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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2019;141(12):121101-121101-9. doi:10.1115/1.4043776.

Entrapped air in pipeline systems can compromise the operation of the system by blocking flow and raising pumping costs. Fluid transients are a potential tool for characterizing entrapped air pockets, and a numerical model which is able to accurately predict transient pressures for a given air volume represents an asset to the diagnostic process. This paper presents a detailed study on our current capability for modeling and predicting the dynamics of an inline air pocket, and is one of a series of articles within a broader context on air pocket dynamics. This paper presents an assessment of the accuracy of the variable wave speed and accumulator models for modeling air pockets. The variable wave speed model was found to be unstable for the given conditions, while the accumulator model is affected by amplitude and time-delay errors. The time-delay error could be partially overcome by combining the two models.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2019;141(12):121201-121201-17. doi:10.1115/1.4043576.

This study quantified the performance and fluid disbursal capabilities of several fluidic oscillator variations injecting from the face of a backward-facing step. These devices were designed as a replacement for a pair of nonoscillating fuel injector jets in an ultra-compact combustor. However, these results have relevance whenever fluid is injected from the face of a backward-facing step making the oscillator performance widely applicable. The oscillators were tested with and without coflow and at varying coflow velocities, which controlled the strength of the recirculation behind the backward-facing step. The fluidic oscillators investigated included single as well as paired oscillators that produced in-phase and out-of-phase synchronized jets. The injected fluid disbursal was found to be dependent on the velocity ratio of the freestream air and the injecting jet velocity. Additionally, the oscillation angle was found to be a function of Reynolds number due to the interaction of the oscillating jet with the walls of the models used in the present study. Finally, the oscillation frequency was found to be independent of Reynolds number, throat aspect ratio, working gas, and model scale, which resulted in a Strouhal number of 0.017. This result was supported by nondimensionalizing the published data from several other studies.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(12):121202-121202-11. doi:10.1115/1.4043707.

An optimization study based on computational fluid dynamics (CFD) in combination with Stratford's analytical separation criterion was developed for the design of piecewise conical contraction zones and nozzles. The risk of flow separation was formally covered by a newly introduced dimensionless separation number. The use of this separation number can be interpreted as an adaption of Stratford's separation criterion to piecewise conical nozzles. In the nozzle design optimization process, the risk of flow separation was reduced by minimizing the separation number. It was found that the flow-optimized piecewise conical nozzle did not correspond to a direct geometric approximation of an ideal polynomial profile. In fact, it was beneficial to reduce the flow deflection in the outlet region for a piecewise conical nozzle to increase the nozzle performance. In order to validate the novel design method, extensive tests for different nozzle designs were conducted by means of wind tunnel tests. The measured velocity profiles and wall pressure distributions agreed well with the CFD predictions.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2019;141(12):121301-121301-6. doi:10.1115/1.4043592.

We have used the integral form of the conservation equations, to find a cubic formula for the drop size during in liquid sprays in coflow of air (air-blast atomization). Similar to our previous work, the energy balance dictates that the initial kinetic energy of the gas and injected liquid will be distributed into the final surface tension energy, kinetic energy of the gas and droplets, and viscous dissipation. Using this approach, the drop size can be determined based on the basic injection and fluid parameters for “air-blast” atomization, where the injected liquid is atomized by high-speed coflow of air. The viscous dissipation term is estimated using appropriate velocity and length scales of liquid–air coflow breakup. The mass and energy balances for the spray flows render to an expression that relates the drop size to all of the relevant parameters, including the gas- and liquid-phase velocities and fluid properties. The results agree well with experimental data and correlations for the drop size. The solution also provides for drop size–velocity cross-correlation, leading to computed drop size distributions based on the gas-phase velocity distribution. This approach can be used in the estimation of the drop size for practical sprays and also as a primary atomization module in computational simulations of air-blast atomization over a wide range of injection and fluid conditions, the only caveat being that a parameter to account for the viscous dissipation needs to be calibrated with a minimal set of observational data.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(12):121302-121302-16. doi:10.1115/1.4043580.

Solid particle erosion is a serious issue in centrifugal pumps that may result in economic losses. Erosion prediction in centrifugal pump is complex because the flow field inside it is three-dimensional (3D) unsteady and erosion can be affected by numerous factors. In this study, solid particle erosion of the entire centrifugal pump for liquid–solid flow is investigated numerically. Two-way coupled Eulerian–Lagrangian approach is adopted to calculate the liquid–solid interaction. The reflection model proposed by Grant and Tabakoff and the erosion model proposed by the Erosion/Corrosion Research Center are combined to calculate the erosion rate and predict the erosion pattern. Results show that for the baseline case, the inlet pipe is the least eroded component, whereas the impeller is the most eroded component. The highest average and maximum erosion rates occur at the hub of impeller. The most severe erosion region of a blade is the leading edge with a curvature angle that varies from 55 deg to 60 deg. The most severe erosion region of a volute is in the vicinity of a curvature angle of 270 deg. The impeller erosion pattern, especially the middle part of the hub and the vicinity of the blade pressure side, can be greatly influenced by operation parameters, such as flow rate, particle concentration, and particle size.

Commentary by Dr. Valentin Fuster

Research Papers: Techniques and Procedures

J. Fluids Eng. 2019;141(12):121401-121401-8. doi:10.1115/1.4043706.

The effects of a laser beam propagating through atmospheric turbulence are investigated using the phase screen approach. Turbulence effects are modeled by the Kolmogorov description of the energy cascade theory, and outer scale effect is implemented by the von Kármán refractive power spectral density. In this study, we analyze a plane wave propagating through varying atmospheric horizontal paths. An important consideration for the laser beam propagation of long distances is the random variations in the refractive index due to atmospheric turbulence. To characterize the random behavior, statistical analysis of the phase data and related metrics are examined at the output signal. We train three different machine learning algorithms in tensorflow library with the data at varying propagation lengths, outer scale lengths, and levels of turbulence intensity to predict statistical parameters that describe the atmospheric turbulence effects on laser propagation. tensorflow is an interface for demonstrating machine learning algorithms and an implementation for executing such algorithms on a wide variety of heterogeneous systems, ranging from mobile devices such as phones and tablets to large-scale distributed systems and thousands of computational devices such as GPU cards. The library contains a wide variety of algorithms including training and inference algorithms for deep neural network models. Therefore, it has been used for deploying machine learning systems in many fields including speech recognition, computer vision, natural language processing, and text mining.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(12):121402-121402-13. doi:10.1115/1.4043539.

The importance of the turbulence closure to the modeling accuracy of the partially-averaged Navier–Stokes equations (PANS) is investigated in prediction of the flow around a circular cylinder at Reynolds number of 3900. A series of PANS calculations at various degrees of physical resolution is conducted using three Reynolds-averaged Navier–Stokes equations (RANS)-based closures: the standard, shear-stress transport (SST), and turbulent/nonturbulent (TNT) k–ω models. The latter is proposed in this work. The results illustrate the dependence of PANS on the closure. At coarse physical resolutions, a narrower range of scales is resolved so that the influence of the closure on the simulations accuracy increases significantly. Among all closures, PANS–TNT achieves the lowest comparison errors. The reduced sensitivity of this closure to freestream turbulence quantities and the absence of auxiliary functions from its governing equations are certainly contributing to this result. It is demonstrated that the use of partial turbulence quantities in such auxiliary functions calibrated for total turbulent (RANS) quantities affects their behavior. On the other hand, the successive increase of physical resolution reduces the relevance of the closure, causing the convergence of the three models toward the same solution. This outcome is achieved once the physical resolution and closure guarantee the precise replication of the spatial development of the key coherent structures of the flow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(12):121403-121403-10. doi:10.1115/1.4043773.

A feedback controlled thermal wall plate designed to investigate thermal boundary layer flows is described and validated. The unique capabilities of the design are the ability to modify the thermal boundary conditions in a variety of ways or to hold the wall-temperature fixed even when the flow above the wall is unsteady and strongly three-dimensional. These capabilities allow for the generation and study of thermal transport in nonequilibrium boundary layer flows driven by different perturbations and of varying complexity. The thermal wall plate and the experimental facility in which the thermal wall plate is installed are first described. The wall-plate is then validated in a zero-pressure-gradient (ZPG) boundary layer flow for conditions of a uniform wall temperature and a temperature step. It is then shown that the wall temperature can be held constant even when a hemisphere body is placed on the wall that produces large localized variations in the convective heat transfer coefficient. Last, since the thermal wall plate is intended to support the study of thermal transport in a variety of nonequilibrium boundary layer flow, several possible experimental configurations are presented and described.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(12):121404-121404-7. doi:10.1115/1.4043717.

This paper demonstrates the usefulness of treating subsonic Fanno flow (adiabatic flow, with friction, of a perfect gas in a constant-area pipe) as a polytropic process. It is shown that the polytropic model allows an explicit equation for mass flow rate to be developed. The concept of the energy transfer ratio is used to develop a close approximation to the polytropic index. Explicit equations for mass flow rate and net expansion factor in terms of upstream properties and pressure ratio are developed for Fanno and isothermal flows. An approximation for choked flow is also presented. The deviation of the results of this polytropic approximation from the values obtained from a traditional gas dynamics analysis of subsonic Fanno flow is quantified and discussed, and a typical design engineering problem is analyzed using the new method.

Commentary by Dr. Valentin Fuster

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