Numerical Study of the Subsonic Base Flow with a Side Support

2012 ◽  
pp. 427-437 ◽  
Author(s):  
Yancheng You ◽  
Kai Oßwald ◽  
Heinrich Lüdeke ◽  
Volker Hannemann
Keyword(s):  
Numerical Study ◽  
Base Flow

scholarly journals Adjoint-based parametric sensitivity analysis for swirling M-flames

2018 ◽  
Vol 859 ◽  
pp. 516-542 ◽  
Author(s):  
Calum S. Skene ◽  
Peter J. Schmid
Keyword(s):  
Sensitivity Analysis ◽  
Frequency Response ◽  
Stokes Equations ◽  
Numerical Study ◽  
Computational Cost ◽  
Base Flow ◽  
Sensitivity Analyses ◽  
Perturbation Expansions ◽  
Mean Flow ◽  
Adjoint Solutions

A linear numerical study is conducted to quantify the effect of swirl on the response behaviour of premixed lean flames to general harmonic excitation in the inlet, upstream of combustion. This study considers axisymmetric M-flames and is based on the linearised compressible Navier–Stokes equations augmented by a simple one-step irreversible chemical reaction. Optimal frequency response gains for both axisymmetric and non-axisymmetric perturbations are computed via a direct–adjoint methodology and singular value decompositions. The high-dimensional parameter space, containing perturbation and base-flow parameters, is explored by taking advantage of generic sensitivity information gained from the adjoint solutions. This information is then tailored to specific parametric sensitivities by first-order perturbation expansions of the singular triplets about the respective parameters. Valuable flow information, at a negligible computational cost, is gained by simple weighted scalar products between direct and adjoint solutions. We find that for non-swirling flows, a mode with azimuthal wavenumber $m=2$ is the most efficiently driven structure. The structural mechanism underlying the optimal gains is shown to be the Orr mechanism for $m=0$ and a blend of Orr and other mechanisms, such as lift-up, for other azimuthal wavenumbers. Further to this, velocity and pressure perturbations are shown to make up the optimal input and output showing that the thermoacoustic mechanism is crucial in large energy amplifications. For $m=0$ these velocity perturbations are mainly longitudinal, but for higher wavenumbers azimuthal velocity fluctuations become prominent, especially in the non-swirling case. Sensitivity analyses are carried out with respect to the Mach number, Reynolds number and swirl number, and the accuracy of parametric gradients of the frequency response curve is assessed. The sensitivity analysis reveals that increases in Reynolds and Mach numbers yield higher gains, through a decrease in temperature diffusion. A rise in mean-flow swirl is shown to diminish the gain, with increased damping for higher azimuthal wavenumbers. This leads to a reordering of the most effectively amplified mode, with the axisymmetric ($m=0$) mode becoming the dominant structure at moderate swirl numbers.

Relevance of local parallel theory to the linear stability of laminar separation bubbles

2012 ◽  
Vol 698 ◽  
pp. 468-478 ◽  
Author(s):  
Sourabh S. Diwan ◽  
O. N. Ramesh
Keyword(s):  
Linear Stability ◽  
Strong Interaction ◽  
Numerical Study ◽  
Separation Bubble ◽  
Global Analysis ◽  
Base Flow ◽  
Initial Portion ◽  
Laminar Separation ◽  
Separation Bubbles ◽  
Laminar Separation Bubbles

AbstractLaminar separation bubbles are thought to be highly non-parallel, and hence global stability studies start from this premise. However, experimentalists have always realized that the flow is more parallel than is commonly believed, for pressure-gradient-induced bubbles, and this is why linear parallel stability theory has been successful in describing their early stages of transition. The present experimental/numerical study re-examines this important issue and finds that the base flow in such a separation bubble becomes nearly parallel due to a strong-interaction process between the separated boundary layer and the outer potential flow. The so-called dead-air region or the region of constant pressure is a simple consequence of this strong interaction. We use triple-deck theory to qualitatively explain these features. Next, the implications of global analysis for the linear stability of separation bubbles are considered. In particular we show that in the initial portion of the bubble, where the flow is nearly parallel, local stability analysis is sufficient to capture the essential physics. It appears that the real utility of the global analysis is perhaps in the rear portion of the bubble, where the flow is highly non-parallel, and where the secondary/nonlinear instability stages are likely to dominate the dynamics.

Experimental and numerical study of electrically driven magnetohydrodynamic flow in a modified cylindrical annulus. I. Base flow

2015 ◽  
Vol 27 (7) ◽  
pp. 077101 ◽  
Author(s):  
Zacharias Stelzer ◽  
David Cébron ◽  
Sophie Miralles ◽  
Stijn Vantieghem ◽  
Jérôme Noir ◽  
...  
Keyword(s):  
Numerical Study ◽  
Base Flow ◽  
Magnetohydrodynamic Flow ◽  
Cylindrical Annulus

scholarly journals Numerical Study of the Supersonic Base Flow around Aircraft Model

2018 ◽  
Vol 1009 ◽  
pp. 012005 ◽  
Author(s):  
Ya V Khankhasaeva ◽  
V E Borisov ◽  
A E Lutsky
Keyword(s):  
Numerical Study ◽  
Base Flow ◽  
Aircraft Model

scholarly journals Experimental and Numerical Study of Laminar and Turbulent Base Flow on a Spherical Capsule

2009 ◽  
Author(s):  
Matthew MacLean ◽  
Erik Mundy ◽  
Timothy Wadhams ◽  
Michael Holden ◽  
Michael Barnhardt ◽  
...  
Keyword(s):  
Numerical Study ◽  
Base Flow ◽  
Spherical Capsule

scholarly journals Global stability of the rotating-disc boundary layer with an axial magnetic field

2013 ◽  
Vol 724 ◽  
pp. 510-526 ◽  
Author(s):  
Christian Thomas ◽  
Christopher Davies
Keyword(s):  
Magnetic Field ◽  
Boundary Layer ◽  
Linear Stability ◽  
Axial Magnetic Field ◽  
Numerical Study ◽  
Control Strategies ◽  
Base Flow ◽  
Global Instability ◽  
Rotating Disc ◽  
Radially Inhomogeneous

AbstractA numerical study is conducted to investigate the influence of a uniform axial magnetic field on the global linear stability of the rotating-disc boundary layer. Simulation results obtained using a radially homogenized base flow were found to be in excellent agreement with an earlier linear stability analysis, which indicated that an axial magnetic field can locally suppress both convective and absolute instabilities. However, the numerical results obtained for the genuine, radially inhomogeneous, flow indicate that a global form of instability develops for sufficiently large magnetic fields. The qualitative nature of the global instability is similar to that which was observed in a previous study, where mass suction was applied at the rotating disc surface. It is shown that, just as for the case with mass suction, it is possible to explain the promotion of global instability by considering a model that includes detuning effects, which are associated with the radial variation of locally defined absolute temporal frequencies. The recurrence of the same type of instability behaviour when two distinct flow control strategies are implemented, one using suction and the other an axial magnetic field, indicates that the phenomena described by the model may be considered generic.

Enhancement of Turbulent Mixing by Embedded Longitudinal Vorticity: A Numerical Study and Experimental Comparison

2006 ◽  
Author(s):  
Hakim Mohand Kaci ◽  
Thierry Lemenand ◽  
Dominique Della Valle ◽  
Hassan Peerhossaini
Keyword(s):  
Turbulent Mixing ◽  
Test Section ◽  
High Efficiency ◽  
Numerical Study ◽  
Cylindrical Tube ◽  
Base Flow ◽  
Vortex Generators ◽  
Experimental Comparison ◽  
Turbulence Structure ◽  
Static Mixer

This work concerns the characterization of turbulent flow underlying the mixing phenomenon in a static mixer-reactor HEV (high-efficiency vortex). An experimental test section made of a cylindrical tube equipped with seven rows of vortex generators was designed and constructed for this purpose. Each row has four vortex generators fixed symmetrically on the tube wall. This new type of mixer generates coherent structures in the form of longitudinal counter-rotative vortices. The resulting flow enhances radial mass transfer and thus facilitates the dispersion and mixing of the particles. The energy cost of this mixer is 1000 times less than that of other mixers for a given interface area [1, 2]. The aim of this work is to study numerically and experimentally the turbulence structure of the flow generated by the mixer, in particular the more energetic structures present in the base flow. Numerical simulations of the velocity distribution and turbulence levels inside the static mixer were conducted for various turbulence models by using the commercial mesh-generator code Gambit coupled with the CFD package Fluent. Attention was focused on the evolution and distribution of the rate of turbulent kinetic energy dissipation as the underlying mechanism for turbulent mixing. Experiments were carried out on the test section in a flow loop by using LDA. Mean and turbulent quantities were measured and numerical results were compared with experimental results. This study provides a basis for understanding the physical mechanisms in the mixing and homogenising of the flow and therefore the efficiency of the mixer.

scholarly journals Elliptic instability in a strained Batchelor vortex

2007 ◽  
Vol 577 ◽  
pp. 341-361 ◽  
Author(s):  
LAURENT LACAZE ◽  
KRIS RYAN ◽  
STÉPHANE LE DIZÈS
Keyword(s):  
Reynolds Number ◽  
Strain Field ◽  
Flow Parameter ◽  
Numerical Study ◽  
Normal Modes ◽  
Axial Flow ◽  
Base Flow ◽  
Large Reynolds Number ◽  
Resonant Coupling ◽  
Flow Parameters

The elliptic instability of a Batchelor vortex subject to a stationary strain field is considered by theoretical and numerical means in the regime of large Reynolds number and small axial flow. In the theory, the elliptic instability is described as a resonant coupling of two quasi-neutral normal modes (Kelvin modes) of the Batchelor vortex of azimuthal wavenumbers m and m + 2 with the underlying strain field. The growth rate associated with these resonances is computed for different values of the azimuthal wavenumbers as the axial flow parameter is varied. We demonstrate that the resonant Kelvin modes m = 1 and m = −1 which are the most unstable in the absence of axial flow become damped as the axial flow is increased. This is shown to be due to the appearance of a critical layer which damps one of the resonant Kelvin modes. However, the elliptic instability does not disappear. Other combinations of Kelvin modes m = −2 and m = 0, then m = −3 and m = −1 are shown to become progressively unstable for increasing axial flow. A complete instability diagram is obtained as a function of the axial flow parameter for several values of the strain rate and Reynolds number.The numerical study considers a system of two counter-rotating Batchelor vortices in which the strain field felt by each vortex is due to the other vortex. The characteristics of the most unstable linear modes developing on the frozen base flow are computed by direct numerical simulations for two axial flow parameters and compared to the theory. In both cases, a very good agreement is obtained for the most unstable modes. Less unstable modes are also identified in the numerics and shown to correspond to peculiar resonances involving Kelvin modes from branches of different labels.

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