Acoustic radiation from the end of two-dimensional duct, effects of uniform flow and duct lining

A study is presented of the radiation of acoustic modes from the end of a duct immersed in a uniformly moving medium. It is shown that the uniform flow has roughly the same effect as an increase in frequency at constant mode number: the number of lobes of the radiation pattern increases, and the radiation maximum is slightly displaced towards the duct axis. When the mode is near cut-off the forward radiation for an inlet is enhanced. The acoustic radiation characteristics of ducts with soft or absorbing walls and hard, perfectly-reflecting walls are then compared. It is shown, and this is of technological interest, that the side radiation from the end of an acoustically soft duct is greatly reduced for lower-order modes

Acoustic transmission and reflection by a shear discontinuity separating hot and cold regions

Acoustic transmission and reflection is analyzed for plane waves propagating from a hot moving medium, impinging on a plane shear discontinuity into a cold stationary region. It is shown that incident waves originating in the hot region and propagating in the flow direction are transmitted into the cold region at almost right angles to the interface. The result is employed to examine the strong side radiation of internal noise transmitted through the exhaust duct of a turbojet engine

Acoustic Attenuation in Fans and Ducts by Vaporization of Liquid Droplets

A cloud of small water droplets in saturated air attenuates acoustic disturbances by viscous drag, heat transfer, and vapor exchange with the ambient gas. The viscous and heat transfer phenomena attenuate at frequencies above 104 Hz for I-J.l droplets. The processes associated with phase exchange attenuate at a much lower frequency that may he controlled by choice of the liquid mass fraction. The strength of this attenuation is proportional to the mass of water vapor in the air, a factor controlled by air temperature. For plane waves, the attenuation magnitude e~ceeds 5 db!m ~t a temperature of 25°C with a cloud of 0.7 J.l radius droplets constituting 1 % of the gas mass. ThiS attenuation mcreases to more than 7 dbjm at frequencies above 1000 Hz where viscous and heat transfer mechanisms contribute significantly. The attenuation of higher order duct modes is strongly increased over the above values, similarly to the attenuation by duct lining. When the droplet cloud occupies only a fraction of the duct height close to the walls, the droplet clond may be up to twice as elfective as the uniform cloud, and a significant saving is possible in the water required to saturate the air and furnish the water droplets

Preliminary Water Assessment Reports of The Test Basins of The Watch Project

This report presents the initial plans of the case studies how they link to rest of the Watch project and on which water resources they will focus. This report will function as the basis for further discussions on how to improve the integration of the case studies within the project and to develop a more general protocol for each of the case studies. Currently 5 catchments are used within the Watch project, they differ in climatic and hydro-geological features and expected climate changes: the Glomma River basin (Eastern Norway), the upper Guadiana basin (Central Spanish Plateau), the Nitra River basin (central Slovakia), the Upper-Elbe basin (part of the Elbe River) and the island of Crete. Also the water resources issues vary over these cases. Agricultural (and domestic) water use is under pressure in the Mediterranean catchments probably aggravating with the expected increase in drought frequency under future climate. The Norwegian catchment provides hydropower services under threat of precipitation increase rather than decrease. The central European catchments are threatened mainly by increased variability, i.e. increased frequencies of extremes in a densely populated environment, and river flow may need additional buffers (reservoirs) to reduce floodrisk and store water for dry period

Experimental and theoretical investigation of chiral separation by crystallisation

Chiral molecules often show different pharmacological and toxicological properties, making their separation crucial for pharmaceutical companies. The resolution of racemic mixtures is often achieved via crystallisation methods. The lack of experimental data has been a major constraint in validating proposed computational methods for aiding the design of crystallisation processes for chiral resolution. This thesis provides both structural and thermodynamic data, and uses it to assess the limitations of current computer modelling methods. Progress in computational methods might eventually result in the design of resolving agents and hence reduce production costs of drugs and fine chemicals. Previous studies of naproxen have concentrated on the marketed enantiopure form of this anti-inflammatory drug. A crystallisation screen was conducted to identify all possible crystal phases of racemic and enantiopure naproxen. No polymorphs were detected and the crystal structure of the racemic compound was solved from powder X-ray diffraction data. The nature of the racemic species was confirmed with thermal methods, and differential scanning calorimetric and solubility measurements were used to estimate the enthalpy difference between the crystals at 156 °C and in the range of 10 to 40 °C. These data were used to test the different approximations involved in determining the energy differences between the racemic and enantiopure crystals. An extensive crystallisation screen was also performed for (1R,2S)-ephedrine 2-phenylpropionate salts. The crystal structure of the least soluble salt and three polymorphs of the most soluble salt were determined by low temperature single crystal X-ray diffraction or powder X-ray diffraction. Solubility measurements and differential scanning calorimetry were used to determine the relative stability of the salt pairs and polymorphs. These results showed the inadequacies of lattice energy calculations of the diastereomeric salt pair and their polymorphs. Experimental work on related diastereomeric salt pairs emphasised the difficulty in fully structurally and thermodynamically characterising these systems

The dynamics of quasi-isometric foliations

If the stable, center, and unstable foliations of a partially hyperbolic system are quasi-isometric, the system has Global Product Structure. This result also applies to Anosov systems and to other invariant splittings. If a partially hyperbolic system on a manifold with abelian fundamental group has quasi-isometric stable and unstable foliations, the center foliation is without holonomy. If, further, the system has Global Product Structure, then all center leaves are homeomorphic.Comment: 18 pages, 1 figur

Flame Describing Function analysis of spinning and standing modes in an annular combustor and comparison with experiments

This article reports a numerical analysis of combustion instabilities coupled by a spinning mode or a standing mode in an annular combustor. The method combines an iterative algorithm involving a Helmholtz solver with the Flame Describing Function (FDF) framework. This is applied to azimuthal acoustic coupling with combustion dynamics and is used to perform a weakly nonlinear stability analysis yielding the system response trajectory in the frequency-growth rate plane until a limit cycle condition is reached. Two scenarios for mode type selection are tentatively proposed. The first is based on an analysis of the frequency growth rate trajectories of the system for different initial solutions. The second consid- ers the stability of the solutions at limit cycle. It is concluded that a criterion combining the stability analysis at the limit cycle with the trajectory analysis might best define the mode type at the limit cy- cle. Simulations are compared with experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global FDF and it is considered that the spacing between burners is such that coupling with the mode takes place without mutual interac- tions between adjacent burning regions. Depending on the nature of the mode being considered, two hypotheses are made for the FDFs of the burners. When instabilities are coupled by a spinning mode, each burner features the same velocity fluctuation level implying that the complex FDF values are the same for all burners. In case of a standing mode, the sixteen burners feature different velocity fluctua- tion amplitudes depending on their relative position with respect to the pressure nodal line. Simulations retrieve the spinning or standing nature of the self-sustained mode that were identified in the exper- iments both in the plenum and in the combustion chamber. The frequency and amplitude of velocity fluctuations predicted at limit cycle are used to reconstruct time resolved pressure fluctuations in the plenum and chamber and heat release rate fluctuations at two locations. For the pressure fluctuations, the analysis provides a suitable estimate of the limit cycle oscillation and suitably retrieves experimental data recorded in the MICCA setup and in particular reflects the difference in amplitude levels observed in these two cavities. Differences in measured and predicted amplitudes appear for the heat release rate fluctuations. Their amplitude is found to be directly linked to the rapid change in the FDF gain as the velocity fluctuation level reaches large amplitudes corresponding to the limit cycle, underlying the need of FDF information at high modulation amplitudes