Forcing symmetry exchanges and flow reversals in turbulent wakes

2017 ◽  
Vol 829 ◽  
Author(s):  
Diogo Barros ◽  
Jacques Borée ◽  
Olivier Cadot ◽  
Andreas Spohn ◽  
Bernd R. Noack
Keyword(s):  
Pressure Gradient ◽  
Bluff Body ◽  
Separated Flow ◽  
High Sensitivity ◽  
Normal Pressure ◽  
Bluff Bodies ◽  
Turbulent Wakes ◽  
Symmetry Properties ◽  
And Control ◽  
Flow Symmetry

Turbulent wakes past bluff bodies commonly present asymmetric flow states reminiscent of bifurcations in the laminar regime. Understanding the sensitivity of these states to flow forcing is crucial to the modelling and control of flow symmetry properties. In this study, the near wake of a rectangular bluff body in proximity to a wall is disturbed by the use of passive devices located between the model and the wall, upstream of the massive flow separation occurring at the blunt trailing edges. Due to the proximity to the boundary, the wake initially presents wall-normal asymmetry and a negative wall-normal pressure gradient along the base. The application of disturbances with variable size, however, sets flow symmetry along the wall-normal plane, leading to the intermittent spanwise wake reversals reported recently in the literature. A further increase in the size of perturbation suppresses wake switching, and wall-normal asymmetry is recovered, but with a positive wall-normal pressure gradient. The dynamical features of this bifurcation scenario can be retrieved using two coupled symmetry-breaking models for spanwise and wall-normal pressure gradients. This confirms the high sensitivity of the separated flow to external perturbations. More importantly, the results unify observations of the bluff-body wake topologies covered in previous investigations.

Bluff-Body Simulation by SPH Method With Relatively High Reynolds Number in Laminar Flow Regime

2010 ◽  
Author(s):  
Mostafa Safdari Shadloo ◽  
Amir Zainali ◽  
Mehmet Yildiz
Keyword(s):  
Bluff Body ◽  
Separated Flow ◽  
Fracture Repair ◽  
Solid Boundary ◽  
Bluff Bodies ◽  
Element Mesh ◽  
Particle Hydrodynamics ◽  
Repair Procedure ◽  
High Reynolds ◽  
Wcsph Method

In this work, we present solutions for flow over an airfoil and square obstacle using Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) method. For the solution of these two problems, we present an improved WCSPH algorithm that can handle complex geometries with the usage of multiple tangent solid boundary method, and eliminate particle clustering induced instabilities with the implementation of particle fracture repair procedure as well as the corrected SPH discretization scheme. We have shown that the improved WCSPH method can be effectively used for flow simulations over bluff-bodies with Reynolds numbers as high as 1400, which is not achievable with standard WCSPH formulations. Our simulation results are validated with a Finite Element mesh-dependent Method (FEM), and excellent agreements among the results were observed. We illustrated that the improved WCSPH method is able to capture the complex physics of bluff-body flows naturally such as flow separation, detachment of separated flow, wake formation at the trailing edge, and vortex shedding without any extra effort to increase the particle resolution in some specific areas of interest.

A Data-Driven Approach for the Stability Analysis of Vortex-Induced Vibration

2018 ◽  
Author(s):  
Sandeep B. Reddy ◽  
Allan Ross Magee ◽  
Rajeev K. Jaiman
Keyword(s):  
Stability Analysis ◽  
Bluff Body ◽  
Data Driven ◽  
Bluff Bodies ◽  
Stability Boundaries ◽  
Data Driven Approach ◽  
Offshore Industry ◽  
The Stability ◽  
Lock In ◽  
And Control

In this paper, a general data-driven approach to construct a reduced-order model (ROM) for the coupled fluid-structure interaction (FSI) problem of a transversely vibrating bluff body in an incompressible flow is presented. The proposed data-driven approach relies on the Eigensystem Realization Algorithm (ERA) to design ROM models in a state-space format. The stability boundaries of the coupled FSI system are obtained by examining the eigenvalue trajectories of the ERA-based ROM. These stability boundaries provide us valuable quantitative insights into the lock-in phenomenon of the bluff-body vibration. We demonstrate the present ERA-based ROM technique for various configurations of bluff bodies such as an isolated single cylinder, the side-by-side and the tandem cylinder arrangements. A comparative study on the effect of different appendages to suppress the VIV of a cylinder is also presented using the ERA-based stability analysis. The validity of the proposed method for the FSI stability analysis on such variety of configurations has not been presented before and is the novel contribution of this paper. Overall, the proposed data-driven framework is found to be much more effective in terms of computational costs and the predicted lock-in regions are comparable to high-fidelity full-order simulations. This work has a potential for a profound impact on the design optimization and control of bluff body structures used in offshore industry.

Asymptotic Theory of Global Separation

1998 ◽  
Vol 51 (9) ◽  
pp. 523-536 ◽  
Author(s):  
S. I. Chernyshenko
Keyword(s):  
Turbulent Flows ◽  
Bluff Body ◽  
Asymptotic Theory ◽  
General Structure ◽  
Separated Flow ◽  
Bluff Bodies ◽  
Mathematical Methods ◽  
Plane Flow ◽  
Flow Past ◽  
High Reynolds

This article aims to review the recent achievements and the state of the art in the high Reynolds number asymptotic theory of steady separated flow past bluff bodies for a general reader specializing in fluid dynamics who is not necessarily familiar with modern asymptotic techniques. A short historical overview is given. The ideas of the mathematical methods used are briefly outlined. Then the general structure of the solution for a plane flow past a bluff body is described. The physical mechanisms of such a flow are discussed, and quantitative results are given and compared with numerical calculations. Existing extensions of the theory and the latest results for axisymmetric flows are described. In conclusion, the relationship between asymptotic theory and real turbulent flows is discussed. This review article contains 76 references.

scholarly journals A Generalized Numerical Method for Bluff Body and Stalling Aerofoil Flow

1982 ◽  
Author(s):  
R. I. Lewis ◽  
D. T. C. Porthouse
Keyword(s):  
Numerical Method ◽  
Bluff Body ◽  
Vortex Motion ◽  
Separated Flow ◽  
Theoretical Method ◽  
Circular Cylinders ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Vortex Streets

A numerical method has been developed for predicting the two-dimensional incompressible separated flow from bluff bodies and aerofoils with preliminary extension to cascades. The paper will present a brief outline of the theoretical method accompanied by illustrations. These include predictions of the vortex streets downstream of circular cylinders and wedge shaped bodies, the stalling behavior of an aerofoil and initial attempts to calculate the vortex motion of a stalling cascade. The method makes use of the surface vorticity technique extended to include vorticity shedding and convection with the mainstream.

Characteristics of a separated flow past a semicircular leading-edge airfoil model under different imposed pressure gradient

2021 ◽  
pp. 095441002110212
Author(s):  
K Anand ◽  
KT Ganesh
Keyword(s):  
Boundary Layer ◽  
Particle Image Velocimetry ◽  
Pressure Gradient ◽  
Particle Image ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Field Measurements ◽  
Bench Mark ◽  
Image Velocimetry

The effect of pressure gradient on a separated boundary layer past the leading edge of an airfoil model is studied experimentally using electronically scanned pressure (ESP) and particle image velocimetry (PIV) for a Reynolds number ( Re) of 25,000, based on leading-edge diameter ( D). The features of the boundary layer in the region of separation and its development past the reattachment location are examined for three cases of β (−30°, 0°, and +30°). The bubble parameters such as the onset of separation and transition and the reattachment location are identified from the averaged data obtained from pressure and velocity measurements. Surface pressure measurements obtained from ESP show a surge in wall static pressure for β = −30° (flap deflected up), while it goes down for β = +30° (flap deflected down) compared to the fundamental case, β = 0°. Particle image velocimetry results show that the roll up of the shear layer past the onset of separation is early for β = +30°, owing to higher amplification of background disturbances compared to β = 0° and −30°. Downstream to transition location, the instantaneous field measurements reveal a stretched, disoriented, and at instances bigger vortices for β = +30°, whereas a regular, periodically shed vortices, keeping their identity past the reattachment location, is observed for β = 0° and −30°. Above all, this study presents a new insight on the features of a separation bubble receiving a disturbance from the downstream end of the model, and these results may serve as a bench mark for future studies over an airfoil under similar environment.

scholarly journals Gold–Carbon Nanocomposites for Environmental Contaminant Sensing

Micromachines ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 719
Author(s):  
Shahrooz Rahmati ◽  
William Doherty ◽  
Arman Amani Babadi ◽  
Muhamad Syamim Akmal Che Mansor ◽  
Nurhidayatullaili Muhd Julkapli ◽  
...  
Keyword(s):  
Environmental Pollutants ◽  
High Sensitivity ◽  
Environmental Crisis ◽  
World Population ◽  
Chemical Contaminants ◽  
Carbon Nanocomposites ◽  
Minimal Sample ◽  
Wide Range ◽  
And Control ◽  
Analytical Tools

The environmental crisis, due to the rapid growth of the world population and globalisation, is a serious concern of this century. Nanoscience and nanotechnology play an important role in addressing a wide range of environmental issues with innovative and successful solutions. Identification and control of emerging chemical contaminants have received substantial interest in recent years. As a result, there is a need for reliable and rapid analytical tools capable of performing sample analysis with high sensitivity, broad selectivity, desired stability, and minimal sample handling for the detection, degradation, and removal of hazardous contaminants. In this review, various gold–carbon nanocomposites-based sensors/biosensors that have been developed thus far are explored. The electrochemical platforms, synthesis, diverse applications, and effective monitoring of environmental pollutants are investigated comparatively.

scholarly journals Pressure gradient tailoring effects on the mechanisms of bluff-body flame extinction

2020 ◽  
Vol 215 ◽  
pp. 224-237 ◽  
Author(s):  
Anthony J. Morales ◽  
Jonathan Reyes ◽  
Peter H. Joo ◽  
Isaac Boxx ◽  
Kareem A. Ahmed
Keyword(s):  
Pressure Gradient ◽  
Bluff Body ◽  
Flame Extinction

On turbulent separation in the flow past a bluff body

1992 ◽  
Vol 241 ◽  
pp. 443-467 ◽  
Author(s):  
A. Neish ◽  
F. T. Smith
Keyword(s):  
Large Scale ◽  
Bluff Body ◽  
Separated Flow ◽  
Small Scale ◽  
Supporting Evidence ◽  
Turbulent Regime ◽  
Trailing Edge ◽  
Flow Past ◽  
Starting Point ◽  
Turbulence Factor

The basic model problem of separation as predicted by the time-mean boundary-layer equations is studied, with the Cebeci-Smith model for turbulent stresses. The changes between laminar and turbulent flow are investigated by means of a turbulence ‘factor’ which increases from zero for laminar flow to unity for the fully turbulent regime. With an attached-flow starting point, a small increase in the turbulence factor above zero is found to drive the separation singularity towards the trailing edge or rear stagnation point for flow past a circular cylinder, according to both computations and analysis. A separated-flow starting point is found to produce analogous behaviour for the separation point. These findings lead to the suggestion that large-scale separation need not occur at all in the fully turbulent regime at sufficiently high Reynolds number; instead, separation is of small scale, confined near the trailing edge. Comments on the generality of this suggestion are presented, along with some supporting evidence from other computations. Further, the small scale involved theoretically has values which seem reasonable in practical terms.

Aerodynamic Characteristics of Asymmetric Bluff Bodies

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
J. C. Hu ◽  
Y. Zhou
Keyword(s):  
Bluff Body ◽  
Laser Doppler Anemometry ◽  
Vortex Formation ◽  
Aerodynamic Characteristics ◽  
Bluff Bodies ◽  
Reynolds Stresses ◽  
Freestream Velocity ◽  
Drag And Lift ◽  
Particle Imaging ◽  
Visualization Techniques

The wake of asymmetric bluff bodies was experimentally measured using particle imaging velocimetry, laser Doppler anemometry, load cell, hotwire, and flow visualization techniques at Re=2600–8500 based on the freestream velocity and the characteristic height of the bluff bodies. Asymmetry is produced by rounding some corners of a square cylinder and leaving others unrounded. It is found that, with increasing corner radius, the flow reversal region is expanded, and the vortex formation length is prolonged. Accordingly, the vortex shedding frequency increases and the base pressure rises, resulting in a reduction in the mean drag as well as the fluctuating drag and lift. It is further found that, while the asymmetric cross section of the cylinder causes the wake centerline to shift toward the sharp corner side of the bluff body, the wake remains globally symmetric about the shifted centerline. The near wake of asymmetric bluff bodies is characterized in detail, including the Reynolds stresses, characteristic velocity, and length scale, and is further compared with that of the symmetric ones.

Sign in / Sign up

Export Citation Format

Share Document