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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;140(8):081101-081101-8. doi:10.1115/1.4039510.

In the presence of mean strain or rotation, the anisotropy of turbulence increases due to the rapid pressure strain term. In this paper, we consider the modeling of the rapid pressure strain correlation of turbulence. The anisotropy of turbulence in the presence of mean strain is studied and a new model is formulated by calibrating the model constants at the rapid distortion limit. This model is tested for a range of plane strain and elliptic flows and compared to direct numerical simulation (DNS) results. The present model shows agreement with DNS and improvements over the earlier models like those by Launder et al. (1975, “Progress in the Development of a Reynolds-Stress Turbulence Closure,” J. Fluid Mech., 68(3), pp. 537–566.) and Speziale et al. (1991, “Modelling the Pressure–Strain Correlation of Turbulence: An Invariant Dynamical Systems Approach,” J. Fluid Mech., 227(1), pp. 245–272.) that have been reported to give satisfactory performance for hyperbolic flows but not satisfactory for elliptic flows.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081102-081102-6. doi:10.1115/1.4039509.

Commercial water tunnels typically generate a momentum thickness based Reynolds number (Reθ) ∼1000, which is slightly above the laminar to turbulent transition. The current work compiles the literature on the design of high-Reynolds number facilities and uses it to design a high-Reynolds number recirculating water tunnel that spans the range between commercial water tunnels and the largest in the world. The final design has a 1.1 m long test-section with a 152 mm square cross section that can reach speed of 10 m/s, which corresponds to $Reθ=15,000$. Flow conditioning via a tandem configuration of honeycombs and settling-chambers combined with an 8.5:1 area contraction resulted in an average test-section inlet turbulence level <0.3% and negligible mean shear in the test-section core. The developing boundary layer on the test-section walls conform to a canonical zero-pressure-gradient (ZPG) flat-plate turbulent boundary layer (TBL) with the outer variable scaled profile matching a 1/7th power-law fit, inner variable scaled velocity profiles matching the log-law and a shape factor of 1.3.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081103-081103-12. doi:10.1115/1.4039413.

It is a well-known fact that the diffuser of a centrifugal fan plays a vital role in the energy transformation leading to better static pressure rise and efficiency. Many researchers have worked on modified geometry with respect to both impeller and diffuser so as to extract better efficiency of the fan. This paper highlights a unique numerical study on the performance of a centrifugal fan, which has a diffuser having nonparallel shrouds. The shroud geometry is parametrically varied by adopting various convergence ratios (CR) for the nonparallel shrouds encompassing the diffuser passage. It is revealed in the study that there exists an optimal CR for which the performance is improved over the regular parallel shrouded diffuser passage (base model). It is observed from the numerical analysis that for a nonparallel convergent shroud corresponding to a CR of 0.35, a relatively higher head coefficient of 3.6% is obtained when compared to that of the base model. This configuration also yields a higher theoretical efficiency of about 2.1% corroborating the improvement in head coefficient. This study predicts a design prescription for nonparallel diffuser shrouds of a centrifugal fan for augmented performance due to the fact that the converging region accelerates and guides the flow efficiently by establishing radial pressure equilibrium.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081104-081104-13. doi:10.1115/1.4039254.

Due to the practical space limitation, the control valve in industrial utilities is usually immediately followed by a short flow passage, which would introduce considerable complexity into highly unsteady flow behaviors, along with the flow noise and structure vibration. In the present study, the unsteady behaviors of the steam flow inside a control valve with a T-junction discharge, when the valve operates under the choked condition, are numerically simulated. Toward this end, the detached eddy simulation (DES) is used to capture the spatiotemporally varying flow field in the serpentine flow passage. The results show periodic fluctuations of the aerodynamic forces on the valve spindle and periodic fluctuations of the pressure and flow rate at the two discharge outlets. Subsequently, proper orthogonal decomposition (POD) analysis is conducted using the velocity field and pressure field, identifying, respectively, the dominant coherent structures and energetic pressure fluctuation modes. Finally, the extended-POD method is used to delineate the coupling between the pressure fluctuations with the dominant flow structures superimposed in the highly unsteady flow field. The fourth velocity mode at St = 0.1, which corresponds to the alternating oscillations of the annular wall-attached jet, is determined to cause the periodic flow imbalance at the two discharge outlets, whereas signatures of the first three modes are found to be dissipated in the spherical chamber. Such findings could serve as facts for vibration prediction and optimization design. Particularly, the POD and extended-POD techniques were demonstrated to be effective methodologies for analyzing the highly turbulent flows in engineering fluid mechanics.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081105-081105-16. doi:10.1115/1.4039517.

The problem of rising droplets in liquids is important in physics and has had many applications in industries. In the present study, the rising pattern of oil droplets has been examined using the smoothed particle hydrodynamics (SPH), which is a fully Lagrangian meshless method. The open-source SPHysics2D code is developed to two phase by adding the effects of surface tension and an added pressure term to the momentum equation. Several problems of droplet dynamics were simulated, and the performance of the developed code is evaluated. First, the still water–oil tank problem was solved to examine the hydrostatic pressure, especially at the interface, for different density ratios. Then, the rising patterns of an oil droplet of different densities are simulated and the time evolutions of the rising velocity and center of mass are shown. It is shown that the shape and behavior of the droplet rising depend on the balance between viscous, surface tension, and dynamic forces. Afterward, the flow morphologies of multiple droplet rising are shown where the density ratio causes negligible effects on the droplet shape, but it has large effects on the dynamics behavior of rising process.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081106-081106-8. doi:10.1115/1.4039368.

In the current semiconductor industrial scenario, wafers are rinsed in an overflow rinsing tank while being mounted on several lifters prior to most of its manufacturing processes. However, a major drawback of this overflow rinsing methodology is that some of the processing fluid stagnates due to the generated vortices in the regions between the side and middle lifters which entrap some of the flushed particles that further adhere and deteriorate the surface of the wafers. In this work, the hydrodynamics of the flow field inside the wafer rinsing tank with this original lifter orientation setup was studied and compared through numerical simulation and flow visualization using particle image velocimetry (PIV) method, and a strong agreement was found between them in terms of velocity calculation. A new lifter orientation setup was initiated and it was evidenced by the numerical simulation that with this new setup, the generated vortices which are situated opposite to the lifters tilting direction can be displaced significantly in terms of magnitude and distribution. This work presents a new wafer cleaning concept which shows its great potentials in improvement and implementation to the current in-line wafer batch fabrication process without modifying the original design of the rinsing tank.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081107-081107-8. doi:10.1115/1.4039521.

The external characteristics of a superheated water jet released into water at ambient conditions are dominated by the vapor bubble formation, which results in an unsteady flow dynamics. This hinders the use of classical methods to assess the mean flow and the turbulence characteristics. Here, the proper orthogonal decomposition (POD) technique was employed on the velocity measurements obtained using particle image velocimetry (PIV) to quantify the external characteristics of a superheated water jet released into water. This was done at three different inlet pressure ratios. From the energy modes obtained using the POD technique, it was observed that the first mode well represents the mean flow, while subsequent higher modes show the fluctuating nature. The phase-averaged properties were calculated by considering only the first mode. Unlike a canonical jet, the maximum value of the mean centerline velocity for a superheated jet occurs far downstream from the nozzle, at x/D ≈ 15, due to the thermal nonequilibrium in the jet attributed to the formation of vapor bubbles. The turbulent kinetic energy (TKE), size of the coherent structures (CS), and swirling strength showed a nonmonotonic decrease in the downstream direction, indicating that the vapor formation has significant influence on the jet dynamics. The novel aspect of this work is the use of POD technique for phase averaging, using which dynamics of a superheated jet have been quantified. The distribution of vapor bubbles in the flow field was also measured using the Shadowgraphy technique to substantiate the above observations.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;140(8):081201-081201-15. doi:10.1115/1.4039257.

Flow in turbomachines is generally highly turbulent. Nonetheless, boundary layers may exhibit laminar-to-turbulent transition, and relaminarization of the turbulent flow may also occur. The state of flow of the boundary layer is important since it influences transport phenomena like skin friction and heat transfer. In this paper, relaminarization in accelerated flat-plate boundary-layer flows is experimentally investigated, measuring flow velocities with laser Doppler anemometry (LDA). Besides the mean values, statistical properties of the velocity fluctuations are discussed in order to understand the processes in relaminarization. It is shown that strong acceleration leads to a suppression of turbulence production. The velocity fluctuations in the accelerated boundary layer flow “freeze,” while the mean velocity increases, thus reducing the turbulence intensity. This leads to a laminar-like velocity profile close to the wall, resulting in a decrease of the local skin friction coefficient. Downstream from the section with enforced relaminarization, a rapid retransition to turbulent flow is observed. The findings of this work also describe the mechanism of retransition.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081202-081202-7. doi:10.1115/1.4039251.

The effect of trimming of radial impellers on the hydraulic performance of low specific-speed centrifugal pumps is studied. Prediction methods from literature, together with a new prediction method that is based on the simplified description of the flow field in the impeller, are used to quantify the effect of trimming on the hydraulic performance. The predictions by these methods are compared to measured effects of trimming on the hydraulic performance for an extensive set of pumps for flow rates in the range of 80% to 110% of the best efficiency point. Of the considered methods, the new prediction method is more accurate (even for a large impeller trim of 12%) than the considered methods from literature. The new method generally overestimates the reduction in the pump head after trimming, and hence results less often in impeller trims that are too large when the method is used to determine the amount of trimming that is necessary in order to attain a specified head.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081203-081203-7. doi:10.1115/1.4039294.

Velocity fluctuations are widely used to identify the behavior of developing turbulent flows. The pressure on the other hand, which is strongly coupled with the gradient of the mean velocity and fluctuations, is less explored. In this study, we report the results of wall pressure measurements for the development of pipe flow at high Reynolds numbers along the axial direction. It is found that the pressure fluctuations increase exponentially along the pipe with a self-similarity scaling. The exponential growth of the pressure fluctuations along the pipe saturates after reaching a critical position around 50 diameters from the inlet. It qualitatively agrees with the critical position usually adopted for fully developed turbulence, which was obtained from earlier velocity fluctuations at various locations along the pipe centerline. Results also show that the exponential growth of the pressure fluctuations is weakly affected by the presence of ring obstacles placed close to the pipe inlet. Finally, it is found that the pressure fluctuations decrease as a function of Reynolds number, contrary to the boundary layer flow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081204-081204-8. doi:10.1115/1.4039253.

In this work, we have performed the flume study to analyze the high-order velocity moments of turbulent boundary layer with and without downward seepage. Sediment transport experiments were done in the laboratory for no seepage (NS), 10% seepage (10%S), and 15% seepage (15%S) cases. Measures of streamwise velocity variance were found increasing with seepage, which lead to increase in sediment transport with seepage. Results show that the variance of streamwise velocity fluctuation follows logarithmic law with distance away from the bed, within inner layer. This observation is also valid for even-order moments obtained in this work. The results show that the (2p-order moments)1/p also follows logarithmic law. The slopes Ap in the turbulent boundary layer seem fairly unaffected to NS and seepage flow but follows nonuniversal behavior for NS and seepage runs. The computed slope based on the Gaussian statistics does not agree well with the slope obtained from the experimental data and computed slope are reliable with sub-Gaussian performance for NS flow and super-Gaussian behavior for seepage flow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081205-081205-12. doi:10.1115/1.4039369.

This paper describes a numerical simulation of the interaction of a single nonlinear wave with a solid vertical surface in three dimensions. A coupled volume of fluid (VOF) and level set method (LSM) is used to simulate the wave-body interaction. A Cartesian-grid method is used to model immersed solid boundaries with constant grid spacing for simplicity and lower storage requirements. Mesh refinement is implemented near the wall boundaries due to the complex behavior of the free surface around the body. The behavior of the wave impact, the water sheet, and the high-speed jet arising from the wave impact are all captured with these methods. The numerical scheme is implemented using parallel computing due to the high central processing unit and memory requirements of this simulation. The maximum wave run-up velocity, instant wave run-up velocity in front of the vertical surface, the sheet break-up length, and the maximum impact pressure are computed for several input wave characteristics. Results are compared with a laboratory experiment that was carried out in a tow tank in which several generated waves were impacted with a fixed flat-shaped plate model. The numerical and experimental data on sheet breakup length are further compared with an analytical linear stability model for a viscous liquid sheet, and good agreement is achieved. The comparison between the numerical model and the experimental measurements of pressure, the wave run-up velocity, and the break-up length in front of the plate model shows good agreement.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2018;140(8):081301-081301-10. doi:10.1115/1.4039514.

The separation of a shear-driven thin liquid film from a sharp corner is studied in this paper. Partial or complete mass separation at a sharp corner is affected by two different mechanisms: liquid film inertia, which affects liquid mass separation through force imbalance at the sharp corner, and large amplitude waves (LAW) at the interface, which contributes to liquid instability at the corner. Experimental results for liquid Ref number that varies from 100 to 300 and mean film thickness from 130 to 290 μm show that both film inertia and LAW effects correlate to mass separation results. The results suggest that while both inertia of the film substrate and LAW effects enhance the mass separation, the correlations between LAW characteristics and mass separation results provide better insight into the onset of separation and the impact of the gas phase velocity on separation for the conditions studied.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(8):081302-081302-9. doi:10.1115/1.4039520.

Water removal and behavior, in proton exchange membrane fuel cell (PEMFC) gas flow channel has been investigated in this work. Single serpentine gas flow channel has been simulated. Hydrodynamics of water drops in a serpentine channel are studied as a function of nature of gas diffusion layer (GDL) surface wettability. In one case, the surface becomes gradually hydrophobic starting from 90 deg to 170 deg. In this second case, the value of contact angle reduces to 10 deg. A three-dimensional model has been developed by using cfd software. Two different drop of diameter 0.2 mm and 0.4 mm are simulated for all the cases. It is observed that, water coverage is always lesser for a gradual hydrophobic surface. Also at low air velocity and gradual hydrophobic GDL surface results in lesser pressure drop as well as water coverage.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Fluids Eng. 2018;140(8):084501-084501-3. doi:10.1115/1.4039252.

According to several known experiments, an increase of the incoming flow air content can increase the hydrofoil lift coefficient. The presented theoretical study shows that such increase is associated with the decrease of the fluid density at the cavity surface. This decrease is caused by entrainment of air bubbles to the cavity from the surrounding flow. The theoretical results based on such explanation are in a good agreement with the earlier published experimental data for NACA0015.

Commentary by Dr. Valentin Fuster