Two-component Particle Image Velocimetry Research Articles

PaIRS-UniNa: A Robust And Accurate Free Tool For Digital Particle Image Velocimetry And Optical Camera Calibration

This contribution presents PaIRS-UniNa (PArticle Image Reconstruction Software – University of Naples “Federico II”), a free software application designed to perform digital particle image velocimetry (PIV). PaIRS-UniNa relies on a powerful C library designed for speed, robustness and accuracy, and a user-friendly graphical interface (Figure 1) running in Python environment on all operating systems, including Windows, Linux and MacOS. At the current stage, it features modules to perform two-component (2C) PIV, stereoscopic PIV and optical camera calibration for multi-camera systems. The processing of PIV and stereoscopic PIV data is based on an iterative image deformation method, which can be customized by the user varying different parameters in such a way to find the best compromise between accuracy and computational speed. These parameters include the size, shape and number of interrogation windows, the number of process iterations and the type of interpolating functions for both image deformation and displacement interpolation (such as bilinear, bicubic, methods based on simplexes or the FFT shift theorem and B-splines of varying orders). Additionally, it is possible to select a range of weighting windows for cross-correlation estimation and velocity field filtering, including rectangular, Blackman, Gaussian, Harris, Hann windows, among others, and several vector validation strategies for outlier detection and replacement. Such a level of customization enables to effectively adjust the modulation transfer function of the PIV process to meet specific application requirements. For the stereoscopic PIV process two different approaches are possible: mapping, which involves applying PIV to de-warped images for 2D-2C displacement fields, and warping, which applies PIV to raw images and combine 3C reconstruction and image dewarping in a single subsequent step. In order to accurately estimate the laser sheet plane location and correct errors due to misalignment and inaccurate displacement vector positions, a disparity correction procedure is also available. The calibration module (CalVi) supports accurate calibration with the camera models mostly used in the PIV community: polynomials, rational functions and the pinhole camera model. Moreover, it supports the integration of the pinhole camera model with a refractive correction model for cylindrical geometries and camera calibration procedures working with unknown positions and orientations of the calibration target. In such a way, the target may be moved by hand without losing accuracy in comparison with a conventional calibration, in which the target is displaced with the aid of translational and rotational stages and knowledge of the target positions is strictly required. Already used in numerous experimental studies by the Experimental Thermo-Fluid Dynamics group of University of Naples “Federico II” and international collaborating groups, PaIRS-UniNa offers a rapid and effective free alternative to commercial PIV software for research and industrial use.

Fluid Dynamics Effects of a Porous Blockage on Laminar Flow in the Interior Subchannels of a 61-Pin Wire-Wrapped Rod Bundle

Flow blockages in Liquid Metal Fast Reactor (LMFR) fuel assemblies that can cause fuel pin cladding failures require further investigation for the development and optimization of these Generation IV reactor designs. The objective of this study was to experimentally evaluate the effects of a porous blockage accident scenario on laminar flow behavior in the interior subchannels of a prototypical 61-pin wire-wrapped rod bundle. The laminar flow condition is considered because of its importance to natural circulation and loss-of-coolant-accident operating scenarios as well as the limited availability of experimental data. The matched-index-of-refraction method was utilized to obtain two-dimension two-component time-resolved particle image velocimetry (TR-PIV) measurements at three planes centered on blocked and neighboring subchannel regions. Time-averaged first-order and second-order statistics, computed by Reynolds decomposition, were visualized including the mean and fluctuating streamwise and spanwise velocity components. Line profiles described the evolution of flow from upstream regions, to separated flow, and recombination downstream, while a modified Galilean decomposition distinguished differences between flow in the blocked measurement plane and flow in its neighboring counterpart region. Spatial-temporal cross-correlations were performed for the streamwise velocity fluctuations to characterize the convection velocity and decay of traveling vortices in the wake of the porous blockage. The isothermal TR-PIV measurements from this study provide high-fidelity experimental data sets for the validation of computational models and numerical studies to characterize complex fluid phenomena in wire-wrapped rod bundles during the potential accident scenario of a porous, interior flow blockage.

Investigation on the turbulent structures in combined wave-current boundary layers

The problem of turbulent vortices and vortex dynamics under the influence of ocean waves is a subject of continuing research in the field of wave-current interaction. Thus, this study investigates the evolution of turbulence structures and vortex interactions in combined wave-current boundary layers, where water waves propagate with currents. Experiments were performed in two glass-walled laboratory channels with water depths of 160 and 400 mm. A two-dimensional two-component (2D2C) Particle Image Velocimetry (PIV) system was used to measure the velocities. Periodic variations in turbulence are observed for combined wave-current flows. Pairs of counter-rotating vortices are identified in the outer region when the combined flow is reversed, leading to destructive vortex interactions. This process leads to a reduction in the turbulent kinetic energy (TKE) and TKE production and dissipation during the passage of the wave trough. A conceptual model of hairpin vortices in combined wave-current flows is proposed in this study. The results of this study provide novel insights into the hydrodynamics of combined wave-current flows.

Data Assimilation of Ideally Expanded Supersonic Jet Using RANS Simulation for High-Resolution PIV Data

Data assimilation using particle image velocimetry (PIV) and Reynolds-averaged Navier–Stokes (RANS) simulation was performed for an ideally expanded supersonic jet flying at a Mach number of 2.0. The present study aims to efficiently reconstruct all the physical quantities in the aeroacoustic fields that match well with a realistic, experimentally obtained flow field. The two-dimensional, two-component PIV measurement was applied to the jet axis plane, and the time-averaged velocity field was obtained using single-pixel ensemble correlation. Two-dimensional axisymmetric RANS simulation using the Menter shear stress transport (SST) model was also performed, and the parameters of the SST model were optimized via data assimilation using the ensemble Kalman filter. The standard deviation of the observation noise σ, which is a parameter of the ensemble Kalman filter, is estimated by the previously proposed method (Nakamura et al., Low-Grid-Resolution-RANS-Based Data Assimilation of Time-Averaged Separated Flow Obtained by LES. Int. J. Comp. Fluid. Dyn., 2022), and its effectiveness was investigated for the first time. This method effectively estimated the magnitude of σ at each generation without tuning the hyperparameters. The assimilated flow fields exhibited similar flow structures observed in PIV such as the potential core length or shear layer. Therefore, the present framework can be used to estimate time-averaged full flow fields that match well with experimentally observed flow fields, and has the potential to construct a database for the Navier-Stokes-based stability analysis that requires a full flow field.

On the wake characteristics of a model wind turbine and a porous disc: Effects of freestream turbulence intensity

Porous discs are the experimental counterparts of the numerical actuator disc concept and are frequently used in wind tunnel studies to reproduce the far-wake characteristics of wind turbines. The potential of using porous discs instead of wind turbines comes with the benefits such as operational easiness and design simplicity. This paper presents the results of an experimental study focusing on the wake development characteristics of a single porous disc and a single wind turbine under different freestream turbulence intensities but at similar Reynolds number conditions. The measurements are performed using two-dimensional two-component particle image velocimetry (2D2C PIV) technique up to seven diameters downstream distance in the wake of the models. The wake flows are analyzed in terms of the development of mean wake velocity deficit, wake decay, and wake turbulence characteristics. The results show that the wake recovery occurs faster for both the porous disc and the turbine with increasing freestream turbulence intensity. The wake width increases with the freestream turbulence intensity, indicating faster diffusion of the wake for both models. On the other hand, the wake width of the porous disc and the turbine is similar at the same turbulence intensity conditions, although they operate with slightly different thrust coefficients. Turbulent kinetic energy profiles in the far wake of the porous disc and the turbine, which are significantly different for the low turbulence intensity conditions, become similar for the high turbulence intensity case. The comparison of the turbulent kinetic energy, decay rate and production characteristics points to different turbulence mechanisms in the wake of the porous disc and the turbine. In the near wake, the wake decay rate and turbulent kinetic energy levels are higher in the porous disc case due to enhanced mixing levels associated with the grid-generated turbulence. In the far wake, the decay rate of the turbine wake is higher, most probably due to the breakdown of the tip vortices. When the freestream turbulence is increased, the differences in the turbulent wake flow characteristics diminish in the far wake, particularly after a 4.5-diameter downstream distance.

Proper Orthogonal Decomposition (POD) of the Wake Flow Field of a Model Wind Turbine and a Porous Disc under Different Freestream Turbulence Intensity Conditions

The effects of freestream turbulence intensity on the wake development of a model wind turbine and a porous disc are investigated through Proper Orthogonal Decomposition (POD) analysis. The capability of porous discs for reproducing far-wake characteristics of a model wind turbine is examined through coherent structures both in the near-wake and far-wake regions. Instantaneous velocity fields are obtained through wake measurements using two-dimensional two-component particle image velocimetry (2D2C PIV). These velocity fields are considered snapshots of the spatial domain. Results show inherent differences between coherent structures of a model wind turbine and porous disc in the near-wake region, especially when the freestream turbulent intensity level is low. However, these differences reduce, and coherent structures become more comparable when the freestream turbulence intensity level is higher. It is shown that the first five streamwise components of POD modes are paired for the model wind turbine and the porous disc cases under high freestream turbulence intensity conditions in the far-wake region.

Experimental study of velocity and turbulence fields in a square mixer-settler tank: Comparison of shake the box PTV and 2D2C PIV

The characterization of flows in agitated vessels is essential for a good understanding of physical and chemical processes and to validate computational fluid dynamics code. The hydrodynamics of cylindrical vessels have been well studied at various scales, mostly in the turbulent regime, typically using particle image velocimetry (PIV). Studies of square tanks or at moderate Reynolds numbers are much rarer however, and 3D Shake-The-Box (STB) Lagrangian particle tracking has never been used in this context. The objective of the present work was to compare two-dimensional, two-component PIV (2D2C-PIV) and STB particle tracking in characterizing the hydrodynamics of a stirred 20 cm sided (8 L) square tank at Reynolds numbers ranging from 1700 to 4300. The velocity fields, velocity profiles, turbulent kinetic energy fields and turbulent kinetic energy dissipation rates obtained using the two approaches were compared. The velocity fields measured with the two methods were found to be in good agreement, except for a few discrepancies in the velocity profiles of the turbine discharge at 250 rpm. The dimensionless velocity profiles were similar above 100 rpm and indicated that the flow was turbulent at the Reynolds numbers considered. The turbulent kinetic energy fields obtained with the two approaches were globally similar, although with STB, the energy tended to be underestimated, especially in the jet discharge area. The STB method was also found to have lower spatial resolution and precision in measuring turbulent kinetic energy dissipation rates, a consequence of a too large interrogation window. Recommendations are proposed to improve the precision of STB measurements.

Resonator-like behavior of a wall-bounded precessing vortex core in a diffuser with wall asymmetries

This paper reports a detailed investigation of the interaction between a wall-bounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and two-component particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonator-like behavior of a wall-bounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure anti-node) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of ±π. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wall-bounded PVC in asymmetric geometries.

Co-flow jet diffusion flames in a multi-slot burner: Flow field and emissions

Motivated by the application of internally air-assisted flares and recent anomalous data that black carbon (BC) emissions changes are not monotonic with the amount of air added inside the fuel stream, a burner was designed to study the flow, emissions, and stability aspects of the existence of both normal and inverse jet diffusion flames in close proximity. Since the radius of curvature of a burner affects all aspects of combustion, such as dynamics, stability, flame structure, and emissions, a slot burner configuration, inspired by the Wolfhard-Parker burner, was adopted. This multi-slot burner consists of five parallel rectangular slots (i.e., the central slot for the inner air, sandwiched between two fuel slots, and surrounded by two outer air slots). When operated with laminar fuel and air flows, this burner produced flame sheets at each fuel-air mixing layer with open optical access to all the flows. Flow fields were characterized using two-dimensional two-component particle image velocimetry, while simultaneous single-lens reflex photography was used to establish the overall height and the location of the flames. Emission measurements specifically targeted BC, a precursor of soot, and oxides of nitrogen (NOx). The experimental test conditions involved constant flows of outer air and propane, and variable inner air flow. At one extreme, i.e., zero inner air flow, two flame bases were produced at the interfaces between the fuel and outer air streams. These flames merged to produce a normal diffusion flame. With the addition of inner air, two more flame bases appeared (referred to as the inner flame) and formed an inverse diffusion flame. With increasing inner air flow, the inner flame transitioned from a closed-tip flame, to an open-tip flame, and eventually to a lifted flame. With increasing inner air flow, the NOx emissions remained constant while the BC emissions, which were 3 g/kg-fuel with no inner air, rose by an order of magnitude as it became an open-tip flame. Only when the inner flame finally lifted off due to the inner air flow, did the BC emissions collapse to near zero. Phenomenological models associated with the importance of partial premixing were proposed to explain this collapse, thereby generalizing this finding to other combustion systems when attempting to reduce BC emissions through secondary internal air addition.

Characterizing Streamwise Development of Surface Roughness Effects on a Supersonic Boundary Layer

Two-component particle image velocimetry was employed to measure the influence of surface roughness on a Mach 2 boundary layer. Three surface conditions were examined: a smooth wall, patterned diamond-shaped roughness, and an approximately random realistic roughness. The response of the boundary layer was characterized in ensemble-averaged measurements of the mean velocity and Reynolds stresses. The streamwise development of the mechanical distortions introduced by the diamond-shaped roughness was also explored through observation of roughness-generated shock waves and changes in the boundary-layer profile. Measurements acquired at three streamwise locations are consistent with the trends observed in other studies when scaled by the roughness friction velocity. Ensemble-averaged outer-scaled streamwise velocity profiles showed a velocity deficit in the near-wall region. Inner-scaled boundary-layer profiles showed a downward vertical shift in the scaled velocities of 4.5 and 3 for the diamond roughness and nonuniform/realistic roughness topologies, respectively. These values show similar responses to the results seen in the literature for similar test conditions. The roughness effects were shown to increase in magnitude along the streamwise distance.

Design of a gust generator and comparison of model wind turbine and porous disc wake flows in a transverse gust

This paper outlines the similarities and differences between the wake characteristics of a porous disc and model wind turbine under transverse gust loading which is generated via a two-vane wind tunnel gust generator. Phase-locked two-dimensional two-component (2D2C) particle image velocimetry (PIV) measurements show that the capability of a porous disc in representing a wind turbine increases under gust loading. Compared to the uniform inflow case, normalized streamwise velocity plots of the porous disc and model turbine are in a better agreement under gusty inflow condition particularly in the near wake region. Comparison of the wake centerlines reveals a similar wavy wake pattern with no significant phase lag suggesting that the gust dynamics is the dominant factor in the determination of the wake dynamics.

Effects of freestream turbulence on the wake growth rate of a model wind turbine and a porous disc

This study presents the results of an experimental investigation that focuses on quantifying the differences between the spreading rates of a model wind turbine wake and a porous disc wake at different freestream turbulence intensity levels. Two-dimensional two-component particle image velocimetry (2D2C PIV) measurements are performed within the wakes of a model wind turbine and a porous disc (up to 7D downstream) of the same diameter and a matching thrust coefficient. The wind turbine is operated at a Tip Speed Ratio (TSR) of 2 in order to have matching thrust coefficient conditions for a consistent wake comparison. The results show that the mean wake flow field (both near and far wake) is significantly different for the wind turbine compared to the porous disc even if they are operating at similar, high or low, freestream turbulence levels. The wake of the wind turbine recovers much faster than that of a porous disc with a matching thrust coefficient especially in the far wake region at both low and high freestream turbulence levels. On the other hand, the data shows that the far wake of the turbine operating at low freestream turbulence is very similar to that of the disc operating at high freestream turbulence. This suggests caution and stresses the importance in choosing the freestream turbulence intensity level when using porous discs to represent wind turbines in wind tunnel studies.

Generation and characterization of fully developed state in open channel flow

A fully developed approach flow is necessary in open channel studies to maintain commonality among datasets obtained from different facilities. Two-component planar particle image velocimetry is used to study the characteristics of fully developed smooth open channel flow at a constant Reynolds number of 3.9 × 104 based on the maximum velocity and flow depth. The near-bed boundary layer is tripped to achieve a fully developed state and compared with the under- and over-tripped cases. The Reynolds stresses and higher-order moments are used as indicators to establish the fully developed state. Flow properties are explored by identifying uniform momentum zones (UMZs) using the probability density function of streamwise velocities. The instances are grouped based on the number of UMZs (NUMZ) and conditional averaging of flow variables of each group is used to evaluate the difference in flow properties between the developed and the developing flow. Large-scale ejections are found in the logarithmic layer when NUMZ is higher, whereas a lower number indicates the existence of large-scale sweeping motions. The distribution of the conditionally averaged ratio of the shear contribution from ejections and sweeps and velocity deficits shows a vertical variability in the fully developed state. The large-scale and pointwise quadrant events are used simultaneously to depict variability in inner flow properties between developing and fully developed flow which cannot be recognized in the mean flow characteristics. The sweep events have much higher shear generation in the outer flow in the fully developed state whereas the shear stress contribution from ejection is lower than that in developing flow.

90° 曲がりを有する円形断面ノズル内の非定常流がオイルジェット挙動へ与える影響

Methods of decreasing the CO2 emissions of the internal combustion engine have been suggested. However, these methods generate high combustion temperatures that increase the heat load in general. Therefore, the piston cooling gallery has been proposed as a system for cooling. For supplying the oil to cooling gallery, an oil jet injected from a nozzle placed under the piston flows into the cooling gallery through an entrance hall. It may thus be desirable to control the shape of the oil jet such that it is stable and straight. However, the interface of the ambient air and oil jet may have unstable, and the mechanism is unclear. In this paper, we measured nozzle internal flow using two-dimensional two-component time-resolved particle image velocimetry and visualization of the jet time-synchronously. As the result, we clarified that the characteristic POD (Proper Orthogonal Decomposition) modes of nozzle internal flow makes oil jet wavy.

Flow Separation Control over an Airfoil Using Plasma Co-Flow Jet

A NACA0025 coflow jet airfoil model, including the inner and outer airfoils, was designed and manufactured through 3D printing technology. Using the duct connecting the leading- and trailing-edge slots formed by the inner and outer airfoils, eight plasma actuators are arranged in the duct and synchronously driven by two high-voltage alternating-current sources to realize the plasma coflow jet (PCFJ) flow control technique. The test angle of attack range is 8–30°, and the Reynolds number is 68,000. The induced airflow characteristics were studied, and the flow control mechanism of the PCFJ airfoil was explored using the detailed two-component particle image velocimetry technique. The results in the quiescent air show that in addition to the injection at the leading-edge slot and the suction at the trailing-edge slot, the actuators on the upper surface of the inner airfoil provide local flow acceleration and momentum injection. The results in the wind tunnel show that the PCFJ airfoil has a better performance in delaying flow separation, and its time-averaged flow separation position is delayed by 46% compared with the baseline airfoil. The mixing shear layer that becomes turbulent in advance and the subsequent coherent vortex structures, the injection at the leading edge and suction at the trailing edge, and local flow acceleration and momentum injection of the plasma actuators are the key factors for the mechanism of separation delaying of the PCFJ flow control technique.

Distortion correction of two-component two-dimensional PIV using a large imaging sensor with application to measurements of a turbulent boundary layer flow at $$\hbox {Re}_{\tau } = 2386$$

In the past decade, advances in electronics technology have made larger imaging sensors available to the experimental fluid mechanics community. These advancements have enabled the measurement of 2-component 2-dimensional (2C-2D) velocity fields using particle image velocimetry (PIV) with much higher spatial resolution than previously possible. However, due to the large size of the sensor, the lens distortion needs to be taken into account as it will now have a more significant effect on the measurement quality that must be corrected to ensure accurate high-fidelity 2C-2D velocity field measurements. In this paper, two dewarping models, a second-order rational function (R2) and a bicubic polynomial (P3) are investigated with regards to 2C-2D PIV measurements of a turbulent boundary layer (TBL) using a large imaging sensor. Two approaches are considered and compared: (i) dewarping the images prior to the PIV cross-correlation analysis and (ii) undertaking the PIV cross-correlation analysis using the original recorded distorted images then followed by using the mapping functions derived for image dewarping to provide the correct spatial location of the velocity measurement point. The results demonstrate that the use of P3 dewarping model to correct lens distortion yields better results than the R2 dewarping model. Furthermore, both approaches for the P3 dewarping model yield results which are statistically indistinguishable.

Analysis of the Contribution of Large Scale Motions to the Skin Friction of a Zero-Pressure-Gradient Turbulent Boundary Layer Using the Renard-Deck Decomposition

Coherent flow structures in turbulent boundary layers have been an active field of research for many decades, as they might be the key to reveal the mechanics of turbulence production and transport in turbulent shear flows. Renard and Deck (2016) proposed a theoretical decomposition for the mean skin-friction coefficient based on the mean kinetic energy budget in the streamwise direction. This decomposition, referred to as the Renard-Deck (RD) decomposition, decomposes the mean skin friction generation into three physical mechanisms in an absolute reference frame, namely, direct viscous dissipation, turbulent kinetic energy production, and spatial growth. In this study, the large scale motions (LSMs) are extracted using a proper orthogonal decomposition (POD) of the velocity field based on high-spatial-resolution two-dimensional – two-component particle image velocimetry (HSR 2C-2D PIV) of a zero-pressure-gradient turbulent boundary layer (ZPG-TBL), and their effect on the skin friction via RD decomposition.

PIV Measurements of the Wake Formation from a Rough Flat Plate

Wake flows are prevalent in a wide range of engineering applications and their behaviour can significantly impact engineering design and performance. A considerable body of work exists on smooth body wake structures and flows over rough bodies, however, there is a lack of fundamental physical understanding of the amalgamation of the two fields. Two-component two-dimension particle image velocimetry (2C-2D PIV) is used to investigate the effect of surface roughness on the formation of large scale structures in the near wake of a thin flat plate. Both high-speed and low-speed, high-resolution PIV setups have been used to investigate the effect of surface roughness on the boundary layer and the near wake of the plate to gain insight into the underlying physical connection between these regions.

Effects of inflow boundary layer on the wake of a radially non-uniform porous disk

This study presents the results of an experimental investigation focusing on the effects of the inflow boundary layer on the wake characteristics of a 0.12 m diameter porous disk with radially non-uniform porosity in terms of mean flow, turbulence, and wake scaling. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disk is lowered deeper into a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain. Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disk centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production, and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disk such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., toward the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake.

Experimental and numerical investigation of pulsed flows in asevere aortic stenosed model

Steady and pulsatile aortic stenotic flows through stenosis tubes were experimentally and numerically investigated. The objective was the understanding of the fluid dynamics in arterial geometries most relevant in the context of atherosclerosis. Axisymmetric phantoms corresponding to significant artery stenosis of 50% in diameter and severe aortic stenosis of 75% were respectively machined from silicon. A water flow circuit was established, a steady flow was provided by gravity and a pulsed flow by a pulsatile pump. At inlet Reynolds numbers in the range of 85 to 1125, flows at the stenosis region were investigated using two-component Particle Image Velocimetry (PIV). For the unsteady flow, three different heartbeats (60, 69 and 90 beats per minute) were considered. The k-ω shear-stress-transport first-order turbulence model in Computational Fluid Dynamics (CFD) commercial software was adopted for simulations. Experimental measurements of the velocity fields show good agreements with CFD for both steady and pulsed flows. Recirculation regions were found near the stenosis in both cases. Reverse flow through the stenosis was also observed in pulsatile flow during the end diastolic phase of the cycle. CFD simulations allowed us to accurately assess wall shear stress in the stenotic region where the optical measurements are very noisy. High values of wall shear stress (with high variations both in space and time), are observed, which are indicators of possible future aortic wall damage.