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### Review Article

J. Fluids Eng. 2018;141(2):020801-020801-19. doi:10.1115/1.4040501.

The ground-effect diffuser has become a major aerodynamic device on open-wheel racing and sports cars. Accordingly, it is widely considered to be indispensable to their aerodynamic performance, largely due to its significant downforce contribution. However, the physics and characteristics that determine how it generates downforce and its application in the auto racing industry require an in-depth analysis to develop an understanding. Furthermore, research that could generate further performance improvement of the diffuser has not been defined and presented. For these reasons, this review attempts to create a systematic understanding of the physics that influence the performance of the ground-effect diffuser. As a means of doing this, the review introduces research data and observations from various relevant studies on this subject. It then investigates advanced diffuser concepts mainly drawn from the race car industry and also proposes a further research direction that would advance the aerodynamic performance of the diffuser. It is concluded that although the diffuser will continue to be paramount in the aerodynamic performance of racing cars, research is needed to identify means to further enhance its performance.

Commentary by Dr. Valentin Fuster

### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;141(2):021101-021101-10. doi:10.1115/1.4040557.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021102-021102-15. doi:10.1115/1.4040523.
OPEN ACCESS

The propulsors of organisms from paramecia to dolphins have ball-and-socket jointed bases that allow large-amplitude, low-friction swings. Their olivo-cerebellar control also remains unchanged. Yet, the propulsive surfaces of small animals vary widely from flagellar filaments (0 < Re < 5) to flapping fins (Re > 20) with an intermediate range of Reynolds number (5 < Re < 20) where both types are present in the same swimming animal. Analysis suggests that these unsteady surfaces are mechanical oscillators coupled to their nonlinear wakes. A low-friction-driven oscillator that can interact with the oscillators of models or live swimming and flying animals could help us understand the hydro-structural events prompting the evolution of such surfaces at specific Re values. A gearless underdamped (in air) hemispherical motor oscillator is described where energetic efficiency increases by a factor of eight as the forces drop by a factor of ten from 10 N. The electrical efficiencies at 0.8 N are comparable to the total thermal efficiencies of flies, and the quality factor is comparable. The continuously varying fin oscillation of penguin fins and abruptly varying fin oscillations of Clione antarctica and flies are reproduced. When flapping at 0.3 Hz, the oscillator would respond to all wake nonlinearities. Abrupt fin turning is modeled by switching the roll and pitch phase difference between $−π/2$ and $π/2$ in successive quadrants. Defining the fish-wake lock-in error as the difference between Triantafyllou's fish Strouhal number and the tangent of the vortex-shedding angle, an experiment is discussed for measuring the minimum drag of live fish.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021103-021103-17. doi:10.1115/1.4040502.

The cavitating flow around the asymmetric leading edge (ALE) 15 hydrofoil is investigated through large eddy simulation with the modified Schnerr–Sauer cavitation model, which considers the effect of noncondensable gas. The statistical average velocity profiles obtained by simulation and experimentation show good agreement. The time evolution of cavity shape shows that cavity growth and separation start from the short side and spread toward the long side due to a side-entrant jet. The variation frequency of the cavity length of ALE15 hydrofoil at the long side is 163.93 Hz, and the cavitation shedding frequency at the short side is 306.67 Hz, which is about twice the value of the former. The filtered vorticity transport equation is employed to investigate the cavitation–vortex–turbulence interaction. Results indicate that vortex stretching is the major promoter of cavitation development, and vortex dilatation links vapor cavity and vortices. Baroclinic torque is noticeable at the liquid–vapor interface, and turbulent stress is related to cavitation inception. Moreover, a one-dimensional model for predicting pressure fluctuation is proposed, and results show that the model can effectively predict cavitation-induced pressure fluctuation on a hydrofoil, even on a three-dimensional ALE15 hydrofoil.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021104-021104-13. doi:10.1115/1.4040594.

Enhancing mixing in heat exchangers for low Re regimes is vital. A better mixing may be achieved by using corrugated plates. In this work, the flow patterns between corrugated plates with a novel egg-carton geometry were studied. Three-dimensional (3D) numerical models were developed for the steady laminar flow between two corrugated plates having 180 deg or 0 deg phase angles. The Reynolds number (Re ≤ 600) was defined as a function of the average distance between the corrugated plates. The numerical models were strictly developed and corroborated to achieve global convergence, local convergence, and grid-size independence. For both phase angles, it was determined that “close recirculations” decrease in size downstream and finally disappear becoming “open recirculations” due to the flow developing characteristics; the secondary flow regions were found to grow downstream; interestingly, increments on the Reynolds number favor recirculation growth and early flow detachment; the behavior and geometry of the recirculation were in line with previous flow visualization results. The recirculations were determined to be z-symmetric with respect to the channel center only for the 180 deg model. The recirculations in the 0 deg model were smaller and became “open recirculations” earlier than in the 180 deg model. Convex geometries on the transversal direction were found to favor detachment, while concave geometries inhibit it. The capability of the numerical methods to track flow paths in any direction showed a complex three-dimensional flow causing 3D-interaction among secondary flows and the main flow not reported before for these channels and just hinted by previous flow visualization studies.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021105-021105-13. doi:10.1115/1.4040928.

An experimental study aimed at evaluating the influence of Mach number on the base pressure fluctuations of a cylindrical afterbody was performed over a wide range of Mach numbers from subsonic to supersonic speeds. Time-averaged results indicate that the coefficient of base pressure drops with the increase in the freestream Mach number at subsonic speeds and increases at supersonic Mach numbers. The coefficient of root-mean-square of the pressure fluctuations follows a decreasing trend with the increase in the Mach number. Examination of the spectra reveals different mechanisms dominate the pressure fluctuations from the center to the periphery of the base as well as with the change in the Mach number. Analysis of the azimuthal coherence indicates that all the dominant tones in the spectra can be classified either into a symmetric or an antisymmetric mode at subsonic Mach numbers. However, at supersonic Mach numbers, all the dominant tones in the spectra are symmetric in nature. The results from the cross-correlation suggest that two possible mechanisms of recirculation bubble pulsing and convective motions/vortex shedding are driving the dynamics on the base at subsonic Mach numbers. However, at supersonic Mach numbers, only single mechanism of the recirculation bubble pulsing dominates. Moreover, it indicates that the symmetric mode is associated with the dynamics of the recirculation bubble and the antisymmetric mode is related to the convective motions/vortex shedding.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021106-021106-11. doi:10.1115/1.4040930.

The vertically launched underwater vehicle always suffers various hydrodynamic disturbances in its water-emerging process due to the uncertainty of the launch platform motion. Based on the nested sparse grid based stochastic collocation method (NSSCM) and nonintrusive polynomial chaos method, the effect of uncertainty of platform velocity and yaw angle on robustness of vehicle's trajectory and attitude is numerically studied. Results indicate that the uncertainty stemming from platform motion propagates along vehicle's water-emerging process. As the negative horizontal velocity of vehicle gradually changes to positive direction, the uncertainty bar of horizontal velocity presents contracting-expanding mode with an “hourglass” shape while the uncertainty bar of horizontal displacement experiences a “spindle-shaped” one (expanding-contracting-expanding), which is a half cycle later compared with the velocity. The uncertain motion of platform enlarges the uncertainty bar of bottom force via its impact on the gas-leakage process of trail bubble, resulting in the increasing of uncertainty of vertical velocity. Pitching angle (attitude of vehicle) and pitching angular velocity of vehicle persist getting worse driven by the pressure difference between vehicle's front and back sides especially on head part. And their continuous increasing uncertainty bars are formed mainly due to the condition that pressure uncertainty of front side is larger than that on back side, which also leads to the increasing of uncertainty of horizontal force.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;141(2):021201-021201-9. doi:10.1115/1.4040568.

Conditional statistics are employed in analyzing wake recovery and Reynolds shear stress (RSS) and flux directional out of plane component preference. Examination of vertical kinetic energy entrainment through describing and quantifying the aforementioned quantities has implications on wind farm spacing, design, and power production, and also on detecting loading variation due to turbulence. Stereographic particle image velocimetry measurements of incoming and wake flow fields are taken for a 3 × 4 model wind turbine array in a scaled wind tunnel experiment. Reynolds shear stress component is influenced by $⟨uv⟩$ component, whereas $⟨vw⟩$ is more influenced by streamwise advection of the flow; u, v, and w being streamwise, vertical, and spanwise velocity fluctuations, respectively. Relative comparison between sweep and ejection events, $ΔS⟨uiuj⟩$, shows the role of streamwise advection of momentum on RSS values and direction. It also shows their tendency to an overall balanced distribution. $⟨uw⟩$ intensities are associated with ejection elevated regions in the inflow, yet in the wake, $⟨uw⟩$ is linked with sweep dominance regions. Downward momentum flux occupies the region between hub height and top tip. Sweep events contribution to downward momentum flux is marginally greater than ejection events'. When integrated over the swept area, sweeps contribute 55% of the net downward kinetic energy flux and 45% is the ejection events contribution. Sweep dominance is related to momentum deficit as its value in near wake elevates 30% compared to inflow. Understanding these quantities can lead to improved closure models.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021202-021202-11. doi:10.1115/1.4040572.

This paper discusses an unsteady separated stagnation-point flow of a viscous fluid over a flat plate covering the complete range of the unsteadiness parameter β in combination with the flow strength parameter a (>0). Here, β varies from zero, Hiemenz's steady stagnation-point flow, to large β-limit, for which the governing boundary layer equation reduces to an approximate one in which the convective inertial effects are negligible. An important finding of this study is that the governing boundary layer equation conceives an analytic solution for the specific relation β = 2a. It is found that for a given value of $β (≥0)$ the present flow problem always provides a unique attached flow solution (AFS), whereas for a negative value of β the self-similar boundary layer solution may or may not exist that depends completely on the values of a and β (<0). If the solution exists, it may either be unique or dual or multiple in nature. According to the characteristic features of these solutions, they have been categorized into two classes—one which is AFS and the other is reverse flow solution (RFS). Another interesting finding of this analysis is the asymptotic solution which is more practical than the numerical solutions for large values of β (>0) depending upon the values of a. A novel result which arises from the pressure distribution is that for a positive value of β the pressure is nonmonotonic along the stagnation-point streamline as there is a pressure minimum which moves toward the stagnation-point with an increasing value of β > 0.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021203-021203-11. doi:10.1115/1.4041066.

The goal of this study is to provide pump users a simple means to predict a pump's performance change due to changing fluid viscosity. During the initial investigation, it has been demonstrated that pump performance can be represented in terms of the head coefficient, flow coefficient, and rotational Reynolds number with the head coefficient data for all viscosities falling on the same curve when presented as a function of $ф*Rew−a$. Further evaluation of the pump using computational fluid dynamics (CFD) simulations for wider range of viscosities demonstrated that the value of a (Morrison number) changes as the rotational Reynolds number increases. There is a sharp change in Morrison number in the range of $104 indicating a possible flow regime change between laminar and turbulent flow. The experimental data from previously published literature were utilized to determine the variation in the Morrison number as the function of rotational Reynolds number and specific speed. The Morrison number obtained from the CFD study was utilized to predict the head performance for the pump with known design parameters and performance from published literature. The results agree well with experimental data. The method presented in this paper can be used to establish a procedure to predict any pump's performance for different viscosities; however, more data are required to completely build the Morrison number plot.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2018;141(2):021301-021301-10. doi:10.1115/1.4040465.

A state-of-the-art, portable dispersion characterization rig (P-DCR) is used to investigate the effect of nanoparticles (NP) on oil-water emulsion formation and stabilization. Spherical silica NP of different wettabilities were used to investigate their effect on separation kinetics of solid stabilized emulsions in terms of solid particle concentration, wettability, initial dispersion phase, water-cut, and shearing time. The main findings of the study include the following: NP, even at concentrations as low as 0.005% or 0.01% (by weight), can significantly increase separation time of oil/water emulsions from a few minutes to several hours or even days. The P-DCR is recommended as an effective inline tool to measure emulsion stability in the field.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021302-021302-13. doi:10.1115/1.4040929.

Isothermal transient Eulerian–Lagrangian simulation of the turbulent gas–solid flow in a cyclone gasifier with two inlet tubes at 890 °C has been performed. The single-phase gas flow is modeled using SSG Reynolds stress turbulence model. Ten thousand representative solid particles of different sizes are injected from each inlet continuously at every second of simulation time. Particles are finally stopped as soon as they arrive at the outlet or reach the bottom plate of the gasifier. The effect of particle-to-gas coupling on the pressure and velocity of the flow and particles motion inside the gasifier is studied. The numerical approach can reasonably predict the impact of particle load on the gas flow as presented in the experimental results. Single particles are traveled throughout the transient gas flow field by using Lagrangian approach. High temperature of the gas flow inside the gasifier has significant effects on the swirl intensity reduction, damping the turbulence in the core region, pressure, and particle behaviors. However, the presence of solid particles does not have a notable influence on the swirl intensity and turbulence.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;141(2):021303-021303-15. doi:10.1115/1.4040933.

A new approach for obtaining a normalized closed-form frequency domain analytical model for the non-Newtonian shear thinning effects on the pressure and shear stress transients in a pretransient turbulent flow of fluids in smooth circular lines is formulated. The Oldroyd-B model is utilized to analyze these shear thinning effects on these transients. The process of converting the analytical frequency domain model to the time domain using an inverse frequency algorithm commonly used in system identification is explained and demonstrated. The boundary conditions at the ends of the line are defined by the flow and pressure variables, which are in general functions of time or defined by causality relationships. Corresponding equations for the transient changes in the velocity profile and shear stress are also formulated. Two examples demonstrating the application versatility of the model and the sensitivity of the transients to the shear thinning parameters are included. For these specific examples, the sensitivity of the pressure and velocity transients is observed to be relatively low compared to the sensitivity of the wall shear stress. Insight into when the non-Newtonian complexities associated with shear thinning need to be included in a model for fluid transients considering the mode frequencies and/or the input frequencies is provided. The analytical model can easily be simplified for laminar flow and Newtonian fluids.

Commentary by Dr. Valentin Fuster

### Research Papers: Techniques and Procedures

J. Fluids Eng. 2018;141(2):021401-021401-12. doi:10.1115/1.4040464.

Large eddy simulation (LES) is conducted for the flow over the shell side of a helical coil steam generator (HCSG) heat exchanger. Simulations are conducted on a simplified experimental test section that represents a one-column region of the helical coils using half-rods. Although the rods are wall-bounded, the flow still exhibits the turbulent characteristics and fluctuations from vortex shedding that one would expect from crossflow around a cylinder. The spectral element, computational fluid dynamics (CFD) code Nek5000, is used to capture the physics, and the results are compared with particle image velocimetry (PIV) measurements. In order to ensure that the turbulence is resolved, analysis is conducted by using the Taylor length scales and normalized wall distance. Sensitivity to the inlet boundary conditions (BCs) and the spatial discretization for different polynomial order solutions are also studied, finding only minor differences between each case. Pressure drop and velocity statistics show reasonable agreement with PIV. Proper orthogonal decomposition (POD) analysis reveals that the primary modes are similar between experiment and simulation, although the LES predicts higher turbulent kinetic energy than does PIV. Overall, the study establishes the resolution and resources required in order to conduct a high-fidelity simulation over 12 helical rods.

Commentary by Dr. Valentin Fuster