Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;140(10):101101-101101-11. doi:10.1115/1.4039862.

An effective boundary potential has been proposed to solve nonperiodic boundary condition (NPBC) of hybrid method. The optimized hybrid method is applied to investigate the influences of the channel height and solid–liquid interaction parameters on slip characteristics of Couette flows in micro/nanochannels. By changing the channel height, we find that the relative slip lengths show the obvious negative correlation with the channel height and fewer density oscillations are generated near the solid wall in the larger channel height. Moreover, we continue to investigate the solid–liquid interaction parameters, including the solid–liquid energy scales ratio (C1) and solid–liquid length scales ratio (C2). The results show that the solid–liquid surface changes from hydrophobic to hydrophilic with the increase of C1, the arrangement of liquid particles adjacent to the solid particles is more disorganized over the hydrophobic solid–liquid surface compared with the hydrophilic surface, and the probability of the liquid particles that appear near the solid particles becomes smaller. Meanwhile, the relative slip lengths are minimum when the liquid and solid particles have the same diameter. Furthermore, the relative slip lengths follow a linear relationship with the shear rate when the solid–liquid interaction parameters change. The plenty computational time has been saved by the present hybrid method compared with the full molecular dynamics simulation (FMD) in this paper.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101102-101102-22. doi:10.1115/1.4039908.

In this paper, a computational fluid dynamics model of flashing flow, which considers the thermal nonequilibrium effect, has been proposed. In the proposed model, based on the two-phase mixture approach, the phase-change process depends on the difference between the vaporization pressure and the vapor partial pressure. The thermal nonequilibrium effect has been included by using ad hoc modeling of the boiling delay. The proposed model has been applied to the case of two-dimensional axisymmetric convergent-divergent nozzle, which is representative of well-known applications in nuclear and energy engineering applications (e.g., the primary flow in the motive nozzle of ejectors). The numerical results have been validated based on a benchmark case from the literature and have been compared with the numerical results previously obtained by different research groups. The proposed approach has shown a good level of agreement as regards the global and the local experimental fluid dynamic quantities. In addition, sensitivity analyses have been carried out concerning (a) grid independency, (b) turbulence modeling approaches, (c) near-wall treatment approaches, (d) turbulence inlet parameters, and (e) semi-empirical coefficients. In conclusion, the present paper aims to provide guidelines for the simulation of flash boiling flow in industrial applications.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101103-101103-6. doi:10.1115/1.4039946.

This investigation demonstrates microfluidic synthesis of monodisperse hydrogel beads with controllable electromechanical properties. Hydrogel beads were synthesized using aqueous monomer solutions containing difunctional macromer, ionic liquid monomer, and photoinitiator. Electromechanical properties of these beads were measured at compression ratios up to 20% to examine their potential use in vibrational energy harvesters. Bead stiffness decreased dramatically as water content increased from 19% to 60%. As water content and compression ratio increased, electrical permittivity of beads increased, while resistivity decreased. As ionic liquid monomer concentration increased from 0% to 4%, relative permittivity increased by 30–45% and resistivity decreased by 70–80%.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101104-101104-5. doi:10.1115/1.4039792.

A novel variant of a synthetic jet actuator (SJA) has been designed, manufactured, and tested. The novelty consists in a bio-inspired nozzle whose oscillating lip is formed by a flexible diaphragm rim. The working fluid is air, and the operating frequency is 65 Hz. The proposed SJA was tested by three experimental methods: phase-locked visualization of the nozzle lips, hot-wire anemometry, and momentum flux measurement using a precision scale. The results demonstrate advantages of the proposed SJA, namely, an increase in the momentum flux by 18% compared with that of a conventional SJA.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101105-101105-11. doi:10.1115/1.4040016.

Wire and nonparallel plate electrode-type electrostatic air accelerators have attracted significant interest. The physical process involved in using accelerators is complicated. Moreover, mechanisms are unclear, especially for accelerators with double- and multiwire electrodes. In this study, the two-dimensional (2D) model of a wire–nonparallel plate-type accelerator validated by experiments is established with a finite element method. Onset voltage, average current, and outlet average velocity are analyzed with respect to different parameters. Onset voltage is derived by the proposed quadratic regression extrapolation method. Moreover, current is affected by interference and discharge effects, while velocity is also influenced by the suction effect. For the single-wire electrode, high wind speed can be obtained by either increasing channel slope or placing the wire near the entry section. For the double-wire electrode, velocity can be further increased when one of the wires is placed near the inlet and the distance between the two wires is widened. Comparatively, the velocity of the three-wire electrode is higher with larger gaps between wires and stronger discharge effect. The highest velocity is obtained by the four-wire electrode. Comparisons indicate that higher velocity can be obtained with weaker interference effect, stronger suction effect, and intensified discharge effect. Optimum parameter combinations are considered by the Taguchi method. Consequently, velocity can be enhanced by more than 39% after optimization compared with the reference design.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101106-101106-13. doi:10.1115/1.4040015.

Wave rotors are periodic-flow devices that provide dynamic pressure exchange and efficient energy transfer through internal pressure waves generated due to fast opening and closing of ports. Wave turbines are wave rotors with curved channels that can produce shaft work through change of angular momentum from inlet to exit. In the present work, conservation equations with averaging in the transverse directions are derived for wave turbines, and quasi-one-dimensional model for axial-channel non-steady flow is extended to account for blade curvature effects. The importance of inlet incidence is explained and the duct angle is optimized to minimize incidence loss for a particular boundary condition. Two different techniques are presented for estimating the work transfer between the gas and rotor due to flow turning, based on conservation of angular momentum and of energy. The use of two different methods to estimate the shaft work provides confidence in reporting of work output and confirms internal consistency of the model while it awaits experimental data for validation. The extended wave turbine model is used to simulate the flow in a three-port wave rotor. The work output is calculated for blades with varying curvature, including the straight axial channel as a reference case. The dimensional shaft work is reported for the idealized situation where all loss-generating mechanisms except flow incidence are absent, thus excluding leakage, heat transfer, friction, port opening time, and windage losses. The model developed in the current work can be used to determine the optimal wave turbine designs for experimental investment.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101107-101107-12. doi:10.1115/1.4040038.

Complex energy conversion and energy dissipation occur in pump-turbines during the load rejection process. However, the underlying fluid mechanism is not clear. In order to solve these problems, in this study, a three-dimensional (3D) transient turbulent flow in a pump-turbine, with clearance during the load rejection process, was simulated using the method of coupling of the rigid rotor motion with flow and dynamic mesh technology. The simulated rotational speed shows good agreement with the experimental data. Most of the differences of rotational speed between simulations and experiments are very small and lower than 5%. Based on the numerical simulation, the energy conversion process, loss distribution, and flow mechanism in a pump-turbine were analyzed using the method of coupling of the entropy production analysis with the flow analysis. The results indicate that the load rejection process of a pump-turbine is an energy-dissipation process where the energy is converted among various energy forms. After load rejection, the hydraulic loss in the reverse pump process distributes primarily in the stay/guide vanes (GV), the vaneless space, and near draft tube inlet. While the hydraulic losses in the runaway process and the braking process are distributed mainly in the elbow section of the draft tube, the clearance of runner (RN), and the vaneless space, the hydraulic losses are mainly caused by viscous dissipation effects of the vortex flows, including the flow separation vortices, the shedding vortices of flow wake, the secondary flow, and the backflow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101108-101108-13. doi:10.1115/1.4040070.

A Reynolds-averaged Navier–Stokes (RANS) computational study was conducted to investigate the effect of various variable camber continuous trailing edge flap (VCCTEF) configurations on the lift and drag of a NASA generic transport model (GTM) wing section. Out of the five two-dimensional (2D) VCCTEF configurations considered with varying camber in the three-segment flap region, with a total deflection of 6 deg, the best stall performance was exhibited by the circular and parabolic arc camber flaps. Both circular and parabolic arc flaps give similar lift performance, with the circular arc yielding a higher lift coefficient and parabolic arc resulting in the lowest drag and hence the best L/D performance at design Cl. Analysis of results based on linear theory shows excellent agreement between computed and theoretical incremental lift.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;140(10):101201-101201-11. doi:10.1115/1.4039945.

This paper focuses on the lateral jetting commencing points associated with the peak pressure when an arc-curved jet impacts flat, concave, convex, and inclined solid surfaces, respectively. A theoretical method based on a shock wave background is used to establish models for these situations, which indicate that the critical radius for the initiation of lateral jetting is dependent on the combined actions of the jet velocity, surface shape, and surface angle. Arbitrary Lagrangian–Eulerian (ALE) formulations are then used to model the process of arc-curved jets impacting varied solid surfaces. The numeric simulation results are found to be in good agreement with the theoretical models.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101202-101202-12. doi:10.1115/1.4039793.

The Eulerian–Eulerian two-fluid model (EE) is a powerful general model for multiphase flow computations. However, one limitation of the EE model is that it has no ability to estimate the local bubble sizes by itself. In this work, we have combined the discrete phase model (DPM) to estimate the evolution of bubble sizes with the EE model. In the DPM, the change of bubble size distribution is estimated by coalescence, breakup, and volumetric expansion modeling of the bubbles. The time-varying bubble distribution is used to compute the local interface area between gas and liquid phase, which is then used to estimate the momentum interactions such as drag, lift, wall lubrication, and turbulent dispersion forces for the EE model. In this work, this newly developed hybrid model Eulerian–Eulerian discrete-phase model (EEDPM) is applied to compute an upward flowing bubbly flow in a vertical pipe and the results are compared with previous experimental work of Hibiki et al. (2001, “Axial Interfacial Area Transport of Vertical Bubbly Flows,” Int. J. Heat Mass Transfer, 44(10), pp. 1869–1888). The EEDPM model is able to reasonably predict the locally different bubble size distributions and the velocity and gas fraction fields. On the other hand, the standard EE model without the DPM shows good comparison with measurements only when the prescribed constant initial bubble size is accurate and does not change much. Parametric studies are implemented to understand the contributions of bubble interactions and volumetric expansion on the size change of bubbles quantitatively. The results show that coalescence is larger than other effects, and naturally increases in importance with increasing gas fraction.

Topics: Bubbles , Bubbly flow
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(10):101203-101203-13. doi:10.1115/1.4039711.

The study of transient pressure waves in both low- and high-frequency domains has become a new research area to provide potentially high-resolution pipe fault detection methods. In previous research works, radial pressure waves were evidently observed after stopping the laminar pipe flows by valve closures, but the generation mechanism and components of these radial pressure waves are unclear. This paper intends to clarify this phenomenon. To this end, this study first addresses the inefficiencies of the current numerical scheme for the full two-dimensional (full-2D) water hammer model. The modified efficient full-2D model is then implemented into a practical reservoir-pipeline-valve (RPV) system, which is validated by the well-established analytical solutions. The generation mechanism and components of the radial pressure waves, caused by different flow perturbations from valve operations, in transient laminar flows are investigated systematically using this efficient full-2D model. The results indicate that nonuniform changes in the initial velocity profile form pressure gradients along the pipe radius. The existence of these radial pressure gradients is the driving force of the formation of radial flux and radial pressure waves. In addition, high radial modes can be excited, and the frequency of flow perturbations by valve oscillation can redistribute the energy entrapped in each high radial mode.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2018;140(10):101301-101301-11. doi:10.1115/1.4039710.

Simplifying assumptions and empirical closure relations are often required in existing two-phase flow modeling based on first-principle equations, hence limiting its prediction accuracy and in some instances compromising safety and productivity. State-of-the-art models used in the industry still include correlations that were developed in the sixties, whose prediction performances are at best acceptable. To better improve the prediction accuracy and encompass all pipe inclinations and flow patterns, we propose in this paper an artificial neural network (ANN)-based model for steady-state two-phase flow liquid holdup estimation in pipes. Deriving the best input combination among a large reservoir of dimensionless Π groups with various fluid properties, pipe characteristics, and operating conditions is a laborious trial-and-error procedure. Thus, a self-adaptive genetic algorithm (GA) is proposed in this work to both ease the computational complexity associated with finding the elite ANN model and lead to the best prediction accuracy of the liquid holdup. The proposed approach was implemented using the Stanford multiphase flow database (SMFD), chosen for being among the largest and most complete databases in the literature. The performance of the proposed approach was further compared to that of two prominent models, namely a standard empirical correlation-based model and a mechanistic model. The obtained results along with the comparison analysis confirmed the enhanced accuracy of the proposed approach in predicting liquid holdup for all pipe inclinations and fluid flow patterns.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Fluids Eng. 2018;140(10):104501-104501-3. doi:10.1115/1.4039864.

A simple procedure for calculating the pressure at the onset and termination of condensation shocks that occur in steam nozzles and steam turbine blade passages is presented. In addition, the location of the termination of the condensation shock with reference to the throat location is also predicted. The procedure is based entirely on thermodynamic and gas dynamic considerations, without using a model for droplet nucleation and growth and the nozzle profile. The only input required is the stagnation condition at the inlet to the nozzle. The procedure requires the solution of a system of algebraic equations which can be accomplished quite easily. Calculations have been carried out for several inlet stagnation conditions and the predictions are compared with the available experimental data. The agreement is seen to be reasonable considering the simplicity of the procedure.

Commentary by Dr. Valentin Fuster

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