Complex Unsteady Flow Research Articles

PIV와 CFD에 의한 산업용 교반기 Impeller 형상 선정에 관한 연구

The Industrial Mixers are used in various industrial fields where they are necessary to intimately mix two reactants in a short period of time. However, despite their widespread use, complex unsteady flow characteristics of industrial mixers are not systematically investigated. The present study aims for clarify unsteady flow characteristics induced by various impellers in a tank. Impellers are pitched blade turbine(PBT) types, Screw type and Rushton turbine type. In this study choice of the industrial mixer's impeller type using PIV and CFD method. The rotating speed of impellers are fixed by 100RPM.

Numerical simulation of 3D fluid–structure interaction flow using an immersed object method with overlapping grids

The newly developed immersed object method (IOM) [Tai CH, Zhao Y, Liew KM. Parallel computation of unsteady incompressible viscous flows around moving rigid bodies using an immersed object method with overlapping grids. J Comput Phys 2005; 207(1): 151–72] is extended for 3D unsteady flow simulation with fluid–structure interaction (FSI), which is made possible by combining it with a parallel unstructured multigrid Navier–Stokes solver using a matrix-free implicit dual time stepping and finite volume method [Tai CH, Zhao Y, Liew KM. Parallel computation of unsteady three-dimensional incompressible viscous flow using an unstructured multigrid method. In: The second M.I.T. conference on computational fluid and solid mechanics, June 17–20, MIT, Cambridge, MA 02139, USA, 2003; Tai CH, Zhao Y, Liew KM. Parallel computation of unsteady three-dimensional incompressible viscous flow using an unstructured multigrid method, Special issue on “Preconditioning methods: algorithms, applications and software environments. Comput Struct 2004; 82(28): 2425–36]. This uniquely combined method is then employed to perform detailed study of 3D unsteady flows with complex FSI. In the IOM, a body force term F is introduced into the momentum equations during the artificial compressibility (AC) sub-iterations so that a desired velocity distribution V 0 can be obtained on and within the object boundary, which needs not coincide with the grid, by adopting the direct forcing method. An object mesh is immersed into the flow domain to define the boundary of the object. The advantage of this is that bodies of almost arbitrary shapes can be added without grid restructuring, a procedure which is often time-consuming and computationally expensive. It has enabled us to perform complex and detailed 3D unsteady blood flow and blood–leaflets interaction in a mechanical heart valve (MHV) under physiological conditions.

Experimental methods for multi-diagnostics of flow fields in wind tunnels

Advanced optical and acoustical measurement techniques as developed during the last decade allow new insight into complex unsteady flow fields and sound fields in aerodynamics. Parallel application of such experimental diagnostics methods allows a more complete description of complex flow phenomena and provides high quality data for validation of numerical calculations. In particular, three examples for multi-diagnostics of the flow field above a delta wing and within the slat of a high lift configuration by means of PSP and PIV, PIV and PDMS, and by means of PIV and the Acoustic Microphone Array Technique will be given during the presentation.

Roughness-induced flow instability: a lattice Boltzmann study

Effects of wall roughness/topography on flows in strongly confined (micro-)channels are studied by means of lattice Boltzmann simulations. Whereas wall roughness in macroscopic channels is considered to be relevant only for high-Reynolds-number turbulent flows (where the flow is turbulent even for smooth walls), it is shown in this paper that, in micro-channels, surface roughness may even modify qualitative features of the flow. In particular, a transition from laminar to unsteady flow is observed. It is found that this roughness-induced transition is strongly enhanced as the channel width is decreased. The reliability of our results is checked by computing the viscous shear stress and the Reynolds stress across the channel, their sum following the theoretical prediction for the stress balance perfectly. Furthermore, the solutions obtained obey the transformation rules of the Navier–Stokes equation: When expressed in reduced (dimensionless) units, results for various channel dimensions, forcing term or dynamic viscosity are identical provided that the channel shape and the Reynolds number are unchanged. The time evolution of the velocity fluctuations at the initial stages of the transition to flow instability is monitored. It is found that fluctuations first occur in the vicinity of the rough wall, supporting the interpretation of wall roughness as a source of fluctuations and thus flow instability. In addition to their physical significance, our results provide further evidence for the reliability of the lattice Boltzmann method in dealing with complex unsteady flows.

Comparison of zonal RANS and LES for a non-isothermal ribbed channel flow

Numerical simulations for a non-isothermal ribbed channel flow are made. Due to the relatively low Reynolds number ( Re = 14,200, based on the channel height), the results have relevance to higher Reynolds number electronics cooling systems. Through considering a k– l based zonal large Eddy simulation (ZLES) method and the S–A based detached Eddy simulation (DES) approach, the predictive accuracy of hybrid methods is assessed. This is assisted through comparisons with Reynolds-averaged Navier–Stokes (RANS) model results and also measurements. The RANS models used are the zonal k– l/EASM (explicit algebraic stress model) and k– l/cubic (non-linear cubic). The k– l modelling in these is just used near walls. Comparison reveals that both ZLES and DES are capable of capturing complex unsteady flow features associated with flow separation and reattachment. In general, for the present case, ZLES and DES perform similarly and are in better agreement with measurements than results from the zonal RANS. ZLES and DES heat transfer predictions are encouraging, the model sensitivity of results displayed when using RANS approaches being much less marked.

DETACHED-EDDY SIMULATION FOR SYNTHETIC JETS WITH MOVING BOUNDARIES

A Detached-Eddy Simulation (DES) with moving boundaries has been developed and applied to synthetic jet flows. Complex unsteady flow patterns of the flows were revealed from the simulation. Comparisons between the simulation and experiments showed reasonably good agreements, which indicates that, as a hybrid RANS/LES method, DES is capable of handling unsteady separations with turbulent boundary layers and time-dependent moving boundary conditions.

Visualization and computational steering of fluid motion in Czochralski crucibles during silicon crystal growth

This paper describes the authors' utilization of both post- and co-visualization techniques to indicate details of the fluid motion in a Czochralski crucible. Different visualization methods (particle tracing, isosurface extraction and line integral convolution) are discussed and applied to time-dependent flow in the crucible. Co-visualization is used to investigate the dynamic evolution of the flow structures at high temporal resolution. Moreover, the co-visualization approach also allows for interactive online visualization and computational steering, which strongly increases the user's ability to visualize the results and to supervise and dynamically control the entire computational process. By means of the resulting pictures and animations, the interaction of the different forces, which are responsible for the complex flow in the crucible is illuminated. In this way, the transport processes in the melt flow are characterized. Special attention is paid to the Marangoni-driven convection close to the interface between the melt and the crystal. The methodology for visualization and computational steering described in the paper can be applied to all kinds of complex three-dimensional and unsteady flow simulations. Its application is essential for handling the overwhelming amount of data produced by modern flow predictions. Consequently, it will play a decisive role in the next generation of CFD codes.

Reverse flow in channel with obstruction at entry

In this investigation the complex unsteady flow through and around a channel in the presence of an obstruction at the entry is studied by solving directly the unsteady Navier–Stokes equations. Depending on the location of the obstruction, the flow through the channel either reverses its direction, stagnates or continues in the same direction as that of the flow outside but with reduced magnitude. The Reynolds number of 4000 is considered, as experimental results are available for comparison. The computed results are in close agreement with experiments. The computations help with better understanding of the phenomenon of reverse flow and fluid pumping.

Syringomyelia Hydrodynamics: An In Vitro Study Based on In Vivo Measurements

A simplified in vitro model of the spinal canal, based on in vivo magnetic resonance imaging, was used to examine the hydrodynamics of the human spinal cord and subarachnoid space with syringomyelia. In vivo magnetic resonance imaging (MRI) measurements of subarachnoid (SAS) geometry and cerebrospinal fluid velocity were acquired in a patient with syringomyelia and used to aid in the in vitro model design and experiment. The in vitro model contained a fluid-filled coaxial elastic tube to represent a syrinx. A computer controlled pulsatile pump was used to subject the in vitro model to a CSF flow waveform representative of that measured in vivo. Fluid velocity was measured at three axial locations within the in vitro model using the same MRI scanner as the patient study. Pressure and syrinx wall motion measurements were conducted external to the MR scanner using the same model and flow input. Transducers measured unsteady pressure both in the SAS and intra-syrinx at four axial locations in the model A laser Doppler vibrometer recorded the syrinx wall motion at 18 axial locations and three polar positions. Results indicated that the peak-to-peak amplitude of the SAS flow waveform in vivo was approximately tenfold that of the syrinx and in phase (SAS approximately 5.2 +/- 0.6 ml/s, syrinx approximately 0.5 +/- 0.3 ml/s). The in vitro flow waveform approximated the in vivo peak-to-peak magnitude (SAS approximately 4.6 +/- 0.2 ml/s, syrinx approximately 0.4 +/- 0.3 ml/s). Peak-to-peak in vitro pressure variation in both the SAS and syrinx was approximately 6 mm Hg. Syrinx pressure waveform lead the SAS pressure waveform by approximately 40 ms. Syrinx pressure was found to be less than the SAS for approximately 200 ms during the 860-ms flow cycle. Unsteady pulse wave velocity in the syrinx was computed to be a maximum of approximately 25 m/s. LDV measurements indicated that spinal cord wall motion was nonaxisymmetric with a maximum displacement of approximately 140 microm, which is below the resolution limit of MRI. Agreement between in vivo and in vitro MR measurements demonstrates that the hydrodynamics in the fluid filled coaxial elastic tube system are similar to those present in a single patient with syringomyelia. The presented in vitro study of spinal cord wall motion, and complex unsteady pressure and flow environment within the syrinx and SAS, provides insight into the complex biomechanical forces present in syringomyelia.

Two-dimensional simulation of unsteady marine propulsor blade flow using dynamic meshing techniques

The unsteady flow around a marine propeller blade section embedded in a traveling wave field is simulated and the results are validated against the well-known experimental data. Unstructured dynamic meshing techniques based on smoothing and remeshing are implemented and applied. The Massachusetts Institute of Technology Flapping Foil Experiment is selected for the validation, as it reveals the complex unsteady flow physics on a typical marine propeller blade section. The comparison with experimental data is good in terms of the first harmonic amplitudes and phases of the surface pressure coefficient and streamwise velocity component around and in the boundary layer of the blade section. The validation results suggest that the present computational approach with the unstructured dynamic meshing techniques, which are also available for three-dimensional problems, is both easy to apply and accurate enough for more realistic problems.

Proper orthogonal decomposition of direct numerical simulation data of by-pass transition

Reduced Order Modeling (ROM) of by-pass transition over a zero pressure gradient shear layer data is presented here in a Proper Orthogonal Decomposition (POD) framework. Here, by-pass transition is triggered by a convecting vortex outside the shear layer. POD analysis is relevant for those fluid-structure interaction problems where the fluid dynamical analysis necessarily requires solving the full Navier–Stokes equation. Two types of cases are reported here for which the vortex is either stationary or it convects with a constant speed. Analysis of the complex unsteady flow is performed by locating the eigenvalues and eigenvectors of the covariance matrix by using Lanczos iteration. The local description produces eigenvectors that match seamlessly due to the linearity property of POD.

Stability of preconditioned Navier–Stokes equations associated with a cavitation model

A 3D numerical model is proposed to simulate complex unsteady cavitating flows. The final objective is to predict instabilities due to cavitation in turbopump inducers. It was previously applied to simpler two-dimensional simulations such as a Venturi type section [Int. J. Numer. Meth. Fluids, in press] and foil sections [Int. J. JSME B 45(3) (2002)]. The model is based on the code FineTurbo significantly modified to take into account the cavitation process. The numerical scheme consists in a time-marching algorithm initially devoted to compressible flows. A low-speed preconditioner is applied to treat low Mach number flows. This numerical resolution is coupled to a single-fluid model of cavitation. The evolution of the density is governed by a barotropic state law proposed and validated previously by Delannoy and Kueny [Proc 1990 ASME Cavitation Multiphase Flow Forum 98 (1990) p. 153] and Coutier-Delgosha et al. [Int. J. Numer. Meth. Fluids 42 (2003) 527]. The present work focuses on the numerical stability of the Navier–Stokes equations associated to the barotropic state law. Fourier footprint representations are applied to several two-dimensional non-cavitating and cavitating flow field configurations, and the influence of the numerical and physical parameters on the stability is investigated. The influence of the preconditioner is also discussed: a modification is proposed in the two-phase areas, and its effect is tested in a two-dimensional Venturi type section flow configuration. A significant improvement is obtained concerning both the convergence level and speed.

A semi-implicit discontinuous Galerkin finite element method for the numerical solution of inviscid compressible flow

The paper is concerned with the numerical solution of an inviscid compressible flow with the aid of the discontinuous Galerkin finite element method. Since the explicit time discretization requires a high restriction of the time step, we propose semi-implicit numerical schemes based on the homogeneity of inviscid fluxes, allowing a simple linearization of the Euler equations which leads to a linear algebraic system on each time level. Numerical experiments performed for the Ringleb flow problem verify a higher order of accuracy of the presented method and demonstrate lower CPU-time costs in comparison with an explicit method. Then the method is tested on more complex unsteady Euler flows.

Finite-volume compact schemes on staggered grids

Compact finite-difference schemes have been recently used in several Direct Numerical Simulations of turbulent flows, since they can achieve high-order accuracy and high resolution without exceedingly increasing the size of the computational stencil. The development of compact finite-volume schemes is more involved, due to the appearance of surface and volume integrals. While Pereira et al. [J. Comput. Phys. 167 (2001)] and Smirnov et al. [AIAA Paper, 2546, 2001] focused on collocated grids, in this paper we use the staggered grid arrangement. Compact schemes can be tuned to achieve very high resolution for a given formal order of accuracy. We develop and test high-resolution schemes by following a procedure proposed by Lele [J. Comput. Phys. 103 (1992)] which, to the best of our knowledge, has not yet been applied to compact finite-volume methods on staggered grids. Results from several one- and two-dimensional simulations for the scalar transport and Navier–Stokes equations are presented, showing that the proposed method is capable to accurately reproduce complex steady and unsteady flows.

A DPIV Study of Liquid Steel Flow in a Wide Thin Slab Caster Using Four Ports Submerged Entry Nozzles

Influence of SEN design and its depth below the meniscus on liquid steel flow inside a large width thin slab mold was studied using a 1:1 scale water model. Fluid flow characterization was performed through Digital Particle Image Velocimetry techniques, measurements of bath oscillations and tracer dispersion experiments. Two designs of SEN with four ports (two lateral ports with two bottom ports) were investigated. SEN-1 has a wider bottom base and SEN-2 has a narrow one. Both SEN's provide fluid flows in the mold with upper and lower recirculating flows. Jets emerging from the lateral and bottom ports do not overlap although the surface area of the lateral jet of SEN-1 works inefficiently as compared with that of SEN-2. SEN-1 promotes higher bath oscillations than SEN-2 because the lateral jets travel almost horizontally toward the narrow face. This enhances fluid velocities along the narrow wall until the bath surface. A recirculating flow is formed close to the SEN-1 in the upper bath surface that provokes also high bath oscillations. Design of SEN-2 avoids these flow defects yielding more stable fluid flows. Rising the SEN depths alters negatively the flow characteristics, although SEN-2 is better than SEN-1. Generally speaking both designs yield complex turbulent and unsteady flows and a better SEN design is suggested.

The Flow Rate Influence on the Interaction of a Radial Pump Impeller and the Diffuser

This article presents the results of a detailed flow investigation within a centrifugal pump equipped with a vaned diffuser. The measurements were made with a laser Doppler velocimeter and were carried out at different operating points. The flow was investigated for different rotor–stator relative positions.Unsteady velocity measurements, obtained in phase with the rotor angular position, gave access to the flow inside the impeller channels where three sections were explored. In the diffuser, five sections were studied. Thus, time resolved details of the flow were examined for a better understanding of the complex unsteady flow existing between the two interacting blade rows.Results obtained at different operating conditions show that the rotor‐stator interaction is affected by the diffuser position and the flow rate.

Integrated measurement system for high speed unsteady plasma flow and its application to electric arcs

An integrated measurement system for the study of high speed and high temperature unsteady plasma flows, such as those found in the vicinity of high current switching arcs, is presented. An optical high-speed imaging system is integrated with pressure sensing and spectroscopic measurement to gain insight into the complex unsteady flows in applications such as circuit breakers and internal combustion engines. The system permits direct and non-intrusive measurement to obtain new data on these transient events, which occur over a period of a few milliseconds. The imaging system captures at a rate of 1000 000 images per second. Software techniques have been developed to control light sensitivity, to generate dynamic images of the high-speed flows; to measure trajectories, times, velocities, pressure change, and to analyse gaseous composition in the tested device simultaneously. The range of applications for this system is very wide. For demonstration purposes, a study of the electric arcing process within a low-voltage circuit breaker and an aeroplane engine ignition unit is presented. This is the first time that a system has been developed to provide detailed measurement of arc root immobility time and arc motion.

A computational investigation of unsteady turbulent wake–boundary-layer interaction

The relative motion of rotor and stator blade rows in a turbomachine generates periodically unsteady flow on the blades due to travelling wake perturbations. To better understand the attendant wake–boundary-layer interaction a calculation procedure was developed to model the behaviour of this complex unsteady flow. Due to nonlinear interactions with the boundary layer, the travelling discrete frequency wakes were found to decrease the velocity profile shape factor. For the range of reduced frequencies examined ( ϖ=0.33–9.33) the skin-friction coefficient was found to be frequency dependent. The calculated results for both steady and unsteady velocity profiles, and for skin friction compared well with experimental data. Although the agreement between measured and calculated velocity phase shift was poor, in both experimental and model results the negative phase shift throughout the boundary layer due to the travelling-wave fluctuations has been captured.

Two-dimensional free surface flow in branch channels by a finite-volume TVD scheme

Free surface flow, in particular caused by dam-breaks in branch channels or other arbitrary geometrical rivers is an attention getting subject to the engineering practice, however the studies are few to be reported. In this paper a finite-volume total variation diminishing (TVD) scheme is presented for modeling unsteady free surface flows caused by dam-breaks in branch channels. In order to extend the finite-difference TVD scheme to finite-volume form, a mesh topology is defined relating a node and an element. The solver is implemented for the 2D shallow water equations on arbitrary quadrilateral meshes, and based upon a second-order hybrid TVD scheme with an optimum-selected limiter in the space discretization and a two-step Runge–Kutta approach in the time discretization. Verification for two typical dam-break problems is carried out by comparing the present results with others and very good agreement is obtained. The present algorithm is then used to predict the characteristics of free surface flows due to dam breaking in branch channels, for example, in a symmetrical trifurcated channel and a natural bifurcated channel, on coarse meshes and fine meshes, respectively. The characteristics of complex unsteady free surface flows in these examples are clearly shown.

Multigrid CCDPIV measurements of accelerated flow past an airfoil at an angle of attack of 30°

Multigrid cross-correlation digital PIV (MCCDPIV) technique is used to measure the flow field around a NACA 0015 airfoil at a 30° angle of attack when the flow is accelerated to a constant velocity from a quiescent state. The experiments were conducted in an acceleration water tunnel using uniform accelerations of 50 and 100 mm/s 2. The final uniform velocity was 100 mm/s for both cases, and the Reynolds number based on this velocity and the chord length of the airfoil was about 8000. The MCCDPIV measurements were carried out using a digital 2048 px×2048 px CCD camera to record single exposed images of seed particles illuminated with a Nd:YAG laser. These measurements have revealed a rich and complex unsteady flow structure, during both the acceleration phase and the constant velocity post-acceleration phase.