Application of fractional calculus methods to viscoelastic behaviours of solid propellants.

A three-branch viscoelastic model based on fractional derivatives is proposed for the viscoelastic behaviours of solid propellants. The simulation results show a satisfactory agreement with the stress relaxation modulus and complex modulus of solid propellants. As a comparison, the static modulus is also characterized by traditional viscoelastic model with integer-order derivatives. Results show that the application of the fractional derivatives to the viscoelastic constitutive model can effectively reduce the number of the required parameters while giving an accurate prediction of viscoelastic behaviours of solid propellants. Moreover, a simple and effective direct search method based on simulated annealing and Powell's mothed is proposed for the data fitting. This article is part of the theme issue 'Advanced materials modelling via fractional calculus: challenges and perspectives'.

A comparison of ultrasound and magnetic resonance imaging to assess visceral fat in the metabolic syndrome.

Visceral adipose tissue (VAT) plays a key role in the metabolic syndrome. Easy detection of VAT could be an important tool to increase understanding of the metabolic syndrome. To study the relationship between the area of the inferior part of the perirenal fat (AIPPF) and anthropometric, imaging and cardiovascular risk factors of metabolic syndrome, seventy two subjects with metabolic syndrome were recruited including 44 men and 28 women (age: 26-68 yr). Each subject underwent ultrasound detection of AIPPF, intraabdominal fat thickness and magnetic resonance imaging (MRI) to calculate abdominal VAT (MRI VAT). Anthropometric and cardiovascular risk factors were also evaluated. AIPPF measured by ultrasonography demonstrated excellent reproducibility. Receiver operating characteristic analysis revealed that AIPPF has the best sensitivity for women, specificity for men and accuracy of the various measures to predict visceral obesity (MRI VAT value > or = 110 cm2) for both genders. AIPPF was related to MRI VAT, ultrasound measured intraabdominal fat, waist circumference, the ratio of waist and hip circumferences (of men), body mass index and the main cardiovascular risk factors of metabolic syndrome. Multiple stepwise linear regression analysis suggested that MRI VAT affected AIPPF independent of other investigated obesity indices. This study showed that AIPPF could be applied as an easy and reliable imaging indicator of visceral obesity and cardiovascular risk factors in the metabolic syndrome.