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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(6):061101-061101-13. doi:10.1115/1.4035876.

This paper considers the inherent unsteady behavior of the three-dimensional (3D) separation in the corner region of a subsonic linear compressor cascade equipped of 13 NACA 65-009 profile blades. Detailed experimental measurements were carried out at different sections in spanwise direction achieving, simultaneously, unsteady wall pressure signals on the surface of the blade and velocity fields by time-resolved particle image velocimetry (PIV) measurements. Two configurations of the cascade were investigated with an incidence of 4 deg and 7 deg, both at $Re=3.8×105$ and Ma = 0.12 at the inlet of the facility. The intermittent switch between two statistical preferred sizes of separation, large, and almost suppressed, is called bimodal behavior. The present PIV measurements provide, for the first time, time-resolved flow visualizations of the separation switch with an extended field of view covering the entire blade section. Random large structures of the incoming boundary layer are found to destabilize the separation boundary. The recirculation region, therefore, enlarges when these high vorticity perturbations blend with larger eddies situated in the aft part of the blade. Such a massive detached region persists until its main constituting vortex suddenly breaks down and the separation almost completely vanishes. The increase of the blockage during the separation growth phase appears to be responsible for this mechanism. Consequently, the proper orthogonal decomposition (POD) analysis is carried out to decompose the flow modes and to contribute to clarify the underlying cause-effect relations, which predominate the dynamics of the present flow scenario.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061102-061102-14. doi:10.1115/1.4035877.

A numerical-based (Reynolds-averaged Navier–Stokes (RANS)) investigation into the role of span and wing angle in determining the performance of an inverted wing in ground effect located forward of a wheel is described, using a generic simplified wheel and NACA 4412 geometry. The complex interactions between the wing and wheel flow structures are investigated to explain either increases or decreases for the downforce and drag produced by the wing and wheel when compared to the equivalent body in isolation. Geometries that allowed the strongest primary wing vortex to pass along the inner face of the wheel resulted in the most significant reductions in lift and drag for the wheel. As a result, the wing span and angle combination that would produce the most downforce, or least drag, in the presence of the wheel does not coincide with what would be assumed if the two bodies were considered only in isolation demonstrating the significance of optimizing these two bodies in unison.

Topics: Vortices , Wheels , Wings
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061103-061103-13. doi:10.1115/1.4036084.

Effectiveness of ventilated helmets in providing thermal comfort to a motorcycle rider is studied. Computational fluid dynamics (CFD) simulations of human thermoregulation system and the air flow in the air gap of a full-face motorcycle helmet are carried out. The thermal comfort of a rider is predicted using apparent temperature (AT) and wet-bulb global temperature (WBGT) heat indices. The effect of an increase in ambient temperature and relative humidity (RH) of air on the air flow and temperature in the region above the head is studied to predict the thermal comfort of the rider wearing full-face helmets. The effect of increasing the air gap between the head and the helmet is also studied. The results are then compared with the conditions when the rider is not wearing helmet. It is observed that the ventilated helmet is effective in providing thermal comfort to the rider only if the ambient air temperature is less than normal body temperature. For air temperature higher than the body temperature, vents do not provide any cooling to the head and the nonventilated helmet is more comfortable. Furthermore, CFD simulations are performed to investigate the effect of increase in RH in the ambient air on the thermal comfort of the rider. The increase in RH of air from 50% to 90% at a fixed ambient air temperature leads to an increase in AT and WBGT, indicating reduced thermal comfort of the rider.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061104-061104-19. doi:10.1115/1.4035952.

Hybrid bearings are mostly used in high-speed and load situations due to their better stability and loading capacity. They are typically designed with recess grooves to enhance both static and dynamic performance of the bearing. Previous theoretical studies on the influence of the recess geometrical shapes often utilize the Reynolds equation method. The aim of this paper is to analytically study the influence of various recess geometrical shapes on hybrid journal bearings. A three-dimensional (3D) computational fluid dynamics (CFD) model of a hybrid journal bearing is built, and a new method of response surface model is employed to determine the equilibrium position of the rotor. Based on the response surface model, an optimization scheme is used to search around the equilibrium position to get an accurate solution. The current analysis includes the geometries of rectangular, circular, triangular, elliptical, and annular shaped recesses. All these different shapes are studied assuming the same operating conditions, and static properties are used as the indices of the bearing performance. This study proposes a new design process using a CFD method with the ability of calculating the equilibrium position. The flow rate, fluid film thickness, and recess flow pattern are analyzed for various recess shapes. The CFD model is validated by published experimental data. The results show that the response surface model method is fast and robust in determining the rotor equilibrium position, even though a 3D-CFD model is utilized. The results suggest that recess shape is a dominant factor in hybrid bearing design.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;139(6):061201-061201-9. doi:10.1115/1.4035636.

In this paper, the effect of transverse magnetic field on a laminar liquid lead lithium flow in an insulating rectangular duct is numerically solved with three-dimensional (3D) simulations. Cases with and without buoyancy force are examined. The stability of the buoyant flow is studied for different values of the Hartmann number from 0 to 120. We focus on the combined influence of the Hartmann number and buoyancy on flow field, flow structure in the vicinity of walls and its stability. Velocity and temperature distributions are presented for different magnetic field strengths. It is shown that the magnetic field damps the velocity and leads to flow stabilization in the core fluid and generates magnetohydrodynamic (MHD) boundary layers at the walls, which become the main source of instabilities. The buoyant force is responsible of the generation of vortices and enhances the velocities in the core region. It can act together with the MHD forces to intensify the flow near the Hartmann layers. Two critical Hartmann numbers (Hac1 = 63, Hac2 = 120) are found. Hac1 is corresponding to the separation of two MHD regimes: the first one is characterized by a core flow maximum velocity, whereas the second regime is featured by a maximum layer velocity and a pronounced buoyancy effect. Hac2 is a threshold value of electromagnetic force indicating the onset of MHD instability through the generation of small vortices close to the side layers.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061202-061202-14. doi:10.1115/1.4035947.

This study reports an efficient reduction of the drag exerted by a flow on a cylinder when the former is forced with a plasma actuator. A three-electrode plasma device (TED) disposed on the surface of the body is considered, and the effect of the actuation frequency and amplitude is studied. Particle image velocimetry (PIV) measurements provided a detailed information that was processed to obtain the time-averaged drag force and to compare the performances of TED actuator and the canonical dielectric discharge barrier actuator. For the Reynolds number considered (Re = 5500), excitations with the TED actuator were more efficient, achieving drag reductions that attained values close to 40% with high net energy savings. The reduction of coherent structures using the instantaneous vorticity fields and a clustering technique allowed us to gain insight into the physical mechanisms involved in these phenomena. This highlights that the symmetrical forcing of the wake flow at its resonant frequency with the TED promotes symmetrical vorticity patterns which favor drag reductions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061203-061203-9. doi:10.1115/1.4035945.

Aspect ratio is an important parameter in the study of flow through noncircular microchannel. In this work, three-dimensional numerical study is carried out to understand the effect of cross aspect ratio (height to width) on flow in diverging and converging microchannels. Three-dimensional models of the diverging and converging microchannels with angle: 2–14 deg, aspect ratio: 0.05–0.58, and Reynolds number: 130–280 are employed in the simulations with water as the working fluid. The effects of aspect ratio on pressure drop in equivalent diverging and converging microchannels are studied in detail and correlated to the underlying flow regime. It is observed that for a given Reynolds number and angle, the pressure drop decreases asymptotically with aspect ratio for both the diverging and converging microchannels. At small aspect ratio and small Reynolds number, the pressure drop remains invariant of angle in both the diverging and converging microchannels; the concept of equivalent hydraulic diameter can be applied to these situations. Onset of flow separation in diverging passage and flow acceleration in converging passage is found to be a strong function of aspect ratio, which has not been shown earlier. The existence of a critical angle with relevance to the concept of equivalent hydraulic diameter is identified and its variation with Reynolds number is discussed. Finally, the effect of aspect ratio on fluidic diodicity is discussed which will be helpful in the design of valveless micropump. These results help in extending the conventional formulae made for uniform cross-sectional channel to that for the diverging and converging microchannels.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061204-061204-9. doi:10.1115/1.4035944.

The compressible Rayleigh–Taylor (RT) instability is studied by performing a suite of large eddy simulations (LES) using the Miranda and Ares codes. A grid convergence study is carried out for each of these computational methods, and the convergence properties of integral mixing diagnostics and late-time spectra are established. A comparison between the methods is made using the data from the highest resolution simulations in order to validate the Ares hydro scheme. We find that the integral mixing measures, which capture the global properties of the RT instability, show good agreement between the two codes at this resolution. The late-time turbulent kinetic energy and mass fraction spectra roughly follow a Kolmogorov spectrum, and drop off as k approaches the Nyquist wave number of each simulation. The spectra from the highest resolution Miranda simulation follow a Kolmogorov spectrum for longer than the corresponding spectra from the Ares simulation, and have a more abrupt drop off at high wave numbers. The growth rate is determined to be between around 0.03 and 0.05 at late times; however, it has not fully converged by the end of the simulation. Finally, we study the transition from direct numerical simulation (DNS) to LES. The highest resolution simulations become LES at around t/τ ≃ 1.5. To have a fully resolved DNS through the end of our simulations, the grid spacing must be 3.6 (3.1) times finer than our highest resolution mesh when using Miranda (Ares).

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2017;139(6):061301-061301-8. doi:10.1115/1.4035928.

A transport-equation-based homogeneous cavitation model previously assessed and validated against experimental data is used to investigate and explain the efficiency alteration mechanisms in Kaplan turbines. On the one hand, it is shown that the efficiency increase is caused by a decrease in energy dissipation due to a decreased turbulence production driven by a drop in fluid density associated with the cavitation region. This region also entails an increase in torque, caused by the modification of the pressure distribution throughout the blade, which saturates on the suction side. On the other hand, the efficiency drop is shown to be driven by a sharp increase in turbulence production at the trailing edge. An analysis of the pressure coefficient distribution explains such behavior as being a direct consequence of the pressure-altering cavitation region reaching the trailing edge. Finally, even though the efficiency alteration behavior is very sensitive to the dominant cavitation type, it is demonstrated that the governing mechanisms are invariant to it.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061302-061302-12. doi:10.1115/1.4035946.

Chemical reactors, air lubrication systems, and the aeration of the oceans rely, either in part or in whole, on the interaction of bubbles and their surrounding liquid. Even though bubbly mixtures have been studied at both the macroscopic and bubble level, the dissipation field associated with an individual bubble in a shear flow has not been thoroughly investigated. Exploring the nature of this phenomenon is critical not only when examining the effect a bubble has on the dissipation in a bulk shear flow but also when a microbubble interacts with turbulent eddies near the Kolmogorov length scale. In order to further our understanding of this behavior, this study investigated these interactions both analytically and experimentally. From an analytical perspective, expressions were developed to model the dissipation associated with the creeping flow fields in and around a fluid particle immersed in a linear shear flow. Experimentally, tests were conducted using a simple test setup that corroborated the general findings of the theoretical investigation. Both the analytical and experimental results indicate that the presence of bubbles in a shear flow causes elevated dissipation of kinetic energy.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061303-061303-15. doi:10.1115/1.4035929.

A novel scaling law for the tip vortex cavitation (TVC) noise was determined, employing the Rankine vortex model, the Rayleigh–Plesset equation, the lifting surface theory, the boundary layer effect, and the number of bubbles generated per unit time $(N0)$. All terms appearing in the final derived scaling law are well known three-dimensional (3D) lifting surface parameters, except for $N0$. In this study, the dependence of $N0$ with inflow velocity and hydrofoil dimension is investigated experimentally while trying to retain the same TVC patterns among different experimental conditions. Afterward, the effect of $N0$ on the TVC noise is analyzed. Optimal TVC observation conditions are determined from consideration of cavitation number and Reynolds number of two comparable conditions. Two geometrically scaled hydrofoils are concurrently placed in a cavitation tunnel for the hydrofoil size variation experiment. Wall effects and flow field interaction are prevented with the aid of computational fluid dynamics. Images taken with a high‐speed camera are used to count $N0$ by visual inspection. The noise signals at all conditions are measured and an acoustic bubble counting technique, to supplement visual counting, is devised to determine $N0$ acoustically from the measured noise data. The broad-band noise scaling law incorporating $N0$ and the International Towing Tank Conference (ITTC) cavitation noise estimation rule for hydrofoil are both applied to estimate the TVC noise level for comparison with the measured noise level. The noise level estimated by the broad-band noise scaling law accounting for the acoustically estimated $N0$ gives the best agreement with the measured noise level.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(6):061304-061304-6. doi:10.1115/1.4035949.

Flow-induced vibration of hydrofoils affects pressure pulsations on their surfaces and influences cavitation inception and desinence. As these pulsations depend on the hydrofoil material, cavitation inception and desinence numbers for hydrofoils of the same shape made from different metals can be substantially different. This conclusion is based on the comparison of the multistep numerical analysis of fluid–structure interaction for hydrofoils Cav2003 with earlier obtained experimental data for them. The material impact on cavitation must be taken into account in future experiments.

Commentary by Dr. Valentin Fuster