CFD Study of Ejector Efficiencies

2014 ◽  
Author(s):  
Giorgio Besagni ◽  
Riccardo Mereu ◽  
Emanuela Colombo
Keyword(s):  
Thermal Field ◽  
Turbulence Models ◽  
Navier Stokes ◽  
Validation Process ◽  
Local Flow ◽  
Convergent Nozzle ◽  
Flow Parameters ◽  
Rans Turbulence Models ◽  
Flow Phenomena ◽  
Computational Fluid Dynamics Cfd

This paper presents a method to evaluate ejector efficiency in function of local flow parameters. The paper is divided into two parts. In the first part, a Computational Fluid-Dynamics (CFD) approach for convergent nozzle ejectors is presented and computational results are validated using experimental velocity and temperature profiles at different sections. The validation process includes the evaluation of seven Reynolds-Averaged Navier–Stokes (RANS) turbulence models: the Spalart-Allmaras and the k–omega SST models show better performance in terms of convergence capability and flow and thermal field prediction. In the second part, local flow phenomena and their influence on ejector component efficiencies are investigated. The validated CFD approach is used to determine the efficiencies of the ejector primary nozzle, suction chamber, and mixing zone. Efficiency maps, regressing equation linking efficiencies, and local flow quantities are proposed and discussed. Finally, global ejector performance is mapped and considerations are outlined.

Minimum Wall Distance Computations With Time-Dependent Geometry for CFD

2021 ◽  
Author(s):  
Liwu Wang ◽  
Jian Feng ◽  
Yu Liu ◽  
Sijun Zhang
Keyword(s):  
Present Method ◽  
Turbulence Models ◽  
Time Dependent ◽  
Navier Stokes ◽  
Field Function ◽  
Convection Diffusion ◽  
Convection Diffusion Equation ◽  
Wall Distance ◽  
Rans Turbulence Models ◽  
Computational Fluid Dynamics Cfd

Abstract This paper presents an efficient and scalable method to calculate the minimum wall distance (MWD), which is necessary for the Reynolds-Averaged Navier-Stokes (RANS) turbulence models. The MWD is described by the distance field function which is essentially a partial differential equation (PDE). The PDE is a type of convection-diffusion equation and can be solved by existing computational fluid dynamics (CFD) codes with minor modifications. Parallel computations for the PDE are conducted to study its efficiency and scalability. Encouraging results are obtained and demonstrate the present method is more efficient than all the alternate methods.

scholarly journals Assessment of Numerical Methods for Plunging Breaking Wave Predictions

2021 ◽  
Vol 9 (3) ◽  
pp. 264
Author(s):  
Shanti Bhushan ◽  
Oumnia El Fajri ◽  
Graham Hubbard ◽  
Bradley Chambers ◽  
Christopher Kees
Keyword(s):  
Solitary Wave ◽  
Wave Breaking ◽  
Large Scale ◽  
Turbulence Models ◽  
Navier Stokes ◽  
Wave Crest ◽  
Breaking Wave ◽  
Rans Turbulence Models ◽  
Run Up ◽  
Better Than

This study evaluates the capability of Navier–Stokes solvers in predicting forward and backward plunging breaking, including assessment of the effect of grid resolution, turbulence model, and VoF, CLSVoF interface models on predictions. For this purpose, 2D simulations are performed for four test cases: dam break, solitary wave run up on a slope, flow over a submerged bump, and solitary wave over a submerged rectangular obstacle. Plunging wave breaking involves high wave crest, plunger formation, and splash up, followed by second plunger, and chaotic water motions. Coarser grids reasonably predict the wave breaking features, but finer grids are required for accurate prediction of the splash up events. However, instabilities are triggered at the air–water interface (primarily for the air flow) on very fine grids, which induces surface peel-off or kinks and roll-up of the plunger tips. Reynolds averaged Navier–Stokes (RANS) turbulence models result in high eddy-viscosity in the air–water region which decays the fluid momentum and adversely affects the predictions. Both VoF and CLSVoF methods predict the large-scale plunging breaking characteristics well; however, they vary in the prediction of the finer details. The CLSVoF solver predicts the splash-up event and secondary plunger better than the VoF solver; however, the latter predicts the plunger shape better than the former for the solitary wave run-up on a slope case.

Investigation of the Flow Structures in Supersonic Free and Impinging Jet Flows

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
C. Chin ◽  
M. Li ◽  
C. Harkin ◽  
T. Rochwerger ◽  
L. Chan ◽  
...  
Keyword(s):  
High Speed ◽  
Numerical Study ◽  
Experimental Studies ◽  
Turbulence Models ◽  
Impinging Jet ◽  
Optical Methods ◽  
Navier Stokes ◽  
Jet Flows ◽  
Shock Cell ◽  
Rans Turbulence Models

A numerical study of compressible jet flows is carried out using Reynolds averaged Navier–Stokes (RANS) turbulence models such as k-ɛ and k-ω-SST. An experimental investigation is performed concurrently using high-speed optical methods such as Schlieren photography and shadowgraphy. Numerical and experimental studies are carried out for the compressible impinging at various impinging angles and nozzle-to-wall distances. The results from both investigations converge remarkably well and agree with experimental data from the open literature. From the flow visualizations of the velocity fields, the RANS simulations accurately model the shock structures within the core jet region. The first shock cell is found to be constraint due to the interaction with the bow-shock structure for nozzle-to-wall distance less than 1.5 nozzle diameter. The results from the current study show that the RANS models utilized are suitable to simulate compressible free jets and impinging jet flows with varying impinging angles.

Numerical Simulation of Turbulent Flows in Ribbed Pipes

2005 ◽  
Author(s):  
Sowjanya Vijiapurapu ◽  
Jie Cui
Keyword(s):  
Turbulent Flows ◽  
Turbulence Models ◽  
Numerical Data ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Friction Resistance ◽  
Circular Pipes ◽  
Computational Fluid Dynamics Cfd ◽  
Stress Models ◽  
Type Intermediate

The Reynolds averaged Navier-Stokes (RANS) equations were solved along with three turbulence models, namely κ-ε, κ-ω, and Reynolds stress models (RSM), to study the fully developed turbulent flows in circular pipes roughened by repeated square ribs. The spacing between the ribs was varied to form three representative types of surface roughness; d–type, intermediate, and k–type. Solutions of these flows at two Reynolds numbers were obtained using the commercial computational fluid dynamics (CFD) software Fluent. The numerical results were validated against experimental measurements and other numerical data published in literature. Extensive investigation of effects of rib spacing and Reynolds number on the pressure and friction resistance, flow and turbulence distribution was presented. The performance of three turbulence models was also compared and discussed.

INVESTIGATION OF INLET EFFECTS ON BACKWARD-FACING STEP FLOW PREDICTION

2016 ◽  
Vol 40 (3) ◽  
pp. 317-329 ◽  
Author(s):  
Mustafa Kemal Isman
Keyword(s):  
Experimental Data ◽  
Turbulence Models ◽  
Navier Stokes ◽  
Backward Facing Step ◽  
Step Flow ◽  
Uniform Velocity ◽  
Flow Prediction ◽  
Rans Turbulence Models ◽  
Negative Effect ◽  
Inlet Boundary Condition

The turbulent flow over backward-facing step (BFS) is numerically investigated by using FLUENT® code. Both uniform and non-uniform velocity profiles are used as inlet boundary condition. Five different Reynolds averaged Navier–Stokes (RANS) turbulence models are employed. The Std. k–ω model shows the best agreement with the experimental data among the models used under the conditions considered in this study. The results show that using a uniform velocity profile has a negative effect on predictions if the domain is not sufficiently extended upstream from the inlet. To eliminate this effect, the domain should be extended upstream by about 10Dh from the inlet. However, results show that this extension causes absorption effects of inlet parameters such as inlet turbulence intensity.

Visualization and Validation of Ejector Flow Field With Computational and First-Principles Analysis

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Adrienne B. Little ◽  
Yann Bartosiewicz ◽  
Srinivas Garimella
Keyword(s):  
Flow Field ◽  
First Principles ◽  
Large Scale ◽  
Turbulence Models ◽  
Ideal Gas ◽  
Analytical Models ◽  
Local Flow ◽  
Flow Features ◽  
Flow Phenomena ◽  
Air Ejector

Passive, heat actuated ejector pumps offer simple and energy-efficient options for a variety of end uses with no electrical input or moving parts. In an effort to obtain insights into ejector flow phenomena and to evaluate the effectiveness of commonly used computational and analytical tools in predicting these conditions, this study presents a set of shadowgraph images of flow inside a large-scale air ejector and compares them to both computational and first-principles-based analytical models of the same flow. The computational simulations used for comparison apply k-ε renormalization group (RNG) and k-ω shear stress transport (SST) turbulence models to two-dimensional (2D), locally refined rectangular meshes for ideal gas air flow. A complementary analytical model is constructed from first principles to approximate the ejector flow field. Results show that on-design ejector operation is predicted with reasonable accuracy, but accuracy with the same models is not adequate at off-design conditions. Exploration of local flow features shows that the k-ω SST model predicts the location of flow features, as well as global inlet mass flow rates, with greater accuracy. The first-principles model demonstrates a method for resolving the ejector flow field from relatively little visual data and shows the evolving importance of mixing, momentum, and heat exchange with the suction flow with distance from the motive nozzle exit. Such detailed global and local exploration of ejector flow helps guide the selection of appropriate turbulence models for future ejector design purposes, predicts locations of important flow phenomena, and allows for more efficient ejector design and operation.

scholarly journals Analysis of Applicability of CFD Numerical Studies Applied to Problem When Pump Working as Turbine

Water ◽  
2021 ◽  
Vol 13 (15) ◽  
pp. 2134
Author(s):  
Frank Plua ◽  
Victor Hidalgo ◽  
P. Amparo López-Jiménez ◽  
Modesto Pérez-Sánchez
Keyword(s):  
Turbulence Models ◽  
Experimental Tests ◽  
Navier Stokes ◽  
Variable Speed ◽  
Reynolds Average Navier Stokes ◽  
Academic Literature ◽  
Numerical Studies ◽  
Speed Rotation ◽  
Computational Fluid Dynamics Cfd ◽  
Reynolds Average

The present research depicts an analysis of the implementation of computational fluid dynamics (CFD) in the study of pumps such as turbines and PATs. To highlight the benefits of CFDs for PAT studies, results from both experimental tests have been compared to better understand the reproduction error phenomena. For this, data analysis used in successful models has been applied to determine variables and parameters, and to report a low relative error. The results show that most of the studies focused on fixed speed rotation with some cases of variable speed rotation. Furthermore, there is not enough information in the academic literature for PAT of axial and mixed flows with fixed and variable speed. Finally, turbulence models based on Reynolds average Navier–Stokes (RANS) have been used to simulate PATs with fixed speed rotation in most cases.

A Deterministic Stress Model for Rotor-Stator Interactions in Simulations of Average-Passage Flow

2002 ◽  
Vol 124 (2) ◽  
pp. 550-554 ◽  
Author(s):  
Charles Meneveau and ◽  
Joseph Katz
Keyword(s):  
Turbulence Models ◽  
Navier Stokes ◽  
Stress Model ◽  
Direct Measurements ◽  
Blade Row ◽  
Model Predictions ◽  
Stator Vane ◽  
Inflow Condition ◽  
Flow Phenomena ◽  
Inflow Conditions

A procedure for modeling deterministic stresses for average-passage simulations of flow in multiple blade-row turbomachines is proposed and tested. This method uses the results of several (two or more) steady Reynolds-averaged Navier-Stokes (RANS) simulations with boundary conditions that are representative of different inflow conditions encountered during the passage of a neighboring blade-row. The deterministic stresses are calculated by averaging the steady results while weighting them with the approximate duration of each inflow condition. This approach incorporates important rotor-stator interactions that are neglected in models based on a swept-wake approximation. The model is tested successfully by computing the deterministic stresses in the stator vane passage of a centrifugal pump, and comparing them with direct measurements using PIV data. Remaining discrepancies between model predictions and experimental data are probably linked to the inability of the turbulence models to account for flow phenomena at each phase, such as mid-vane separation.

Synthesis of a CFD Benchmark for the Thermal Mixing in a Sharp Corner T-Junction With a Wall

2018 ◽  
Author(s):  
Afaque Shams ◽  
Nicolas Edh ◽  
Kristian Angele
Keyword(s):  
Thermal Field ◽  
Turbulence Models ◽  
Temperature Fluctuations ◽  
Reynolds Numbers ◽  
Navier Stokes ◽  
Sharp Corner ◽  
Cfd Validation ◽  
Thermal Mixing ◽  
Eddy Simulation ◽  
Rans Models

This article reports a CFD-benchmark with the purpose of validating different turbulence modelling approaches for the transient heat transfer due to mixing of hot and cold flow in a T-junction including the wall. This validation exercise has been carried out within the MOTHER project. In the framework of the project, new experiments were performed with a novel measurement sensor allowing the measurements of the fluctuating wall temperature inside the solid pipe wall. The tests were performed for two different Reynolds numbers (Re) 40000 and 60000 and for two different T-junction geometries; a sharp corner and a round corner. The present article reports the synthesis of the CFD validation for a sharp corner T-junction for Re = 40 000. The CFD validation study has been performed using four different CFD softwares, namely STAR-CCM+, Code_Saturne, LESOCC2 and Fluent. In addition, five different turbulence models i.e. wall-function Large Eddy Simulation (LES), Deatched Eddy Simulation (DES), Partially Resolved Numerical Simulation (PRNS), Unsteady Reynolds Averaged Navier-Stokes URANS and RANS were used to perform the CFD computations. The validation exercise has shown that LES gives the best agreement with the experimental data followed by hybrid (LES/RANS), URANS and RANS models, respectively. The velocity and the thermal fields in the fluid region are correctly predicted by the proper use of the LES modelling, whereas, the accurate prediction of the thermal field in the solid requires very long sampling time in order to achieve a statistically converged solution, which of course requires an enormous computational power. Therefore, the statistical convergence of the thermal field in the solid has been found to be a bottleneck in order to accurately predict the temperature fluctuations in the wall. However, measuring the small amplitude temperature fluctuations is also associated with an uncertainty so the disagreement between CFD and measurements (of the order of 10 %) can also be attributed, in part, to uncertainties in the measurements.

A Methodology for Simulating Compressible Turbulent Flows

2005 ◽  
Vol 73 (3) ◽  
pp. 405-412 ◽  
Author(s):  
Hermann F. Fasel ◽  
Dominic A. von Terzi ◽  
Richard D. Sandberg
Keyword(s):  
Turbulent Flows ◽  
Compressible Flows ◽  
Bluff Body ◽  
Turbulence Modeling ◽  
Flow Behavior ◽  
Flow Simulation ◽  
Turbulence Models ◽  
Navier Stokes ◽  
Fine Grid ◽  
Local Flow

A flow simulation Methodology (FSM) is presented for computing the time-dependent behavior of complex compressible turbulent flows. The development of FSM was initiated in close collaboration with C. Speziale (then at Boston University). The objective of FSM is to provide the proper amount of turbulence modeling for the unresolved scales while directly computing the largest scales. The strategy is implemented by using state-of-the-art turbulence models (as developed for Reynolds averaged Navier-Stokes (RANS)) and scaling of the model terms with a “contribution function.” The contribution function is dependent on the local and instantaneous “physical” resolution in the computation. This physical resolution is determined during the actual simulation by comparing the size of the smallest relevant scales to the local grid size used in the computation. The contribution function is designed such that it provides no modeling if the computation is locally well resolved so that it approaches direct numerical simulations (DNS) in the fine-grid limit and such that it provides modeling of all scales in the coarse-grid limit and thus approaches a RANS calculation. In between these resolution limits, the contribution function adjusts the necessary modeling for the unresolved scales while the larger (resolved) scales are computed as in large eddy simulation (LES). However, FSM is distinctly different from LES in that it allows for a consistent transition between RANS, LES, and DNS within the same simulation depending on the local flow behavior and “physical” resolution. As a consequence, FSM should require considerably fewer grid points for a given calculation than would be necessary for a LES. This conjecture is substantiated by employing FSM to calculate the flow over a backward-facing step and a plane wake behind a bluff body, both at low Mach number, and supersonic axisymmetric wakes. These examples were chosen such that they expose, on the one hand, the inherent difficulties of simulating (physically) complex flows, and, on the other hand, demonstrate the potential of the FSM approach for simulations of turbulent compressible flows for complex geometries.

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