oil recovery. The application of bio-based materials is a novel interest, and foam forming enables these materials to be used more flexibly. For efficient industrial usage, the flow of such materials must be well understood and characterized. This work measured the velocity field of nanocellulose laden foam in a two-dimensional Hele-Shaw cell with a constriction, using optical imaging and particle image velocimetry. The measurements showed that the addition of cellulose increased the effective viscosity of the liquid films. In a numerical simulation using the Giesekus polymer model, the experimental trend was reproduced through increasing the fluid’s viscosity. Adding highly viscoelastic nanofibrillated cellulose suspension to foam affected only the viscous component of the foam. The delayed elastic response did not change.