On the Acoustical Dynamics of Bubble Clouds

Recently, Morch [1,2,3,4] Chahine [5,6] and others have focused attention on the dynamics of a cloud or cluster of cavitating bubbles and have expanded on the work of van Wijngaarden [7,8] and others. Unfortunately, there appear to be a number of inconsistencies in this recent work which will require further study before a coherent body of knowledge on the dynamics of clouds of bubbles is established. For example, Morch and his co-workers [1,2,3] have visualized the collapse of a cloud of cavitating bubbles as involving the inward propagation of a shock wave; it is assumed that the bubbles collapse virtually completely when they encounter the shock. This implies the virtual absense of non-condensable gas in the bubbles and the predominance of vapor. Yet in these circumstances the mixture in the the cloud will not have any real sonic speed. As implied by a negative L.H.S. of equation (9), the fluid motion equations for the mixture would be elliptic not hyperbolic and hence shock wave solutions are inappropriate

A local composition model for the prediction of mutual diffusion coefficients in binary liquid mixtures from tracer diffusion coefficients

In a recent publication (Moggride, 2012a), a simple equation was shown to accurately predict the mutual diffusion coefficients for a wide range of non-ideal binary mixtures from the tracer diffusion coefficients and thermodynamic correction factor, on the physical basis that the dynamic concentration fluctuations in the liquid mixture result in a reduction of the mean thermodynamic correction factor relative to the hypothetical case in which such fluctuations do not occur. The analysis was extended to cases where strong molecular association was hypothesised to occur in the form of dimerization of a polar species in mixtures with a non-polar one. This required modification of the average molecular mobility in the form of doubling the tracer diffusivity of the dimerized species (Moggridge, 2012b). Predictions were found to show good accuracy for the mixtures investigated. One of the difficulties with this approach is that it is an a posteriori correction: there is no a priori way of knowing whether strong cluster formation influences the observed molecular mobility, or what the appropriate size of the cluster is. In this work, a modification is made to the average molecular mobility in the original equation by replacing the bulk mole fraction with local mole fraction calculated using the NRTL (non-random two liquid) model, to take account of strong molecular association that results in highly correlated movement during diffusion. The new equation enables an accurate description of mutual diffusion coefficients in mixtures of one strongly self-associating species and one non-polar species, as well as in non-ideal, non-associating mixtures. This result is significant because in this way there is no need of any prior knowledge on the degree of molecular association in the mixture for the prediction of mutual diffusion coefficients from tracer diffusivities.Carmine D’Agostino would like to acknowledge Wolfson College, University of Cambridge, for supporting his research activities.This is the accepted manuscript. The final version is available at http://www.sciencedirect.com/science/article/pii/S0009250915002821

Palmitoylethanolamide is a disease-modifying agent in peripheral neuropathy : pain relief and neuroprotection share a PPAR-alpha-mediated mechanism

Peer reviewedPublisher PD

Influence of the rainfall measurement interval on the erosivity determinations in the Mediterranean area

The single-storm erosion index, EI, of the USLE and RUSLE models may vary appreciably with the rainfall measurement interval, Dt. However, the effect of Dt on EI has not been investigated in the Mediterranean area. Approximately 700 erosive events and 1.5 years of rainfall energies measured by a rainfall impact measurement device were used to evaluate the effect of the rainfall measurement interval (5 min 6 Dt 6 60 min) on the erosivity determinations in the Mediterranean semi-arid area of Sicily. According to both literature and practical considerations, a reference time interval equal to 15 min was used in this investigation. Hourly rainfall data led to an appreciable underestimation of the mean value of EI (i.e., by also a factor of two, depending on the location). In the range 5 min 6 Dt 6 15 min, the effect of the rainfall measurement interval on the predicted erosivity was negligible (i.e., mean values differing by a maximum factor of 1.10) as compared with the uncertainties in the soil loss predictions. Two methods were developed for estimating the reference single-storm erosion index, (EI)15, from hourly rainfall data in Sicily. Method 1 converts the erosion index calculated on a 60- min measurement interval basis to (EI)15. Method 2 estimates (EI)15 by using the storm rainfall depth and the maximum rainfall intensity. Testing the two methods against two independent data sets produced a maximum difference between the estimated and the calculated mean values of (EI)15 equal to 7% for method 1 and 11% for method 2. Both methods may be applied in practice, depending on the available rainfall data. For a given rainfall intensity, the specific power, P, measured at eight time intervals (5 min 6 Dt 6 60 min) was in the range ±10% of the mean of the eight P values

Size and asymmetry of the reaction entrance channel: influence on the probability of neck production

The results of experiments performed to investigate the Ni+Al, Ni+Ni, Ni+Ag reactions at 30 MeV/nucleon are presented. From the study of dissipative midperipheral collisions, it has been possible to detect events in which Intermediate Mass Fragments (IMF) production takes place. The decay of a quasi-projectile has been identified; its excitation energy leads to a multifragmentation totally described in terms of a statistical disassembly of a thermalized system (T$\simeq$4 MeV, E$^*\simeq$4 MeV/nucleon). Moreover, for the systems Ni+Ni, Ni+Ag, in the same nuclear reaction, a source with velocity intermediate between that of the quasi-projectile and that of the quasi-target, emitting IMF, is observed. The fragments produced by this source are more neutron rich than the average matter of the overall system, and have a charge distribution different, with respect to those statistically emitted from the quasi-projectile. The above features can be considered as a signature of the dynamical origin of the midvelocity emission. The results of this analysis show that IMF can be produced via different mechanisms simultaneously present within the same collision. Moreover, once fixed the characteristics of the quasi-projectile in the three considered reactions (in size, excitation energy and temperature), one observes that the probability of a partner IMF production via dynamical mechanism has a threshold (not present in the Ni+Al case) and increases with the size of the target nucleus.Comment: 16 pages, 7 figures, accepted for publication on Nuclear Physics

On the linearized dynamics of two-dimensional bubbly flows over wave-shaped surfaces

In the last decades the technological importance or bubbly flows has generated considerable efforts to achieve a better understanding of their properties, [1],[2]. However, the presence or two interacting phases so much increases the complexity or the problem that a satisfactory mathematical model of these flows has been possible only in special cases under fairly restrictive simplifying assumptions. The main purpose of the present note is to investigate the effects due to the inclusion or bubble dynamic response in two-dimensional flows over wave-shaped surfaces. The earlier studies of bubbly flows based on space averaged equations for the mixture in the absence or relative motion between the two phases, [5], [6], do not consider bubble dynamic effects. This approach simply leads to an equivalent compressible homogeneous medium and has been used to analyze the behaviour or one-dimensional bubbly flows through converging-diverging nozzles. In order to account for bubble dynamic response, in a classical paper by Foldy, [7], each individual bubble is described as a randomly distributed point scatterer. Assuming that the system is ergodic, the collective effect of bubble dynamic response on the flow is then obtained by taking the ensemble average over all possible configurations. An alternative way to account for bubble dynamic effects would be to include the Rayleigh-Plesset equation in the space averaged equations. Both methods have been successfully applied to describe the propagation or one-dimensional perturbances through liquids containing small gas bubbles, [8], [9], [10], [11]. However, because of their complexity, there are not many reported examples of the application to specific flow geometries of the space averaged equations which include the effects of bubble response, [12]. In an earlier note, [13], we considered the one-dimensional time dependent linearized dynamics or a spherical cloud of bubbles. The results clearly show that the motion of the cloud is critically controlled by bubble dynamic effects. Specifically, the dominating phenomenon consists of the combined response of the bubbles to the pressure in the surrounding liquid, which results in volume changes leading to a global accelerating velocity field. Associated with this velocity field is a pressure gradient which in turn determines the pressure encountered by each individual bubble in the mixture. Furthermore, it can be shown that such global interactions usually dominate any pressure perturbations experienced by one bubble due to the growth or collapse or a neighbor (see section 5). In the present note the same approach is applied to the two-dimensional case or steady flows over wave-shaped surfaces (for which there exist well established solutions for compressible and incompressible flow), With the aim, as previously stated, of assessing the effects due to the introduction or bubble dynamic response. Despite its intrinsic limitations, the following linear analysis indicates some of the fundamental phenomena involved in such flows and provides a useful basis for the study of the same flows with non-linear bubble dynamics, which we intend to discuss in a later publication. The present extention to the case of bubbly flows over arbitrarily shaped surfaces also constitutes the starting point for the investigation or such flows, a problem of considerable technical interest, for example in cavitating flows past lifting surfaces

Estimate of average freeze-out volume in multifragmentation events

An estimate of the average freeze-out volume for multifragmentation events is presented. Values of volumes are obtained by means of a simulation using the experimental charged product partitions measured by the 4pi multidetector INDRA for 129Xe central collisions on Sn at 32 AMeV incident energy. The input parameters of the simulation are tuned by means of the comparison between the experimental and simulated velocity (or energy) spectra of particles and fragments.Comment: To be published in Phys. Lett. B 12 pages, 5 figure

The 2009 L' Aquila Earthquake (Central Italy): a source mechanism and implications for seismic hazard

We use InSAR and body-wave seismology to determine independent source\ud parameters for the 6th April 2009 Mw 6.3 L'Aquila earthquake and con¯rm\ud that the earthquake ruptured a SW-dipping normal fault with »0.6{0.8 m\ud slip. The causative Paganica fault had been neglected relative to other nearby\ud range-frontal faults, partly because it has a subdued geomorphological ex-\ud pression in comparison with these faults. The L'Aquila earthquake occurred\ud in an area with a marked seismic de¯cit relative to geodetically determined\ud strain accumulation. We use our source model to calculate stress changes on\ud nearby faults produced by the L'Aquila earthquake and we ¯nd that several\ud of these faults have been brought closer to failure

Assessing the effect of reducing agents on the selective catalytic reduction of NO<inf>x</inf> over Ag/Al<inf>2</inf>O<inf>3</inf> catalysts

The selective catalytic reduction (SCR) of NOx in the presence of different reducing agents over Ag/Al2O3 prepared by wet impregnation was investigated by probing catalyst activity and using NMR relaxation time analysis.We gratefully acknowledge funding for this work from the EPSRC CASTech grant (EP/G012156/1). Carmine D’Agostino would like to acknowledge Wolfson College, Cambridge, for supporting his research activities. The authors would also like to thank Dr Jonathan Mitchell for useful discussions.This is the final version of the article. It first appeared from RSC via http://dx.doi.org/10.1039/C5CY01508