0

### Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;139(4):041101-041101-12. doi:10.1115/1.4035217.

In this paper, a new cubic subgrid-scale (SGS) model is proposed to capture the rotation effect. Different from the conventional nonlinear model with second-order term, the new model contains a cubic term which is originated in the Reynolds stress closure. All the three model coefficients are determined dynamically using the Germano’s identity. The model is examined in the rotating turbulent channel flow and the Taylor–Couette flow. Comparing with the linear model and the second-order model, the new model shows better performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041102-041102-14. doi:10.1115/1.4035224.

Francis turbine working at off-design operating condition experiences high swirling flow at the runner outlet. In the present study, a high head model Francis turbine was experimentally investigated during load rejection, i.e., best efficiency point (BEP) to part load (PL), to detect the physical mechanism that lies in the formation of vortex rope. For that, a complete measurement system of dynamic pressure, head, flow, guide vanes (GVs) angular position, and runner shaft torque was setup with corresponding sensors at selected locations of the turbine. The measurements were synchronized with the two-dimensional (2D) particle image velocimetry (PIV) measurements of the draft tube. The study comprised an efficiency measurement and maximum hydraulic efficiency of 92.4 ± 0.15% was observed at BEP condition of turbine. The severe pressure fluctuations corresponding to rotor–stator interaction (RSI), standing waves, and rotating vortex rope (RVR) have been observed in the draft tube and vaneless space of the turbine. Moreover, RVR in the draft tube has been decomposed into two different modes; rotating and plunging modes. The time of occurrence of both modes was investigated in pressure and velocity data and results showed that the plunging mode appears 0.8 s before the rotating mode. In the vaneless space, the plunging mode was captured before it appears in the draft tube. The physical mechanism behind the vortex rope formation was analyzed from the instantaneous PIV velocity vector field. The development of stagnation region at the draft tube center and high axial velocity gradients along the draft tube centerline could possibly cause the formation of vortex rope.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041103-041103-8. doi:10.1115/1.4035299.

Heavy cavitation in torque converters can have a significant effect on hydrodynamic performance, particularly with regards to the torque capacity. The objective of this study is to therefore investigate the effects of pump and turbine blade geometries on cavitation in a torque converter and improve the torque capacity without increasing the torus dimension. A steady-state homogeneous computational fluid dynamics (CFD) model was developed and validated against test data at stall operating condition. A full flow passage with a fixed turbine-stator domain was used to improve the convergence and accuracy of the cavitation model. Cavitation analysis was carried out with various pump and turbine blade geometries. It was found that there is a threshold point for pump blade exit angle in terms of its effect on torque capacity due to heavy cavitation. Further increasing the pump blade exit angle past this point will worsen cavitation condition and decrease torque capacity. The study also shows that a higher turbine blade exit angle, i.e., lower stator incidence angle, could reduce flow separation at the stator suction surface and consequently abate cavitation. A base high-capacity torque converter was upgraded utilizing the cavitation model, and the resulting design exhibited a 20.7% improvement in capacity constant without sacrificing other performance metrics.

Commentary by Dr. Valentin Fuster

### Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;139(4):041201-041201-7. doi:10.1115/1.4035115.

A solution of the problem of Poiseuille slip flow through an eccentric cylindrical annulus is obtained in bipolar coordinates. The solution is in excellent agreement with the two published limiting cases of slip flow through concentric annuli and no-slip flow through eccentric annuli. It is shown that for a fixed aspect ratio, fully eccentric channels sustain the maximum average velocity (flow rate) under the same pressure gradient and slip conditions. For a given channel geometry, the average velocity varies linearly with Knudsen number except for small aspect ratio. It is also shown that the extrema of the friction factor Reynolds number product is determined by how this product is defined or scaled.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041202-041202-11. doi:10.1115/1.4035242.

This article reports the results of a numerical computation of the length and total pressure drop in the entrance region of a circular tube with laminar flows of pseudoplastic and dilatant fluids at high Reynolds numbers (i.e., approximately 400 or higher). The analysis utilizes equations for the apparent viscosity that span the entire shear rate regime, from the zero to the infinite shear rate Newtonian regions, including the power law and the two transition regions. Solutions are thus reported for all shear rates that may exist in the flow field, and a shear rate parameter is identified that quantifies the shear rate region where the system is operating. The entrance lengths and total pressure drops were found to be bound by the Newtonian and power law values, the former being approached when the system is operating in either the zero or the infinite shear rate Newtonian regions. The latter are approached when the shear rates are predominantly in the power law region but only if, in addition, the zero and infinite shear rate Newtonian viscosities differ sufficiently, by approximately four orders of magnitude or more. For all other cases, namely, when more modest differences in the limiting Newtonian viscosities exist, or when the system is operating in the low- or high-shear rate transition regions, then intermediate results are obtained. Entrance length and total pressure drop values are provided in both graphical form, and in tabular and correlation equation form, for convenient access.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041203-041203-13. doi:10.1115/1.4035223.

The principal purpose of this study is to understand the entropy generation rate in bypass, transitional, boundary-layer flow better. The experimental work utilized particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to measure flow along a flat plate. The flow past the flat plate was under the influence of a negligible “zero” pressure gradient, followed by the installation of an adverse pressure gradient. Further, the boundary layer flow was artificially tripped to turbulence (called “bypass” transition) by means of elevated freestream turbulence. The entropy generation rate was seen to behave similar to that of published computational fluid dynamics (CFD) and direct numerical simulation (DNS) results. The observations from this work show the relative decrease of viscous contributions to entropy generation rate through the transition process, while the turbulent contributions of entropy generation rate greatly increase through the same transitional flow. A basic understanding of entropy generation rate over a flat plate is that a large majority of the contributions come within a wall coordinate less than 30. However, within the transitional region of the boundary layer, a tradeoff between viscous and turbulent dissipation begins to take place where a significant amount of the entropy generation rate is seen out toward the boundary layer edge.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041204-041204-9. doi:10.1115/1.4035465.

In the present study, we visualize the turbulent wake of various bluff bodies placed inside a circular pipe. The objective is to identify shapes which are strong vortex generators and incur minimum irrecoverable pressure loss. The shapes of the bluff bodies are chosen such that they exhibit distinct separation point for strong and stable vortex shedding. The dye used for flow visualization is a shear-thickening and high extension viscosity fluid, which can sustain turbulent separated flows. The planar illumination of the flow field with laser sheet improves the visibility of coherent structures for qualitative and quantitative assessment of the images. The vortex shedding frequency, wake width, and vortex formation length are computed from image analysis. The results highlight that streamlined shapes possess lower wake width and larger vortex formation length, whereas blunt shapes (like triangle and trapezoid) show a larger wake width and a shorter vortex formation length. The vortex shedding frequency is also measured with a piezoelectric sensor to aid flow visualization studies. The optimum location of the piezoelectric sensor is explained based on the vortex formation length to obtain high amplitude signals. Among all the shapes studied, trapezoidal bluff body appears to be the most appropriate shape with strong and stable vortex shedding. This information is useful in the design of vortex flowmeters and other similar applications.

Commentary by Dr. Valentin Fuster

### Research Papers: Multiphase Flows

J. Fluids Eng. 2017;139(4):041301-041301-8. doi:10.1115/1.4035113.

A fluid–structure interaction (FSI) system has been solved using the coupled acoustic structural finite element method (FEM) to simplify the cavitating flow conditions around a hydrofoil. The modes of vibration and the added mass effects have been numerically simulated for various flow conditions including leading edge attached partial cavitation on a two-dimensional NACA0009 hydrofoil. The hydrofoil has been first simulated surrounded by only air and by only water. Then, partial cavities with different lengths have been modeled as pure vapor fluid domains surrounded by the corresponding water and solid domains. The obtained numerical added mass coefficients and mode shapes are in good agreement with the experimental data available for the same conditions. The study confirms that the fluid added mass effect decreases with the cavitation surface ratio (CSR) and with the thickness of the cavitation sheet. Moreover, the simulations also predict slight mode shape variations due to cavitation that have also been detected in the experiments. Finally, the effects of changes in cavity location have been evaluated with the previously validated model.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041302-041302-10. doi:10.1115/1.4035218.

Unsteady cavitating flows around propellers become increasingly prominent on large-scale and high-speed ships, but large eddy simulations (LES) are limited in the literature. In this study, numerical simulation of an unsteady cavitating flow around a highly skewed propeller in a nonuniform wake is performed based on an explicit LES approach with $k−μ$ subgrid model. Kunz cavitation model, volume of fluid (VOF) method, and a moving mesh scheme are adopted. The predicted evolution of the unsteady cavitating flow around a highly skewed propeller in a nonuniform ship wake is in good agreement with experimental results. An analysis of the factors affecting the cavitation on the propeller is conducted based on numerical simulation. Furthermore, the influences between cavitation structures and vortex structures are also briefly analyzed.

Commentary by Dr. Valentin Fuster

### Research Papers: Techniques and Procedures

J. Fluids Eng. 2017;139(4):041401-041401-9. doi:10.1115/1.4035243.

A blow-down wind tunnel is a typical nonlinear time-varying system facing the coupling effects between the pressure and temperature during the short-time test procedure. The control of blow-down wind tunnels has been discussed for a long time and a satisfactory general solution to this problem is still of interests. This paper aims to model the internal relationship of the state variables of the wind tunnel by using thermodynamic theories. With the developed model, a new control method combining extended Kalman filter (EKF) together with auxiliary nonlinear predictive filter (NPF) is proposed to improve the control performance of the blow-down wind tunnel controller, in terms of accuracy and robustness. The transient coupling effects between the pressure and temperature are fully considered in the proposed approach. The results from the simulation and experiments are consistent and demonstrate that the controller based on EKF combined together with NPF can work better than previously proposed EKF-based controller.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):041402-041402-19. doi:10.1115/1.4035302.

In the current paper nondeterministic computational fluid dynamics (CFD) computations of three-dimensional (3D), developing, and statistically steady turbulent flow through an asymmetric diffuser with moderate adverse pressure gradient are presented. The inflow condition is assumed to be uncertain. The inlet streamwise velocity is supposed to be a stochastic process and described by the Karhunen–Loève (KL) expansion. In addition, the inlet turbulence intensity and turbulent length scale are assumed to be uncertain. The nonintrusive polynomial chaos (NIPC) expansion is used to propagate the inflow uncertainties in the flow field. The developed code is verified using a Monte Carlo (MC) simulation with 1000 Latin Hypercube samples on a planar asymmetric diffuser. A very good agreement is observed between the results of MC and polynomial chaos expansion methods. The verified uncertainty quantification method is then applied to stochastic developing turbulent flow through a 3D asymmetric diffuser. It was observed that the eigenvalues of covariance kernel rapidly decay due to the large correlation lengths and thus a few terms in the truncated KL expansion are used to describe the stochastic inlet velocity. For the KL expansion, the mean and the standard deviation are set to those measured experimentally. The uncertain inlet condition has a significant influence on the numerical results of velocity and turbulence fields specially in the developing region before the shear layers meet. It is concluded that one of the reasons for discrepancies between experimental and deterministic CFD results is the uncertainty in inflow condition. A sensitivity analysis is also performed using the Sobol’ indices and contribution of each uncertain parameter on outputs variance is presented.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Fluids Eng. 2017;139(4):044501-044501-7. doi:10.1115/1.4035114.

A modified partially averaged Navier–Stokes model (MPANS) is proposed by treating the standard k–ε model as the parent model and formulating the unresolved-to-total kinetic energy ratio fk as a function of the local grid size and turbulence length scale. Flows over a backward facing step are used to evaluate the performance of MPANS mode. Computations of the standard k–ε model, the constant fk partially averaged Navier–Stokes (PANS) models (fk = 0.6, 0.7), and the two-stage PANS model are carried out for comparisons. Based on the detailed analyses of calculated results and experimental data, the MPANS model performs better to predict the reattachment length together with the corner vortex and provides overall improved statistics of skin frictions, pressures, velocity profiles, and Reynolds stresses, demonstrating its promising applications in industrial turbomachines that often encounter with flow separations.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):044502-044502-5. doi:10.1115/1.4035301.

The Kelvin–Helmholtz instability of viscoelastic flows was examined through a linear instability analysis. Due of the position change of viscoelastic effects, different unstable responses of liquid elastic effects and medium viscous effects were fully investigated. Finally, a comparison of gas/liquid shearing and inviscid aerodynamic effects on sheet instability is conducted.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;139(4):044503-044503-6. doi:10.1115/1.4035467.

In this paper, we consider the evolution of decaying homogeneous anisotropic turbulence without mean velocity gradients, where only the slow pressure rate of strain is nonzero. A higher degree nonlinear return-to-isotropy model has been developed for the slow pressure–strain correlation, considering anisotropies in Reynolds stress, dissipation rate, and length scale tensor. Assumption of single length scale across the flow is not sufficient, from which stems the introduction of length scale anisotropy tensor, which has been assumed to be a linear function of Reynolds stress and dissipation tensor. The present model with anisotropy in length scale shows better agreement with well-accepted experimental results and an improvement over the Sarkar and Speziale (SS) quadratic model.

Commentary by Dr. Valentin Fuster