Investigation Of Flow Field Research Articles

Computational and experimental investigation of flow fields in a Rushton turbine stirred tank with shear‐thinning fluid

AbstractThe flow feature and local shear rate distribution near the impeller blade in a standard Rushton turbine stirred tank with the aqueous solution of sodium polyacrylate were studied by using particle image velocimetry experiments and computational fluid dynamics simulations. Experimental and simulated results, including average velocity and local shear rate distribution at different rotational speeds, were compared, and good agreement was achieved. On this basis, the numerical methods were used to predict the effects of liquid rheology on the velocity and shear rate distribution. The consistency coefficient affects obviously the velocity fluctuation level but has a limited impact on the flow structure. The power‐law index influences significantly the radial jet structure. In addition, the liquid rheology almost has no effect on the average value of the local shear rate near the impeller region, which is proportional to the rotational speed.

Investigation of fine viscous flow fields in ship planar motion mechanism tests by DDES and RANS methods

The fine large separated flow is a focus in the naval architecture and ocean engineering. In the present study, the CFD solver, naoe-FOAM-SJTU, coupled with delayed detached-eddy simulation (DDES) and Reynolds Averaged Navier-Stokes (RANS) is adopted to simulate the fine viscous flow field in the ship planar motion mechanism (PMM) tests for the benchmark model Yupeng Ship. This paper compares the time histories of forces/moment acting on the hull and their oscillation frequency obtained by Fast Fourier Transform. The predicted results are in good agreement with the experimental data. By comparing the turbulent kinetic energy (TKE) and eddy viscosity coefficient calculated by both numerical methods, the results show that the TKE and eddy viscosity coefficient obtained by DDES method are much less than that achieved by RANS approach. The capacity of four vortex identification methods to capture vortex structures is analyzed in this paper, indicating that the third generation of vortex identification methods (ΩR and Liutex methods) are more suitable for analyzing the flow mechanism in the viscous large separated flow field. The axial Liutex and streamlines on the cutting planes are used to depict the flow around the hull.

Experimental and Computational Investigation of Flow Fields using Accelerated Erosion Test Ring (AETR)

The erosion of the blades of hydraulic pumps and turbines, caused due to the water that has enormous suspended particles and erosive agents, is a severe global challenge among scientists and engineers. In the present work, an attempt has been made to compute the extent of erosion caused by water when it passes through devices like hydraulic machines, centrifugal pumps and turbines. Experimental and computational techniques using pitched turbine blades with 45o with the horizontal plane under down pumping condition is employed for investigation of the suspension phenomena for calculating erosion wear by sand suspended in water in river, canal on turbine, pump blades. An experimental set up, named as Accelerated Erosion Test Rig (AETR), is developed through dimensional analysis. For experimental analysis, the sand particle size and propeller dimensions were varied while running the propeller at different speeds. Experimental results revealed that the propeller speed must be maintained at an optimum value, preferably lower speeds, to ensure maximum lifting of impurities and sand particles from the base of the cylinder.

Comparison of Techniques for the Estimation of Flow Parameters of Fan Inflow Turbulence from Noisy Hot-Wire Data

Turbulence parameters, in particular integral length scale (ILS) and turbulence intensity (Tu), are key input parameters for various applications in aerodynamics and aeroacoustics. The estimation of these parameters is typically performed using data obtained via hot-wire measurements. On the one hand, hot-wire measurements are affected by external disturbances resulting in increased measurement noise. On the other hand, commonly applied turbulence parameter estimators lack in robustness. If not addressed correctly, both issues may impede the accuracy of the turbulence parameter estimation. In this article, a procedure consisting of several signal processing steps is presented to filter non-turbulence related disturbances from the unsteady velocity data. The signal processing techniques comprise time- and frequency-domain approaches. For the turbulence parameter estimation, two different models of the turbulence spectra—the von Kármán model and the Bullen model—are fitted to match the spectrum of the measured data. The results of several parameter estimation techniques are compared. Computational Fluid Dynamics (CFD) data are used to validate the estimation techniques and also to assess the influence of the variation in window size on the estimated parameters. Additionally, hot-wire data from a high-speed fan rig are analyzed. ILS and Tu are assessed at several radial positions for two fan speeds. It is found that most techniques yield similar values for ILS and Tu. The comparison of the fitted spectra with the spectra of the measured data shows a good agreement in most cases provided that a sufficiently fine frequency resolution is applied. The ratio of ILS and Tu of the velocity components in longitudinal and transverse direction allows the assessment of flow-isotropy. Results indicate that the turbulence is anisotropic for the investigated flow fields.

Numerical investigation of a muzzle multiphase flow field using two underwater launch methods

A two-dimensional axisymmetric model, employing a dynamic mesh and user-defined functions, is used to numerically simulate the transient multiphase flow field produced by an underwater gun. Furthermore, a visualized shooting experiment platform with a high-speed camera is built to observe the evolution process of such a multiphase flow field. The simulated phase distribution diagram is agreed well with the shadow photo of the experiment, indicating that the numerical model is reasonable. Further examinations of the multiphase flow fields by using the submerged and sealed launch methods show that use of the sealed launch can significantly improve the interior ballistic performance of an underwater gun. In the cases by using these two types of underwater launch methods, the displacement of the projectile within the range of the muzzle flow field meets the exponential law over time. Moreover, a not fully developed bottle-shaped shock wave is formed when t = 0.4 ms, but this bottle-shaped shock wave expands more rapidly for the sealed launch. In addition, the amplitude of pressure oscillation for the sealed launch is larger than that of the submerged launch, but the pressure oscillation of the sealed launch lasts shorter.

Thrust Characteristics of Ducted Propeller and Hydrodynamics of an Underwater Vehicle in Control Motions

A numerical technique to simulate the hydrodynamic behavior of ducted propellers attached to an underwater vehicle traveling under the mutually interacting flow fields of the vehicle and the propellers is proposed; the hydrodynamic performance of the propellers and the hydrodynamic loading on the main body of the vehicle when it is in different kinds of motion is investigated numerically. In the research, 3D geometric models of the duct, propeller, and main body of the vehicle are first constructed according to their geometrical features. A computational fluid dynamics (CFD) technique based on the hybrid algorithm of dynamic mesh-nested sliding mesh is applied to solve the Navier–Stokes equations that govern the fluid motion around the duct, propeller, and main body of the vehicle when it is in motion. These equations are solved numerically with the CFD code Fluent. With the proposed numerical simulation technique, the hydrodynamic characteristics of the thrusts generated by the ducted propellers and the loading on the main body in the vehicle system under the mutually interacting flow fields are observed. The results of our numerical simulation indicate that the hybrid algorithm of dynamic mesh-nested sliding mesh can simulate multiple degrees of freedom of motion in underwater vehicle systems. In different motion states, the main body exerts a significant influence on the investigated flow fields; during the vehicle motions, negative wakes are formed on both sides of the main body, which lead to a decrease in the thrusts generated by the propellers on both sides. The thrust of the middle propeller is greater than that of the normal single one because of the obstructing effect in the tunnel caused by the main body.

Numerical investigation of flow field around T-shaped spur dyke in a reverse-meandering channel

Abstract In this paper, the flow characteristics around T-shape spur dyke situated in the reverse meandering channel with a rigid bed is simulated using renormalization group (RNG) turbulence model with ANSYS 2018 Fluent software. To solve the model in 3D ways we used Navier-Stroke's equation based on the principle of conservation of mass and momentum within a moving fluid. For studying the flow characteristics, computational fluid dynamics were applied with all geometric parameters and the turbulence was simulated using (RNG) equations of model. In this simulation, the structured meshes were used with different diameters and diameters of meshes are high at the exit channel for obtaining accuracy in the result. In this study, we mainly focus on the effect of Froude number on flow pattern and several other characteristics like velocity distribution, flow separation, bed-shear-stress distribution. The final result of this research work is compared with the condition when no structure is present in the channel.

Preliminary BCP Flow Field Investigation by CFD Simulations and PIV in a Transparent Model of a SRF Elliptical Low Beta Cavity

Preliminary BCP Flow Field Investigation by CFD Simulations and PIV in a Transparent Model of a SRF Elliptical Low Beta Cavity

Time-resolved flow field investigation in an industrial centrifugal compressor application involving TR-PIV synchronized with unsteady pressure measurements

We report on combined velocity and unsteady pressure measurements obtained on an radial compressor with vaneless diffuser and asymmetric volute. Time-resolved PIV recordings were acquired at 26 kHz both upstream of the impeller as well as within the vaneless diffusor at several rotation speeds at clean conditions and prior to the onset of instabilities within the rotor. The velocity data was acquired with a high-repetition rate, double-pulse laser system consisting of two combined DPSS lasers and a high-speed CMOS camera that was synchronized with multi-point unsteady pressure measurements. Details on the facility, the utilized instrumentation and data processing are provided with particular focus on the spectral and coherence analysis. Power spectra obtained from time records of the inlet velocity and unsteady pressure reveal an increase of low-frequency fluctuations below the blade passing frequency and the occurrence of a mode-locked behaviour indicating the presence of rotating instabilities. High levels of correlation between velocity and unsteady pressure signals not only confirm the temporal coherence of the acquired data but also reveal a direct coupling between flow field and pressure signature that is more prominent upstream of the rotor rather than in the diffusor.

Comprehensive particle image velocimetry measurement and numerical model validations on the gas–liquid flow field in a lab-scale cyclonic flotation column

The investigation of flow field can provide strong theoretical support for improving the flotation performance of fine particles in a cyclonic flotation column. In this study, particle image velocimetry (PIV) is combined with endoscopic measurement and phase discrimination technique to measure the cross section and axial section of gas–liquid two-phase flow field in a lab-scale cyclonic flotation column. The axial, radial, and tangential velocity of both gas and liquid phases are obtained. Based on the PIV experimental data, computational fluid dynamics (CFD)-based numerical models are validated. The results show the good prediction ability of Eulerian–Eulerian multiphase model, Reynolds stress model (RSM), and Tomiyama drag model. Then the simulated results with validated models are displayed to indicate the flow characteristics of the flotation column. The gas concentration is observed to be higher in the center and low on the sides. The gas phase always moves upward, while the liquid phase moves upward at the axis, downward between the axis and the wall, and upward again near the wall. Overall, this study provides an innovative approach for PIV measurement of complex multiphase flow field, and a useful reference for appropriate numerical models selection.

Flow and thermal field investigation of rarefied gas in a trapezoidal micro/nano-cavity using DSMC

The impetus of the this study is to investigate flow and thermal field in rarefied gas flows inside a trapezoidal micro/nano-cavity using the direct simulation Monte Carlo (DSMC) technique. The investigation covers the hydrodynamic properties and thermal behavior of the flow. The selected Knudsen numbers for this study are arranged in the slip and transition regimes. The results show the center of the vortex location moves by variation in the Knudsen numbers. Also, as the Knudsen number increases, the non-dimensional shear stress increases, but the distribution deviates from a symmetrical profile. The cold to hot transfer, which is in contrast with the conventional Fourier law, is observed. We show that the heat transfer is affected by the second derivative of the velocity. By increasing the Knudsen number, the transferred heat through the walls decreases, but the contraction/expansion effects on the temperature in the corner of the cavity become higher.

Unsteady mixed convection flow of magneto-Williamson nanofluid due to stretched cylinder with significant non-uniform heat source/sink features

This analysis reports an unsteady and incompressible flow of Williamson nanoliquid in presence of variable thermal characteristics are persuaded by a permeable stretching cylinder. The flow field investigation is established with the effect of mixed convection and non-uniform heat source/sink on flow and heat transfer. On the cylinder surface, the analysis is inspected with utilization of zero mass flux constraints. By using the appropriate similarity variables, the framed equations for the energy, momentum and mass is converted into non-linear ODEs. The numerical communication of the boundary value problem is successfully implemented using a computer algorithm programmed into the fifth Runge-Kutta scheme. Additionally, the wall shear factor and rate of heat transfer are calculated in two different cases namely, with curvature and without curvature. In addition, the results obtained are confirmed by making comparisons with previously published articles and we found an excellent match that guarantees the indemnity of current communication. A comprehensive change in velocity, temperature and concentration is examined for involved parameters like local Weissenberg number, space dependent heat source constant, magnetic number, curvature constant, thermophoretic parameter, buoyancy parameter, Brownian motion parameter, Prandtl number, Schmidt number, unsteadiness parameter, reaction rate parameter, activation energy parameter and temperature difference parameter. A reduction in velocity is observed for unsteady parameter and buoyancy constant. An enhanced nanofluid temperature is noted for space dependent heat source parameter, time dependent heat source parameter and unsteady parameter. Moreover, the nanofluid concentration is increases for temperature difference parameter while reverse observations are noticed for chemical reaction rate.

An analysis of the imperfections and defects inside composite bipolar plates using X-Ray computer tomography and resistivity simulations

With the increasing mass production, the quality control of bipolar plates (BPPs) becomes more important. A classification and understanding of imperfections and defects is necessary for the design of quality assurance measures within the manufacturing process. In this paper, a combined X-Ray computer tomography (CT) and resistivity simulation approach is used to investigate graphite composite BPPs. With this non-destructive approach, the morphology and composition of a detailed BPP section, as well as an entire flow field, can be analyzed for its characteristics and defects. The different detected imperfections occurring in injection-molded and compression-molded BPP samples include cracks, air bubbles and agglomerations of voids and foreign particles. The investigated flow field geometry has a major impact and increases the electrical resistance by about 19% compared to a bulk material body. The investigated imperfections inside the BPP material have a minimal impact on the electrical resistance of the BPP.

Experimental Flow Field Investigation of the Bio-Inspired Corrugated Wing for MAV Applications

This is an experimental flow field study of a bio-inspired corrugated finite wing from the dragonfly intended to assess the flow behavior over the wing and compare it with a wing of the same geometry with filled corrugation, at low Reynolds numbers 46000 and 67000. The work purpose is to explore the potential application of such types of wings for Micro Air Vehicles (MAVs) or micro sized Unmanned Air Vehicles (UAVs). Two types of wings are taken into account: first wing was a bio-inspired corrugated wing which was obtained from the mid span of the dragonfly, and the second wing was the same geometry with filled corrugation. Both wings were fabricated by using 3-D printing machine. The tufts were glued at three different locations i.e. at center, 30%, and 60% of the semi-span towards the right side of the wing at the trailing edge. The boundary layers were measured by using boundary layer rakes inside the open-end low speed wing tunnel with varied angles of attack. The results of the tuft flow visualization showed that the flow pattern at different span locations was different at different angles of attack and different wing velocities (Reynolds number). The fluctuations of the two different wings at the same angle of attack and Reynolds number were found different. Also, the directions of the flow for both wings were found to be different at different span locations. The boundary layer measurement results for both wings were found to be different at the same angles of attack and Reynolds numbers. The flow pattern also showed that the wing’s upper as well as lower surface behaved differently on the same wing under the same measurement conditions. The results showed that the corrugated wing outperformed the conventional wing at low Reynolds number and the stall angle of the corrugated wing was more than the conventional wing.

Particle image velocimetry and constant temperature anemometer measurements of the jet produced by a centrifugal fan

This article presents a detailed investigation of the three-dimensional flow field produced by a small centrifugal fan used for cooling of electronic components, for instance, in the automotive industry. A particle image velocimetry (PIV) system captured numerous cross sections of the fan jet for analysis of velocity distribution and jet development. General parameters, such as the spreading rate, and flow specifics, such as the full width at half maximum contours in planes parallel to the fan outlet and the jet's rotation rate, were thus evaluated. In addition, constant temperature anemometry enabled a thorough investigation of the flow field at the fan's outflow port. Hot-wire measurements complement the PIV results by providing the power spectral density of the turbulent kinetic energy at several locations. Our results demonstrate that the cross-flow profile at the fan outlet consists of two counter-rotating vortices. This leads to a jet with a nearly elliptical, rotating cross section, which does not propagate in a direction perpendicular to the outlet plane. Several aspects of these distinctive flow features are evaluated and presented to provide both fluid mechanics and heat transfer engineers with (i) data on the air jet produced by a small centrifugal fan and (ii) reference data for computational fluid dynamics simulations. Despite complex flow development in the near field of the jet and the nonaxisymmetric jet contour in the intermediate field, our results are in good agreement with published data on axisymmetric jets in terms of spreading rate and the development of turbulence spectra observed in the intermediate field.

Honeycomb-generated Reynolds-number-dependent wake turbulence

ABSTRACT We present an experimental and numerical study of the flow downstream of honeycomb flow straighteners for a range of Reynolds numbers, covering both laminar and turbulent flow inside the honeycomb cells. We carried out experiments with planar particle image velocimetry (PIV) in a wind tunnel and performed numerical simulations to perform an in-depth investigation of the three-dimensional flow field. The individual channel profiles downstream of the honeycomb gradually develop into one uniform velocity profile. This development corresponds with an increase in the velocity fluctuations which reach a maximum and then start to decay. The position and magnitude of the turbulence intensity peak depend on the Reynolds number. By means of the turbulence kinetic energy (TKE) budget it is shown that the production of TKE is dominated by the shear layers corresponding to the honeycomb walls. The near-field and far-field decay of the turbulence intensity can be described by power laws where we used the position where the production term of the TKE reaches its maximum as the virtual origin.

Extensive Experimental and Analytical Investigation of the Aerodynamic Flow Field of Labyrinth Seals with Innovative Liner Configurations

The state-of-the-art sealing systems in aircraft engines are labyrinth seals to regulate mass flow between rotating components. To prevent structural damage of vital parts during a rubbing process, abradable liner configurations with hollow body structures are applied on the stator. Compared to smooth stator surfaces, these structures increase the cross-sectional area of the sealing gap and, thus, lead to a higher leakage. The aim of this paper is to identify the main geometrical parameters influencing the aerodynamic behavior of various liner configurations. As a basis of an analytical study, an extensive experimental database is required. For this purpose, a test rig is set up at the Institute of Thermal Turbomachinery (ITS). Three liner structures (honeycomb, rhombus, and polyhedral structures), two seal fin configurations, and the effect of the adjusted nominal clearances (0.1-1.0 mm) and different pressure conditions are investigated. The experimental results will be presented, discussed, and evaluated by means of established parameters. The focus is on the leakage of the seal, the discharge coefficient, and the equivalent gap width. Additionally, the influence of the geometric shape of the liner structures is discussed. Flow separation and flow direction play the main role. With the results of this paper, optimization guidelines are derived for the design of innovative liner configurations.

Investigation of Unsteady Pressure Fluctuations in a Simplified Steam Turbine Control Valve

Abstract Steam turbines are among the most important systems in commercial and industrial power conversion. As the amount of renewable energies increases, power plants formerly operated at steady-state base load are now experiencing increased times at part load conditions. Besides other methods, the use of control valves is a widely spread method for controlling the power output of a steam turbine. In difference to other throttling approaches, the control valve enables fast load gradients as the boiler can be operated at constant conditions and allows a quicker response on variable power requirements. At part load, a significant amount of energy is dissipated across the valve, as the total inlet pressure of the turbine is decreased across the valve. At these conditions, the flow through the valve becomes trans- and supersonic and large pressure fluctuations appear within the downstream part of the valve. As a result, unsteady forces are acting on the valve structure and vibrations can be triggered, leading to mechanical stresses and possible failures of the valve. Besides more complex valve geometries, a spherical valve shape is still often used in smaller and industrial steam turbines. Because of the smooth head contour, the flow is prone to remain attached to the head surface and interact with the flow coming from the opposite side. This behavior is accompanied by flow instabilities and large pressure fluctuations, leading to unsteady forces and possible couplings with mechanical frequencies. The spherical valve shape was therefore chosen as the experimental test geometry for the investigation of the unsteady flow field and fluid–structure interactions within a scaled steam turbine control valve. Using numerical methods, the test valve is investigated and the time dependent pressure distribution in the downstream diffuser is evaluated. The evolution of the flow stability will be discussed for different pressure ratios (PRs). Pressure signals retrieved from the control valve test rig will be used to compare the numerical results with the experimental data.

Continuous synthesis of monodisperse silica microspheres over 1 μm size

This study presents a robust and scalable method for synthesizing continuously the silica microspheres via the Stober method. Owing to the superb mixing accessible with gas-liquid segmented flow in coiled microtube, satisfactory morphology and mono-dispersity of silica spheres was obtained. The continuous method was firstly optimized through investigating the effects of various operation parameters on the morphology, particle size, size distribution and chemical structure of silica spheres. Without the surfactant, the size of silica spheres was limited < 850 nm due to the partial blockage of microchannel when synthesizing larger silica spheres. The addition of surfactant migrated the clogging issue, enabling the reliable synthesis of microspheres with diameters over 1 μm within a four times shorter residence time (30 min). Moreover, the mono-dispersity of silica microspheres could be further improved by increasing the gas/liquid ratio in segmented flow due to the more intensified mixing in liquid segments, as verified in visual flow field investigation.

Intermittent breakdown of the tip leakage vortex and the resultant flow unsteadiness in the tip-region of a subsonic compressor cascade

In axial-flow compressor rotors, the breakdown of the tip leakage vortex (TLV) occurs in an intermittent manner under some conditions. In order to gain better knowledge of this unsteady phenomenon, a numerical investigation of the tip-region flow field in a subsonic compressor cascade has been conducted using unsteady Reynold-averaged Navier-Stokes simulations. First of all, the reason why TLV breakdown exhibits intermittency has been clarified through the analysis of time-resolved flow fields. It was found that the switch between the breakdown and no-breakdown states is triggered by a feedback mechanism: when the TLV breaks down, either an unsteady induced vortex or a backflow vortex forms and induces low-pressure on the pressure surface of the adjacent blade, which unloads the blade tip and subsequently suppresses the TLV breakdown; as the unsteady induced vortex or backflow vortex migrates downstream, the pressure on the pressure surface increases again so that the loading at the blade tip is increased again and the TLV breakdown is re-established. The amplitude and frequency characteristics of the unsteadiness associated with the intermittent TLV breakdown have been analyzed and presented. It was found that the backflow vortex-dominant breakdown induces much stronger flow fluctuation and has a much lower characteristic frequency than the unsteady induced vortex-dominant breakdown. The underlying flow physics responsible for this difference have been discussed based on the understanding of the feedback mechanism for the intermittent TLV breakdown.