CFD of Vortical Flows: Requirements and Mesh Adaption Techniques

2015 ◽  
Author(s):  
Martino Reclari ◽  
Shinji Fukao ◽  
Masamichi Iino ◽  
Takeshi Sano ◽  
Yuuki Nakamura
Keyword(s):  
Turbulence Models ◽  
Air Entrainment ◽  
Correct Prediction ◽  
Vortex Identification ◽  
Two Phase ◽  
Erosion Risk ◽  
Mesh Adaption ◽  
Mesh Resolution ◽  
Identification Techniques ◽  
Wing Tip

In most hydraulic applications (turbines, pumps, water intakes, propellers) the appearance of gas filled vortices, either caused by cavitation or air entrainment from a free surface, is usually associated with increase of losses, vibrations, noise and erosion risk. However, a correct prediction of the vortex characteristics (most importantly, of the pressure at the core) by numerical simulations may be challenging. A common example is the over-prediction of the vortex dissipation, which leads to wrong estimation of the gas core collapse location. In the present paper we assess the numerical requirements necessary to compute vortex characteristics comparable to experimental results. As a first step, we evaluate the influence of the mesh resolution for different turbulence models (SST, SAS and RMS), in the case of a vortex generated by an elliptical wing. Secondly, we compare the efficiency of several popular vortex identification techniques (helicity, Q, λ, Δ, etc…) to designate the mesh refinement regions, thus adapting the mesh to successfully compute the vortex characteristics, in the case of a vortex created in a cylindrical container with tangential inflow and central outflow. Therefore, we are able to present effective guidelines for the correct computation of the above mentioned two phase problems, that can also be applied to leakage flow in gas turbomachines, wing-tip vortices, and more generally all computations where a high quality resolution of the vortices is necessary.

A Time-Resolved PIV Study of Vortical Structures in the Near-Wall Region of an Impinging Round Jet

2009 ◽  
Author(s):  
Khaled J. Hammad ◽  
Ivana M. Milanovic
Keyword(s):  
Flow Field ◽  
Turbulence Models ◽  
Wall Region ◽  
Vortex Identification ◽  
Time Resolved ◽  
Vortical Structures ◽  
Reynolds Decomposition ◽  
Gradient Based ◽  
Pod Analysis ◽  
Identification Techniques

Time-Resolved Particle Image Velocimetry (TR-PIV) was used to study the vortical structures resulting from a submerged water jet impinging normally on a smooth and flat surface. A fully developed turbulent jet, exiting a long pipe, and a semi-confined flow configuration ensured properly characterized boundary conditions, which allows for straightforward assessment of turbulence models and numerical schemes. The Reynolds number based on jet mean exit velocity was 23,000. The pipe-to-plate separation was varied between 2D and 7.6D. Turbulent velocity fields are presented using Reynolds decomposition into mean and fluctuating components. Proper Orthogonal Decomposition (POD) analysis was used to identify the most energetic coherent structures of the turbulent flow field. Three velocity gradient-based vortex identification techniques, 2nd invariant Q, λ2, and swirling strength, were found to perform equally well in identifying vortical structures along the impingement wall. The results clearly demonstrate the shortcomings of local vorticity as a vortex identifier in an impinging jet flow field.

scholarly journals Numerical Study on an Interface Compression Method for the Volume of Fluid Approach

Fluids ◽  
2021 ◽  
Vol 6 (2) ◽  
pp. 80
Author(s):  
Yuria Okagaki ◽  
Taisuke Yonomoto ◽  
Masahiro Ishigaki ◽  
Yoshiyasu Hirose
Keyword(s):  
Linear Theory ◽  
Volume Fraction ◽  
Numerical Study ◽  
Volume Of Fluid ◽  
Interfacial Wave ◽  
Validation Process ◽  
Two Phase ◽  
Numerical Diffusion ◽  
Mesh Resolution ◽  
Water Reactors

Many thermohydraulic issues about the safety of light water reactors are related to complicated two-phase flow phenomena. In these phenomena, computational fluid dynamics (CFD) analysis using the volume of fluid (VOF) method causes numerical diffusion generated by the first-order upwind scheme used in the convection term of the volume fraction equation. Thus, in this study, we focused on an interface compression (IC) method for such a VOF approach; this technique prevents numerical diffusion issues and maintains boundedness and conservation with negative diffusion. First, on a sufficiently high mesh resolution and without the IC method, the validation process was considered by comparing the amplitude growth of the interfacial wave between a two-dimensional gas sheet and a quiescent liquid using the linear theory. The disturbance growth rates were consistent with the linear theory, and the validation process was considered appropriate. Then, this validation process confirmed the effects of the IC method on numerical diffusion, and we derived the optimum value of the IC coefficient, which is the parameter that controls the numerical diffusion.

Flow structure segmentation for vortex identification using butterfly convolutional neural networks

2020 ◽  
Vol 34 (14n16) ◽  
pp. 2040121 ◽  
Author(s):  
Zhi-Xian Ye ◽  
Qian Chen ◽  
Bing-Hua Li ◽  
Jian-Feng Zou ◽  
Yao Zheng
Keyword(s):  
Flow Field ◽  
Physical Mechanism ◽  
Image Data ◽  
Global Information ◽  
Vortex Identification ◽  
Feature Maps ◽  
Gradient Tensor ◽  
The Common ◽  
Original Size ◽  
Identification Techniques

Vortex identification is important for understanding the physical mechanism of turbulent flow. The common vortex identification techniques based on velocity gradient tensor such as [Formula: see text] criterion will consume a lot of computing resources for processing great quantity of experimental data. To improve the vortex identification efficiency and achieve real-time recognition, we present a novel vortex identification method using segmentation with convolutional neural network (CNN) based on flow field image data, which is named “Butterfly-CNN”. Considering that the view of flow field is small, it is necessary to integrate both the local and global feature maps to achieve higher precision. The architecture consists of an encoded–decoded path, which is similar to [Formula: see text]-net but with different superimposed network part. In the Butterfly-CNN, the cross-expanding paths are designed with the global information to enable precise localization, and the feature maps after each convolution are regarded as the original pictures, then convolute to the size of the last feature map and upsample to the original size again. Finally, the decoded and cross-expanding networks are added up. The Butterfly-CNN can be trained end-to-end from a few images, and it is useful and efficient for vortex identification.

Development and Validation of a Three-Dimensional Multiphase Flow CFD Analysis for Journal Bearings in Steam and Heavy Duty Gas Turbines

2012 ◽  
Author(s):  
Stephan Uhkoetter ◽  
Stefan aus der Wiesche ◽  
Michael Kursch ◽  
Christian Beck
Keyword(s):  
Experimental Data ◽  
Multiphase Flow ◽  
Gas Turbines ◽  
High Speed ◽  
Three Dimensional ◽  
Air Entrainment ◽  
Journal Bearings ◽  
Heavy Duty ◽  
Present Contribution ◽  
Two Phase

The traditional method for hydrodynamic journal bearing analysis usually applies the lubrication theory based on the Reynolds equation and suitable empirical modifications to cover turbulence, heat transfer, and cavitation. In cases of complex bearing geometries for steam and heavy-duty gas turbines this approach has its obvious restrictions in regard to detail flow recirculation, mixing, mass balance, and filling level phenomena. These limitations could be circumvented by applying a computational fluid dynamics (CFD) approach resting closer to the fundamental physical laws. The present contribution reports about the state of the art of such a fully three-dimensional multiphase-flow CFD approach including cavitation and air entrainment for high-speed turbo-machinery journal bearings. It has been developed and validated using experimental data. Due to the high ambient shear rates in bearings, the multiphase-flow model for journal bearings requires substantial modifications in comparison to common two-phase flow simulations. Based on experimental data, it is found, that particular cavitation phenomena are essential for the understanding of steam and heavy-duty type gas turbine journal bearings.

Entrainment of Air at the Transoms of Full-Scale Surface Ships

2015 ◽  
Vol 59 (01) ◽  
pp. 49-65
Author(s):  
Eric J. Terrill ◽  
Genevieve R.L. Taylor
Keyword(s):  
Initial Conditions ◽  
Air Entrainment ◽  
Full Scale ◽  
Far Field ◽  
Two Phase ◽  
Cfd Models ◽  
Surface Ship ◽  
Full Speed ◽  
Bubbly Wake ◽  
Scale Surface

We report on the results from a series of full-scale trials designed to quantify the air entrainment at the stern of an underway vessel. While an extremely complex region to model air entrainment due to the confluence of the breaking transom wave, bubbles from the bow, turbulence from the hull boundary layer, and bubbles and turbulence from propellers, the region is a desirable area to characterize and understand because it serves as the initial conditions of a ship's far-field bubbly wake. Experiments were conducted in 2003 from R/V Revelle and 2004 from R/VAthena II using a custombuilt conductivity probe vertical array that could be deployed at the blunt transom of a full-scale surface ship to measure the void fraction field. The system was designed to be rugged enough to withstand the full speed range of the vessels. From the raw timeseries data, the entrainment of air at speeds ranging from 2.1 to 7.2 m/s is computed at various depths and beam locations. The data represent the first such in-situ measurements from a full-scale vessel and can be used to validate two-phase ship hydrodynamic CFD codes and initialize far-field, bubbly wake CFD models.

Flow over a stepped chute with and without macro-roughness elements

2004 ◽  
Vol 31 (5) ◽  
pp. 880-891 ◽  
Author(s):  
Mehmet Ali Kökpinar
Keyword(s):  
Water Flow ◽  
High Speed ◽  
Length Distribution ◽  
Air Entrainment ◽  
Air Concentration ◽  
Bubble Frequency ◽  
Two Phase ◽  
Air Bubble ◽  
Roughness Elements ◽  
Stepped Chute

High-speed two-phase flows over a 30° stepped flume were experimentally investigated using macro-roughness elements. The roughness elements included combinations of steps and horizontal strips. Local values of air concentration, air bubble frequency, and mean chord lengths were measured by a fiber-optical instrumentation system in the air–water flow region. The range of unit discharge of water was varied from 0.06 to 0.20 m2/s. Three step configurations were studied: (i) without macro-roughness elements, (ii) with macro-roughness elements on each step, and (iii) with macro-roughness elements on each second step (AMR configuration). The results were compared in terms of onset flow conditions and internal air–water flow parameters such as local air concentration, mean air bubble chord length distribution, and air bubble frequency in the skimming flow regime. It was observed that the AMR configuration produced the maximum free-surface aeration among the other configurations. This alternative step geometry has potential for less cavitation damage than conventional step geometry because of the greater air entrainment.Key words: stepped chute, air-entrainment, air-water flow properties, macro-roughness elements, skimming flow.

scholarly journals A review of multiphase flow and deposition effects in film-cooled gas turbines

2018 ◽  
Vol 22 (5) ◽  
pp. 1905-1921 ◽  
Author(s):  
Jin Wang ◽  
Milan Vujanovic ◽  
Bengt Sunden
Keyword(s):  
Film Cooling ◽  
Gas Turbines ◽  
Particle Deposition ◽  
Experimental Studies ◽  
Turbulence Models ◽  
Phase Flow ◽  
Two Phase ◽  
Film Cooling Effectiveness ◽  
Factors Affecting ◽  
Multi Phase Flow

This paper presents a review of particle deposition research in film-cooled gas turbines based on the recent open literature. Factors affecting deposition capture efficiency and film cooling effectiveness are analyzed. Experimental studies are summarized into two discussions in actual and virtual deposition environments. For investigation in virtual deposition environments, available and reasonable results are obtained by comparison of the Stokes numbers. Recent advances in particle deposition modeling for computational fluid dynamics are also reviewed. Various turbulence models for numerical simulations are investigated, and solutions for treatment of the particle sticking probability are described. In addition, analysis of injecting mist into the coolant flow is conducted to investigate gas-liquid two-phase flow in gas turbines. The conclusion remains that considerable re-search is yet necessary to fully understand the roles of both deposition and multi-phase flow in gas turbines.

Application of Wray-Agarwal turbulence model for numerical simulation of gas-solid flows in CFB risers

2021 ◽  
pp. 1-25
Author(s):  
Yali Shao ◽  
Ramesh K. Agarwal ◽  
Xudong Wang ◽  
Baosheng Jin
Keyword(s):  
Turbulence Model ◽  
Circulating Fluidized Bed ◽  
Volume Fraction ◽  
Solid Phase ◽  
Solid Volume Fraction ◽  
Turbulence Models ◽  
Two Phase ◽  
Efficient Simulation ◽  
Pressure Profiles ◽  
First Time

Abstract In recent decades, increasing attention has been focused on accurate modeling of circulating fluidized bed (CFB) risers to provide valuable guidance to design, optimization and operation of reactors. Turbulence model plays an important role in accurate prediction of complex gas-solid flows. Recently developed Wray-Agarwal (WA) model is a one-equation turbulence model with the advantages of high computational efficiency and competitive accuracy with two-equation models. In this paper for the first time, Eulerian-Eulerian approach coupled with different turbulence models including WA model, standard κ-ε model and shear stress transport (SST) κ-ω model is employed to simulate two-phase flows of gas phase and solid phase in two CFB risers, in order to assess accuracy and efficiency of WA model compared to other well-known two-equation models. Predicted gas-solid flow dynamic characteristics including the gas-solid volume fraction distributions in radial and axial directions, pressure profiles and solid mass flux distributions are compared with data obtained from experiment in detail. The results demonstrate WA model is very promising for accurate and efficient simulation of gas-solid multiphase flows.

scholarly journals Wake behind a three-dimensional dry transom stern. Part 1. Flow structure and large-scale air entrainment

2019 ◽  
Vol 875 ◽  
pp. 854-883 ◽  
Author(s):  
Kelli Hendrickson ◽  
Gabriel D. Weymouth ◽  
Xiangming Yu ◽  
Dick K.-P. Yue
Keyword(s):  
Froude Number ◽  
Flow Structure ◽  
Large Scale ◽  
Three Dimensional ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Variable Density ◽  
Entrainment Rate ◽  
Two Phase ◽  
Density Spectrum

We present high-resolution implicit large eddy simulation (iLES) of the turbulent air-entraining flow in the wake of three-dimensional rectangular dry transom sterns with varying speeds and half-beam-to-draft ratios $B/D$. We employ two-phase (air/water), time-dependent simulations utilizing conservative volume-of-fluid (cVOF) and boundary data immersion (BDIM) methods to obtain the flow structure and large-scale air entrainment in the wake. We confirm that the convergent-corner-wave region that forms immediately aft of the stern wake is ballistic, thus predictable only by the speed and (rectangular) geometry of the ship. We show that the flow structure in the air–water mixed region contains a shear layer with a streamwise jet and secondary vortex structures due to the presence of the quasi-steady, three-dimensional breaking waves. We apply a Lagrangian cavity identification technique to quantify the air entrainment in the wake and show that the strongest entrainment is where wave breaking occurs. We identify an inverse dependence of the maximum average void fraction and total volume entrained with $B/D$. We determine that the average surface entrainment rate initially peaks at a location that scales with draft Froude number and that the normalized average air cavity density spectrum has a consistent value providing there is active air entrainment. A small parametric study of the rectangular geometry and stern speed establishes and confirms the scaling of the interface characteristics with draft Froude number and geometry. In Part 2 (Hendrikson & Yue, J. Fluid Mech., vol. 875, 2019, pp. 884–913) we examine the incompressible highly variable density turbulence characteristics and turbulence closure modelling.

Features of Automotive Gas Tank Filler Pipe Two-Phase Flow: Experiments and Computational Fluid Dynamics Simulations

2002 ◽  
Vol 124 (2) ◽  
pp. 412-420 ◽  
Author(s):  
R. Banerjee ◽  
K. M. Isaac ◽  
L. Oliver ◽  
W. Breig
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Air Entrainment ◽  
Imaging Features ◽  
Two Phase ◽  
Progressive Development ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition ◽  
Extensive Flow ◽  
Dynamics Simulations

Extensive flow visualization in an automotive fuel filler pipe made visible by introducing dyes and smoke in water and air, respectively, were conducted for nominal flow rates of 4–18 liters per minute. Video and still cameras were used for imaging. Features of the flow such as laminar-to-turbulent transition, progressive development of strong swirl along filler pipe axis, air entrainment, and mixing with the liquid were observed in the experiments. The experimental observations were supported by computational fluid dynamics (CFD) simulations of the flow which also showed features such as swirl and air entrainment.

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