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.

Effect of Reynolds Number on the Turbulent Flow Structure in the Near-Wall Region of an Impinging Round Jet

2011 ◽  
Author(s):  
Khaled J. Hammad ◽  
Ivana Milanovic
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Flow Structure ◽  
Turbulence Models ◽  
Separation Distance ◽  
Wall Region ◽  
Time Resolved ◽  
Reynolds Decomposition ◽  
Turbulent Flow Structure ◽  
Pod Analysis

Time-Resolved Particle Image Velocimetry was used to study the effect of the Reynolds number on the turbulent flow structure of a submerged water jet impinging normally on a smooth and flat surface. A fully developed turbulent jet and a semi-confined flow configuration ensured properly characterized boundary conditions allowing for straightforward assessment of turbulence models and numerical schemes. The Reynolds number based on jet mean exit velocity was 5,000, 10,030 and 15,050 while the pipe-to-plate separation distance was fixed at two diameters. Turbulent velocity fields are presented using Reynolds decomposition into mean and fluctuating components while Proper Orthogonal Decomposition (POD) analysis identified the most energetic coherent structures in the stagnation and wall-jet regions.

Effect of Asymmetric Jet Placement on Turbulent Flow Structure Inside a Jet-Stirred Reactor

2010 ◽  
Author(s):  
Khaled J. Hammad ◽  
Ivana M. Milanovic
Keyword(s):  
Flow Field ◽  
Turbulent Flow ◽  
Flow Structure ◽  
Turbulence Models ◽  
Vortex Identification ◽  
Reynolds Decomposition ◽  
Fully Developed Turbulent Flow ◽  
Identification Technique ◽  
Turbulent Flow Structure ◽  
Wall Interaction

Particle Image Velocimetry (PIV) was used to investigate the turbulent flow structure inside a jet-stirred cylindrical vessel. The submerged jet issued vertically downward from a long pipe ensuring fully developed turbulent flow conditions at the outlet. The Reynolds number based on jet mean exit velocity was 15,000. The effect of symmetric and asymmetric nozzle placement within the vessel on the resulting flow patterns was also studied. The measured turbulent velocity fields are presented using Reynolds decomposition into mean and fluctuating components, which, for the selected flow configuration, inflow and boundary conditions, allow for straightforward assessment of turbulence models and numerical schemes. The flow field was subdivided into three regions: the jet, the jet-wall interaction and bulk of vessel. Proper Orthogonal Decomposition (POD) analysis was applied to identify the most energetic coherent structures of the turbulent flow field in the bulk of tank region. The swirling strength vortex identification technique was used to detect the existence and strength of vortical structures in the jet region.

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.

scholarly journals An Arbitrary Hybrid Turbulence Modeling Approach for Efficient and Accurate Automotive Aerodynamic Analysis and Design Optimization

Fluids ◽  
2021 ◽  
Vol 6 (11) ◽  
pp. 407
Author(s):  
Saule Maulenkul ◽  
Kaiyrbek Yerzhanov ◽  
Azamat Kabidollayev ◽  
Bagdaulet Kamalov ◽  
Sagidolla Batay ◽  
...  
Keyword(s):  
Flow Field ◽  
Design Optimization ◽  
Automotive Industry ◽  
Turbulence Modeling ◽  
Turbulence Models ◽  
Analysis And Design ◽  
Eddy Simulation ◽  
Gradient Based ◽  
Automotive Aerodynamics ◽  
Single Flow

The demand in solving complex turbulent fluid flows has been growing rapidly in the automotive industry for the last decade as engineers strive to design better vehicles to improve drag coefficients, noise levels and drivability. This paper presents the implementation of an arbitrary hybrid turbulence modeling (AHTM) approach in OpenFOAM for the efficient simulation of common automotive aerodynamics with unsteady turbulent separated flows such as the Kelvin–Helmholtz effect, which can also be used as an efficient part of aerodynamic design optimization (ADO) tools. This AHTM approach is based on the concept of Very Large Eddy Simulation (VLES), which can arbitrarily combine RANS, URANS, LES and DNS turbulence models in a single flow field depending on the local mesh refinement. As a result, the design engineer can take advantage of this unique and highly flexible approach to tailor his grid according to his design and resolution requirements in different areas of the flow field over the car body without sacrificing accuracy and efficiency at the same time. This paper presents the details of the implementation and careful validation of the AHTM method using the standard benchmark case of the Ahmed body, in comparison with some other existing models, such as RANS, URANS, DES and LES, which shows VLES to be the most accurate among the five examined. Furthermore, the results of this study demonstrate that the AHTM approach has the flexibility, efficiency and accuracy to be integrated with ADO tools for engineering design in the automotive industry. The approach can also be used for the detailed study of highly complex turbulent phenomena such as the Kelvin–Helmholtz instability commonly found in automotive aerodynamics. Currently, the AHTM implementation is being integrated with the DAFoam for gradient-based multi-point ADO using an efficient adjoint solver based on a Sparse Nonlinear optimizer (SNOPT).

Modification of Turbulence in Boundary Layers by Mean and Fluctuating Pressure Gradients

2012 ◽  
Author(s):  
Pranav Joshi ◽  
Xiaofeng Liu ◽  
Joseph Katz
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Wall Region ◽  
Pressure Gradients ◽  
Two Dimensional ◽  
Time Resolved ◽  
Freestream Velocity ◽  
Vortical Structures ◽  
Fluctuating Pressure ◽  
Near Wall

In this study we focus on the effect of mean and fluctuating pressure gradients on the structure of boundary layer turbulence. Two dimensional, time-resolved PIV measurements have been performed upstream of and inside an accelerating sink flow for inlet Reynolds number of Reθ = 3071, and acceleration parameter of K=1.1×10−6. The time-resolved data enables us to calculate the planer projection of pressure gradient by integrating the in-plane components of the material acceleration of the fluid (neglecting out-of-plane contribution). We use it to study the effect of boundary layer scale fluctuating pressure gradients ∂p′~/∂x, which are expected to be mostly two-dimensional, on the flow structure. Due to the imposed mean favorable pressure gradient (FPG) within the sink flow, the Reynolds stresses normalized by the local freestream velocity decrease over the entire boundary layer. However, when scaled by the inlet freestream velocity, the stresses increase close to the wall and decrease in the outer part of the boundary layer. This trend is caused by the confinement of the newly generated vortical structures in the near-wall region of the accelerating flow due to combined effects of downward mean flow, and stretching by velocity gradients. Within both the zero pressure gradient (ZPG) and FPG boundary layers, sweeping motions mostly occur during positive fluctuating pressure gradients ∂p′~/∂x>0 as the fluid moving towards the wall is decelerated by the presence of the wall. Vorticity is depleted in the near-wall region, as the wall absorbs −ω′ from the flow by viscous diffusion. On the other hand, ejections occur mostly during periods of favorable fluctuating pressure gradients ∂p′~/∂x<0. During these periods, there is more viscous flux of vorticity −ω′ into the flow, since ∂−ω′/∂y<0 at the wall. Large scale ejection motions associated with ∂p′~/∂x<0 are more likely to transport smaller scale turbulence to the outer region of the boundary layer, while turbulence remains largely confined close to the wall due to the sweeping motions accompanying ∂p′~/∂x>0. During periods of ∂p′~/∂x>0 in the ZPG boundary layer, sweeps tend to increase the momentum in the near-wall region, whereas the adverse pressure gradient decelerates the fluid. These competing effects result in an unstable ω′<0 shear layer which rolls up into coherent vortical structures and increases ω′ω′ near the wall as compared to periods of ∂p′~/∂x<0. Due to the strong mean acceleration of the flow and weaker sweeps in the FPG boundary layer, the formation of an unstable shear layer, and hence vortical structures, is suppressed, decreasing the enstrophy close to the wall as compared to periods of ∂p′~/∂x<0.

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 Novel Vortex Identification Technique Applied to the 3D Flow Field of a High-Pressure Turbine

2019 ◽  
Author(s):  
Marek Pátý ◽  
Sergio Lavagnoli
Keyword(s):  
High Pressure ◽  
Flow Field ◽  
Pure Shear ◽  
Three Dimensional ◽  
Secondary Flows ◽  
Vortex Identification ◽  
3D Flow ◽  
High Pressure Turbine ◽  
Vortical Structures ◽  
Identification Technique

Abstract The efficiency of modern axial turbomachinery is strongly driven by the secondary flows within the vane or blade passages. The secondary flows are characterized by a complex pattern of vortical structures that origin, interact and dissipate along the turbine gas path. The endwall flows are responsible for the generation of a significant part of the overall turbine loss because of the dissipation of secondary kinetic energy and mixing-out of non-uniform momentum flows. The understanding and analysis of secondary flows requires a reliable vortex identification technique to predict and analyse the impact of specific turbine designs on the turbine performance. However, literature shows a remarkable lack of general methods to detect vortices and to determine the location of their cores and to quantify their strength. This paper presents a novel technique for the identification of vortical structures in a general 3D flow field. The method operates on the local flow field and it is based on a triple decomposition of motion proposed by Kolář. In contrast to a decomposition of velocity gradient into the strain and vorticity tensors, this method considers a third, pure shear component. The subtraction of the pure shear tensor from the velocity gradient remedies the inherent flaw of vorticity-based techniques which cannot distinguish between rigid rotation and shear. The triple decomposition of motion serves to obtain a 3D field of residual vorticity whose magnitude is used to define vortex regions. The present method allows to locate automatically the core of each vortex, quantify its strength and determine the vortex bounding surface. The output may be used to visualize the turbine vortical structures for the purpose of interpreting the complex three-dimensional viscous flow field, as well as to highlight any case-to-case variations by quantifying the vortex strength and location. The vortex identification method is applied to a high-pressure turbine with three optimized blade tip geometries. The 3D flow-field is obtained by CFD computations performed with Numeca FINE/Open. The computational model uses steady-state RANS equations closed by the Spalart-Allmaras turbulence model. Although developed for turbomachinery applications, the vortex identification method proposed in this work is of general applicability to any three-dimensional flow-field.

Periodic Unsteady Tip Clearance Vortex Development in a Low Speed Axial Research Compressor at Different Tip Clearances

2017 ◽  
Author(s):  
Martin Lange ◽  
Matthias Rolfes ◽  
Ronald Mailach ◽  
Henner Schrapp
Keyword(s):  
Flow Field ◽  
Research Work ◽  
Pressure Rise ◽  
Turbulence Models ◽  
Tip Clearance ◽  
Tip Leakage Flow ◽  
Time Resolved ◽  
Low Speed ◽  
Sst Model ◽  
Tip Clearance Vortex

Since the early work on axial compressors the penalties due to radial clearances between blades and side walls are known and an ongoing focus of research work. The periodic unsteadiness of the tip clearance vortex, due to its interaction with the stator wakes, has only rarely been addressed in research papers so far. The current work presents experimental and numerical results from a four stage low speed research compressor modeling a state of the art compressor design. Time-resolved experimental measurements have been carried out at three different rotor tip clearances (gap to tip chord: 1.5%, 2.2%, 3.7%) to cover the third rotor’s casing static pressure and exit flow field. These results are compared with either steady simulations using different turbulence models or harmonic RANS calculations to discuss the periodical unsteady tip clearance vortex development at different clearance heights. The prediction of the local tip leakage flow is clearly improved by the EARSM turbulence model compared to the standard SST model. The harmonic RANS calculations (using the SST model) improve the prediction of time-averaged pressure rise and are used to analyze the rotor stator interaction in detail. The interaction of the rotor tip flow field with the passing stator wakes cause a segmentation of the tip clearance vortex and result in a sinusoidal variation in blockage downstream the rotor row.

Vortex Identification Study of a Turbulent Cavity Flow

2018 ◽  
Author(s):  
Khaled J. Hammad
Keyword(s):  
Particle Image ◽  
Vortex Identification ◽  
Cavity Depth ◽  
Piv Measurements ◽  
Vortical Structures ◽  
Image Velocimetry ◽  
Flow Features ◽  
Shallow Cavity ◽  
Pod Analysis ◽  
Proper Orthogonal

A combined vortex identification and Proper Orthogonal Decomposition (POD) analysis is applied to high-resolution Particle Image Velocimetry (PIV) measurements of a turbulent flow past an open shallow cavity. The PIV measurements, at a cavity depth based Reynolds number of 42,000, capture the flow structure and turbulence, upstream, over, and downstream an open cavity having a length-to-depth ratio of four. Vorticity and second invariant Q of the velocity gradient tensor analysis are used to identify the vortical structures and the overall flow field features. POD analysis is applied to the vorticity and Q fields to identify the most energetic vortical structures and flow features. The results demonstrate the superiority of the combined Q-criterion and POD analysis in identifying distinct vortical structures and their evolution.

Full Field Algebraic Anisotropic Eddy Viscosity Model for the Film Cooling Flows

2012 ◽  
Author(s):  
Xueying Li ◽  
Jing Ren ◽  
Hongde Jiang
Keyword(s):  
Flow Field ◽  
Film Cooling ◽  
Eddy Viscosity ◽  
Turbulence Models ◽  
Viscosity Model ◽  
Wall Region ◽  
Mean Flow ◽  
Experiment Data ◽  
Anisotropic Models ◽  
Eddy Viscosity Model

The algebraic anisotropic eddy viscosity model proposed by the authors is further developed to make it suitable to the full flow field in order to focus not only in the near wall region but also in the main flow field. The three anisotropic eddy viscosity ratios for u′v′, u′w′, v′w′ are determined from the eddy viscosity hypothesis and algebraic Reynolds stress transport equations and expressed in Cartesian coordinate system. This model is applied to four isotropic two-equation turbulence models to make them anisotropic. These anisotropic models are validated with the experiment data from Sinha et al.[1]. Thorough tests are performed with all these isotropic and anisotropic turbulence models for film cooling on a flatplate with different blowing ratios. Detailed analyses of computational simulations are presented. The predicted adiabatic film cooling effectiveness and mean flow field show that the algebraic anisotropic eddy-viscosity turbulence models agree better with the experiment data. Among the four anisotropic models, the anisotropic models based on the realizable k-ε and RNG k-ε models stand out as the most promising models for flatplate film cooling predictions. It’s a big advantage of this model that it deals with the whole flow field and can be combined with different turbulence models.

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