Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;141(3):031101-031101-12. doi:10.1115/1.4041120.

New apparatus is described to simulate a compliant seal interface, allowing the percolation of liquid to be viewed by a fluorescence microscope. A model, based on the boundary element (BE) methodology, is used to provide a theoretical explanation of the observed behavior. The impact of contact pressure, roughness, and surface energy on percolation rates are characterized. For hydrophilic surfaces, percolation will always occur provided a sufficient number of roughness length scales are considered. However, for hydrophobic surfaces, the inlet pressure must overcome the capillary pressure exerted at the minimum channel section before flow can occur.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031102-031102-15. doi:10.1115/1.4041112.

Analytical and computational fluid dynamics (CFD) analyses confirmed the presence of apparent slip for water flow in microchannels with equivalent hydraulic diameter, Dh < 103μm, markedly decreasing the friction number, fRein. The determined values of the slip length, β, from reported measurements of pressure losses in microchannels with aspect ratio, α = 1, 1.74, 2, and 40, are 0.9, 3.5, 1.6, and 0.125 μm, respectively. For Dh > 103μm, the apparent slip in microchannels diminishes, and the friction number approaches the theoretical Hagen–Poiseuille with no slip. The analytical solution for fully developed flow successfully benchmarked the CFD approach, which is subsequently used to investigate fRein and the flow development length, Le, for uniform inlet velocity in microchannels. For fully developed flow, the analytical and CFD values of fRein are in excellent agreement. For microchannels with Dh < 103μm, fRein decreases below that of the theoretical Hagen–Poiseuille with no slip, almost exponentially with decreased Dh. The difference increases with decreased Dh, but increased α and β. The friction number for uniform inlet velocity is identical to that for fully developed flow when Dh ≤ 100 μm, but is as much as 9% higher for larger Dh. For uniform inlet velocity, Le negligibly depends on α and β, but increases with increased Rein. The obtained values are correlated as: Le/Dh = 0.068 Rein.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031103-031103-8. doi:10.1115/1.4041228.

In order to help the design of miniature centrifugal pumps, the design method for macrosize centrifugal pumps is reviewed and the critical parameter, the flow coefficient, is examined in this paper for the miniature centrifugal pumps. The performance of the pumps designed is analyzed theoretically, numerically, and experimentally. Both numerical and theoretical results show that the value of the optimized flow coefficient is approximately 1.47. This value is about five times larger than the recommended value using conventional design technique for macrosize pumps. The optimum radius ratio obtained numerically is approximately 0.4. It can be concluded that the design approach for macrosize pumps is not applicable for pumps in the scale of decimeters. The results obtained in the present study provide us guidelines on the design and performance study of the miniature centrifugal pump.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031104-031104-13. doi:10.1115/1.4041231.

Wind farms have often been located in close proximity to coastal cliffs to take advantage of the consistent wind regimes associated with many coastal regions, as well as to extract any available increase in flow speed that might be generated by such cliffs. However, coastal cliffs are often rugged as a result of erosion and the natural shape of the landform. This research explores the impact of the three-dimensional cliff topography on the wind flow. Specifically, wind tunnel testing is conducted, modeling the naturally occurring ruggedness as sawtooth lateral variations of various amplitudes applied to a forward facing step (FFS). Surface shear stress visualization techniques have been employed to derive the flow topology associated with different topographies, while pressure probe measurements are used to measure the development of wind speed and turbulence intensity (TI). Pressure probe measurements and surface pressure taps also assist to determine the lateral and vertical extents of the vortex structures identified. In particular, flow fields characterized by the probe measurements were consistent with vortex bursting that is described by various researchers in the flow over delta wings. Such bursting is observed as a stagnation and corresponding expansion of the vortex. Based on these observations, recommendations are provided for the siting of wind turbines near analogous cliffs.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031105-031105-8. doi:10.1115/1.4041391.

Ionic wind pumps have attracted considerable interest because of their low energy consumption, compact structures, flexible designs, and lack of moving parts. However, large cross-sectional ionic wind pumps have yet to be numerically analyzed and experimentally optimized. Accordingly, this study develops a large cross-sectional ionic wind pump with multiple needles-to-mesh electrode, as well as analyzes its flow characteristics using a proposed full three-dimensional simulation method validated with experimental data. To obtain a considerably high outlet average velocity, experimental studies and numerical methods are employed to optimize the pump's configuration parameters, including needle electrode configuration, needle diameter, grid size, and gap between electrodes. The breakdown voltage and highest velocity corresponding to the breakdown voltage increase with an increase in the needle tip-to-mesh gap. After parametric optimization, a maximum velocity of 2.55 m/s and a flow rate of 2868 L/min are achieved.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031106-031106-13. doi:10.1115/1.4041393.

The present research applied a mixed-fidelity approach to examine the fan–intake interaction. Flow separation induced by a distortion generator (DG) is either resolved using large eddy simulation (LES) or modeled using the standard k–ω model, Spalart–Allmaras (SA) model, etc. The immersed boundary method with smeared geometry (immersed boundary method with smeared geometry (IBMSG)) is employed to represent the effect of the fan and a wide range of test cases is studied by varying the (a) height of the DG and (b) proximity of the fan to the DG. Comparisons are drawn between the LES and the Reynolds-averaged Navier–Stokes (RANS) approaches with/without the fan effect. It is found that in the “absence of fan,” the discrepancies between RANS and LES are significant within the separation and reattachment region due to the well-known limitations of the standard RANS models. “With the fan installed,” the deviation between RANS and LES decreases substantially. It becomes minimal when the fan is closest to the DG. It implies that with an installed fan, the inaccuracies of the turbulence model are mitigated by the strong flow acceleration at the casing due to the fan. More precisely, the mass flow redistribution due to the fan has a dominant primary effect on the final predictions and the effect of turbulence model becomes secondary, thereby suggesting that high fidelity eddy resolving simulations provide marginal improvements to the accuracy for the installed cases, particularly for the short intake–fan strategies with fan getting closer to intake lip.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;141(3):031201-031201-7. doi:10.1115/1.4040972.

Starting flow due to a suddenly applied pressure gradient in a circular tube containing two immiscible fluids is solved using eigenfunction expansions. The orthogonality of the eigenfunctions is developed for the first time for circular composite regions. The problem, which is pertinent to flow lubricated by a less viscous near-wall fluid, depends on the ratio of the radius of the core region to that of the tube, and the ratios of dynamic and kinematic viscosities of the two fluids. In general, a higher lubricating effect will lead to a longer time for the starting transient to die out. The time development of velocity profile and slip length are examined for the starting flows of whole blood enveloped by plasma and water enveloped by air in a circular duct. Owing to a sharp contrast in viscosity, the starting transient duration for water/air flow can be ten times longer than that of blood/plasma flow. Also, the slip length exhibits a singularity in the course of the start-up. For blood with a thin plasma skimming layer, the singularity occurs very early, and hence for the most part of the start-up, the slip length is nearly a constant. For water lubricated by air of finite thickness, the singularity may occur at a time that is comparable to the transient duration of the start-up, and hence, an unsteady slip length has to be considered in this case.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031202-031202-14. doi:10.1115/1.4041121.

Laboratory experiments were conducted to study the dynamics of particle clouds in viscous fluids. Different shapes of frontal head and trailing stems were observed, and particle clouds were classified using data mining methodology. The stability of the frontal head of particle clouds was found to be correlated with the nozzle diameter and mass of sand particles in the form of an initial aspect ratio. The formation of particle clusters into a torus and the split of the frontal head into two or three clusters were investigated in detail. The cluster of particles flow through viscous fluid experienced partial separation due to the release of air bubbles from the rear of frontal head. It was observed that the time and location of major particle separation increase linearly with the aspect ratio. The oscillatory motion of the frontal head, caused by an uneven release of air bubbles from the rear of the frontal head, was found to be correlated with the initial aspect ratio. Both amplitude and wavelength exhibited a linear relationship with nondimensional time. The average drag coefficient of particle clouds Cd in viscous fluids was calculated for different aspect ratios, and the results were compared with the drag coefficient of individual particles. It was found that the averaged drag coefficients of particle clouds were smaller than the drag coefficient of individual particles, and Cd slightly increases with the increasing initial aspect ratio.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031203-031203-14. doi:10.1115/1.4041230.

Centrifugal pumps operate below their nominal capacity when handling gas–liquid flows. This problem is sensitive to many variables, such as the impeller speed and the liquid flow rate. Several works evaluate the effect of operating conditions in the pump performance, but few bring information about the associated gas–liquid flow dynamics. Studying the gas phase behavior, however, can help understanding why the pump performance is degraded depending on the operating condition. In this context, this paper presents a numerical and experimental study of the motion of bubbles in a centrifugal pump impeller. The casing and the impeller of a commercial pump were replaced by transparent components to allow evaluating the bubbles' trajectories through high-speed photography. The bubble motion was also evaluated with a numerical particle-tracking method. A good agreement between both approaches was found. The numerical model is explored to evaluate how the bubble trajectories are affected by variables such as the bubble diameter and the liquid flow rate. Results show that the displacement of bubbles in the impeller is hindered by an increase of their diameter and impeller speed but facilitated by an increase of the liquid flow rate. A force analysis to support understanding the pattern of the bubble trajectories was provided. This analysis should enlighten the readers about the dynamics leading to bubble coalescence inside an impeller channel, which is the main reason behind the performance degradation that pumps experience when operating with gas–liquid flows.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031204-031204-13. doi:10.1115/1.4041390.

A small-disturbance model to study transonic steady condensing flow of pure steam around a thin airfoil is developed. Water vapor thermodynamics is described by the perfect gas model and its dynamics by the compressible inviscid flow equations. Classical nucleation and droplet growth theory for homogeneous and nonequilibrium condensation is used to compute the condensate mass fraction. The model is derived from an asymptotic analysis of the flow and condensation equations in terms of the proximity of upstream flow Mach number to 1, the small thickness ratio of airfoil, the small quantity of condensate, and the small angle-of-attack. The flow field may be described by a nonhomogeneous and nonlinear partial differential equation along with a set of four ordinary differential equations for calculating condensate mass fraction. The analysis provides a list of similarity parameters that describe the flow physics. A numerical scheme, which is composed of Murman and Cole's algorithm for the computation of flow parameters and Simpson's integration method for calculation of condensate mass fraction, is applied. The model is used to analyze the effects of heat release due to condensation on the aerodynamic performance of airfoils operating in steam at high temperatures and pressures near the vapor–liquid saturation dome.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2018;141(3):031301-031301-11. doi:10.1115/1.4041065.

An accurate and practical approach is necessary for predicting oil fraction in horizontal oil–water flows. In this study, a concept of a drift-flux model is adopted to develop a predictive method for the oil fraction in the horizontal oil–water flows due to its simplicity and practicality. A new drift-flux correlation for the horizontal oil–water flows is developed based on the least square method using collected experimental data. The distribution parameter is determined to be 1.05 for the data with the ratio of oil density to water density ranging from 0.787 to 1.00, whereas the oil fraction weighted mean drift velocity is set at 0 m/s due to the flow direction perpendicular to the gravity direction. The physical meaning for the order of unity of the distribution parameter is explained by introducing a simple model. The predictive capability of the new drift-flux correlation is examined using the collected database of oil–water flows in horizontal pipes under a variety of test conditions. It is demonstrated that the new drift-flux correlation can predict the existing oil fractions in the horizontal pipe channels with the mean absolute error, standard deviation, mean relative deviation, and mean absolute relative deviation being −0.0124, 0.0338, −3.25%, and 9.57%, respectively.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031302-031302-14. doi:10.1115/1.4041361.

Aquatic invasive species (AIS) have spread throughout the United States via major rivers and tributaries. Locks and dams positioned along affected waterways, specifically lock chambers, are being evaluated as potential management sites to prevent further expansion into new areas. Recent research has shown that infusion of chemicals (e.g., carbon dioxide) into water can block or kill several invasive organisms and could be a viable option at navigational structures such as lock chambers because chemical infusion would not interfere with vessel passage or lock operation. Chemical treatments near lock structures will require large-scale fluid-mechanic systems and significant energy. Mixing must extend to all stagnation regions within a lock structure to prevent the passage of an invasive fish. This work describes the performance of both wall- and floor-based CO2-infused-water to water injection manifolds targeted for lock structures in terms of mixing time, mixing homogeneity, injection efficiency, and operational power requirements. Both systems have strengths and weaknesses so selection recommendations are given for applications such as open systems and closed systems.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031303-031303-6. doi:10.1115/1.4041456.

This investigation demonstrates that metallization can be used to tailor the electromechanical properties of polymer beads. Rigid ion exchange resin beads and softer microfluidically synthesized polyionic liquid hydrogel beads were metallized using an ion exchange process. Metallization increased bead stiffness and dielectric coefficient while reducing resistivity in all beads examined here. Gold-filled beads were preferable over platinum-filled beads as they generated greater changes in electrical properties with smaller increased stiffness. These properties could be further altered by performing multiple metallization steps, but diminishing returns were observed with each step. Ion exchange resin beads were always stable after multiple metallization steps, but polyionic beads would often rupture when repeatedly compressed. Polyionic beads with higher ionic liquid (IL) content were more fragile, and beads synthesized from monomer solutions containing 1% IL were mechanically robust after three metallization steps. These 1% IL beads delivered similar electrical properties as the IONAC beads that also underwent three metallization steps at a significantly reduced stiffness.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(3):031304-031304-14. doi:10.1115/1.4041986.

This paper presents computational simulations of flows in packed beds and compares the computational pressure-drop results with those given by the Ergun correlation. The computational methodology used in this work follows the combined discrete element method (DEM) and computational fluid dynamics (CFD) technique. DEM is used to predict the locations and packing structure of the particles in the bed, while CFD is used to predict the flow field in the void space surrounding the packed particles. The computational results obtained for irregular packed beds show that the local packing-structure parameters have significant effects not only on the local velocity and pressure fields but also on macroscopic quantities, such as the average pressure gradient along the length of the packed column. The computational results also show that classical correlations based on averaged values, such as the Ergun correlation, have poor predictive accuracy for macroscopic variations along a packed column, and this is mainly because such correlations do not account for local packing-structure parameters. The computational results confirm the existence of sections with linear variation of macroscopic parameters along the length of the packed column, and this leads to the conclusion that accurate results from DEM-CFD methods on shortened columns can be extrapolated to full-length columns. Moreover, it was found that unlike regularly packed beds, the predicted pressure for randomly packed beds experiences an apparent strong recovery near the downstream end of the packed bed, and then experiences a strong dip down to the plateau leading to the exit pressure.

Commentary by Dr. Valentin Fuster

Research Papers: Techniques and Procedures

J. Fluids Eng. 2018;141(3):031401-031401-12. doi:10.1115/1.4041362.

The eigenvalue approach is a recently developed compressor stability model used to predict stall onset. In this model, the flow field from a Reynolds-averaged Navier–Stokes (RANS) simulation provides the basic flow. This paper presents the effect of the RANS methods (including the computational grid, the turbulence model, and the spatial discretization scheme) on the eigenvalue and investigates the most influencing flow structures to the eigenvalue. The test compressor was the transonic compressor of NASA Rotor 37. Three individual meshes with different grid densities were used to validate the grid independence, and the results indicated that RANS simulation and eigenvalue calculation obtain grid independence at the same grid density. Then, the effect of four turbulence models (including Spalart–Allmaras (SA) turbulence model, two different k–ε models with the extended wall function model (EWFKE), and the Yang–Shih model (YSKE), and k–ω shear stress transport (SST) model), and three spatial discretization schemes (the central scheme, the flux difference splitting (FDS) scheme, and the symmetric total variation diminishing (STVD)) was also studied. Further investigation showed that the SA turbulence model combined with the STVD scheme provided the best stall point prediction, with a relative error of 0.05%. Detailed exploration of the three-dimensional flow field revealed that there were two flow patterns near the blade tip necessary for precisely predicting stall onset: the flow blockage generated by the shockwave-tip leakage vortex (TLV) interaction, and the trailing edge separation and corresponding wake flow. The effect of the blockage was greater than the effect of the trailing edge flow.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Fluids Eng. 2018;141(3):034501-034501-7. doi:10.1115/1.4041111.

We aim to perform a series of field measurements for high-speed railway tunnels in China to obtain the micropressure wave (MPW) at the tunnel exit and the transient pressures near the tunnel portals. The relationship between the MPW and the nose-entry wave and the effects of train speed and the tunnel exit hole on the MPW are analyzed. The results show that the MPW decreases with increasing distance from the tunnel exit, but increases rapidly with increasing train speeds. Additionally, holes in the hoods near the tunnel exit could decrease the MPW near the tunnel exit by 10–20%.

Commentary by Dr. Valentin Fuster

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