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### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;140(1):011101-011101-17. doi:10.1115/1.4037500.

Dynamic stability of the high-head Francis turbines is one of the challenging problems. Unsteady rotor–stator interaction (RSI) develops dynamic stresses and leads to crack in the blades. In a high-head turbine, vaneless space is small and the amplitudes of RSI frequencies are very high. Credible estimation of the amplitudes is vital for the runner design. The current study is aimed to investigate the amplitudes of RSI frequencies considering a compressible flow. The hydro-acoustic phenomenon is dominating the turbines, and the compressibility effect should be accounted for accurate estimation of the pressure amplitudes. Unsteady pressure measurements were performed in the turbine during the best efficiency point (BEP) and part load (PL) operations. The pressure data were used to validate the numerical model. The compressible flow simulations showed 0.5–3% improvement in the time-averaged pressure and the amplitudes over incompressible flow. The maximum numerical errors in the vaneless space and runner were 6% and 10%, respectively. Numerical errors in the instantaneous pressure amplitudes at the vaneless space, runner, and draft tube were ±1.6%, ±0.9%, and ±1.8%, respectively. In the draft tube, the incompressible flow study showed the pressure amplitudes up to eight times smaller than those of the compressible. Unexpectedly, the strong effect of RSI was seen in the upper and lower labyrinth seals, which was absent for the incompressible flow.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011102-011102-9. doi:10.1115/1.4037504.

In this paper, the analysis of fast laminar transients in pressurized pipes is developed using a computational fluid dynamics (CFD) model, combined with the Zielke model and laboratory data. The systematic verification of the performance of the CFD model executed in the first part of the paper allows defining the most efficient set of the discretization parameters capable of capturing the main features of the examined transient. In this framework, the crucial role of radial discretization is pointed out. In the second part of the paper, the refined and efficient CFD model is used to examine some aspects of interest for understanding the dynamics of transients. Specifically, the uniformity of the instantaneous pressure distributions along the pipe radius, which validates the results of the most popular quasi-two-dimensional (2D) models, has been revealed. Moreover, it has been shown that the strongest link between the wall shear stress and the axial component of the velocity occurs in the region close to the pipe wall as well as that the time-shift between the wall shear stress and the local instantaneous flow acceleration increases significantly as time elapses.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011103-011103-5. doi:10.1115/1.4037505.

Cavitation within regions of flow separation appears in drifting vortices. A two-part computational method is employed for prediction of cavitation inception number there. The first part is an analysis of the average flow in separation regions without consideration of an impact of vortices. The second part is an analysis of equilibrium of the bubble within the core of a vortex located in the turbulent flow of known average characteristics. Computed cavitation inception numbers for axisymmetric flows are in the good agreement with the known experimental data.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011104-011104-10. doi:10.1115/1.4037681.

Despite the big interest in both, micro-air vehicles (MAV) and flow-control strategies, only few studies have investigated the flow-control possibilities over low aspect ratio (LAR) wings flying at low Reynolds numbers (Re). The present study verified the LAR thick airfoils' conformity with the nonlinear lift approximation equation. Then, a moving-wall flow control method was designed and tested over an LAR thick airfoil (0.57 aspect ration (AR), NACA0015 shaped) performing at a chord-based Re of 4 × 104. The moving belt control postponed the stall onset by 25 deg and produced a 103% gain in lift without any saturation signs at a control speed ratio of Ub/U = 6. Particle image velocimetry (PIV) measurements confirmed the effectiveness of the moving-wall control strategy on the upper surface flow reattachment. Moreover, other quantities such as the, vortices, and the swirling strength are investigated.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011105-011105-10. doi:10.1115/1.4037661.

This study investigates energy harvesting of a two-dimensional foil in the wake downstream of a cylinder. The foil is passively mobile in the transverse direction. An immersed boundary (IB) method with a fluid–structure interaction (FSI) model is validated and employed to carry out the numerical simulation. For improving numerical stability, this study incorporates a modified low-storage first-order Runge–Kutta scheme for time integration and demonstrates the performance of this temporal scheme on reducing spurious pressure oscillations of the IB method. The simulation shows the foil emerged in a vortical wake achieves better energy harvesting performance than that in a uniform flow. The types of the dynamic response of the energy harvester are identified, and the periodic response is desired for optimal energy harvesting performance. Last, the properties of vortical wakes are found to be of pivotal importance in obtaining this desired periodic response.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;140(1):011201-011201-9. doi:10.1115/1.4037677.

The present study deals with the generalization of a macroscopic turbulence model in porous media using a capillary model. The additional source terms associated with the production and dissipation of turbulent kinetic energy due to the presence of solid matrix are calculated using the capillary model. The present model does not require any prior pore scale simulation of turbulent flow in a specific porous geometry in order to close the macroscopic turbulence equations. Validation of the results in packed beds, periodic arrangement of square cylinders, synthetic foams, and longitudinal flows such as pipes, channels, and rod bundles against available data in the literature reveals the ability of the present model in predicting turbulent flow characteristics in different types of porous media. Transition to the fully turbulent regime in porous media and different approaches to treat this phenomenon are also discussed in the present study. Finally, the general model is modified so that it can be applied to lower Reynolds numbers below the range of fully turbulent regime in porous media.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011202-011202-9. doi:10.1115/1.4037522.
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For scientist, the Coanda effect has been an object of interest for a long time. All the time, some new applications of it are found although it has been more than a hundred years since Henri Coanda got a patent that was critical for that issue. Apart from aviation, it is more and more often used in ventilation systems to control the manner of air division and the design nozzles and ventilators. It is surprising, however, that a good command of that phenomenon and a need to apply it in different solutions did not entail a significant increase of the interest in the Coanda effect hysteresis, although it was mentioned for the first time by Newman in 1961. This article presents results of experimental measurements for a two-dimensional incompressible plane jet by an inclined plate. The hysteresis has been observed as a different jet behavior (a free jet or a jet attached to a flat plate) depending on the direction in which the plate deflection angle changes. The observed hysteresis area, defined by critical values for the αca attachment and αcd detachment angles, spanned from 8 deg to 14 deg. Its dependency on the Reynolds number has also been examined for Re ranging from 3500 to 26,500. Considering the Coanda effect hysteresis, a pressure distribution on the plate and the xR reattachment distance has been examined. The distribution of forces on a plate has been identified, which has facilitated a graphical mirroring of the Coanda effect hysteresis loop.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011203-011203-12. doi:10.1115/1.4037506.

Owing to the limiting effect of energy, vortex rings cannot grow indefinitely and thus pinch off. In this paper, experiments on the vortex rings generated using a piston-cylinder apparatus are conducted so as to investigate the pinch-off mechanisms and identify the limiting effect of energy. Both theoretical and experimental results show that the generated vortex rings share a unified energy feature, regardless of whether they are pinched-off or not. Moreover, the unified energy feature is quantitatively described by a dimensionless energy number γ, defined as $γ=(E/I2Γωmax)$ and exhibiting a critical value γring = 0.14 ± 0.01 for the generated vortex rings. This unified energy feature reflects the limiting effect of energy and specifies the target of vortex ring formation. Furthermore, based on the tendency of γ during vortex ring formation, criteria for determining the two timescales, i.e., pinch-off time and separation time, which correspond to the onset and end of pinch-off process, respectively, are suggested.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011204-011204-12. doi:10.1115/1.4037675.

In the present study, the optimal two-dimensional (2D) tripping technique for inducing a naturally fully developed turbulent boundary layer in wind tunnels has been investigated. Various tripping techniques were tested, including wires of different diameters and changes in roughness. Experimental measurements were taken on a flat plate in a wind tunnel at a number of locations along the flat plate and at a variety of flow speeds using hot-wire anemometry to measure the boundary layer resulting from each tripping method. The results have demonstrated that to produce a natural turbulent boundary layer using a 2D protuberance, the height of the trip must be less than the undisturbed boundary layer thickness. Using such a trip was shown to reduce the development length of the turbulent boundary layer by approximately 50%. This was shown to hold true for all Reynolds numbers investigated $(Rex=1.2×105−1.5×106)$. The present study provides an insight into the effect of the investigated trip techniques on the induced transition of a laminar boundary layer into turbulence.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(1):011205-011205-10. doi:10.1115/1.4037676.

An experimental study was conducted to investigate the breakup of a liquid sheet produced by oblique impingement of a liquid jet on a plane solid surface. Experiments are carried out over a wide range of jet Weber number (80–6300) and various jet impingement angles (30 deg, 45 deg, and 60 deg) are employed to study the sheet dynamics. The breakup of a liquid sheet takes place in three modes, closed rim, open rim, and perforated sheet, depending upon the Weber number. The transitions across the modes are also influenced by the impingement angle with the transition Weber number reducing with increase in impingement angle. A modified regime map is proposed to illustrate the role of impingement angle in breakup transitions. A theoretical model based on force balance at the sheet edge is developed to predict the sheet parameters by taking the shear interaction between the sheet and the solid surface into account. The sheet shape predicted by the model fairly matches with the experimentally measured sheet shape. The breakup length and width of the sheet are measured and comparisons with the model predictions show good agreement in closed rim mode of breakup.

Commentary by Dr. Valentin Fuster