Time-resolved Tomographic Particle Image Velocimetry Research Articles

Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

The near wake of a hemisphere immersed in a laminar boundary layer is studied utilizing time-resolved tomographic particle image velocimetry (TPIV). Focus is placed on the three-dimensional vortical structures and the formation details of hairpin vortices before the onset of transition. The three-dimensional instantaneous pressure field of the hemisphere wake is reconstructed for better understanding the flow mechanism. Experiments are carried out with Reynolds number $Re_{r}=1370$ , based on the hemisphere radius $R$ . Features of periodicity of the near wake are analysed using proper orthogonal decomposition and Fourier transformation. The velocity fluctuation in the wall-normal direction is shown to be crucial to the formation of hairpin vortices in the near wake. By investigating the transport of mass and vorticity, and the correlation between pressure and hairpin vortex strength, the formation mechanism is revealed clearly. Specifically, the main hairpin vortices (MHVs) are formed within the reaction of outer high-speed flow and near-wall flow. The formation of the head portion is followed by the leg portion formation. The shedding of the MHVs is highly correlated with the pressure, as well as the pressure gradient in the wall-normal direction. For the side hairpin vortices (SHVs), the leg portion is formed first, followed by the generation of the head portion thanks to induction of the re-oriented standing vortices. The generation of the SHVs can be regarded as the downstream bridging of the standing vortices, similar to the generation of hairpin vortices due to the connection of streamwise vortices in turbulent boundary layers.

Turbulent boundary layer over porous media with wall-normal permeability

Porous walls are a widely used passive flow control technique, which shows potential in reducing skin friction and mitigating flow-introduced noise. In the present study, porous media with wall-normal permeability is applied to a flat plate to investigate its interaction with the turbulent boundary layer at the Reynolds number based on friction velocity of Reτ=225. Time-resolved planar and tomographic particle image velocimetry were employed to identify the impact on mean statistics and coherent structures. An overall skin friction reduction of 22% is achieved. The porous wall induces counter-rotating streamwise vortex pairs at the spanwise sides of each pore, leading to momentum transport and the generation of alternative low- and high-speed regions close to the wall. Slip velocity is obtained, associated with the reduction in turbulent fluctuations and Reynolds shear stress. The streamwise velocity streaks and the hairpin vortices are significantly distorted compared with the smooth wall condition due to the downwash and upwash motion, featuring a notable reduction in the number and scale of the coherent structures, in which the skin friction reduction mechanism is related to. The proper orthogonal decomposition analysis returns the most energetic unsteady modes. Although the wall-coherent mode type remains to dominate the production of turbulent fluctuations, the scale and energy content of wall-incoherent modes increase, confirming the modification of the distribution and scale of near-wall turbulent structures.

Dynamics of cavitating tip vortex

The three-dimensional dynamic behaviour of a tip vortex generated by a NACA $66_2$ -415 hydrofoil is investigated under wetted flow and cavitating conditions using time-resolved tomographic particle image velocimetry. Two main cavitation modes are studied, namely the breathing and double-helical modes. The time-averaged flow field consists of a system of two streamwise vortices for all three conditions. The shape of the tip vortex resembles that of the cavitation mode, instead of a circular cylinder. Multi-scale vortical structures are captured in the instantaneous flow field. The surface oscillation of the cavity contributes to the growth of Kelvin–Helmholtz instability over the tip vortex, leading to the onset of hairpin vortices. Stronger spanwise interaction between the tip vortex and flow separation of the hydrofoil is produced by cavitation, further intensifying the perturbation growth. The proper orthogonal decomposition analysis gives insight into the relationship between cavity oscillation and vortex instability. Two major types of unstable modes of the tip vortex are obtained, leading to serpentine centreline displacement and elliptical deformation motion. The selection of the dominant unstable mode is associated with cavity surface oscillation. For the wetted flow condition, the displacement mode dominates the growth of vortex perturbation, while for breathing mode cavitation, the most energetic unstable mode changes into an elliptical deformation pattern, the disturbance energy of which is negligible in the wetted flow condition. Consistency is found between the peak frequency of the deformation mode and the cavity resonance frequency, indicating the contribution of cavity oscillation to the disturbance growth and breakdown of the tip vortex.

Hydrodynamics characteristics of non-uniform inflow in reactor coolant pump based on time-resolved tomographic particle image velocimetry

Non-uniform inflow at the inlet of a reactor coolant pump generates performance variations, such as in the head, efficiency, cavitation, and vibrations. The three-dimensional velocity and pressure for non-uniform inflow are reconstructed via time-resolved tomographic particle image velocimetry to evaluate its effect on the reactor coolant pump. Five volume velocity fields were reconstructed through the multiplicative algebraic reconstruction technique, which constitutes the entire non-uniform flow field. The statistics of the velocity fields were used to study the non-uniform inflow characteristics. The non-uniform inflow contains two large-scale vortices in the form of counter-rotating vortex pairs. The influence of non-uniform inflow on the performance of the pump may be caused by the non-uniform pressure field. For power fluctuations during reactor operations, this non-uniform pressure distribution may generate a low-pressure region on the impeller, which induces cavitation. The non-uniform inflow shown by inhomogeneous vectors was evaluated statistically through the turbulent kinetic energy, which represents the velocity variance in each direction. For a constant head, the non-uniformity of the flow field increased with the flow rate, and the scale of small-scale turbulent vortices decreased. With proper orthogonal decomposition analysis, 90% of the energy region and flow structures were dominated by the previous 412 modes. Furthermore, the temporal modes 1, 2, 3, 100, 200, and 400 show that the frequency of large-scale eddy turnover was about 6.6–13.2 Hz. However, the large-scale eddy could be characterized within the first mode of the spatial distribution.

Simultaneous visualization of the velocity and wall temperature fields in impinging swirling jets

Laser-based three-dimensional velocimetry techniques have been recently undergone a fast and significant development enabled by the improvements in both hardware and processing methods. The benefits of this progress are huge especially in the field of the investigation of complex turbulent flows. In this paper, the application of time-resolved tomographic particle image velocimetry (TPIV) to the analysis of submerged impinging swirling jets is presented. Swirling flows recur both in natural phenomena and industrial processes and impinging swirling jets have been extensively investigated for heat transfer applications; however, three-dimensional and time-resolved investigations on the impinging flow fields of swirling jets are rare in the literature. The current study focuses mainly on the effects of the swirl number on the formation and development of the coherent vortex structures arising in this kind of flow. Five values of the swirl number, including the non-swirling case (S=0, 0.23, 0.43, 0.61 and 0.74) are investigated for fixed values of the Reynolds number (Re=5,100) and the impingement distance (equal to two nozzle exit diameters). TPIV measurements are carried out both in the near field (in proximity of the nozzle exit) and in the impingement zone (in proximity of the wall). In the latter case, the water jet is set to a temperature higher than the ambient one and the wall temperature distribution is measured via time-resolved infrared (IR) thermography with the aid of a transparent IR window. The simultaneous velocity and temperature measurements elucidate the role of the vortex structures in both the flow dynamics and transport of internal energy, which is a passive scalar in the present operating conditions. Modal decomposition techniques, in particular the proper orthogonal decomposition, are also applied to identify the most characteristic structures of the turbulent flows.

Combined three-dimensional flow field measurements and motion tracking of freely moving spheres in a turbulent boundary layer

A combination of time-resolved tomographic particle image velocimetry, refractive index matching technique and machine vision algorithms was used to measure the translational and rotational motion of freely moving, nearly neutrally buoyant spheres in a fully developed turbulent boundary layer (TBL). Located in the buffer and logarithmic layers, the hydrogel spheres ( $\sim$ 70 inner wall units in diameter) were refractive index matched with the water and tagged by ‘spokes’. Besides translational motion, the spheres exhibited significant rotation. The spheres were surrounded by typical coherent structures observed in TBLs, among them hairpin packets and transverse and longitudinal vortices that induced ejections and sweeps. While the majority of instantaneous sphere Reynolds numbers did not exceed 100, and vortex shedding was not observed, the results showed that the spheres may affect the evolution of hairpin packets in TBLs due to their finite size. The instantaneous rotation-, wall- and shear-induced lift forces, as well as the drag forces, acting on the spheres were estimated using available correlations for the lift and drag coefficients. Results hinted at negative shear-induced lift due to flow separation at a smaller critical Reynolds number than incorporated in the correlations that do not include the effect of ambient turbulence. The results indicated further that the drag force aided by the rotation-induced lift force was instrumental in keeping one of the spheres aloft. For the wall-ward moving spheres, lift forces opposed sphere motion. As a result, the spheres approached the wall with velocities lower than their quiescent settling velocity.

Comparison of waveform estimated using inverse filtering with direct measurement of the volume flow at the glottal exit

Inverse filtering is a signal processing technique used to estimate the glottal waveform from the speech signal. Although this technique is regularly used in research studies, it has never been formally validated because of the difficulties in obtaining direct measurements of the glottal airflow. The objective of this study is a first step towards validating this technique by comparing its estimated glottal waveform with flow measurements taken simultaneously at the glottal exit. The setup is based on synthetic vocal folds connected to a vocal tract model. Direct measurements of the volume flow at the glottal exit are taken using time-resolved tomographic particle image velocimetry. A circumferentially vented pneumotachograph (i.e., Rothenberg) mask is connected to the vocal tract and used with the Glottal Enterprise system to calculate the glottal waveform using inverse filtering. Effect of varying subglottal pressures and effect of near-field (i.e., the minimal gap between the false vocal folds) and far-field (i.e., oral opening) constrictions were also investigated. Results show that overall using inverse filtering gives a good approximation of the glottal flow waveform, but its accuracy can change depending on the constrictions in the vocal tract. The clinical implication of these findings will be further discussed.

Experimental study on a zigzagging bubble using tomographic particle image velocimetry with shadow image reconstruction

For decades, it has been proven by numerous experiments and simulations that a single bubble freely rises in an unstable path and shape in a surface tension force dominant regime. Using time-resolved tomographic particle image velocimetry combined with three-dimensional shadow image reconstruction, the present study experimentally provides a full three-dimensional diagnosis of the shape and wake structures of a zigzagging bubble. An ellipsoidal bubble with an equivalent diameter of deq = 5.47 mm freely rising in stagnant water is investigated at a terminal Reynolds number of 1390 with a zigzag path. The results show a typical double-threaded vortex structure generated during the initial ascending stage. In the regular zigzagging stage, a four-ring mode of vortex generation is observed, which is composed of alternatively discharged and induced hairpin vortices. Thanks to the volumetric measurement, the shedding or inducing mechanism of complicated wake structures is clearly achieved. We speculate that the secondary shape oscillation of the bubble is excited by the shedding of the primary hairpin vortex. Frequencies of the bubble trajectory, variation of velocity, and bubble shape oscillation are analyzed in detail. Their associated harmonics are classified to indicate the interactions between the bubble and the wakes.

Analysis of the 3D flow of an impinging jet on a slotted plate using TR-Tomo PIV and Proper Orthogonal Decomposition

Impinging jets on plates with specific geometries, found in habitable enclosures, require a fundamental knowledge of the flow topology in order to reduce any acoustic or aeraulic discomfort. However, the flow in this type of jets is three-dimensional, time-dependent and turbulent. Thus, a high-speed volumetric velocity measurement technique is required to investigate the complete topology of the three-dimensional unsteady coherent flow structures. The research in the present paper has as aim the analysis of the flow from a volumetric point of view by using Time Resolved-Tomographic Particle Image Velocimetry (TR-TPIV) measurements. For this reason, an impinging jet on a slotted plate experimental set up has been developed and implemented. The kinematic fields of the studied volume are measured by TPIV acquisitions. Afterwards, the flow’s most energetic features were understood by the use of the snapshot Proper Orthogonal Decomposition (POD) technique which was applied on the kinematic fields. The results have shown that the first two modes were the most energetic and reflected the convective motion through the volume.

Experimental study on low-speed streaks in a turbulent boundary layer at low Reynolds number

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The interactions of rectangular synthetic jets with a laminar cross-flow

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Flow measurements in the near wake of a smooth sphere and one mimicking a pine cone

The flow field in the sphere wake may be affected by roughness patterns such as those found on pine cones. These may enhance pollination by generating favorable flow patterns that capture conspecific pollen. A combination of time-resolved particle image velocimetry (PIV) and tomographic PIV is used to study the instantaneous three-dimensional vortex shedding, as well as mean velocities and turbulent stresses, in the near wake of a smooth and a rough (mimicking a pine cone) sphere, at intermediate Reynolds numbers (from 200 to 5000), where little quantitative flow field data are available.

A novel subcritical transition to turbulence in Taylor–Couette flow with counter-rotating cylinders

The transition to turbulence in Taylor-Couette flow often occurs via a sequence of supercritical bifurcations to progressively more complex, yet stable, flows. We describe a subcritical laminar-turbulent transition in the counter-rotating regime mediated by an unstable intermediate state in a system with an axial aspect ratio of $\Gamma=5.26$ and a radius ratio of $\eta=0.905$. In this regime, flow visualization experiments and numerical simulations indicate the intermediate state corresponds to an aperiodic flow featuring interpenetrating spirals. Furthermore, the reverse transition out of turbulence leads first to the same intermediate state, which is now stable, before returning to an azimuthally-symmetric laminar flow. Time-resolved tomographic particle image velocimetry is used to characterize the experimental flows; these measurements compare favorably to direct numerical simulations with axial boundary conditions matching those of the experiments.

Structure evolution at early stage of boundary-layer transition: simulation and experiment

The beginning of laminar–turbulent transition is usually associated with a wave-like disturbance, but its evolution and role in precipitating the development of other flow structures are not well understood from a structure-based view. Nonlinear parabolized stability equations (NPSE) were solved numerically to simulate the transition of K-regime, N-regime and O-regime. However, only the K-regime transition was examined experimentally using both hydrogen bubble visualization and time-resolved tomographic particle image velocimetry (tomo-PIV). Based on the ‘NPSE visualization’ and ‘tomographic visualization’, at least four common characteristics of the generic transition process were identified: (i) inflectional regions representing high-shear layers (HSL) that develop in vertical velocity profiles, accompanied by ejection–sweep behaviours; (ii) low-speed streak (LSS) patterns, manifested in horizontal timelines, that seem to consist of several three-dimensional (3-D) waves; (iii) a warped wave front (WWF) pattern, displaying multiple folding processes, which develops adjacent to the LSS in the near-wall region, prior to the appearance of 𝛬-vortices; (iv) a coherent 3-D wave front, similar to a soliton, in the upper boundary layer, accompanied by regions of depression along the flanks of the wave. It was determined that the amplification and lift-up of a 3-D wave causes the development of the HSL, WWF and multiple folding behaviour of material surfaces, that all contribute to the development of a 𝛬-vortex. The amplified 3-D wave is hypothesized as a soliton-like coherent structure. Based on our results, a path to transition is proposed, which hypothesizes the function of the WWF in boundary-layer transition.

Volume velocity in a canine larynx model using time‑resolved tomographic particle image velocimetry.

In the classic source-filter theory, the source of sound is flow modulation. "Flow" is the flow rate (Q) and flow modulation is dQ/dt. Other investigators have argued, using theoretical, computational, and mechanical models of the larynx, that there are additional sources of sound. To determine the acoustic role of dQ/dt in a tissue model, Q needs to be accurately measured within a few millimeters of the glottal exit; however, no direct measures of Q currently exist. The goal of this study is to obtain this waveform in an excised canine larynx model using time-resolved tomographic particle image velocimetry. The flow rate data are captured simultaneously with acoustic measurements to determine relations with vocal characteristics. The results show that glottal waveform characteristics such as maximum flow declination rate are proportional to the subglottal pressure, fundamental frequency, and acoustic intensity. These findings are important as they use direct measurements of the volume flow at the glottal exit to validate some of the assumptions used in the source-filter theory. In addition, future work will address the accuracy of indirect clinical measurement techniques, such as the Rothenberg mask.

Turbulent Kinetic Energy and self-sustaining tones in an impinging jet using High Speed 3D Tomographic-PIV

Impinging jets are encountered in many ventilation systems, and these can have a major impact on the acoustic environment and energy performance. Self-sustaining tones produced by aero-acoustic coupling can occur in impinging jets when a feedback loop develops between the jet exit and the surface of impact. (Powell, 1964) developed an analogy that takes into account the sound sources created by vortices, and (Howe, 1975) was the first to use this analogy in the case of a near-wall flow. Howe’s energy corollary makes it possible to calculate acoustic power using three quantities: vorticity, flow velocity and acoustic velocity, using experimental or numerical data and taking the aerodynamic field to be the main source of energy. In this study, the velocity and the acoustic fields in a rectangular jet impinging on a slotted plate were measured simultaneously using High Speed 3D Tomographic time-resolved particle image velocimetry (Tomographic-PIV) and a microphone. The 3D Tomographic time-resolved has the advantage of providing three components of velocity in a volume. Thus, we inspect the interaction between turbulent energy produced by the flow and the acoustic field in the presence of self-sustaining tones in order to have a better comprehension of the aero-acoustic coupling. Results were obtained for a Reynolds numbers Re = 5294 and Re = 5956 which are configurations that induces acoustic tones. The spectrum of the Turbulent Kinetic Energy (TKE) had peaks of frequencies such that the period of the acoustic signal was smaller than that of the TKE in presence of self-sustaining tones. The findings of this work may serve to develop new techniques of control to reduce the acoustic generation.

The alignment of vortical structures in turbulent flow through a contraction

We investigate experimentally the turbulent flow through a two-dimensional contraction. Using a water tunnel with an active grid we generate turbulence at Taylor microscale Reynolds number $Re_{\unicode[STIX]{x1D706}}\sim 250$ which is advected through a 2.5 : 1 contraction. Volumetric and time-resolved tomographic particle image velocimetry and shake-the-box velocity measurements are used to characterize the evolution of coherent vortical structures at three streamwise locations upstream of and within the contraction. We confirm the conceptual picture of coherent large-scale vortices being stretched and aligned with the mean rate of strain. This alignment of the vortices with the tunnel centreline is stronger compared to the alignment of vorticity with the large-scale strain observed in numerical simulations of homogeneous turbulence. We judge this by the peak probability magnitudes of these alignments. This result is robust and independent of the grid-rotation protocols. On the other hand, while the pointwise vorticity vector also, to a lesser extent, aligns with the mean strain, it principally remains aligned with the intermediate eigenvector of the local instantaneous strain-rate tensor, as is known in other turbulent flows. These results persist when the distance from the grid to the entrance of the contraction is doubled, showing that modest transverse inhomogeneities do not significantly affect these vortical-orientation results.

Single- and double-helix vortex breakdown as two dominant global modes in turbulent swirling jet flow

In this paper, we study the shape and dynamics of helical coherent structures found in the flow field of an annular swirling jet undergoing vortex breakdown. The flow field is studied by means of time-resolved tomographic particle image velocimetry measurements. The obtained flow fields are analysed using both classic and spectral proper orthogonal decomposition. Despite the simple geometrical set-up of the annular jet, the flow field is very complex. Two distinct large-scale helical flow structures are identified: a single and a double helix, both co-rotating with the swirl direction, and it is revealed that these structures are not higher harmonics of each other. The structures have a relatively low energy content which makes it hard to separate them from other dynamics of the flow field, notably turbulent motions. Because of this, classic proper orthogonal decomposition fails to identify both structures properly. Spectral proper orthogonal decomposition, on the other hand, allows them to be identified accurately when the filter size is set at around eight times the precession period. The precession frequencies of the single and double helices correspond to Strouhal numbers of 0.273 and , respectively. A global stability analysis of the mean flow field shows that these structures correspond to two separate global modes. The precessing frequencies obtained by the stability analysis and the related spatial structures match very well with the experimental observations. The current work extends our knowledge on turbulent vortex breakdown and on mean field global stability theory in general. It leads to the following conclusions. Firstly, single- and double-helix vortex breakdown are both manifestations of global modes. Previous studies have shown that both and modes can coexist in swirling jets. However, the mode has been identified as a second harmonic of the first mode, while this study identifies both as two independent global modes. Secondly, this work shows that the simultaneous occurrence of multiple helical global modes is possible within a turbulent flow and their shapes and frequencies are very well predicted by mean field stability analysis. The latter finding is of general interest as it applies to a wide class of fluid problems dominated by multiple oscillatory structures.

Characterization of hop-and-sink daphniid locomotion

AbstractThis study characterizes the hop-and-sink locomotion of Daphnia magna, a zooplankton species widely studied in a variety of biological fields. Time-resolved tomographic particle image velocimetry (tomo-PIV) is used to obtain 3D kinematics and flow field data with high spatial and temporal resolution. The kinematics data show that the daphniid’s velocity quickly increases during the power stroke, reaching maximum accelerations of 1000 body lengths/s2, then decelerates during the recovery stroke to a steady sinking speed. The hop-and-sink locomotion produces a viscous vortex ring located under each second antennae. These flow structures develop during the power stroke, strengthen during the recovery stroke, and then decay slowly during the sinking phase. The time records of vortex circulation are self-similar when properly normalized. The flow fields were successfully modeled using an impulsive stresslet, showing good agreement between the decay of circulation and a conceptual model of the impulse. While no relationships were found between kinematics or flow field parameters and body size, the total energy dissipated by the daphniid hop-and-sink motion was found to scale exponentially with the vortex strength.

Revisiting coherent structures in low-speed turbulent boundary layers

Coherent structures are essential for the momentum exchange and turbulence production in wall-bounded turbulent flows. Diversified coherent structures have been observed in turbulent boundary layers, and hairpin-based vortices dominate most of the relevant literature. However, there is no consensus yet on the origin and forming mechanism of hairpin vortices. Herein, five cornerstones pertaining to the framework of hairpin-based coherent structures are reviewed, and three different hairpin generation mechanisms are clarified. Next, the time-resolved tomographic particle image velocimetry (Tomo-PIV) is used in an early turbulent boundary layer (Reθ= 420) to investigate the origin of hairpin vortices. The timelines reveal a triangular bulge in the low-speed streak (LSS), and the initial roll-up occurs at two sides of it. Meanwhile, the material surfaces manifest as a three-dimensional (3D) wave structure in the LSS, which may support the model of a soliton-like coherent structure (SCS). Subsequently, the method of Lagrangian-averaged vorticity deviation is used to detect early vortices. We find that the 3D wave structure is flanked by two vortices, thus confirming the roll-up of timelines and demonstrating the advantage of the Lagrangian criteria in capturing structures in complex flows. These results suggest that various coherent structures may evolve from the metamorphosis of 3D wave structures and their later interaction. Finally, the limitations of traditional experimental and post-processing tools are discussed.