Instantaneous Velocity Fields Research Articles

Multi-scale reconstruction of turbulent rotating flows with proper orthogonal decomposition and generative adversarial networks

Data reconstruction of rotating turbulent snapshots is investigated utilizing data-driven tools. This problem is crucial for numerous geophysical applications and fundamental aspects, given the concurrent effects of direct and inverse energy cascades. Additionally, benchmarking of various reconstruction techniques is essential to assess the trade-off between quantitative supremacy, implementation complexity and explicability. In this study, we use linear and nonlinear tools based on the proper orthogonal decomposition (POD) and generative adversarial network (GAN) for reconstructing rotating turbulence snapshots with spatial damages (inpainting). We focus on accurately reproducing both statistical properties and instantaneous velocity fields. Different gap sizes and gap geometries are investigated in order to assess the importance of coherency and multi-scale properties of the missing information. Surprisingly enough, concerning point-wise reconstruction, the nonlinear GAN does not outperform one of the linear POD techniques. On the other hand, the supremacy of the GAN approach is shown when the statistical multi-scale properties are compared. Similarly, extreme events in the gap region are better predicted when using GAN. The balance between point-wise error and statistical properties is controlled by the adversarial ratio, which determines the relative importance of the generator and the discriminator in the GAN training.

Direct numerical simulation of turbulent convection in a channel roughened with circular-arc ribs

Turbulent flows through a channel roughened with circular-arc ribs of different pitch-to-height ratios (P/H=3.0, 5.0 and 7.5) at a fixed Reynolds number are studied using direct numerical simulations (DNS). It is interesting to observe that in contrast to the classical channel flow over transverse square bars, the flow separation point in a channel roughened with circular-arc ribs is sensitive to the testing conditions and varies along the rib arc. The influences of the pitch-to-height ratio on the flow separation, statistical moments of the velocity field, and the anisotropy of turbulence stresses are investigated. The dynamics of coherent structures are studied by examining the skewness of the flow field, characteristics of the joint probability density functions (JPDF) of velocity fluctuations, quadrant analysis of Reynolds stresses, instantaneous velocity field, swirling strength, and temporal–spatial two-point auto-correlations. It is found that the flow anisotropy becomes suppressed as the value of P/H increases due to significant enhancement of the spanwise Reynolds normal stress 〈w′w′〉. It is also interesting to observe that the lifespan of turbulence structures becomes increasingly shortened at the midspan between two adjacent ribs as the pitch-to-height ratio increases.

Synchronized measurement of instantaneous convective heat flux and velocity fields in wall-bounded flows

This work presents an experimental setup for acquiring synchronized wall and flow field measurements in a turbulent boundary layer (TBL). Instantaneous measurements of the convective heat transfer distribution at the wall and of the flow field are carried out simultaneously, using synchronized wall-parallel planar particle image velocimetry and infrared thermography. A low-thermal-inertia heated-thin-foil sensor is embedded in the wall beneath the TBL to measure the wall temperature maps with enough temporal resolution. The unsteady energy balance of the heated foil can be solved to restore the instantaneous value of the convective heat transfer coefficient on the wall. A detailed description of the sensor design is included. Furthermore, owing to the relatively low signal-to-noise ratio of instantaneous temperature fluctuation measurements, a recipe for data processing is proposed. A study to characterize the uncertainty of the experimental setup design is also performed. The overall agreement in the correlation between wall heat transfer and velocity fields with the literature supports the validity of the proposed approach. This solution is potentially interesting for flow control purposes, where sensing is performed at the wall.

Experimental study on the effect of hydrodynamic conditions on flocculation and settling properties of fine-grain sediment

The flocculation of cohesive sediment particles is a function of the collision efficiency of sediment particles, which is mainly influenced by local flow hydrodynamics. A detailed study on local hydrodynamic characteristics in a novel stirred tank was done to measure the flow field of a turbulent flow. Instantaneous flow velocity fields were obtained and processed, and the average flow field was computed. Particle image velocimetry (PIV) results measuring the hydrodynamic characteristics (expressed as turbulent kinetic energy (TKE), local dissipation rate of TKE, velocity gradient or shear rate, turbulent shear stress, etc.) were analyzed and discussed. Particle tracking and automated image processing techniques were utilized to analyze the impact of the hydrodynamic conditions on the floc and settling properties. The differences in floc properties under the two turbulent shear states (i.e., steady-state (SS) and unsteady-state (US)) hydrodynamic conditions were examined. The results show that floc size and floc size distribution (FSD) of pure kaolin clay flocs are significantly influenced by the local dissipation rate of TKE and the turbulent intensity states. The floc size, df, 84, at local dissipation rate of TKE of 11 × 10−4, 29 × 10−4, 44 × 10−4, 83 × 10−4, and 142 × 10−4 m2/s3 is 97, 108, 107, 124, and 150 μm, respectively. It was found that by increasing the local dissipation rate of TKE, the FSD was skewed left. The US shear condition generated larger flocs (and in higher proportions) relative to the SS condition. The settling velocity and flux settling velocities of flocs formed under SS conditions are higher than those formed under US conditions. The fractal dimension of SS flocs also is higher than the fractal dimension of US flocs. These results highlight a significant impact of the states of hydrodynamics on floc properties such as floc size and FSD as well as the settling and morphological properties of flocs.

Experimental Study on the Flow Pattern of Double-hemisphere Rough Elements with Different Streamwise Spacing

Particle image velocimetry is used to study the variation around single- and double-hemisphere rough elements with different streamwise spacings immersed in the boundary layer. Instantaneous velocity field information in the streamwise–normal and streamwise–spanwise directions is collected at a Reynolds number of 1800. 3R, 5R, and 7R are determined as the rough element spacing used in the double-hemisphere rough element experiment, representing the smaller, medium transition, and larger rough element spacing, respectively. The average velocity, Reynolds shear stress, shedding frequency, and proper orthogonal decomposition results of the flow field around the rough elements under various working conditions were compared. The downstream hemisphere will encroach on the streamline following the upstream hemisphere, and changing the spacing means changing the position of the encroached area. When the spacing is smaller, the streamline reattachment is destroyed, and the momentum and mass exchange between the two-hemisphere rough elements decreases. The double-hemisphere rough element is a slender, blunt, rough element. At the medium transition spacing, the shear layer and vortex structure shed from the upstream hemisphere are over the downstream hemisphere, and the double-hemisphere rough elements will cause disturbance in a larger wall-normal range. The streamline has been reattached at the larger streamwise spacing, and the interaction between the two hemispheres is the weakest. Here, the double-hemisphere rough element will form the recirculation zone, recirculation arch vortex, and periodic hairpin vortex.

Experimental study on the wake control of a square cylinder mounted with dual rigid/flexible splitter plates in the subcritical regime

The wake control of a bluff body is vital to the safety of structures in the field of coastal and ocean engineering. The quantitative statistics wake control and flow field in the subcritical regime are relatively unexplored. The purpose of this research is to find an optimum arrangement for the wake control of a square cylinder mounted by dual flexible/rigid splitter plates and uncover the physical mechanisms. The Reynolds number based on the side length of the square cylinder is 4.2×103≤Re≤1.0×104. Splitter plates with two thicknesses were adopted, which represents the flexible and rigid splitters, respectively. The splitter length was considered over the range l/L=1∼2 with an increment of Δl/L=0.25. The force exerting on the bluff body and the instantaneous velocity field were measured by the triaxial load cell and particle image velocimetry techniques. The results indicate the modulation of the flapping splitter as well as the splitting effect of the tip vortices determine the lift suppression and the gap flow and streamlined trailing play a vital role in drag reduction. The maximum drag reduction and lift suppression reach ηmax=58.3% and 90.6%, respectively. The flexible splitter outperforms its rigid counterpart, especially at high Reynolds numbers, which is beneficial for those structures in marine engineering. Given the hydrodynamic characteristics and sensitivity to incoming flow conditions, the optimum arrangement is the flexible splitter configuration with splitter length of l/L=1.25∼1.5.

Estimation of turbulent dissipation rates and its implications for the particle-bubble interactions in flotation

Increasing turbulence intensity has been traditionally applied to the beneficiation of fine particles for process intensification with higher flotation recovery. Nevertheless, quantifying turbulent dissipation rate directly from local velocity gradients is challenging as it requires measurements of flow fields down to the Kolmogorov length scales. Spatial resolution in velocity measurements using existing experimental techniques is insufficient. In this paper, we studythe effects of spatial resolution on the estimated turbulent dissipation rate and its effects on predicted particle-bubble interactions are further discussed. Direct numerical simulation of a homogeneous isotropic turbulence has been performed using the pseudo-spectral method. An instantaneous 3D velocity field is used to calculate true values of turbulent dissipation rate, which is used as benchmark to compare turbulent dissipation rates estimated from the same velocity fields of different resolutions. This velocity field is then coarsened to different levels relative to the Kolmogorov length scale, which are used to calculate turbulent dissipation rate from local velocity gradients of different spatial resolution. The underestimation of turbulent dissipation rate is quantified as a function of spatial resolution relative to the Kolmogorov length scales. This clarifies that special care should be paid to the understanding of turbulence effects on particle-bubble interactions.

Engine combustion and emission analysis using optical methods: An overview

Measurements in combustion places important role in design of reciprocating and turbo hot engines. The available methods of combustion analysis of engines have gained much attention and are capable of giving good results in determining the fuel characteristics and engine design optimizations. However, the full understanding of engine combustion can be made through optical image processing in addition to the existing methods. Optical tools are used efficiently to predict the combustion phenomena in internal combustion engine. In order to observe typical regimes of combustion most widely used techniques are Schlieren and shadowgraph. The devices used in optical measurements are high speed CCD camera and Constant Volume Spray Chambers (CVSCs). The analysis is carried to know the spray characteristics at atmospheric and elevated pressure conditions.The luminosity techniques are widely used to capture the combustion flames with high speed camera in optically accessible engines. The present engines have to be redesigned for making optical access for the desired measurements. Several endoscopy techniques are used for combustion visualization. Phase Doppler Interferometer (PDI) is used for the quantitative spray measurements. Particle Image Velocimetry is an intrusive technique which is capable of characterizing instantaneous velocity fields in a fluid flow. This paper presents the works on different optical techniques available and is in use to diagnose the engine combustion especially for reciprocating engines.

Upper part-load instability in a reduced-scale Francis turbine: an experimental study

Francis turbines with medium or high specific speeds may experience a particular type of instability in the upper part load in which the precessing vortex has an elliptical shape. The occurrence of the upper part-load instability (UPLI) is accompanied by large-amplitude pressure fluctuations at a distinct frequency between 2 and 4 times the runner rotational speed. This paper experimentally investigates UPLI for a reduced-scale Francis turbine. To investigate the causal factors of this instability, draft tube pressure measurements, particle image velocimetry, and high-speed flow visualizations have been performed at several operating points under cavitation and cavitation-free conditions. It is shown for the first time that for an operating point within the UPLI range, the vortex always features a circular section in cavitation-free conditions, which is preserved even after the initial appearance of cavitation. It is only below a certain Thoma number that the vortex section turns into an ellipse and shows an abrupt increase in pressure fluctuations. Analysis of the phase-averaged velocity fields reveals that a concentrated vortex with a large precession radius is a prerequisite for UPLI, while the instantaneous velocity fields clearly illustrate the asymmetric velocity distribution around the elliptical vortex. The existence of a breathing mode and the intermittent formation of two side vortices along the elliptical vortex rope are also evidenced by high-speed flow visualizations. These results provide a much deeper insight into the flow structures that favor the development of UPLI and help delimit its thresholds to higher precision, and thus, prevent its occurrence during turbine operations.

Decomposition of power number in a stirred tank and real time reconstruction of 3D large-scale flow structures from sparse pressure measurements

We consider the flow inside an unbaffled stirred tank at Re = 600 and decompose the pressure field into components that are constant, linear and quadratic with respect to the temporal coefficients of velocity POD (proper orthogonal decomposition) modes. The pressure components are used to analyse the power number of individual blades and blade combinations. We also employ an identification algorithm to derive a linear dynamic estimator that allows the reconstruction (in real time) of the instantaneous 3D velocity field from sparse pressure measurements at the impeller blades. The first pair of POD modes are reconstructed with reasonable accuracy using the pressure signal from a single sensor. The results improve with 6 or 12 sensors. Application of the estimator to Re=500 and 700 shows that it is robust to changes in operating conditions. The work opens the possibility for real time control of mixing with targeted feeding location and rate.

Turbulent flow around a short rectangular cylinder in uniform flow at moderate Reynolds numbers

The influence of Reynolds number (Re) on the wake characteristics, Kelvin-Helmholtz instabilities and von-Kármán vortex shedding process around a short rectangular cylinder with length to height ratio of 0.5 was studied using particle image velocimetry. The Reynolds numbers based on cylinder height and free-stream velocity ranged from 3000 to 21000. The flow characteristics were analyzed using the instantaneous and mean velocity fields, as well as the Reynolds stresses, while the unsteady dynamics were elucidated using frequency spectra, proper orthogonal decomposition (POD) and reverse flow area. The results demonstrate that the size of the wake vortex reduced but the magnitudes of the Reynolds stresses increased with increasing Re, until Reynolds number independence is reached at Re = 10000. Regardless of Reynolds number, spectral analyses of the velocity fluctuations, POD mode coefficients and fluctuating reverse flow area showed dominant peaks at the same fundamental vortex shedding frequency. Meanwhile, instantaneous flow visualization showed that the structural coherency of the large-scale vortices increased with increasing Reynolds number. Cross-correlation analysis also revealed that the odd and even POD mode coefficients of the first two mode pairs, respectively, acted to contract and expand the instantaneous reverse flow area at the higher Reynolds numbers.

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

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

Investigation of turbulent flows using spillway models aiming to aid downstream migration of fish

AbstractDams and other flow control structures constitute obstacles to upstream and downstream aquatic habitats, reducing life‐cycle success and even eliminating diadromous and potamodromous fish species from river basins. Providing effective passage past hydroelectric dams is critical for restoring habitat connectivity and sustaining populations of fish species. Surface water release over spillways is effective in diverting fish from turbines and establishing a safe bypass for downstream migrants. Developing an effective passage system over spillways requires both biological knowledge of fish behavior and abilities, as well as hydraulic knowledge of spillway flows to provide hydrodynamic conditions that fish may exploit. The present study experimentally investigates several depths, velocity, and turbulence fields intended to promote the downstream migration of fish over spillways. Instantaneous velocity fields were measured in an open recirculating rectangular water channel. A 90° standard spillway model, as well as a spillway model with a modified upstream face slope of 45°, were tested for two water depths. The results show the presence of a distinct recirculation bubble at the heel of the 90° standard spillway with the shallower water depth. It was concluded that the upstream recirculation bubble may be suppressed by either increasing the water depth or decreasing the upstream face slope. Decreasing the upstream face slope also resulted in a more uniform distribution of acceleration and lower turbulence along the upstream face of the spillway. The modified spillways provide more suitable hydrodynamic conditions for downstream migrants.

Assessment of LES Dynamic Smagorinsky-Lilly model resolution for combustion engineering applications

Large Eddy Simulation (LES) has become a powerful tool for dealing with turbulence. Nevertheless, mesh resolution of the simulated domain under the LES approach was shown to be a key matter. Critical regions should maintain an adequate mesh resolution and also the highest possible quality. LES with the Dynamic Smagorisky-Lilly sub-grid model was used for the simulation of SMA1-flame, to assess the influence of the mesh resolution on the instantaneous velocity fields, species profiles, and temperatures profiles. The results of the simulation suggest a comprehensible agreement with experimental data. Nevertheless, some areas in the highly rotational velocity field are not properly solved, mainly due to a poor mesh resolution of such areas. As a result, the central jet’s decay rate was not accurately predicted. On the other hand, the temperature and species profiles were reasonably computed, considering the simple chemistry provided by the Eddy Dissipation Model (EDM).

A transformer-based synthetic-inflow generator for spatially developing turbulent boundary layers

This study proposes a newly developed deep-learning-based method to generate turbulent inflow conditions for spatially developing turbulent boundary layer (TBL) simulations. A combination of a transformer and a multiscale-enhanced super-resolution generative adversarial network is utilised to predict velocity fields of a spatially developing TBL at various planes normal to the streamwise direction. Datasets of direct numerical simulation (DNS) of flat plate flow spanning a momentum thickness-based Reynolds number, $Re_\theta = 661.5\unicode{x2013}1502.0$ , are used to train and test the model. The model shows a remarkable ability to predict the instantaneous velocity fields with detailed fluctuations and reproduce the turbulence statistics as well as spatial and temporal spectra with commendable accuracy as compared with the DNS results. The proposed model also exhibits a reasonable accuracy for predicting velocity fields at Reynolds numbers that are not used in the training process. With the aid of transfer learning, the computational cost of the proposed model is considered to be effectively low. Furthermore, applying the generated turbulent inflow conditions to an inflow–outflow simulation reveals a negligible development distance for the TBL to reach the target statistics. The results demonstrate for the first time that transformer-based models can be efficient in predicting the dynamics of turbulent flows. They also show that combining these models with generative adversarial networks-based models can be useful in tackling various turbulence-related problems, including the development of efficient synthetic-turbulent inflow generators.

Multi-fidelity vortex simulations of rotor flows: Validation against detailed wake measurements

Two flow models with different fidelity of the DTU vortex solver MIRAS have been used to simulate the wake generated by a model wind turbine with various levels of asymmetry. Predictions are validated against experimental Particle Image Velocimetry measurements and dye visualizations. The experiments were conducted in a recirculating free-surface water channel with an immersed two-bladed rotor mounted on a shaft. The blades were designed to approximate a Joukowsky rotor. A detailed comparison between the measurements and the simulations is first performed for an unperturbed baseline case at different tip speed ratios. The analysis consists of a tip vortex characterization, including the vortex core profile, and a comparison of the instantaneous and mean velocity and vorticity fields, along with the mean wake profile at different downstream locations. Overall, good agreement is obtained between measurements and simulations, especially with the higher-fidelity particle-mesh model, which is capable of very closely predicting many of the flow features observed in the experiments. Rotor asymmetry triggers a vortex instability, commonly known as leapfrogging, which accelerates the breakdown of tip vortices, enhancing the mixing of wake structures and promoting a faster wake recovery. The prediction accuracy of this instability by the solvers is analyzed for different tip speed ratios and perturbation amplitudes. This work aims at setting the groundwork for future flow instability studies with the MIRAS solver.

Measuring Instantaneous Velocity Fields Remotely Using a 2-D Power Spectral Density Technique

Surface currents are a fundamental hydrodynamic variable that are essential in estimating volumetric discharge, material, and sediment transport in riverine environments. Existing methods of remotely measuring currents are met with steep challenges when applied in the field. Here we adapt a two-dimensional power spectral density (2D PSD) technique for use in a riverine setting, where wind waves dominate the free surface. The technique relies on quantifying the Doppler shift of surface gravity waves, which are omnipresent on most free surfaces. Therefore, typical problems such as insufficient and uneven distribution of tracer material that frequently plague other remote methods in the field are not a concern with this method. We demonstrate the ability of the 2D PSD technique to remotely measure surface and sub-surface mean currents. Moreover, because this technique analyzes image pairs, temporal statistics of the velocity field (i.e., time series, velocity spectra, and turbulence quantities) can easily be extracted. Given its capabilities, the 2D PSD current measuring technique could profoundly impact the status quo of remote sensing of hydrodynamic variables in a variety of aquatic settings.

Experimental Study of the Particles Influence on the Pyramid Wake within the Turbulent Boundary Layer

Particle imaging velocimetry (PIV) was used to study the near-field variation of a pyramid rough element in clear water and a liquid–solid boundary layer (thickness: 60 mm). Particles with an average diameter of 355 µm and Stokes number of 4.3 were injected into a 1:1000 mass ratio (solid particles: water) liquid–solid two-phase solution. Experiments were conducted to collect instantaneous velocity field information in the streamwise–normal direction and streamwise–spanwise direction at a Reynolds number of 8350. Then, the average velocity field and turbulence intensity of the rough element wake under single-phase and two-phase conditions were compared, and the morphology and periodicity of the shedding structure were analyzed by using proper orthogonal decomposition (POD) combined with the power spectral density function (PSD). Particles were shown to have no significant impact on the recirculation area in the streamwise–spanwise plane but did result in a reduction of the recirculation zone in the streamwise–normal plane and a 0.2h closer location of the streamline's origin to the obstacle. Along with the weakening of the upwash structure, the particle phase diminishes the velocity gradient along the span direction and turbulence intensity. Structural shedding at the top of the pyramid and near the wall occurred simultaneously, and the same shedding period was maintained. Particularly, in the first two POD modes, the energy of the shedding structure near the wall was higher than that at the obstacle tip, with a maximum energy differential of approximately 6%. The Strouhal number of the shedding structure decreased by particles from 0.217 to 0.209. The concentration distribution and degree of dispersion in the particle-laden flow illustrate different results, with lower statistics in the wake flow field.

Study on the flow mixing of hot and cold fluids in the T-junction under rolling motion condition

The present study numerically investigates the flow mixing characteristics in the T-junction with momentum ratio (MR) of 0.95 under rolling motion condition. The effects of different inflow rates of 0.47–1.9 kg·s−1 and the synergistic effects between velocity and temperature are discussed. The results show that the rolling motion significantly changes the upstream instantaneous velocity field, especially that near the wall, which induces the variations of the instantaneous MR and the mixing length of the temperature field. The rolling motion increases the range of the velocity fluctuation while decreasing that of the temperature fluctuation. Under rolling motion condition, increasing the inflow rates decreases the range of the velocity fluctuations and inhibits the variation of MR. This weakens the flow mixing of main and branch fluids thereby increasing the range of the temperature fluctuations. In addition, it is found that the synergistic effects between velocity and temperature occur at the junction of the main and branch fluids, where the power spectrum densities (PSDs) of y velocity component fluctuation and temperature fluctuation exhibit the same frequency peaks of 0.2 Hz, 5.8 Hz, 14.7 Hz, and 31.2 Hz. By comparing the PSDs along with the y-axis, it is revealed that the high temperature fluctuation area of y/D = 0–0.33 has a faster energy cascade with a larger slope of −5/3 in the low-frequency area (0.1–10 Hz) and −13/2 slope in the high-frequency area (>10 Hz).

Simultaneous In Situ Characterization of Turbulent Flocculation and Reactor Mixing Using Image Analysis and Particle Image Velocimetry in Unison

Floc characteristics, including their size distribution, mean size, and fractal dimension, are impacted by mixing intensity and duration, coagulant dosing method, dose, type, and pH. Herein, we directly employ flocs inherently generated during treatment to determine instantaneous velocity fields, which were further utilized to estimate local velocity gradients and turbulent kinetic energy dissipation (TKED) rates. This novel non-intrusive methodology, which combines particle image velocimetry (PIV) and an imagery-based sizing scheme, was examined through the characteristics of reactor mixing and flocculation from in situ measurements for conventional FeCl3 chemical coagulation and iron electrocoagulation (EC). Our first-of-its-kind procedure avoids the need to externally add artificial seeding particles for PIV analysis, automatically reducing the number of experiments, and simultaneously improves both accuracy and precision by linking fluid dynamics and particle characteristics with only a single experiment. The TKED rates estimated using flocs were largely similar with conventional artificial seeding at the early stage of flocculation when flocs were smaller. However, the new method is expected to estimate turbulence more accurately in the flocs’ microenvironment during the later stages of tapered flocculation when particle sizes are similar to dissipation eddy dimensions, especially at high particle concentrations (i.e., highly turbid waters). Measured size distributions, mean sizes, and fractal dimensions confirm the robustness of the present imagery-based sizing scheme and showed that EC produced numerous and more compact flocs than conventional coagulation.