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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;140(9):091101-091101-12. doi:10.1115/1.4039707.

Swirl inlet distortion is usually encountered in modern flight vehicles since their inlet ducts usually consist of one or two bends, such as S-inlet duct. An experimental device is first designed to simulate the swirl inlet distortion and then used to test the effectiveness of a novel casing treatment (CT) on a low-speed compressor under the swirl distortions of various intensities. The influences of co- and counter-rotating swirl inlet distortion on the test compressor and the stabilization ability of this novel CT are well demonstrated by the illustrations of static pressure rise curves and efficiency curves. The dynamic prestall pressure signals are also captured to reflect the perturbation energy in the flow field through which the mechanism of the novel CT will be indicated. The relevant results show that counter-rotating swirl distortion in small intensity could increase the compressive ability of compressor with small efficiency loss, and the co-rotating swirl distortion always brings about detrimental effects on compressor performance. At the same time, the distortion of twin swirls can cause nonuniform total pressure profile which can seriously damage the compressor performance. Besides, the stall precursor-suppressed (SPS) CT shows a good capability of stall margin (SM) enhancement no matter what swirl inlet distortions are encountered in the test compressor.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091102-091102-11. doi:10.1115/1.4039518.

A ground-effect diffuser is an upward-sloping section of the underbody of a racing car that enhances aerodynamic performance by increasing the downforce, thus improving tire grip. The downforce generated by a diffuser can be increased by geometric modifications that facilitate passive flow control. Here, we modified a bluff body equipped with a $17deg$ diffuser ramp surface (the baseline/plane diffuser) to introduce a convex bump near the end of the ramp surface. The flow features, force, and surface pressure measurements determined in wind-tunnel experiments agreed with previous studies but the bump favorably altered the overall diffuser pressure recovery curve by increasing the flow velocity near the diffuser exit. This resulted in a static pressure drop near the diffuser exit followed by an increase to freestream static pressure, thus increasing the downforce across most of the ride heights we tested. We observed a maximum $4.9%$ increase in downforce when the modified diffuser was compared to the plane diffuser. The downforce increment declined as the ride height was gradually reduced to the low-downforce diffuser flow regime.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091103-091103-14. doi:10.1115/1.4039519.

Pressure-equalizing film is a slice of air film generated through exhausting and attached to the vehicle's exterior with nearly uniform inner pressure. Similar to ventilated cavity in composition, but of interest, here is the weakening of pitching moment and environment disturbance that the film offers, the film's forming speed and covering range upon vehicle determine the improvement effect of vehicle's trajectory stability as it emerges from water. This paper established a numerical approach to investigate the effect of single and double rows of venting holes on the evaluation of air film along vehicle's exterior, at the same time its influence on the trajectory stability of vehicle with three degrees-of-freedom (3DOF) motion is also analyzed. Results indicate that reverse flow forms between row-to-row spacing when exhausting with two rows of holes, which enhances the exhausting process with the film's size enlarged and axial length extended, meanwhile it brings about more complex vortices structure near venting holes compared to the single-row hole case. As for the 3DOF cases, the pressure difference between vehicle's front and back sides is dramatically reduced attributing to the existence of attached air film, consequently the rotation of vehicle is weaken, leaving a better attitude to vehicle after it piercing water surface. Besides, the rapid formation of air film in double-row hole cases is advantage for the timely inhibiting of vehicle's pitching motion compared to the single-row hole cases, and their weaker stagnation high pressure near film's closure region is also good for the reduction of vehicle's lateral load.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091104-091104-9. doi:10.1115/1.4039512.

Efficient actuation of liquid slugs in microfluidic circuits is a matter of interest in droplet-based microfluidic (DMF) applications. In this paper, the electrowetting on dielectric (EWOD) actuation of a liquid slug fully confined in a microchannel is studied. A set of experiments are conducted in which the mean transport velocity of a liquid slug enclosed in a microchannel of rectangular cross section and actuated by EWOD method is measured. A printed circuit board-based (PCB-based) microfluidic chip is used as the platform, and the transport velocity of the slug is measured by processing the images recorded by a high-speed camera while the slug moves in the channel. To investigate the effect of microchannel geometry on the mean transport velocity of the slugs, different channel heights and widths (ranging between $250−440μm$ and 1–2 mm, respectively) as well as different liquid volumes (ranging between $2.94and5.15μL$) are tested and slug velocities up to 14.9 mm/s are achieved. A theoretical model is also developed to analyze the effect of involved parameters on the transport velocity. The results show that, within the range of design parameters considered in this study, for a constant slug volume and channel width, increasing the channel height enhances the velocity. Moreover, keeping the slug volume and channel height fixed, the transport velocity is increased by enlarging the channel width. An inverse proportionality between the slug length and velocity is also observed. These results are also shown to agree with the theoretical model developed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091105-091105-12. doi:10.1115/1.4039708.

This paper uses eigenexpansion technique to describe electro-osmotic effect on unsteady intrusion of a viscous liquid driven by capillary action in a narrow channel. It shows how the dynamics can be manipulated by imposing an electric field along the flow direction in the presence of free charges. Similar manipulation can generate controlled transiency in motion of a complex fluid in a tube by nondestructive forcing leading to efficient rheological measurement. Existing theories analyze similar phenomena by accounting for all involved forces among which the viscous contribution is calculated assuming a steady velocity profile. However, if the transport is strongly transient, a new formulation without an underlying quasi-steady assumption is needed for accurate prediction of the time-dependent penetration. Such rigorous mathematical treatment is presented in this paper where an eigenfunction expansion is used to represent the unsteady flow. Then, a system of ordinary differential equations is derived from which the unknown time-dependent amplitudes of the expansion are determined along with the temporal variation in encroached length. The outlined methodology is applied to solve problems with both constant and periodically fluctuating electric field. In both cases, simplified and convenient analytical models are constructed to provide physical insight into numerical results obtained from the full solution scheme. The detailed computations and the simpler reduced model corroborate each other verifying accuracy of the former and assuring utility of the latter. Thus, the theoretical findings can render a new rheometric technology for effective determination of fluid properties.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091106-091106-12. doi:10.1115/1.4039712.

This study investigates the flow-mediated interaction between two vibrating cylinders of the same size immersed in an otherwise still fluid. The master cylinder carries out forced vibration, while the slave cylinder is elastically mounted with one degree-of-freedom along the centerline between the two cylinders. We examined the stabilized vibration of the slave cylinder. In total, 6269 two-dimensional (2D) cases were simulated to cover the parameter space, with a fixed Reynolds number of 100, the structural damping factor of the slave cylinder ranging from 0 to 1.4, the mass ratio of the slave cylinder ranging from 1.5 to 2.5, the initial gap ratio ranging from 0.2 to 1.0, the vibration amplitude ratio of the master cylinder ranging from 0.025 to 0.1, and the vibration frequency ratio ranging from 0.05 to 2.4. We found that the vibration amplitude of the slave cylinder is highly sensitive to damping when the damping coefficient is small. The two cylinders' vibration is in antiphase at low frequencies but in phase at high frequencies. The phase of the slave cylinder changes abruptly at resonance when it has little damping, but the phase change with the frequency becomes increasingly gradual with increasing damping. With a nonzero damping factor, the maximum vibration amplitude of the slave cylinder is inversely correlated with its mass ratio. The response of the slave cylinder is explained by examining the pressure distribution and velocity field adjacent to it.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;140(9):091201-091201-16. doi:10.1115/1.4039515.

The maximum impingement plate skin friction and flow field is measured for an acoustically forced planar impinging gas jet using oil film interferometry (OFI) and particle image velocimetry (PIV), respectively. The study is performed at a jet Reynolds number of Rejet = 11,000 and an impingement distance H, which is set to eight times the nozzle width W. The planar impinging gas jet is forced at the jet nozzle exit using Strouhal numbers StH = 0.39, 0.76, and 1.1, which are similar to those associated with the jet-plate tones measured in air-knife wiping experiments. The flow-field measurements indicate that the jet column oscillates at the applied forcing frequency, and depending on the forcing frequency, organized vortex structures can be identified in the shear layers that impinge on the plate surface. Both of these jet oscillation features result in a reduction in the time-averaged maximum impingement plate skin friction. This skin friction reduction is attributed to momentum loss at the jet centerline caused by increased levels of fluid entrainment and mixing of the surrounding quiescent fluid.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091202-091202-14. doi:10.1115/1.4039511.

This work presents a three-objective design optimization of a centrifugal pump impeller to reduce flow recirculation and cavitation using three-dimensional (3D) Reynolds-averaged Navier–Stokes equations. A cavitation model was used to simulate the multiphase cavitating flow inside the centrifugal pump. The numerical results were validated by comparing them with experimental data for the total head coefficient and critical cavitation number. To achieve the optimization goals, blockage at 50% of the design flow rate, hydraulic efficiency at the design flow rate, and critical cavitation number for a head-drop of 3% at 125% of the design flow rate were selected as the objective functions. Based on the results of the elementary effect (EE) method, the design variables selected were the axial length of the blade, the control point for the meridional profile of the shroud, the inlet radius of the blade hub, and the incidence angle of tip of the blade. Kriging models were constructed to approximate the objective functions in the design space using the objective function values calculated at the design points selected by Latin hypercube sampling (LHS). Pareto-optimal solutions were obtained using a multi-objective genetic algorithm (MOGA). Six representative Pareto-optimal designs (POD) were analyzed to evaluate the optimization results. The PODs showed large improvements in the objective functions compared to the baseline design. Thus, both the hydraulic performance and the reliability of the centrifugal pump were improved by the optimization.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091203-091203-10. doi:10.1115/1.4039516.

Self-sustaining oscillations of flow over ducted cavities and in corrugated pipes are a known source of tonal noise and excessive vibration in industrial applications. Corrugated pipes can be modeled as a series of axisymmetric cavities. In the current study, the aero-acoustic sources generated by one-, two-, and three-cavity configurations have been experimentally investigated by means of the standing wave method (SWM) for a wide range of Strouhal numbers and acoustic excitation levels. The source strength is found to increase in a nonlinear manner with increasing the number of cavities. Moreover, the self-excited acoustic resonances of the same cavity combinations are investigated. The source characteristics are compared with the observed lock-in range from the self-excited experiments. A prediction model is also developed to utilize the measured source characteristics for estimating the amplitude of the cavities self-sustained oscillations. The self-excited experimental data are used to assess the effect of acoustic absorption at the cavity edges. This absorption is found to be substantial and must be accounted for in the prediction model. When the model is supplemented with appropriate loss coefficients, it predicts fairly well the pulsation amplitude within the resonance lock-in range of the studied multiple cavity configurations.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091204-091204-13. doi:10.1115/1.4039714.

Prewhirl regulation by inlet guide vanes (IGVs) has been proven as an effective method for operation regulation of centrifugal pumps. By contrast, the influence of the geometry of IGVs on operation stability of centrifugal pump remains unknown. The pressure fluctuations and flow patterns in a centrifugal pump without and with two-dimensional (2D) or three-dimensional (3D) IGVs are investigated numerically at 1.0Qd, 0.6Qd, and 1.2Qd. Renormalization group (RNG) k–ε turbulence model is used as turbulence model, and fast Fourier transform (FFT) method is used to analyze the pressure fluctuations. The dominant frequency of pressure fluctuations in impellers is either the rotational frequency fi or twice thereof for pumps without and with IGVs at three flow rates, while the dominant frequency is constantly the blade passing frequency in volute. For 1.0Qd, the comparison of pumps without IGVs indicates that the maximum amplitude of pressure fluctuations at fi in pumps with 2D IGVs is decreased by an average of 22.2%, and the amplitude is decreased by an average of 44.9% in pumps with 3D IGVs. The IGVs mainly influence the pressure fluctuations at fi but indicate minimal influence at 2fi. For 0.6Qd, the comparison of pumps without IGVs denotes that the maximum amplitudes of pressure fluctuations at fi in pumps with 2D or 3D IGVs both increase; the maximum increase is 2.01%. For 1.2Qd, the comparison of pumps without IGVs indicates that the maximum amplitudes of pressure fluctuations at fi in pumps with 2D or 3D IGVs both decrease; the maximum decline is 15.9%.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2018;140(9):091301-091301-13. doi:10.1115/1.4039715.

The entrainment, breakup, and interplay of air bubbles were observed in a vertical, two-dimensional supported jet at low impact velocities. Ultra-high-speed movies were analyzed both qualitatively and quantitatively. The onset velocity of bubble entrainment was between 0.9 and 1.1 m/s. Most bubbles were entrained as detached bubbles from elongated air cavities at the impingement point. Explosion, stretching, and dejection mechanisms were observed for individual bubble breakup, and the bubble interaction behaviors encompassed bubble rebound, “kiss-and-go,” coalescence and breakup induced by approaching bubble(s). The effects of jet impact velocity on the bubble behaviors were investigated for impact velocities from 1.0 to 1.36 m/s, in the presence of a shear flow environment.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(9):091302-091302-6. doi:10.1115/1.4039709.

Viscoelastic solution is encountered extensively in microfluidics. In this work, the particle movement of the viscoelastic flow in the contraction–expansion channel is demonstrated. The fluid is described by the Oldroyd-B model, and the particle is driven by dielectrophoretic (DEP) forces induced by the applied electric field. A time-dependent multiphysics numerical model with the thin electric double layer (EDL) assumption was developed, in which the Oldroyd-B viscoelastic fluid flow field, the electric field, and the movement of finite-size particles are solved simultaneously by an arbitrary Lagrangian–Eulerian (ALE) numerical method. By the numerically validated ALE method, the trajectories of particle with different sizes were obtained for the fluid with the Weissenberg number (Wi) of 1 and 0, which can be regarded as the Newtonian fluid. The trajectory in the Oldroyd-B flow with Wi = 1 is compared with that in the Newtonian fluid. Also, trajectories for different particles with different particle sizes moving in the flow with Wi = 1 are compared, which proves that the contraction–expansion channel can also be used for particle separation in the viscoelastic flow. The above results for this work provide the physical insight into the particle movement in the flow of viscous and elastic features.

Commentary by Dr. Valentin Fuster