Instantaneous Field Research Articles

Research on circulation control mechanisms based on Lagrangian dynamics

Circulation control technology greatly improves an aircraft's aerodynamic efficiency by employing high-speed jets at the wing's trailing edge, which generates the Coanda effect. This paper analyzes and reveals the mechanism of circulation control from the perspective of Lagrangian dynamics. First, simulate the circulation control airfoil using the shear stress transport turbulence model to obtain instantaneous flow field data under various jet momentum coefficients (Cμ) and angles of attack. Subsequently, from the perspective of Lagrangian dynamics, the mixing, material transport, and momentum transfer processes between the jet and the trailing-edge separation zone are analyzed using attracting Lagrangian Coherent Structures (aLCSs) under different jet momentum coefficients. Next, through the perspective of particle motion in the Lagrangian framework, the study reveals the accelerating effect of the Coanda jet on the low-speed fluid over the upper wing surface. Additionally, repelling Lagrangian Coherent Structures are used to characterize the downward movement of the leading-edge stagnation point after applying circulation control, and the impact of this movement on the flow state is analyzed. Finally, the analysis using aLCSs assesses the flow control effectiveness of Coanda jets as “virtual aerodynamic flaps.” This analysis helps to further understand the advantages of the “virtual rudder surface” formed by Coanda jet.

Turbulence model adaptability at critical Reynolds numbers and applications in wake control via fairings

The water-drop shaped fairings with varying shape angles are attached to a circular cylinder to achieve wake control and vortex suppression at critical Reynolds numbers. To ensure the capability of Reynolds averaged Navier–Stokes (RANS), detached eddy simulation (DES) and large eddy simulation (LES) models at the critical Reynolds number region, three representative turbulence models are employed: LES with σ subgrid-scale (SGS) model, delayed DES model with improved wall-modeling capability (IDDES) and shear stress transport (SST) k−ω RANS model. These models are utilized to simulate flow around a circular cylinder at Reynolds number Re=2.5×105. The solver used in this paper is further developed based on the high-resolution algorithm platform for incompressible flow (HRAPIF). The comparative analysis of the results from the three turbulence models has been rigorously validated and investigated. An exhaustive examination of the mean flow field, Reynolds stresses, characteristic lengths, and instantaneous flow fields among the models reveals instructive insights. The IDDES and σ-LES models predict the hydrodynamic forces, the so-called ‘drag crisis’, alongside the pressure distribution and skin friction coefficient with high precision. The σ-LES model stands out for its superior accuracy, while the IDDES model is also a viable alternative, offering commendable accuracy with a reduced demand for mesh density. Subsequently, the IDDES model is selected for wake control calculations using fairings with five distinct shape angles (30∘,45∘,60∘,75∘ and 90∘) at Re=2.5×105. In-depth comparisons to the bare cylinder and subcritical results reveal that the wake control effect varies at the critical Reynolds region. The fairing with a 30° shape angle substantially suppresses hydrodynamic forces. The lift coefficient experiences a remarkable decrease of approximately 96%, while the drag coefficient diminishes by about 90%. Concurrently, fairings with angles from 45° to 90° lead to reductions in drag coefficient of 11.6%, 10%, 3% and 4%, respectively. A 75% lowering in the lift force coefficient is found even with a short fairing with shape angle α=90∘, which may be a meaningful instruction to industrial design.

Application of the FDTD Method to Analyze the Influence of Brick Complexity on Electromagnetic Wave Propagation

This article presents a numerical analysis of the effects related to the propagation of electromagnetic waves in an area containing a non-ideal, non-uniform, and absorbing dielectric. The analysis concerns the influence of electrical parameters, the structure of the building material, and the layering of the wall on the values of the electric field intensity. A multivariate analysis was carried out with different conductivity values. Homogeneous materials (e.g., solid brick) can be analyzed using the analytical method. In the case of complex materials containing, e.g., hollows (brick with hollows, hollow block), it is necessary to use the numerical method. The FDTD (finite difference time domain) method was used to assess the dependence of the electric field intensity on the layering, the length of hollows in bricks, and the material loss. In order to check the correctness of the adopted numerical assumptions, a series of tests related to the discretization of the model was carried out. The article also presents the influence of changing the length of hollows in bricks on the values of the electric field intensity at a frequency of 2.4 GHz. The instantaneous field distributions and maximum values of the electric field intensity are presented. In the model with a two-layer wall, regardless of the conductivity, the field values were the same for the two models, where the difference in the percentage of ceramic mass in the brick was 8%. A 12% decrease in the percentage of ceramic mass in the brick resulted in a 15% increase in the value of the area between a single-layer and a double-layer wall made of clinker bricks. At a conductivity of 0.04 S/m for a single-layer wall, the field values were similar for all brick variants.

Spatio-temporal averaging of jets obscures the reinforcement of baroclinicity by latent heating

Abstract. Latent heating modifies the jet stream by modifying the vertical geostrophic wind shear, thereby altering the potential for baroclinic development. Hence, correctly representing diabatic effects is important for modelling the mid-latitude atmospheric circulation and variability. However, the direct effects of diabatic heating remain poorly understood. For example, there is no consensus on the effect of latent heating on the cross-jet temperature contrast. We show that this disagreement is attributable to the choice of spatio-temporal averaging. Jet representations relying on averaged wind tend to have the strongest latent heating on the cold flank of the jet, thus weakening the cross-jet temperature contrast. In contrast, jet representations reflecting the two-dimensional instantaneous wind field have the strongest latent heating on the warm flank of the jet. Furthermore, we show that latent heating primarily occurs on the warm flank of poleward directed instantaneous jets, which is the case for all storm tracks and seasons.

Flow around triangular prisms with varying vertex angle at low Reynolds numbers

The present work investigates the unsteady flow around triangular prisms with vertex angles of 30°,45°,60°, and 90° for shedding Reynolds number between 50 and 150. The numerical simulations of flow around triangular prisms at different vertex angles and Reynolds number has been carried out using the open-source code OpenFOAM. The wake of the prisms in different cases has been examined using instantaneous and time-averaged velocity and vorticity fields. The energy dynamics in the wake are demonstrated using enstrophy. The paper explains the shedding around a prism and reports the differences in the wake due to the different vertex angles of the prisms employed in the present work. Strouhal number and force coefficients have been obtained and compared for different prisms. The coefficient of lift and drag phase plot indicates a higher spread for prisms with larger vertex angles at higher Reynolds number. The shedding frequency has a linear variation with Reynolds numbers for the prisms. The obtained results were compared with earlier works on square cylinders and 45° oriented square cylinder.

Estimation of IFOV Inter-Channel Deviation for Microwave Radiation Imager Onboard FY-3G Satellite

The Microwave Radiation Imager (MWRI) onboard the FengYun satellite plays a crucial role in global change monitoring and numerical weather prediction. Estimating and correcting geolocation errors are important to retrieving accurate geophysical variables. However, the instantaneous field of view (IFOV) inter-channel deviation, which is mainly caused by the structure mounting error and measurement error of feedhorns, is less studied. In this present study, we constructed a general theoretical model to automatically estimate the IFOV inter-channel deviations suitable for conical-scanning instruments. The model can automatically detect the along-track and across-track vectors that pass through the land–sea boundary points and are perpendicular to the actual coastlines. Regarding the midpoints of the vectors as the brightness temperature (Tb) inflection points, the IFOV inter-channel deviation is the pixel offset or distance of the maximum gradients of the Tb near the inflection points for each channel relative to the 89-GHz V-pol channel. We tested the model’s operational performance using the FY-3G/MWRI-Rainfall Mission (MWRI-RM) observations. Considering that parameter uploading adjusted the IFOV inter-channel deviations, the model’s validity was verified by comparing the adjustments calculated by the model with the theoretical changes caused by parameter uploading. The result shows that the differences between them for all window channels are less than 100 m, indicating the model’s effectiveness in evaluating the IFOV inter-channel deviation for the MWRI-RM. Furthermore, the estimated on-orbit IFOV inter-channel deviations for the MWRI-RM show that all channel deviations are less than 1 km, meeting the instrument’s design requirement of 2 km. We believe this study will provide a foundation for IFOV inter-channel registration of passive microwave payloads and spatial matching of multiple payloads.

Large eddy simulation of in-hole blockage effects on cooling performance of fan-shaped holes

Film-cooling and thermal barrier coating approaches are commonly used in gas turbines to protect the turbine parts from the high-temperature flow of gases. However, one drawback of this method is that the coatings can partially obstruct the cooling hole exit on the turbine blade surfaces, resulting in reduced cooling performance. Therefore, this study aimed to investigate the effects of in-hole blockage thickness on cooling effectiveness and flow characteristics of fan-shaped holes using large eddy simulations (LES). A laidback fan-shaped hole located on a flat plate was the reference cooling hole. Numerical simulations were conducted for two different blocked cooling hole configurations with various blockage thicknesses (t/D = 0.5 and 0.9) at three different blowing ratios (0.5, 1.0, and 2.0). The cooling effectiveness computed by the LES method was validated for both unblocked and blocked cooling holes, and the results were compared to the experiments at various blowing ratios. It was revealed that an increase in blockage thickness had a significant impact on the area-averaged cooling effectiveness, particularly at high blowing ratios. The overall area-averaged cooling effectiveness for the t/D = 0.9 case was approximately 75 % lower than that of the unblocked cooling hole. Moreover, assessment of the instantaneous velocity field inside the hole over time revealed that larger blockage thicknesses resulted in greater velocity fluctuations and a more unsteady flow.

Schlieren Image Velocimetry and Modal Decomposition Study of Preheated Isothermal Flow From a Generic Multi-Swirl Burner

Abstract This study presents an experimental investigation into the turbulent flow characteristics of an unconfined counter-rotating dual swirl burner under external acoustic excitation. Utilizing Schlieren image velocimetry (SIV), we capture the velocity field of the swirling jets. Mean velocity field analysis reveals the upstream propagation of the central recirculation zone within the burner passages. Through proper orthogonal decomposition (POD) analysis on instantaneous axial velocity fields, coherent structures are identified and the impact of different actuation methods on spatial modes is illustrated. Spatial modes of the unforced (natural) flow show the presence of a single and double helical precessing vortex core (PVC) modes at St = 0.53. Low-frequency acoustic actuation (St = 0.46) effectively suppresses the PVC mode, while high-frequency (St = 2) actuation stabilizes it. Broadband excitation of the flow field, however, induces the excitation of both single and double helical PVC modes.

The MISTRAL Instrument and the Characterization of Its Detector Array

AbstractThe MIllimeter Sardinia radio Telescope Receiver based on Array of Lumped elements KIDs, MISTRAL, is a cryogenic LEKID camera, operating in the W band ($${77}{-}{103\,\textrm{GHz}}$$ 77 - 103 GHz ) from the Gregorian focus of the 64-m aperture Sardinia Radio Telescope (SRT), in Italy. This instrument features a high angular resolution ($$\sim {12\,\textrm{arcsec}}$$ ∼ 12 arcsec ) and a wide instantaneous field of view ($$\sim {4\,\textrm{arcmin}}$$ ∼ 4 arcmin ), allowing continuum surveys of the mm-wave sky with many scientific targets, including observations of galaxy clusters via the Sunyaev–Zel’dovich effect. In May 2023, MISTRAL has been installed at SRT for the technical commissioning. In this contribution, we will describe the MISTRAL instrument focusing on the laboratory characterization of its focal plane: a $$\sim {400}$$ ∼ 400 -pixel LEKID array. We will show the optical performance of the detectors highlighting the procedure for the identification of the pixels on the focal plane, the measurements of the optical responsivity and NEP, and the estimation of the optical efficiency.

A New Method for Calculating Instantaneous Atmospheric Heat Transport

Abstract Atmospheric heat transport (AHT) is an important piece of our climate system but has primarily been studied at monthly or longer time scales. We introduce a new method for calculating zonal-mean meridional AHT using instantaneous atmospheric fields. When time averaged, our calculations closely reproduce the climatological AHT used elsewhere in the literature to understand AHT and its trends on long time scales. In the extratropics, AHT convergence and atmospheric heating are strongly temporally correlated suggesting that AHT drives the vast majority of zonal-mean atmospheric temperature variability. Our AHT methodology separates AHT into two components (eddies and the mean meridional circulation) which we find are negatively correlated throughout most of the mid- to high latitudes. This negative correlation reduces the variance in the total AHT compared to eddy AHT. Last, we find that the temporal distribution of the total AHT at any given latitude is approximately symmetric.

Direct numerical simulation of fully-developed supersonic turbulent channel flows with dense vapors

This work aims to investigate the impact of the molecule-complexity effect and the non-ideal effect on wall-bounded turbulent flows by applying direct numerical simulation (DNS) to fully-developed channel flows of two typical organic vapors: R1233zd(E) and octamethyltrisiloxane (MDM). For each vapor, three thermodynamic states are analyzed: one in the dilute-gas region, one near the saturation line, and one in the supercritical region. For mean flow fields, it is found that, due to smaller Prandtl and Eckert numbers, both the molecule-complexity effect and the non-ideal effect reduce the mean temperature rise from the cold wall to the channel center. Meanwhile, the molecule-complexity effect weakens the mean density drop, while the non-ideal effect strengthens the drop. Furthermore, once the density and viscosity variations are considered, the mean streamwise velocity profiles of dense vapors are practically the same as the ideal gas. For turbulent fluctuations, it is found that the correlations between T′, p′, and ρ′ in dense vapors are more complicated than the ideal gas: for the ideal gas, fluctuations are dominated by “vorticity mode”; hence, ρ′ and T′ are strongly related to u′ but independent of p′; however, for dense vapors, “acoustic mode” can also play an important role. A newly derived equation illustrates that, through the “acoustic mode,” the molecule-complexity effect obviously enhances the positive correlation between ρ′ and p′, while the non-ideal effect can enhance the positive correlation between T′ and p′. Further analysis of instantaneous flow fields shows that p′ is isotropic. The isotropic character affects fluctuation magnitudes but has limited effect on the specified wall-direction turbulent transport. Consequently, Walz's equation and Reynolds analogy in terms of enthalpy are still valid. Finally, a comparison between the DNS energy budget and k equation of Reynolds-averaged Navier–Stokes (RANS) model has been carried out. Results show that obvious deviation happens on the production term in spite of the careful selection of eddy viscosity model.

CONCERTO at APEX On-sky performance in continuum

Context. CarbON CII line in post-rEionisation and ReionisaTiOn epoch (CONCERTO) instrument is a low-resolution mapping spectrometer based on lumped element kinetic inductance detector (LEKIDs) technology, operating at 130-310 GHz. It was installed on the 12-metre APEX telescope in Chile in April 2021 and was in operation until May 2023. CONCERTO’s main goals were the observation of [CII]-emission line fluctuations at high redshift and of the Sunyaev–Zel’dovich (SZ) signal from galaxy clusters. Aims. We present the data processing algorithms and the performance of CONCERTO in continuum by analysing the data from the commissioning and scientific observations. Methods. We developed a standard data processing pipeline to proceed from the raw data to continuum maps. Using a large dataset of calibrators (Uranus, Mars, and quasars) acquired in 2021 and 2022 at the APEX telescope across a wide range of atmospheric conditions, we measured the CONCERTO continuum performance and tested its stability against observing conditions. Further, using observations on the COSMOS field and observations targeting a distant sub-millimetre galaxy in the UDS field, we assessed the robustness of the CONCERTO performance on faint sources and compared our measurements with expectations. Results. The beam pattern is characterised by an effective full width at half maximum (FWHM) of 31.9 ± 0.6″ and 34.4 ± 1.0″ for high-frequency (HF) and low-frequency (LF) bands, respectively. The main beam is slightly elongated with a mean eccentricity of 0.46. Two error beams of ~65″ and ~130″ are characterised, allowing us to estimate a main beam efficiency of ~0.52. The field of view is accurately reconstructed and presents coherent distortions between the HF and LF arrays. LEKID parameters were robustly determined for 80% of the read tones. Cross-talks between LEKIDs are the first cause of flagging, followed by an excess of eccentricity for ~10% of the LEKIDs, all located in a given region of the field of view. Of the 44 scans of Uranus selected for the absolute photometric calibration, 72.5% and 78.2% of the LEKIDs were selected as valid detectors with a probability >70%. By comparing the Uranus measurements with a model, we obtain calibration factors of 19.5±0.6 Hz Jy−1 and 25.6±0.9 Hz Jy−1 for HF and LF, respectively. The point-source continuum measurement uncertainties are 3.0% and 3.4% for the HF and LF bands, ignoring the uncertainty in the model (which is <2%). This demonstrates the accuracy of the methods we deployed to process the data. Finally, the RMS of CONCERTO maps is verified to evolve as proportional to the inverse square root of the integration time. The measured noise-equivalent flux densities (NEFDs) for HF and LF are 115±2 mJy beam−1 s1/2 and 95±1 mJy beam−1 s1/2, respectively, obtained using CONCERTO data on the COSMOS field for a mean precipitable water vapour (pwv) and elevation of 0.81 mm and 55.7 deg. Conclusions. CONCERTO has unique capabilities in fast dual-band spectral mapping at ~30 arcsec resolution and with a ~18.5 arcmin instantaneous field of view. CONCERTO’s performance in continuum is perfectly in line with expectations.

Low-frequency broadband acoustic metamaterial absorber based on nested resonator and synergistic coupled weak resonances

Utilizing absorbers with sub-wavelength thickness for low-frequency broadband noise control is a challenging problem, and rapid on-demand design of structures based on target noise frequency has received continuous attention. In this work, nested cavities are introduced into the typical neck-embedded Helmholtz resonator (NEHR) to obtain the nested neck-embedded Helmholtz resonator (NNEHR), which exhibits better cavity partitioning volume fraction and covers wider sound absorption band. Both theoretical analysis and simulations demonstrate that NNEHR possesses superior acoustic properties: generating an additional absorption peak, lowering the position of the first absorption peak by 11 %, and increasing the absorption coefficient by 4.7 %. Instantaneous velocity fields and thermal-viscous energy dissipation density illustrate that the efficient sound absorption of NNEHR originates from resonant dissipation in the neck and surrounding areas. Furthermore, by combining the theoretical model with genetic algorithms, three sets of NNEHR metasurfaces are optimized. The optimized structures achieve average absorption coefficients of 0.831, 0.913, and 0.885 in the range of 250–550 Hz, 400–1000 Hz, and 500–2000 Hz, with a thickness of only 50 mm, reflecting sub-wavelength, low-frequency broadband sound absorption characteristics. The excellent absorption stems from synergistic coupled weak resonances and higher-order absorption peaks of constituent units, and the overdamping characteristic ensures the realization of the optimal structure with minimal thickness. The contribution of weak resonant units to the total sound absorption performance can be measured by the surface admittance normalized by the total incident area, and the essence of the coupling effect is the strong interaction between units with adjacent absorbing frequencies. The proposed structure exhibits good sound absorption performance and great adjustability, while the genetic algorithm optimization based on the theoretical model demonstrates high efficiency and robustness, providing a methodology for the design and on-demand optimization of low-frequency broadband structures.

Designing turbulence with entangled vortices

Matter entanglement is a common chaotic structure found in both quantum and classical systems. For classical turbulence, viscous vortices are like sinews in fluid flows, storing and dissipating energy and accommodating strain and stress throughout a complex vortex network. However, to explain how the statistical properties of turbulence arise from elemental vortical structures remains challenging. Here, we use the quantum vortex tangle as a skeleton to generate an instantaneous classical turbulent field with intertwined vortex tubes. Combining the quantum skeleton and tunable vortex thickness makes the synthetic turbulence satisfy key statistical laws, offering valuable insights for elucidating energy cascade and extreme events. By manipulating the elemental structures, we customize turbulence with desired statistical features. This bottom-up approach of designing turbulence provides a testbed for analyzing and modeling turbulence.

Simulation Analysis Based on Sodium-Cooled Fast Reactor Fuel Assembly Under Blockage Accident

This paper employs computational fluid dynamics to investigate the fuel assemblies within a sodium-cooled fast neutron reactor. To begin with, we developed computational models for seven wire spacer fuel rods under both normal operating conditions and transient blockage conditions. We conducted separate analyses to assess the impacts of normal operation, blockage thickness, and blockage area. This work allowed us to acquire data on the heat transfer properties and the flow field variations for the coolant, blockages, and fuel rods across different conditions. Subsequently, we leveraged these flow field alterations to examine the resulting temperature distributions. Analyses were conducted to evaluate the effects of normal operation, blockage thickness, and blockage area. The research acquired the heat transfer characteristics and flow field distribution variations of the coolant, blockages, and fuel rods under different operational conditions and utilized these variations to analyze the temperature distribution. Through research analysis, the following conclusions have been reached. Under normal operating conditions, the temperature and flow fields of the fuel components exhibit cyclic variations along the axial length, corresponding to the pitch of the wire spacers. Heat exchange between the internal and external subchannels occurs independently, which further substantiates that the incorporation of wire spacers strengthens lateral flow disturbances. This effect is more pronounced within the internal subchannels, thereby leading to a marked difference in the flow fields on either side of the wire spacers. In the case of blocked conditions, an increase in blockage thickness and area both lead to higher temperatures for the coolant, the blockages themselves, and the fuel rods. The temperature in the recirculation zone behind the blockages also rises with increasing blockage thickness and area, although the magnitude of this increase is not significant. After the onset of blockage conditions, the instantaneous temperature tends to increase. As time progresses, the instantaneous temperature fields following the blockage do not display greater fluctuations in temperature change than those observed after the system has stabilized. For the temperature parameters of the convective field, differences arising from an increase in blockage area are more significant than those caused by an increase in blockage thickness. When blockage area and blockage thickness are increased by the same multiple, an increase in blockage area results in a higher temperature peak. Increasing the area of blockage necessitates a longer duration for both the temperature and the velocity fields to revert to equilibrium.

Experimental research on the flow field and flame geometry of free buoyant diffusion flames with low dimensionless heat release rates

The flame geometry of turbulent diffusion flames is dominated by the turbulent mixing between the gaseous fuel and the entrained air. The significant variations of the power law for the mean flame height regarding the dimensionless heat release rate (Q˙*) in different Q˙* ranges implied that the turbulent mixing characteristics would change greatly among these Q˙* regimes. This paper presents a combined experimental and analytical study to reveal the turbulent transport properties and flame shape of free turbulent buoyant diffusion flames of relatively low Q˙* prevalent in large-scale fires. The instantaneous velocity fields of free buoyant methane diffusion flames (burner diameter: d = 0.30 m, Q˙*= 0.22–0.40) were measured by the stereo particle image velocimetry (SPIV) technique. The results indicate that the radial profiles of the time-averaged axial velocity, axial normal stress, radial normal stress, shear stress, and turbulent kinetic energy exhibited self-similarity at different heights under all the heat release rates. The turbulent flow in the buoyant flame was found to be anisotropic. Based on the gradient transport approximation, the mean turbulent viscosity within the mean flame height (νt=) was scaled by g1/2d3/2, where g is the gravitational acceleration. The semi-physical correlations for the mean flame height and width were derived based on the concepts of turbulent mixing and equal axial convection and radial diffusion times, respectively. The two correlations agreed reasonably with the experimental data of this work.

Wall Modeling of Turbulent Flows with Varying Pressure Gradients Using Multi-Agent Reinforcement Learning

We propose a framework for developing wall models for large-eddy simulation that is able to capture pressure-gradient effects using multi-agent reinforcement learning. Within this framework, the distributed reinforcement learning agents receive off-wall environmental states, including pressure gradient and turbulence strain rate, ensuring adaptability to a wide range of flows characterized by pressure-gradient effects and separations. Based on these states, the agents determine an action to adjust the wall eddy viscosity and, consequently, the wall-shear stress. The model training is in situ with wall-modeled large-eddy simulation grid resolutions and does not rely on the instantaneous velocity fields from high-fidelity simulations. Throughout the training, the agents compute rewards from the relative error in the estimated wall-shear stress, which allows them to refine an optimal control policy that minimizes prediction errors. Employing this framework, wall models are trained for two distinct subgrid-scale models using low-Reynolds-number flow over periodic hills. These models are validated through simulations of flows over periodic hills at higher Reynolds numbers and flows over the Boeing Gaussian bump. The developed wall models successfully capture the acceleration and deceleration of wall-bounded turbulent flows under pressure gradients and outperform the equilibrium wall model in predicting skin friction.

Kármán Street-Boundary Layer Interactions In Turbulent Flow

Understanding the intricate interplay between turbulent boundary layers and wake structures caused by obstacles in the flow is key to assessing the local shear stress fields as well as the drag. These types of flows can be relevant to natural phenomena including sediment and nutrient transport. While existing literature predominantly explores the flow downstream of cylinders mounted perpendicular to bounding surfaces or downstream of cylinders positioned outside the boundary layer region, this work experimentally studies the turbulent boundary layer passing over a cylindrical obstacle positioned near and parallel to the surface of an underlying solid wall. Planar particle image velocimetry (PIV) was performed in an open recirculating water channel for three test cases at a friction Reynolds number Reτ = 1320. Key parameters investigated include the cylinder diameter relative to the boundary layer thickness (D/δ = 0.09 and 0.01) and the gap between the cylinder and the wall (G/δ = 0.09), where the range of D/δ used is smaller than most examples found in the literature. Instantaneous and averaged velocity and vorticity fields are examined based on the PIV measurements. The results in Fig. 1 show how von Kármán vortex streets and turbulent boundary layer structures interact to form unique flow features. The larger cylinder generates a clear separation downstream of its axis as well as a wake that remains coherent over a distance δ downstream. The smaller cylinder with D/δ = 0.01 generates a much narrower and weaker wake, but still shows a clear influence on time-averaged velocity profile. Future experiments will evaluate flow conditions further downstream as well as additional values of G/δ for the same D/δ values.

Endoscopic Stereoscopic PIV Measurements Within An Enclosed And Tight Area Of A Hydraulic Turbine

The increasingly common utilization of hydraulic turbines in off design operating conditions in recent years have affected their lifetime due to damaging fluid-structure interactions. To decrease the downtime associated to more frequent maintenance of these turbines, a thorough understanding of the damaging flow phenomena is required. This may eventually lead to the development of mitigation strategies. However, measuring velocity at the entrance of the rotor of Francis turbines is difficult because of the confined spaces and limited optical access. This paper presents a stereoscopic PIV methodology enabling measurements at the inlet of a reduced-scale Francis turbine, along with multiple novelties developed to make accurate measurements possible. It elaborates on the solutions devised to address the challenges of performing optical measurements in highly confined spaces. The measured velocity fields cover the vaneless space and a large section of the interblade channels. In the phase-averaged velocity fields, three circulation zones are detected at the speed-no load operating point, representing the combined effect of numerous vortices and flow structures observed in the instantaneous velocity fields. The flow is characterized by a significant backflow within the runner, extending to the leading edge of the runner blades and overflowing into the vaneless space.

Flow dynamics and turbulent coherent structures around sediment reduction plates of a sewer system

In the management of urban drainage networks, great interest has been generated in the removal of sediments from sewer systems. The unsteady three-dimensional (3D) flow and turbulent coherent structures surrounding sediment reduction plates in a sewer system are investigated by means of the detached-eddy simulation (DES). Particular emphasis is given to detailing the instantaneous velocity and vorticity fields within the grooves, along with an examination of the three-dimensional, long-term, average flow structure at a Reynolds number of approximately 105. Velocity vectors demonstrate continuous flapping of the flow on the groove wall, periodically interacting with ejections of positive and negative vorticity originating from the grooves. The interaction between the three-dimensional groove flow and the shear flow leads to the downstream transport of patches of positive and negative vorticity, which significantly influence sediment transport. The high-velocity shear flows and strong vortices generated in undulating topography, as identified by the Q-criteria, are the key factors contributing to the efficient sediment reduction capabilities of the sediment reduction plates. The sediment reduction plates with partially enclosed structures exhibit low sedimentation rates in grooves on the plate, a broader acceleration region, and a lesser impact on the flow capacity. The results improve the understanding of the hydrodynamics and turbulent coherent structures surrounding the sediment reduction plates while elucidating the driving factors behind the enhancement of sediment scouring and suspension capacities. These results indicate that the redesign of the plates as partially enclosed structures contributes to further improving their sediment reduction performance.