Accepted Manuscripts

Technical Brief  
Ye Hu, Xiaohui Liu, Wei Shyy and Huihe Qiu
J. Fluids Eng   doi: 10.1115/1.4042667
The investigation focused on the conversions of flow structures with a change in angle of attack (AOA) for a flexible cantilever wing, which experienced a self-excited vibration. Stereoscopic particle imaging velocimetry (Stereo-PIV) was utilized to measure the velocity field in the wing-tip region as AOA varied from 0° to 12°. At the Reynolds number (Re) of 3×104, instability waves shedding from the wing were amplified as they propagated and developed into Karman Vortex Street in the far downstream region at low AOAs (AOA=4° & 6°). As AOA increased to 8° with the wing model was still steady, the Karman Vortex Street no longer existed. The wing started to vibrate at AOA=10° owing to the self-excited vibration, and the Karman Vortex Street appeared again. The inception location of the Karman Vortex Street moved further upstream than in the cases at AOA=4° & 6°. A new vortex structure, secondary vortex-pairs, appear outside the main wing-tip vortex (WTV).
TOPICS: Flow (Dynamics), Cantilevers, Wings, Vortex street, Vortices, Vibration, Imaging, Particulate matter, Reynolds number, Waves
Dr. John P Abraham, Dr. Ephraim Sparrow, John M. Gorman, Yu Zhao and W. J. Minkowycz
J. Fluids Eng   doi: 10.1115/1.4042664
A turbulent transition model has been applied to fluid flow problems that can be laminar, turbulent, transitional, or any combination. The model is based on a single additional transport equation for turbulence intermittency. While the original model was developed for external flows, a slight modification in model constants has enabled it to be used for internal flows. It has been successfully applied to such flows for Reynolds numbers that ranged from 100 to 100,000 in circular tubes, parallel plate channels, and circular tubes with an abrupt change in diameters. The model is shown to predict fully developed friction factors for the entire range of Reynolds numbers as well as velocity profiles for both laminar and turbulent regimes.
TOPICS: Turbulence, Internal flow, Reynolds number, Flow (Dynamics), Friction, Fluid dynamics
Javid Zohrabi Chakaneh, Seyed Javad Pishbin, Alireza Sheikhi Lotfabadi and Mohammad Passandideh-Fard
J. Fluids Eng   doi: 10.1115/1.4042666
In this paper, the impact of distilled water drops on hydrophobic cylinders is characterized using both experiments and numerical simulations. Water drops of 2.54mm in diameter impact with a velocity of 1m/s on hydrophobic cylinders. The corresponding Reynolds and Weber numbers are 2800 and 34, respectively. Three different stainless steel cylinders with diameters of 0.48 mm, 0.88mm and 1.62mm were used. The surfaces of the cylinders were made hydrophobic using a special coating spray. An experimental setup consisting of a drop generator, a high speed camera, a lighting system and a photoelectric sensor was used to capture images of the impact with a time step of 1ms. The images were then analyzed using an image processing technique implemented in the MATLAB software. Both the centric and off-centric impacts were studied for each cylinder diameter. A numerical simulation of the impact was also obtained using an open-source code called OpenFOAM by employing its InterFoam solver. The numerical scheme used by the solver is the volume-of-fluid (VOF) method. The predicted images of the simulations were compared well with those of the captured photographs both qualitatively and quantitatively for the entire experiments. The behavior of the drop after the impact and the subsequent deformation on hydrophobic cylinders including flow instabilities, liquid breakup and secondary drops formation were observed from both simulations and experiments. By decreasing the cylinder diameter, the breakup occurs sooner and a smaller number of secondary drops are formed.
TOPICS: Cylinders, Water, Computer simulation, Simulation, Engineering simulation, Flow instability, Sprays, Computer software, Deformation, Fluids, Coating processes, Coatings, Sensors, Generators, Image processing, Matlab, Stainless steel
Farzam Mortazavi and Alan Palazzolo
J. Fluids Eng   doi: 10.1115/1.4042559
Modern high performance turbomachines frequently operate in supercritical condition above their first critical speed, rendering these machines prone to rotordynamic instability. The American Petroleum Institute (API) standards require advanced simulation models for level II stability analysis of impellers. Such data is then incorporated into rotor-bearing vibration response models. Despite recent advancements in high fidelity, general modeling (i.e. 3D viscous transient non-axisymmetric model) of closed impeller rotordynamic forces, no such general model is available for open impellers, especially the centrifugal type. The current paper extends the transient Computational Fluid Dynamics (CFD) model used for closed impellers by the authors (2018) to open impellers. The recent model uses a phase modulated, multi-frequency approach for enhanced computational efficiency and robustness. Results are validated against the experiments of Yoshida et al. (1999) at design and off-flow condition. The model is further applied to a spectrum of specific speeds to extract the dimensionless rotordynamic forces for each class of impellers at design and off-flow conditions. Such dimensionless force data can be used to estimate the rotordynamic forces of impellers with similar specific speed. Depending on specific speed and the relative flow coefficient, many of these impellers are found to be excited by forward or backward whirl. Strong interaction with rotating stall typically appears in the force data at off-flow condition. Simulations of the isolated leakage path model for equivalent closed impellers reveals similar bumps and dips associated with highly swirling inflow which naturally occurs at part flow condition.
TOPICS: Computational fluid dynamics, Impellers, Transients (Dynamics), Flow (Dynamics), American Petroleum Institute, Design, Engineering simulation, Modeling, Rotors, Vibration, Rendering, Robustness, Simulation models, Swirling flow, Turbomachinery, Whirls, Leakage, Inflow, Stability, Machinery, Simulation, Bearings
Rajan Ray, Paul F. Henshaw and Nihar Biswas
J. Fluids Eng   doi: 10.1115/1.4042562
In automotive engineering, paint evaporation is considered one of the major sources of volatile organic compound (VOC) emission. The Rotary Bell Atomizer (RBA) is a special atomizer used for improved automotive painting. A model to predict the droplet size was developed by combining two already-reported models. Further, the calculated values of evaporation rate, K, and the RBA nozzle parameter, K1, for water and clearcoat at room temperature (24°C) are presented here. For water, K1 was dependent on the flow rate and bell speed, but had a unvarying value for clearcoat. The numbers for K were 1.3x10E-5 - 2.0x10E-5 cm2/s for clearcoat and 8.6x10E-6 -1.27x10E-5 cm2/s for water. The Sauter mean diameter of the particle leaving the RBA at different flow rates and bell speeds can be predicted using K1. A generic model which can anticipate the evaporation rate of droplets with axial length was also introduced.
TOPICS: Drops, Sprays, Evaporation, Water, Flow (Dynamics), Temperature, Automotive engineering, Particulate matter, Nozzles, Organic compounds, Painting, Emissions
Xiaohua Liu, Jinfang Teng, Jun Yang, Xiaofeng Sun, Dakun Sun, Chen He and Juan Du
J. Fluids Eng   doi: 10.1115/1.4042561
Although steady micro injection is experimentally validated as an attractive method in improving the stall margin of axial compressors, up to now a fast prediction of stall boundary remains some way off. This investigation is to propose such a prediction model. A flow stability model is developed to further consider the effect of high speed micro injection. After the base flow field is calculated by steady CFD simulation, a body force model is applied to reproduce the effect of blade on the flow turning and loss. A group of homogeneous equations are obtained based on linearized N-S equations and harmonic decomposition of small flow disturbance. The stall onset point can be judged by the imaginary part of the resultant eigenvalue. After the existing experimental results are summarized, an unsteady numerical simulation reveals that the computed characteristics and radial profile of pressure rise coefficient are almost unchanged. The unsteady response of compressor to the micro injection is preliminarily verified based on the observation of the disturbed spillage of tip leakage flow. It is verified that this approach can provide a qualitative assessment of stall point with acceptable computational cost. Both high injection velocity and short axial gap between injector and rotor leading edge are beneficial for the stall margin extension. These theoretical findings agree well with experimental measurements. It is inferred that the spillage of tip clearance flow, which is inward pushed by higher speed injection with shortened distance away from rotor, could lead to further stable flow field.
TOPICS: Compressors, Flow (Dynamics), Rotors, Blades, Eigenvalues, Leakage flows, Flow turning, Computer simulation, Pressure, Stability, Simulation, Clearances (Engineering), Computational fluid dynamics, Ejectors
Ali Shakiba and A. B. Rahimi
J. Fluids Eng   doi: 10.1115/1.4042563
The steady, viscous flow and mixed convection heat transfer of an incompressible electrically conducting fluid within a vertical cylindrical annulus with moving walls are investigated. This annulus is under the influence of a radial magnetic field and the fluid is suctioned/injected through the cylinders' walls. An exact solution of the Navier-Stokes equations and energy equation is derived in this problem where heat is transferred from the hot cylinder walls with constant temperature to the cooler moving fluid. The role of the movement of the annulus walls is studied on the flow and heat transfer of the fluid within the annulus, for the first time. The effects of other parameters, including Prandtl number, Hartman number, mixed convection parameter, suction/injection parameter and ratio of the radius, on the behavior of the flow and heat transfer of the fluid is also considered. The results indicate that if, for example, the internal cylinder wall moves in the direction of z-axis and the external cylinder is stationary, the maximum and minimum heat transfer occur on the walls of internal and external cylinders, respectively. Moreover, the augmentation of the radius ratio between the two cylinders increases the rate of heat transfer and decreases the shear stress on the wall of the internal and external cylinders, however, the results on the wall of external cylinder are exactly the reverse. Consequently, by changing the effective parameters used in this paper, the flow of the fluid can be controlled and the heat transfer of the fluid can be improved.
TOPICS: Flow (Dynamics), Fluids, Magnetic fields, Annulus, Transpiration, Cylinders, Heat transfer, Mixed convection, Viscous flow, Navier-Stokes equations, Suction, Heat, Temperature, Prandtl number, Shear stress

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