Investigating flow patterns in a channel with complex obstacles using the lattice Boltzmann method

In this work, mesoscopic modeling via a computational lattice Boltzmann method (LBM) is used to investigate the flow pattern phenomena and the physical properties of the flow field around one and two square obstacles inside a two-dimensional channel with a fixed blockage ratio, β=1/4, centered inside a 2D channel, for a range of Reynolds numbers (Re) from 1 to 300. The simulation results show that flow patterns can initially exhibit laminar flow at low Re and then make a transition to periodic, unsteady, and, finally, turbulent flow as the Re get higher. Streamlines and velocity profiles and a vortex shedding pattern are observed. The Strouhal numbers are calculated to characterize the shedding frequency and flow dynamics. The effect of the layouts or configurations of the obstacles are also investigated, and the possible connection between the mixing process and the appropriate design of a chemical mixing system is discussed.     

Numerical simulation of the aeroacoustic noise in the separated laminar boundary layer

The unsteady flow characteristics and the related noise of separated incompressible laminar boundary layer flows (Re _δ*=614, 868, and 1,063) are numerically investigated. The characteristic lines of the wall pressure are examined to identify the primary noise source, related with the unsteady motion of the vortex at the reattachment point of the separation bubble. The generation and propagation of the vortex-induced noise in the separated laminar boundary layer are computed by the method of Computational Aero-Acoustics (CAA), and the effects of Reynolds number, Mach number and adverse pressure gradient strength are examined.     

Effects of fluid-structure interaction on trailing-edge noise

This study numerically investigates the effects of fluid-structure interaction (FSI) on the trailing-edge noise, particularly for the cases of wake instability and Karman vortex shedding. The trailing edge is modeled as a flat plate with an elastic cantilever end and its flow-induced vibration is solved by an eigenmode analysis with the Galerkin method. The flow and sound coupled in the FSI analysis are computed on the moving grid by a direct numerical simulation (DNS) procedure. The computed result of wake instability shows that when the first-eigenmode natural frequency ω _n of the cantilever is close to be resonant with the wake characteristic frequency ω _c , the sound pressure level (SPL) is significantly reduced by 20 dB at ω _n /ω _c =0.95, or increased by 15 dB at ω _n /ω _c =1.05, for all angles. For the Karman vortex shedding, a similar frequency modulation occurs via FSI, if ω _n is close to ω _c . The flow and acoustic details are somewhat different for this case but a considerable noise reduction was also possible for angles from −120° to +120°.     

Elliptic feature of coherent fine scale eddies in turbulent channel flows

Direct numerical simulations (DNS) of turbulent channel flows up to Reτ=1270 are performed to investigate an elliptic feature and strain rate field on cross sections of coherent fine scale eddies (CFSEs) in wall turbulence. From DNS results, the CFSEs are educed and the strain rate field around the eddy is analyzed statistically. The principal strain rates (i.e. eigenvalues of the strain rate tensor) at the CFSE centers are scaled by the Kolmogorov length ηand velocity u_k. The most expected maximum (stretching) and minimum (compressing) eigenvalues at the CFSE centers are independent of the Reynolds number in each y⁺ region (i. e. near-wall, logarithmic and wake regions). The elliptic feature of the CFSE is observed in the distribution of phase-averaged azimuthal velocity on a plane perpendicular to the rotating axis of the CFSE (ω _c ). Except near the wall, phase-averaged maximum (γ^*/γ) and minimum (α^*/a_c ^*) eigenvalues show maxima on the major axis around the CFSE and minima on the minor axis near the CFSE center. This results in high energy dissipation rate around the CFSE.     

Numerical study on laminar flow over three side-by-side cylinders

The present study has numerically investigated two-dimensional flow over three circular cylinders in an equidistant side-by-side arrangement at a low Reynolds number. For the study, numerical simulations are performed, using the immersed boundary method, in the range ofg* < 5 at Re=100, whereg* is the spacing between two adjacent cylinder surfaces divided by the cylinder diameter. Results show that the flow characteristics significantly depend on the gap spacing and a total of five kinds of wake patterns are observed over the range: modulationsynchronized (g*≥2), inphase-synchronized(g*≈l.5), flip-flopping (0.3<g*≲1.2), deflected (g*≈0.3), and single bluff-body patterns (g*<0.3). Moreover, the parallel and symmetric modes are also observed depending ong* in the regime of the flip-flopping pattern. The corresponding flow fields and statistics are presented to verify the observations.     

An analysis of turbulent flow around a NACA4412 airfoil by using a segregated finite element method

A turbulent flow around a NACA4412 airfoil is simulated by a segregated finite element method based on the SIMPLE algorithm and the low Reynolds numberk-ω turbulence model. The originalk-ω model and a modified version of thek-ω, model (shear stress transport model) are adopted, for which grid independent solutions are obtained, respectively. From the present numerical experiment, it has been shown that the segregated finite element method with thek-ω turbulence model can predict the turbulent flow leading to separation satisfactorily with apparently reduced memories compared with the mixed integrated formulation. It is also recommended that for the analysis of external flows a modifiedk-ω model should be used instead of the originalk-ω model, which combines the features of both the standardk-ε model and the originalk-ω model.     

Drag reduction for flow past a square cylinder through corner chamfering

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 10⁴. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction.

     

An instrument for measuring the intensity and dose of cancerigenic ultraviolet radiation

An instrument for measuring the intensity and dose of cancerigenic radiation from the Sun and artificial sources has been developed. A 4H-SiC Schottky barrier photodetector is used as the sensor. The sensitivity spectrum of the photoelectric transducer lies in the region 240–300 nm and has a maximum at 260 nm. The quantum efficiency is ≈0.3 electrons/photon (at λ = 254 nm), and its temperature coefficient at T = 250–310 K is < 0.1%/°C. The dimensions of the device are 3 × 6 × 11 cm.     

Aeroacoustic computation of cavity flow in self-sustained oscillations

A computational aero-acoustic (CAA) method is used to predict the tonal noise generated from a cavity of automobile door seals or gaps at low flow Mach numbers (M_∞=0.077 and 0.147). In the present method, the acoustically perturbed Euler equations are solved with the acoustic source term obtained from the unsteady incompressible Navier-Stokes calculations of the cavity flow in self-sustained oscillations. The aerodynamic and acoustic fields are computed for the Reynolds numbers based on the displacement thickness, Re_δ*=850 and 1620 and their fundamental mode characteristics are investigated. The present method is also verified with the experimentally measured sound pressure level (SPL) spectra.     

Dynamical research on spherical micro actuator with piezoelectric ceramic stacks drivers

This paper develops a 30 mm × 30 mm × 50 mm spherical micro actuator driven by piezoelectric ceramic stacks (PZT), and analyzes its dynamic performances. First, the space coordinate relationship of the spherical micro actuator and a dynamic model are set up. Second, The Runge-Kutta arithmetic is used to calculate the dynamical parameters of the micro actuator; the SIMULINK module of MATLAB is used to build the dynamical simulating model and then simulate it. Third, an experimental sample of the spherical micro actuator is developed, a micromanipulator is integrated with a micro-gripper based on the sample spherical micro actuator, and the experimental research on the micro assembly is conducted between a micro shaft of Φ180 μm and a micro spindle sleeve of Φ200 μm. Finally, the characteristics of the spherical micro actuator influenced by the mass of the metal sphere of the micro actuator, driving signal frequency, friction coefficient of the contact surface between the metal sphere and the friction block of the micro driving unit are analyzed. The experimental results indicate that the rotation resolution of the micro actuator reaches 0.000 1°, the rotation positioning precision reaches 0.000 5°, and the maximum working frequency is about 1200 Hz. The experimental results validate the back rotation vibration model of the spherical micro actuator. The micromanipulator integrated by the spherical micro actuator can meet the requirements of precise micro operation and assembly for micro electro mechanical systems (MEMS) or other microelements in micro degree fields. __________ Translated from Optics and Precision Engineering, 2007, 15(2): 248–253 [译自: 光学精密工程]     

Analysis of two dimensional and three dimensional supersonic turbulence flow around tandem cavities

The supersonic flows around tandem cavities were investigated by two-dimensional and three-dimensional numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) equation with thek-ω turbulence model. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves, and the acoustic effect transmitted from wake flow to upstream. The upwind TVD scheme based on the flux vector split with van Leer’s limiter was used as the numerical method. Numerical calculations were performed by the parallel processing with time discretizations carried out by the 4th-order Runge-Kutta method. The aspect ratios of cavities are 3 for the first cavity and 1 for the second cavity. The ratio of cavity interval to depth is 1. The ratio of cavity width to depth is 1 in the case of three dimensional flow. The Mach number and the Reynolds number were 1.5 and 4.5 × 10⁵, respectively. The characteristics of the dominant frequency between twodimensional and three-dimensional flows were compared, and the characteristics of the second cavity flow due to the first cavity flow was analyzed. Both two dimensional and three dimensional flow oscillations were in the ‘shear layer mode’, which is based on the feedback mechanism of Rossiter’s formula. However, three dimensional flow was much less turbulent than two dimensional flow, depending on whether it could inflow and outflow laterally. The dominant frequencies of the two dimensional flow and three dimensional flows coincided with Rossiter’s 2nd mode frequency. The another dominant frequency of the three dimensional flow corresponded to Rossiter’s 1st mode frequency.     

Numerical assessment of turbulent models at a critical regime on unstructured meshes

The present study mainly aims to investigate the performances of different turbulent models for the flow simulation around a circular cylinder at a critical Reynolds number regime (Re = 8.5×10⁵, Tu = 0.7%). A hybrid RANS/LES model (SAS model), a correlation-based transition model ( $\gamma - \widetilde{\operatorname{Re} }_{\theta t} $ model), and a fully turbulent RANS model (SST model) were used to simulate various flow features, such as laminar-turbulence transition inside the boundary layer and the unsteady vortex shedding in the wake region, and their feasibilities for the flow simulation at a critical Reynolds number regime were demonstrated. A vertex-centered finite-volume method was used to discretize the incompressible Navier-Stokes equations, and an unstructured mesh technique was used to discretize the computational domain. The inviscid fluxes were evaluated using 2^nd-order Roe??s flux difference splitting, and the viscous fluxes were computed based on central differencing. A dual time-stepping method and the Gauss-Seidel iteration were used for unsteady time integration. The parallelization strategy using METIS and MPI libraries was used to reduce computational costs. The unsteady characteristics and the time-averaged quantities of the flow fields were compared between turbulent models. The numerical results were also compared with experimental results. The turbulent models showed quite different results at the critical regime because of the different abilities of each model to predict various flow features, such as laminar-turbulence transition and unsteady vortex shedding.     

Numerical simulation of flow past rectangular cylinders with different aspect ratios using the incompressible lattice Boltzmann method

This paper presents a numerical study of a uniform flow past a rectangular cylinder using the incompressible lattice Boltzmann method (ILBM). Firstly, we use the ILBM to simulate the flow past a square cylinder symmetrically placed in a two-dimensional channel and results are validated against the well-resolved results obtained using finite-difference method and finite-volume method. Secondly, the effects of the aspect ratio defined as R = width/height on the fluid forces, vortex shedding frequency and the flow structures in the wake are investigated. Aspect ratios ranging from 0.15 to 4.00 and four Reynolds numbers Re = 100, 150, 200 and 250 are selected for the investigation. The results show that the effects of aspect ratio on physical quantities such as drag and lift coefficients, Strouhal number and the vortex shedding mechanism are very notable in the range between 0 and 2. In general, the drag coefficient decreases with the aspect ratio and the decreasing rate is more distinct in the range of 0.15 ≤ R ≤ 2.0. There is no local maximum found at around R = 0.6 in the drag coefficient as reported for higher Reynolds numbers in the literature. However the root-mean-square value of the lift coefficient shows a maximum value at R ≈ 0.5 for all Reynolds numbers selected. The variation of Strouhal number with R appears to be different for four selected Reynolds numbers. Especially for Re = 250, a discontinuity in St, as has been observed for higher Reynolds numbers, is observed at around R = 1.45 where multiple peaks are found in the result of Fourier spectrum analysis of the lift force and irregular vortex shedding behavior with no fixed shedding frequency is observed from the instantaneous vorticity contours. Such discontinuity is not observed for Re = 100, 150 and 200. The present results using the LBM are compared with some existing experimental data and numerical studies. The comparison shows that the LBM can capture the characteristics of the bluff body flow well and is a useful tool for bluff body flow studies.     

Numerical analysis of the subsonic flow around a three-dimensional cavity

Flight vehicles such as wheel wells and bomb bays have many cavities. The flow around a cavity is characterized as an unsteady flow because of the formation and dissipation of vortices brought about by the interaction between the free stream shear layer and the internal flow of the cavity. The resonance phenomena can damage the structures around the cavity and negatively affect the aerodynamic performance and stability of the vehicle. In this study, a numerical analysis was performed for the cavity flows using the unsteady compressible three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equation with Wilcox’s turbulence model. The Message Passing Interface (MPI) parallelized code was used for the calculations by PC-cluster. The cavity has aspect ratios (L/D) of 2.5 ∼ 7.5 with width ratios (W/D) of 2 ∼ 4. The Mach and Reynolds numbers are 0.4 ∼ 0.6 and 1.6×10 ⁶ , respectively. The occurrence of oscillation is observed in the “shear layer and transient mode” with a feedback mechanism. Based on the Sound Pressure Level (SPL) analysis of the pressure variation at the cavity trailing edge, the dominant frequencies are analyzed and compared with the results of Rossiter’s formula. The dominant frequencies are very similar to the result of Rossiter’s formula and other experimental data in the low aspect ratio cavity (L/D = ∼ 4.5). In the large aspect ratio cavity, however, there are other low dominant frequencies due to the leading edge shear layer with the dominant frequencies of the feedback mechanism. The characteristics of the acoustic wave propagation are analyzed using the Correlation of Pressure Distribution (CPD). This paper was recommended for publication in revised form by Associate Editor Do Hyung Lee Hong-il CHOI received the B.S and M.S degrees in Aerospace Engineering from Chosun University, Korea in 2005 and 2008, respectively. He currently work at KOREA Electric Power Research Institute in Korea Pa-ul MUN received the B.S in Aerospace Engineering from Chosun University, Korea in 2008. He is currently Candidate for the degree of master of Aerospace Engineering at Chosun University in Korea. Jae-soo KIM received the B.S in Aerospace Engineering from Seoul National University in Korea in 1981. He then received his M.S and Ph.D. degree in Aerospace Engineering from KAIST in Korea in 1983 and 1987, respectively. He spent one year at Cornell university(USA) as a Post Doc. He worked at Korea Aerospace Research Institute for eight years. Dr. Kim is currently a Professor at the Department of Aerospace Engineering at Chosun University in Korea     

Effect of process parameters on the macrostructure and defect formation in friction stir lap welding of AA5456 aluminum alloy

Friction stir welding could be considered as a suitable technique for joining of aluminum alloys due to the emerging of different problems in fusion welding of these alloys, especially in lap joint designs. For this purpose, it is necessary to optimize the process parameters while in this study, the combined effects of tool rotation and welding travel speed on the macrostructure and defect formation of friction stir lap welding of AA5456 was investigated. The rotating tool was plunged from the 5 mm-thick AA5456-H321 (top sheet) surface into the 2.5 mm-thick AA5456-O (bottom sheet) and lap joints were fabricated by rotational speeds of 300, 600, 800 and 1000 rpm and welding speeds of 15, 30, 60 and 100 mm min⁻¹. The effect of tool rotation and welding speed on the macrostructure, material flow and defect formation, i.e. hooking, kissing-bond and cavity, were studied by optical microscopy and scanning electron microscope. The results declared that hooking height decreased as the welding speed increased while kissing-bond was formed at higher welding speeds. Moreover, hooking region was extended as the tool rotational speed increased. However, at a high rotational speed, cavity was even created.     

Effect of boundary condition history on the symmetry breaking bifurcation of wall-driven cavity flows

A symmetry breaking nonlinear fluid flow in a two-dimensional wall-driven square cavity taking symmetric boundary condition after some transients has been investigated numerically. It has been shown that the symmetry breaking critical Reynolds number is dependent on the time history of the boundary condition. The cavity has at least three stable steady state solutions for Re=300-375, and two stable solutions if Re>400. Also, it has also been showed that a particular solution among several possible solutions can be obtained by a controlled boundary condition.     

Study on the unsteady wakes past a square cylinder near a wall

Experimental and numerical studies on the unsteady wake field behind a square cylinder near a wall were conducted to find out how the vortex shedding mechanism is correlated with gap flow. The computations were performed by solving unsteady 2-D Incompressible Reynolds Averaged Navier-Stokes equations with a newly developed ε-SST turbulence model for more accurate prediction of large separated flows. Through spectral analysis and the smoke wire flow visualization, it was discovered that velocity profiles in a gap region have strong influences on the formation of vortex shedding behind a square cylinder near a wall. From these results. Strouhal number distributions could be found, where the transition region of the Strouhal number was atG/D=0.5 - 0.7 above the critical gap height. The primary and minor shedding frequencies measured in this region were affected by the interaction between the upper and the lower separated shear layer, and minor shedding frequency was due to the separation bubble on the wall. It was also observed that the position(y/G) and the magnitude of maximum average velocity (u/u∞) in the gap region affect the regular vortex shedding as the gap height increases.     

Experimental study on the vortex flow in a concentric annulus with a rotating inner cylinder

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0-600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Re_c decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.     

Experimental investigation of natural convection from a horizontal ice surface melting in pure water

A study on natural convection from a horizontal ice surface melting in pure water was conducted experimentally for the ambient water temperature from 2▿C to 10°C. Natural convection flow around upward-or downward-facing horizontal ice plate was divided into three regions according to the temperature variation of ambient water. The flow patterns of three regions were no flow, two-dimensional steady laminar flow and unsteady flow. Mean Nusselt number for the upward-facing surface had its maximum value at about 3°C of ambient water temperature. However, in the case of the downward-facing surface it increased as the ambient water temperature increased.     

Application of AZ4903 to production of stator windings

To satisfy the needs of the high deep-width ratio and thickness of a planar micromotor’s stator windings, a process method to produce an electromagnetic planar micromotor with AZ4903 is proposed. Optimum relationships, such as the desired rotation speed vs. thickness of the coating process, and the temperature vs. time of pre-baking, are obtained. The appropriate time of lithography and development are also achieved. The thickness of the produced windings is 40 μm with ideal perpendicularity. Finally, stators and rotors are fabricated. The rotation speed and pull-out torque of the micromotor are tested. The experiment proved that the micromotor worked with a steady speed and a low ripple of pull-out torque. __________ Translated from J Tsinghua Univ (Sci & Tech) 2005, 45(8): 1062–1065 [译自: 清华大学学报(自然科学版)]