Theory and application of reverberated direct and indirect noise

The generation of a temperature disturbance in a flow is accompanied by the production of acoustic waves (direct noise), and of an entropy perturbation. If this entropy perturbation is accelerated or decelerated (as is the case through a nozzle or flow restriction), additional acoustic waves are generated (indirect noise). Several studies have demonstrated this mechanism in controlled conditions by using Entropy Wave Generators, in which entropy waves are generated and convected through a nozzle, leading to direct and indirect noise. An analytical analysis of the direct and indirect noise produced by the generation and acceleration of entropy waves in a reflective environment is presented. The e ect of reverberation (repeated acoustic reflections) on low-frequency perturbations (characteristic of Entropy Wave Generators) is determined analytically. These results are then implemented in a set of limit cases, showing the limit behaviours of such systems. The analytical model is applied to the case of the Cambridge Entropy Wave Generator experiment, in which entropy waves are generated by an electric heater and accelerated through a subsonic ori ce plate. Due to the clear time separation of direct and indirect noise in the experimental results, direct and indirect noise transfer functions can be extracted from the experimental data for the rst time, and compared directly with existing theoretical models. The backward-propagating indirect noise generated at an ori ce plate is shown to be signi cantly higher than predicted by existing theoretical models for isentropic nozzles.EPSRC EP/K02924X/1

Particle interactions and lattice dynamics: Scenarios for efficient bidirectional stochastic transport?

Intracellular transport processes driven by molecular motors can be described by stochastic lattice models of self-driven particles. Here we focus on bidirectional transport models excluding the exchange of particles on the same track. We explore the possibility to have efficient transport in these systems. One possibility would be to have appropriate interactions between the various motors' species, so as to form lanes. However, we show that the lane formation mechanism based on modified attachment/detachment rates as it was proposed previously is not necessarily connected to an efficient transport state and is suppressed when the diffusivity of unbound particles is finite. We propose another interaction mechanism based on obstacle avoidance that allows to have lane formation for limited diffusion. Besides, we had shown in a separate paper that the dynamics of the lattice itself could be a key ingredient for the efficiency of bidirectional transport. Here we show that lattice dynamics and interactions can both contribute in a cooperative way to the efficiency of transport. In particular, lattice dynamics can decrease the interaction threshold beyond which lanes form. Lattice dynamics may also enhance the transport capacity of the system even when lane formation is suppressed.Comment: 25 pages, 17 figures, 2 table

Low frequency generation, transmission and reflection of direct and indirect perturbations through nozzles

Pressure perturbations arise in combustors from direct noise related to the change in density in flames, as well as the indirect (entropic) noise associated with the acceleration of non-homogeneous regions of flow through nozzles. In this paper we review our recent work on quantifying the relative contributions of direct and indirect noise generated from perturbations in temperature and composition, and the resulting transmitted and reflected pressure perturbations. We show that (a) isentropic models are inadequate to capture the acoustic and entropic transfer functions across a nozzle; (b) corrections to non-isentropic behaviour are possible using existing models for orifices using a single parameter accounting for losses; (c) the behaviour of low frequency entropic noise generated in a chamber can be entirely accounted for when reverberation is taken into account; and (d) indirect noise due to compositional fluctuations can be as large as entropic noise arising from temperature fluctuations. The findings have implications for both the study of entropy noise in model systems, as well as for understanding how to separate the origins of noise in practical systems. In particular, the role of compositional noise in gas turbines (regarding for example the role of cooling in rich-quench-lean turbines) and the role of non-isentropic effects is not accounted for in current models, and should be revisited in the light of current findings.QUALCOM

Acoustic and entropic transfer functions of a generalised subsonic nozzle

The knowledge of the acoustic and entropic transfer functions at the boundaries of combustors is crucial to understand the fate of flame-generated pressure perturbations and to predict and prevent the emergence of combustion instabilities. Traditional models often rely on the isentropic assumption for nozzle guide vanes. In real systems, however, pressure losses and local flow recirculations may occur, as evidenced by drops in the static pressure. In this work we relax the isentropic assumption and derive a parametric model to predict the acoustic and entropic transfer functions of generalised convergent-divergent nozzles with subsonic-to-sonic throat conditions in the low frequency domain. By tuning two parameters, this model can retrieve the impedance of three limit cases known from the literature: the isentropic nozzle, the orifice plate and the convergent nozzle duct termination. The generalised model also includes the conversion of entropy to sound through orifice plates and non-isentropic nozzles, yet to be considered in the literature. These analytical results are then compared with the experimental data acquired in the Cambridge Entropy Generator. The comparison highlights the need to correctly account for the losses in the system to properly explain the transfer functions of nozzles, as isentropic predictions differ substantially from the acquired experimental data.Qualcom

Measurements of the effect of boundary conditions on upstream and downstream noise arising from entropy spots

Pressure fluctuations in combustors arise either directly from the heat release rate perturbations of the flame (direct noise), or indirectly from the acceleration of entropy, vorticity or compositional perturbations through nozzles or turbine guide vanes (indirect noise). In this work, the generation of synthetic entropy spots via the Joule effect produces direct noise, and their acceleration through orifice plates and nozzles produces indirect noise. These acoustic waves reverberate, reflecting several times at the boundaries to add up to the measured pressure. Single travelling pulses are isolated by the introduction of a semiinfinite tube that acts as an anechoic termination for a limited time-window. It is shown how the shape of the converging nozzle does not affect the reflection of the direct noise wave, confirming the hypothesis of a compact nozzle. Further, it is demonstrated that the assumption of an isentropic nozzle does not hold, but that an alternative theory which takes into account the partial acoustic energy dissipation offers good agreement with the experiments. Finally, it is shown that the reflected indirect noise is underpredicted by isentropic theories. An extension of the present work is indicated for the measurement of the transmissivity of indirect noise.Francesca De Domenico is supported by the Honorary Vice-Chancellor’s Award and a Qualcomm/DTA Studentship (University of Cambridge). Erwan Rolland is supported by an EPSRC DTA studentship (University of Cambridge). Experiments were partly funded by EPSRC grant EP/K02924X/1

Administrative Law

This is the author accepted manuscript. The final version is available from Oxford University Press via the link in this recor

Weight loss and ethnicity: a cohort study of the effects induced by a very low calorie diet.

We aimed to determine whether British Asians of Indian or Pakistani descent differed in their baseline characteristics and in response to a twelve-week, very-low-calorie diet (VLCD). We then assessed whether or not a VLCD had a different effect on Asians and Caucasians, in terms of changes in weight and waist circumference. Weight loss was achieved using a nutritionally-complete VLCD with an average daily intake of 550kcal, using greater than or equal to 50g protein, greater than or equal to 50g carbohydrate, mean 15.9g fats, and greater than or equal to 100% of the recommended daily allowances (RDA) for key vitamins and minerals. The VLCD was used alongside a unique behaviour-change programme, called LighterLife (LL). Data were analysed for Asians who were recruited onto LL in 2009/2010, and for whom twelve-week weight change information was available. Waist circumference data were available for a subset of Asians, including age, body mass index and gender. These were matched to a Caucasian population and compared by independent t-test. No differences were observed between the Indian and Pakistani group for baseline measurements or weight change at twelve weeks. Caucasians had a greater percent excess body weight loss (%EBWL) than Asians at twelve weeks (72.4 ± 22.1 vs 48.9 ± 18.0, p < 0.0001). However, Asians had a greater waist circumference reduction per kilogram of weight loss when compared to Caucasians (1.16 ± 0.7 vs 0.95 ± 0.3, p = 0.037). It appears that, despite a greater %EBWL for Caucasians, Asians had a greater waist circumference reduction per kilogram of weight loss using a VLCD approach for a twelve-week period

Chaotic properties of systems with Markov dynamics

We present a general approach for computing the dynamic partition function of a continuous-time Markov process. The Ruelle topological pressure is identified with the large deviation function of a physical observable. We construct for the first time a corresponding finite Kolmogorov-Sinai entropy for these processes. Then, as an example, the latter is computed for a symmetric exclusion process. We further present the first exact calculation of the topological pressure for an N-body stochastic interacting system, namely an infinite-range Ising model endowed with spin-flip dynamics. Expressions for the Kolmogorov-Sinai and the topological entropies follow.Comment: 4 pages, to appear in the Physical Review Letter

Compositional and entropy indirect noise generated in subsonic non-isentropic nozzles

Abstract Gonville and Caius College; EPSRC/DTA studentship (University of Cambridge); Qualcomm/DTA Studentship; Royal Academy of Engineering Research Fellowships Scheme; EPSRC grant EP/K02924X/