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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(8):081101-081101-25. doi:10.1115/1.4036244.

The swirling flow exiting the runner of a hydraulic turbine is further decelerated in the discharge cone of the draft tube to convert the excess of dynamic pressure into static pressure. When the turbine is operated far from the best efficiency regime, particularly at part load, the decelerated swirling flow develops a self-induced instability with a precessing helical vortex and the associated severe pressure fluctuations. This phenomenon is investigated numerically in this paper, for a swirl apparatus configuration. The unsteady three-dimensional (3D) flow field is analyzed using a proper orthogonal decomposition (POD), and within this framework we examine the effectiveness of an axial jet injection for mitigating the flow instability. It is shown that a limited number of modes can be used to reconstruct the flow field. Moreover, POD enables to reveal influence of the jet injection on the individual modes of the flow and illustrates continuous suppression of the modes from higher-order modes to lower-order modes as the jet discharge increases. Application of POD offers new view for the future control effort aimed on vortex rope mitigation because spatiotemporal description of the flow is provided. Thereby, POD enables better focus of the jets or other flow control devices.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(8):081102-081102-14. doi:10.1115/1.4036267.

Withdrawal of water-capped viscoplastic fluid was investigated using laboratory experimentation and numerical modeling. The viscoplastic fluid was modeled using a Laponite suspension, which was withdrawn by a vertical pipe intake. Variations of the Laponite–water interface and intake configurations were investigated in this study. The critical submergence, the depth of the intake in the Laponite layer when the upper water begins to withdraw, was studied under different experimental conditions, and the critical depths were measured for different flow rates. An empirical relationship was found between the withdrawal flow rate and the critical submergence. The averaged Laponite velocity was measured at different withdrawal stages to identify the critical stage. A series of numerical simulations were conducted to study the effect of intake structures so that a maximum amount of the Laponite suspension can be withdrawn before the water layer being withdrawn. It was found that a combination of a collar and a cone with an edge length to the intake diameter of 1.5 can increase the pumping duration by 16.7%. The installation of a collar or collar-cone setup can also decrease the disturbance in Laponite layer.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(8):081103-081103-7. doi:10.1115/1.4036264.

Operating hydraulic turbines under part- or over-load conditions leads to the development of the precessing vortex rope downstream of the turbine runner. In a regime close to the best efficiency point (BEP), the vortex rope is very unstable because of the low residual swirl of the flow. However, strong pressure pulsations have been detected in the regime. These oscillations can be caused by self-merging and reconnection of a vortex helix with the formation of a vortex ring. The vortex ring moves along the wall of the draft tube and generates a sharp pressure pulse that is registered by pressure transducer. This phenomenon was investigated on a simplified draft tube model using a swirl generator consisting of a stationary swirler and a freely rotating runner. The experiments were performed at Reynolds number (Re) = 105. The measurements involved a high-speed visualization technique synchronized with pressure measurements on the draft tube wall, which enables an analysis of the key stages of vortex ring formation by comparing it with the pressure on the draft tube wall. Quantitative information regarding the average velocity distribution was obtained via the laser Doppler anemometer (LDA) technique.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;139(8):081201-081201-8. doi:10.1115/1.4036266.

The Jeffery–Hamel problem for laminar, radial flow between two nonparallel plates has been extended to the case of two immiscible fluids in slender channels. The governing continuity and momentum equations were solved numerically using the fourth-order Runge–Kutta method. Solutions were obtained for air–water at standard conditions over the void-fraction range of 0.4–0.8 (due to its practical significance) and the computations were limited to conditions where unique solutions were found to exist. The void fraction, pressure gradient, wall friction coefficient, and interfacial friction coefficient are dependent on the Reynolds numbers of both fluids and the complex nature of this dependence is presented and discussed. An attempt to use a one-dimensional two-fluid model with simplified assumptions succeeded in producing a qualitatively similar form of the void-fraction dependence on the two Reynolds numbers; however, quantitatively there are significant deviations between these results and those of the complete model.

Topics: Fluids , Radial flow
Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(8):081202-081202-16. doi:10.1115/1.4036247.

In this work, we propose a fixed mesh finite element formulation to solve the fluid dynamic on an Eulerian mesh dealing with immersed bodies in motion. The study is focused on the computation of the fluid dynamic forces acting on immersed bodies which strongly depend on the evolution of the vortex shedding. The frequency of vortex detachment for flow past cylinder problems can be modified when the cylinder moves, promoting the modification of the wake of vortices. Synchronization phenomena appear when the frequencies of the resulting flow pattern coincide with the frequency of the imposed body motion. To study this problem, we propose to describe the immersed body surface by a collection of markers that moves according to the imposed body motion. The markers are updated using a Lagrangian scheme. In this framework, a distinct aspect of the present work is the imposition of the body velocity as an internal immersed boundary condition for the fluid dynamic analysis. To transfer the body velocity to the fluid along the fluid–solid interface, a restriction on the flow velocity is added into the weak form of the Navier–Stokes equations by means of a penalty technique. This work encompasses the study of flows past a crossflow, streamwise, and rotational oscillating cylinders. The results are satisfactorily compared with numerical data reported in the literature, showing a proper behavior for the analysis of long-term vibrating systems at low Reynolds numbers.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(8):081203-081203-7. doi:10.1115/1.4036265.

In this paper, we have analyzed an enhanced electroosmotic flow (EOF) by geometric modulation of the surface of a charged nanochannel. Otherwise, flat walls of the channel are modulated by embedding rectangular grooves placed perpendicular to the direction of the applied electric field in a periodic manner. The modulated channel is filled with a single electrolyte. The EOF within the modulated channel is determined by computing the Navier–Stokes–Nernst–Planck–Poisson equations for a wide range of Debye length. The objective of the present study is to achieve an enhanced EOF in the surface modulated channel. A significant enhancement in average EOF is found for a particular arrangement of grooves with the width of the grooves much higher than its depth and the Debye length is in the order of the channel height. However, the formation of vortex inside the narrow grooves can reduce the EOF when the groove depth is in the order of its width. Results are compared with the cases in which the grooves are replaced by superhydrophobic patches along which a zero shear stress condition is imposed.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2017;139(8):081301-081301-12. doi:10.1115/1.4036246.

An algorithm to prevent or delay bubble coalescence for the level set (LS) method is presented. This novel algorithm uses the LS method field to detect when bubbles are in close proximity, indicating a potential coalescence event, and applies a repellent force to simulate the unresolved liquid drainage force. The model is introduced by locally modifying the surface tension force near the liquid film drainage area. The algorithm can also simulate the liquid drainage time of the thin film by controlling the length of time the increased surface tension has been applied. Thus, a new method of modeling bubble coalescence has been developed. Several test cases were designed to demonstrate the capabilities of the algorithm. The simulations, including a mesh study, confirmed the abilities to identify and prevent coalescence as well as implement the time tracking portion, with an additional 10–25% computational cost. Ongoing tests aim to verify the algorithm's functionality for simulations with different flow conditions, a ranging number of bubbles, and both structured and unstructured computational mesh types. Specifically, a bubble rising toward a free surface provides a test of performance and demonstrates the ability to consistently prevent coalescence. In addition, a two bubble case and a seven bubble case provide a more complex demonstration of how the algorithm performs for larger simulations. These cases are compared to much more expensive simulations capable of resolving the liquid film drainage (through very high local mesh resolution) to investigate how the algorithm replicates the liquid film drainage process.

Commentary by Dr. Valentin Fuster

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