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Cambridge, United Kingdom

Juniper M.P.,Trumpington Street
50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition | Year: 2012

Under certain conditions, the flow in a combustion chamber can sustain large amplitude oscillations even when its steady state is linearly stable. Experimental studies show that these large oscillations can sometimes be triggered by very low levels of background noise. This theoretical paper sets out the conditions that are necessary for triggering to occur. It uses a weakly nonlinear analysis to show when these conditions will be satisfied if the heat release rate is a function of the acoustic velocity amplitude. The role played by non-normality is investigated. It is shown that, when a state triggers to sustained oscillations from the lowest possible energy, it exploits transient energy growth around an unstable limit cycle. The positions of these limit cycles in state space is determined by nonlinearity, but the tangled-ness of trajectories in state space is determined by non-normality. When viewed in this dynamical systems framework, triggering in thermoacoustics is seen to be directly analogous to bypass transition to turbulence in pipe flow. Copyright © 2012 by Matthew P Juniper. Source


Longo S.,University of Parma | Liang D.,Trumpington Street | Chiapponi L.,University of Parma | Aguilera Jimenez L.,University of Granada
Coastal Engineering | Year: 2012

This paper is the third part of a report on systematic measurements and analyses of wind-generated water waves in a laboratory environment. The results of the measurements of the turbulent flow on the water side are presented here, the details of which include the turbulence structure, the correlation functions, and the length and velocity scales. It shows that the mean turbulent velocity profiles are logarithmic, and the flows are hydraulically rough. The friction velocity in the water boundary layer is an order of magnitude smaller than that in the wind boundary layer. The level of turbulence is enhanced immediately beneath the water surface due to micro-breaking, which reflects that the Reynolds shear stress is of the order u *w 2. The vertical velocities of the turbulence are related to the relevant velocity scale at the still-water level. The autocorrelation function in the vertical direction shows features of typical anisotropic turbulence comprising a large range of wavelengths. The ratio between the microscale and macroscale can be expressed as λ/Λ=a Re Λ n, with the exponent n slightly different from -1/2, which is the value when turbulence production and dissipation are in balance. On the basis of the wavelength and turbulent velocity, the free-surface flows in the present experiments fall into the wavy free-surface flow regime. The integral turbulent scale on the water side alone underestimates the degree of disturbance at the free surface. © 2012 Elsevier B.V. Source


Campbell A.M.,Trumpington Street
Superconductor Science and Technology | Year: 2011

This paper describes experiments and theories of flux cutting in superconductors. The use of the flux line picture in free space is discussed. In superconductors cutting can either be by means of flux at an angle to other layers of flux, as in longitudinal current experiments, or due to shearing of the vortex lattice as in grain boundaries in YBCO. Experiments on longitudinal currents can be interpreted in terms of flux rings penetrating axial lines. More physical models of flux cutting are discussed but all predict much larger flux cutting forces than are observed. Also, cutting is occurring at angles between vortices of about one millidegree which is hard to explain. The double critical state model and its developments are discussed in relation to experiments on crossed and rotating fields. A new experiment suggested by Clem gives more direct information. It shows that an elliptical yield surface of the critical state works well, but none of the theoretical proposals for determining the direction of E are universally applicable. It appears that, as soon as any flux flow takes place, cutting also occurs. The conclusion is that new theories are required. © 2011 IOP Publishing Ltd. Source


Booth E.,Edmund Booth Consulting Engineer | Saito K.,Cambridge Architectural Research Ltd. | Spence R.,Cambridge Architectural Research Ltd. | Madabhushi G.,Trumpington Street | Eguchi R.T.,ImageCat Inc.
Earthquake Spectra | Year: 2011

Assessments of damage following the 2010 Haitian earthquake were validated by comparing three datasets. The first, for 107,000 buildings, used vertical aerial images with a 15-25 cm spatial resolution. The second, for 1,241 buildings, used Pictometry images (oblique angle shots with a resolution of about 10 cm taken in four directions by aircraft). The third dataset, for 142 buildings, used ground observations. The ground observations confirmed the tendency of remote sensing to underestimate the proportion of heavily damaged and collapsed buildings, and the difficulty of making remote assessments of moderate or low damage. Bayesian statistics and sample surveys made from Pictometry images and ground observations were used to improve remote damage assessments from vertical images. The possibility of developing standard factors to correct remote assessments is discussed. The field exercise pointed to the need to produce an internationally agreed-upon set of damage definitions, suitable for postdisaster needs assessments as well as for other uses. © 2011, Earthquake Engineering Research Institute. Source


Bennett S.,Trumpington Street | Asinari P.,Polytechnic University of Turin | Dellar P.J.,Mathematical Institute
International Journal for Numerical Methods in Fluids | Year: 2012

This paper describes the development of a lattice Boltzmann (LB) model for a binary gas mixture, and applications to channel flow driven by a density gradient with diffusion slip occurring at the wall. LB methods for single component gases typically use a non-physical equation of state in which the relationship between pressure and density varies according to the scaling used. This is fundamentally unsuitable for extension to multi-component systems containing gases of differing molecular masses. Substantial variations in the species densities and pressures may exist even at low Mach numbers; hence, the usual linearized equation of state for small fluctuations is unsuitable. Also, existing methods for implementing boundary conditions do not extend easily to novel boundary conditions, such as diffusion slip. The new model developed for multi-component gases avoids the pitfalls of some other LB models. A single computational grid is shared by all the species, and the diffusivity is independent of the viscosity. The Navier-Stokes equation for the mixture and the Stefan-Maxwell diffusion equation are both recovered by the model. Diffusion slip, the non-zero velocity of a gas mixture at a wall parallel to a concentration gradient, is successfully modelled and validated against a simple one-dimensional model for channel flow. To increase the accuracy of the scheme, a second-order numerical implementation is needed. This may be achieved using a variable transformation method that does not increase the computational time. Simulations were carried out on hydrogen and water diffusion through a narrow channel for varying total pressure and concentration gradients. © 2011 John Wiley & Sons, Ltd. Source

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