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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2016;138(6):061101-061101-12. doi:10.1115/1.4032034.

This paper proposes two different noise cancellation techniques for cleaning wall-pressure fluctuations signals. These fluctuations are measured around a circular cylinder with laminar and turbulent flow separation. The noise cancellation techniques are based on Wiener and adaptive filters and use the signals of pressure transducers mounted in a cross section of the cylinder and the signal of a free-field sensor opportunely located upstream. First, synthetic signals are used in order to validate the procedure. Then, both techniques are applied to the experimental data. Specific attention is paid to the filter order, optimized by a method introduced in this paper. Both filter types showed a selective behavior preserving the essence of the fluid dynamic phenomena characterizing the flow fields at each Reynolds number tested, especially when laminar separation occurs.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061102-061102-10. doi:10.1115/1.4032293.

For the cloud cavitation around slender axisymmetric projectiles, a two-dimensional (2D) numerical method was based on the mixture approach with Singhal cavitation model and modified renormalization-group (RNG) k–ε turbulence model, and a three-dimensional (3D) method was established with large-eddy simulation (LES) and volume of fraction (VOF) approach. The commercial computational fluid dynamic (CFD) software fluent is used for the 2D simulation, and the open source code OpenFOAM is adopted for the 3D calculation. Experimental and numerical results were presented on a typical case, in which the projectile moves with a quasi-constant axial speed. Simulation results agree well with experimental results. An analysis of the evolution of cavitating flow was performed, and the related physical mechanism was discussed. Results demonstrate that shedding cavity collapse plays an important role in the generation and acceleration of re-entry jet, which is the main reason for the instability of cloud cavitation. The 2D Reynolds-Averaged Navier–Stokes (RANS) method can represent the physical phenomena effectively. The 3D LES method can give an efficient simulation on the shedding vortices, and considerable accurate shapes of shedding cavities are captured.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061103-061103-11. doi:10.1115/1.4032295.

In this paper, a three-dimensional (3D) computational fluid dynamics (CFD) methodology to improve the performance of hydraulic components will be shown, highlighting the importance that a study in the fluid mechanics field has for their optimization. As known, the valve internal geometry influences proportional spool valve hydraulic performance, axial flow forces, and spin effects on the spool. Axial flow forces and spin effects interact directly with the position control performance of a direct actuating closed-loop control valve, reducing its capability. The goal of this activity is the study of the torque on the spool induced by the flow and using a CFD 3D methodology to identify causes of this phenomenon and to find a general mathematical solution to minimize the spool spin effect. The baseline configuration and the new ones of the proportional four-way three-position closed-loop control spool valve have been studied with a mathematical model. The models were also validated by the experimental data performed in the Hydraulic Lab of the University of Naples. In particular, the tests allowed to measure the torque on the spool varying the oil flow rate, using a dedicated test bench layout where the spool was directly connected to a torque meter. Several geometries have been analyzed to find the best one to minimize spool spin behavior while maintaining an acceptable pressure drop. The study results confirmed the significant improvement of overall component performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061104-061104-9. doi:10.1115/1.4032301.

The impact of Gurney flaplike strips, of different geometric configurations and heights, on the aerodynamic characteristics and the tip vortices generated by a reverse delta wing (RDW) was investigated via force-balance measurement and particle image velocimetry (PIV). The addition of side-edge strips (SESs) caused a leftward shift of the lift curve, resembling a conventional trailing-edge flap. The large lift increment overwhelmed the corresponding drag increase, thereby leading to an improved lift-to-drag ratio compared to the baseline wing. The lift and drag coefficients were also found to increase with the strip height. The SES-equipped wing also produced a strengthened vortex compared to its baseline wing counterpart. The leading-edge strips (LESs) were, however, found to persistently produce a greatly diffused vortex flow as well as a small-than-baseline-wing lift in the prestall α regime. The downward LES delivered a delayed stall and an increased maximum lift coefficient compared to the baseline wing. The LESs provide a potential wingtip vortex control alternative, while the SESs can enhance the aerodynamic performance of the RDW.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061105-061105-14. doi:10.1115/1.4032562.

This paper presents an experimental investigation of the flow fields in a centrifugal pump by particle image velocimetry (PIV) technique with two different tracer particles, all designed for the same operating point. In order to systematically analyze the tracking characteristics of tracer particles once used in centrifugal pump by Basset–Boussinesq–Oseen (BBO) equation, aluminum powder (AP, with high density ratio and small diameter) and hollow glass spheres (HGS, with neutral density and large diameter) were selected. The velocity fields obtained for AP and HGS were presented and compared, in order to enhance the understanding of their tracking properties in rotating impeller. The results show that AP and HGS give nearly the same phase-averaged velocity fields except at two small regions. BBO extended equation by the phase average theory in a centrifugal pump was applied to explain the first difference, namely, why the velocity of HGS is higher than that of AP in the low-speed zone. In addition, the mean vorticity distributions for AP and HGS show high strength velocity micelles with different directions of development and dissemination, which causes the second difference in energy exchange. As a consequence, HGS tends to conglomerate closer to the pressure side (PS) near the impeller outlet than AP

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061106-061106-9. doi:10.1115/1.4032570.

The use of supercritical carbon dioxide (SC-CO2) as a working fluid in energy conversion systems has many benefits, including high efficiency, compact turbomachinery, and the abundance of CO2. A very important issue for design optimization and performance analysis of future SC Brayton cycles is concerned with the SC-CO2 flow inside high-speed compressors and turbines. The objective of this paper is to present a novel modeling approach to, and its use in numerical simulations of, SC-CO2 flow inside a high-speed compact compressor. The proposed approach capitalizes on using three different physical and mathematical formulations of one-dimensional (1D) models, i.e., compressible and incompressible flow models using actual properties of SC-CO2 and a compressible ideal gas model, as a reference to verify the predictive capabilities of a three-dimensional (3D) incompressible flow model. The incompressible model has been used to perform simulations for a complete detailed multidimensional model of an SC-CO2 high-speed compact compressor. The advantages of the new model include numerical stability, computational efficiency, and physical accuracy. In particular, it has been shown that the model's predictions are consistent with selected published technical data.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2016;138(6):061201-061201-15. doi:10.1115/1.4032298.

Similarities of the flow in the rotor–stator interaction (RSI) affected region (stay vanes, guide vanes, and runner domain) in prototype and model Francis pump-turbines are analyzed using numerical simulations with special attention on the influence of Reynolds number. The ratios of characteristic length and velocity between the prototype and the model are 10.97 and 2.54; thus, the Reynolds numbers differ by about 28 times. Detailed flow analysis argues for higher partial load condition, Q = 0.8Qd, and severe partial load condition, Q = 0.45Qd. The flows in the distributor (spiral casing, stay vanes, and guide vanes domain) are well-behaved for both conditions with no separation, presenting high level of similarity in both space and time domain. The flows in the runners are well-behaved at higher partial load, Q = 0.8Qd, and present good flow similarity and weak Reynolds number effects between the model and the prototype. At severe partial load, Q = 0.45Qd, flow separation develops on the blade pressure sides and partially blocks the runner passages, showing prominent flow discrepancy and stronger Reynolds number effects between the two turbines. For the prototype flow of high Reynolds number, viscous effects have a minor role and less momentum is lost in the boundary layer. Therefore, the flow deceleration is less severe and the emergence of separation is restrained, presenting spatially delayed separation and a less disorganized flow pattern in the prototype. Validated by the model tests and on-site measurements, pressure fluctuations recorded in the vaneless space show that the relative fluctuation amplitudes in the model are slightly higher than those in the prototype. Resorting to dimensionless analytical equations and simulation results, the deviation in pressure fluctuations is found out to be influenced by Reynolds number effects. The research provides an improved understanding of Reynolds number effects on the flow discrepancy and pressure fluctuation difference in the RSI-affected region, which will facilitate better estimations of performance from scale model to prototype.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061202-061202-4. doi:10.1115/1.4032300.

This paper determines natural and resonant frequencies of radial–circular oscillations (waves) on the nonviscous, incompressible fluid partially and evenly filling similar compartments in a rotor that has the cylindrical chamber, solid radial baffles, and constant angular velocity. It is assumed as follows: influence of the gravity and surface tension is negligibly small as compared to the centrifugal effect; configuration of dynamic equilibrium (unperturbed motion) of fluid is an annular rigid body rotation; and the fluid motion perturbed by small lateral deflections of the rotor does not depend on the axial coordinate (plane motion).

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061203-061203-11. doi:10.1115/1.4032535.

An algebraic relationship between turbulent dissipation rate and von Karman length are used to dismiss the transport equation of turbulent dissipation rate in standard $k−ε$ (SKE) turbulence model. Meanwhile, a recalibrated Bradshaw's assumption is built based on the data from a boundary layer flow of turbulent flat plate simulated by direct numerical simulation (DNS). The JL model is reformed to a one-equation model which only depends on the turbulent energy, so the new model can also be called kinetic-energy dependent only (KDO) turbulence model. As the KDO model is using the von Karman length scale, it can automatically adjust to fit the resolved structures of the local flow. Results will be shown for the boundary layer flow on a turbulent flat plate, and the external flows of an NACA4412 airfoil, an ONERA-M6 wing, a three dimension delta wing, and an NACA0012 airfoil at deep stall.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2016;138(6):061301-061301-12. doi:10.1115/1.4032299.

Stiction forces exerted by a fluid in a thin, quickly widening gap to its boundaries can become a strongly limiting factor of the performance of technical devices, like compressor valves or hydraulic on–off valves. In design optimization, such forces need to be properly and efficiently modeled. Cavitation during parts of a stiction process plays a strong role and needs to be taken into account to achieve a meaningful model. The paper presents an approximate calculation method which uses qualitative solution properties of the non cavitating stiction problem, in particular of its level curves and gradient lines. In this method, the formation of the cavitation boundaries is approximated by an elliptic domain. The pressure distribution along its principle axis is described by a directly integrable differential equation, the evolutions of its boundaries is guided just by pressure boundary conditions when the cavitation zone expands and by a nonlinear differential equation when it shrinks. The results of this approximate model agree quite well with the solutions of a finite volume (FV) model for the fluid stiction problem with cavitation.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061302-061302-19. doi:10.1115/1.4032216.

We show how introducing a small amount of gas can completely change the motion of a solid object in a viscous liquid during vibration. We analyze an idealized system exhibiting this behavior: a piston in a liquid-filled housing with narrow gaps between piston and housing surfaces that depend on the piston position. Recent experiments have shown that vibration causes some gas to move below the piston and the piston to subsequently move downward against its supporting spring. We analyze the analogous but simpler situation in which the gas regions are replaced by bellows with similar pressure–volume relationships. We show that the spring formed by these bellows (analogous to the pneumatic spring formed by the gas regions) enables the piston and the liquid to oscillate in a mode with low damping and a strong resonance. We further show that, near this resonance, the dependence of the gap geometry on the piston position produces a large rectified (net) force on the piston. This force can be much larger than the piston weight and tends to move the piston in the direction that decreases the flow resistance of the gap geometry.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2016;138(6):061303-061303-14. doi:10.1115/1.4032487.

Recirculation accompanied by shear cavitation is a key flow feature in annular jet pumps (AJPs). In this study, a high-speed camera was used to capture the recirculation region and various types of cavity clouds. By monitoring the trajectories of the small bubbles, the main recirculation regions under each flow rate ratio were obtained. As the flow rate ratio decreases, the recirculation region continued expanding with the separation point moving upstream, while the reattachment point remained nearly stationary regardless of the decreasing flow rate ratio. Hill's spherical vortex theory was adopted to evaluate the variations of the recirculation regions. Moreover, the minimum local static wall pressure in the recirculation region decreases as well, which can promote the inception and development of shear cavitation. There are numerous vortices simultaneously induced by the large velocity gradient in the shear layer, the core of which becomes a potential site for cavitation. Consequently, with the growth of the recirculation region, three types of cavity clouds, viz., the ribbonlike, annular, and merged cavity clouds, appear in turn. The movement characteristics of these cavity clouds, including their inception, distortion, and collapse, are illustrated based on the high-speed imaging results. The ribbonlike and annular cavity clouds are both induced by the small vortices in the shear layer because of the low local pressure in the vortex core. However, the merged cavity clouds are caused by a combination of several ribbonlike and annular cavity clouds, which provides a larger scale and a longer life span. Hence, the collapse of the merged cavity clouds can cause a large pressure pulsation near the reattachment point of the recirculation region. The corresponding frequency spectra were also demonstrated based on the fast Fourier transform (FFT) method.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Fluids Eng. 2016;138(6):064501-064501-6. doi:10.1115/1.4032296.

The migration of a bubble inside a two-dimensional converging–diverging channel is investigated numerically. A parametric study is conducted to investigate the effects of the Reynolds and Weber numbers and the amplitude of the converging–diverging channel. It is found that increasing the Reynolds number and the amplitude of the channel increases the oscillation of the bubble and promotes the migration of the bubble toward one of the channel wall. The bubble undergoes oblate–prolate deformation periodically at the early times, which becomes chaotic at the later times. This phenomenon is a culmination of the bubble path instability as well as the Segré–Silberberg effect.

Commentary by Dr. Valentin Fuster