Journal Of Applied Fluid Mechanics
Volume:9 Issue: 2, Mar-Apr 2016

In this study, the onset of nanofluid convection confined within a Hele-Shaw cell is investigated by performing a classical linear stability analysis. The model used for nanofluid combines the effects of Brownian motion and thermophoresis, while for Hele-Shaw cell Brinkman model are employed. The new stability equations are formulated by introducing new characteristic dimensionless parameters such as the Hele-Shaw number, the Hele-Shaw Rayleigh number and the nanoparticle concentration Hele-Shaw Rayleigh number. The resulting stability equations are solved numerically by using higher order Galerkin method. It is found that the nanoparticle concentration Hele-Shaw Rayleigh number, the Lewis number and the modified diffusivity ratio hasten the onset of convection, while the Hele-Shaw number delays the onset of convection. A comparison is also made between the existing boundary conditions for nanoparticle and obtained that the zero nanoparticle flux boundary conditions under the thermophoretic effects has more destabilizing effect than the fixed nanoparticle boundary conditions.

Numerical experiments were carried out on the high speed driven cavity flows in 2D curved channels to investigate mainly the pressure field. A density-based algorithm in ANSYS Fluent 13.0 was used in the present URANS simulations. The SST k-w model was used for modeling the turbulence within an unstructured mesh solver. Validation of the numerical code was accomplished, and the results showed a good agreement between the numerical simulation and experimental data. Three channels (straight, concave and convex) with a nominal height of H = 4×10-3 m under the transonic flow conditions were considered in the study. The cavity studied is L = 12×10-3 m long with the depth ranging from D = 12×10-3 m to 48×10-3 m to obtain the length-to-depth ratios of L/D 1 to 1/4. The study comprised the analysis of the cavity surface pressures and the associated flow structures. The channel configuration influenced the cavity flowfield, and that influence finally resulted in a change in the surface pressure fluctuations in the cavity. The deep cavity attenuated the flowfield oscillation inside the cavity.

This work presents a numerical analysis of the ability of the high lift airfoil profile Selig 1223 for working as hydrofoil under water conditions. The geometry of the hydrofoil blade is designed through a suitable airfoil profile and then studied carefully by means of Computational Fluid Dynamics (CFD) in order to check its hydrodynamic behavior, i.e., including lift and drag analysis, and determinations of streamlines velocities and pressures fields. Finally conclusions on the use of this profile in a possible application for hydrokinetic turbine blades are detailed.

In this study we analyzed the momentum and heat transfer behavior of CuO-water and A 2O3-waternanofluids embedded with micrometer sized conducting dust particles towards a porous stretching/shrinking cylinder at different temperatures in presence of suction/injection, uniform magnetic field, shape of nano particles, volume fraction of micro and nano particles. The governing boundary layer equations are transformed to nonlinear ordinary differential equations by using similarity transformation. Numerical solutions of these equations can be obtained by using Runge-Kutta Felhberg technique. The influence of non-dimensional governing parameters on the flow field and heat transfer characteristics are discussed and presented through graphs and tables. Results indicates that spherical shaped nano particles showed better thermal enhancement compared with cylindrical shaped nano particles, increase in volume fraction of nano particles helps to enhance the uniform thermal conductivity. But it does not happen by increase in volume fraction of dust particles. Enhancement in fluid particle interaction reduces the friction factor and improves the heat transfer rate.

In this study we analyzed the momentum and heat transfer characteristics of MHD boundary layer flow over an exponentially stretching surface in porous medium in the presence of radiation, non uniform heat source/sink, external pressure and suction/injection. Dual solutions are presented for both suction and injection cases. The heat transfer analysis is carried out for both prescribed surface temperature (PST) and prescribed heat flux (PHF) cases. The governing equations of the flow are transformed into system of nonlinear ordinary differential equations by using similarity transformation and solved numerically using bvp4c Matlab package. The impact of various non-dimensional governing parameters on velocity, temperature profiles for both PST and PHF cases, friction factor and rate of heat transfer is discussed and presented with the help of graphs and tables. Results indicate that dual solutions exist only for certain range of suction or injection parameters. It is also observed that the exponential parameter have tendency to increase the heat transfer rate for both PST and PHF cases.

The instinct system of cilia motion with magnetic field and slip for Jeffrey fluid model in a symmetric channel is examined. The problem of two-dimensional fluid motion in a symmetric channel with ciliated walls is considered. The structures of ciliary motion are stubborn by the sovereignty of viscid possessions above inertial properties by the long-wavelength and low Reynolds approximation. Exact solutions for the longitudinal pressure gradient, temperature and velocities are obtained. The pressure gradient and volume flow rate for different values of the flow parameters are also discussed. The flow property for the Jeffrey fluid is presented graphically as a function of the cilia and metachronal wave velocity.

The purpose of this work is to provide a flexible thermodynamic model based on the filling and emptying approach for the performance prediction of turbocharged compression ignition engine. To validate the model, comparisons are made between results of a developed a computer program in FORTRAN language and the commercial GT-Power software operating under different conditions. The comparisons show that there is a good concurrence between the developed program and the commercial GT-Power software. The variation of the speed of the diesel engine chosen extends from 800 RPM to 2100 RPM. In this work, we studied the influence of several engine parameters on the power and efficiency. Moreover, it puts in evidence the existence of two optimal points in the engine, one relative to maximum power and another to maximum efficiency. It is found that if the injection time is advanced, so the maximum levels of pressure and temperature in the cylinder will be high.

Icing phenomenon on a natural laminar flow airfoil (NLF-0414) has been experimentally investigated. Double horn glaze ice geometry which was acquired during a 15 minutes spray time a −2.23℃ with liquid water content and a median volumetric diameter of 1.0 g/m3 and 20 μm, has been extracted from database of NASA Lewis Research Center. Pressure distribution over airfoil surfacewas evaluated at angles of attack between -2 to 6 degreesfor both iced and clean airfoils. Aerodynamics performance degradation of the iced airfoil has been studied and it is shown that double horn ice accretion, due to its unique geometry, severely affects aerodynamic characteristics of natural laminar flow airfoils. Reattachment locations have been evaluated for upper and lower separation bubbles. The upper surface separation bubble was seen to increase in size in contrary to the lower surface separation bubble.

In this paper, a numerical investigation is carried out on mixed convection in a vertical vented rectangular enclosure filled with Al2O3-water nanofluid. The mixed convection effect is attained by heating the right wall by a constant hot temperature and cooling the cavity by an injected or sucked imposed flow. The effects of some pertinent parameters such as the Reynolds number, 100≤Re ≤ 5000, the solid volume fraction of the nanoparticles, 0≤ф≤0.1, and the aspect ratio of the cavity, 1≤A ≤ 4, on flow and temperature patterns as well as on the heat transfer rate within the enclosure are presented for the two ventilation modes. For a value of the aspect ratio A = 2, the obtained results demonstrate that the increase of volume fraction of nanoparticles contributes to an enhancement of the heat transfer and to an increase of the mean temperature within the cavity. Also, it was revealed that the fluid suction mode yields the best heat transfer performance. In the case when A is varied from 1 to 4, it was obtained that the heat transfer enhancement, using nanofluids, is more pronounced at shallow enclosures than at tall ones.

This article is intended for investigating the entropy generation analysis for the peristaltic flow of Cu-water nanofluid with magnetic field in a lopsided channel. The mathematical formulation is presented. The resulting equations are solved exactly. The obtained expressions for pressure gradient, pressure rise, temperature and velocity phenomenon are described through graphs for various pertinent parameters. The streamlines are drawn for some physical quantities to discuss the trapping phenomenon.

Recently closed loop pulsating heat pipes have been receiving much attention because of their potential applications in high heat flux micro-electronic systems. They work by self thermal driven oscillation without any mechanical parts. Though they are simple in structure, understanding of the heat transfer mechanism is highly complex having a strong thermo- hydro dynamic coupling governing their performance. In this paper, an experimental study on a closed loop PHP with a single turn has been conducted there by providing vital information regarding parameter dependence on its performance. The PHP is made of brass tube having an internal diameter of 2 mm and outer diameter of 3 mm. The parametric characterization has been done for the variation in internal diameter, fill ratio, working fluid and orientation of the device. The working fluids Acetone, Methanol, Ethanol and Propanol are considered for experimentation with volumetric filling ratios of 50%, 60%, 70% and 80%. Input heat power of 7 to 12 W is varied at the evaporator section. The CLPHP is also verified for its thermal performance at 00, 300 and 600 orientations. The transient and steady state experiments are conducted and operating temperatures are measured using K- type thermocouples. The results highlighted that the thermal performance of a PHP is strongly influenced by change in fill ratios, orientation and heat input. 80% fill ratio yields an effective heat transfer rate for a horizontal mode of operation. Appreciable fluid movement and better heat transfer rate are observed for the 300 orientation of PHP operation. Acetone exhibits better heat transport capability compared to other working fluids in all orientations.

Two-dimensional incompressible fluid flow around a rectangular shape placed over a larger rectangular shape is analyzed numerically. The vortex shedding is investigated at different arrangements of the two shapes. The calculations are carried out for several values of Reynolds numbers from low values up to 52. At low Reynolds number, the flow remains steady. The flow characteristics are analyzed for each configuration. The analysis of the flow evolution shows that with increasing Re beyond a certain critical value, the flow becomes unstable and undergoes a bifurcation. It is observed that the transition to unsteady regime is performed by a Hopf bifurcation. The critical Reynolds number beyond which the flow becomes unsteady is determined for each configuration.

The wake and internal waves of a moving three dimensional (3D) airfoil body in a stratified fluid has been investigated in a large stratified tank with a finite depth using movies of shadowgraphs of the flow fields. Typical Reynolds and Froude numbers of the flow varied between 103 and 104, and 0.3 and 2 respectively. The flows are generated often by towing the body in a uniformly stratified flow, while limited cases are carried out with body stationary and the channel was in recirculating mode. For some experiments the density profile had a stepped like shape. The wake flow is often consisted of internal waves including random and coherent ones. Distortion of density fields was also observed ahead and above the body in cases where the Froude number was subcritical. Results show that as the Froude number (Fr=U/Nh, where U is the body relative velocity, N is buoyancy frequency and h is the thickness of the body) is increased, the flow undergoes from a subcritical narrow wake (for Fr1). Typical wavelength of the exited internal waves is increased with Fr, as the theory predicts. The wake of the flow for Fr>1.4 appeared to collapse and some internal waves emission from it could be observed. Usually two types of internal waves, namely random small scale and large scale, more regular waves are observed.

Various applications of ornithopter have led to research interest in oscillation airfoils which affect on low Reynolds number flight, like; pitching oscillation, heaving oscillation and flapping of a wing. The purpose of this study is investigation of aerodynamic characteristics of NACA0012 airfoil with a simple harmonic pitching oscillation at zero and 10 degrees of mean angle of attack. Therefore the effects of unstable parameters, including oscillation amplitude up to 10 degrees, reduced frequency up to 1.0, center of oscillation up to 6/8 chord length, and Reynolds number up to 5000 have been studied numerically. A pressure based algorithm using a finite volume element method has been used to solve Navier-Stokes equations. According to results, variation of each studied parameters at mean angle of attack of 0 degree do not cause significant changes in flow phenomena on airfoil but at mean angle of attack of 10 degrees, changing in reduced frequency and specially Reynolds number cause variations in flow phenomena. These variations are because of “wake capturing” and/or “added mass” phenomena.

The aim of the present study is to investigate the superiority of steady tests simulations relative to the unsteady experiments, especially planar motion mechanism tests (PMM), for computing velocity-based hydrodynamics coefficients. Using CFD analysis, steady maneuvers including towing with drift and attack angles together with rotating arm tests are simulated in order to calculate the linear damping coefficients of a prototype submarine. Comparisons of the obtained results with available unsteady experimental results of the SUBOFF submarine show the reliability of the methods used in this paper. It also demonstrates the accuracy and simplicity of the present simulations due to the steady nature of simulations. In order to compute the linear damping coefficients, the simulations have been performed in small values of the attack and drift angles and angular velocities for the towing and rotating arm tests, respectively.

In this study, the Least Square Method (LSM) is a powerful and easy to use analytic tool for predicting the temperature distribution in a porous fin which is exposed to uniform magnetic field. Theheat transfer through porous media is simulated using passage velocity from the Darcy’s model. It has been attempted to show the capabilities and wide-range applications of the LSM in comparison with a type of numerical analysis as Boundary Value Problem (BVP) in solving this problem. The results reveal that the present method is very effective and convenient, and it is suggested that LSM can be found widely applications in engineering and physics.

Schemes to write the flow equations in discreet form, solution solvers, pre and post data processing utilities provided by OpenFoam libraries, are used to build a finite volume executable for simulating a low speed, turbulent and rate controlled diffusive CH4-Air combustion. Unsteady Favre’s averaged turbulent conservation equations (total mass, momentum, energy and species mass fractions), are used to describe the combustion gas dynamics, and to handle turbulence a modified k-e model is applied. Several global kinetic mechanisms, one step, two and four steps have been considered to describe the oxidation process of CH4 in a free jet type flame. The interaction between chemistry and turbulence, is modeled according to the partially stirred reactor (PaSR) concept. To improve convergence and accuracy in solving low speed fluid dynamic equations, a pressure implicit with splitting of operators (PISO) technique extended to cover high temperature flows, is utilized. The exponential dependence of the chemical kinetics from temperature, makes stiffs the ODE’s needed to determine source average values with which the species conservation equations are solved. To deal with the stiffness issue, OpenFoam provides numerical schemes that guaranties the stability of the computation. Comparisons between results of numerical simulations and experimental data obtained with the benchmark known as flame “D”, are presented.

The objective of this study is to determine the characteristics of hydromagnetic flow over a slendering stretching sheet in slip flow regime. Steady, two dimensional, nonlinear, hydromagnetic laminar flow of an incompressible, viscous and electrically conducting fluid over a stretching sheet with variable thickness in the presence of variable magnetic field and slip flow regime is considered. Govern- ing equations of the problem are converted into ordinary differential equations utilizing similarity transformations. The resulting non-linear differential equations are solved numerically by utilizing Nachtsheim-swigert shooting iterative scheme for satisfaction of asymptotic boundary conditions along with fourth order Runge-Kutta integration method. Numerical computations are carried out for various values of the physical parameters and their effects over the velocity and temperature are analyzed. Numerical values of dimensionless skin friction coefficient and non-dimensional rate of heat transfer are also obtained.

This article investigates the effect of magnetic field on the viscosity of Fe3O4-water magnetic nanofluid experimentally. Experiments were done in the volume fraction range 0 to 1 vol% and the temperature ranges from 25 to 45 °C. The results showed that the viscosity increased with increasing ofnanoparticle volume fractions and decreased with temperature enhancement with or without of magnetic field. Also, it is observed that the viscosity of the magnetic nanofluid increases with enhancement of magnetic field strength. Thus, magnetic field is a basic factor that influences the viscosity of the magnetic nanofluids and magnetic nanofluid flow can be controlled by applying a magnetic field.

In this paper the results obtained by the numerical method of modeling of Ekibastuz coal burning in BKZ-420 combustion chamber of Kazakhstan Power Plant are presented. They are devoted to the numerical simulation of combustion processes in the furnace boiler BKZ-420. Boiler’s steam generates capacity equal 420 T/h. Boiler has six vertical pulverized coal burners arranged in two levels with three burners on the front wall of the boiler. High ash, low-grade coal from Ekibastuz burned in the furnace. Its ash content is 40%, volatile – 24%, humidity–5%, highest calorific value is 16750 kJ/kg. Milling dispersity of coal was equal to R90 = 15%.It was shown in this research that the most intense burning is observed in the central part of the chamber where the flow temperature reaches about 980 °C and it is seen that the temperature reaches a peak in the cross sections of the burners location. The combustion reaction there occurs more intensively.

This paper proposed a new methodology which was based on computational fluid dynamics for predicting the scouring process of underwater pipeline. By redeveloping a commercial CFD computer code, the governing equations for the flow model was solved by finite volume method and wall shear stress which acted as the key parameter to judge the incipient motion of sediment was firstly calculated. Then the morphological change of the sandy bed was simulated by dynamic mesh technology. Based on the comparisons between experimental results and numerical results, it was confirmed that present numerical modeling method can simulate the flow field and scouring process around underwater pipeline accurately. Besides, the influence of gap ratios on the scour behaviors was investigated by the present simulation.

In the present research work, we introduce a new method for estimating the slip length on superhydrophobic surfaces. Hence, a dynamic force is added to momentum equations and velocity boundary condition is rewritten in a new form. Laminar and turbulent channel flows are considered and two force functions are used with different profiles to investigate their effects on results. The turbulent channel flow is considered at Re=  180 and the Large Eddy Simulation (LES) method has been applied to analyze this flow. All results indicate that this method can predict the streamwise slip length with a good accuracy, which is comparable with the Navier’s method. So, using this numerical solution and also measuring pressure drop and mass flow rate in the channel, slip length can be calculated. Consequently, the errors and difficulties of slip length measurements in typical methods such as AFM and μPIV would be eliminated.

In the present article Williamson nano fluid flow over a continuously moving surface is discussed when the surface is heated due to the presence of hot fluid under it. Governing equations have been developed and simplified using the suitable transformations. Mathematical analysis of various physical parameters is presented and the percentage heat transfer enhancement is discussed due to variation of these parameters. We employed Optimal homotopy analysis method to obtain the solution. It is presented that initial guess optimization will provide us one more degree of freedom to obtain the convergent and better solutions.

Combined effects of magnetic field and thermodiffusion (Soret effect) on natural convection within an electrically conducting binary mixture, confined in a horizontal sparsely packed porous enclosure subjected to uniform fluxes of heat and mass, is studied analytically and numerically. In the limit of a shallow enclosure, an analytical solution is derived using the parallel flow approximation. The approximate analytical solution is validated against the numerical solution of the full governing equations using a finite difference method. Interesting flow bifurcation phenomena are obtained herein and discussed. The linear stability theory and the parallel flow concept are used to determine explicitly the thresholds for the onset of stationary, subcritical and oscillatory convections as functions of the governing parameters. The obtained results showed the existence of different regions in the (N, Le) plane that correspond to different parallel flow behaviors. The number and the locations of these regions depend on the Soret parameter. The existence of a codimension-2 point is demonstrated. The effects of the Hartmann number on the fluid flow intensity and heat and mass transfer characteristics are also discussed.

Present study reports the effects of operating conditions on the mixing of two co-axial streams. Produced mixing layers between the co-axial streams are investigated numerically in the developing regions. Closed form governing equations of the mixing layer flow are solved using Fully Implicit Numerical Scheme (FINS) and Tridiagonal Matrix Algorithm (TDMA). Calculations are made for the mean and turbulence properties, and spatial mixing deficiency (SMD). Obtained results show that increase in flow width does not correspond to increase in spatial mixing while increased level of centerline velocity, centerline concentration, mean vorticity, turbulent shear stress and turbulent kinetic energy (TKE) corresponds to increase in spatial mixing.

This paper experimentally studied the dynamic behavior of a droplet impacting upon a liquid film, by investigating the effects of the droplet velocity and thickness of the liquid film on the impact behavior of the droplet. The formation of the crown, central jet, and disintegrating droplet from the central jet were visualized by time-delay photography. The time evolutions of the diameter and height of the crown and the height of the central jet were obtained, and the size of the disintegrating droplet from the central jet was measured. The crown diameter and the central jet height were mostly affected by the droplet velocity and the thickness of the liquid film, respectively, while the crown height was influenced by both the droplet velocity and the thickness of the liquid film. The diameter and height of the crown were higher for the case of the faster impacting droplet and thinner liquid film. On the other hand, the height of the central jet was higher for the case of the faster impacting droplet and thicker liquid film. The size of the disintegrating droplet from the central jet heavily depends on the velocity of the impacting droplet. Namely, a larger droplet is produced by a faster impacting droplet.

The aim of this study is to investigate the effects of ramification length and angle on pressure drop and heat transfer in a ramified microchannel. The governing equations for the fluid flow were solved by using Fluent CFD code. Computational results were compared with mathematical model values given in the literature for validation. On the basis of a water-cooled (only water and water竘媞) smooth microchannel, ramified plates were designed into the heat sink, and then the corresponding laminar flow and heat transfer were investigated numerically. Four different configurations of ramified plates were considered by adjusting the angle and length of the T profile. Results obtained from the numerical tests show good agreement with the mathematical model and these results also demonstrate that the pressure drop increases with increasing both the ramification length and angle. Moreover, the maximum temperature inside the ramified microchannel increases with increasing the ramification length as well as increasing the ratio volume fraction of ethanol.

An efficient Collocation method based on the shifted Legendre polynomials is implemented for solv- ing the Magnetohydrodynamic Hiemenz flow with variable wall temperature in a porous medium. In the presented method the need for guessing and correcting the initial values during the solution procedure is eliminated and by using the given boundary conditions of the problem astable solution can be derived. Numerical results show influence of the Prandtl number, permeability parameter,Hartmann number and suction/blowing parameter on the velocity and temperature profiles. The skin friction coefficient and the rate of heat transfer given by the Spectral Collocation method are in good agreement with those of the previous studies.

In this contribution a numerical study is carried out to analyze the effect of slip at the boundary of unsteady two-dimensional MHD flow of a non-Newtonian fluid over a stretching surface having a prescribed surface temperature in the presence of suction or blowing at the surface. Casson fluid model is used to characterize the non-Newtonian fluid behavior. With the help of similarity transformations, the governing partial differential equations corresponding to the momentum and heat transfer are reduced to a set of non-linear ordinary differential equations, which are then solved for local similar solutions using the very robust computer algebra software MATLAB. The flow features and heat transfer characteristics for different values of the governing parameters are graphically presented and discussed in detail. Comparison with available results for certain cases is excellent. The effect of increasing values of the Casson parameter is seen to suppress the velocity field. But the temperature is enhanced with increasing Casson parameter. For increasing slip parameter, velocity increases and thermal boundary layer becomes thinner in the case of suction or blowing.

The laminar boundary layer flow and heat transfer of Casson non-Newtonian fluid from an inclined (solar collector) plate in the presence of thermal and hydrodynamic slip conditions is analysed. The inclined plate surface is maintained at a constant temperature. The boundary layer conservation equations, which are parabolic in nature, are normalized into non-similar form and then solved numerically with the well-tested, efficient, implicit, stable Keller-box finite-difference scheme. Increasing velocity slip induces acceleration in the flow near the inclined plate surface. Increasing velocity slip consistently enhances temperatures throughout the boundary layer regime. An increase in thermal slip parameter strongly decelerates the flow and also reduces temperatures in the boundary layer regime. An increase in Casson rheological parameter acts to elevate considerably the velocity and this effect is pronounced at higher values of tangential coordinate. Temperatures are however very slightly decreased with increasing values of Casson rheological parameter.

This work focuses on melting heat transfer in the stagnation point flow of Jeffrey fluid past an impermeable stretching cylinder with homogeneous-heterogeneous reactions. Characteristics of magnetohydrodynamic flow are explored in presence of heat generation/absorption. Diffusion coefficients of species A and B are taken of the same size. Heat released during chemical reaction is negligible. A system of ordinary differential equations is obtained by using suitable transformations. Convergent series solutions are derived. Impacts of various pertinent parameters on the velocity, temperature and concentration distributions are discussed. Numerical values of skin friction coefficient and Nusselt number are computed and analyzed. Present results are compared with the previous published data.

To estimate the maneuverability of a submarine at the early design stage, an accurate evaluation of the hydrodynamic coefficients is important. In a collaborative exercise, the authors performed calculations on the bare hull DRAPA SUBOFF submarine to investigate the capability of viscous-flow solvers to predict the forces and moments as well as flow field around the body. A typical simulation program was performed for both the steady drift tests and rotating arm tests. The same grid topology based on multi-block mesh strategy was used to discretize the computational domain. A procedure designated drift sweep was implemented to automatically increment the drift angle during the simulation of steady drift tests. The rotating coordinate system was adopted to perform the simulation of rotating arm tests. The Coriolis force and centrifugal force due to the computation in a rotating frame of reference were treated explicitly and added to momentum equations as source terms. Lastly, the computed forces and moment as a function of angles of drift in both conditions are compared with experimental results and literature values. They always show the correct trend. Flow field quantities including pressure coefficients and vorticity and axial velocity contours are also visualized to vividly describe the evolution of flow motions along the hull.

Active flow control is experimentally investigated on a car-type bluff body. The actuation is based on a synthetic jet actuator placed at the top of the Ahmed body rear window. In the present paper, a synthetic jet characterization is presented, the frequencies and the optimal amplitudes with regard to the spatial evolution are analyzed. All the measurements are carried out in a wind tunnel at Reynolds numbers based on the body length between 106 and 3106. The bluff body shows a maximum drag reduction of 10% when optimal control is applied. Independent effect of the reduced frequency and the momentum coefficient actuation parameters on the drag reduction are also detailed in the present paper. This reduction induces changes in the flow field due to the piezoelectric actuation. The flow topology modification is investigated via particle image velocimetry measurements in order to estimate the flow response to a local excitation and to understand the mechanism involved in the aerodynamic drag control.

The shear velocity is an important parameter in characterizing the shear at the boundary in open channels and there exist methods to estimate the shear velocity in steady flows, but the application and comparison of these methods to non-uniform unsteady flows is limited. In this study, three artificial triangular shaped hydrographs were generated where the base flow is non-uniform with fine sand bed and the shear velocity was obtained by the methods, u*SV by using the Saint-Venant equations, u*L by using the procedure given by Clauser Method, u*P by using the parabolic law, u*UN by using the momentum equation assuming the slope of energy grade line is equal to bed slope and u*avg by using the average velocity equation are used in this study. The stream-wise components of velocity time series and the velocity profiles were obtained by means of an acoustic Doppler velocity meter. The variation of the shear velocity and the constant for the parabolic law with time is discussed. It is concluded that the shear velocities found by the parabolic law and the average velocity equation can be used interchangeably. Furthermore a hysteresis intensity parameter is proposed in order to examine the depth variation of hysteretic behavior of depth variation both with point velocity and average velocity. It is revealed that the more the unsteady the hydrograph the more the hysteresis both in terms of point velocity and cross-sectional mean velocity.

This paper theoretically analyzes the combined effect of slip velocity and surface roughness on the performance of Jenkins model based ferrofluid squeeze film in curved annular plates. The effect of slip velocity has been studied resorting to the slip model of Beavers and Joseph. The stochastically averaging method of Christensen and Tonders has been deployed for studying the effect of surface roughness. The pressure distribution is derived by solving the associated stochastically averaged Reynolds type equation with suitable boundary conditions, leading to the computation of load car- rying capacity. The graphical representations reveal that the transverse surface roughness adversely affects the bearing performance. However, Jenkins model based ferrofluid lubrication offers some scopes in minimizing this adverse effect when the slip parameter is kept at minimum. Of course, an appropriate choice of curvature parameters adds to this positive effect in the case of negatively skewed roughness. Moreover, it is established that this type of bearing system supports certain amount of load; even when there is no flow which does not happen in the case of conventional lubricant based bearing system.

In the present paper, the hybrid AUSM-van Leer scheme is extended to solve the governing equations of twophase condensing flows. The method of moments with the classical homogeneous nucleation theory is used to model the non-equilibrium condensation phenomenon. Firstly, the hybrid method is validated using two test cases (i.e. Laval nozzle and rotor-tip cascade) and the results are compared with the MacCormack method. Then the hybrid method is used to solve two other problems (i.e. wavy channel and VKI stage). Based on the numerical results of the paper, the hybrid AUSM-van Leer scheme is an accurate method to simulate twophase transonic flows with nucleation. If the super cooling degree reaches to its maximum value, the nonequilibrium condensation begins and wetness fraction increases suddenly. Also across a shock the wetness fraction decreases due to evaporation of the droplets.

An experimental study of stationary and non-stationary dielectric barrier discharge (DBD) plasma actuator is presented to control the flow around a NACA0024 airfoil. First, an induced air velocity of ~5 m/s is generated on a flat plate in still air using an AC-DBD actuator to find the optimal setup of the actuator (voltage, frequency, electrode width and gap size). Using the same actuator in the optimal position/setup on a NACA0024 airfoil at Reynolds number of 0.48×106, we are able to increase the stall angle of the airfoil to 18º, compared to 16º in no-actuator state. Furthermore, during the plasma actuation, the lift is increased by up to 5%. We show that non-stationary actuation, while yielding a performance similar to stationary actuation, leads to a considerable reduction of ~51% in plasma power consumption.

The effects of hall current and rotation on unsteady hydro magnetic free convection flow past an exponentially accelerated infinite vertical plate with uniform temperature and variable mass diffusion has been discussed. The flow is induced by a general time-dependent movement of the vertical plate, and the cases of ramped temperature and isothermal plates are studied. The governing partial differential equations have been derived for the velocity, temperature, concentration profiles by Laplace transform technique. The solutions that have been obtained are expressed in simple forms in terms of elementary function and complementary error function. Expressions for velocity, temperature and concentration fields are obtained. The obtained results are discussed with the effect of various parameters like Rotation parameter, Hall parameter, Hartmann number, Schmidt number, radiation parameter thermal Grashof number and mass Grashof number. The numerical values of primary and secondary velocities are displayed graphically. The temperature and concentration distributions are discussed numerically and presented through graphs.

The effects of three different nanoparticles and magnetic field on the nonlinear Jeffery Hamel flow of water based nanofluid are analyzed in the present study. The basic dimensionless governing equations are solved using series solution which are then analysed to inspect the instability of the problem by a semi-numerical analytical technique called Hermite- Padé approximation. The velocity profiles are presented in convergentdivergent channels for various values of nanoparticles solid volume fraction, Hartmann number, Reynolds number and channel angle. The dominating singularity behavior of the problem is analysed numerically and graphically. The critical relationships among the parameters are also performed qualitatively to observe the behavior of the various nanoparticles.

Three-dimensional unsteady flow field around a finite circular cylinder standing in a flat plate boundary layer is studied. For this purpose, two different numerical turbulence approaches as wall adapted local eddyviscosity LES (LES-WALE) and the zonal hybrid RANS-LES approach of Detached-Eddy Simulation (Zonal-DES) are used. Analysis is carried out for a finite circular cylinder with diameter of D = 3 mm and length-to-diameter ratio of L/D=6 which leads to the Reynolds number 2×104. Numerical simulation has been performed based on the LES-WALE and Zonal-DES turbulence models using coarse and fine grids. Abilityand accuracy of two models in capturing the complex physics of present phenomenon are investigated by comparing their results with each other and validated experimental results. Also, effect of several important parameters such as time-averaged pressure coefficient, velocity, vortex shedding frequency and performance of the LES-WALE and Zonal-DES turbulence models are studied.

In this study, an exact solution of the Navier-Stokes and energy equations is obtained for the problem of unsteady three-dimensional stagnation point flow and heat transfer of viscous, incompressible fluid on a flat plate. An external flow with strain rate a / (1-at) impinges obliquely on the flat plate when the plate is assumed to be with transpiration. This flow consists of an irrotational stagnation-point flow (Hiemenz) and a tangential component. The relative importance of these two flows is measured by a parameter . Appropriate similarity transformations are introduced, for the first time, to reduce the governing Navier-Stokes and energy equations to a coupled system of ordinary differential equations. The fourth-order Runge-Kutta method along with a shooting technique is applied to numerically solve the ordinary differential equations. The results obtained from numerical procedure are presented and discussed for a wide range of parameters characterizing the problem. The results achieved reveal that the transpiration rate has a considerable effect on the distributions of velocity components, temperature and pressure. Moreover, it is shown that the main consequence of the free stream obliqueness is to move the stagnation point away from the origin of the coordinate system.

Bladeless fan is a novel type of fan with an unusual geometry and unique characteristics. This type of fan has been recently developed for domestic applications in sizes typically up to 30cm diameter. In the present study, a Bladeless fan with a diameter of 60cm was designed and constructed, in order to investigate feasibility of its usage in various industries with large dimensions. Firstly, flow field passed through this fan was studied by 3D modeling. Aerodynamic and aeroacoustic performance of the fan were considered via solving the conservation of mass and momentum equations in their unsteady form. To validate the acoustic code, NACA 0012 airfoil was simulated in a two dimension domain and the emitted noise was calculated for Re=2×105. Good agreement between numerical and experimental results was observed by applying FW-H equations for predicting noise of the fan. To validate the simulated aerodynamic results, a Bladeless fan with a 60cm diameter was constructed and experimentally tested. In addition, the difference between the experimental and numerical results was acceptable for this fan. Moreover, the experimental results in the present study showed that this fan is capable to be designed and used for various industrial applications.

The aim of the present numerical study to analyze the conjugate natural convection heat transfer in a rotating enclosure with finite wall thickness. The enclosure executes a steady counterclockwise an- gular velocity about its longitudinal axis. The staggered grid arrangement together with the Marker and Cell (MAC) method was employed to solve the governing equations. The governing parameters considered are the wall thickness, 0:05≤D≤0:2, the conductivity ratio, 0:5≤Kr≤10 and the Taylor number, 8:9104≤Ta≤1:1106, and the centrifugal force is assumed weaker than the Coriolis force. It is found that decreasing the conductivity ratio or/and rotational speed stabilize of the convective flow and heat transfer oscillation. The global quantity of the heat transfer rate increases by increasing the conductivity ratio and it decreases about 12% by increasing 20% wall thickness for the considered rotational speeds.

The problem of an axisymmetric Stokes flow for an incompressible viscous fluid past a swarm of porous cylindrical shells with four known boundary conditions as Happel’s, Kuwabara’s, Kvashnin’s and Cunningham/Mehta-Morse’s is tackled. The Brinkman equation is taken for fluid flow through the porous region and the Stokes equation for fluid region in their stream function formulation are used. Drag force experienced by the porous cylindrical shell within a cell is evaluated. The hydrodynamic permeability of the membrane built by the porous particles is also investigated. For different values of parameters, the variation of drag force and the hydrodynamic permeability are presented graphically and discussed.

This paper proposed a numerical model aiming at coupling the MUltiple-SIze-Group (MUSIG) with the semiempirical air entrainment model based on the Euler-Euler two-fluid framework to handle the bubble transport in hydraulic jump flows. The internal flow structure including the recirculation region, the shear layer region and the jet region was accurately predicted. The flow parameters such as the water velocity and void fraction distributions were examined and compared with the experimental data, validating the effectiveness of the numerical model. Prediction of the Sauter mean bubble diameter distributions by the population balance approach at different axial locations confirmed the dominance of breakage due to the high turbulent intensity in the shear layer region which led to the generation of small gas bubbles at high void fraction. Comparison between different cases indicates that high Froude number not only give rise to longer recirculation region and higher void fraction due to larger air entrainment rate, but also generate larger bubble number density and smaller bubble size because of the stronger turbulence intensity in the same axial position.

The linear and nonlinear stability analysis of double diffusive reaction-convection in a sparsely packed anisotropic porous layer subjected to chemical equilibrium on the boundaries is investigated analytically. The linear analysis is based on the usual normal mode method and the nonlinear theory on the truncated representation of Fourier series method. The Darcy-Brinkman model is employed for the momentum equation. The onset criterion for stationary, oscillatory and finite amplitude convection is derived analytically. The effect of Darcy number, Damkohler number, anisotropy parameters, Lewis number, and normalized porosity on the stationary, oscillatory, and finite amplitude convection is shown graphically. It is found that the effect of Darcy number and mechanical anisotropy parameter have destabilizing effect, while the thermal anisotropy parameter has stabilizing effect on the stationary, oscillatory and finite amplitude convection. The Damkohler number has destabilizing effect in the case of stationary mode, with stabilizing effect in the case of oscillatory and finite amplitude modes. Further, the transient behavior of the Nusselt and Sherwood numbers are investigated by solving the nonlinear system of ordinary differential equations numerically using the Runge-Kutta method.

This paper presents a three dimensional numerical simulation of premixed methane-air low swirl stabilized flames. The computational domain has a simple geometry describing a LBS (low swirl burner) with 50mm of nozzle diameter. RANS Standard κ – ε model to treat turbulence coupled with partially premixed combustion model are used. The purpose is to show the applicability limits and their capacities to predict governing flame parameters by varying swirl intensity and CH4 mass fraction at the inlet, which shows the optimum operating area of the burner in terms of generated energy and flame stability with a particular interest to thermal NOx apparitions. This work is compared and validated with experimental and LES numerical simulation works available in the literature. Results offered good similarity for all flame studied parameters. Swirl number was varied from 0.5 to 1.0 to ensure a wide operating range of the burner. From S=0.6, we observed the onset of recirculation zones, while for the inert flow the appearance of recirculation zones was observed for S=0.9. CH4 equivalence ratio was increased from 0.6 to 1.4. That showed apparition of zones with important NOx mass fraction due to the existence of zones with high temperature. Otherwise, the flow field wasn’t disturbed in terms of recirculation zones apparitions who remained absent for all cases. Actual investigation works to find equilibrium between the maximum of generated temperature and the minimum of NOx emissions for swirled burners. Used models haven’t showed applicability limits, results were clear and precise and offered a significantly gain in computing time and means.

This investigation extends prior work on the use of perturbation techniques in the solution of unsteady boundary layer flows caused by an impulsively stretching sheet. We propose a spectral method based approach to solve the governing sequence of differential equations generated by the perturbation series approximation. The aim of this study is to demonstrate that, in contrast to conclusions drawn from previous research on this subject, the perturbation approach can be used efficiently to obtain very accurate solutions that are valid on the whole problem domain, in both dimensionless space ( 0≤ η

In the present study the fuel spray of a gasoline direct injected engine with multi-hole injector is simulated. Simulation inputs data, injection flow rate and spray cone angle are obtained from previous experimental studies. Log-normal distribution with different standard deviation is used for initial droplet size as the primary break-up model in order to reach the agreement between experimental and calculated spray tip penetration. As the first step, only one plume of spray injected into a quiescent air environment is simulated and validated by varying break-up model and standard deviation. Then, with coefficient obtained from the single jet simulation all six spray jets are simulated based on the injector nozzles geometry. The comparison between single-jet simulation and multi-jet simulation shows that validated model coefficients for the single-jet spray cannot be used for multi-jet spray simulation without significant modifications due to adjacent jet interaction and pressure drag. A set of new coefficients for the multi-jet spray is presented.

The numerical simulation of flow around a three dimensional moving body faces different problems in several methods, such as disruption of the structure of the grid, the need for deletion and insertion of nodes, interpolation, and data transfer between different parts of grid. In order to tackle the above-mentioned problems, a new configuration has been developed for meshing domain, which besides providing the body with the capability of rotational and oscillatory motions in large displacements, saves the grid’s primitive quality. In the introduced method, the grid connections are manipulated with the motion of the body, but the general form of the grid is not changed or disrupted. This needs a special form for nodes of the grid, which is explained in this paper. The three dimensional unsteady form of the Euler equations is solved and the properties over each cell faces are evaluated using an averaging method. For time integration of the equations an implicit dual time method is used. It can prove that the volume of all elements is constant in the introduced grid. Therefore, there is no need to calculate elements volume in every time step. Several test cases are solved and the results are compared with experimental or other numerical data.