Journal Of Applied Fluid Mechanics
Volume:10 Issue: 4, Jul-Aug 2017

To gain insights into ink material deposition behavior during Aerosol Jet® printing, particle deposition patterns on the plate of inertial impactor with circular laminar jet are investigated numerically with a lagrangian solver implemented within the framework of the OpenFOAM® CFD package. Effects of taper angle of the nozzle channel and jet-to-plate distance are evaluated. The results show quite different particle deposition patterns between tapered nozzle and straight nozzle. At jet Reynolds number Re = 1132, a tapered nozzle deposits particles to form a pattern with a high density ring toward the deposition spot edge, especially when the particle Stokes number St > St50, which is absent with a straight nozzle. Increasing the jet-to-plate distance tends to reduce such particle density peak. Reducing Re to 283 yields particle deposition patterns without the high density ring near the spot edge, with the same tapered nozzle. The particle deposition patterns with the straight nozzle at Re = 283 exhibit further reduced particle density around the spot edge such that the particle density profile appears more like a Gaussian function. In general, the effect of reducing Re on particle deposition pattern seems to be similar to increasing the jet-to-plate distance. The computed particle deposition efficiency η shows the fact that those particles around the jet axis, even with very small values of St, always impact the center of plate, as indicated by the nonvanishing value of η with substantial reduction of St. Such a “small particle contamination” typically amounts to ~10% of small particles (with St

An exhaustive investigation of the structure of the turbulence around an asymmetric FX 63-137 wind turbine airfoil is carried out in this paper. Reliable hot-wire velocity measurements, made at the Xixon Aeroacoustic Wind Tunnel, are presented with the aim of analyzing the turbulent flow features. The probe was placed at two different positions along the streamwise direction, one over the airfoil and the other at the wake, both on the suction and pressure side. These measurements were performed in order to capture the evolution of the flow and its behavior at the wake. The experimental data were collected at a Reynolds number of 350000 for several incidence angles to explore their influence in the turbulence characteristics. The data processing from the dual hot-wire, capable of measuring two velocity components, allowed to achieve half set of the Reynolds stresses, the turbulence intensity and the degree of anisotropy. The boundary layer and wake size were estimated from the Reynolds stress components. In addition, the production term of the turbulence kinetic energy budget is calculated to visualize the unsteadiness energy inside the boundary layer. As a result of these analyses, it was observed that the transversal fluctuations were higher than the longitudinal ones. Besides, an alternative description of the turbulence structure is obtained when a frequency analysis of the motion is provided, disclosing a clear change in the spectra tendencies in the wake and boundary layer regions. This analysis, combined with the degree of anisotropy analysis, was helpful to define a transition zone between the clearly distinguishable instability zone and the free-stream zone. Finally, the integral length scale of turbulence was estimated from the area under the autocorrelation function of the velocity fluctuations. The combination of the results of this work have provided a wide description of the turbulent behavior of the flow around the airfoil and present a clearer physical picture of the phenomena.

In this paper, numerical investigation of three-dimensional, developing turbulent flow, subjected to a moderate adverse pressure gradient, has been investigated using various turbulence models, namely: the low- Re k- E, the SST k -W , the v2- f and a variant of Reynolds stress model. The results are compared with the detailed velocity and pressure measurements. Since the inlet condition is uncertain, a study was first performed to investigate the sensitivity of the results to the inlet boundary condition. The results showed the importance of including the contraction effects. It is seen that the developing flow inside the straight duct, is highly sensitive to the inlet boundary condition. The comparisons indicate that all turbulence models are able to predict a correct trend for the centerline velocity and pressure recovery inside the straight duct and diffuser but the low-Re k - E and RSM turbulence models yield more realistic results. The SST k - W model largely overpredicts the centerline velocity and boundary layer thickness in the straight duct. The comparisons of the numerical results also revealed that the RSM model, due to its anisotropic formulation, is able to reproduce the secondary flows. As expected, the RSM model demonstrates the best performance in prediction of the flow field and pressure recovery in the asymmetric diffuser.

The rotor-stator interaction and the corresponding pressure fluctuations represent one of the sources of pressure and load fluctuations on the rotating parts of rotating machineries. The high Reynolds flow is subject to rotation in the comparably large vaneless space of axial turbines, causing wake interaction and wake dissipation in this region. This increases the level of flow complexity in this region. This study examined the effect of the flow condition entering the spiral casing on the flow condition within the distributor and the runner and the physical source of pressure fluctuations exerted on the runner of a Kaplan turbine model. Simulations were performed within the water supply system, including the upstream tank, penstock, and the Francis turbines, the level of entering the spiral casing; the results were compared with laser Doppler anemometry (LDA) results. The results were considered as the inlet boundary condition for simulation of the turbine model from the spiral inlet to the draft tube outlet to investigate the flow condition within the distributor and the runner. The CFD simulations showed that the water supply system induces inhomogeneity to the velocity distribution at the spiral inlet. However, the flow condition does not affect the pressure fluctuations exerted on the runner blades due to the rotor-stator interactions. Moreover, the dominant frequencies exerted on the runner blades were accurately approximated although the amplitudes of the fluctuations were underestimated.

The use of micro shock tubes has become common in many instruments requiring a high velocity and temperature flow field, for example in micro-propulsion systems and drug delivery devices for medical systems. A shock tube has closed ends, and the flow is generated by the rupture of a diaphragm separating a driver gas at high pressure from a driven gas at relatively low pressure. The rupture results in the movement of a shock wave and contact discontinuity into the low-pressure gas, and an expansion wave into the high pressure gas. The characteristics of the resulting unsteady flow for micro shock tubes are not well known as the physics of such tubes includes additional phenomena such as rarefaction and complex viscous effects at low Reynolds numbers. In the present study, computational fluid dynamics (CFD) calculations are made for unsteady compressible flow within a micro shock tube using the van-Leer MUSCL scheme and the two-layer 􀝇-􀟝 turbulence model. Novel results have been obtained and discussed of the effects of using different diaphragm pressure ratios, shock tube diameters and wall boundary conditions, namely no slip and slip walls.

This study proposes a semi-analytic approximation to the laminar boundary layer growth in a polarized pressure field with temperature gradient represented by the joint Blasius-energy equation. We illuminate that f'' ( ) is a probability density function (PDF) approximated by an amended Gaussian PDF with zero mean and standard deviation   2.18 . This implies a diffusive structure for the molecular momentum conversion as well as the energy flux in the boundary layer. A new limit for the boundary layer edge is also presented. Results suggest an augmented boundary layer when compared to accepted values in the literature. We also reproduce the inverse proportionality of the free stream velocity to the diffusion of both momentum and energy.

Cavitation breakdown at various liquid temperatures is one of the major problems encountered in the operations of centrifugal pumps. There are a number of practical cases where the pumps operate at high temperature or near the saturation temperature of the liquid. A detailed understanding of the factors affecting the breakdown of cavitation is essential for accurate performance prediction and design. The purpose of this paper is to present results of a cavitation breakdown investigation which are both experimental and theoretical. The present model based on Rayleigh-Plesset expression for bubble dynamics. The predicted model includes many important parameters controlling the cavitation breakdown such as bubble dynamics, flow rate, rotational speed, temperature and thermodynamics properties of water and the gas pressure inside the cavity. The present model has been tested against extensive present and earlier published experimental results in centrifugal pumps at various operating water temperatures and operating conditions. The comparison between the predicted breakdown blade cavitation number with the present and previous published experimental results showed a surprisingly good agreement. This agreement means that the roles played by many important parameters are consistent with the present model. Therefore, the present model represents an addition to knowledge in this aspect which could help the centrifugal pump user and designer to predict the breakdown cavitation performance at various operating conditions.

Three-dimensional particulate flow has been simulated using Lattice Boltzmann Method (LBM). Solid-fluid interaction was modeled based on Smoothed Profile Method (SPM) (Jafari et. al, Lattice Boltzmann method combined with smoothed-profile method for particulate suspensions, Phys. Rev. E, 2011). In this paper a GPU code based on three-dimensional lattice Boltzmann method and smoothed profile method has been prepared due to the ability of SPM-LBM to perform locally and in parallel mode. Results obtained for sedimentation of one and two spherical particles as well as their behavior in shear flow showed excellent correspondence with previous published works. Computations for a large number of particles sedimentation showed that combination of LBM and SPM on a GPU platform can be considered as an efficient and promising computational frame work in particulate flow simulations.

In this paper, we examine the combined effects of magnetic field, thermal radiation, heat source, velocity slip and thermal jump on peristaltic transport of an electrically conducting Walters-B fluid through a compliant walled channel. Using small wave number approach, the nonlinear model differential equations are obtained and tackled analytically by regular perturbation method. Expressions for the stream function, velocity, temperature, skin-friction coefficient and heat transfer coefficient are constructed. Pertinent results are presented graphically and discussed quantitatively. It is found that the velocity distribution depresses while the fluid temperature rises with an increase in Hartmann number. The trapping phenomenon is observed and the size of trapped bolus increases with an increase in Hartmann number.

Combined solutal and thermal buoyancy–thermocapillary convection in a square open cavity is studied numerically in the present article. The Forchheimer–Brinkman-extended Darcy model is used in the mathematical formulation for the porous layer and the COMSOL Multiphysics software is applied to solve the dimensionless governing equations. The governing parameters considered are the thermal Marangoni number, −1000 ≤ Ma_T ≤ 1000, the Darcy number, 10−5 ≤ Da ≤ 10−2, the porosity of porous medium, 0.4 ≤ ε ≤ 0.99 and the Lewis number, 10 ≤ Le ≤ 200. It is found that the global heat and solute transfer rate decreases by reducing the counteracting surface tension force and increases by augmenting the surface tension force. The minimum values of the global heat and solute transfer rate were obtained about Ma_T = −90 for the all porosities.

A Numerical study is carried out to investigate the effect of Rayleigh number with rotation on the flow and heat transfer characteristics in a differentially heated enclosure rotating about the horizontal axis. A Fortran Code developed based on FVM is used to discretize governing equations. Upwind difference scheme for convective terms and fully implicit scheme for transient terms are used. The SIMPLE algorithm is employed to couple pressure and velocities on staggered grid arrangement. The results were obtained for a Taylor number(103 ≤ Ta ≤ 105), rotation (10 rpm ≤ Ω ≤ 25 rpm), and Rotational Rayleigh number (101 ≤ Raw ≤ 103) for two different Rayleigh number (1.3 × 104 & 1.1 × 105) with fixed Prandtl number (pr = 0.71). The results showed that the Coriolis force first tends to decrease heat transfer to a minimum and then starts to increase it with increase Rayleigh number and rotation. Minimum depends on Rayleigh number and corresponds to the balanced effects of interacting forces at the point of transition. At rotations, below minimum in average heat transfer, the circulations are counter clockwise. The direction of coriolis force is from core region, so both flow and heat transfer is reduced. When coriolis force is much larger than thermal buoyancy, motion is clockwise, and transition is prevented. coriolis force now tends to promote flow circulation and therefore increases the heat transfer. The frequency content of flow pattern reveals the structural changes in the flow and temperature fields with increasing Rayleigh number and rotation. The existence of different flow regimes dominated by these body forces complicates the time average heat transfer characteristics with a different behaviour in each of the regimes.

Smoke temperature distribution in non-smoke evacuation under different mechanical smoke exhaust rates of semi-transverse tunnel fire were studied by FDS numerical simulation in this paper. The effect of fire heat release rate (10MW 20MW and 30MW) and exhaust rate (from 0 to 160m3/s) on the maximum smoke temperature in non-smoke evacuation region was discussed. Results show that the maximum smoke temperature in non-smoke evacuation region decreased with smoke exhaust rate. Plug holing was observed below the smoke vent when smoke exhaust rate increased to a certain value. Smoke spreading distance can be divided into three stages according to changes of smoke exhaust rate. The maximum smoke temperature model concluded that the peak temperature rise at tunnel vault is proportional to 0.75 power of dimensionless fire power. The maximum temperature in non-smoke evacuation region decays exponentially with the increase of smoke exhaust rate. However smoke vent interval influences the dimensionless maximum temperature in nonsmoke evacuation region slightly. Smoke vent interval influences the dimensionless maximum temperature in non-smoke evacuation region slightly.

In the present study, the unsteady magnetrohydrodynamic (MHD) flow of compressible fluid with variable thermal properties has been numerically investigated. The electrically conducting fluid flows through a porous media channel. The uniform magnetic field is applied perpendicular to the direction of the flow. The wall is assumed to be non-conducting and maintained at two different temperatures. The thermal conductivity and viscosity of the fluid change with temperature. Sixth - Order Accurate Compact Finite Difference scheme together with the Third-order Runge-Kutta method is used to solve a set of non-linear equations. The results of the calculation are expressed in the form of the velocity and temperature at different values of the magnetic field and porosity. The proposed mathematical model and numerical methods have been validated by comparing with the results of previously published studies that the compared results reveal the same trends. The difference is due to the compressibility and property variation effects. The results showed that the magnetic field and variable properties considerably influences the flows that is compressible thereby affecting the heat transfer as well as the wall shear stress.

Drag reduction phenomena can be obtained using additive polymer that can generate turbulence damping by fluid movement and characteristic. Measurements were carried out to investigate pressure losses in square and rectangular horizontal ducts coating of additive Agar solution, with aspect ratios (e) = 1.0 and 0.5, respectively. The increment concentrations are ranged up to 2 times and drag reduction effect was obtained up to 1.5 times bigger. This research analysis was done using friction coefficients and Reynolds numbers relation to put forward drag reduction phenomena. The effect of Agar coating delayed the transition regime. The research results using 1 mm of thickness Agar coating with concentration 20% and 40% concentrations were obtained maximum drag reduction phenomena about 19% at the Reynolds number about 2,600.

Cyclones are one of the most widely used gas-solid separators in circulating fluidized bed (CFB) systems. This paper focuses on numerical study of the gas-solid flow in a cyclone attached to the CFB system. The objective was to understand the flow pattern in the cyclone in order to run the CFB setup problem free. The previous works on cyclone separators do not include critical parameter such as coefficient of restitution which is responsible for swirling effect and increase in efficiency. Reynolds stress model (RSM) is used to obtain the gas flow characteristics. The resulting flow and pressure fields are verified by comparing with the measured experimental results and then used in the determination of solids flow that is simulated by the use of a discrete phase model. The simulation results show how the particle trajectories and cyclone efficiency change with varying coefficient of restitution and particle size keeping inlet velocity of gas and mean particle diameter constant. The separation efficiency, pressure drop and particle trapping time from the numerical analysis are shown to be comparable to those observed experimentally. The velocity distribution pattern obtained from the analysis exhibits strong flow recirculation with large turbulent eddies in the cyclone separator. The particle trajectories depend upon relative velocity of fluid/particles and concentration of particles. Efficiency of the cyclone is found to be dependent on particle size and coefficient of restitution. The results obtained are further utilized to optimize the velocity range of gas flow in the loop seal and riser for stable operation of CFB setup.

In studies of self-aspirating impellers found that gas bubbles are not broken down by the impeller blades. Breakup of bubbles is caused by the eddies generated by the blades. Therefore, to describe how the liquid flow near the blades is an important research issue for this type of impellers. Using the PIV method average velocity fields in the axial-radial plane between baffles in the stirred tank were defined for seven different positions of blades of a self-aspirating disk impeller in relation to that plane. It was found that in the small space in blade vicinity, big changes in fluid circulation were observed depending on the position of the blade relative to the baffle. In front of blade the liquid from bottom and from over impeller is directed radially towards the wall of tank and the average axial velocity is zero. Behind the blade the cavern (cavity) is formed, understood as a space of reduced pressure. Underpressure causes suction effect which directs the liquid inside the cavern. In just a few millimeters from the blade tip average axial and radial velocities are equal to zero. In this region the tangential component of velocity is dominant.

In order to promote the windbreak effect of the earth embankment type windbreak wall, enhance the operational speed of the single passenger train and improve the quality of the pantograph-catenary current collection for a locomotive, a three-dimensional RANS turbulence model k-epsilon was used to optimize the shape of windbreak walls. The relationships between the overturning moment of trains, the lateral wind speed at the catenary position and the height (depth) in optimization projects were analyzed. Validation was performed against full-scale experimental data. To understand the flow field around the train with different types of windbreak walls, pressure contours and surface pressure coefficient distributions were investigated. The results show that for the original type windbreak wall, the overturning moment of the passenger car is a little larger. However, for the optimization projects, the trains are basically in a minor negative pressure environment and the aerodynamic forces are much less. The optimal heights of the heightening type (depths for the cutting type) do not change obviously as the train speed increases. When the passenger car stands on the track without movement, the optimal height/depth is the smallest. Behind the original type’s windbreak wall, the lateral wind speed at the catenary position on the leeward line is less than that on the windward line. Meanwhile, as the train runs on the windward or leeward lines, the corresponding lateral wind speed rise sharply by 37.5% and 40.5%, respectively. After the adoption of optimized projects, the speeds of the two lines monotonically decrease. The best height of the heightening type is 0.30 m, and the optimal depth of the cutting type is 1.40 m. From the perspective of engineering application, the heightening type is a more suitable project.

Investigation of the effects blade pitch angle on noise emission from a horizontal axis wind turbine is the goal of this paper. To understand the flow around blade wind turbine, and to reduce noise emission in order to respect noise regulation, especially a residential area , three different pitch angles 0°, 3°, and 6° are tested, using computational aerodynamic and aero acoustic methods. Three dimensional flow simulations are carried out with two unsteady CFD simulations URANS, DES used to calculate the near-field flow around a HAWT of NREL Phase VI small scale. The far field noise is predicted from the simulated sources by the Ffowcs William and Hawkings analogy, and compared and validated with available test data for a small scaled model of the NREL Phase VI. The comparison demonstrates a generally good agreement between DES predicted and measured noise levels. It can be seen that the noise emission increases by decreasing pitch angle. Moreover, the pitch angle control has a significant effect on the noise emission especially in the intermediate frequency range. We show that it is possible to reduce the noise level by control pitch angle without losing too much the power.

A turbulent plane jet impinging on a slotted surface is simulated using Large Eddy Simulation LES. The Reynolds number, based on the jet-exit velocity and width, is equal to 5435. The slotted surface is placed at a distance equal to four times the jet-exit width. Three computational grids were used to assess the accuracy of the LES simulations conducted. The interaction effects of the jet with the slot propagate away from the slot region and manifest into pressure perturbations. Interesting phenomena were observed when linking the dynamic flow features upstream and downstream of the slotted surface. LES predicted three dominant frequencies at different points from time signals of velocities and pressure. The dominant frequency of the pressure field, away from the slot, corresponds to that of coherent vortices which follow a trajectory that is far from being deviated towards the wall jet or into the slot of the impingement wall completely. Among these turbulent structures of interest, pairs of opposite, but in phase, vortices are responsible for promoting the occurrence of the throttling phenomenon. The characteristic frequencies of the pressure field are similar upstream and downstream of the impingement wall. The peaks of the fluctuating pressures, away from the slot, correlate well with the minimum flow rate through the slot which correspond to the throttling phenomenon.

Experimental and numerical investigation of multihole gasoline direct injection (GDI) sprays at high injection pressure and temperature are performed. The primary objective of this study is to analyse the role of gas entrainment and spray plume interactions on the global spray parameters like spray tip penetration, spray angles and atomization. Three-hole 90° spray cone angle and six-hole 60° spray cone angle injectors are used for current work to examine the effect of the geometry of the injector on the spray interactions. The numerical results from Reynolds Average Navier Stokes (RANS) simulations show a reasonable comparison to experiments. The simulations provide further insight to the gas entrainment process highlights the fact that a stagnation plane is formed inside the spray cone which basically governs the semi collapse of spray that in turn affects the spray direction and cone angle.