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research-article  
Maxime Rosello, Guillaume Maîtrejean, Denis C D Roux, Pascal Jay, Bruno Barbet and Jean Xing
J. Fluids Eng   doi: 10.1115/1.4037691
In the present work, the influence of nozzle shape on microfluidic ink jet breakup is investigated. First, an industrial ink used in continuous inkjet printing devices is selected. Ink rheological properties are measured to ensure an apparent Newtonian behavior and a constant surface tension. Then, breakup lengths and shapes are observed on a wide range of disturbance amplitude for four different nozzles. Later on, ink breakup behaviors are compared to the linear theory. Finally, these results are discussed using numerical simulations to highlight the influence of the velocity profiles at the nozzle outlet. Using such computations, a simple approach is derived to accurately predict the breakup length for industrial CIJ nozzles.
research-article  
Sofía S. Sarraf, Ezequiel Jose Lopez, Laura Battaglia, Gustavo A. Rios Rodriguez and Jorge D'Elia
J. Fluids Eng   doi: 10.1115/1.4037690
In the Boundary Element Method (BEM), the Galerkin weighting technique allows to obtain numerical solutions of a Boundary Integral Equation (BIE), giving the Galerkin Boundary Element Method (GBEM). In three-dimensional (3D) spatial domains, the nested double surface integration of GBEM leads to a significantly larger computational time for assembling the linear system than with the standard collocation method. In practice, the computational time is roughly an order of magnitude larger, thus limiting the use of GBEM in 3D engineering problems. The standard approach for reducing the computational time of the linear system assembling is to skip integrations whenever possible. In this work, a modified assembling algorithm for the element matrices in GBEM is proposed for solving integral kernels that depend on the exterior unit normal. This algorithm is based on kernels symmetries at the element level and not on the flow nor in the mesh. It is applied to a BIE that models external creeping flows around 3D closed bodies using second-order kernels, and it is implemented using OpenMP. For these BIEs, the modified algorithm is on average 32% faster than the original one.
research-article  
Mehdi Esmaeilpour, J. Ezequiel Martin and Dr. Pablo Carrica
J. Fluids Eng   doi: 10.1115/1.4037693
The dead water problem, in which under certain conditions a vessel advancing in a stratified fluid experiences a considerable increase in resistance respect to the equivalent case without stratification, was studied using computational fluid dynamics (CFD). The advance of a vessel in presence of a density interface (pycnocline) results in the generation of an internal wave that in the most adverse conditions can increase the total resistance coefficient by almost an order of magnitude. This paper analyses the effects of stratification on total and friction resistance, the near field wake, internal and free surface waves, and resistance dynamics. Some of these effects are reported for the first time, as limitations of previous efforts using potential flow are overcome by the use of a viscous, free surface CFD solver. A range of densimetric Froude numbers from subcritical to supercritical are evaluated changing both the ship speed and pycnocline depth, using as platform the Research Vessel Athena. It was found that the presence of the internal wave causes a favorable pressure gradient, acceleration of the flow in the downstream of the hull, resulting in thinning of the boundary layer and increases of the friction resistance coefficient of up to 30%. The total resistance presents an unstable region that results in a hysteretic behavior, though the characteristic time to establish the speed-resistance curve, dominated by the formation of the internal waves, is very long and unlikely to cause problems in modern ship speed controllers.
TOPICS: Computational fluid dynamics, Water, Vessels, Waves, Flow (Dynamics), Friction, Ships, Surface waves (Fluid), Pressure gradient, Hull, Fluids, Control equipment, Wakes, Boundary layers, Density, Dynamics (Mechanics)
research-article  
Ramesh A S and Joseph Sekhar S
J. Fluids Eng   doi: 10.1115/1.4037692
In this experimental study, the impact of Suction Chamber Angle (SCA) on the entrainment ratio of a steam ejector refrigeration system of 700 W was investigated. The basic dimensions of the ejector were derived from the compressible fluid flow equations using MatLab. The system was tested with 6 different suction chamber angles with various operating conditions, and its performance was analysed. It is inferred that the entrainment of passive fluid from the evaporator is the strong function of the SCA. For all the active steam pressures, the entrainment of the passive fluid increased up to 12° of SCA and above that the performance decreased significantly. Optimum angle of suction chamber increases the entrainment ratio for at least 49.96%. It is also found that the SCA has a minor influence on the back pressure.
research-article  
Anton Silvestri, Farzin Ghanadi, Maziar Arjomandi, Benjamin S. Cazzolato and Anthony C. Zander
J. Fluids Eng   doi: 10.1115/1.4037675
In the present study, the optimal 2D tripping technique for inducing a naturally fully developed turbulent boundary layer in wind tunnels has been investigated. Various tripping techniques were tested, including wires of different diameters and changes in roughness. Experimental measurements were taken on a flat plate in a wind tunnel at a number of locations along the flat plate and at a variety of flow speeds using hotwire anemometry to measure the boundary layer resulting from each tripping method. The results have demonstrated that to produce a natural turbulent boundary layer using a 2D protuberance, the height of the trip must be less than the undisturbed boundary layer thickness. Using such a trip was shown to reduce the development length of the turbulent boundary layer by approximately 50%. This was shown to hold true for all Reynolds numbers investigated (Re_x=1.2×?10?^5-Re_x=1.5×?10?^6). The present study provides an insight into the effect of the investigated trip techniques on the induced transition of a laminar boundary layer into turbulence.
research-article  
Nima F. Jouybari, Mehdi Maerefat, Staffan Lundstrom, Majid Eshagh Nimvari and Zahra Gholami
J. Fluids Eng   doi: 10.1115/1.4037677
The present study deals with the generalization of a macroscopic turbulence model in porous media using a capillary model. The additional source terms associated with the production and dissipation of turbulent kinetic energy due to the presence of solid matrix are calculated using the capillary model. The present model does not require any prior pore scale simulation of turbulent flow in a specific porous geometry in order to close the macroscopic turbulence equations. Validation of the results in packed beds, periodic arrangement of square cylinders, synthetic foams and longitudinal flows such as pipes, channels and rod bundles against available data in the literature reveals the ability of the present model in predicting turbulent flow characteristics in different types of porous media. Transition to the fully turbulent regime in porous media and different approaches to treat this phenomenon are also discussed in the present study. Finally, the general model is modified so that it can be applied to lower Reynolds numbers below the range of fully turbulent regime in porous media.
research-article  
Apoloniusz Kodura, Pawel Stefanek and Katarzyna Weinerowska-Bords
J. Fluids Eng   doi: 10.1115/1.4037678
Fine solid materials can be transported with the use of water as a carrier liquid. From the practical point of view the economy of designing and maintenance is usually the most important factor. That way of transport has a lot of advantages for many industry processes. However, the problems of pressure flow are more complicated for slurries than for liquids. The transient flow is one of the most difficult problem to describe. A deep analysis of transients in slurries is crucial, both theoretical and practical. In this paper, the analysis of the transient flow in HDPE pressure pipelines is described. At the first stage a laboratory model was build. Experiments made for different volume concentrations were performed. The results were used to build a numerical model of transient flow, which was the second stage of investigation. Due to relatively difficult description of the volumetric concentration bottom layer depth, these parameters vary in time and volume of slurry, an alternative approach was proposed. The equivalent density was introduced to express unknown parameters. Performed numerical simulations lead to promising results. In all analyzed episodes the calculated pressure characteristics demonstrated satisfactory coincidence with observations.
research-article  
Boukenkoul Mohammed Amin, Feng-Chen Li, Wenli Chen and Hongna Zhang
J. Fluids Eng   doi: 10.1115/1.4037681
Despite the big interest in both, micro air vehicles (MAV) and flow-control strategies, only few studies have investigated the flow-control possibilities over low aspect ratio (LAR) wings flying at low Reynolds numbers (Re). The present study verified the LAR thick airfoils' conformity with the nonlinear lift approximation equation. Then, a moving-wall flow control method was designed and tested over an LAR thick airfoil (0.57 AR, NACA0015 shaped) performing at a chord-based Re of 4×104. The moving belt control postponed the stall onset by 25 degrees and produced a 103% gain in lift without any saturation signs at a control speed ratio of Ub/U = 6. PIV measurements confirmed the effectiveness of the moving-wall control strategy on the upper surface flow reattachment. Moreover, other quantities such as the boundary layer, vortices and the swirling strength are investigated.
TOPICS: Flow control, Airfoils, Belts, Swirling flow, Wings, Flow (Dynamics), Reynolds number, Chords (Trusses), Boundary layers, Vehicles, Vortices, Approximation
research-article  
Thunivumani G and Hrishikesh Gadgil
J. Fluids Eng   doi: 10.1115/1.4037676
An experimental study was conducted to investigate the breakup of a liquid sheet produced by oblique impingement of a liquid jet on a plane solid surface. Experiments are carried out over a wide range of jet Weber number (80-6300) and various jet impingement angles (30, 45 and 60 degrees) are employed to study the sheet dynamics. The breakup of a liquid sheet takes place in three modes, closed rim, open rim and perforated sheet, depending upon the Weber number. The transitions across the modes are also influenced by the impingement angle with the transition Weber number reducing with increase in impingement angle. A modified regime map is proposed to illustrate the role of impingement angle in breakup transitions. A theoretical model based on force balance at the sheet edge is developed to predict the sheet parameters by taking the shear interaction between the sheet and the solid surface into account. The sheet shape predicted by the model fairly matches with the experimentally measured sheet shape. The breakup length and width of the sheet are measured and comparisons with the model predictions show good agreement in closed rim mode of breakup.
TOPICS: Dynamics (Mechanics), Shapes, Shear (Mechanics)
research-article  
Niraj Shah, Abhimanyu Gavasane, Dr. Amit Agrawal and Upendra Bhandarkar
J. Fluids Eng   doi: 10.1115/1.4037679
Three dimensional Direct Simulation Monte Carlo (DSMC) has been used to simulate flow in a straight microchannel using an in-house parallelized code. In the present work a comparative study of seven boundary conditions is carried out with respect to time required for achieving steady state, accuracy in predicting the specified pressure at the boundaries, and the total simulation time required for attaining a statistical error within one percent. The effect of changing the Knudsen number, pressure ratio and cross aspect ratio on these parameters is also studied. The presence of a boundary is seen to affect the simulated pressure in a cell when compared to the specified pressure; the difference being highest for corner cells and least for cells away from walls. All boundary conditions tested work well at the inlet boundary however similar results are not obtained at the outlet boundary. For the same cell size, the schemes that employ first- and second-order corrections lead to a smaller pressure difference compared to schemes applying no corrections. The best predictions can be obtained by using first-order corrections with finer cell size close to the boundary. For most of the simulated cases, the boundary condition employing the characteristic scheme with non-equilibrium effect leads to the minimum simulation time. Considering the non-equilibrium effect, prediction of inlet and outlet pressures and the speed of simulation, the characteristic scheme with non-equilibrium effect performs better than all the other schemes, at least over the range of parameters investigated herein.
TOPICS: Simulation, Boundary-value problems, Pressure, Equilibrium (Physics), Corners (Structural elements), Knudsen number, Errors, Steady state, Microchannels, Flow (Dynamics)
research-article  
Zhenglun Alan Wei and Zhongquan Charlie Zheng
J. Fluids Eng   doi: 10.1115/1.4037661
This study investigates energy harvesting of a two-dimensional foil in the wake downstream of a cylinder. The foil is passively mobile in the transverse direction. An immersed-boundary method with a fluid-structure interaction model is validated and employed to carry out the numerical simulation. For improving numerical stability, this study incorporates a modified low-storage 3rd-order Runge-Kutta scheme for time integration and demonstrates the performance of this temporal scheme on reducing spurious pressure oscillations of the immersed-boundary method. The simulation shows a foil, as the energy harvester, emerged in a vortical wake achieves better energy harvesting performance than that in a uniform flow. The types of the dynamic response of the energy harvester are identified, and the periodic response is desired for optimal energy harvesting performance. Last, the properties of vortical wakes are found to be of pivotal importance in obtaining this desired periodic response.
research-article  
James Sucec
J. Fluids Eng   doi: 10.1115/1.4037523
The integral form of the equation for x momentum is solved for the skin friction coefficient on surfaces whose technical roughness elements' size is given. This is done by using a "roughness depression function" in the Law of the Wall and Wake which serves as the needed velocity profile. The method uses the equivalent sand grain size concept in its calculations. Predictions are made of the friction coefficient, Cf, as a function of momentum thickness Reynolds number and also, of Cf's dependence on the ratio of momentum thickness to the size of the technical (actual) roughness elements. In addition, boundary layer thicknesses and velocity profiles on rough surfaces are calculated and, when available, comparisons are made with the experimental data from a number of sources in the literature. Also, comparisons are made with the results of another major predictive scheme which does not use the equivalent sand grain concept.
TOPICS: Turbulence, Surface roughness, Momentum, Sands, Friction, Reynolds number, Skin friction (Fluid dynamics), Wakes, Boundary layers, Grain size
research-article  
Aldona Skotnicka-Siepsiak
J. Fluids Eng   doi: 10.1115/1.4037522
For scientist, the Coanda effect has been an object of interest for a long time. All the time, some new applications of it are found although it has been more than a hundred years since Henri Coanda got a patent that was critical for that issue. Apart from aviation, it is more and more often used in ventilation systems to control to manner of air division and to design nozzles and ventilators. It is surprising, however, that a good command of that phenomenon and a need to apply it in different solutions did not entail a significant increase of the interest in the Coanda effect hysteresis, although it was mentioned for the first time by B. G. Newman in 1961. The article below presents results of experimental measurements for a two-dimensional incompressible plane jet by an inclined plate. The hysteresis has been observed as a different jet behaviour (a free jet or a jet attached to a flat plate) depending on the direction in which the plate deflection angle changes. The observed hysteresis area, defined by critical values for the ?ca attachment and ?cd detachment angles, spanned from 8° to 14°. Its dependency on the Reynolds number has also been examined for Re ranging from 3,500 to 26,500. Considering the Coanda effect hysteresis, a pressure distribution on the plate and the xR reattachment distance have been examined. The distribution of forces on a plate has been identified, which has facilitated a graphical mirroring of the Coanda effect hysteresis loop.
TOPICS: Pressure, Reynolds number, Ventilation equipment, Design, Nozzles, Deflection, Flat plates, Patents, Aviation
research-article  
Gabriel Naccache and Marius Paraschivoiu
J. Fluids Eng   doi: 10.1115/1.4037490
Small Vertical Axis Wind Turbines (VAWTs) are good candidates to extract energy from wind in urban areas because they are easy to install, service and do not generate much noise; however, the efficiency of small turbines is low. Here-in a new turbine, with high efficiency, is proposed. The novel design is based on the classical H-Darrieus VAWT. VAWTs produce the highest power when the blade chord is perpendicular to the incoming wind direction. The basic idea behind the proposed turbine is to extend that said region of maximum power by having the blades continue straight instead of following a circular path. This motion can be performed if the blades turn along two axes; hence it was named Dual Vertical Axis Wind Turbine (D-VAWT). The analysis of this new turbine is done through the use of Computational Fluid Dynamics (CFD) with 2D and 3D simulations. While 2D is used to validate the methodology, 3D is used to get an accurate estimate of the turbine performance. The analysis of a single blade is performed and the turbine shows that a power coefficient of 0.4 can be achieved. So far, reaching performance levels high enough to compete with the most efficient VAWTs. The D-VAWT is still far from full optimization, but the analysis presented here shows the hidden potential and serves as proof of concept.
TOPICS: Computational fluid dynamics, Vertical axis wind turbines, Turbines, Blades, Wind, Cities, Simulation, Noise (Sound), Chords (Trusses), Design, Engineering simulation, Optimization
research-article  
Marc Sanchez, Adrien Toutant and Françoise Bataille
J. Fluids Eng   doi: 10.1115/1.4037507
Hybrid low pressure air extractors are an economic way to enhance indoor air quality. The evaluation of their energetic performances needs the analysis of flow parameters that is typically done with wind tunnel data and numerical simulations. The purpose of this study is to analyse, numerically and experimentally, the flow and the energetic performances of a hybrid rooftop extractor. This innovative extractor has two main features : it works at low difference of pressure, below 50 Pa, and its fan is placed far above the duct outlet, out of the fluid flow. The hybrid extractor works following three modes of operation: stack effect, Venturi effect and fan rotation. The two first modes of operation allow large energy saving. To analyse the three modes of operation, three sets of corresponding RANS simulations are developed. The first one allows to estimate the pressure drop due to the geometry of the air extractor. The second one is used to check the ability of the extractor to generate a suction into the duct in presence of wind. The final one involves Multiple Reference Frame modelling in order to study the flow when the electric motor drives the fan. The numerical simulation configurations are validated with experimental data. A good behaviour of the extractor is found for simulations of stack effect mode and Venturi effect mode. The stack effect and the Venturi effect allow the hybrid extractor to work most of the time without electric power. Finally, energetic comparisons are given.
TOPICS: Computer simulation, Flow (Dynamics), Venturi tubes, Pressure, Ducts, Simulation, Engineering simulation, Modeling, Air pollution, Electricity (Physics), Electric motors, Suction, Geometry, Pressure drop, Reynolds-averaged Navier–Stokes equations, Rotation, Fluid dynamics, Wind, Wind tunnels
research-article  
Chirag Trivedi
J. Fluids Eng   doi: 10.1115/1.4037500
Dynamic stability of the high head Francis turbines is one of the challenging problems. Unsteady rotor-stator interaction (RSI) develops dynamic stresses and leads to crack in the blades. In a high head turbine, vaneless space is small and the amplitudes of RSI frequencies are very high. Credible estimation of the amplitudes is vital for the runner design. Current study is aimed to investigate the amplitudes of RSI frequencies considering a compressible flow. Hydro acoustic phenomenon is dominating the turbines, and the compressibility effect should be accounted for accurate estimation of the pressure amplitudes. Unsteady pressure measurements were performed in the turbine during the best efficiency point, and part load operations. The pressure data were used to validate the numerical model. The compressible flow simulations showed 0.5-3% improvement in the time-average pressure and the amplitudes over incompressible flow. The maximum numerical errors in the vaneless space and runner were 6% and 10%, respectively. Numerical errors in the instantaneous pressure amplitudes at the vaneless space, runner and draft tube were ±1.6%, ±0.9% and ±1.8%, respectively. In the draft tube, the incompressible flow study showed the pressure amplitudes up to 8 time smaller than those of the compressible. Unexpectedly, strong effect of RSI was seen in the upper and lower labyrinth seals, which was absent for the incompressible flow.
TOPICS: Turbulence, Francis turbines, Pressure, Flow (Dynamics), Turbines, Compressible flow, Stress, Errors, Stators, Fracture (Materials), Design, Engineering simulation, Rotors, Dynamic stability, Blades, Pressure measurement, Compressibility, Acoustics, Computer simulation, Simulation
research-article  
Nuno M. C. Martins, Bruno Brunone, Silvia Meniconi, Helena M. Ramos and Didia Covas
J. Fluids Eng   doi: 10.1115/1.4037504
In this paper, the analysis of fast laminar transients in pressurized pipes is developed using a Computational Fluid Dynamics model (CFD), combined with the Zielke model and laboratory data. The systematic verification of the performance of the CFD model executed in the first part of the paper allows defining the most efficient set of the discretization parameters capable of capturing the main features of the examined transient. In this framework, the crucial role of radial discretization is pointed out. In the second part of the paper, the refined and efficient CFD model is used to examine some aspects of interest for understanding the dynamics of transients. Specifically, the uniformity of the instantaneous pressure distributions along the pipe radius, which validates the results of the most popular quasi-2D models, has been revealed. Moreover, it has been shown that the strongest link between the wall shear stress and the axial component of the velocity occurs in the region close to the pipe wall as well as that the time-shift between the wall shear stress and the local instantaneous flow acceleration increases significantly as time elapses.
TOPICS: Wave propagation, Laminar flow, Transients (Dynamics), Computational fluid dynamics, Modeling, Pressure, Pipes, Shear stress, Flow (Dynamics), Dynamics (Mechanics)
research-article  
Eduard Amromin
J. Fluids Eng   doi: 10.1115/1.4037505
Cavitation within regions of flow separation appears in drifting vortices. A two-part computational method is employed for prediction of cavitation inception number there. The first part is an analysis of the average flow in separation regions without consideration of an impact of vortices. The second part is an analysis of equilibrium of a bubble within the core of a vortex located in the turbulent flow of known average characteristics. Computed cavitation inception numbers for axisymmetric flows are in the good agreement with the known experimental data.
TOPICS: Cavitation, Flow separation, Vortices, Flow (Dynamics), Separation (Technology), Turbulence, Equilibrium (Physics), Bubbles, Computational methods
research-article  
Yang Xiang, Hong Liu and Suyang Qin
J. Fluids Eng   doi: 10.1115/1.4037506
Owing to the limiting effect of energy, vortex rings cannot grow indefinitely and thus pinch off. In this paper, experiments on the vortex rings generated using a piston-cylinder apparatus are conducted so as to investigate the pinch-off mechanisms and identify the limiting effect of energy. Both theoretical and experimental results show that the generated vortex rings share a unified energy feature, regardless of whether they are pinched-off or not. Moreover, the unified energy feature is quantitatively described by a dimensionless energy number ?, defined as ? = E/(I^2G?_max) and exhibiting a critical value ?_ring =0.14 for the generated vortex rings. This unified energy feature reflects the limiting effect of energy and specifies the target of vortex ring formation. Furthermore, based on the tendency of ? during vortex ring formation, criteria for determining the two timescales, i.e pinch-off time and separation time, which correspond to the onset and end of pinch-off process respectively, are suggested.
TOPICS: Pinch effect, Vortices, Cylinders, Pistons, Separation (Technology)
Technical Brief  
C.Y. Wang
J. Fluids Eng   doi: 10.1115/1.4037495
The starting flow due to a sudden pressure gradient in a channel containing two layers of different fluids is studied for the first time. The necessary eigenvalues and eigenfunctions, including orthogonality, for the composite regions are developed. Infinite series analytic solution are obtained for the starting transient. The properties of the instantaneous velocity profiles depend on the thickness ratio of the layers, the viscosity ratio and the density ratio. Starting times are determined for the important cases of air over water and oil over water. The bulk flow is greatly increased when there exists a low- viscosity layer buffeting the channel wall. An important conclusion is that, in general, Navier's partial slip condition cannot be applied to unsteady starting flows.
TOPICS: Flow (Dynamics), Fluids, Water, Viscosity, Transients (Dynamics), Eigenfunctions, Eigenvalues, Pressure gradient, Density, Composite materials

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