Dissipation of vibration in rough contact

The relationship which links the normal vibration occurring during the sliding of rough surfaces and the nominal contact area is investigated. Two regimes are found. In the first one, the vibrational level does not depend on the contact area, while in the second one, it is propor- tional to the contact area. A theoretical model is proposed. It is based on the assumption that the vibrational level results from a competition between two processes of vibration damping, the internal damping of the material and the contact damping occurring at the interface

Modelling digestible phosphorus content of salmonid fish feeds

Accurate estimation of digestible phosphorus (P) content of fish feeds is essential to formulating feeds that meet nutritional requirements of fish but minimize P waste output from fish culture operations. Phosphorus is a component of different chemical compounds in feeds. Differences in chemical characteristics of these compounds may result in different P digestion dynamics. A model to estimate digestible P content of salmonid fish feed based on levels of different P types was constructed by integrating data from 22 studies. Phosphorus types present in feed ingredients were classified into broad chemical categories: bone-P, phytate-P, organic P, Ca monobasic/Na/K Pi supplement, and Ca dibasic Pi supplement. The relationship between digestible P content of feeds and various P chemical compound contents was modelled through a multiple regression approach. Multiple regression analysis yielded the following model: digestible P=0.68 bone-P+0 phytate-P+0.84 organic P+0.89 Ca monobasic/Na/K Pi supplement+0.64 Ca dibasic Pi supplement+0.51 phytase/phytate-0.02 (phytase/phytate)(2)-0.03 (bonep)(2)-0.14 bone-P*CamonobasicNa/K Pi supplement (P < 0.0001, R-2 = 0.96). The results suggest that the digestibility of different P types differ significantly and the digestibility of bone-P is not additive. A subsequent digestibility trial validated the model and suggested that the model accurately predicted digestible P content of diets formulated with a wide variety of ingredients commonly used in fish feeds

Quantification of differences in digestibility of phosphorus among cyprinids, cichlids, and salmonids through a mathematical modelling approach

Accurate estimates of phosphorus (P) digestibility are essential to enable the formulation of cost-effective diets meeting P requirement of fish but resulting in minimal P waste outputs. Evidence suggests that differences in P digestibility exist among fish species but they have not been quantified. A modelling approach was employed in this study to quantify these differences. As a first step, an existing mathematical model of P digestibility developed for salmonids was statistically evaluated to determine if this model could be extrapolated to predict digestible P content in feeds of two widely cultured groups of freshwater fish species, tilapias (cichlids species) and carps (cyprinid fish species) or if modification was required. Subsequently, two novel mathematical models specific for tilapias and carps were developed. Results suggest that significant differences in P apparent digestibility exist among cyprinids, cichlids, and salmonids. Cyprinid species appear to have low ability to digest P compounds of low solubility. They do not appear to be able to effectively digest bone P (digestibility of bone P was estimated to be nil), and their ability to digest dibasic calcium phosphates is low (30%) in comparison to that of cichlids and salmonids (which is in the ranges of 62-64%). These differences could be attributable to the absence of an acid stomach in cyprinids. Cyprinids and salmonids appear to be unable to digest phytate-P from plant ingredients, while tilapias seem to be able to digest phytate-P to a certain degree (approximately 27%). These mathematical models could be useful tools to estimate the digestible phosphorus content of feeds formulated with a wide variety of ingredients fed to fish species with different digestive anatomy and physiology