scholarly journals Stochastic modelling and feedback control of bistability in a turbulent bluff body wake

2016 ◽  
Vol 802 ◽  
pp. 726-749 ◽  
Author(s):  
R. D. Brackston ◽  
J. M. García de la Cruz ◽  
A. Wynn ◽  
G. Rigas ◽  
J. F. Morrison
Keyword(s):  
Reynolds Number ◽  
Feedback Control ◽  
Symmetry Breaking ◽  
Large Scale ◽  
Bluff Body ◽  
Three Dimensional ◽  
Power Saving ◽  
Stochastic Modelling ◽  
Bluff Body Wake ◽  
Modelling Approach

A specific feature of three-dimensional bluff body wakes, flow bistability, is a subject of particular recent interest. This feature consists of a random flipping of the wake between two asymmetric configurations and is believed to contribute to the pressure drag of many bluff bodies. In this study we apply the modelling approach recently suggested for axisymmetric bodies by Rigaset al.(J. Fluid Mech., vol. 778, 2015, R2) to the reflectional symmetry-breaking modes of a rectilinear bluff body wake. We demonstrate the validity of the model and its Reynolds number independence through time-resolved base pressure measurements of the natural wake. Further, oscillating flaps are used to investigate the dynamics and time scales of the instability associated with the flipping process, demonstrating that they are largely independent of Reynolds number. The modelling approach is then used to design a feedback controller that uses the flaps to suppress the symmetry-breaking modes. The controller is successful, leading to a suppression of the bistability of the wake, with concomitant reductions in both lateral and streamwise forces. Importantly, the controller is found to be efficient, the actuator requiring only 24 % of the aerodynamic power saving. The controller therefore provides a key demonstration of efficient feedback control used to reduce the drag of a high-Reynolds-number three-dimensional bluff body. Furthermore, the results suggest that suppression of large-scale structures is a fundamentally efficient approach for bluff body drag reduction.

scholarly journals On three-dimensional bluff body wake symmetry breaking with free-stream turbulence and residual asymmetry

2020 ◽  
Vol 348 (6-7) ◽  
pp. 509-517
Author(s):  
Olivier Cadot ◽  
Maha Almarzooqi ◽  
Antoine Legeai ◽  
Vladimir Parezanović ◽  
Luc Pastur
Keyword(s):  
Symmetry Breaking ◽  
Bluff Body ◽  
Free Stream ◽  
Three Dimensional ◽  
Free Stream Turbulence ◽  
Bluff Body Wake

WAVELET-BASED INVESTIGATION OF THE HIGH REYNOLDS NUMBER BLUFF BODY WAKE TOPOLOGY

2006 ◽  
Vol 04 (02) ◽  
pp. 321-332
Author(s):  
ADRIAN DOBRE ◽  
HORIA HANGAN
Keyword(s):  
Reynolds Number ◽  
High Reynolds Number ◽  
Bluff Body ◽  
Three Dimensional ◽  
Square Prism ◽  
Wavelet Techniques ◽  
High Reynolds ◽  
Bluff Body Wake ◽  
Topological Models ◽  
Recognition Technique

The high Reynolds number wake topology of a square prism is experimentally investigated using wavelet analysis. It is shown that a systematic application of one-dimensional continuous wavelet techniques, including a relatively new wavelet pattern recognition technique can reveal important three-dimensional features of the flow, assessing the validity of previously proposed wake topological models. Present results suggest that the high Reynolds number turbulent wakes are topologically similar, but not identical, to their laminar counterparts.

Wavelet-based adaptive simulations of three-dimensional flow past a square cylinder

2014 ◽  
Vol 748 ◽  
pp. 433-456 ◽  
Author(s):  
Giuliano De Stefano ◽  
Oleg V. Vasilyev
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Numerical Solutions ◽  
Three Dimensional ◽  
Adaptive Methods ◽  
Three Dimensional Flow ◽  
Dimensional Flow ◽  
Eddy Simulation ◽  
Experimental Findings ◽  
Bluff Body Flows

AbstractThe wavelet-based eddy capturing approach is extended to three-dimensional bluff body flows, where the flow geometry is enforced through Brinkman volume penalization. The wavelet-collocation/volume-penalization combined method is applied to the simulation of vortex shedding flow behind an isolated stationary prism with square cross-section. Wavelet-based direct numerical simulation is conducted at low supercritical Reynolds number, where the wake develops fundamental three-dimensional flow structures, while wavelet-based adaptive large-eddy simulation supplied with the one-equation localized dynamic kinetic-energy-based model is performed at moderately high Reynolds number. The present results are in general agreement with experimental findings and numerical solutions provided by classical non-adaptive methods. This study demonstrates that the proposed hybrid methodology for modelling bluff body flows is feasible, accurate and efficient.

Vortex Dynamics and Instability Mechanisms in a Radially Lobed Nozzle

2021 ◽  
Author(s):  
Aarthi Sekaran ◽  
Noushin Amini
Keyword(s):  
Reynolds Number ◽  
Vortex Dynamics ◽  
Large Scale ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Combustion Chambers ◽  
Passive Flow Control ◽  
Free Jet ◽  
Passive Flow ◽  
Lobed Nozzle

Abstract The application of radially lobed nozzles has seen renewed challenges in the recent past with their roles in combustion chambers and passive flow control. The free jet flow from such nozzles has been studied for different flow conditions and compared to jets from round nozzles, verifying their improved mixing abilities. The precise mixing mechanisms of these nozzles are, however, not entirely understood and yet to be analyzed for typical jet parameters and excitation modes. The present study carries out three-dimensional Large Eddy Simulations (LES) of the flow from a tubular radially lobed nozzle to identify instability mechanisms and vortex dynamics that lead to enhanced mixing. The flow is studied at two Reynolds numbers of around 6000 and 75,000, based on the effective jet diameter. The low Reynolds number jet is compared to that from a round nozzle and experimental data to demonstrate changes in mixing mechanisms. The present simulations confirmed the presence of K-H-like modes and their evolution. The analysis also confirms the evolution of three distinct types of structures - the large-scale streamwise modes at the lobe crests, corresponding K-H structures at the troughs and an additional set of structures generated from the lobe walls. The higher Reynolds number simulations indicate changes in the mechanics with a subdued role of the lobe walls.

Numerical Design and Study of a MEMS-Based Micro Turbine

2005 ◽  
Author(s):  
Tatsuo Onishi ◽  
Ste´phane Burguburu ◽  
Olivier Dessornes ◽  
Yves Ribaud
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Skin Friction ◽  
Large Scale ◽  
Low Reynolds Number ◽  
Turbine Blades ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Low Reynolds ◽  
Micro Turbine

A full three dimensional Navier-Stokes solver elsA developed by ONERA is used to design and study the aerothermodynamics of a MEMS-based micro turbine. This work is performed in the framework of micro turbomachinery project at ONERA. A few millimeter scale micro turbine is operated in a low Reynolds number regime (Re = 5,000∼50,000), which implies a more important influence of skin friction and heat transfer than the conventional large-scale gas turbine. The 2D geometry constraints due to the limitation of fabrication technology also distinguish the aerothermodynamic characteristics of a micro turbine from that of conventional turbomachinery. Thus, for the foundation of aerothermodynamic design of micro turbomachinery, understanding of low Reynolds number effects on the performance is required and then the design of the turbine geometry can be optimized. In this study, aero-thermodynamic effects at low Reynolds number and different stator/rotor configurations are examined with a prescribed wall temperature. Losses due to heat transfer to walls and skin friction are estimated and their effects on the operating performance are discussed. Power delivery to turbine blades is checked and found satisfactory to give the objective design value of more than 100W. The effects of turbine exhaust geometry and the number of blades on turbine performance are also discussed.

Experimental investigation of a three-dimensional bluff-body wake

AIAA Journal ◽  
1993 ◽  
Vol 31 (3) ◽  
pp. 559-563 ◽  
Author(s):  
A. Ahmed ◽  
M. J. Khan ◽  
B. Bays-Muchmore
Keyword(s):  
Experimental Investigation ◽  
Bluff Body ◽  
Three Dimensional ◽  
Bluff Body Wake

A study of three-dimensional aspects of vortex shedding from a bluff body with a mild geometric disturbance

1997 ◽  
Vol 330 ◽  
pp. 85-112 ◽  
Author(s):  
N. TOMBAZIS ◽  
P. W. BEARMAN
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Base Pressure ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Trailing Edges ◽  
Mode Transitions

Experiments have been carried out to study the three-dimensional characteristics of vortex shedding from a half-ellipse shape with a blunt trailing edge. In order to control the occurrence of vortex dislocations, the trailing edges of the models used were constructed with a series of periodic waves across their spans. Flow visualization was carried out in a water tunnel at a Reynolds number of 2500, based on trailing-edge thickness. A number of shedding modes were observed and the sequence of mode transitions recorded. Quantitative data were obtained from wind tunnel measurements performed at a Reynolds number of 40000. Two shedding frequencies were recorded with the higher frequency occurring at spanwise positions coinciding with minima in the chord. At these same positions the base pressure was lowest and the vortex formation length longest. Arguments are put forward to explain these observations. It is shown that the concept of a universal Strouhal number holds, even when the flow is three-dimensional. The spanwise variation in time-average base pressure is predicted using the estimated amount of time the flow spends at the two shedding frequencies.

Numerical study of three-dimensional Kolmogorov flow at high Reynolds numbers

1996 ◽  
Vol 306 ◽  
pp. 293-323 ◽  
Author(s):  
Vadim Borue ◽  
Steven A. Orszag
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Eddy Viscosity ◽  
Stokes Equations ◽  
Numerical Study ◽  
Three Dimensional ◽  
Large Reynolds Number ◽  
Kolmogorov Flow ◽  
Turbulence Decay ◽  
High Reynolds

High-resolution numerical simulations (with up to 2563 modes) are performed for three-dimensional flow driven by the large-scale constant force fy = F cos(x) in a periodic box of size L = 2π (Kolmogorov flow). High Reynolds number is attained by solving the Navier-Stokes equations with hyperviscosity (-1)h+1Δh (h = 8). It is shown that the mean velocity profile of Kolmogorov flow is nearly independent of Reynolds number and has the ‘laminar’ form vy = V cos(x) with a nearly constant eddy viscosity. Nevertheless, the flow is highly turbulent and intermittent even at large scales. The turbulent intensities, energy dissipation rate and various terms in the energy balance equation have the simple coordinate dependence a + b cos(2x) (with a, b constants). This makes Kolmogorov flow a good model to explore the applicability of turbulence transport approximations in open time-dependent flows. It turns out that the standard expression for effective (eddy) viscosity used in K-[Escr ] transport models overpredicts the effective viscosity in regions of high shear rate and should be modified to account for the non-equilibrium character of the flow. Also at large scales the flow is anisotropic but for large Reynolds number the flow is isotropic at small scales. The important problem of local isotropy is systematically studied by measuring longitudinal and transverse components of the energy spectra and crosscorrelation spectra of velocities and velocity-pressure-gradient spectra. Cross-spectra which should vanish in the case of isotropic turbulence decay only algebraically but somewhat faster than corresponding isotropic correlations. It is verified that the pressure plays a crucial role in making the flow locally isotropic. It is demonstrated that anisotropic large-scale flow may be considered locally isotropic at scales which are approximately ten times smaller than the scale of the flow.

Linear instability of low Reynolds number massively separated flow around three NACA airfoils

2016 ◽  
Vol 811 ◽  
pp. 701-741 ◽  
Author(s):  
W. He ◽  
R. S. Gioria ◽  
J. M. Pérez ◽  
V. Theofilis
Keyword(s):  
Reynolds Number ◽  
Short Wavelength ◽  
Large Scale ◽  
Initial Conditions ◽  
Turbine Blades ◽  
Three Dimensional ◽  
Separated Flow ◽  
Linear Stability Theory ◽  
Instability Analysis ◽  
Stall Cells

Two- and three-dimensional modal and non-modal instability mechanisms of steady spanwise-homogeneous laminar separated flow over airfoil profiles, placed at large angles of attack against the oncoming flow, have been investigated using global linear stability theory. Three NACA profiles of distinct thickness and camber were considered in order to assess geometry effects on the laminar–turbulent transition paths discussed. At the conditions investigated, large-scale steady separation occurs, such that Tollmien–Schlichting and cross-flow mechanisms have not been considered. It has been found that the leading modal instability on all three airfoils is that associated with the Kelvin–Helmholtz mechanism, taking the form of the eigenmodes known from analysis of generic bluff bodies. The three-dimensional stationary eigenmode of the two-dimensional laminar separation bubble, associated in earlier analyses with the formation on the airfoil surface of large-scale separation patterns akin to stall cells, is shown to be more strongly damped than the Kelvin–Helmholtz mode at all conditions examined. Non-modal instability analysis reveals the potential of the flows considered to sustain transient growth which becomes stronger with increasing angle of attack and Reynolds number. Optimal initial conditions have been computed and found to be analogous to those on a cascade of low pressure turbine blades. By changing the time horizon of the analysis, these linear optimal initial conditions have been found to evolve into the Kelvin–Helmholtz mode. The time-periodic base flows ensuing linear amplification of the Kelvin–Helmholtz mode have been analysed via temporal Floquet theory. Two amplified modes have been discovered, having characteristic spanwise wavelengths of approximately 0.6 and 2 chord lengths, respectively. Unlike secondary instabilities on the circular cylinder, three-dimensional short-wavelength perturbations are the first to become linearly unstable on all airfoils. Long-wavelength perturbations are quasi-periodic, standing or travelling-wave perturbations that also become unstable as the Reynolds number is further increased. The dominant short-wavelength instability gives rise to spanwise periodic wall-shear patterns, akin to the separation cells encountered on airfoils at low angles of attack and the stall cells found in flight at conditions close to stall. Thickness and camber have quantitative but not qualitative effect on the secondary instability analysis results obtained.

Large-scale vortex structures in turbulent wakes behind bluff bodies. Part 2. Far-wake structures

1987 ◽  
Vol 174 ◽  
pp. 271-298 ◽  
Author(s):  
T. R. Steiner ◽  
A. E. Perry
Keyword(s):  
Large Scale ◽  
Bluff Body ◽  
Three Dimensional ◽  
Bluff Bodies ◽  
Mean Velocity ◽  
Wake Flows ◽  
Wake Patterns ◽  
Large Scale Vortex ◽  
High Reynolds ◽  
Far Wake

An investigation of a selection of high-Reynolds-number bluff-body flows was conducted. Here in Part 2 phase-averaged velocity-field results will be presented for several far-wake flows generated by nominally two-dimensional and three-dimensional bodies. In these far-wake flows the shed vortices have approached a nearly constant convection velocity. Some mean velocity and phase-averaged and global Reynoldsstress measurements are also presented. The turbulent wake of a lift-producing three-dimensional body has been examined. Also included are the phase-averaged wake patterns behind a flapping flag and a windmill. The topological structure of these patterns is discussed and a preliminary classification of wake patterns is presented.

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