Non-uniform Velocity Distribution Research Articles

A numerical study on the interaction between two cross-flow turbines in tandem configuration to support a simplified turbine model approach

In the planning of hydrokinetic turbine array deployment and for the performance prediction of its constituent turbines, the use of simplified turbine models is essential to alleviate the computational costs. The Effective Performance Turbine Model (EPTM) introduced in 2018 is a promising tool for that purpose, allowing array analysis and optimization. Its performance predictions scale with a local flow velocity characterization, which ensures to take into account inherently blockage effects and mean, local flow conditions. However, the characteristics of the local flow within an array also include different types of perturbation such as shear, large-scale temporal fluctuations, and turbulence. To ensure that the model is still reliable in those conditions, this paper presents a validation of the EPTM through the analysis of a large-scale tandem turbine configuration. In fact, three unsteady-Reynolds-averaged-Navier–Stokes simulations of a cross-flow turbine tandem configuration with a longitudinal spacing of six diameters have been conducted in this study, in addition to a simulation of a single turbine with turbulent ambient conditions. This set of simulations allows us to study independently the different types of perturbations associated with array deployment. Whereas the slow-varying large-scale upstream velocity fluctuations do not seem to affect significantly the turbine operation, the upstream non-uniform velocity distribution affects appreciably the extracted power. We find also that the value of the effective power coefficient associated with the EPTM needs to be adapted to the array simulation. Compared to the case of a single turbine in a uniform flow with a low turbulence level, we show that a smaller effective power coefficient value must be used in array simulations. An important result is that the different types of perturbations are found to yield similar effective performance coefficients, which suggests that the same set of values can be used throughout the array. With the appropriate set of values, we show that the EPTM succeeds to predict accurate downstream turbine performance.

Investigation on the Transient Thermal Performance of a Mini-Channel Cold Plate for Battery Thermal Management

Cold plate is an important component for a liquid battery thermal management system. In order to study the transient thermal performance of the cold plate under conditions with sharply increasing heat loads, the numerical model of a battery cold plate is established. The validation experiment shows that the error between the simulation and experiment is around 2.5% to 5%. Effects of the coolant flow rate, the increase in heat flux, and the channel number are analyzed to study the transient thermal performance of the cold plate. Results show that the average temperature of the cold plate at 540 s is lowered from 28.3°C to 26.9°C when the coolant flow rate is raised from 0.065 kg/s to 0.165 kg/s. The temperature deviation is decreased when the coolant flow rate is increased from 0.065 kg/s to 0.115 kg/s; however, it is slightly increased if the coolant flow rate is further increased. Both average temperature and temperature deviation are raised if the final heat flux is increased from 11 000 W/m2 to 16 500 W/m2, which are 2 and 3 times of the initial, respectively. In addition, increasing the channel number has slightly positive effect on the average temperature of the cold plate, while the temperature deviation is increased when the channel number is increased from 3 to 11 due to the non-uniform velocity distribution between each channel. The results of this study will be helpful during the design of cold plate for battery thermal management, especially for transient conditions with sudden rising heat loads like thermal runaway.

Numerical Analysis of the Particle-Induced Effect on Gas Flow in a Supersonic Powder-Laden Oxygen Jet

The article addresses the particle-induced effect on gas flow by a developed mathematical model simulating the supersonic oxygen–limestone powder mixture jet from a newly designed, freely swinging oxygen lance for dephosphorization in a converter. The model was validated first and then employed to analyze the gas flow dynamics with respect to the jet structure, pressure wave sequence and velocity distribution, and particle motion behaviors over a range of powder particle sizes and mass flowrates. Numerical results reveal that the powder causes significant changes of the oxygen jet structure and weakens the pressure wave sequence outside the nozzle despite the higher gas flow static pressure of the powder-laden oxygen jet than that of the single-phase oxygen jet there. Furthermore, the powder creates nonuniform radial distribution of the gas velocity as an “M-type” curve and greatly restricts the gas jet flowing along the nozzle axis, which results in the smaller gas flow velocity but slower velocity attenuation. These phenomena become increasingly remarkable as the powder particle size is decreased or the powder mass flowrate is increased. The particles are accelerated but the acceleration declines gradually along the nozzle axis. The greater powder mass flowrate or particle size induces the lower particle velocity.

Influence of non-uniform inflow swirl and hot streak on turbine vane

The turbine vane bears higher and higher temperature of upstream flow with the higher requirements for the engine thrust. The heat resistance of turbine vanes is a severe problem for the lifetime of turbine vanes. Therefore, it is essential to study the flow field and heat transfer of the vane to find better ways to improve the heat resistance of the turbine and increase the thrust of the engine. To improve the efficiency of the turbine vanes, the present paper conducted researches on the heat transfer and the pressure loss of turbine vanes under different radius ratio of swirl and hot streak. The numerical applies CFD 3D RANs solver with the SST turbulent model to get the influence of non-uniform inflow velocity and temperature distribution on turbine vanes. The present numerical model utilizes turbine vane C3X. Considering the viscosity of the fluid causes shear stress on the wall, the paper set the boundary layers mesh in the numerical model. The size of the first boundary layer mesh is 10–6 m. The results show that the hot fluid on the suction side migrates towards the hub, and the hot fluid on the pressure side migrates towards the shroud while there are coupling effect of the swirl and hot streak. The larger the radius ratio of the hot streak and the swirl (RHS) is, the higher the temperature of the local high-temperature regions exist on the suction side of leading edge and the pressure side of the trailing edge is, and the higher the pressure loss is.

Influence of thermodynamics inside bubble and flow nonuniformity on weakly nonlinear waves in bubbly liquids

An effect of the liquid viscosity and the thermal conductivity and nonuniform distribution of initial flow velocity on weakly nonlinear propagation of pressure waves in an initially quiescent liquid containing many small spherical gas bubbles is theoretically elucidated. Based on the derivation of the KdV–Burgers equation for a long wave and the nonlinear Schrodinger equation for a short carrier wave, the following results are obtained: (i) the installation of the energy equation affects nonlinear, dispersion, and dissipation effects; (ii) the liquid viscosity and the thermal conductivity lead to change considerably the explicit form of the coefficient in the dissipation term; and (iii) the weak nonuniform effect appears in a far field.

공기유속 분포와 열전달 향상조건을 고려한 발전 플랜트용 공랭식 응축기(ACC)의 성능에 관한 수치적 연구

공기유속 분포와 열전달 향상조건을 고려한 발전 플랜트용 공랭식 응축기(ACC)의 성능에 관한 수치적 연구

Optimization of die design for thin-walled flat multi-port tube with the aid of finite element simulation

Abstract Die design plays a critical role in aluminum alloy profile production, which directly influences the quality of the final products and the service life of the die. In the present study, a porthole die for a multi-hole extrusion process of a thin-walled flat multi-port tube is designed and optimized through numerical simulations. The simulation results show that with the original die, the extruded tube has a non-uniform velocity distribution in its cross-section and bends outward when exiting the die orifice. Meanwhile, a severe stress concentration in the mandrel teeth causes the premature failure of the die, and an uneven deflection of the mandrel teeth and bearing causes the arc-shaped wall thickness of the tube to exceed the tolerance level, which is validated by corresponding extrusion experiments. To solve these problems, two periods of die structure modifications are proposed, and their extrusion processes are simulated. The optimized die is confirmed to exhibit better performance than the original one in terms of lowered maximum die stress, increased bearing pressure, and decreased distortion of the final products. The results of the extrusion experiments indicate that, with the optimized die, a highly qualified tube with good appearance, desired dimensional accuracy, fine-grained microstructure, and high-pressure bearing capacity is manufactured successfully, and the die failure is delayed effectively.

Hydrodynamic loads and wake dynamics of ducted propeller in oblique flow conditions

ABSTRACT The hydrodynamic loads and wake dynamics of the ducted propeller in oblique flow are investigated by detached eddy simulation (DES), with particular emphasis on the characteristics of wake fields and kinetic energy (KE) spectra. The influence of the duct, together with the strengthened induction of the propeller, cause the non-uniform velocity distributions in the inflow plane, resulting in unstable and periodic hydrodynamic loads on the blades. Shedding vortices from the leading edge of the duct dominate the leeward wake, and strengthen the self- and mutual induction of tip vortices. Large deformation of the tip vortices is observed due to the complex interference of the shear-layer vortices, shedding vortices and secondary vortices. The trajectories of the tip vortices are found distorted but still recognisable on the windward wake. The spiral-to-spiral distances of tip vortices increase and the re-alignment effect of the main flow enhances in the downstream wake.

Migration of a red blood cell in a permeable microvessel

One of the important features of the microvessels is with permeable vessels. The hyperpermeablility is especially prounounced due to the fast grow of the microvessel in the tumor tissues. Flow field induced by leakage often presents a different figure compared to that of the health vessel. Non-uniform velocity and pressure distribution are found in the simulation by using Lattice Boltzmann Method combined with Starling law. This non-uniform flow field has significant influence on the mass transport in permeable mocrovessel, such as the motion of red blood cell. Using 2D spring network model to express the red blood cell and immersed boundary method to implement the coupling of fluid and structure, numerical simulations were carried out to study the migration of a red blood cell in a straight permeable channel. The results show that the staying time of red blood cell in the leakage region increases with the intensity of leakage. We also found that the leakage can weaken the lateral migration of the red blood cell from the wall to the channel center, and even tune the migration towards the wall when the leakage is reasonably strong.

A field-segmentation-based variational optical flow method for PIV measurements of nonuniform flows

Air flow fields have nonuniform velocity distributions because of the compressibility of air. Current optical flow-based particle image velocimetry (PIV) methods cannot thoroughly solve these distributions because of the global optical flow formulation. In this study, we propose a novel field-segmentation-based variational optical flow (FS-VOF) method for preserving the spatially discontinuous characteristics of nonuniform flow fields. The proposed method is designed based on the level-set segmentation method, in which the particle image is segmented according to the velocity distribution of the fluid flow. Subsequently, a segmented smoothness constraint term built from the assumption that velocity varies continuously in each segmented particle image region and the discontinuous structures of the velocity field are preserved at the boundaries of the segmented regions. In addition, the data term of the proposed method is based on the segmented regions and derived from the physics-based optical flow equation. The proposed method is evaluated over synthetic and experimental images, and the comparison with advanced variational optical flow algorithms demonstrates the effectiveness of our method in preserving the spatially discontinuous characteristics of nonuniform flow fields.

Natural frequency analysis of curved pipe conveying non-uniform axial flow

In this article, the natural frequencies of curved pipe conveying non-uniform axial flow are calculated. The non-uniformity of velocity distribution on the section is considered and the parabola fluid is adopted in the section. The Flügge curved beam is used to build the pipe model. The dynamic equation of pipe conveying non-uniform fluid is derived based on Hamilton principle. The wave propagation method is used to solve the equation. With different flow velocity, the natural frequency is computed in the assumption of non-uniform fluid and uniform fluid. Compared with the uniform fluid, the non-uniform fluid will have a higher frequency value.

Effect of Laser-Matter Interaction on Molten Pool Flow and Keyhole Dynamics

In laser-based welding and additive manufacturing, the interaction of the laser with the metal leads to the formation of a cavity known as a keyhole, which can fluctuate unstably during the process. This work significantly advances our understanding of laser-induced keyholes and their dynamics, by combining state-of-the-art dynamic x-ray radiography with multiphase, multiphysics modeling. Numerical simulations of keyhole morphologies are validated by experiment, then leveraged to predict transient nonuniform distributions of laser absorption, temperature, and flow velocity in the complex multiphase process.

Effect of Orifice Geometry on Synthetic Jet Evolution

The present work discusses an experimental investigation into the effect of orifice geometry on the formation and advection of vortical structures associated with a synthetic jet issued into a quiescent flow. Three orifice geometries (namely, rectangular, trapezoidal, and triangular shapes) having the same aspect ratio () and area were tested at a hydraulic diameter-based Reynolds number and a Strouhal number of 7000 and 0.066, respectively. Using a single hot wire, the spanwise distribution of the velocity at the jet orifice plane was first measured. The trapezoidal and triangular orifices exhibited a nonuniform velocity distribution along the orifice, where the velocity was 25 and 130% higher at the smaller cross-section side as compared to the large section side, respectively. Stereoscopic particle image velocimetry was employed to investigate the velocity and vorticity fields downstream of the synthetic jet’s orifice. Vortical, ringlike structures were created at each orifice, where the trapezoidal and triangular orifice geometries resulted in vectoring of these structures toward the larger cross-sectional area of the shape. Axis switching was also observed by each orifice; however, it occurred closer to the orifice for the triangular and trapezoidal orifices when compared to the rectangular orifice. Jet spreading and vorticity dissipation are also presented and were shown to be highly dependent on the orifice geometry.

Effect of flow behavior in extra-column volumes on the retention pattern of proteins in a small column

Experimental and theoretical analysis of deformation of band profiles in extra-column volumes (ECV) was performed, and its influence on the retention pattern of proteins in a small chromatographic column was quantified. Several macromolecule and small-molecule compounds, and their mixtures were eluted from a chromatographic system in the absence and presence of the column. The peak deformation in ECV was attributed to non-uniform velocity distribution in the radial direction in connecting capillaries. The phenomenon enhanced with increasing molecular weight of the model compound, when radial diffusion dominated the mechanism of band spreading. The band shape was also affected by the geometry of the injection system used, i.e., an injection loop capillary or a superloop. The phenomenon vanished for a small molecule compound, for which plug flow conditions could be established. The difference in flow behaviour of the macromolecule and small-molecule compounds caused them to migrate with different velocities in ECV, which resulted in partial separation of their bands. The ECV effect influenced the retention behaviour of macromolecules in a small column; it caused tailing of peaks and asymmetry of breakthrough curves. To describe the elution profiles in ECV and in the column, a mathematical model was used which accounted for non-ideality of the flow pattern. The model reproduced accurately band profiles of macromolecules within a range of relatively low velocities, typical however for protein chromatography.

Estimating the non-uniform air velocity distribution for the optimal design of a heat exchanger

The maldistribution of flow in a heat exchanger usually has negative effects on the heat exchanger specifications such as the effectiveness, annual cost, and pressure drop. This paper examines the issue to improve the thermoeconomic performance. For this purpose, a plate-fin heat exchanger was selected and optimized by considering both the effectiveness and total annual cost as objective functions. The design parameters were the parameters of the heat exchanger and fin, as well as the non-uniform inlet flow profiles for both sides of the hot and cold streams. To generalize the results, the optimization was applied at different mass flow rates. The optimum results of the non-uniform profile were compared with those obtained with a uniform profile of the inlet flow. The optimum results reveal that thermoeconomic improvement occurred in the case of non-uniform flow compared with the case of uniform flow. For example, a 12.7% reduction for the total annual cost was found in the case of non-uniform flow in comparison to the uniform flow for a fixed effectiveness of 0.92. At low effectiveness, no significant improvement was found in the case of non-uniform flow. Nevertheless, the suggested profiles could be considered as an alternative to uniform inlet profiles.

Research and optimization of performances of a pump turbine in pump mode

For reversible pump turbines, a sufficient hump safety margin has to be provided under small-flow and high-head conditions to avoid instability resulting from operations in the hump-shaped zone. On the other hand, under large flow conditions, cavitation easily occurs on the low-pressure edge of runner blades due to effects of low pressure and flow separation, sharply reducing the efficiency and lift head. Therefore, the hydraulic design of the pump turbine in pump mode is crucial for the properties of energy, cavitation and stability of the whole unit. A model runner (runner A) designed for a pumped storage power plant in China was demonstrated with insufficient hump safety margin and poor cavitation performance. To improve those deficiencies, optimization design was carried out on the geometric dimensions of the runner A, based on the computational fluid dynamics (CFD) analyses and model test results. The final model runner B was designed with less blade number Z and large blade wrap angle θ, to both alleviate the flow separation around the lower pressure edge of the blade and the non-uniform velocity distribution in the outlet area around high pressure edge. The model test results show that in comparison with the original runner A, the performances, such as efficiency, hump safety margin, cavitation and stability are improved in runner B.

Numerical analysis of water exit for a sphere with constant velocity using the lattice Boltzmann method

In this paper, the three-dimensional water exit of a sphere with different vertical velocities is investigated numerically using the lattice Boltzmann method (LBM). In this method, the liquid-gas two-phase flow is simplified as a single-phase free surface flow. To capture the free surface, a mass tracking algorithm is incorporated into the LBM. The gravity as a body force is introduced in the form of calculating the equilibrium distribution with an altered velocity, while the surface tension is neglected. Besides, the employed bounce-back boundary conditions are used for a moving sphere. What’s more, the Wall-Adapting Local Eddy (WALE) viscosity model is employed to capture the turbulent structures of the flow and stabilize the simulation. The accuracy of the numerical results is demonstrated through comparisons with the previous numerical and experimental results in the literature. The results show that the spike height is significantly influenced under the Froude number (Fr) below 4.12 and slightly affected under the Fr varying from 4.12 to 8.24. After the sphere exits water totally, the evolution of the free surface waterfall can be described as two phases and becomes more intense with the Froude number increasing. The non-uniform distribution of velocity results in the breaking of the free surface after the sphere completely exits the water. Moreover, the Reynolds number greatly affects the wake dynamics and hydrodynamics acting on the sphere when it moves beneath the water surface.

ALMA observations of the “fresh” carbon-rich AGB star TX Piscium

Aims. The carbon-rich asymptotic giant branch (AGB) star TX Piscium (TX Psc) has been observed multiple times during multiple epochs and at different wavelengths and resolutions, showing a complex molecular CO line profile and a ring-like structure in thermal dust emission. We investigate the molecular counterpart in high resolution, aiming to resolve the ring-like structure and identify its origin. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) observations have been carried out to map the circumstellar envelope (CSE) of TX Psc in CO(2–1) emission and investigate the counterpart to the ring-like dust structure. Results. We report the detection of a thin, irregular, and elliptical detached molecular shell around TX Psc, which coincides with the dust emission. This is the first discovery of a non-spherically symmetric detached shell, raising questions about the shaping of detached shells. Conclusions. We investigate possible shaping mechanisms for elliptical detached shells and find that in the case of TX Psc, stellar rotation of 2 km s−1 can lead to a non-uniform mass-loss rate and velocity distribution from stellar pole to equator, recreating the elliptical CSE. We discuss the possible scenarios for increased stellar momentum, enabling the rotation rates needed to reproduce the ellipticity of our observations, and come to the conclusion that momentum transfer of an orbiting object with the mass of a brown dwarf would be sufficient.

Two-stage velocity distribution measurement from multiple projections by tunable diode laser absorption spectrum

A novel approach to using tunable diode laser absorption spectrum (TDLAS) is developed for nonuniform velocity distribution measurement by Doppler effect. An analysis of the energy in direct absorption spectrum at low frequencies is made by Fourier transform, because the TDLAS method offers the advantages in using Beer law to deal with coupling relations between velocity distribution and corresponding length of velocity region. By comparing with traditional TDLAS-Doppler velocity measurement, advantages of this approach to the more exact solution of core flow velocity by signal process without using extra lasers and detectors are explored. Following the published theory, between velocity regions at multiple projections the absorbance about average in frequency offsets and the absorbance about difference in frequency offsets are incorporated into an improved fitting model. A solution to obtaining changes of absorbance energy at low frequencies by Fourier transform is used to demonstrate the ability to recover minor change in absorbance under different conditions, inferring a better method to realize the simultaneous measurement of velocity distribution. The influences of these parameters, such as projection angles and noise during absorption, are investigated by the multiple projection simulations at rovibrational transitions of H<sub>2</sub>O near 7185.6 cm<sup>–1</sup> from three projections. This approach is validated in a two-stage velocity distribution model, demonstrating the ability to exactly measure core flow, with a precision of 0.9% RMS (root mean square). The high velocity in the core flow is less influenced by the random noise in absorption due to nearly linear relationship between the difference in frequency offsets and the ratio of length of velocity region. Some satisfied results can be obtained when larger angles of projection are arranged. The combination of 0°, 30°, and 60° will be a reasonable optic design considering the limitation of spatial resolution. In conclusion, the novel approach to velocity distribution measurement based on TDLAS-Doppler from multiple projections has great potential applications in engine diagnosis and gas dynamic research.

On the fully nonlinear water entry of a cone in Stokes wave

Motivated by the wave entry problems of the three-dimensional (3D) ship bow, the paper concentrates on the interaction between an incident wave and a falling cone. The nonlinear 5th order Stokes wave is used to model the undisturbed fluid field, and the wave disturbed by the falling cone satisfies the fully nonlinear boundary conditions. The numerical model is calculated by boundary element method. During the numerical procedure, stretched coordinated system and improved Eulerian method are adopted to trace the highly distorted wave surface. Water entry with different wave heights has been compared with calm water case, oblique entry has been simulated, and the flow behaviors on the up-wave, down-wave and lateral side of the body are investigated. The relative deadrise angle, the relative velocity between body and fluid particles, the propagation direction of the wave as well as the non-uniform distribution of flow velocity are considered together to investigate the 3D pressure field.