Vol14 , No 6 (in press)
Comparison of Pulsatile Pressure Flow through a ‎Specific Patient's Rigid and Deformable ‎Arteriovenous Fistula: In Vitro Study
Pages : 1593-1601
Authors : Jonhattan Rangel,  Willyan Santos,  Luiz Henrique Pinheiro,  Thercio Costa,  Kleiber Bessa,  Jayme Ortiz, 
Arteriovenous fistula (AVF) is the most widely used vascular access by patients undergoing hemodialysis, however, even though the most widely used access to AVF has a high failure rate, and can be affected by problems during its use, among the most common highlights intimal hyperplasia, thrombosis and stenosis. Studies suggest that recurrent problems in this type of access are directly linked to geometry, flow conditions and stiffness of the vascular wall by the vessels that compose it. The present work seeks to analyze the variation of pressure and flow in rigid and flexible AVF models manufactured from data from an actual patient undergoing treatment. The study was carried out from the acquisition and processing of the patient’s medical examinations (computed tomography), the creation of the geometry, treatment and modeling of said patient, the manufacturing of the AVF models by 3D printing and injection in mold, experimental analysis with pulsatile flow conditions, close to the real physiological conditions, and data analysis. The results obtained show the influence of vascular wall stiffness on flow conditions. In the rigid and flexible model, pressure peaks ranged from 170.98 mmHg to 172.44 mmHg (± 0.02) and 69.83 mmHg to 116.63 mmHg (± 0.03), respectively. The pressure drop between entry and exit in the AVF was also analyzed, presenting a greater value in the flexible model, being approximately three times that of the rigid model. The observed results show the direct relation of the deformation in the flow conditions in the system, and consequently, its direct influence on the pathologies of the vascular system, especially the AVF.

Fluid Dynamic Characteristics and Flow Distribution ‎Structure Optimization of Axial Piston Pump Considering ‎Cavitation Bubble Evolution
Pages : 1603-1616
Authors : Yang Pan,  Anhua Chen,  Zhineng Wang, 
An axial piston pump can produce a serious cavitation phenomenon in the high- and low-pressure ‎transition process. Cavitation bubbles expand, compress, rebound and collapse when they enter the high-‎pressure oil drainage area. This affects the outlet flow ripple as well as the pressure pulsation of the ‎piston pump. However, the effect of the cavitation bubbles is ignored in the current outlet flow ripple ‎model of axial piston pumps. It affects the optimization design of the axial piston pump distribution area ‎structure parameters with the objective of reducing the pressure and flow rate. Therefore, a method of ‎optimizing the fluid dynamic characteristics and the flow distribution area structure parameters of an axial ‎piston pump considering the cavitation bubble evolution is proposed. A single-cavity dynamic model was ‎established to study the bubble evolution as the piston chamber pressure changes. According to the ‎cavitation cloud (group cavitation) characteristics of the axial piston pump, theoretical models of the ‎outlet flow ripple and the pressure pulsation of a piston pump were established considering the cavitation ‎bubble characteristics. The influence of cavitation characteristics on the outlet flow ripples and pressure ‎pulsation of the axial piston pump was analyzed and compared with that without cavitation. Comparison ‎with the experimental results, verified that the outlet flow ripple model becomes more accurate when ‎cavitation bubble characteristics are considered. Based on the multi-agent particle swarm optimization ‎‎(MAPSO) algorithm, an optimization model of the piston pump outlet flow ripple was established ‎considering the cavitation bubble characteristics. The optimized design parameters for the flow ‎distribution area of the axial piston pump were evaluated. The proposed method can provide theoretical ‎guidance for the design of a low flow ripple axial piston pump.‎

Optimization of a Draft Tube using Statistical Techniques- DOE and 2D Computational Fluid Dynamic Analysis
Pages : 1617-1633
Authors : Antonio Carlos Barkett Botan,  Ramiro Gustavo Ramirez Gustavo,  Geraldo Lucio Tiago Filho,  Edna Raimunda da Silva, 
The draft tube is one of the main components that integrate a turbine, since it has the function of recovering the residual kinetic energy after the runner by the pressure energy. The search for a draft tube design that increases the efficiency of the turbine is always an engineering challenge. The hydromechanics components geometry optimization can be accomplished through the integration of optimization methods and CFD tools. In this work, the geometric optimization of a double diffuser draft tube of a Bulb turbine applied to ultra-low heads is presented, with the objectives of maximizing the pressure recovery coefficient, Cp, and increasing the hydraulic efficiency of the turbine, ηh. These improvements would make it possible to reduce the longitudinal length of the draft tube, thereby, making an easier insertion of this kind of turbines in water transport systems, with pressures around 3 [mH2O]. The optimization methodology was performed in the meridional plane, using twelve geometric variables in the draft tube through the integration of optimization methods and computational fluid dynamics. The optimized geometry obtained showed an increase in the Cp value of 0.71516, from the original geometry, to 0.83080. The results were extended to the 3D flow analysis, where the optimized turbine showed efficiency gains of 82% to 84%, when compared to the original turbine considering that its total length was reduced and its geometry simplified, resulting in a more compact and versatile equipment. The study also concluded that the applied methodology can be extended to other similar optimization problems in the design of hydraulic machines‎.

Effect of a Pylon Mounted Cavity-Based Flameholder on ‎the Combustor Flow Characteristics
Pages : 1635-1642
Authors : Sanchayata Nayal,  Devabrata Sahoo, 
This paper aims to study the effect of a pylon mounted cavity-based flameholder on the combustor flow ‎characteristics. Computational analysis of two different models of flameholder configurations is ‎performed. The novel cavity design 110_90 has a fore-wall ramp angle of 110 degrees and an aft-wall ‎ramp angle of 90 degrees and this design which shows a comparatively better combustor performance is ‎adopted and mounted with a pylon. The flow features over the high performance base cavity 110_90 is ‎compared with the flow features obtained by adding a pylon on the upstream of the base cavity. The two ‎cases are compared qualitatively as well as quantitatively based on the temperature distribution, pressure ‎distribution, recirculation zones and drag experienced by the model. These compared parameters helped ‎us to identify whether the mentioned combination is favorable and augments the flameholder ‎performance‎‎.

Effect of Geometry Design on Mixing Performance of Newtonian Fluids using Helical Overlapped Mixer Elements in Kenics Static Mixer
Pages : 1643-1656
Authors : Ahmed TALHAOUI,  Belkacem Draoui,  Abdelkader YOUCEFI, 
The laminar flow pattern and mixing behavior of incompressible Newtonian fluids in different modified mixer ‎configurations were numerically investigated using Computational Fluid Dynamics (CFD) simulations in the range of ‎Re=0.15-100. The governing equations were solved by ANSYS Fluent 14 using the second-order finite volume method ‎‎(FVM) and the SIMPLE algorithm scheme. The computational model is assessed by comparing the predicted pressure ‎drop results to empirical correlations in the literature. The effects of incorporated helical overlapped mixer elements and ‎the diameter aspect ratio (C) on the mixing efficiency for different mixer geometries were examined and evaluated by ‎characteristics measures of Intensity of Segregation (IOS), pressure drop, extensional efficiency, and G-factor. The ‎performance of new modified mixers is evaluated via comparison with the standard industrial Kenics static mixer. The ‎static mixers with modified internal geometry achieved fast mixing and better mixing quality than the Kenics mixer. ‎Besides, an increase in diameter aspect ratio C benefited from a decrease in pressure drop within the static. The ‎modified mixer: C=1.5 was found to have the highest mixing efficiency, concerning short mixing length with marginally ‎higher pressure drop than the other mixers. In contrast, the mixer: C=2 is the most efficient based on low pressure drop ‎and energy requirement with slightly greater mixing length‎‎.

Wall Shear Stress-Based Hemodynamic Descriptors in the ‎Abdominal Aorta Bifurcation: Analysis of a Case Study
Pages : 1657-1668
Authors : A Soares,  Filipa Carvalho,  Argetina Leite, 
The knowledge of hemodynamic behaviour in the abdominal aorta artery bifurcation is of great ‎importance for the early diagnosis of several cardiovascular diseases common in this bifurcation. The ‎work developed focuses on a case study of hemodynamic in the abdominal aorta artery bifurcation, ‎based on a realistic 3D geometric model reconstructed from 2D medical images of a real patient. ‎Hemodynamic quantities based on the wall shear stress (WSS) of the abdominal aorta bifurcation are ‎analysed and is presented an alternative analysis of the well-established stress hemodynamic descriptors ‎to identify specific zones of the artery with a higher probability of developing cardiovascular diseases. ‎The individual analysis of different zones of the artery allowed to obtain information that can remain ‎masked when whole artery is considered as a single zone. The reported results provide a correlation ‎between the analysed stress hemodynamic descriptors and the area of the wall artery. Then, the aim of ‎this work is the identification of regions at the luminal surface subject to atherosusceptible WSS ‎phenotypes. For the patient studied, the analysis presented allowed the identification of the patient's ‎propensity to develop atherosclerosis, according to the hemodynamic descriptors time-averaged WSS ‎‎(TAWSS), oscillatory shear index (OSI), and relative residence time (RRT). Thus, this work offers a new ‎way of looking to the stress hemodynamic descriptors.‎

Vortex Evolution and Energy Production in the Blade ‎Channel of a Francis Turbine Operating at Deep Part ‎Load Conditions
Pages : 1669-1678
Authors : An YU,  Yongshuai Wang,  Daqing Zhou, 
The blade vortex evaluation in Francis Turbine under deep part load conditions generates severe pressure ‎fluctuations in the runner. The complex flow in a model turbine is numerically investigated based on a ‎modified Partially Averaged Navier-Stokes method. The main emphasis is focused on revealing the ‎correlation mechanism of blade vortex evolution and energy production. The results indicate that the ‎modified PANS method shows significant advantages in hydro turbine’s simulation than the traditional ‎RANS method. At deep part load conditions, the vorticity formed at the leading edge of the suction ‎surface and the trailing edge of the pressure surface in the blade channels. The stretching term provides ‎the most vorticity increments while the dilation term inhibiting part which only provides a decrement of ‎the vorticity evolution. Based on the entropy production theory, the total entropy production distribution ‎is consisting with the distribution of vorticity. At deep part load condition, direct dissipation and turbulent ‎dissipation provide the most entropy, while at part load condition the proportion of these two-part ‎decreased‎.

Aeroelastic Simulation of Stall Flutter Undergoing High ‎and Low Amplitude Limit Cycle Oscillations
Pages : 1679-1689
Authors : Khairil Hafizi Zorkipli,  A. Abas,  N. Razak, 
The aeroelastic behaviour of an airfoil oscillating in large and small pitch amplitudes due to nonlinearity ‎in aerodynamics is examined. The phenomenon of stall flutter resulted in the limit cycle oscillations of ‎NACA 0012 at low to intermediate Reynolds number is investigated numerically through the unsteady ‎two-dimensional aeroelastic simulation. The simulations employed unsteady Reynolds Average Navier ‎Stokes shear stress transport k-ω turbulent model with the low Reynolds number correction. The ‎simulations of the fluid-structure interaction were performed by coupling the structural equation of ‎motion with a fluid solver through the user-defined function utility. Numerical simulations were executed ‎at three different elastic axis positions; the leading-edge, 18% and 36% of the airfoil chord length. The ‎airfoil chord measures 0.156 m. The simulations were executed at the free stream velocity ranging from ‎‎5.0 m/s to 13 m/s corresponding to the Reynolds number between 51618 and 134207. Two types of ‎oscillation amplitudes were observed at each elastic axis position. At the leading-edge and 18% case, ‎small amplitude oscillations were observed while at 36%, the system underwent high amplitude ‎oscillations. The analysis revealed the cause for small oscillation amplitude is due to the separation of the ‎laminar boundary layer on the suction side of the airfoil starting at the trailing edge. High amplitude ‎oscillations occurred due to the existence of the dynamic stall phenomenon beginning at the leading-edge. ‎Small amplitude LCOs only occurred within a limited range of airspeed before it disappeared due to ‎increasing airspeed‎‎.

Mechanism of Stability Enhancement with Shallow ‎Reversed Slot-Type Casing Treatment in a Transonic ‎Compressor ‎
Pages : 1691-1704
Authors : Zhidong Chi,  W. L. Chu,  Haoguang Zhang, 
In this paper, the influence of a shallow reversed slot-type casing treatment on the performance of a tip-critical transonic compressor has been numerically investigated. Firstly, the complex flow fields in the rotor tip region are studied in details. It shows the severe blockage induced by suction surface boundary separation triggers compressor stall at 100% design speed, while the blockage due to tip leakage vortex dominates at 80% design speed. Secondly, the mechanism of stability extension is presented at different rotating speeds. The casing treatment alleviates greatly the tip blockage by manipulating the tip leakage flow, accompanied by the redistribution of aerodynamic loading and mass flux. As a result, the casing treatment is more efficient for the blockage induced by tip leakage vortex (at 80% design speed). Further analysis of the pressure field and passage shock distribution demonstrates that the passage shock intensity and its location will affect the effectiveness of casing treatment. Finally, the instability characteristics of compressor with casing treatment are revealed. The numerical results reflect when the mass flow approaching the instability boundary, the stator passage blockage presumably is dominant for triggering the compressor stall.

Analytical and Numerical Study on the Performance of ‎Supersonic Ejector at Different Operating Conditions ‎and Working Fluids
Pages : 1705-1716
Authors : SURENDRA YADAV,  K. M. Pandey,  Rajat Gupta, 
The computation of ejector geometry for a given fluid is essential and plays a crucial role in creating an ejector profile and performance analysis. The current paper discusses the constant rate of momentum change (CRMC) approach using a real gas equation for ejector design. The numerical analysis is carried to validate the analytical geometry and the effect of operating parameters on the entrainment ratio. The variation in entrainment ratio for the different working fluids has also been studied on the geometry computed for water-vapour. It is observed that the entrainment ratio of the ejector significantly varies with the change in operating conditions and working fluids. The numerically predicted entrainment ratio is ~0.354 compared to the on-design entrainment ratio 0.4 for water vapor, while the predicted entrainment ratio for other working fluids is ~0.319, ~0.314, and ~0.36 for air, N2, and CO2, respectively‎.

Pore Network Modeling to Study the Impacts of ‎Geometric Parameters on Water Transport inside Gas ‎Diffusion Layers
Pages : 1717-1730
Authors : Hamed Gholipour,  M.J Kermani,  Rahim Zamanian, 
A pore network model (PNM) is proposed for the simulation of water transport inside the cathode side ‎gas diffusion layer (GDL) of polymer electrolyte fuel cells (PEFCs) during the transient start-up period as ‎well as the steady state. Numerous two-dimensional random networks representing GDL are generated ‎followed by statistical averaging of the results (Monte Carlo methods) to circumvent the uncertainties ‎imposed by random pore size distributions. The resulting liquid water saturation profiles within GDLs ‎exhibit concave patterns which is typically encountered in capillary fingering flow regimes in porous ‎media. The effect of GDL thickness and current collector rib width as two geometric parameters on water ‎transport dynamics are separately investigated. It turns out that thin and thick GDLs compared to the base ‎case can have contradicting outcomes on the account of total water saturation in the network. On the ‎other hand, wide current collector ribs give rise to liquid water saturation and build-up within GDL which ‎can lead to flooding. At the end, three-dimensional networks are generated demonstrating higher pore ‎connectivity which results in higher percolation times and different invasion patterns.‎

A Model for Shelled Micro-Bubble in Geometric Confinement under Acoustics Field‎
Pages : 1731-1740
Authors : Adnan Qamar, 
A theoretical model to predict the dynamics of a shelled micro-bubble driven by acoustic field in a tubular geometric confinement is proposed in the present study. The model is derived from first principle and may not be considered as a variant of Rayleigh-Plesset solution. A semi-analytical model is derived in the form of an ordinary differential equation connecting all parameters involved. Results obtained are in agreement with the available experimental data. The model is further linearized to obtain expression for the forced resonant frequency, which is shown to depend on geometric parameter of confinement as D/ where D and L are the tube diameter and length, respectively. Further, linear viscous damping coefficient is also studied and is found that an overdamped or an underdamped state exist base on shelled micro-bubble size and parameters of geometric confinement (L and D). The state of damping clearly indicate when the shelled micro-bubble in confinement would respond linearly or non-linearly under the influence of acoustic field‎.

Effects of an Axial Magnetic Field on Vortex Breakdown and Fluid Layer
Pages : 1741-1753
The effects of an axial magnetic field on both the vortex breakdown process and fluid layers development in a cylindrical container filled with a conducting viscous fluid are numerically analyzed by using the Generalized Integral Transform Technique (GITT) with a stream function-only formulation. A temperature gradient is imposed in the axial direction on the swirling flow which is advanced by the rotation of the bottom disk under the stabilizing effect of the external magnetic field. Flows are studied for a range of parameters: the Richardson number, Ri, 0 ≤Ri ≤2.0; and three values of the Prandtl number are investigated, Pr = 0.025 (liquid Mercury), 0.032 ( PbLi 17 alloy), and 0.065 (the molten lithium). Three combinations of aspect ratios (H/R) and Reynolds numbers are compared: (case A: Re=1500, H/R=1.5); (case B: Re=1855, H/R=2.0) and (case C: Re=2400, H/R=2.5). The results reveal that the increase in the values of Hartmann number, Ha suppresses the vortex breakdown in the isothermal case and reduces the number of fluid layers in the layering case. The stability diagram (Hacr–Ri) corresponding to the transition from the multiple fluid layers zone to the one fluid layer zone for increasing Prandtl number is obtained‎.

Boundary Condition Effect on Two-Phase Fluid Flow ‎and Heat Transfer inside 3-D Microchannels
Pages : 1755-1765
Authors : Chandrasekhar S,  V. R. K. Raju, 
Two-phase Taylor flows play a vital role in dissipating heat effectively for the proper functioning of electronic systems. In the present study, the thermal performance of liquid-liquid Taylor flow was carried out in a 3D microchannel with uniform wall heat flux boundary for five different cases: uniform heat flux on the four walls, three walls, two opposite walls, single wall, and two adjacent walls, and the aspect ratio of microchannel was varied in the range of 0.2-5. The length of the microchannel was 4 mm, height and width were 0.1 mm each for a square microchannel. For varying aspect ratios of the microchannel, the height and width of the microchannel were taken in the range of 0.06-0.3 mm to keep the hydraulic diameter constant. Dodecane and water were the working fluids in the study and assumed to be Newtonian, incompressible, immiscible, and the properties of fluids were assumed to be independent of temperature. The pressure distribution in the microchannel was investigated under five thermal boundary cases and the aspect ratio effect on pressure drop was also discussed. Results showed Nusselt number of two-phase flow with four-wall heat flux increases up to 280% compared to liquid-only flow and it has been validated with standard heat transfer correlation available in the literature. A higher heat transfer rate (Nu=10.41) was recorded in the opposite walls boundary condition and the heat transfer rate (Nu=7.81) was minimum when the adjacent walls were subjected to uniform heat flux. The effect of microchannel aspect ratio on Taylor flow heat transfer under thermal boundary conditions was also analyzed‎.

Multiscale Hydrodynamics in Line Contacts with Different ‎Fluid-Contact Interactions
Pages : 1767-1773
Authors : Jian Li,  Y Zhang, 
Multiscale hydrodynamics in line contacts were analyzed from the newly developed ‎multiscale flow equations by respectively considering the weak, medium and strong fluid-‎contact interactions. In the studied line contact, the surface separation is on the same scale ‎with the thickness of the adsorbed layer on the contact surface, and between the coupled ‎adsorbed layers occurs the continuum fluid flow. The present study shows that when using ‎the developed multiscale flow equations to calculate the surface separation for the weak or ‎medium fluid-contact interactions, the value of the parameter k , which is in the important ‎formulation ‎ ‎, should be taken as about 5.0, different from its value 1.0 for the ‎strong fluid-contact interaction. The study also shows that owing to the adsorbed layer effect, ‎for a given operating condition, stronger the fluid-contact interaction, greater the surface ‎separation. The results show the significant effect of the fluid-contact interaction in ‎hydrodynamic line contacts in a quite wide range of the surface separation because of the ‎multiscale hydrodynamic effect‎.

Journal of Applied Fluid Mechanics

The Journal of Applied Fluid Mechanics (JAFM) is an international, peer-reviewed journal which covers a wide range of theoretical, numerical and experimental aspects in fluid mechanics. The emphasis is on the applications in different engineering fields rather than on pure mathematical or physical aspects in fluid mechanics. Although many high quality journals pertaining to different aspects of fluid mechanics presently exist, research in the field is rapidly escalating.