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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;140(6):061101-061101-9. doi:10.1115/1.4038761.

The paper describes the modeling and the experimental tests of a variable displacement vane pump for engine lubrication. The approach used for the simulation has involved three-dimensional (3D) commercial tools for tuning a zero-dimensional (0D) customized model implemented in the LMS Amesim® environment. Different leakage paths are considered and the axial clearances are variable to take into account the deformation of the pump cover, calculated through a finite element analysis with ANSYS. The vane tip clearances are calculated as function of the dynamic equilibrium equation of the vanes. The displacement control takes into account the internal forces on the stator due to the pressure in all variable chambers and to the contact force exerted by the vanes. The discharge coefficients in the resistive components have been tuned by means of a complete 3D transient model of the pump built with PumpLinx®. The tuned 0D model has been proved to be reliable for the determination of the steady-state flow-speed and flow-pressure curves, with a correct estimation of the internal leakages and of the pressure imposed by the displacement control. The pump has been also tested using a simplified circuit, and a fair agreement has been found in the evaluation of the delivery pressure ripple.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061102-061102-7. doi:10.1115/1.4038986.

The pressure drop across 90deg sharp-angled miter elbows connecting straight circular pipes is studied in a bespoke experimental facility by using water and air as working fluids flowing in the range of bulk Reynolds number 500<Re<60,000. To the best of our knowledge, the dependence on the Reynolds number of the pressure drop across the miter elbow scaled by the dynamic pressure, i.e., the pressure-loss coefficient K, is reported herein for the first time. The coefficient is shown to decrease sharply with the Reynolds number up to about Re=20,000 and, at higher Reynolds numbers, to approach mildly a constant K=0.9, which is about 20% lower than the currently reported value in the literature. We quantify this relation and the dependence between K and the straight-pipe friction factor at the same Reynolds number through two new empirical correlations, which will be useful for the design of piping systems fitted with these sharp elbows. The pressure drop is also expressed in terms of the scaled equivalent length, i.e., the length of a straight pipe that would produce the same pressure drop as the elbow at the same Reynolds number.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061103-061103-10. doi:10.1115/1.4039089.

Flexible electricity demand and variability of the electricity produced by wind turbines and photovoltaic affect the stable operations of power grids. Pump-turbines are used to stabilize the power grid by maintaining a real-time electricity demand. Consistently, the machines experience transient conditions during the course of operation, such as start-up, load acceptance, load rejection, and shutdown, which induce high amplitude pressure pulsations and affect operating lifespan of the components. During the closure of the wicket gates, the transient flow characteristics is analyzed for a Francis-type reversible pump-turbine in generating mode by three-dimensional (3D) numerical simulation with a moving mesh technique and using detached eddy simulation (DES) turbulent model. Mesh motion is carried out in the region of wicket gates during the load rejection by a moving, sliding mesh, which makes dynamic flow simulation available, instead of building various steady models with different guide vanes angles. The transient flow characteristics are illustrated by analyzing the flow, torque, and pressure fluctuations signals by frequency and time–frequency analyses. The flow field analysis includes the onset and strengthening of unsteady phenomena during the turbine load reduction. The flow pattern in return channel maintained a quite stable flow field, whereas the flow pattern in the runner and draft tube emphasized its instability with the flow rate decreased. Influence of 3D unsteady flow structures on runner is determined, and its evolution is characterized spectrally during fast closure of wicket gates.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;140(6):061201-061201-7. doi:10.1115/1.4038730.

Cavitation leads to rapid degassing of fluids. Up to date, there is a lack of model approaches of cavitation-induced degassing. The aim of the present study is to gain a more thorough knowledge of the process. Therefore, the relation between cavitation intensity and air release is investigated experimentally for an orifice flow as function of cavitation number. For this, shadowgraphy imaging is used to visualize regions of steam and air volume downstream of the orifice. Analysis of the images shows a strongly nonlinear scaling behavior for both cavitation intensity and air release as a function of cavitation number. Three distinct regimes could be identified for cavitation-induced gas release. While an exponential scaling was found at high cavitation intensities, degassing rates appear to be nearly constant in the intermediate cavitation number range. Empirical scaling laws are given here that may serve as first model approach for the prediction of cavitation induced air release behind flow constrictions.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061202-061202-10. doi:10.1115/1.4039032.

Two-dimensional (2D) numerical simulations of multiphase flows past a circular cylinder close to free surface waves were performed to investigate an interaction of vortex formation around a cylinder with periodic waves by utilizing waves2Foam. The lock-on responses of lift coefficient at low frequencies were lower than those of the natural response due to the existence of “out-of-phase” between the fluctuations of lift coefficient by vortex formation and the vertical force fluctuations by wave motions. The resonant effect of an external excitation by the periodic waves on the lift coefficient fluctuations was not significant despite the occurrence of lock-on.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061203-061203-8. doi:10.1115/1.4038668.

Numerical simulations of laminar pipe and channel flows were carried out: (i) to understand the effect of inlet conditions, viz., flat inlet and streamtube inlet, on entry lengths, and (ii) to investigate the flow development in radial/transverse locations. Results show that hydrodynamic entry lengths from the streamtube inlet simulations are significantly lower compared to the entry lengths from the flat inlet simulations for low Reynolds numbers. Moreover, results from the current study (Newtonian flow with no-slip) as well as the results from the literature (non-Newtonian flow with no-slip) showed that for many flow situations, the slowest development of axial velocity in the transverse location happens to be very near to the wall. For the above cases, the existing entry length criteria (centerline as well as global entry length) are not appropriate to define the entry length. We have proposed a new entry length criterion based on the displacement thickness which is an integral measure of the velocity profile. A new entry length correlation using the displacement thickness criterion is proposed for Newtonian flows in pipe and channel based on simulations with the streamtube inlet condition.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061204-061204-10. doi:10.1115/1.4039120.

Transient pressure peak values and decay rates associated with water hammer surges in fluid lines are investigated using an analytical method that has been formulated, in a previous publication, to simulate pressure transients in turbulent flow. The method agrees quite well with method of characteristics (MOC) simulations of unsteady friction models and has been verified with experimental data available for Reynolds numbers out to 15,800. The method is based on the formulation of ordinary differential equations from the frequency response of a pressure transfer function using an inverse frequency algorithm. The model is formulated by dividing the line into n-sections to distribute the turbulence resistance along the line at higher Reynolds numbers. In this paper, it will be demonstrated that convergence of the analytical solution is achieved with as few as 5–10 line sections for Reynolds numbers up to 200,000. The method not only provides for the use of conventional time domain solution algorithms for ordinary differential equations but also provides empirical equations for estimating peak surge pressures and transient decay rates as defined by eigenvalues. For typical sets of line and fluid properties, the trend of the damping ratio of the first or dominate mode of the pressure transients transfer function is found to be an approximate linear function of a dimensionless parameter that is a function of the Reynolds number. In addition, a reasonably accurate dimensionless trend formula for estimates of the normalized peak pressures is formulated and presented.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2018;140(6):061301-061301-13. doi:10.1115/1.4038753.

A twin-fluid atomizer configuration is predicted by means of the two-dimensional (2D) weakly compressible smooth particle hydrodynamics (SPH) method and compared to experiments. The setup consists of an axial liquid jet surrounded by a high-speed air stream (Ug ≈ 60 m/s) in a pressurized reactor, which is operated at up to 11 bar. Two types of liquid are investigated: a viscous Newtonian liquid (μl = 200 mPa·s) consisting of glycerol/water mixture and a viscous non-Newtonian liquid (μ1,apparent. ≈ 150 mPa·s), which is a carboxymethyl cellulose solution. Three-dimensional (3D) effects are taken into account in the 2D code by introducing: (i) a surface tension term, (ii) a cylindrical viscosity operator, and (iii) a modified velocity accounting for the divergence of the volume in the radial direction. The numerical results at high pressure show a good qualitative agreement with experiment, i.e., a correct transition of the different atomization regimes with regard to pressure, and similar dynamics and length scales of the generated ligaments. The propagation velocity of the Kelvin–Helmholtz (KH) instability is well predicted, but its frequency needs a correction factor to be globally well recovered for the Newtonian liquid. The Sauter mean diameter (SMD), calculated from the spray size distribution, shows similar trends of the reactor pressure dependency. The simulation of the non-Newtonian liquid at high pressure shows the same breakup regime with finer droplets compared to Newtonian liquids, and the simulation at atmospheric pressure shows an apparent viscosity similar to the experiment.

Commentary by Dr. Valentin Fuster

Research Papers: Techniques and Procedures

J. Fluids Eng. 2018;140(6):061401-061401-5. doi:10.1115/1.4038802.

Based on computational fluid dynamics and the finite volume method, water droplet erosion in the last stage of an industrial steam turbine was researched and the trajectories of the water droplets were traced by using the Lagrange method. Under steady conditions, the influence of variant bowed vane designs was compared based on the distribution and movement trends of the secondary water droplets. In addition, the effects of the bowed blades on water movement at the vane surfaces and their impact areas and intensity on the blades were analyzed. The results showed that: (1) a negatively bowed blade can reduce the speed of the secondary water droplets at the mid span of the blade, which are also effective for water droplets on the surface of the vanes and (2) a negatively bowed blade improves the speed of the secondary droplets on the end walls of vanes, which is advantageous to the secondary droplets through blade passage and reduction of secondary droplet impulse on the blades.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061402-061402-10. doi:10.1115/1.4038866.

This paper is concerned with the study of a kind of discrete forcing immersed boundary method (IBM) by which the loosely aero-elasticity coupled method is developed to analyze turbine blade vibration. In order to reduce the spurious oscillations at steep gradients in the compressible viscous flowing field, a five orders weighted essentially nonoscillatory scheme (WENO) is introduced into the flow solver based on large eddy simulation (LES). The three-dimensional (3D) full-annulus domain of the last two stages of an industrial steam axial turbine is adopted to validate the developed method. By the method, the process of grid generation becomes very simple and the unsteady data transferring between stator and rotor is realized without the process of being averaged or weighted. Based on the analysis of some important aerodynamic parameters, it is believed that hypothesis of azimuthal periodicity is not reasonable in this case and full-annulus passages model is more feasible and suitable to the research of turbine blade vibration. Meanwhile, the blade vibration data are also discussed. It is at about 65% of rotor blade height of the last stage that an inflection point is observed and the midspan region of the blade is the vulnerable part damaged potentially by the blade vibration.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(6):061403-061403-10. doi:10.1115/1.4039087.

The Spalart–Allmaras (SA) is one of the most popular turbulence models in the aerospace computational fluid dynamics (CFD) community. In its original (low-Reynolds number) formulation, it requires a very tight grid spacing near the wall to resolve the high flow gradients. However, the use of wall functions with an automatic feature of switching from the wall function to the low-Reynolds number approach is an effective solution to this problem. In this work, we extend Menter's automatic wall treatment (AWT), devised for the k–ω-shear stress transport (SST), to the SA model in our in-house developed three-dimensional unstructured grid density-based CFD solver. It is shown, for both momentum and energy equations, that the formulation gives excellent predictions with low sensitivity to the grid spacing near the wall and allows the first grid point to be placed at y+ as high as 150 without loss of accuracy, even for the curved walls. In practical terms, this means a near-wall grid 10–30 times as coarse as that required in the original model would be sufficient for the computations.

Commentary by Dr. Valentin Fuster

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