Basic Flow Characteristics Research Articles

Experimental research on characteristics of condensation induced water hammer in natural circulation systems

Abstract Condensation induced water hammer (CIWH) is a common phenomenon possibly appearing in industrial pipelines, which may cause pipeline oscillation and even serious damage to the components. Previous research on the CIWH mostly focuses on the steam-water counterflow under forced flow conditions. For new floating nuclear power plants proposed recently, the CIWH may occur in some passive natural circulation safety systems. Nevertheless, the CIWH in natural circulation systems (NCS) has never been reported publicly yet. In this paper, a series of experiments are performed to investigate the flow characteristics during the CIWH in a NCS. The basic flow and temperature characteristics of the CIWH caused by counterflow of saturated steam-water mixtures and subcooled water in the NCS are studied in detail. Thereinto, the steam condensation induced flashing (SCIF) along with CIWH is firstly observed and analyzed. Moreover, the influences of CIWH on the flow behavior of the system are discussed under natural circulation and forced circulation conditions. The experimental results indicate that the occurrence of CIWH can result in further reverse flow of the subcooled water, which causes a sudden decrease in the flow rate of NCS. Moreover, the flow rate is more unstable under natural circulation than under forced flow.

HTR-PM-based 3D pebble flow simulation on the effects of base angle, recirculation mode and coefficient of friction

This work shows basic features of pebble flow in HTR-PM via DEM simulation, in accompany with pebbled beds of varied base angles, friction coefficients and recirculation modes. The effects of base angles, friction coefficients and recirculation modes on the characteristics of pebble flows are explored. Beside phenomenological observation by pebble stripes, the three-dimensional pebble spindles and pebble streamlines defined in the Lagrangian and Eulerian framework respectively are all computed and analyzed quantitatively to give full description of pebble flows. The probability density function of horizontal distribution is computed to quantify pebble diffusion in the pebble spindle. The pebble streamlines are also fitted with modeling functions well to give useful indications for reactor design, and the fitting coefficients under various conditions are provided. The simulation results will be useful for better understanding and predictions of the real pebble beds, as well as their influencing factors.

Characteristics and Prevention Measures of Loess Geological Disasters in Longdong Region

In recent years, loess geological disasters in Longdong region occurred more and more frequently. On the basis of investigation on loess geological disasters in Longdong region and analysis of basic physical characteristics of loess geological disasters, landslide, collapse, debris flow and loess collapsibility are identified as the main geological disasters in Longdong region. The causes of four types of loess geological disasters in Longdong region are studied and corresponding prevention measures are proposed.

Computational Investigation of Vertical Stabilizer with Vortex Generators and Dorsal Fin

This paper describes methods to improve the performance of a vertical stabilizer in terms of yawing moment with corotating blade-type vortex generators and a dorsal fin over a wide range of sideslip angles. A vertical stabilizer designed by ONERA was mounted on the NASA Common Research Model to investigate the performance of sets of vortex generators and dorsal fins at several rudder deflection angles. Computational fluid dynamics simulations were performed to reveal basic flow characteristics of the vertical stabilizer, including its rudder, and to understand how vortices generated around the vortex generators and dorsal fins interact with the leading-edge separation vortex and boundary layer on the vertical stabilizer and how the vortices improve the lateral performance of the vertical stabilizer. Wind-tunnel tests were also conducted to validate the computational results. Vortex generators improved the performance of the vertical stabilizer slightly at low sideslip angles by reducing flow separation on the rudder, and a dorsal fin assisted greatly at high sideslip angles due to two vortices it induced: the primary dorsal fin vortex and the leading-edge separation vortex.

Traffic flow analysis in Ahmedabad (India)

In terms of traffic flow analysis, there are many research studies since 1935 when Bruce Douglas Greenshields introduced traffic flow theory with traffic density and vehicle speed using photographic measurement methods for the first time. There are several theoretical proposals followed by Greenshields, such as Greenberg (1959), Underwood (1961) etc. In these studies, there are several limitations like, particular expressways, time period to be measured and so on. While Greenshields’ model is not perfect, it is fairly accurate and relative simple. Therefore Greenshields’ model is widely used in the traffic flow analysis. In advanced countries Europe, United State of America and Japan, there are several variations for their analysis and certain confidence based on Greenshields’ model. However there is quite few traffic flow analysis in developing countries because of their difficulty for measuring traffic condition and lack of infrastructure system for road operation. The purpose of this study is the first time to analyze actual traffic flow based on more than one month collecting traffic date in Ahmedabad city of Gujarat state India.In order to analyze traffic data quantitatively in Ahmedabad city, we are able to collect actual traffic flow data. However the data is spread out widely which is most unlikely in the advanced countries. After creating traffic flow with measured data such as traffic density (k) to traffic volume (q) and traffic density (k) to speed (v), there are clear boundary line each traffic characteristic which divides from data existence area and no existence area in every traffic flow characteristics. The Data Envelopment Analysis (DEA) allows to make traffic characteristics of each roads uniquely when the theoretical traffic flow equations fit along with those Envelopment lines. Once the traffic flow equation is fixed, we are able to get traffic parameters such as free flow speed (vf), jam traffic density (kj) etc. In order to understand traffic congestion, we create speed ratio which is average speed (vave) to free speed (vf) which makes generic value for comparing traffic condition among measured roads.As the result of DEA, we have the traffic flow equation for each road as the basic traffic flow characteristics and understand inside of those Envelopment line is traffic congestion. And speed ratio parameter under 0.65 shows congested condition by real traffic condition monitoring. This phenomena reminds some similarity of the parallel flat plates viscous fluids model characteristics. The flow speed ratio between average flow speed to maximum flow speed becomes two third provides threshold between turbulent flow and viscous flow. This study is the beginning of developing country with quantitative analysis by DEA and helps the understanding traffic flow characteristics.

Numerical Simulation of Turbulent Incompressible Flow with Increasing Adverse Pressure Gradient

Consideration is given to separation-free plane flow in a boundary layer developing on a smooth diffuser wall under the action of a pressure gradient p(x), where both quantities dp/dx and d2p/dx2 are positive (adverse pressure gradient). The adverse pressure gradient arose in a diffuser with plane walls (experiment F0141A). The results of numerical calculations of flow with a nonzero pressure gradient in the boundary layer of an incompressible liquid with the use of the near-wall k–e model of turbulence are given. A comparison of the basic characteristics of flow with tested experimental and theoretical data is made.

Machine learning–based analysis of in-cylinder flow fields to predict combustion engine performance

Cycle-to-cycle variations in an optically accessible four-stroke direct injection spark-ignition gasoline engine are investigated using high-speed scanning particle image velocimetry and in-cylinder pressure measurements. Particle image velocimetry allows to measure in-cylinder flow fields at high spatial and temporal resolution. Binary classifiers are used to predict combustion cycles of high indicated mean effective pressure based on in-cylinder flow features and engineered tumble features obtained during the intake and the compression stroke. Basic in-cylinder flow features of the mid-cylinder plane are sufficient to predict combustion cycles of high indicated mean effective pressure as early as 180 degree crank angle before the top dead center at 0 degree crank angle. Engineered characteristic tumble features derived from the flow field are not superior to the basic flow features. The results are independent of the tested machine learning method (multilayer perceptron and boosted decision trees) and robust to hyper-parameter selection.

Selective upstream influence on the unsteadiness of a separated turbulent compression ramp flow

A study of the shock-wave/boundary-layer interaction induced by a compression ramp was carried out using high-fidelity simulations. The objective was to investigate the influence of upstream disturbances on low-frequency separation unsteadiness. Two computations were performed for a 24° compression ramp at Mach 2.25, one highly resolved case and one reduced-resolution case. The reduced-resolution case was run for an extended duration to capture many cycles of low-frequency unsteadiness. Basic flow characteristics, including statistics on the boundary layer, wall pressure, and skin friction, were computed. Frequency spectra were calculated to confirm the presence of low-frequency unsteadiness. The influence of upstream disturbances on large-scale separation unsteadiness was investigated using correlations, filtering, and conditional averaging based on the position of the primary separation shock. Low-frequency unsteadiness was found to be related to structures near the wall (y/δ < 0.5) with a time scale greater than 20δ/U∞, whereas higher frequency separation motion could be attributed to turbulent boundary layer structures with a time scale on the order of δ/U∞. The finding that the separation region responds selectively to certain large-scale, near-wall perturbations in the incoming flow supports a model of separation unsteadiness in which external forcing by certain components of boundary layer turbulence drives a weakly damped global mode of the separation bubble. This contrasts with suggestions that have been made in the literature that the separation region oscillates on its own, as in an amplified global mode.

Jet Flow Issuing from Deforming Nozzle

The purpose of this study is to control the jet diffusion by using the deforming nozzle. The shape of polypropylene nozzle can be changed from square shape to cruciform shape, again to square one smoothly and periodically. The velocity measurement of the jet flow was carried out using a PIV method, by changing the nozzle shape variously. The basic characteristics of the jet flow, such as the time averaging velocity distributions and the half value widths of the jet have been obtained. Due to the periodic deformation of the shape of the nozzle exit, the inducement of the ambient fluid into the jet becomes more activated. As a result, it has been found that the jet diffusion issuing from the deforming nozzle is consequently promoted in xOy-plane, but is suppressed in xOy'-plane.

Buoyancy-induced convection of water-based nanofluids from an enclosed heated cylinder

PurposeLaminar natural convection of nanofluids in a square cooled cavity enclosing a heated horizontal cylinder is studied numerically. This paper aims to investigate in what measure the nanoparticle size and average volume fraction, the cavity width, the cylinder diameter and position, the average temperature of the nanofluid and the temperature difference imposed between the cylinder and the cavity walls, affects the basic heat and fluid flow features, as well as the thermal performance of the nanofluid relative to that of the base liquid.Design/methodology/approachThe four-equation system of the mass, momentum and energy transfer governing equations has been solved using a computational code incorporating three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity and the coefficient of thermophoretic diffusion, all based on a high number of experimental data available in the literature. The SIMPLE-C algorithm has been used to handle the pressure-velocity coupling. Simulations have been performed using Al2O3 + H2O, for different values of the average volume fraction of the suspended solid phase in the range 0-0.04, the diameter of the nanoparticles in the range 25-75 nm, the temperature difference imposed between the cylinder and the cavity walls in the range 5-20 K, the average nanofluid temperature in the range 300-330 K, the ratio between the cylinder diameter and the cavity width in the range 0.1-0.5 m, the ratio between the distance of the cylinder axis from the bottom wall and the cavity width in the range 0.2-0.8 and the ratio between the distance of the cylinder axis from the left sidewall and the cavity width in the range 0.2-0.5.FindingsThe main results obtained may be summarized as follows: the overall solid phase migration from hot to cold results in a cooperating solutal buoyancy force which tends to compensate the friction increase consequent to the viscosity growth due to the dispersion of the nanoparticles into the base fluid; the effect of the increased thermal conductivity consequent to the nanoparticle dispersion into the base fluid plays the major role in determining the heat transfer enhancement of the nanofluid, at least in the upper range of the investigated average temperatures; at high temperatures, the nanofluid heat transfer performance relative to that of the pure base liquid increases with increasing the average volume fraction of the suspended solid phase, whereas at low temperatures, it has a peak at an optimal particle loading; the relative heat transfer performance of the nanofluid increases notably with increasing the average temperature, and just moderately as the imposed temperature difference, the width of the cavity and the distance of the cylinder from the bottom of the cavity, are increased; the relative heat transfer performance of the nanofluid increases as the nanoparticle size, the cylinder diameter and the distance of the cylinder from the sidewall, are decreased; as a consequence of the local competition between the thermal and the solutal buoyancy forces, a periodic flow arises when the cylinder is located in the vicinity of one of the cooled walls of the enclosure.Originality/valueFramed in this general background, a comprehensive numerical study on buoyancy-driven convection of alumina-water nanofluids inside a cooled square cavity containing a heated circular cylinder is executed by the way of a two-phase model based on the double-diffusive approach accounting for the effects of the Brownian diffusion and thermophoresis.

Modelling of flow around hexagonal and textured cylinders

The flow regime around a hexagonal polygon with low Reynolds numbers Re < 200 is numerically investigated in two different orientations namely face- and corner oriented. The basic flow characteristics, including drag coefficient, lift coefficient, Strouhal number and critical Reynolds number of the hexagonal cylinders, are calculated by solving the Navier–Stokes and mass conservation (continuity) equations, using the Simple (semi-implicit method for pressure-linked equations) algorithm. Within the studied range of Re, the predicted lift coefficient and Strouhal number of the face-oriented hexagon were higher than those of the corner-oriented hexagon. In contrast, the predicted drag coefficient and critical Reynolds number of the corner-oriented hexagon were greater than those of the face-oriented one. Flow characteristics of a novel textured geometry are also studied using three-dimensional transient analysis. The Strouhal number St of the textured geometry was found to be in between the St of both the hexagonal cylinders, and its lift coefficient is lower than that of the hexagonal cylinders. The computational fluid dynamics results show that, within the studied Reynolds number range, the drag coefficient of the textured pipe is almost equal to that of the circular cylinder while its lift coefficient is substantially smaller than that of circular and face- and corner-oriented hexagon pipes.

Numerical analyses and optimizations on the flow in the nacelle region of a wind turbine

Abstract. The present study investigates flow dynamics in the hub region of a wind turbine focusing on the influence of nacelle geometry on the root aerodynamics by means of Reynolds averaged Navier–Stokes simulations with the code FLOWer. The turbine considered is a generic version of the Enercon E44 converter incorporating blades with flat-back-profiled root sections. First, a comparison is drawn between an isolated rotor assumption and a setup including the baseline nacelle geometry in order to elaborate the basic flow features of the blade root. It was found that the nacelle reduces the trailed circulation of the root vortices and improves aerodynamic efficiency for the inner portion of the rotor; on the other hand, it induces a complex vortex system at the juncture to the blade that causes flow separation. The origin of these effects is analyzed in detail. In a second step, the effects of basic geometric parameters describing the nacelle have been analyzed with the purpose of increasing the aerodynamic efficiency in the root region. Therefore, three modification categories have been addressed: the first alters the nacelle diameter, the second varies the blade position relative to the nacelle and the third comprises modifications in the vicinity of the blade–nacelle junction. The impact of the geometrical modifications on the local flow physics are discussed and assessed with respect to aerodynamic performance in the blade root region. It was found that increasing the nacelle diameter deteriorates the root aerodynamics, since the flow separation becomes more pronounced. Possible solutions identified to reduce the flow separation are a shift of the blade in the direction of the rotation or the installation of a fairing fillet in the junction between the blade and the nacelle.

Numerical methods study of solving transient flow of impeller machinery under mutational working conditions

The transient performances of impeller machinery under typical mutational working conditions play an important role in the area of engineering and have attracted extensive attention. In order to fully understand transient behaviors of impeller machinery under mutational working conditions, it is very necessary to confirm an efficient and reliable solving method. In this paper, the dynamic mesh method and the sliding mesh method are respectively used to be solved the inner transient flow of a 2D centrifugal pump model during abrupt start-up period. The Renormalization-group (RNG) k– ε turbulence model in eddy viscosity models is chosen to close the Reynolds-average equation and to complete the turbulence calculation. Moreover, the SIMPLE algorithm is also adapted to accomplish the coupling calculation between velocity and pressure. The study shows that in theory, the most appropriate method to solve transient flow is the dynamic mesh method, but it has the shortage of low mesh quality during mesh reconstruction. The sliding mesh method with high mesh quality is an ideal solving method under mutational working conditions. In general, both of the methods would reflect the basic characteristics of inner flow, while the sliding mesh method has higher solution precision.

Superplasticity in Ultrafine-Grained Materials.

Abstract Superplasticity refers to the ability of a polycrystalline solid to exhibit a high elongation, of at least 400% or more, when testing in tension. The basic characteristics of superplastic flow are now understood and a theoretical model is available to describe the flow process both in conventional superplastic materials where the grain sizes are a few micrometers and in ultrafinegrained materials processed by severe plastic deformation where the grain sizes are in the submicrometer range. This report describes the basic characteristics of superplastic metals, gives examples of flow in ultrafine-grained materials, demonstrates the use of deformation mechanism mapping for providing a visual display of the flow processes and provides a direct comparison with the conventional model for superplastic flow. The report also describes the potential for using nanoindentation to obtain detailed information on the flow properties using only exceptionally small samples.

Modeling and Analysis of Traffic Flow Considering Automatic Gap Control

SUMMARYIn recent years, the number of vehicles with automatic gap control (AGC) (e.g., adaptive cruise control) has increased with the development of vehicle control technology. AGC mainly helps to ease driver fatigue on highways. Vehicles equipped with AGC can maintain an appropriate distance between two vehicles. Therefore, the traffic flow can be improved by AGC‐equipped vehicles. In this research, we develop a traffic flow model that considers AGC‐equipped vehicles, and is based on the Nagel–Schreckenberg model, which is a probabilistic cellular automaton model. The Nagel–Schreckenberg model can reproduce the basic features of traffic flow. On analyzing the mathematical model for the traffic including AGC‐equipped vehicles, we can obtain analytical solutions of the traffic flow and the average velocity.

A Demonstrative Study on the Two-phase vs. Single-phase Modeling of Buoyancy-driven Flows of Enclosed Nanofluids

ABSTRACTA demonstrative numerical study on natural convection of water-based nanofluids in square enclosures with different boundary conditions imposed at the walls, and different orientations with respect to the gravity vector, is performed using both the single-phase and the two-phase approaches, with the main scope to evaluate in what measure the single-phase approach fails in describing the basic heat and fluid flow features, as well as in determining the thermal performance of nanofluids. The system of the mass, momentum and energy transfer governing equations is solved by way of a computational code based on the SIMPLE-C algorithm. Empirical correlations are used for the calculation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient. The following configurations are investigated: a tilted cavity differentially-heated at two opposite walls; a vertical cavity partially-heated at the bottom wall and cooled at both sides; and a vertical cavity differentially-heated at the vertical and horizontal walls. It is found that the non-uniform distribution of the suspended solid phase throughout the enclosure gives rise to a solutal buoyancy force, whose competition with the thermal buoyancy force results in a periodic flow detectable only if the two-phase approach is applied. Moreover, the impact of the dispersion of the nanoparticles into the base liquid, which turns out to be notably higher at higher average temperatures, is found to be systematically underestimated by the single-phase approach.

Numerical study of patterns and influencing factors on flow boiling in vertical tubes by thermal LBM simulation

Heat transfer and flow pattern of flow boiling in vertical tube are investigated numerically based on the phase-change Lattice Boltzmann method (LBM) which includes an improved pseudo-potential LB model and a thermal LB model. Two-dimensional numerical simulations are carried out under constant heat flux conditions for the first time. The processes of growth, slippage, detachment and coalescence of the bubbles are captured to verify the correctness of the model. The effects of gravity, contact angle and wall superheat on bubble departure diameter and nucleation waiting time are illustrated. The multiple flow patterns, single-phase flow, bubble flow, slug flow and DNB have been illustrated with the behaviors of bubble nucleation, growth, departure, and coalescence. Some basic features of flow boiling have been clearly observed in the simulation. The influences of several factors such as heat flux, Reynolds number, the width of flow channel, and the width of nucleation point on flow boiling especially on the point of DNB are investigated. The numerical results show that the DNB could be avoided by reducing the heating density, increasing the Reynolds number, increasing the width of the tube and reducing the heating concentration.

Invariant submodels of the model of thermal motion of gas in a rarefied space

A model describing the thermal motion of a gas in a rarefied space is investigated. This model can be used in the study of the motion of gas in outer space, and the processes occurring inside the tornado, and the state of the medium behind the shock front of the wave after a very intense explosion. For a given initial pressure distribution, a special choice of mass Lagrange variables leads to a reduced system of differential equations describing this motion, in which the number of independent variables is one less than the original system. This means that there is a stratification of a highly rarefied gas with respect to pressure. Namely, in a strongly rarefied space for each given initial pressure distribution, at each instant of time all gas particles are localized on a two-dimensional surface moving in this space. At each point of this surface, the acceleration vector is collinear with its normal vector. The resulting system admits an infinite Lie transformation group. All significantly various submodels that are invariant with respect to the subgroups of its eight-parameter subgroup generated by the transfer, extension, rotation, and hyperbolic rotation operators (the Lorentz operator) are found. For invariant submodels of rank 1, the basic mechanical characteristics of the gas flow described by them are obtained. Conditions for the existence of these submodels are given. For invariant submodels of rank 2, integral equations describing these submodels are obtained. For some submodels, the problem of describing the gas flow from the initial location of its particles and the distribution of their velocities has been investigated.

Measurement of Basic Geometric Characteristics of Particle Flow Between Surfacing Head and Substrate

We present a method of determining the geometric characteristics of gas-powder flow during laser cladding with coaxial injection. We evaluate the effect of the operating parameters (feed rate and channel width of powder injection) on the geometry of particle flow. Optical diagnostics of the laser cladding is carried out by an infrared camera.

Effects of rotational speed on the leakage behavior, temperature increase, and swirl development of labyrinth seal in a compressor stator well

The stator well in a compressor is the space between the rotor and stator inside the mainstream annulus flow. Labyrinth seals are normally used to control the internal flow in the stator well. The upstream and downstream rotating cavities of the labyrinth seal can lead to substantial temperature rise and swirl development in this region. Additionally, due to the centrifugal expansion and thermal expansion, the tip clearance of labyrinth seal changes dramatically at different rotational speeds and temperatures in the stator well. A test rig capable to establish different rotational speeds and pressure ratios was designed according to the simplified model of the labyrinth seal in a compressor stator well (one stage). The leakage flow rate and change in total temperature across the stator well were measured. This paper also contains the experimental results of swirl ratios in the outlet rotating cavity to reveal the swirl development. Special emphasis in this work lies on acquiring the working tip clearance precisely. The set up tip clearance was measured with plug gauges, while the radial displacements of labyrinth ring and stator casing were measured separately with two high precision laser distance sensors. Two-dimensional, axisymmetric swirl flow numerical simulations were performed to get a further understanding about the basic flow characteristics and to evaluate their ability to predict the experimental results. The computational results of discharge coefficient, windage heating, and swirl ratio were compared to those obtained from test rig measurements. Particularly, when calculating, the tip clearance, the inlet parameters, and the outlet parameters of numerical model at a specific rotating speed were set to be the same with the experimental conditions.