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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(11):111101-111101-9. doi:10.1115/1.4037060.

A new approach based on the open source tool OpenFOAM is presented for the numerical simulation of a mini gerotor pump working at low pressure. The work is principally focused on the estimation of leakage flow in the clearance disk between pump case and gears. Two main contributions are presented for the performance of the numerical simulation. On one hand, a contact point viscosity model is used for the simulation of solid–solid contact between gears in order to avoid the teeth tip leakage. On the other hand, a new boundary condition has been implemented for the gear mesh points motion in order to keep the mesh quality while moving gears with relative velocity. Arbitrary coupled mesh interface (ACMI) has been used both in the interface between clearance disk in inlet/outlet ports and between clearance disk and interteeth fluid domain. Although the main goal of the work is the development of the numerical method rather than the study of the physical analysis of the pump, results have been compared with experimental measurement and a good agreement in volumetric efficiency and pressure fluctuations has been found. Finally, the leakage flow in the clearance disk has been analyzed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111102-111102-9. doi:10.1115/1.4037035.

Wall shear stress is characterized for underexpanded axisymmetric impinging jets for the application of aerodynamic particle resuspension from a surface. Analysis of the flow field resulting from normally impinging axisymmetric jets is conducted using computational fluid dynamics (CFD). A normally impinging jet is modeled with a constant area nozzle while varying the height to diameter ratio (H/D) and the inlet pressures. Schlieren photography is used to visualize the density gradient of the flow field for validation of the CFD. A dimensionless jet parameter (DJP) is developed to describe flow regimes and characterize shear stress. The DJP is defined as being proportional to the jet pressure ratio divided by the H/D ratio squared. Maximum wall shear stress is examined as a function of DJP with three distinct regimes: (i) subsonic impingement (DJP < 1), (ii) transitional (1 < DJP < 2), and (iii) supersonic impingement (DJP > 2). It is observed that wall shear stress is limited to a finite value due to jet energy dissipation in shock structures, which become a dominant dissipation mechanism in the supersonic impingement regime. Additionally, the formation of shock structures in the wall flow was observed for DJP > 2, resulting in difficulties with dimensionless analysis. In subsonic impingement and transitional regimes, equations as a function of the DJP are obtained for the maximum wall shear stress magnitude, maximum shear stress location, and shear stress decay. Using these relationships, wall shear stress can be predicted at all locations along the impingement surface.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111105-111105-11. doi:10.1115/1.4037171.

Compared with traditional speed regulation (SR) approaches like variable frequency and hydraulic coupling, magnetorheological clutch (MRC) provides a more superior solution for high-efficiency energy saving SR. However, recent developments have demonstrated that severe heating is an outstanding challenge for MRC, especially in high-power applications. Among commonly used cooling methods, liquid cooling offers a viable alternative for the problem. Aiming at pre-evaluating the cooling efficiency of a liquid-cooled MRC in high-power situations, this study introduces a heat-flow coupling simulation method. In this paper, theoretical basis for the simulation is presented first, which is followed by an illustration of the heat-flow coupling simulation. This paper details the simulation model establishment, finite element meshing (FEM), boundary conditions, and simulation parameters. After the simulations, the results concerning the steady flow field of the internal coolant, along with the steady-state temperature fields of MRC, magnetorheological (MR) fluids and the coolant are presented and discussed. Finally, several heating tests of an MRC prototype under various operation conditions are performed and the results verify the correctness and rationality of the simulation.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111106-111106-9. doi:10.1115/1.4037172.

The present work shows that weak blast waves that are considered as being harmless can turn to become fatal upon their reflections from walls and corners inside a building. In the experimental part, weak blast waves were generated by using an open-end shock tube. A three level building model was placed in vicinity to the open-end of the used shock tube. The evolved wave pattern inside the building rooms was recorded by a sequence of schlieren photographs; also pressure histories were recorded on the rooms' walls. In addition, numerical simulations of the evolved flow field inside the building were conducted. The good agreement obtained between numerical and experimental results shows the potential of the used code for identifying safe and dangerous places inside the building rooms penetrated by the weak blast wave.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;139(11):111201-111201-10. doi:10.1115/1.4037083.

An investigation is conducted on the flow in a moderately wide gap between an inner rotating shaft and an outer coaxial fixed tube, with stationary end-walls, by three-dimensional Reynolds-averaged Navier–Stokes (RANS) computational fluid dynamics (CFD), using the realizable model. This approach provides three-dimensional spatial distributions of static and dynamic pressures that are not directly measurable in experiment by conventional nonintrusive optics-based techniques. The nonuniform pressure main features on the axial and meridional planes appear to be driven by the radial momentum equilibrium of the flow, which is characterized by axisymmetric Taylor vortices over the Taylor number range $2.35×106≤Ta≤6.47×106$. Regularly spaced static and dynamic pressure maxima on the stationary cylinder wall follow the axial stacking of the Taylor vortices and line up with the vortex-induced radial outflow documented in previous work. This new detailed understanding has potential for application to the design of a vertical turbine pump head. Aligning the location where the gauge static pressure (GSP) maximum occurs with the central axis of the delivery pipe could improve the head delivery, the pump mechanical efficiency, the system operation, and control costs.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111202-111202-7. doi:10.1115/1.4037058.

Mathematical relations are obtained for velocities and pressure distribution for a fluid entering the peripheral clearance of a pair of rotating concentric disks that converges and discharges through an opening at the center. Both, the flows in the gap of corotating disks and in the gap of contrarotating disks can be predicted using the present analytical solutions. The prediction of instability of radial velocity for corotating disks at the speed ratio of unity is very important for practical applications. The radial velocity profile is similar to a parabolic profile exactly at speed ratio of unity. The profile drastically changes with the small difference of $±1%$ in the disks’ rotation. The radial convection was observed in the tangential velocity at a low radius. Centrifugal force caused by disk rotation highly influences the flow resulting in backflow on the disks. The pressure consists of friction losses and convective inertia. Therefore, the pressure decrease is high for increased speed ratio, throughflow Reynolds number, and rotational Reynolds number. The pressure decrease for the flow between contrarotating disks is lesser than that for the flow between corotating disks due to decreased viscous losses in the tangential direction. This study provides valuable guidance for the design of devices where disks are rotated independently by highlighting the instabilities in the radial velocity at the speed ratio of unity.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111203-111203-10. doi:10.1115/1.4037044.

This study presents experimental results on the effects of riblets on the coherent structures of turbulence within a turbulent spot. The riblet spacings of the study correspond to 0.5 and 1.5 times the natural spacing of the low-speed streak. The cross-sectional dimensions of the riblets were chosen to control the spatial distribution of wave packets consisting of streamwise-aligned hairpin vortices. Both riblet spacings demonstrated effective control on the spanwise positioning of the wave packets. The wider-spaced riblets reduced spanwise mutual interaction between wave packets. The closer-spaced riblets promoted this interaction via spanwise-oriented vortical structures which produced stronger turbulent fluctuations.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111204-111204-8. doi:10.1115/1.4037140.

This paper expands on the numerical simulation of entropy noise by performing a comparison of two commonly used models for resolving turbulent flow field: large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS). A brand new numerical procedure was developed allowing an accurate reproduction of two-dimensional spatial and temporal temperature variations of a nonuniform temperature profile. Experimental investigation was performed for the same nonuniform temperature profile, and comparison of the entropy noise level measured experimentally and evaluated numerically using the two models was performed. It was shown that large eddy simulation allows a better prediction of entropy noise within the developed numerical procedure than URANS.

Commentary by Dr. Valentin Fuster

### Research Papers: Techniques and Procedures

J. Fluids Eng. 2017;139(11):111401-111401-13. doi:10.1115/1.4037034.

Foam structures are a class of modern microporous media that possesses high thermal conductivity, large accessible specific surface area, and high porosities. Nowadays, industrial applications, such as filtration, heat exchange and chemical reaction, etc., utilize porous media such as open-cell foams. Knowledge of pressure drop induced by these foam matrices is essential for successful design and operation of high-performance industrial systems. The homogenized pressure drop data in the literature are widely dispersed (up two orders of magnitude) despite numerous researches has been conducted since two decades. Most of the empirical pressure drop correlations were derived using Ergun-like approach. In this view, a careful evaluation of empirical correlations as well as the relationship of intrinsic flow law characteristics (permeability and inertia coefficient) with morphological parameters is imperative. This paper presents the start-of-the-art of various pressure drop correlations as well as highlights the ambiguities and inconsistencies in various definitions of several key parameters. The applicability of the empirical correlations presented in the literature was examined by comparing them against numerically calculated pressure drop data of open-cell foams (metal and ceramic) for the porosities ranging from 0.60 up to 0.95. A comprehensive study has been conducted to identify the reasons of dispersed pressure drop data in the literature. Although substantial progress has been made in the field of fluid flow in open-cell foams, it is yet difficult to predict pressure drop data from a given set of morphological parameters.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(11):111402-111402-12. doi:10.1115/1.4036590.

Despite recent interests in complex fluid–structure interaction (FSI) problems, little work has been conducted to establish baseline multidisciplinary FSI modeling capabilities for research and commercial activities across computational platforms. The current work investigates the fluid modules of contemporary FSI methodologies by solving a purely fluid problem at low Reynolds numbers to improve understanding of the fluid dynamic capabilities of each solver. By incorporating both monolithic and partitioned solvers, a holistic comparison of computational accuracy and time-expense is presented between lattice-Boltzmann methods (LBM), coupled Lagrangian–Eulerian (CLE), and smoothed particle hydrodynamics (SPH). These explicit methodologies are assessed using the classical square lid-driven cavity for low Reynolds numbers (100–3200) and are validated against an implicit Navier–Stokes solution in addition to established literature. From an investigation of numerical error associated with grid resolution, the Navier–Stokes solution, LBM, and CLE were all relatively mesh independent. However, SPH displayed a significant dependence on grid resolution and required the greatest computational expense. Throughout the range of Reynolds numbers investigated, both LBM and CLE closely matched the Navier–Stokes solution and literature, with the average velocity profile error along the generated cavity centerlines at 1% and 4%, respectively, at Re = 3200. SPH did not provide accurate results whereby the average error for the centerline velocity profiles was 31% for Re = 3200, and the methodology was unable to represent vorticity in the cavity corners. Results indicate that while both LBM and CLE show promise for modeling complex fluid flows, commercial implementations of SPH demand further development.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Fluids Eng. 2017;139(11):114501-114501-6. doi:10.1115/1.4036666.

This study reports the results of turbulent flows forward facing steps (FFS) in pressure gradients using a particle image velocimetry (PIV) technique to obtain data up to 68 step heights downstream. The contours of two-point velocity correlations indicate that regardless of the pressure gradients, the physical size of the coherent structures characterized by the autocorrelations grows as the flow develops downstream along the step. Additionally, adverse pressure gradient (APG) elevates the size of the autocorrelations.

Commentary by Dr. Valentin Fuster