Similar Velocity Distributions Research Articles

Soliton sheets of Bose–Einstein condensates in optical lattices

Soliton sheets are observed in Bose–Einstein condensates in optical lattice which are formed by superposition of condensates occupying different single-particle states. These structures consist of one-dimensional stationary solitons distributed in the x-direction arranged continuously along the peaks of the optical lattice in the y-direction. Notably, the phase difference across the soliton sheets is periodic and varies linearly with y within each period. So, we refer to this configuration as a ‘soliton sheet’. A velocity difference in the y-component is observed between the two sides of the soliton sheets. Similar velocity distributions can be achieved by aligning an infinite number of isotropic vortices along the peaks of the optical lattice. And the soliton sheets are distinguished by their lack of dependence on phase singularities. This independence enables the formation of soliton sheets even in the absence of phase singularities, highlighting a unique aspect of this structure.

Scale-up of Interdigitated Design from Small Cell to Large Cell for Aqueous Redox Flow Batteries

Redox flow batteries present attributes for large-scale energy storage. Interdigitated flow configuration is a promising structure due to its higher power density and lower pressure loss. The screening for new electrolyte is usually first tested on small cell (<10 cm2) due to its easy operation and less electrolyte consumption. To test the electrolyte functionality in practical application condition, large-scale (i.e., 780 cm2) testing is required conventionally in laboratory or industry. The twice testing is due to the fact that the testing results in small cell are different from that for large-scale interdigitated cell. In order to reduce the testing cycles and quickly estimate the application of new electrolytes, scale-up of interdigitated design from small cell to large cell is investigated in this work.Numerical simulation on different scales of interdigitated cells (10cm2, 78cm2, 130cm2, 780cm2) is conducted to assess the polarization. The local velocity distribution and redox-active species concentration are extracted to show their dominance on the cell performance. It is shown that with similar velocity and concentration distribution, cell with different scales present similar limiting current density and power density, regardless of active area. It implies that the results in small cells can be directly scaled up for large cells without further testing. It provides an avenue for quick estimation of new electrolyte in application condition.Experimental testing is conducted on vanadium redox flow battery for different scale of interdigitated cell. The experimental results will demonstrate this scale-up from small-scale to middle-scale interdigitated cell.

Effects of discharge-to-submergence ratio on evolution of air-core vortex

An air-core vortex is generated in a tidal power station when the discharge increases or the submergence decreases at the intake. To better understand the complex evolution of the air-core vortex, we conduct high-fidelity coupled level-set and volume-of-fluid and wall-modeled large-eddy simulations for various ratios of discharge and submergence. We characterize the air-core vortex evolution process into two stages: the inception stage and the propagation stage. Our results show that the streamwise and spanwise velocity fluctuations exhibit similar distributions and temporal evolutions and are responsible for generating asymmetrical flow transitioned from a completely symmetrical pattern in the inception stage. The turbulence quadrant decomposition analysis in the confluence region quantifies the velocity fluctuation signal and Reynolds shear stress distribution. Under a specific Reynolds shear stress distribution, eddies converge and evolve into a main vortex. We introduce a vortex burst index to quantitatively characterize the contribution of the Reynolds shear stress to the main vortex evolution. The vortex burst index is positively correlated with the discharge-to-submergence ratio. In the propagation stage, the enstrophy stretching mechanism dominates the air-vortex evolution. The tilting terms nearly cancel each other because of the similar tangential velocity distribution in the axial direction. The results show that the increase in the discharge-to-submergence ratio accelerates the formation of the air-core vortex, owing to the increase in the vertical velocity gradient and the enhancement of the stretching effect.

Positive correlation between wet-day frequency and intensity linked to universal precipitation drivers

Understanding precipitation is essential for quantifying weather and climate-related risks. Changes in precipitation climatology are typically based on independent analysis of precipitation frequency and intensity. Here we show that where it rains more often, it also rains harder. When grouping global precipitation reanalysis data and observations from the past 40 years into regions of similar wet-day frequency, regardless of geographical separation, there is a strong correlation with wet-day intensity distributions. These wet-day-frequency regions are also more physically coherent than regions based on geographical location. We find the coherent relationship between wet-day frequency and intensity distributions is partially explained by wet-day-frequency regions having similar vertical velocity and convective available potential energy distributions, once polar regions are excluded. These represent dynamic and thermodynamic processes that indicate how conducive wet-day-frequency regions are to large-scale and convective precipitation. This suggests that the main drivers of precipitation are universal. We also show that extreme-precipitation metrics are dependent on wet-day frequency within our framework. Our results imply that wet-day frequency could be used to derive estimates of extreme-precipitation climate indices and corresponding uncertainties, these uncertainties being related to local processes.

Determining 3D Distributions of Pulsatile Blood Flow Using Orthogonal Simultaneous Biplane High-Speed Angiography (SB-HSA) with 1000 fps CdTe Photon Counting Detectors for 3D X-ray Particle Image Velocimetry (3D-XPIV) compared to Results Using Computational Fluid Dynamics (CFD).

3D hemodynamic distributions are useful for the diagnosis and treatment of aneurysms. Detailed blood-flow patterns and derived velocity maps can be obtained using 1000 fps High Speed Angiography (HSA). The novel orthogonal Simultaneous Biplane High-Speed Angiography (SB-HSA) system enables flow information to be quantified in multiple planes, and with additional components of flow at depth, accurate 3D flow distributions are available. Computational Fluid Dynamics (CFD) is the current standard for derivation of volumetric flow distributions, but obtaining solution convergence is computationally expensive and time intensive. More importantly, matching in-vivo boundary conditions is non-trivial. Therefore, an experimentally derived 3D flow distribution method could offer realistic results with less computation time. Using SB-HSA image sequences, 3D X-Ray Particle Image Velocimetry (3D-XPIV) was explored as a new method for assessing 3D flow. 3D-XPIV was demonstrated using an in-vitro setup, where a patient-specific internal carotid artery aneurysm model was attached to a flow loop, and an automated injection of iodinated microspheres was used as a flow tracer. Two 1000 fps photon-counting detectors were placed orthogonally with the aneurysm model in the FOV of both planes. Frame-synchronization of the two detectors made correlation of single-particle velocity components at a given timepoint possible. With frame-rates of 1000 fps, small particle displacements between frames resolved realistic time varying flow, where accurate velocity distributions depended on near-instantaneous velocities. 3D-XPIV velocity distributions were compared to CFD velocity distributions, where the simulation boundary conditions matched the in-vitro setup. Results showed similar velocity distributions between CFD and 3D-XPIV.

The circumgalactic medium of Milky Way-like galaxies in the TNG50 simulation – II. Cold, dense gas clouds and high-velocity cloud analogs

ABSTRACT We use the TNG50 simulation of the IllustrisTNG project to study cold, dense clouds of gas in the circumgalactic media (CGM) of Milky Way-like galaxies. We find that their CGM is typically filled with of order one hundred (thousand) reasonably (marginally) resolved clouds, possible analogs of high-velocity clouds (HVCs). There is a large variation in cloud abundance from galaxy to galaxy, and the physical properties of clouds that we explore – mass, size, metallicity, pressure, and kinematics – are also diverse. We quantify the distributions of cloud properties and cloud-background contrasts, providing cosmological inputs for idealized simulations. Clouds characteristically have subsolar metallicities, diverse shapes, small overdensities (χ = ncold/ nhot ≲ 10), are mostly inflowing, and have sub-virial rotation. At TNG50 resolution, resolved clouds have median masses of ∼ $10^6\, \rm {M_\odot }$ and sizes of ∼10 kpc. Larger clouds are well converged numerically, while the abundance of the smallest clouds increases with resolution, as expected. In TNG50 MW-like haloes, clouds are slightly (severely) underpressurized relative to their surroundings with respect to total (thermal) pressure, implying that magnetic fields may be important. Clouds are not distributed uniformly throughout the CGM but are clustered around other clouds, often near baryon-rich satellite galaxies. This suggests that at least some clouds originate from satellites, via direct ram-pressure stripping or otherwise. Finally, we compare with observations of intermediate and high velocity clouds from the real Milky Way halo. TNG50 shows a similar cloud velocity distribution as observations and predicts a significant population of currently difficult-to-detect low velocity clouds.

Methane scattering on porous kerogen surfaces and its impact on mesopore transport in shale

Revealing the scattering behaviour of gas molecules on porous surfaces is essential to develop accurate boundary conditions for kinetic transport models that describe the gas dynamics in shale reservoirs. Here, we use high-fidelity molecular dynamics simulations to resolve the gas–surface interactions between methane molecules and realistic organic kerogen surfaces, and to assess the applicability of the widely used scattering kernels. Our results show that the tight matrix porosities have a negligible effect on the timescale and lengthscale of the scattering process, which can be considered instantaneous in time and local in space. Although reflected velocity distributions reveal that the common Maxwell, Cercignani–Lampis and Yamamoto scattering models fail to fully capture the scattering details of methane on kerogen, especially when the incident molecular speeds are high, the Maxwell model predicts best the reflected angular beam pattern and the overall reflected velocity distribution for rough kerogen surfaces. However, for low-speed impingement, more characteristic of shale applications, all scattering models give similar velocity distributions, which are driven by the high degree of gas–surface accommodation observed. We find that a Maxwell model with a calibrated tangential momentum accommodation coefficient, which approaches unity as the surface roughness increases to ∼2 nm, is enough to reproduce comparable velocity profiles and mass flow rates inside moderately confined kerogen mesopores. Deviations between the Maxwell model and our molecular simulations are only observed for highly rarefied transport problems, but this rarefaction lies beyond the realm of shale reservoir applications. This paper, therefore, reports the first scattering study on porous and rough kerogen surfaces, and demonstrates the applicability of the Maxwell model, which can be readily incorporated into gas kinetic solvers to predict the apparent permeability of shale with mesopore and macropore networks.

Direct numerical simulations of turbulent periodic-hill flows with mass-conserving lattice Boltzmann method

The multi-relaxation time lattice Boltzmann method is used to perform direct numerical simulations of laminar and turbulent pressure-driven flows within a channel with hill-shape periodic constriction for the first time. The simulations are conducted on a graphics processing unit cluster with two-dimensional domain decomposition to accelerate the computation. The hill-shape boundary is represented using the interpolated bounce back scheme. However, the scheme generates mass leakage across the boundary, which is more pronounced in the turbulent flow regime, and this may produce diverging solutions for turbulent flows. The mass leakage due to the local mass imbalance along the curved boundary is solved by modifying the distribution functions locally or globally, and both predict similar velocity distributions. Since the global correction method is more computing time-consuming, the local correction method is adopted. The present numerical implementation’s capability is first validated by performing direct numerical simulations of turbulent channel flow at Reτ = 180, and the current predicted results agree well with the benchmark solutions. Direct numerical simulations are further conducted for the turbulent flow over the periodic hill at Reh = 2800. Both the mean velocity and turbulent stress compare favorably with the benchmark solutions. The present simulation also correctly predicts the turbulence splatting effect near the windward hill. Both phenomena are in good accordance with the benchmark solutions.

Nascent Flux Rope Observations at Earth's Dayside Magnetopause

AbstractTwo THEMIS satellites, separated by only 0.38 Earth radii (RE), traversed two consecutive dayside magnetopause (MP) current sheets at 3.5 &lt; ZGSM &lt; 3.8 RE on 15 November 2010. An early‐stage crater‐like flux transfer event (FTE) with weakly enhanced total pressure is sampled at the first, complete outbound MP crossing. A mature FTE flux rope is observed just 70 s later across the second, inbound crossing of the MP current sheet. Two counterstreaming magnetosheath ion beams at the interface of two converging reconnection exhausts earthward of the MP current sheet provide direct evidence of two X‐lines across the early‐stage crater FTE. A D‐shaped ion beam observed along the southward magnetic field in the magnetosheath boundary layer and northward exhausts observed earthward of the MP provide evidence of two X‐lines across the mature FTE flux rope. Tripolar out‐of‐plane magnetic field perturbations (ΔBM) of substantial magnitudes were sampled across the MP current sheet in both stages of the FTE flux rope evolution. These dayside observations suggest an association between a tripolar ΔBM perturbation with two magnetic reconnection X‐lines and FTE flux rope formation. A dedicated particle‐in‐cell numerical simulation of magnetic reconnection reproduces similar ion velocity distributions and out‐of‐plane magnetic field perturbations at an asymmetric current sheet as those observed across a nascent flux rope between two X‐lines at the dayside asymmetric MP.

Numerical Investigation on the Thermodynamic Characteristics of a Liquid Film upon Spray Cooling Using an Air-Blast Atomization Nozzle.

This paper developed a three-dimensional model to simulate the process of atomization and liquid film formation during the air-blast spray cooling technological process. The model was solved using the discrete phase model method. Several factors including the thermodynamic characteristics of the liquid film as well as the spray quality with different spray mass flow rates under different spray heights were numerically investigated and discussed. The results show that the varied spray height has little effect on the Sauter Mean Diameter (d32) of the spray droplet, while the thermodynamic characteristics of liquid film including the liquid film height, the liquid film velocity, and the liquid film generation rate are sensitive to the change of the spray height. With the growth of spray mass flow rates, d32, the liquid film generation rate and liquid film height become larger, while the liquid film velocity with different spray mass flow rates has a similar velocity distribution, indicating that the spray mass flow rate has little effect on the liquid film velocity. The average d32 of droplet size shows a sharp drop when sprayed from the nozzle in a short period of time (<1.5 ms), then approaching smoothness, below a value of 40 , the spray status tends to be stable.

Experimental study of the interaction between thermal plumes and human breathing in an undisturbed indoor environment

This study characterized the interaction between human thermal plume and breathing air flow while people are sitting in a quiescent indoor environment. A sitting breathing thermal manikin was developed to mimic a real human, and data from real human subjects was collected to verify the breathing thermal manikin. In this study the velocity and temperature were measured in front of and above the breathing thermal manikin with and without breathing. In addition, the breathing modes through mouth and nose were studied to investigate the influence of breathing mode on the development of thermal plume, respectively. The ambient temperature was set at 23 ± 0.5 °C, and the surface temperature of manikin varied between 33–34 °C. Without breathing, a maximum value of 0.19 m/s was reached at around 35 cm height above the head which was similar with the mouth-breathing case (0.19 m/s) and higher than the nose breathing case (0.17 m/s). However, the two breathing mode would mitigate the thermal gradient and lower the maximum velocity and had little influence on the height of maximum velocity. Above the manikin's head, non-breathing thermal manikin can produce very similar velocity distribution compared to the breathing modes. But the velocity above the head in the non-breathing case reported the highest maximum value and developed faster to reach the maximum velocity. Breathing through the nose had much more impact on thermal plumes around manikin than breathing through the mouth, even change the flow direction.

Understanding the Interplay between Electrolyte Velocity Distribution and Current Distribution in Vanadium Flow Battery Electrode

Vanadium redox flow batteries (VRFBs) are considered a promising candidate among grid-scale energy storage technologies due to their flexibility and scalability, high coulombic efficiency, and long cycle life. However, system costs and relatively low energy and power density compared to other electrochemical energy storage devices are major challenges to commercialization. At the cell level, mass transport losses are one of the major contributors to performance losses; this mechanism is associated with inadequate delivery of active species to electrode surfaces. Improving flow field design and electrode structure or simply increasing flow rate can mitigate mass transport losses. However, due to parasitic pumping losses, optimization is not straightforward. As yet unpublished research in our lab has shown that increased convection improves mass transport of the active species through the porous electrode and it is possible to achieve better electrochemical performance with lower pressure drop in convection-dominated VRFB cells. In this study, the impact of convection on electrochemical performance is investigated using computational fluid dynamics (CFD) model and in-situ localized current distribution diagnostics. Polarization curve analysis and pressure drop measurements are also used to support the conclusions. Experiments and simulations are conducted on strip cell architecture with varying channel depths (0.25, 0.50, 1.00, 2.50 mm) and flow rates (10-50 ml min-1). The simulation domain includes both channel and porous electrodes seen in Figure 1. Conservation of mass and momentum are solved to obtain velocity and pressure distributions. It is found that there is a strong relationship between electrolyte velocity in the electrode and electrochemical performance. Similar electrolyte velocity distribution (in the electrode) predicted for two configurations: 1 mm channel depth at 40 mL min-1 flow rate and 0.50 mm channel depth at 10 mL min-1 have similar polarization curves and current distributions down the channel. However, the model predicts very different velocity distributions for these configurations in channel. Higher velocity distribution predicted for 1 mm configuration has also higher pressure drop than 0.5 mm configuration. The results of the computational and experimental study as related to the trade-off between pumping power requirements and electrochemical performance in VRFBs will be discussed in this talk. Figure 1

Analytical and numerical study of Stokes flow problems for Hausdorff fluids

Flow problem for non-Newtonian fluid has drawn considerable attention over past decades. In this study, we theoretically and numerically investigate the unsteady Stokes’ flow problem of the viscoelastic fluid. The constitutive equation of the viscoelastic fluid is modified from the Newtonian fluid by introducing the Hausdorff derivative, called the Hausdorff fluid. It should be noted that the Hausdorff fluid degenerates to the Newtonian fluid when the derivative order α equals to unity (α = 1). The analytical solutions for velocity field are derived by employing the Fourier sine transform and transformation of variables. The obtained analytical solutions are validated by the finite element simulation. Results show that the Hausdorff fluid with small derivative order α has high propagation velocity and flow velocity. Compared with Stokes’ flow problems for other non-Newtonian fluids, the Hausdorff fluid has the similar velocity distributions with the second grade (SG) fluid but different from the Maxwell (MX) fluid. In addition, the SG fluid has decreased propagation velocity while the MX fluid has increased propagation velocity in comparison with the Hausdorff fluid.

Mathematical modelling of swirl oxy-fuel burner flame characteristics

Abstract Oxy-fuel combustion is the most promising carbon capture and storage technology, which eliminates carbon dioxide emissions into the atmosphere and also decreases nitrogen oxides emissions thereby lowering global warming potential. In order to implement oxy-fuel combustion technology in full scale power plants, its costs, mainly connected with the amount of pure oxygen produced, must be lowered. The main hypothesis is that it is possible to maintain similar velocity and heat transfer distribution while maintaining stable and efficient burner operation during both combustion technologies modifying burner aerodynamics. Excess oxygen is chosen as a representative parameter of burner’s performance and investigation is carried out for four different oxy-fuel burner oxygen excess ratios (λ: 0.8, 0.98, 1.07, and 1.24) together with reference air combustion case. This study suggests a workflow, based on semi-industrial experimental investigations and computational fluid dynamics model composed of advanced sub-models for different combustion phases for development of real scale dual-mode coal swirl burners able for efficient operation during both combustion regimes. The results show that the temperature in near-burner zone and nitrogen oxides emissions increase, while carbon monoxide emissions decrease with the increase of burner oxygen excess ratio, and stable combustion with similar velocity and temperature distributions for both combustion modes is achieved for oxygen excess ratio of 1.07, with decrease in nitrogen oxides and carbon monoxide emissions during oxy-fuel combustion. The performed study demonstrates that it is possible to choose the appropriate burner settings regarding nitrogen oxides and carbon monoxide emissions and burner’s ability to operate stably in both air and oxy-fuel combustion modes.

The metal-poor stellar halo in RAVE-TGAS and its implications for the velocity distribution of dark matter

The local velocity distribution of dark matter plays an integral role in interpreting the results from direct detection experiments. We previously showed that metal-poor halo stars serve as excellent tracers of the virialized dark matter velocity distribution using a high-resolution hydrodynamic simulation of a Milky Way-like halo. In this paper, we take advantage of the first Gaia data release, coupled with spectroscopic measurements from the RAdial Velocity Experiment (RAVE), to study the kinematics of stars belonging to the metal-poor halo within an average distance of ∼5 kpc of the Sun. We study stars with iron abundances [Fe/H] < −1.5 and −1.8 that are located more than 1.5 kpc from the Galactic plane. Using a Gaussian mixture model analysis, we identify the stars that belong to the halo population, as well as some kinematic outliers. We find that both metallicity samples have similar velocity distributions for the halo component, within uncertainties. Assuming that the stellar halo velocities adequately trace the virialized dark matter, we study the implications for direct detection experiments. The Standard Halo Model, which is typically assumed for dark matter, is discrepant with the empirical distribution by ∼6σ, predicts fewer high-speed particles, and is anisotropic. As a result, the Standard Halo Model overpredicts the nuclear scattering rate for dark matter masses below ∼10 GeV. The kinematic outliers that we identify may potentially be correlated with dark matter substructure, though further study is needed to establish this correspondence.

A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities

Abstract The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure) and A13 are stellar overdensities at estimated heliocentric distances of d ∼ 11 kpc and 15 kpc observed at low Galactic latitudes toward the anticenter of our Galaxy. While these overdensities were initially thought to be remnants of a tidally disrupted satellite galaxy, an alternate scenario is that they are composed of stars from the Milky Way (MW) disk kicked out to their current location due to interactions between a satellite galaxy and the disk. To test this scenario, we study the stellar populations of the Monoceros Ring and A13 by measuring the number of RR Lyrae and M giant stars associated with these overdensities. We obtain low-resolution spectroscopy for RR Lyrae stars in the two structures and measure radial velocities to compare with previously measured velocities for M giant stars in the regions of the Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars (f RR:MG) in A13 and Mon/GASS. We perform velocity modeling on 153 RR Lyrae stars (116 in the Monoceros Ring and 37 in A13) and find that both structures have very low f RR:MG. The results support a scenario in which stars in A13 and Mon/GASS formed in the MW disk. We discuss a possible association between Mon/GASS, A13, and the Triangulum-Andromeda overdensity based on their similar velocity distributions and f RR:MG.

Global kinetic hybrid simulation for radially expanding solar wind

AbstractWe present the results of a 1‐D global kinetic simulation of the solar wind in spherical coordinates without a magnetic field in the region from the Sun to the Earth's orbit. Protons are considered as particles while electrons are considered as a massless fluid, with a constant temperature, in order to study the relation between the hybrid and hydrodynamic solutions. It is shown that the strong electric field in the hybrid model accelerates the protons. Since the electric field in the model is related to electron pressure, each proton in the initial Maxwellian velocity distribution function moves under the same forces as in the classical Parker Solar wind model. The study shows that the hybrid model results in very similar velocity and number density distributions along the radial distance as in the Parker model. In the hybrid simulations, the proton temperature is decreased with distance in 1 order of magnitude. The effective polytropic index of the proton population slightly exceeds 1 at larger distances with the maximum value ∼1.15 in the region near the Sun. A highly non‐Maxwellian type of distribution function is initially formed. Further from the Sun, a narrow beam of the escaping protons is created which does not change much in later expansion. The results of our study indicates that already a nonmagnetized global hybrid model is capable of reproducing some fundamental features of the expanding solar wind shown in the Parker model and additional kinetic effects in the solar wind.

Simulation of velocity and shear stress distributions in granular column collapses by a mesh-free method

To describe granular flows in the dense regime, the μ(I) rheology model was proposed. It has been proven to be effective in reproducing flow dynamics in the dense regime caused by dry granular materials. For continuum modeling, the mesh-free method can easily handle flows with interface such as free surface, which commonly exists in most granular flows. In this study, the μ(I) rheology model is coupled with a mesh-free method, Moving Particle Semi-implicit method (MPS). The coupled model is used to analyze velocity and shear stress distributions in granular column collapses. To validate the model, velocity measurements were conducted on two aspect ratios a = 1.25 and 5.0. The coupled model is verified by the measured velocity profiles. Both horizontal and vertical velocity distributions are examined in the validation. A linear relationship on the velocity distribution is observed in the flowing region in the collapses both experimentally and numerically. Another larger aspect ratio a = 7.0 were then simulated and a similar linear velocity distribution was obtained. On the basis of the velocity analysis, the tangential shear stress was analyzed and discussed in the three collapses. It showed that the distribution of the shear stress is symmetrical with the opposite direction. In the core quasi-static region, the shear stress was larger than that in the flowing region. In the free fall of the upper portion for the large aspect ratios such as a = 7.0, there was very small shear stress. In the center of the column in the collapses, the shear stress almost remains zero with some fluctuations.

Effects of sediment erosion in guide vanes of Francis turbine

Guide vanes (GV) in Francis turbines are often reported to have a severe material erosion in case of sediments laden projects. A small clearance gap (CG) between GV wall and facing plate is allowed from the design to pivot GV as a part of flow regulating mechanism. Erosion in GV causes to increases the CG, which induces a leakage flow through the CG due to the inherent pressure difference between GV surfaces. Limited scientific studies on understanding the effects of the leakage flow from the eroded GV on turbine performance have been documented. Presented work aims to characterize the velocity conditions at the runner inlet of Francis turbine due to increase in the CG. Pressure and velocity measurements are done inside a GV cascade producing the similar velocity distribution as that in a reference prototype turbine. With a 2mm CG, the relative velocity close to shroud of runner inlet is found to increase up to three folds of its nominal value. A vortex filament is found to be developed due to the mixing of the leakage flow with the main flow and is observed to pass into the runner with the flow.

Velocity and pressure measurements in guide vane clearance gap of a low specific speed Francis turbine

In Francis turbine, a small clearance gap between the guide vanes and the cover plates is usually required to pivot guide vanes as a part of governing system. Deflection of cover plates and erosion of mating surfaces causes this gap to increase from its design value. The clearance gap induces the secondary flow in the distributor system. This effects the main flow at the runner inlet, which causes losses in efficiency and instability. A guide vane cascade of a low specific speed Francis turbine has been developed for experimental investigations. The test setup is able to produce similar velocity distributions at the runner inlet as that of a reference prototype turbine. The setup is designed for particle image velocimetry (PIV) measurements from the position of stay vane outlet to the position of runner inlet. In this study, velocity and pressure measurements are conducted with 2 mm clearance gap on one side of guide vane. Leakage flow is observed and measured together with pressure measurements. It is concluded that the leakage flow behaves as a jet and mixes with the main flow in cross-wise direction and forms a vortex filament. This causes non-uniform inlet flow conditions at runner blades.