Recent Volume
Vol14 , No 2 (in press)
A Block–Interface Approach for High–Order Finite–Difference Simulations of Compressible Flows
Pages : 345-359
Authors : Majid Allahyari,  Kianoosh Yousefi,  Vahid Esfahanian,  Milad Darzi, 
The application of the high-order accurate schemes with multi-block domains is essential in problems with complex geometries. Primarily, accurate block-interface treatment is found to be of significant importance for precisely capturing discontinuities in such complex configurations. In the current study, a conservative and accurate multi-block strategy is proposed and implemented for a high-order compact finite-difference solver. For numerical discretization, the Beam-Warming linearization scheme is used and further extended for three-dimensional problems. Moreover, the fourth-order compact finite-difference scheme is employed for spatial discretization. The capability of the high-order multi-block approach is then evaluated for the onedimensional flow inside a Shubin nozzle, two-dimensional flow over a circular bump, and three-dimensional flow around a NACA 0012 airfoil. The results showed a reasonable agreement with the available exact solutions and simulation results in the literature. Further, the proposed block-interface treatment performed quite well in capturing shock waves, even in situations that the location of the shock coincides with block interfaces.
   PDF XML

Experimental Investigation on Effects of Elastic Agitator to Turbulence Enhancement
Pages : 361-373
Authors : Tshun Howe Yong,  Hiang Bin Chan,  S. S. Dol,  S. K. Wee,  Shaharin Anwar Sulaiman, 
The conventional method to promote mass, momentum and energy transport between fluid particles is to introduce a disturbance to the flow. An ultrasonic velocity profiler, UVP experiment was used to study the mean and fluctuating flow properties in the near wake of the rigid and flexible protruding surface in a water tunnel under the Reynolds number of 4000, 6000 and 8000. In the current study, circular finite cylinders (cantilevers) with various aspect ratios (AR = 10, 12, 14 and 16) and materials were used as the geometry of the rigid and protruding surface. The motion of the cylinder alters the fluid flow significantly. The increment of the wake region (~10% larger in the flexible cylinder compared to the rigid case, for AR = 16) is due to the weakening of the influence of downwash caused by the stream-wise deflection of the flexible cylinder. As a mean to quantify turbulence, the turbulent intensity, Ti, was studied. In general, the flexible cylinders show better capability in augmenting the turbulence than the rigid cylinder. The stream-wise turbulent intensity for AR = 16 and Re = 8000 can be as high as 97% for the flexible cylinder compared to only 26% for the rigid case. The normalized amplitude response graph, which records the cross-flow oscillation of the flexible cylinder was also analyzed. Under the same Reynolds number, the turbulence enhancement increases with the structural velocity. An organized oscillating motion is in favor of a higher performance of turbulence enhancement.
   PDF XML

Drag and Heat Flux Reduction using Counterflow Jet and Spike - Analysis of their Equivalence for a Blunt Cone Geometry at Mach 8
Pages : 375-388
Authors : B. John,  Divik Bhargava,  Saumya Punia,  Parv Rastogi, 
This study aims to explore equivalence between active and passive flow control techniques in reducing the wave drag and surface heat flux over a blunt cone model kept in Mach 8 stream. Computational investigations were carried out by using finite volume-based compressible flow solver. Throughout the study, the solution of governing equations is sought by assuming two dimensional-axisymmetric nature of the flowfield. Both counter flow-stagnation point injection and forward facing-physical spike are considered to mitigate the excess drag and heat flux experienced by a blunt body representing the nose cone section of a hypersonic vehicle. Eventually, based on identified drag reductions, the present study proposes equivalence cases between these two methods. It is shown that a pointed spike of L/D=1 provides almost the same drag reduction as the counterflow injection jet with a pressure ratio of 8.25. Similarly, other equivalence cases are identified and the physics behind them is explored. The identified equivalence is expected to help the designers in effectively replacing one technique with another according to the requirement. Equivalence matrix is presented for different spike cases in terms of injection ratios of counterflow injection.
   PDF XML

Numerical Investigation on Periodically Time-Varying Dynamic Characteristics of Static Eccentric Squeeze Film Damper
Pages : 389-399
Authors : hailun zhou,  Xi Chen,  Ming Zhang, 
To study the periodically time-varying dynamic characteristics of static eccentric squeeze film damper (SFD), two models of computational fluid dynamics were established by dynamic mesh method of transient flow field. Numerical investigation was carried out for periodically time-varying dynamic characteristics of static eccentric SFD and the results were verified by theoretical formulas. As shown by the study, due to the presence of static eccentricity, oil-film damping and stiffness present distinct periodically time-varying characteristics with precession angle and their peak values occur in different circumferential positions; excessive static eccentricity will significantly alter the distribution of oil film pressure and direction of radial force of oil film. For SFD in practice, when dynamic eccentricity is smaller, the periodically time-varying characteristics are subjected to the dual influence of oil supply hole and static eccentricity. With the augment of dynamic eccentricity, the influence from oil supply hole is reduced, and the periodically time-varying characteristics are mainly embodied as the influence of static eccentricity.
   PDF XML

Numerical and Experimental Study of the Stern Wedge Effects on the Hydrodynamics Performance of a Semi-Displacement Catamaran in Calm Water
Pages : 401-415
Authors : mahdi yousefifard,  Ashkan Maboodi, 
In this article, the calm water resistance and dynamic instabilities of a semi-displacement catamaran fitted with a stern wedge is investigated using an experimental method and numerical technique. This is accomplished in order to probe into the effects of aft geometry modification on semi-displacement ship dynamic characteristics, especially at medium and high speeds. An advanced 6-DOF model that takes into consideration the dynamic mesh method has been utilized in open source code OpenFOAM. Reynolds-Average Navier-Stokes (RANS) equations are solved using standard k-ε turbulence model and VOF method. The accuracy of the current numerical method is investigated by the calm water test in National Persian Gulf Towing Tank. The resistance, trim and sinkage of the ship were monitored during the experiments. The experimental analysis was performed on the initial model and a modified model with 8º wedge at different Froude numbers. After that, the wedges were mounted at different angles at the transom of the vessel and the effect of the angle change for 4 different angles was evaluated using numerical solution. The results show that fitting a stern wedge to this type of ship causes an intense pressure at the stern bottom. Also, it decreases the dynamic trimming and forward resistance of the craft. As well as, stern wedge causes increasing the lift force which affects the reduction of dynamic instabilities. It is concluded that numerical model presented here is quite suitable for accurately predicting dynamic characteristics of a semi-planing twin-hull ships at medium and high Froude numbers. As a result, 14% reduction in total resistance was observed due to the installation of a 6 degree stern wedge.
   PDF XML

Computations of Flow past the Corrugated Airfoil of Drosophila Melanogaster at Ultra Low Reynolds Number
Pages : 417-427
Authors : Benjamin Rohit,  Swathi Reddy,  Siddharth Ghosh,  Mohammed Shakil, 
The study of corrugated wings has become acquainted in the field of insect flight in recent times. Recent studies on the aerodynamic effects of a corrugated wing are based on insects like the Dragonfly; whereas the likes of Fruitfly (Drosophila Melanogaster) usually go unobserved due to their smaller size. Consequently, the behaviour of these corrugations is found to be anomalous especially in the low and ultra-low Reynolds number region. Therefore, the main aim of this study is to understand the aerodynamic effects of the corrugated airfoil present in the wing of a Fruitfly; by conducting a geometric parametric study during a static non-flapping flight at 1000 Re. In this computational study, a 2-D section of the corrugated wing along the chord is considered. The parametric study helps in understanding the effects of varying number of corrugations, angle of corrugations and the presence of a hump at the trailing edge. The dimensions were scaled to a suitable reference value to additionally compare the corrugated airfoil of Fruitfly to that of a Dragonfly. The present study shows that the aerodynamic performance of the corrugated wing in terms of cl and cd are predominantly governed by the subtle geometric variations that can largely impact the formation of bubbles, vortex zones, and their mutual interaction. The reduction in the number of leading edge corrugations improved the cl/cd ratio and reduction in the corrugation angle helped produce higher lift. The presence of a trailing edge hump also improved the stall angle with a better flow re-attachment. The presence of corrugation at the trailing edge proved to be more beneficial compared to the model with corrugations at the leading edge. This also helped in understanding, the aerodynamic superiority of the trailing edge corrugations present in the Dragonfly's wing when compared to the Fruitfly's.
   PDF XML

Effect of Diffuser Height on Thermocline in Stratified Chilled Water Storage Tank
Pages : 429-438
Authors : Firas Hassan,  Maathe Theeb, 
Chilled water energy storage using thermal stratification technique currently used in the vast area because it contributes to reducing energy consumption and refrigeration capacity as well as its maintenance, operating and capital costs are low. In this paper, experimental tests were carried out on a small-scale vertical cylindrical storage tank equipped with an elbow-type conventional diffuser at inlet heights of 20, 170, 320 and 470mm for flow charging rates from 1.5-7.5l/min. in order to obtain a good thermal separation. The degree of stratification was estimated by means of temperature distributions and performance metrics, which involve thermocline thickness, the half-cycle figure of merit and equivalent lost tank height. The results show that the decrease in diffuser height above the tank floor tends to the steep thermocline or satisfactory thermal separation, the stratification and thermal performance were obtained at diffuser height of 20 mm within the limiting volume flow rates 1.5-4.5 l/min. better than those at volume flow rates ranging from 5.5-7.5l/min. and much better than at diffuser heights of 170, 320 and 470mm for various flow rates.
   PDF XML

Experimental Study of Self-starting Characteristics for H-Type Wind Turbine
Pages : 439-446
Authors : J. Y. Zhu,  P. Q. Liu, 
In order to study self-starting characteristics for H-type wind turbine, firstly, the effect of low Reynolds number and large separated flow on aerodynamic characteristics of airfoil were analyzed in detail, then two H-type wind turbines with different aerodynamic configurations were tested in a low speed wind tunnel for collecting the static torques at different phase angles and time-rotating speed curves in starting process. Based on theoretical analysis and experimental data, the cause of self-stating problem of H-type wind turbine has been revealed. The aerodynamic profile parameters of the wind turbine are closely related to the dependency of starting on initial phase position, and the minimum static torque determines whether the wind turbine has potential to start from rest. The time-rotating speed curves exhibit two different starting behaviour features, determined by the minimum dynamic torque in driving force conversion stage. Unless both the minimum static torque and minimum dynamic torque in driving force conversion stage are greater than the friction torque, the self-starting of the wind turbine cannot be realized. The typical self-starting behavior characteristics is that the time-rotating speed curve includes four different stages of initial linear acceleration, plateau, rapid acceleration and stable equilibrium with the final tip speed ratio more than 1.
   PDF XML

Numerical Investigation of Active Flow Control on Laminar Forced Convection over a Backward Facing Step Surrounded by Multiple Jets
Pages : 447-458
Authors : Umut Can Coskun,  Sertac Cadirci,  H. Gunes, 
Laminar, transient forced convection problem over a 2D backward facing step (BFS) at an inlet Reynolds number (Re) of 400 is investigated numerically using OpenFOAM. To increase the Nusselt number (Nu) along the bottom wall, active flow control is applied by zero-net-mass-flux (ZNMF) combinations of suction and injection through three thin slits which are placed on the top, step and the bottom walls in the vicinity of the BFS. The combinations of each jet velocity is determined by jet to inlet mean velocity ratios which are limited to integer numbers between -2 and 2 and satisfying ZNMF condition where negative and positive values indicate suction and injection, respectively. All 19 cases which satisfy these rules are investigated. Average Nusselt number, friction coefficient and recirculation zone lengths are calculated along the bottom wall from time averaged flow fields. Among 19 cases with each having different jet configuration, some cases converged to steady state solution while others indicated temporal effects and converged to periodic solutions. To understand these transient effects, velocity oscillation magnitude and Strouhal number which are monitored at a selected critical point are evaluated. It is shown that temporal interaction of chosen active flow control methodology has significant effect on enhancing mixing which results in an increase of Nusselt number. Among all cases, the best case concerning thermal improvement has an increase of 78.5% in Nu number while the best aerodynamic improvement is achieved for another case with a decrease of 81% in total recirculation zone length compared to the reference case where no control is applied.
   PDF XML

Natural Convection of Power Law Fluid through a Porous Deposit: MRT-LBM Approach
Pages : 459-472
Authors : Abderrahmane Bourada,  BOUTRA Abdelkader,  Kaoutar BOUARNOUNA,  Djamel Eddine Ameziani,  Y. K. Benkahla, 
In this research, natural convection of power law fluid in a square cavity, with a porous deposit in the shape of a semi-cylinder is studied numerically, using the multiple-relaxation-time lattice Boltzmann method. The modified Darcy-Brinkman model is applied for modelling the momentum equations in porous medium and the Boussinesq assumption is adopted to model the buoyancy force term. The influences of power law index (0.6 ≤ n ≤ 1.4), Darcy number (10−5 ≤ Da ≤ 10−2), Rayleigh number (103 ≤ Ra ≤ 106) and the radius ratio of the semi-cylindrical porous deposit (0.05 ≤ R ≤ 0.5) on hydrodynamic and heat transfer are studied. The obtained results show that these parameters have an important effect, on the structure of hydrodynamic and thermal transfer. The improvement of the power law index leads to a decrease in the heat transfer rate, illustrated by the average Nusselt number, and the augmentation in Darcy number induces an increase in that rate. Moreover, the variation of the Rayleigh number and the porous deposit radius has a significant effect on the transfer rate and convective structure. Besides, an unusual phenomenon is noticed for high Rayleigh numbers, where a better heat evacuation from the porous deposit is noticed for the dilatant fluid compared to the pseudoplastic one.
   PDF XML

Effects of False-Ceiling on Critical Ventilation Velocity and Maximum Gas Temperature in Tunnel Fires
Pages : 473-483
Authors : Hamidreza Savalanpour,  Bijan Farhanieh,  Hossein Afshin, 
In the present study, the effect of the use of false-ceiling on fire-induced smoke flow characteristics in tunnels is investigated using a 3D developed computational fluid dynamics tool. The critical velocity, the minimum required tunnel ventilation velocity to stop the smoke flow from moving toward the tunnel inlet (toward the upstream of the fire source), and the maximum gas temperature beneath the ceiling are selected to evaluate the smoke flow control in presence of the false-ceiling. The hydraulic height of the cross-sectional geometry of the tunnel is used as the characteristic length in order to dimensional analyze and compare the non-dimensional results. The results indicate that the use of the false-ceiling reduces the critical velocity and the smoke backlayering, while increases the maximum ceiling gas temperature. Reducing the critical velocity results in ventilation cost reduction (positive impact), while increasing the maximum gas temperature beneath the ceiling increases the risk of instrumental and life damages (negative impact). The detailed results and corresponding physical discussions are presented to clarify the reason for the significant differences between the results of the tunnels with and without false-ceiling.
   PDF XML

Effects of Non-Sinusoidal Motion and Effective Angle of Attack on Energy Extraction Performance of a Fully-Activated Flapping Foil
Pages : 485-498
Authors : A. Boudis,  H. Oualli,  A. Benzaoui,  O. Guerri,  A. C. Bayeul-Lainé,  O. Coutier Delgosha, 
Flapping foil energy harvesting systems are considered as highly competitive devices for conventional turbines. Several research projects have already been carried out to improve performances of such new devices. This paper is devoted to study effects of non-sinusoidal heaving trajectory, non-sinusoidal pitching trajectory, and the effective angle of attack on the energy extraction performances of a flapping foil operating at low Reynolds number (Re=1100). An elliptic function with an adjustable parameter S (flattening parameter) is used to simulate various sinusoidal and non-sinusoidal flapping trajectories. The flow around the flapping foil is simulated by solving Navier–Stokes equations using the commercial software Star CCM+ based on the finite-volume method. Overset mesh technique is used to model the flapping motion. The study is applied to the NACA0015 foil with the following kinetic parameters: a dimensionless heaving amplitude h0 = 1c, a shift angle between heaving and pitching motions = 90°, a reduced frequency f* = 0.14, and an effective angle of attack αmax varying between 15° and 50°, corresponding to a pitching amplitude in the range 0 = 55.51° to 99.51°. The results show that, the non-sinusoidal trajectory affects considerably the energy extraction performances. For the reference case (sinusoidal heaving and pitching motions, Sh = S =1), best performances are obtained for the effective angle of attack, αmax = 40°. At small effective angle of attack αmax <30°, the non-sinusoidal pitching motion combined with a sinusoidal heaving motion, greatly improves energy extraction performances. For αmax = 15°, Sh = 1 and S = 2, energy extraction efficiency is improved by 52.22% and the power coefficient by 70.40% comparatively to sinusoidal pitching motion. At high effective angles of attack (αmax > 40°), non-sinusoidal pitching motion has a negative effect. Performances improvement is quite limited with the combined motions non-sinusoidal heaving/sinusoidal pitching.
   PDF XML

Computational Investigation of the Effect of Wall Thickness on Rupture Risk in Abdominal Aortic Aneurysms
Pages : 499-513
Authors : Huseyin Enes Salman,  H. Yalcin, 
Cardiovascular disorders are among the most important causes of sudden death and adult disability worldwide. Abdominal aortic aneurysm (AAA) is a critical clinical condition where the aorta dilates beyond 50% of its normal diameter and leads to a risk of rupture. In this study, we performed fluid-structure interaction (FSI) analysis on an eccentric computational AAA model in order to investigate the effects of wall thickness on AAA wall stresses, which are critically important to estimate the rupture risk. For this purpose, we modeled the problem domain using finite element analysis, and coupled the solutions of fluid and structure domains for improving the accuracy of results. ANSYS commercial finite element analysis software was used for modeling, solving, and post-processing the results. Expanded diameter in AAA sac resulted in altered hemodynamics. Wall shear stresses (WSS) caused by the flow are quite low on the AAA sac, which may deteriorate the endothelial cell regeneration and vascular remodeling in the long term. It is concluded that the most critical region for the rupture risk is the posterior distal end of AAA sac due to being exposed to peak mechanical stresses during the cardiac cycle. Obtained results shed light in understanding the rupture risk assessment of AAA.
   PDF XML

Effects of Prandtl Number on Three Dimensional Coherent Structures in the Wake behind a Heated Cylinder
Pages : 515-526
Authors : AJITH KUMAR S,  Anil S, 
Flow past a heated cylinder kept at constant surface temperature is computationally simulated and analyzed in the laminar regime at moderate buoyancy. In this study, we have restricted to moderate Reynolds numbers to completely eliminate the presence of mode-A and mode-B instabilities. The three dimensional transition due to the mode E instability is captured using a cell-centered finite volume method. The present study reveals the existence of two different kinds of coherent structures - the “surface plumes” and the “mushroom structures”. The role of these mushroom structures in the heat transfer mechanism and the changes that the Prandtl number would bring into this coherent structure are discussed. The mushroom structures observed show high dependency on the changes in Prandtl number whereas the surface plumes are found almost unaffected.
   PDF XML

Coupled Analysis of Transient Aerodynamic Characteristics of the Coach under Crosswind in Different Situations
Pages : 527-539
Authors : Kai SUN,  gu zhengqi,  Jun LIU,  Ledian ZHENG,  Hongbo HU,  Jun GAO, 
The purpose of this work is to investigate transient aerodynamic characteristics of the coach under the crosswind in straight-line situations with different uniform speeds and uniform accelerations. The transient aerodynamics caused by different speed changes are analyzed using the real-time interaction between aerodynamic simulation and dynamic simulation. The target model is a simplified coach on a full scale. The SST (Menter) K-Omega Improved Delayed Detached Eddy Simulation and overset mesh technique are used to predict the transient aerodynamic loads. The accuracy of the turbulence model is verified by a wind tunnel experiment of the 1/7th scaled coach model. The present results show that the transient aerodynamic loads have different locations of maximum side force and the holding duration of yaw moment for different constant speeds. The speed becomes larger, and the position where the side force is maximum becomes farther away. The holding duration of the top yaw moment is larger simultaneously. Moreover, proper acceleration for low initial driving speed and crosswind of small influence range could build up stability. High speed driving in gust wind is not suggested for unskilled drivers.
   PDF XML

Uncertainty Analysis of Experiments of Vortex-Induced Vibrations for Circular Cylinders
Pages : 541-553
Authors : Onur Usta,  Aytekin Duranay, 
In this study, uncertainty analysis of the vortex-induced vibration (VIV) tests, using a VIV test rig is presented. The VIV test rig is set up on the circulation channel in Ata Nutku Ship Model Testing Laboratory at Istanbul Technical University (ITU). The tests are performed using an elastically mounted rigid and smooth circular cylinder in low mass-damping and high Reynolds numbers conditions. The cylinder has one-degree-of freedom. It is allowed to move perpendicular to the flow while inline vibrations are constrained. The aim of the study is to demonstrate and establish a repeatable procedure to predict the uncertainty of VIV tests, utilizing some example applications of existing ITTC recommendations. Within this aim, five distinct VIV tests are carried out following ITTC guidelines and procedures measuring the amplitude (A*) and frequency response (f*) data. Uncertainty analysis study is performed for three different flow velocities, chosen from VIV tests and total uncertainty is calculated by root mean square values of precision and bias uncertainties. The precision uncertainty is predicted using response amplitude values obtained from five sets of VIV tests. The bias uncertainty is predicted utilizing the basic measurements and test results of the components of response amplitude for the cylinder. The results have demonstrated that the current test rig has low uncertainty level. Additionally, it has succeeded to reflect the characteristics of VIV phenomenon in the studied Reynolds number range, which is in the Transition Shear Layer 3 (TrSL3) flow regime. Consequently, it is believed that this study would help in spreading the application of the uncertainty analysis for VIV tests in the future.
   PDF XML

Research on the Flow Characteristic in a Multi-Stage Multiphase Pump by Numerical Simulations
Pages : 555-566
Authors : YI SHI,  Hongwu Zhu, 
As a cost-effective option for subsea gas and oil fields development, multiphase pump has been widely used and plays an important role in promoting field production especially for those brownfields. The objective of this research is to study the variations of flow parameters at the inlet of each stage in a five-stage helico-axial pump under design conditions by using commercial CFD packages. The numerical results show that inlet volume flow rate, gas volume fraction (GVF) and flow angle of each stage both decreases from the first stage to the fifth stage because of the compression. The variations of these parameters along the flow direction are susceptible to the effect of rotational speed and initial gas volume fraction at inlet of the first stage. These inlet flow parameters from inlet to outlet of the five-stage pump just change little when the initial inlet GVF is lower than 10% or higher than 90%. While the variation is obvious when inlet GVF is from 20% to 80%. Based on numerical simulation results, the stage-by-stage design method is proposed and geometry parameters such as inlet impeller hub diameter and inlet angle of blade in the second stage are modified according to its inlet flow conditions. The comparison results between the modified pump and original pump show that pressure distribution at leading edge of impeller blades in each stage becomes uniform in the modified multistage pump. The inlet incident flow loss of each stage is diminished and internal flow conditions has been significantly improved. Thus, the pump’s boosting capacity is enhanced. These research results in this paper are instructive for the performance optimization of multistage pumps used in gas oil fields.
   PDF XML

Experimental Study on Slug Flow Characteristics and its Suppression by Microbubbles in Gas-Liquid Mixture Pipeline
Pages : 567-579
Authors : Tianxiang Ling,  Tianxiang Wang,  Gang Lei,  Zhenlv Fang,  Luhaibo Zhao,  Chuanlong Xu, 
In the oil-gas mixture transportation system of offshore oilfields, it is of great theoretical and practical significance to study the flow characteristics of the slug flow and its suppression or elimination. In this paper, the characteristics of the slug flow and microbubbles in a laboratory-scale rig are experimentally studied and analyzed by a multi-parameters measurement system including electrical resistance tomography (ERT), high-speed camera, and traditional pressure sensor. The suppression of the microbubbles on the slug flow formation is further investigated. Experimental results showed that the bubbles with different sizes from the microbubble generator have different aggregation and dispersion characteristics. The microbubbles can suppress the formation of the slug flow by increasing the liquid slug pressure and further affect the motion of the slug by enhancing the disturbance effect of the boundary layer, so as to achieve the suppression on slug flow.
   PDF XML

Experimental Studies of the Evaporation of Pure Liquid Droplets in a Single-Axis Non-Resonant Levitator
Pages : 581-587
Authors : Marija Radmilovic-Radjenovic,  Dimitrije Radjenovic,  Miodrag Mitric,  Branislav Radjenovic, 
Though a simple daily observation, evaporation of drops is still poorly understood due to the complex nature that involves hydrodynamic effects in the bulk fluids and transport phenomena at the liquid-vapor interface. This paper reports on the evaporation of single component droplets (water, ethanol, acetone, and glycerol) levitated in a single-axis non-resonant levitator. It was observed that the acetone and ethanol evaporated faster than water, although the acetone is the most volatile. The estimated lifetime of acetone is less than 5min, which is much shorter as compared to 56min for ethanol or about 90min for water droplets. On the other hand, glycerol showed no tendency to evaporate. With increasing the evaporation time, the ratio of large and small semi-axis decreases and tends to 1 corresponding to changes in drops shape from oblate ellipsoid to a sphere. Based on the classical D2-law, the surface regression rates have been estimated.
   PDF XML

Research on the Rotor-Stator Interaction of Centrifugal Pump based on Sinusoidal Tubercle Volute Tongue
Pages : 589-600
Authors : Peifeng Lin,  PengFei Song,  Zuchao Zhu,  Xiaojun Li, 
The rotor-stator interaction between the impeller and the volute is the main reason for the pump pressure pulsation and vibration. This work aims at designing a new type of tongue to minimize pressure pulsation, reduce vibration noise and energy loss. Inspired by the humpback pectoral fin, four volute tongues are investigated in this paper, three of which are sinusoidal tubercle volute tongues (STVT) and one is the original volute tongue (OVT). Based on the detached-eddy simulation (DES) turbulence model, the influence of the sinusoidal tubercle volute tongues on the pressure pulsation was investigated, and the flow structure and enstrophy of the four pumps were analyzed, aiming to minimize pressure pulsation, maximize hydraulic performance and reduce the energy dissipation in centrifugal pumps. The results show that the pressure pulsations of the STVT profiles are all lower. The reductions of the average pressure pulsation at the monitoring points are 13.3% (STVT-1), 20.6% (STVT-2), and 16.2% (STVT-3), respectively. The difference in pressure pulsation at the monitoring points closer to the tongue is more obvious. At the design flow rate, the efficiencies of the three bionic pumps are increased by 0.5% (STVT-1), 1.5% (STVT-2), 0.9% (STVT-3), respectively. The STVT profiles change the vortex structure near the tongue and minimized the vortex strength. Meanwhile, the flow in the pump with the STVT profiles have lower enstrophy. The enstrophy of the flow with the STVT-2 profile is the lowest, which is reduced by about 8%. This reduces the dissipation of mechanical energy. The results can be used as a guide for pump design optimization.
   PDF XML

Mixing and Interpenetration in a Three-Dimensional Buoyancy-Driven Flow of Two Immiscible Liquids: A GPU Based LBM Approach
Pages : 601-613
Authors : Prasanna Redapangu,  Tsegay G.Kidan,  Kiros Berhane, 
The Buoyancy-driven flow of two immiscible liquids having varying density and viscosity is studied in a three-dimensional inclined confined channel. Initially, the heavier/lighter liquids occupy the upper/lower parts of the channel, respectively, which is an unstable configuration. The numerical simulations are performed using a multiphase lattice Boltzmann method (LBM) that is further implemented on the graphics processing unit (GPU). The three-dimensional flow dynamics and the associated physics are studied based on various parameters such as viscosity ratios (m), Atwood numbers (At) and Reynolds numbers (Re). The results were presented in the form of iso-surface/contour plots, average density profiles, and lengths of interpenetration. It is observed that larger interpenetration occurs with iso-viscous liquids having higher density gradients (higher At). The Reynolds number had a non-monotonic effect on the axial lengths of interpenetration (Lp∗); Lp∗ increases till Re = 500 and then decreases for Re = 1000. At larger Re, due to the development of Kelvin-Helmholtz instabilities higher transverse interpenetration is observed.
   PDF XML