Time-resolved Particle Image Velocimetry Measurements Research Articles

Experimental study of turbulent flow in lower plenum with flow skirt of reactor pressure vessel of pressurized water reactor

The turbulent flow in lower plenum of reactor pressure vessel (RPV) of pressurized water reactor (PWR) is an important phenomenon for design of flow mixing device. A high-precision matched index of refraction (MIR) technique is requisite for time-resolved particle image velocimetry (TR-PIV) measurements in RPV with complicated internals. In this study, turbulent flow in a scaled-down RPV model was measured by TR-PIV under different Reynolds numbers (Re) and asymmetrical flow rate with aid of the high-precision MIR technique of sodium iodide (NaI) solution and acrylic. The time-averaged flow structures include flow separation at the corner of lower support plate, fluid bifurcation around the lower edge of flow skirt, flow turning upward below the vortex suppression plate, horizontal jets at holes of flow skirt, vertical jets at holes of vortex suppression plate, improving turbulent mixing and Reynolds stresses in the lower plenum. The coherent structures were studied by proper orthogonal decomposition (POD) analysis. The flow skirt and vortex suppression plate break down large vortices through them. With Re increasing, Reynolds stresses decrease fast, and the energy fractions of low-order modes transfer into high-order modes, because turbulent vortices break up. In the case of asymmetrical flow rate, the time-averaged velocity and Reynolds stress decrease slightly, and the cumulative energy fraction decreases at flow skirt and vortex suppression plate.

Columnar-jet to wall-jet state transition in transversely excited swirling flow

The current work investigates the transition of a swirling jet between two flow states, namely, columnar-jet (CJ) and wall-jet (WJ) states, under the influence of external transverse acoustic forcing. Here, we have performed simultaneous time-resolved stereoscopic particle image velocimetry and dynamic pressure measurements to understand the structure and dynamics of these swirling jets under external forcing. It is observed that at low Reynolds number (Re), the swirl flow transitions from CJ to WJ state due to transverse forcing. And the flow returns to the CJ state when the acoustic is turned OFF. However, above a critical flow Re, the swirl flow does transition from CJ to WJ state when subjected to transverse forcing, but upon turning OFF the acoustics, the flow stays in the WJ state. Thus, the swirling flow demonstrates bistability in the flow states only above a critical flow Re. It is observed that upon forcing, there is an increase in the streamwise velocity fluctuations (u′x) near the centerline and in the radial velocity fluctuations (u′r) near the injector lip. This finding is also confirmed through spectral proper orthogonal decomposition analysis. In addition, it is observed that as the flow transitions from CJ to WJ state, the relative contribution of the convective term Ur¯∂Ur¯∂r toward the radial pressure gradient (∂P¯∂r) increases in comparison to the centrifugal force term (U¯θ2r). The study highlights the effect of acoustic-induced velocity fluctuations on the bistability of swirl flows over a range of flow Re.

Flow structure and impinging interactions of two confined turbulent converging jets in crossflow

In this work, time-resolved particle image velocimetry (TR-PIV) measurements are carried out to study the flow structure and impinging interactions of two confined turbulent converging jets in crossflow, with the upstream and downstream jets inclined at 20° and 170° with respect to the oncoming crossflow direction. Experiments are conducted for fixed dimensionless nozzle separation distance of s=L/D=3, fixed dimensionless nozzle-to-target surface distance of H/D=6.73, fixed duct Reynolds number of Re=3500 and two jets' Reynolds numbers of Rej=3000 and 6000 that correspond to jets'-to-crossflow velocity ratios of rj=3.45 and 6.91, respectively. Flow visualization images showing ensemble-averaged and instantaneous flow distributions and turbulent characteristics including Reynolds stresses and turbulence intensities for equal and non-equal discharge velocities of the jets are presented on several orthogonal planes of the test section. The swirling strength criterion has been employed to identify coherent structures, and cross spectral analysis of the fluctuating velocity signals detecting subharmonic and harmonic sideband frequencies identify the merging and breakup interactions of the evolving coherent structures. The snapshot proper orthogonal decomposition technique (POD) has been applied to extract the dominant vortical structures and to quantify their relative and cumulative energy contributions to the total kinetic energy fluctuation. The spatial structure of the main four POD modes and the temporal projections of these POD coefficients have been explored to understand the spatio-temporal characteristics of the dominant turbulent fluctuations and their spectral characteristics. Our results show the effect of the velocity ratios on the jets' trajectories, penetration, spreading characteristics, crossflow entrainment through the inner and outer shear layers, transverse velocity decay, size of the converging region and location of the combining point. In addition, the dimensionless shedding frequencies of the oscillating combined jet, the developing wall jets and wake vortices have been obtained. Furthermore, it has been shown that for all values of the jets' velocity ratios employed in this study, a recirculation region does not develop in the converging region.

Self-sustained azimuthal aeroacoustic modes. Part 1. Symmetry breaking of the mean flow by spinning waves

In this paper, we study the aeroacoustic instability which occurs in a deep axisymmetric cavity in a turbulent pipe flow. This phenomenon is the axisymmetric counterpart of the classical whistling of a rectangular deep cavity subject to a grazing flow. The whistling of such axisymmetric cavity originates from the interaction of the coherent fluctuations of the vorticity at the cavity's opening with one of its trapped azimuthal or radial acoustic modes. We focus here on the situation involving the first pure azimuthal mode, which is trapped in the cavity. As a consequence of the rotational symmetry of the configuration, azimuthal modes are actually pairs of degenerate eigenmodes, or almost degenerate in the presence of small asymmetries. Therefore, the aeroacoustic instabilities exhibit more complex mechanisms than in the case of a rectangular deep cavity. In particular, we show that self-sustained spinning modes induce a symmetry breaking of the mean flow and we will elucidate the details of this phenomenon. To that end, simultaneous acoustic and time-resolved stereoscopic particle image velocimetry (PIV) measurements are performed. They reveal that when large-amplitude aeroacoustic waves spin around the cavity, a quasi-steady mean flow starts whirling slowly in the opposite direction to the wave propagation. A linear perturbation analysis around an axisymmetric mean flow confirms the experimental observations: although the incoming pipe flow is not swirling, the hydrodynamic component of the aeroacoustic wave induces such whirling motion of the mean flow because of the forcing from the steady part of the coherent Reynolds stress tensor.

An experimental investigation of rotor wakes by means of time-resolved PIV measurements

Rotor wakes generated by a four-bladed isolated rotor of a scaled helicopter model in hover have been investigated by time-resolved particle image velocimetry. Two rotor speeds, i.e., Ω/2π equal to 29.9 Hz and 26.3 Hz, have been considered resulting in flow regimes defined by a tip Mach number and a chord Reynolds number of ∼0.20 and ∼1.50×105 or ∼0.17 and ∼1.32×105, respectively. The rotor disk generates aerodynamic loads for a thrust coefficient of CT=5.7×10−3, and a rotor solidity of σ=0.116. It is found an intensification of thrust fluctuations as the rotor speed increases, i.e., the amplitude of the standard deviation at 29.9 Hz intensifies approximately of 3.5 times more than that observed at 26.3 Hz. This enables the inspection of how aerodynamic loads, featured by two levels of fluctuation amplitudes, impact the spatial organization and dynamics of the wake structures. Data-driven decomposition methods, i.e., Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD), have been exploited for analyzing the most prominent coherent structures. A wake contraction inward the radial direction occurs suddenly below the rotor disk. The early wake ages feature overlapped trajectories of the tip vortices indicating a regular formation process. Conversely, as the tip vortices advect further downstream, their trajectories progressively depart from each other, and engulfment events appear. The dominance of the periodic shedding of the tip vortices was captured by both modal decompositions. The first pair of POD modes describe the spatial organization of the tip vortices and the engulfment. The corresponding time coefficients include multiple frequency peaks discerning the most prominent contribution ascribed to the tip vortex shedding for both rotor speeds. POD analysis highlights slightly higher efficiency at slower rotor speed, exhibiting a more spectral purity for the first couple of time coefficients. Despite multiple harmonics featuring the most energetic flow structures, the dominant pulsation attains a Blade Passing Frequency BPF=1 for both rotor speeds. Similarly, DMD analysis captures the tip vortices at BPF=1. However, the shear layer embeds additional flow structures pulsating at different harmonics, that include wake characteristics at BPF=0.25, secondary representation of the tip vortices scattering in the shear layer (BPF=1.5 and 2).

Dynamics and entrainment mechanism of the jet flows from an elliptical nozzle: time-resolved tomographic PIV measurements

Dynamics and entrainment mechanism of the jet flows from an elliptical nozzle: time-resolved tomographic PIV measurements

Flow measurements of jet impingement upon a semicylinder with crossflow

In this work, time-resolved particle image velocimetry (TR-PIV) measurements have been carried out to investigate the structural features of a submerged transverse round liquid turbulent jet discharged into a confined laminar crossflow. The duct Reynolds number based on crossflow velocity is fixed at Re=3000, and the impinging jet is directed to a semicylindrical concave surface located in one of the walls of a vertical square duct. The spatial development and flow dynamics are analyzed along several orthogonal planes parallel/perpendicular to the jet axis for jet Reynolds numbers based on the jet exit diameter of Rej=3000, 6000 and 9000 (jet-to-crossflow velocity ratios of r=3.27, 6.53 and 9.80), respectively. For this purpose, mean and ensemble-averaged velocity and vorticity distributions, turbulent statistics, and Reynolds shear stresses are obtained for various injection rates. Spectral analysis reveals the shedding frequencies of the wake vortices, and the pairing and breakup processes of the evolving large-scale coherent structures have been related to the measured wake Strouhal numbers. Our results indicate that due to the curvature of the target surface and the upwash of the jet impact, the size of coherent structures decreases with increase in the value of Rej. Proper orthogonal decomposition (POD) of the flow using the snapshot method is applied to extract the dominant features of the turbulent flow. It is observed that at the symmetry plane and for Rej=3000, 6000 and 9000, the energy collected by the first two dominant eddies contribute to 14.98%, 18.68%, and 17.89% of the turbulence kinetic energy, respectively. Our results show that the evolution of the time-resolved temporal coefficients of degenerate pairs of modes (1, 2) represent the flapping motion of the jet. Moreover, the amplitude of the oscillations of the flapping jet and the complexity of the flow structure increase for increasing values of Rej.

Vortex-induced sound prediction of slat noise from time-resolved particle image velocimetry data

A data-driven method for predicting the vortex-induced sound from time-resolved velocimetry data is presented and applied to the sound generated by flow passing through the slat in a multi-element high-lift airfoil. The time-dependent velocity fields in the slat-cove region of the 30P30N multi-element airfoil are obtained from time-resolved particle image velocimetry measurements, and a low-order reconstruction is achieved by using the rank-one vbnm modes from spectral proper orthogonal decomposition. The pressure force and associated dipole sound are then computed via the application of the force and acoustic partitioning methods (Seo et al. in Phys Fluids 34(5):053607, 2002) which involve volume integrals of the product of the second invariant of the velocity gradient tensor and geometry-dependent influence fields. The method enables estimation of the dipole sound generated by local flow structures, and the results are shown to be consistent with theory of vortex sound. Comparison with the measured sound data suggests that while the shear layer modes are responsible for the tonal noise, the interactions between the shear layer modes and other parts of the wing also generate a substantial level of flow noise.

Experimental investigation on cylinder noise and its reductions by identifying aerodynamic sound sources in flow fields

Through anechoic wind tunnel tests, this study comprehensively investigates the noise and drag reductions on a circular cylinder with dimples. Dimples built on a surface pattern fabric cover the cylinder surface as one of the passive flow control methods. The force, noise, and flow field measurements are performed at diameter-based Reynolds numbers ranging from 3×104 to 1.3×105, covering the sub-critical, critical, and supercritical regimes. The force and noise measurement results show that dimple fabric simultaneously reduces noise and drag in the critical regime. The changes in flow structures were characterized by the Time-resolved Particle Image Velocimetry (TR-PIV) measurements. Based on the vortex sound theory, the flow analysis shows that the dominant sound sources are found to be concentrated near the cylinder surface, which is caused by the unsteady vortex motions near the separation locations during the process of vortex shedding. The cross-correlation between the synchronized TR-PIV and microphone measurements further supports the conclusions. Moreover, the cylinder noise reductions controlled by the dimples are directly associated with the reduced sound sources in the critical and supercritical regimes, corresponding to the reduced strength of the vortex shedding.

Data-driven determination of low-frequency dipole noise mechanisms in stalled airfoils

An aeroacoustic investigation of planar time-resolved particle image velocimetry (PIV) measurements in the streamwise surface-normal plane of a NACA 0012 airfoil in static stall is presented at chord-based Reynolds number textrm{Re}_c=7.1times 10^4. Instantaneous planar pressure reconstructions are obtained using a Poisson solver and the dipole noise emanating from the surface is extrapolated via Curle’s acoustic analogy. To correlate structure in the velocity field to the generation of noise, a data-driven framework utilising the proper orthogonal decomposition (POD) and the spectral Linear Stochastic Estimation (sLSE) is employed. The flow structures responsible for noise are found to concentrate in proximity to the trailing edge. In addition, a conditional analysis for the extreme noise events reveals that downwash and upwash events in proximity to the trailing edge, coupled with slow and fast-moving fluid at the incipient shear layer, are correlated to local maxima and minima in the acoustic fluctuations, respectively.Graphical abstract

Tail-shake risks assessment & mitigation by wind-tunnel tests on air-intake installation on a heavy-weight H/C configuration

AbstractIn this work, the key role of the upper-deck design including engine installation as a potential source of tail-shake is at focus. The work is based on a Wind-Tunnel Test (WTT) campaign performed at the Airbus Helicopters’ Marignane wind-tunnel facilities on a high-fidelity minibody fuselage at scale 1:3.5 representing a generic heavy-helicopter upper deck.Two different engine intake installations for a Power Unit (PU) have been investigated; in a first configuration, the air intake is implemented at the pylon-fairing trailing edge. The second configuration consists in positioning two air intakes on each side of the pylon fairing, close to the maximum cross-section location. Different measurement methods to evaluate aerodynamic interactions and wake sources are proposed: flow-separation assessments from surface oil flow visualisations, time-resolved Particle Image Velocimetry (PIV) measurements and unsteady skin-pressure measurements at the cowlings. Tail-shake-related indicators are then proposed. Basically, a configuration that produces strong vortices characterised by a broadband spectral signature is believed to gather all the conditions for tail-shake to emerge.The flow over the clean configuration is first analysed for various combinations of angle-of-attack and sideslip, highlighting four different areas of flow separation at the cowlings. The complex flow topology around the upper deck is then assessed, which includes a spectral analysis of the flow in the PIV planes. The influence of the air intakes (operating or not) is then evaluated. When located at the pylon-fairing trailing edge and operating, the air intake has a spectacular impact on the flow-field topology. It is responsible for the generation of an intense broadband wake interacting with the pylon-fairing lip vortices, which is believed to be a potential source of tail-shake. The second air-intake configuration is also not favourable, as it requires enlarging the pylon fairing by 100mm, which causes an intense wake similarly to a blunt body. At last, a mitigation mean is proposed for the first configuration. It demonstrates a significant reduction of the wake intensity and broadband signature at the source.

Effects of sheltering on the unsteady wake dynamics of tandem cylinders mounted in a turbulent boundary layer

The unsteady wake dynamics of two finite wall-mounted cylinders of unequal height and arranged in tandem are investigated using time-resolved particle image velocimetry (TR-PIV). The cylinders were fully submerged in a turbulent boundary layer with a Reynolds number based on the cylinder diameter (d) of 5540 and boundary layer thickness of$\delta /d = 8.7$. The centre-to-centre spacing between the cylinders was fixed at 4d. The degree of sheltering was studied by varying the height of the upstream cylinder (UC)$(h/d \in [0.7\unicode{x2013} 7.0])$while keeping the height of the downstream cylinder (DC) constant at$H/d\; = \; 7.0$. The resulting height ratios were$h/H = 0.10,0.25,0.50,0.75$and$1.00$denoted as HR10, HR25, HR50, HR75 and HR100, respectively. The wake dynamics of the DC were also compared to those of an isolated single cylinder (SC) with similar Reynolds number, aspect ratio and submergence ratio. TR-PIV measurements were performed in the symmetry plane for all test cases and five spanwise planes along the height of HR75. The results showed that as the height ratio increases, the downwash from the free end of the UC impinges directly on the frontal surface of the DC and induces a strong upwash on the opposite rear side of the DC. The induced upwash impedes the downwash from the free end of the DC much earlier than observed behind the SC, resulting in a reduced reverse flow area and high velocity deficit in the sheltered portion of the DC. Nonetheless, the reverse flow area behind the UC and DC undergoes a quasi-periodic pumping motion with frequencies that are synchronized for HR25 to HR100. Spectral analysis and proper orthogonal decomposition also reveal that the attachment of the shear layers of the UC on the DC also promotes a lock-in anti-symmetric vortex shedding behaviour.

Evolution and control of multiscale vortical structures in a wall-mounted cube wake

In this study, multiscale flow features in a wall-mounted cube wake are investigated experimentally based on two-dimensional time-resolved particle image velocimetry measurements and wavelet transform. Moreover, the control mechanism of the horizontal control hole (HCH) on the cube wake is studied. The width of the cube model is D = 50 mm, and the corresponding Reynolds number is ReD=7800. The flow control cases include nine kinds of HCHs with three different diameters and three different heights. The results show that the shear layer contains a continuous merging process of multiscale vortices, which leads to the momentum deficit. Particularly, the evolution of large-scale vortices causes exponential growth of momentum deficit. In the xy-plane, the large spanwise vortices cause fluctuations in the near wake (x/D ≲ 3), which are stronger than those in the shear layer but are not present in the xz-plane. The downstream wake is anisotropic due to its strong downwash flows and weak inward flow. The HCH issuing flow weakens the intermediate- and large-scale vortices in the shear layer and hinders the interacting shear flows in the wake, thus reducing the momentum deficit in the near wake. In the downstream wake, the effect of HCH is also anisotropic: in xz-plane, the momentum recovery is slow due to the decrease in the downwash flow by HCH; in the xy-plane, the momentum recovery is fast due to the increase in the range of the inward flow by HCH.

Time-resolved PIV measurement of coherent structures downstream of step blockage in a narrow rectangular channel

Experimental study of coherent structures in a narrow rectangular channel with 50 percent single side entrance blockage was conducted by time-resolved particle image velocimetry (TR-PIV) under Reynolds numbers from 2,180 to 21,800. With error and uncertainty analysis, the maximum standard uncertainty is less than 2% of bulk velocity (Wb) for time-averaged velocity and less than 2% Wb2 for Reynolds stress.The blocked channel accelerates fluid and forms strong streamwise velocity gradient, which dominates the flow field downstream the blockage. The shear-flow generates a large reverse flow vortex downstream blockage, which forms a second reverse flow in the corner of narrow channel near blockage. The streamwise flow reattaches the side channel wall downstream the reverse flow vortex. The vortices shedding from blockage interact with the reverse flow, forming a long and narrow region of high Reynolds stress near the inlet. The time-averaged velocity and Reynolds stress are strong in shear-layer and weak in the reverse flow and second reverse flow. The streamwise velocity gradient and Reynolds stress decay in streamwise direction. With Reynolds number increasing, normalized spanwise velocity and reattachment length increase, while normalized streamwise velocity decreases.The coherent structures downstream blockage has different PSD peak frequencies in shear-layer, reverse flow and second reverse flow. The Strouhal number decreases with Reynolds number increasing. The integral spatial–temporal length scales in the coherent structures are determined by the velocity gradient, which forms large turbulent fluctuations in the shear layer and reverse flow. With Reynolds number increasing, the spatial–temporal length scales increase because stronger velocity gradient generates larger coherent structures downstream blockage. The strong anisotropic turbulent flow in narrow rectangular channel with 50% single side entrance blockage exists in terms of Reynolds stress and spatial–temporal length scales.The experimental findings improve understandings of coherent structures in narrow channel with 50% single side entrance blockage. The experimental data can be used for mechanism study of entrance blockage accident and validations of turbulent models.

A novel double-image-sequence correlation method for time-resolved particle image velocimetry

This work reports a novel method with a goal of improving the accuracy and robustness of time-resolved particle image velocimetry (TR-PIV) measurements. This method, named double-image-sequence correlation (DISC), accomplishes this goal by utilizing the temporal information stored in TR-PIV measurements. The key to such utilization is that the DISC method constructs two separate image sequences with well-selected sequence length and separation time. As a result, multiple cross-correlation maps from consecutive image pairs can be averaged to reduce the random measurement noises (e.g., due to laser shot-to-shot variation and out-of-plane flow motion), and also each individual cross-correlation map can be obtained using the image pair separated with optimal time to further reduce the uncertainties due to other factors such as non-optimal in-plane velocity gradients and flow accelerations. The DISC method was validated both numerically and experimentally. The results illustrated that this method was able to enhance the accuracy of velocity measurements by 20.4–47.2% in terms of velocity error and by 7.8–22.9% in terms of non-physical Eulerian acceleration, compared to past multi-frame PIV methods.

Time-resolved PIV measurements inside and downstream of a 180° bend

The flow in a 180° bend with a radius of curvature 1.5 time the pipe diameter (Rc/D of 1.5) was studied for a Reynolds number of 34,800 to characterize the development of the mean velocity and Reynolds stresses and the unsteady motions that may affect the heat or mass transfer in the latter part of the bend. Two-component, time-resolved measurements were performed on the axial symmetry plane and on cross sectional planes in the bend and up to 1D downstream of the bend using refractive index matched Particle Image Velocimetry. The unsteady flow structures were examined using the Snapshot Proper Orthogonal Decomposition (POD) technique. Frequency spectra of the POD coefficients, transients of the coefficients and low-order reconstructions were used to characterize the time scales and dynamics of the motions. The results showed that the initial development of the flow was similar to that in a 90° bend but the development of the mean secondary flow, and the low-speed region associated with this feature, appeared delayed relative to the 90° bend in the middle section of the 180° bend. The results in the second half of the 180° bend showed that the mean longitudinal velocity near the inner wall was larger than that near the outer wall in the latter part of the bend. Changes in the unsteady interaction of the flow with the outer wall in the middle of the 180° bend also appeared to cause significant unsteadiness in the flow near the inner wall in the latter part of the bend. The mean secondary flow towards the end of the 180° bend was significantly diminished relative to the flow in the middle of the bend and largely suppressed by 1D downstream of the bend. Reconstructions of the instantaneous secondary flow showed occurrences of large secondary velocities along the sidewalls up to 170°, but these were substantively reduced even at 0.25D into the downstream bend.

Characteristics of very-large-scale motions in gradually varied open-channel flows upstream of a run-of-river dam

The gradually varied open-channel flows (OCF) upstream of a run-of-river (RoR) dam are ubiquitous in natural rivers. In this flow type, the velocity profile shows some similarities to that in uniform open-channel flows, but the turbulence intensity and Reynolds shear stress are slightly greater. However, the presence and properties of very-large-scale motions (VLSMs) in such flows are still unclear. To fill this research gap, time-resolved particle image velocimetry measurements were performed upstream of a modeled RoR dam in an open-channel flume. Based on pre-multiplied spectra analysis, statistical evidence of the presence of VLSMs in the flow type is reported for the first time. The results reveal that although the typical streamwise wavelength of VLSMs in such gradually varied OCF is similar to that in other flows, such as turbulent boundary layers, closed-channel flows, pipe flows, and uniform OCF, the VLSMs in the present gradually varied OCF are stronger and contribute more streamwise turbulent kinetic energy as well as Reynolds shear stress than other flows.

Analysis of an unsteady quasi-capillary channel flow with time-resolved PIV and RBF-based super-resolution

We investigate the interface dynamics in an unsteady quasi-capillary channel flow. The configuration consists of a liquid column that moves along a vertical 2D channel, open to the atmosphere and driven by a controlled pressure head. Both advancing and receding contact lines were analyzed to test the validity of classic models for dynamic wetting and to study the flow field near the interface. The operating conditions are characterized by a large acceleration, thus dominated by inertia. The shape of the moving meniscus was retrieved using Laser-Induced Fluorescence-based image processing, while the flow field near was analyzed via Time-Resolved Particle Image Velocimetry (TR-PIV). The TR-PIV measurements were enhanced in the post-processing, using a combination of Proper Orthogonal Decomposition and Radial Basis Functions to achieve super-resolution of the velocity field. Large counter-rotating vortices were observed, and their evolution was monitored in terms of the maximum intensity of the Q-field. The results show that classic contact angle models based on interface velocity cannot describe the evolution of the contact angle at a macroscopic scale. Moreover, the impact of the interface dynamics on the flow field is considerable and extends to several capillary lengths below the interface.

Outer-layer coherent structures from the turbulent/non-turbulent interface perspective at moderate Reynolds number

This study reports the observation of outer-layer coherent structures in turbulent boundary layer from the turbulent/non-turbulent interface perspective at moderate Reynolds number. The observation relies on time-resolved particle image velocimetry measurements with a large field-of-view. The local turbulent kinetic energy is used to detect the turbulent/non-turbulent interface. In the reference frame of turbulent/non-turbulent interface, raw velocity is decomposed into three components: time average of raw velocity, conditionally averaging components and turbulent fluctuations. The triple velocity decomposition eliminates the bias of Reynolds decomposition fluctuations in the intermittent region. The conditional statistics based on the turbulent/non-turbulent interface is considered to discuss the turbulence characteristics. The basic statistics of mean raw velocity, Reynolds stresses, and swirling strength are observed in conditional statistics. And the results are compared with the results based on the traditional wall surface reference frame. Quadrant analysis is introduced to extract ejections & sweeps and reflect the statistical characteristics of the outer-layer structure. The contribution of ejections and sweeps to the Reynolds shear stresses exhibits significantly different characteristics. Then, the configuration of the turbulent fluctuations and instantaneous structures is presented relied on the uniform momentum zones in the instantaneous frames. The spatial characteristics of coherent structures are reflected by two-point correlation of the streamwise turbulent fluctuations. In addition, the spatial scale of the outer-layer structures is revealed from the turbulent/non-turbulent interface perspective.

Experimental measurements of fluid flow in an 84-pin hexagonal rod bundle with spacer grid for a gas-cooled fast modular reactor

A 50 MWe helium-cooled fast modular reactor (FMR) is under development, and as part of the Department of Energy Integrated Research Project (IRP), Texas A&M University is conducting the thermal-hydraulic characterization of its baseline core configuration. We experimentally investigated the axial and cross flow fields characteristics in the hexagonal fuel rod bundle composed of 84 rods, a central rod, and spacer grids at a Reynolds number of 12,000. A fully transparent experimental facility resembling of one unit of the fuel assembly was constructed. Time-resolved particle image velocimetry (TR-PIV) measurements were performed to characterize hydraulic behavior downstream the spacer grid. Velocity measurements were conducted in the axial and radial direction of the rod bundle. From the PIV velocity vector fields, the full-field flow statistics were computed for the mean velocity, vorticity, and Reynolds stresses. An analysis of the energy decay downstream of the spacer grid was conducted with calculations of secondary-flow intensities, turbulent kinetic energy, power spectrum, and spatial-temporal velocity cross correlations. The vorticity field was obtained and pairs of counter-rotating vortexes were identified using a 3D reconstruction of several measurement planes. The data from this experimental campaign will be used to validate numerical models based on Computational Fluid Dynamics and other codes.