Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;140(11):111101-111101-10. doi:10.1115/1.4040067.

This study deals with the pressure peak position shift with deadrise angle during the initial phase of a two-dimensional (2D) wedge water entry. The finite volume method with volume of fluid (VOF) and dynamic mesh technique is used to simulate the water entry process of the 2D wedges with the moderate deadrise angles within the range α = 20 deg–60 deg. The results show that with the increasing deadrise angle, the pressure peak position shifts from the spray root to the wedge apex. And, the critical deadrise angle of pressure peak position shift is identified in the range between 40.8 deg and 41 deg, which is more precise than previous studies. In the initial stage of water entry of a 2D wedge, the pressure on wedge side is determined by the dynamic pressure term and unsteady term simultaneously. For the spray root position, at small deadrise angles, the unsteady term is stronger than the dynamic pressure term; at large deadrise angles, the former is weaker than the later.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111102-111102-13. doi:10.1115/1.4040069.

Three-dimensional (3D) numerical flow simulations with a mass transfer cavitation model are performed to analyze cloud cavitation at two different flow configurations, i.e., hydrofoil and orifice flows, focusing on the turbulence and cavitation model interaction, including a mixture eddy viscosity reduction and cavitation model parameter modification. For the cavitating flow around the hydrofoil with circular leading edge, a good agreement to the measured shedding frequencies as well as local cavitation structures is obtained over a wide range of operation points, even with a moderate boundary layer resolution, i.e., utilizing wall functions (WF), which are found to be adequate to capture the re-entrant jet reasonably in the absence of viscous separation. Simulations of the orifice flow, that exhibit significant viscous single-phase (SP) flow separation, are analyzed concerning the prediction of choking and cloud cavitation. A low-Reynolds number turbulence approach in the orifice wall vicinity is suggested to capture equally the mass flow rate, flow separation, and cloud shedding with satisfying accuracy in comparison to in-house measurements. Local cavitation structures are analyzed in a time-averaged manner for both cases, revealing a reasonable prediction of the spatial extent of the cavitation zones. However, different cavitation model parameters are utilized at hydrofoil and orifice for best agreement with measurement data.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111103-111103-8. doi:10.1115/1.4040037.

Globe valve is widely used in numerous industries, and its driving energy consumption accounts for high percentages of the whole piping system. In order to figure out novel globe valves with low energy consumption, the pilot control globe valve (PCGV) is proposed, which is made up of a main valve and a pilot valve. By the pressure difference of fluid itself, the opened/closed status of the main valve can be controlled by the pilot valve, which can save driving energy and shorten the response time. In order to fit PCGV in an angle displaced piping system, the pilot control angle globe valve (PCAGV) is developed. In this paper, with validated numerical methods, both steady and transient simulations focusing on the valve core diameter, the single/multi orifices, orifice diameters and their arrangements located on the valve core bottom are presented. The results show that the pressure difference increases with the increase of the valve core diameter and the decrease of the orifice diameter, and large orifice diameters (d > 12 mm) should be avoided in case the valve cannot be opened. As for the multi orifices, it can be treated as a single orifice which having similar cross-sectional area. Meanwhile, the opening time of the main valve also increases with the increase of the valve core diameter correspondingly. Besides, a fitting formula of pressure difference calculation depending on the inlet velocity and the valve core diameter is obtained, which is a power–law relationship.

Topics: Pressure , Valves
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111104-111104-18. doi:10.1115/1.4039713.

Hydraulic turbines are more frequently used for power regulation and thus spend more time providing spinning reserve for electrical grids. Spinning reserve requires the turbine to operate at its synchronous rotation speed, ready to be linked to the grid in what is termed the speed-no-load (SNL) condition. The turbine's runner flow in SNL is characterized by low discharge and high swirl leading to low-frequency high amplitude pressure fluctuations potentially leading to blade damage and more maintenance downtime. For low-head hydraulic turbines operating at SNL, the large pressure fluctuations in the runner are sometimes attributed to rotating stall. Using embedded pressure transducer measurements, mounted on runner blades of a model propeller turbine, and numerical flow simulations, this paper provides an insight into the inception mechanism associated with rotating stall in SNL conditions. The results offer evidence that the rotating stall is in fact associated with an unstable vorticity distribution not associated with the runner blades themselves.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111105-111105-9. doi:10.1115/1.4040107.

The flow around passenger cars is characterized by many different separation structures, typically leading to vortices and areas of reversed flow. The flow phenomena in these patches show a strong interaction and the evolution of flow structures is difficult to understand from a physical point of view. Analyzing surface properties, such as pressure, vorticity, or shear stress, helps to identify different phenomena, but still it is not well understood how these are created. This paper investigates the crossflow separation (CFS) on the A-pillar of a passenger car using numerical simulations. It is discussed how the CFS and the resulting A-pillar vortex can be identified as well as how it is created. Additionally, the vortex strength is determined by its circulation to understand and discuss how the vortex preserves until it merges with the rear wake of the vehicle.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111106-111106-12. doi:10.1115/1.4040224.

This study reports an experimental investigation of two planar jets in a crossflow in a tandem arrangement. Tests were conducted in an open-jet wind tunnel facility using two-dimensional (2D)-particle imaging velocimetry (PIV) measurement. Using the terminology in the dual jets in a quiescent ambient, the mean flow field of the crossflow arrangement was divided into a converging region, a merging region, and a combined region. An approach to determining the range of these three regions was proposed based on the mean characteristics of horizontal velocity profiles of the flow field, validated by the experimental data. The momentum-dominated near field (MDNF) for the rear jet in the dual-jet configuration was recognized using the horizontal offset of mean jet trajectory, which accordingly gives a quantitative definition of the MDNF range. Discussions were made on the effects of different parameters on the three regions and MDNF. Finally, snapshot proper orthogonal decomposition (POD) analysis was conducted, characterizing the coherent structures of the flow field, particularly the large-scale vortices. It was observed that the large-scale vortices mainly occur in the shear layers of the jets and their occurrence is affected by the parameters of the jets. In addition, compared with the single-jet configuration, the introduction of the front jet was found to contribute to the occurrence and development of the large-scale vortices.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111107-111107-9. doi:10.1115/1.4040299.

Efficacy of several large-scale flow parameters as transition onset markers are evaluated using direct numerical simulation (DNS) of boundary layer bypass transition. Preliminary results identify parameters (k2D/ν) and u/U to be a potentially reliable transition onset marker, and their critical values show less than 15% variation in the range of Re and turbulence intensity (TI). These parameters can be implemented into general-purpose physics-based Reynolds-averaged Navier–Stokes (RANS) models for engineering applications.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;140(11):111201-111201-12. doi:10.1115/1.4039865.

An efficient large-eddy simulation (LES) approach is investigated for laminar-to-turbulent transition in boundary layers. This approach incorporates the boundary-layer stability theory. Primary instability and subharmonic perturbations determined by the boundary-layer stability theory are assigned as forcing at the inlet of the LES computational domain. This LES approach reproduces the spatial development of instabilities in the boundary layer, as observed in wind tunnel experiments. Detailed linear growth and nonlinear interactions that lead to the H-type breakdown are well captured and compared well to previous direct numerical simulation (DNS). Requirements in the spatial resolution in the transition region are investigated with connections to the resolution in turbulent boundary layers. It is shown that the subgrid model used in this study is apparently dormant in the overall transitional region, allowing the right level of the growth of small-amplitude instabilities and their nonlinear interactions. The subgrid model becomes active near the end of the transition where the length scales of high-order instabilities become smaller in size compared to the given grid resolution. Current results demonstrate the benefit of the boundary-layer forcing method for the computational cost reduction.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111202-111202-12. doi:10.1115/1.4040031.

The aim of this study is to examine the effects of Reynolds number (Re = 6000–20,000) on mean and turbulent quantities as well as turbulent structures in the near and intermediate regions of equilateral triangular and round sharp contraction jets. The results show shorter potential core length, faster growth of turbulence intensity, and faster diffusion of turbulent structures to the centerline of the triangular jets, implying enhanced mixing in the near field of these jets. On the other hand, the velocity decay and jet spread rates are higher in the round jets. The obtained data in the round jets show that the jet at Re = 6000 has the most effective mixing, while an increase in Reynolds number reduces the mixing performance. In the triangular jets, however, no Reynolds number effects were observed on the measured quantities including the length of the potential core, the decay and spread rates, the axis-switching locations, and the value of the Reynolds number. In addition, the asymptotic values of the relative turbulence intensities on the jet centerline are almost independent of the Reynolds number and geometry. The ratios of transverse and spanwise Reynolds stresses are unity except close to the jet exit where the flow pattern in the major plane of the triangular jet deflects toward the flat side. Proper orthogonal decomposition (POD) analysis revealed that turbulent structures in minor and major planes have identical fractional kinetic energy. The integral length scales increased linearly with the streamwise distance with identical slope for all the test cases.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2018;140(11):111301-111301-8. doi:10.1115/1.4040068.

Cavitation has bothered the hydraulic machinery for centuries, especially in pumps. It is essential to establish a solid way to predict the unsteady cavitation evolution with considerable accuracy. A novel cavitation model was proposed, considering the rotating motion characteristic of centrifugal pump. Comparisons were made with three other cavitation models and validated by experiments. Considerable agreements can be noticed between simulations and tests. All cavitation models employed have similar performance on predicting the pump head drop curve with proper empirical coefficients, and also the unsteady cavitation evolution was well solved. The proposed rotating corrected-based cavitation model (rotating based Zwart-Gerber-Belamri (RZGB)) obtained identical triangle cavity structure with the experiment visualizations, while the others also got triangle structure but with opposite direction. The maximum flow velocity in the impeller passage appears near the shroud, contributing to the typical triangle cavity structure. A preprocessed method for instant rotating images was carried out for evaluating the erosion risk area in centrifugal pump, based on the standard deviation of gray level. The results imply that the unsteady rear part of the attached cavity is vulnerable to be damaged, where the re-entrant flow was noticed. This work presented a suitable cavitation model and reliable numerical simulation approach for predicting cavitating flows in centrifugal pump.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111302-111302-9. doi:10.1115/1.4040066.

Combined damage caused by cavitation and abrasion is a serious problem concerning hydraulic structures and machinery operating in hyper-concentrated sediment-laden rivers. Conceptualization of a model for simulation and assessment of the combined damage, therefore, becomes necessary. Experimental results demonstrate that sediments cast a strong influence on the combined damage caused by cavitation and abrasion. Sediments with size larger compared to a critical size tend to aggravate the combined damage, while sediments with size smaller compared to critical relieve the combined damage effect when compared against cavitation-only damage. Based on these results, a new model has been proposed and built in order to predict the combined damage and assess the range of sediments that relieve or aggravate the damage as sediments pass through the structure and machinery. The model represents an integral with damage as the integrand and sediments representing the domain of integration, and was built in three steps—the first step establishes a relationship between damage and sediments of a single size (SS model); the second step establishes a relationship between damage and sediments from an actual river (MS model); and the third step proposes a standard to assess the damaging effect on hydro machinery (CS model). Model parameters were verified using 74 cases of laboratory experiments. By comparing simulation results against experimental data, it has been inferred that the proposed model can be employed to study practical problems in a predictive manner and promote safe operation of reservoirs by predicting damage characteristics of river water.

Commentary by Dr. Valentin Fuster

Research Papers: Techniques and Procedures

J. Fluids Eng. 2018;140(11):111401-111401-11. doi:10.1115/1.4040215.

The interaction of vortex rings with thin wire mesh screens is investigated using laser-induced fluorescence (LIF) and molecular tagging velocimetry (MTV). The existence of vortex shedding from individual wires of the porous screens, suggested by prior works, is shown and compared to flow visualization results. A range of interaction Reynolds numbers and screen porosities are studied to determine the conditions affecting the interaction. Transmitted vortex (TV) ring formation is shown to be a function of vortex shedding and the shedding Reynolds number, but not a function of porosity. Screen porosity is shown to affect the TV convective speed but did not impact the formation behaviors. Three major flow regimes existed for the interaction: TV formation with no vortex shedding, TV formation with visible vortex shedding, and no downstream formation with strong shed vortices.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(11):111402-111402-10. doi:10.1115/1.4040109.

Preparation of large-scale homogeneous solutions of drag reducing polymers requires an appropriate mixing procedure to ensure full disentanglement of the polymer chains without chain scission due to over-mixing. The latter is known as mechanical degradation and reduces the performance of drag reducing polymers. The dominant large-scale mixing parameters including time, impeller type, impeller speed, and impeller-to-tank diameter ratio are investigated to obtain a recipe for maximum mixing with minimum polymer degradation. Three water-based solutions of 100 ppm Superfloc A-110 (flexible structure), Magnafloc 5250 (flexible structure), and Xanthan Gum (XG) (rigid structure) are considered. The performance of the mixing parameters for each polymer is evaluated based on the solution viscosity in comparison with the highest viscosity (i.e., optimum mixing) obtained by 2 h of low-shear mixing of a small-scale polymer solution using a magnetic stirrer. The results demonstrate that optimum large-scale mixing is obtained at mean and maximum shear rates of ∼17 s−1 and ∼930 s−1, respectively, after 2–2.5 h of mixing for each of the polymers. This shear rate is obtained here using a three-blade marine impeller operating at 75 rpm and at impeller-to-tank diameter ratio of 0.5. The resulting polymer solution has the highest viscosity, which is an indication of minimal degradation while achieving complete mixing. It is also confirmed that chemical degradation due to contact with a stainless steel impeller is negligible.

Commentary by Dr. Valentin Fuster

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