Publications
84 results found
Xu H, Sherwin S, Hall P, et al., 2016, The behaviour of Tollmien-Schlichting waves undergoing small-scale localised distortions, Journal of Fluid Mechanics, Vol: 792, Pages: 499-525, ISSN: 1469-7645
This paper is concerned with the behaviour of Tollmien-Schlichting (T-S) waves experiencingsmall localised distortions within an incompressible boundary layer developingover a flat-plate. In particular, the distortion is produced by an isolated roughness elementlocated at Rexc = 440 000. We considered the amplification of an incoming T-S wavegoverned by the two-dimensional linearised Navier-Stokes equations, where the base flowis obtained from the two-dimensional non-linear Navier-Stokes equations. We comparethese solutions with asymptotic analyses which assume a linearised triple-deck theory forthe base flow and determine the validity of this theory in terms of the height of the smallscalehumps/indentations taken into account. The height of the humps/indentations isdenoted by h which is considered to be less than or equal to xcRe−5/8xc(correspondingto h/δ99 < 6% for our choice of Rexc). The rescaled width ˆd(≡ d/(xcRe−3/8xc)) ofthe distortion is of the order O(1) and the width d is shorter than the T-S wavelength(λT S = 11.3 δ99).We observe that for distortions which are smaller than 0.1 of the inner deck height(h/δ99 < 0.4%) the numerical simulations confirm the asymptotic theory in the vicinityof the distortion. For larger distortions which are still within the inner deck (0.4% <h/δ99 < 5.5%) and where the flow is still attached the numerical solutions show thatboth humps and indentations are destabilising and deviate from the linear theory evenin the vicinity of the distortion.We numerically determine the transmission coefficient which provides the relative amplificationof the T-S wave over the distortion as compared to the flat-plate. We observethat for small distortions, h/δ99 < 5.5%, where the width of the distortion is of orderof the boundary layer a maximum amplification of only 2% is achieved. This amplificationcan however be increased as the width of the distortion is increased or
Wu X, Zhuang X, 2016, Nonlinear dynamics of large-scale coherent structures in turbulent free shear layers, Journal of Fluid Mechanics, Vol: 787, Pages: 396-439, ISSN: 1469-7645
Fully developed turbulent free shear layers exhibit a high degree of order, characterized by large-scale coherent structures in the form of spanwise vortex rollers. Extensive experimental investigations show that such organized motions bear remarkable resemblance to instability waves, and their main characteristics, including the length scales, propagation speeds and transverse structures, are reasonably well predicted by linear stability analysis of the mean flow. In this paper, we present a mathematical theory to describe the nonlinear dynamics of coherent structures. The formulation is based on the triple decomposition of the instantaneous flow into a mean field, coherent fluctuations and small-scale turbulence but with the mean-flow distortion induced by nonlinear interactions of coherent fluctuations being treated as part of the organized motion. The system is closed by employing a gradient type of model for the time- and phase-averaged Reynolds stresses of fine-scale turbulence. In the high-Reynolds-number limit, the nonlinear non-equilibrium critical-layer theory for laminar-flow instabilities is adapted to turbulent shear layers by accounting for (1) the enhanced non-parallelism associated with fast spreading of the mean flow, and (2) the influence of small-scale turbulence on coherent structures. The combination of these factors with nonlinearity leads to an interesting evolution system, consisting of coupled amplitude and vorticity equations, in which non-parallelism contributes the so-called translating critical-layer effect. Numerical solutions of the evolution system capture vortex roll-up, which is the hallmark of a turbulent mixing layer, and the predicted amplitude development mimics the qualitative feature of oscillatory saturation that has been observed in a number of experiments. A fair degree of quantitative agreement is obtained with one set of experimental data.
Huang Z, Wu X, 2015, A non-perturbative approach to spatial instability of weakly non-parallel shear flows, PHYSICS OF FLUIDS, Vol: 27, ISSN: 1070-6631
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- Citations: 10
Dong M, Wu X, 2015, Entrainment of Short-wavelength Free-stream Vortical Disturbances into Boundary Layers, IUTAM/ABCM Symposium on Laminar Turbulent Transition, Publisher: ELSEVIER SCIENCE BV, Pages: 96-104, ISSN: 2210-9838
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- Citations: 1
Marensi E, Ricco P, Wu X, 2015, Nonlinear response of a compressible boundary layer to free-stream vortical disturbances, IUTAM/ABCM Symposium on Laminar Turbulent Transition, Publisher: ELSEVIER SCIENCE BV, Pages: 105-114, ISSN: 2210-9838
Huang Z, Wu X, 2015, Non-parallel-flow effect on compressible boundary layer on a flat plate, 7th International Conference on Fluid Mechanics, Publisher: ELSEVIER SCIENCE BV, Pages: 78-82, ISSN: 1877-7058
Butler A, Wu X, 2015, Non-Parallel-Flow Effects on Stationary Crossflow Vortices at Their Genesis, IUTAM/ABCM Symposium on Laminar Turbulent Transition, Publisher: ELSEVIER SCIENCE BV, Pages: 311-320, ISSN: 2210-9838
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- Citations: 1
Wu X, Assier RC, 2014, Linear and weakly nonlinear instability of a premixed curved flame under the influence of its spontaneous acoustic field, Journal of Fluid Mechanics, Vol: 758, Pages: 180-220, ISSN: 1469-7645
The stability of premixed flames in a duct is investigated using an asymptotic formulation, which is derived from first principles and based on high-activation-energy and low-Mach-number assumptions (Wu et al., J. Fluid Mech., vol. 497, 2003, pp. 23–53). The present approach takes into account the dynamic coupling between the flame and its spontaneous acoustic field, as well as the interactions between the hydrodynamic field and the flame. The focus is on the fundamental mechanisms of combustion instability. To this end, a linear stability analysis of some steady curved flames is undertaken. These steady flames are known to be stable when the spontaneous acoustic perturbations are ignored. However, we demonstrate that they are actually unstable when the latter effect is included. In order to corroborate this result, and also to provide a relatively simple model guiding active control, we derived an extended Michelson–Sivashinsky equation, which governs the linear and weakly nonlinear evolution of a perturbed flame under the influence of its spontaneous sound. Numerical solutions to the initial-value problem confirm the linear instability result, and show how the flame evolves nonlinearly with time. They also indicate that in certain parameter regimes the spontaneous sound can induce a strong secondary subharmonic parametric instability. This behaviour is explained and justified mathematically by resorting to Floquet theory. Finally we compare our theoretical results with experimental observations, showing that our model captures some of the observed behaviour of propagating flames.
Wu X, 2014, On the role of acoustic feedback in boundary-layer instability, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 372, ISSN: 1364-503X
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- Citations: 2
Dong M, Wu X, 2013, On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances, JOURNAL OF FLUID MECHANICS, Vol: 732, Pages: 616-659, ISSN: 0022-1120
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- Citations: 33
Luzzato CM, Assier RC, Morgans AS, et al., 2013, Modelling thermo-acoustic instabilities of an anchored laminar flame in a simple lean premixed combustor: Including hydrodynamic effects
Lean premixed combustors reduce oxide of nitrogen (NOx) emissions but are also prone to self sustained thermo-acoustic instabilities. Attempting to model these instabilities has become a popular research topic, and asymptotic based flame modelling allows us to capture all of the length scales involved in the instability. This paper presents a method for solving the full acoustic, hydrodynamic and flame coupling of an anchored laminar V-flame within a simple combustor. This allows us to model the Darrieus-Landau instability effects, and show how they influence the general behaviour and stability of the combustor when compared to G-Equation numerical results.
Wu X, 2013, The Role of Acoustic Feedback in Boundary-layer Instability, 11th International Conference of Numerical Analysis and Applied Mathematics (ICNAAM), Publisher: AMER INST PHYSICS, Pages: 300-303, ISSN: 0094-243X
Wu X, Tian F, 2012, Spectral broadening and flow randomization in free shear layers, JOURNAL OF FLUID MECHANICS, Vol: 706, Pages: 431-469, ISSN: 0022-1120
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- Citations: 10
, 2011, Nonlinear response of a laminar boundary layer to isotropic and spanwise localized free-stream turbulence, 6th AIAA Theoretical Fluid Mechanics Conference
This paper is concerned with the nonlinear response of a pre-transitional flat-plate boundary layer to isotropic and spanwise localized free-stream turbulence (FST). The turbulence is represented as a superposition of Fourier modes and the displacement effect of the boundary layer on FST is taken into consideration. The responses of the boundary layer to FST are low-frequency streamwise streaks, and their development is obtained by numerically solving the nonlinear unsteady boundary-region (NUBR) equations. Direct numerical simulations (DNS) are carried out to validate the results. Nonlinearity is stabilizing in that it reduces the root mean square (rms) of the perturbation velocity in the boundary layer for small FST Reynolds number RL11, while it is destabilizing for large RL11. The issue of upstream-downstream versus top-down mechanisms is investigated. Streaks primarily develop from the upstream forcing; the top-down forcing plays a minor role. The numerical calculations for isotropic FST are compared with DNS results of Ovchinnikov et al. and experimental data of Roach & Brierley. The computed disturbances do not reach the levels in the DNS and experiment. However, good quantitative agreement is obtained when the anisotropy of FST induced by the blunt leading edge is accounted for. The results suggest that the blunt leading edge can play a key role in explaining the large amplitudes of streaks in that it leads to the deviation from pure isotropy of the FST. The numerical calculation for spanwise localized FST is compared with experimental data of Westin et al. Agreement is obtained except for the amplitude of the disturbances, which is due to the lack of the velocity spectral information of FST in experiment. The viscous secondary instability analysis indicates that there is strong instability in the streaky boundary layer before bypass transition. The maximum growth rate of the unstable modes is larger than that of Tollmien-Schlichting (T-S) waves in the B
Wu X, 2011, On generation of sound in wall-bounded shear flows: back action of sound and global acoustic coupling, JOURNAL OF FLUID MECHANICS, Vol: 689, Pages: 279-316, ISSN: 0022-1120
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- Citations: 14
Wu X, Zhao D, Luo J, 2011, Excitation of steady and unsteady Gortler vortices by free-stream vortical disturbances, JOURNAL OF FLUID MECHANICS, Vol: 682, Pages: 66-100, ISSN: 0022-1120
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- Citations: 60
Ricco P, Luo J, Wu X, 2011, Evolution and instability of unsteady nonlinear streaks generated by free-stream vortical disturbances, JOURNAL OF FLUID MECHANICS, Vol: 677, Pages: 1-38, ISSN: 0022-1120
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- Citations: 62
Zhang Y, Zaki TA, Sherwin SJ, et al., 2011, Nonlinear response of a laminar boundary layer to isotropic and spanwise localized free-stream turbulence
This paper is concerned with the nonlinear response of a pre-transitional flat-plate boundary layer to isotropic and spanwise localized free-stream turbulence (FST). The turbulence is represented as a superposition of Fourier modes and the displacement effect of the boundary layer on FST is taken into consideration. The responses of the boundary layer to FST are low-frequency streamwise streaks, and their development is obtained by numerically solving the nonlinear unsteady boundary-region (NUBR) equations. Direct numerical simulations (DNS) are carried out to validate the results. Nonlinearity is stabilizing in that it reduces the root mean square (rms) of the perturbation velocity in the boundary layer for small FST Reynolds number RL11, while it is destabilizing for large RL11. The issue of upstream-downstream versus top-down mechanisms is investigated. Streaks primarily develop from the upstream forcing; the top-down forcing plays a minor role. The numerical calculations for isotropic FST are compared with DNS results of Ovchinnikov et al. and experimental data of Roach & Brierley. The computed disturbances do not reach the levels in the DNS and experiment. However, good quantitative agreement is obtained when the anisotropy of FST induced by the blunt leading edge is accounted for. The results suggest that the blunt leading edge can play a key role in explaining the large amplitudes of streaks in that it leads to the deviation from pure isotropy of the FST. The numerical calculation for spanwise localized FST is compared with experimental data of Westin et al. Agreement is obtained except for the amplitude of the disturbances, which is due to the lack of the velocity spectral information of FST in experiment. The viscous secondary instability analysis indicates that there is strong instability in the streaky boundary layer before bypass transition. The maximum growth rate of the unstable modes is larger than that of Tollmien-Schlichting (T-S) waves in the B
Wu X, Moin P, 2010, Large-activation-energy theory for premixed combustion under the influence of enthalpy fluctuations, JOURNAL OF FLUID MECHANICS, Vol: 655, Pages: 3-37, ISSN: 0022-1120
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- Citations: 6
Luo J, Wu X, 2010, On the linear instability of a finite Stokes layer: Instantaneous versus Floquet modes, PHYSICS OF FLUIDS, Vol: 22, ISSN: 1070-6631
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- Citations: 28
Wu X, Huerre P, 2009, Low-frequency sound radiated by a nonlinearly modulated wavepacket of helical modes on a subsonic circular jet, JOURNAL OF FLUID MECHANICS, Vol: 637, Pages: 173-211, ISSN: 0022-1120
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- Citations: 28
Wu X, Law CK, 2009, Flame-acoustic resonance initiated by vortical disturbances, JOURNAL OF FLUID MECHANICS, Vol: 634, Pages: 321-357, ISSN: 0022-1120
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- Citations: 8
Sparks CA, Wu X, 2008, Nonlinear development of subsonic modes on compressible mixing layers: a unified strongly nonlinear critical-layer theory, JOURNAL OF FLUID MECHANICS, Vol: 614, Pages: 105-144, ISSN: 0022-1120
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- Citations: 5
Wu X, Crowdy D, 2008, Theoretical fluid dynamics in the twenty-first century - Preface, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 366, Pages: 2645-2648, ISSN: 1364-503X
Wu X, Zhang J, 2008, Instability of a stratified boundary layer and its coupling with internal gravity waves. Part 1. Linear and nonlinear instabilities, Journal Fluid Mechanics, Vol: 595, Pages: 379-408
Wu X, Zhang J, 2008, Instability of a stratified boundary layer and its coupling with internal gravity waves. Part 2. Coupling with internal gravity waves via topography, Journal of Fluid Mechanics, Vol: 595, Pages: 409-433
Wu X, Stewart PA, Cowley SJ, 2008, On the catalytic effect of resonant interactions in boundary layer transition, 14th European Conference for Mathematics in Industry, Publisher: SPRINGER-VERLAG BERLIN, Pages: 146-+, ISSN: 1612-3956
Wu X, Stewart PA, Cowley SJ, 2007, On the catalytic role of the phase-locked interaction of Tollmien-Schlichting waves in boundary-layer transition, JOURNAL OF FLUID MECHANICS, Vol: 590, Pages: 265-294, ISSN: 0022-1120
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- Citations: 20
Ricco P, Wu X, 2007, Response of a compressible laminar boundary layer to free-stream vortical disturbances, 6th Euromech Fluid Mechanics Conference, Publisher: CAMBRIDGE UNIV PRESS, Pages: 97-138, ISSN: 0022-1120
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- Citations: 46
Ricco P, Wu X, 2007, Response of a compressible laminar boundary layer to free-stream vortical disturbances, Journal of Fluid Mechanics, Vol: 587, Pages: 97-138
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