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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;140(3):031101-031101-9. doi:10.1115/1.4037787.

Frequent changes in the operating modes pose significant challenges in the development of a pump-turbine with high efficiency and stability. In this paper, two pump-turbine runners, one with a large positive blade lean and the other with a large negative lean, are investigated numerically and experimentally. These two runners are designed by using the optimum stacking condition at the high pressure edge (HPE). The experimental and the numerical results show that both runners have good efficiency performances, and pressure fluctuations for the runner with a negative blade lean are much lower than those for the runner with a positive blade lean. The internal flow field analyses clarify the effects of the blade lean on the pressure distribution around the runner blades. In the turbine mode at partial load, the negative blade lean can control flow separation in the high pressure side of the runner and then reduce the pressure fluctuations in the vaneless space.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;140(3):031201-031201-13. doi:10.1115/1.4037867.

In this work, the capability and performance of the vorticity confinement (VC) implemented in a high-order accurate flow solver in predicting two-dimensional (2D) compressible mixing layer flows on coarse grids are investigated. Here, the system of governing equations with incorporation of the VC in the formulation is numerically solved by the fourth-order compact finite difference scheme. To stabilize the numerical solution, a low-pass high-order filter is applied, and the nonreflective boundary conditions are used at the farfield and outflow boundaries to minimize the reflections. At first, the numerical results without applying the VC are validated by available direct numerical simulations (DNSs) for a low Reynolds number mixing layer. Then, the calculations using a range of VC levels are performed for a high Reynolds number mixing layer and the results are thoroughly compared with those of available large eddy simulations (LESs). The study shows that, with applying the vortex identification method, more accurate results are obtained in the slow laminar region of the mixing layer. A sensitivity study is also performed to examine the effect of different numerical parameters to reasonably provide more accurate results. It is shown that the local VC introduced based on the artificial viscosity coefficient and the vorticity thickness can improve the accuracy of the results in the turbulent region of the mixing layer compared with those of LESs. It is found that the solution methodology proposed can reasonably preserve the vortices in the flowfield and the results are comparable with those of LESs on fairly coarser grids and thus the computational costs can be considerably decreased.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(3):031202-031202-8. doi:10.1115/1.4037691.

In this work, the influence of nozzle shape on microfluidic ink jet breakup is investigated. First, an industrial ink used in continuous inkjet (CIJ) 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.

Topics: Nozzles , Shapes , Inks , Fluids
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(3):031203-031203-8. 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.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(3):031204-031204-11. doi:10.1115/1.4037946.

In this paper, a theoretical study of ball valves is carried out for investigating the local resistance and pressure drop of ball valves in operating process. An equivalent model of ball valves is proposed based on the inherent mechanism of the resistance loss, which can be divided into three equivalent throttling components: a thick orifice, two variable-opening eccentric orifice plates, and a Z type elbow. Through analysis of the flow resistance of the three components, a general parametric modeling of ball valves is presented for the flow resistance analysis, and then an analytical formula of pressure drop is demonstrated. The results obtained from the presented model are compared with the prior test data to validate this model, and good agreement is observed. Indicate that the presented model has high accuracy in predicting the resistance and pressure loss in various openings. The results show that the influences of thin orifice plates play an important role in the total flow resistance coefficient and pressure drop, especially in the small opening. The effects of thick orifice plates and the Z type elbow gradually increased as the valve opening rises and becomes significant when the opening is more than 70%.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(3):031205-031205-12. doi:10.1115/1.4037679.

Three-dimensional (3D) 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 (PR), and cross aspect ratio (CAR) 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 nonequilibrium effect leads to the minimum simulation time. Considering the nonequilibrium effect, prediction of inlet and outlet pressures and the speed of simulation, the characteristic scheme with nonequilibrium effect performs better than all the other schemes, at least over the range of parameters investigated herein.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2017;140(3):031301-031301-16. doi:10.1115/1.4037944.

Entrained air in oil can cause malfunctions and damages within hydraulic systems. In this paper, we extend existing approaches to reduce the amount of entrained air by separating air bubbles from oil using filter elements. The aim of this study was to investigate the ability of different untreated and surface modified woven and nonwoven fabrics (NWF) to separate air bubbles from oil when directly integrated into an intake socket of an oil pump. An experimental setup was constructed to generate entrained air in oil and to characterize changes in oil aeration and pressure drop induced by the filters. Measurements were conducted at volume flow rates of 2.2 and 5.4 l/min with an inflow angle normal to the filter elements. The developed setup and aeration measurement method proved to be suitable to generate entrained air in oil in a reproducible manner and to accurately characterize aerated oil up to air contents of about 5%. Significant influences on the aeration characteristics were found only for the NWF. Whereas the number of air bubbles decreased by up to 33% relative to the values in the oil reservoir for a flow rate of 2.2 l/min, a significant reduction of the volumetric air ratio could not be achieved as resulting bubble distributions comprised a higher number of large bubbles. We suggest that the lack of effective bubble separation was a result of the flow-induced pressure drop by the filters, which increased with the flow rate.

Commentary by Dr. Valentin Fuster

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