0

### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2019;141(7):071101-071101-13. doi:10.1115/1.4041985.

In this study, a comparison of two different Kaplan turbine runners with differently shaped turbine blades was performed. The two turbines differed in the selection of the hydrofoil, the main hydrofoil parameters of which had been modified including, the position of maximum thickness and curvature and the inlet edge radius. Both turbines (unmodified and modified hydrofoils) were tested on a rig designed for low pressure model turbine acceptance tests. The effect of blade shape on cavitation inception, development, and intensity was demonstrated using computer aided visualization. Visualization was performed on the suction side of Kaplan runner blade where the shape of the blade determines cavitation inception and development. The modified Kaplan turbine reduced the cavitation phenomena, and as a result, both turbine performance and output increased for the selected operating points. This demonstrates that choosing the right turbine blade shape is key for optimal turbine performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071102-071102-13. doi:10.1115/1.4042035.

When an axial flow enters a rotating diffuser or nozzle, a swirl boundary layer appears at the wall and interacts with the axial boundary layer. Below a critical flow number φc, there is a flow separation, known in the turbomachinery context as part load recirculation. This paper extends the previous work for a cylindrical coaxial rotating pipe still considering the influence of the centrifugal force by varying the pipe's radius, yielding a coaxial rotating circular diffuser or nozzle. The integral method of boundary layer theory is used to describe the flow at the inlet of a rotating circular diffuser or nozzle, obtaining a generalized von Kármán momentum equation. This work conducts experiments to validate the analytical results and shows the influence of Reynolds number, flow number, apex angle, and surface roughness on the boundary layers evolution. By doing so, a critical flow number for incipient flow separation is analytically derived, resulting in a stability map for part load recirculation depending on Reynolds number and apex angle. Hereby, positive apex angles (diffuser) and negative apex angles (nozzle) are considered.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071103-071103-11. doi:10.1115/1.4042169.

An upstream cylindrical bluff body connected to a tip body via an aluminum cantilever beam was tested as energy harvester in a wind tunnel. The characteristics and behavior of the different tip body configurations and lengths of aluminum cantilever beam were studied to optimize design to extract wind energy. Particular attention was paid to measure vibration amplitude and frequency response as a function of reduced velocity. Dynamic response showed that the device's behavior was dependent on both tip body shape and cantilever beam length. Flow visualization tests showed that high amplitude vibration was obtainable when a vortex was fully formed on each side of the downstream tip body. This was exemplified in a symmetrical triangular prism tip body at L/D1 = 5, where its structure's vibration frequency was close to its natural frequency. At such configuration, electrical energy was captured using a polyvinylidene fluoride (PVDF) piezoelectric beam of different load resistances, where an optimized load resistance could be found for each Reynolds number. Although power output and efficiency obtained were considerably weak when compared to those of traditional wind turbine, the design merits further research to improve its performance under various circumstances.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071104-071104-9. doi:10.1115/1.4042161.

Oil-lubricated bearings are widely used in high-speed rotating machines such as those found in automotive industries and aerospace. However, environmental issues and risk-averse operations are resulting in the removal of oil and the replacement of all sealed oil bearings with reliable water-lubricated bearings. The low viscosity of water increases Reynolds numbers drastically and therefore makes water-lubricated bearings prone to turbulence effects. This requires finer meshes for finite element modeling when compared to oil-lubricated bearings as the low-viscosity fluid produces a very thin lubricant film. Analyzing water-lubricated bearings can also produce convergence and accuracy issues in traditional oil-based analysis codes. Fitting the velocity profile with experiments having a nondimensional wall distance $y+$ in a certain range results in Ng-optimized Reichardt's constants $k$ and $δ+$. The definition of $y+$ can be used to approximate the first layer thickness calculated for a uniform mesh. On the condition that the $y+$ is fixed to that of a standard oil bearing for which an oil-bearing code was validated, the number of elements across the film thickness and coefficients used in the eddy-viscosity equation can be adjusted to allow for convergence with other fluids other than that which the traditional oil-bearing code was designed for. This study proposed a new methodology to preserve the y+ value to make water-lubricated thrust bearing models valid. A method for determining the required number of cross-film elements in water-lubricated bearings was found. The results of this study could aid in improving future designs and models of water-lubricated bearings.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071105-071105-13. doi:10.1115/1.4042037.

Power plant water usage is a coupling of the energy–water nexus; this research investigates water droplet motion, with implications for water recovery in cooling towers. Simulations of a 2.6 mm-diameter droplet motion on a hydrophobic, vertical surface were conducted in xflow using the lattice Boltzmann method (LBM). Results were compared to two experimental cases; in the first case, experimental and simulated droplets experienced 30 Hz vibrations (i.e., ±0.1 mm x-direction amplitude, ±0.2 mm y-direction amplitude) and the droplet ratcheted down the surface. In the second case, 100 Hz vibrations (i.e., ±0.8 mm x-direction amplitude, ±0.2 mm y-direction amplitude) caused droplet ejection. Simulations were then conducted for a wide range of frequencies (i.e., 10–100 Hz) and amplitudes (i.e., ±0.018–50 mm), resulting in maximum accelerations of 0.197–1970 m/s2. Under low maximum accelerations (e.g., <7 m/s2), droplets rocked upward and downward in rocking mode, but did not overcome the contact angle hysteresis and, therefore, did not move. As acceleration increased, droplets overcame the contact angle hysteresis and entered ratcheting mode. For vibrations that prompted droplet motion, droplet velocities varied between 10–1000 mm/s. At capillary numbers above approximately 0.0044 and Weber numbers above 3.6, liquid breakup was observed in ratcheting droplets (e.g., the formation of smaller child droplets from the parent droplet). It was noted that both x- and y-direction vibrations were required for droplet ejection.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071106-071106-10. doi:10.1115/1.4042162.
FREE TO VIEW

Coaxial rotor uninhabited aerial vehicles (UAVs) are compact compared to single rotor UAVs of comparable capacity. At the low Reynolds numbers (Re) where they operate, the simplifying assumptions from high Re rotor aerodynamics are not valid. The low Re coaxial rotor flowfield is studied including aerodynamic interactions and their effect on performance. The evolution of the wake is captured using high-speed stereo particle image velocimetry (SPIV). Improvement of upper rotor performance due to viscous swirl recovery from the lower rotor is discovered and then verified by analyzing PIV data. Interesting vortex–vortex sheet interactions are observed under the coaxial rotor affecting wake structure spatially and temporally. A qualitative model explaining the observed wake interaction phenomena is presented. Comparison with the performance of high Re rotors shows higher profile and induced drag at low Re for the same thrust coefficient.

Topics: Rotors , Vortices , Wakes , Thrust
Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2019;141(7):071201-071201-11. doi:10.1115/1.4041989.

The objective of this paper is to investigate the effects of nozzle spacing on the mean velocity and higher-order turbulent statistics of free twin round jets produced from sharp contraction nozzles. The experiments were performed in an air chamber where four nozzle spacing ratios, S/d = 2.8, 4.1, 5.5, and 7.1, were investigated at a fixed Reynolds number of 10,000. A planar particle image velocimetry (PIV) system was used to conduct the velocity measurements. The results show that downstream of the potential core, a reduction in spacing ratio leads to an earlier and more intense interaction between the jets, indicated by enhanced half-velocity width spread rate in the inner shear layers and a significant rise of turbulent intensities and vorticity thickness along the symmetry plane. A reduction in spacing ratio, however, confines the ambient fluid entrainment along the inner shear layers leading to a reduced core jet velocity decay rate. The closer proximity of the jets also leads to the decrease of Reynolds stresses in the inner shear layers but not in the outer shear layers. The Reynolds stress ratios along the jet centerline reveal the highest anisotropy in the potential core region.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071202-071202-5. doi:10.1115/1.4042174.

The interfacial instability of Rayleigh–Taylor type at the cylindrical boundary involving the liquid phase and vapor phase of a fluid has been considered when the vapor is warmer than the liquid. We use viscous potential flow theory to include the viscosity at the interface. To examine the stability of the arrangement, the normal-mode analysis is performed together with the effect of heat as well as mass transfer and free swirl. The physical system consists of an annular fluid layer restricted in a cylinder with vapor phase in the core. This work investigates the effect of a variety of variables on the instability of the interface. It is found that when the heat transfer constant increases, the range of stability increases. Also, the range of stability increases faster in the presence of swirling.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2019;141(7):071301-071301-15. doi:10.1115/1.4041988.

A numerical model of a rectangular tank containing a layered liquid is modeled for studying layered sloshing wave. The Arbitrary Lagrangian Eulerian method is used to track the development for both the interfacial and free surface of the fluid domain. A series of cases are simulated for baffled and unbaffled sloshing with various excitation frequencies and various baffle configurations. A case containing a submerged block is also simulated to observe the interfacial wave interaction with the block structure and to observe how the position and size of the block affect the interfacial wave in a fluid. Velocity screenshots are analyzed for observing the velocity distribution in the layers and to observe the behavior of the interfacial layer for baffled and unbaffled tank cases. A fast Fourier transform spectral analysis of the layered liquid sloshing time series for both the interfacial layer and free surface layer is presented to observe the energy in the fluid layers as well as to observe the dominant peak frequency for both the layers.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2019;141(7):071302-071302-8. doi:10.1115/1.4041990.

The impact and bounce of a bubble with a solid surface is of significant interest to many industrial processes such as froth flotation and biomedical engineering. During the impact, a liquid film becomes trapped between the bubble and the solid surface. The pressure buildup in this film leads to the generation of a film force. The drainage rate of this film plays a crucial role in dictating the bouncing process and is known to be a function of the radial film size. However, radial film size is not an easily attained experimental measurement and requires advanced instrumentation to capture. The bouncing process has been characterized using nondimensional numbers that are representative of the bubble collision and film drainage phenomena. These are: Bond number (Bo), Archimedes number (Ar), Froude number (Fr), and the ratio of film force to buoyancy force (FF/FB). These numbers are used to define a predictive function for film radius. Experimentally validated numerical modeling has been implemented to determine the relationship between the four nondimensional numbers, and a quasi-static model is employed to relate the film force to the radial film size. Comparison of our experimental results is in agreement with the predicted film size within ±20%. From these results, the radial film size during bubble impact with a solid surface may be predicted using the easily measurable experimental parameters of bubble size, bubble impact velocity, and the liquid properties.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Fluids Eng. 2019;141(7):074501-074501-7. doi:10.1115/1.4042258.

An experimental study has been conducted to investigate the effects of transitionally rough surface on the flat-plate boundary layer transition. Transitional boundary layers with three different flat plates (ks+ = 0.07 ∼ 0.19, 2.71 ∼ 7.05, and 13.65 ∼ 41.09) have been measured with a single-sensor hot-wire probe. All of the measurements have been conducted under zero pressure gradient (ZPG) at the fixed Reynolds number (ReL) and freestream turbulence intensity (Tu) of 3.05 × 106 and 0.2%. Transitionally, rough surface does not affect the sigmoidal distribution of turbulence intermittency model; but induces earlier transition onset and shortens the transition length. For all surfaces, streamwise turbulence intensity profiles with similar values of turbulence intermittency are similar for the transition length less than 60%. Therefore, mean velocity profiles with the similar values of turbulence intermittency are similar regardless of surface conditions. However, downstream of 60% of the transition length, mean velocity defect increases as the surface roughness increases. Enhanced diffusion of turbulent kinetic energy from the near wall (y/δ < 0.1) to the outer part (y/δ ≈ 0.4) of the boundary layer due to the surface roughness is responsible for the increased momentum deficit.

Commentary by Dr. Valentin Fuster