Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;140(4):041101-041101-11. doi:10.1115/1.4038092.

A naval aircraft has the potential to experience inlet performance decline when taking off from the carrier deck with the steam-driven catapult assistance. The steam ingested into inlet may cause time-dependent rise and spatial distortion of the total temperature on the inlet–exit, which would decrease the compressor stall margin and then lower the performance of the turbine engine. In this paper, these temporal and spatial temperature nonuniformities are numerically studied using the dual-time-step transient method with a real aircraft/inlet model taken into account. The flowfield characteristics of a designed baseline case are first analyzed, indicating that the engine’s suction effect and the wind velocity relative to the aircraft are two key factors affecting the steam ingestion. The former is dominant at the beginning of takeoff since the aircraft's velocity is low, while the latter is increasingly significant as the aircraft accelerates. Next, parametric studies show that the greater the wind speed is, the less significantly the flowfield of the inlet–exit would be influenced by the steam. The effects are also studied among various steam leakage profiles—two are constant in time histories of the steam leakage rate, whereas the other two are nonlinear with the maximum value at different instants. It is found that the temperature rise rate of the inlet–exit would increase apparently if the steam leakage rate reaches the maximum earlier.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041102-041102-10. doi:10.1115/1.4038115.

Cavitation in torque converters may cause degradation in hydrodynamic performance, severe noise, or even blade damage. Researches have highlighted that the stator is most susceptible to the occurrence of cavitation due to the combination of high flow velocities and high incidence angles. The objective of this study is to therefore investigate the effects of cavitation on hydrodynamic performance as well as the influence of stator blade geometry on cavitation. A steady-state homogeneous computational fluid dynamics (CFD) model was developed and validated against test data. It was found that cavitation brought severe capacity constant degradation under low-speed ratio (SR) operating conditions and vanished in high-speed ratio operating conditions. A design of experiments (DOE) study was performed to investigate the influence of stator design variables on cavitation over various operating conditions, and it was found that stator blade geometry had a significant effect on cavitation behavior. The results show that stator blade count and leaning angle are important variables in terms of capacity constant loss, torque ratio (TR) variance, and duration of cavitation. Large leaning angles are recommended due to their ability to increase the cavitation number in torque converters over a wide range of SRs, leading to less stall capacity loss as well as a shorter duration of cavitation. A reduced stator blade count is also suggested due to a reduced TR loss and capacity loss at stall.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041103-041103-12. doi:10.1115/1.4038394.

Pressure-equalizing film is a slice of air layer attached to vehicle exterior with nearly uniform inside pressure, similar to ventilated cavity in composition; it is generated through exhaust process of the inside air chamber as vehicle emerges from deep water, and can reduce the lateral force and pitching moment that vertical launched underwater vehicle suffered. In this work, the emerging process of vehicle from water with pressure-equalizing exhaust was numerically calculated to investigate the evolution and flow characteristics of the generated pressure-equalizing film along its surface. Results indicated that during the whole exhaust process, the film can be obviously classified into different sections according to the distribution of phase volume fraction or pressure. The exhaust velocity ratio and flow rate from vehicle interior chamber were also found to increase as vehicle moves. In the analysis of flow structures, vortex structures such as the horseshoe vortex, “detour-separation” vortex, and counter-rotating vortex pair (CVP) can be figured out in the region of the exhaust hole. Under the effect of re-entrant jet, water around the film tail would be entrained upstream then enter the surface film to mix with the pressure-equalizing air. It leads to the happening of the three-dimensional (3D) wall vortex in the flow field.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041104-041104-13. doi:10.1115/1.4038395.

Two hydraulic losses take effect at the junction point of three cylindrical conduits. These two quantities are considered to be functions of the three signed flow rates and two geometrical parameters: the cross-sectional area ratio and the angle between the main conduit and branch tube. A new design of experiment is developed for exploring the parameter space with continuous response surfaces, which cover both dividing and combining flow regimes with a general trigonometric formula. The loss coefficients are determined by using a steady-state, single-phase, three-dimensional (3D) computational fluid dynamics (CFD) model. To help the analytical treatment, a new reference velocity formulation is introduced. The new loss coefficient formula is validated against known empirical correlations for different junction types and flow directions. The obtained continuous solution promotes the applicability of the resistance model in hydraulic network models.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041105-041105-11. doi:10.1115/1.4038091.

Air traffic volume is expected to triple in the U.S. and Europe by 2025, and as a result, the aerospace industry is facing stricter noise regulations. Apart from the engines, one of the significant contributors of aircraft noise is the deployment of high-lift devices, like leading-edge slats. The unsteady turbulent flow over a leading-edge slat is studied herein. In particular, particle image velocimetry (PIV) measurements were performed on a scale-model wing equipped with a leading-edge slat in the H.J. Irving–J.C.C. Picot Wind Tunnel. Two Reynolds numbers based on wing chord were studied: Re = 6 × 105 and 1.3 × 106. A snapshot proper orthogonal decomposition (POD) analysis indicated that differences in the time-averaged statistics between the two Reynolds numbers were tied to differences in the coherent structures formed in the slat cove shear layer. In particular, the lower Reynolds number flow seemed to be dominated by a large-scale vortex formed in the slat cove that was related to the unsteady flapping and subsequent impingement of the shear layer onto the underside of the slat. A train of smaller, more regular vortices was detected for the larger Reynolds number case, which seemed to cause the shear layer to be less curved and impinge closer to the tail of the slat than for the lower Reynolds number case. The smaller structures are consistent with Rossiter modes being excited within the slat cove. The impingement of the shear layers on and the proximity of the vortices to the slat and the main wing are expected to be strong acoustic dipoles in both cases.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;140(4):041201-041201-7. doi:10.1115/1.4038089.

Three-dimensional (3D) curved wall jets are a significant topic in various applications related to local heat and mass transfer. This study investigates the effects of the impinging angle and Reynolds number with a fixed distance from the nozzle to the surface of a cylinder. The particle image velocimetry (PIV) method was used to measure the mean streamwise velocity profiles, which were normalized by the maximum velocity along the centerline of the impinging jet onto the cylinder. After the impingement of the circular jet, a 3D curved wall jet develops on the cylinder surface due to the Coanda effect. At a given Reynolds number, the initial momentum of the wall jet increases, and flow separation occurs further downstream than in normal impingement as the impinging angle increases. At a given impinging angle, flow separation is delayed with increasing Reynolds number. A self-preserving wall jet profile was not attained in the 3D curved wall jet. The turbulence intensity and the Reynolds shear stress were obtained to analyze the turbulence characteristics. The radial turbulence intensity showed similar tendencies to a two-dimensional (2D) curved wall jet, but the streamwise turbulence intensity was dissimilar. The Reynolds shear stress decreases downstream of the cylinder wall due to the decreased velocity and centrifugal force.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041202-041202-12. doi:10.1115/1.4038241.

Ships and submarines are acoustic hazards to marine life. The rational control of acoustic radiation would be possible at least at low Reynolds numbers if the underlying organization buried in seeming randomness is revealed. We build a novel low-speed propulsor where all blades undergo small-amplitude pitch oscillation while spinning at large pitch angles at transitional chord Reynolds numbers (3.75 × 103 ≤ Rec ≤ 3.75 × 104) and advance ratios (0.51 ≤ J ≤ 4.89). We measure and model time-averaged and temporal thrust. The relationship between the time-averaged and the temporal thrust is observed when the latter is mapped as limit cycle oscillation (LCO), or departure from it. High-thrust coefficients occurring at large (30 deg and 45 deg) angles of amplitude of blade vibration are modeled assuming poststall lift enhancement due to flapping blades when a leading edge vortex (LEV) forms, while the lower thrust coefficients occurring at 20 deg are modeled by its absence. The disorganization in temporal thrust increases with J and Rec. An external orthogonal oscillator, perhaps a vibration, is modeled to couple with the thrust oscillator for temporal control of disorganization. The unfolding disorganization is seen as a departure from LCO, and it is attenuated by smooth-wall boundary-layer fencing, compared to unfenced smooth and rough surfaces. When the fencing properties of the leading edge tubercles of whale fins are recognized, the ratio of the spacing of the fences and chord is found to be similar (0.5–1.0) in both whale flippers and aircraft wings.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041203-041203-11. doi:10.1115/1.4038167.

To provide porous media substrates that are quick to generate and characterize for lattice Boltzmann analysis, we propose a straightforward algorithm. The method leverages the benefits of the lattice Boltzmann method (LBM), and is extensible to multiphysics flows. Several parameters allow for simple customization. The generation algorithm and LBM are reviewed, and suggested implementation covered. Additionally, results are discussed and interpreted to evaluate the approach. Several verification tools are employed such as Darcy's law, the Ergun equation, the Koponen correlation, a newly proposed correlation, and experimental data. Agreement and repeatability are found to be excellent, suggesting this relatively simple method is a good option for engineering studies.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041204-041204-11. doi:10.1115/1.4038242.

Simulations of oscillatory motion in partially filled rectangular tanks with different tank geometries, fullness ratios, and motion frequencies are presented. Smoothed particle hydrodynamics (SPH) method is used to discretize the governing equations together with new velocity variance-based free surface (VFS) and artificial particle displacement (APD) algorithms to enhance the robustness and the accuracy of the numerical scheme. Two-dimensional (2D) oscillatory motion is investigated for three different scenarios where the first one scrutinizes the kinematic characteristics in resonance conditions, the second one covers a wave response analysis in a wide range of enforced motion frequencies, and the last one examines the dynamic properties of the fluid motion in detail. The simulations are carried on for at least 50 periods in the wave response analysis. It is shown that numerical results of the proposed SPH scheme are in match with experimental and numerical findings of the literature.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041205-041205-12. doi:10.1115/1.4038214.

Present investigation deals with the interaction of an incident oblique shock wave on a turbulent boundary layer over a wavy surface. The oblique shock wave was generated by an 8 deg wedge in a freestream Mach number of 2.0. Three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) equations with k–ω shear stress transport (SST) turbulence model were used for numerical computation. The computed results are in good agreement with the experimental measurement and direct numerical simulation (DNS) data in case of the interaction of an oblique shock with plain flat plate. To identify the effect of surface waviness on shock wave/turbulent boundary layer interaction (SWBLI), a section of the flat plate was replaced by a wavy surface. Computations have been conducted for different magnitudes of wavy amplitude. Further, the wavelength of the wavy surface has been varied. Results showed that the presence of wavy surface induces supplementary shock and expansion waves in the flow field, which are referred as topographic waves. This supplementary system of waves interacts with the counterpart of intrinsic SWBLI in a complex manner. Flow structure, separation behavior, and aerodynamic characteristics are studied. It is revealed that the amplitude is dominant than the wavelength of waviness in case of SWBLI on a wavy surface.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041206-041206-10. doi:10.1115/1.4038168.

In this paper, a parametric study showing the impulsive performance of foils with different flexural stiffness pitching in a quiescent fluid is presented. A wide range of Reynolds numbers (different imposed kinematics) and foil rigidities is covered, depicting how flexibility effects on impulse are more important at the largest Reynolds numbers. The impulsive performance of the system is derived from direct thrust force measurements. Passive flexibility alters vortex strength and formation in the wake of the pitching foil. These changes in the wake formation can be used to explain the differences in the measured impulses. The wake dynamics is studied after quantitative analysis of particle image velocimetry data, and it is linked to the momentum transfer generated by the foil.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041207-041207-10. doi:10.1115/1.4038532.

We present simulations of a new experimental platform at the National Ignition Facility (NIF) for studying the hydrodynamic instability growth of a high-energy density (HED) fluid interface that undergoes multiple shocks, i.e., is “reshocked.” In these experiments, indirect-drive laser cavities drive strong shocks through an initially solid, planar interface between a high-density plastic and low-density foam, in either one or both directions. The first shock turns the system into an unstable fluid interface with the premachined initial condition that then grows via the Richtmyer–Meshkov and Rayleigh–Taylor instabilities. Backlit X-ray imaging is used to visualize the instability growth at different times. Our main result is that this new HED reshock platform is established and that the initial data confirm the experiment operates in a hydrodynamic regime similar to what simulations predict. The simulations also reveal new types of edge effects that can disturb the experiment at late times and suggest ways to mitigate them.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(4):041208-041208-9. doi:10.1115/1.4038535.

The pressure fluctuations in both the rotating runner and the other fixed components in a model Francis turbine under various loads were experimentally measured by means of onboard measuring equipment in the runner and data storage device on the shaft in this study. Large pressure fluctuations were observed under both small guide vane opening and large guide vane opening conditions. Flow separation at the blade suction surface led to large pressure fluctuations for small guide vane openings, the unsteady flow around the inlet on the blade pressure side led to large pressure fluctuations for large openings. The pressure fluctuations correlation between the runner and other components of the turbine, mainly the draft tube, was analyzed in detail for both small guide vane opening (12 deg) and large guide vane opening (30 deg). The results show that the pressure fluctuations in the runner space increased by the superposition of draft tube vortex rope pressure fluctuations and runner inter blade vortices pressure fluctuations, resulting in much larger pressure fluctuations in the runner space than in other components.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Fluids Eng. 2017;140(4):044501-044501-7. doi:10.1115/1.4038166.

Savonius vertical axis wind turbines (VAWTs) typically suffer from low efficiency due to detrimental drag production during one half of the rotational cycle. The present study examines a stator assembly created with the objective of trapping cylindrical flow for application in a Savonius VAWT. While stator assemblies have been studied in situ around Savonius rotors in the past, they have never been isolated from the rotor to determine the physics of the flow field, raising the likelihood that a moving rotor could cover up deficiencies attributable to the stator design. The flow field created by a stator assembly, sans rotor, is studied computationally using three-dimensional (3D) numerical simulations in the commercial computational fluid dynamics (CFD) package Star-CCM+. Examination of the velocity and pressure contours at the central stator plane shows that the maximum induced velocity exceeded the freestream velocity by 65%. However, flow is not sufficiently trapped in the stator assembly, with excess leakage occurring between the stator blades due to adverse pressure gradients and momentum loss from induced vorticity. A parametric study was conducted on the effect of the number of stator blades with simulations conducted with 6, 12, and 24 blades. Reducing the blade number resulted in a reduction in the cohesiveness of the internal swirling flow structure and increased the leakage of flow through the stator. Two unique energy loss mechanisms have been identified with both caused by adverse pressure gradients induced by the stator.

Commentary by Dr. Valentin Fuster

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