Modes of Transport to School and Their Associations with Weight Status: A Cross-Sectional Survey of Students in Shanghai, China

Background: Over the past two decades, both transport modes as well as overweight/obesity have changed dramatically among students in China, but their relationships are not clear. This study aimed to investigate modes of transport to school and their associations with the weight status of Chinese students. Methods: A cross-sectional study was conducted with non-resident students aged 6 to 17 years from all 16 districts across Shanghai, China in October and November 2019. Information about sociodemographic characteristics and the models of travel to school among students was investigated using an online, self-administered, structured questionnaire (or those assisted by their parents). Weight and height were measured by school health workers, and the Chinese standard age adjusted BMI (weight/height2) was used to classify students’ weight status. Cumulative logistic regression modelling was used to examine the relationships. Results: The main mode of transport to school was an active mode (46.5%, defined as walking, bicycling, or public transport), followed by an inactive mode of transport (30.5%, defined as a car or bicycle as a passenger), and a combination of both modes (23%). About one-third of the students were overweight or obese and 5% were underweight. No statistically significant association between transport modes and weight status was found in this study. Conclusions: In Shanghai, close to one-third of children travel to school by an inactive mode of transport. The findings of this study did not support the notion that an active mode to school could be beneficial for preventing overweight/obesity in students in China.

An Experimental Study on the Damping of a Steam Generator Tube With Gap Supports in Air and Water

In this study, we present an analysis and experiment on the fluid damping of a Steam Generator (SG) tube with one and several plate-type supports that are submerged under water. For the damping measurement, a short tube of 2.2 meters in length with one gap support as well as a long tube of up to 10 meters with 11 and 13 supports are used. To see a fluid effect through an analytical approach, Mulcahy’s theory on a tube with finite-length gap support and Pettigrew’s empirical correlation for SG design were reviewed. The theoretical model is good at predicting the fluid damping at the first mode in a low frequency range which is an inactive mode at the support, while the practical model is much better at the higher modes, which are active modes, rocking modes, at the support(s). Experimental results from the short length tube with single support shows good agreement with a previous measurement by other researchers. A theoretical model and empirical correlation are in good agreement. Our measurements from a longer tube with several supports are reasonable compared with the others. There is, however, a wide discrepancy between predictions by the two models. As compared with the whole measurements, the theoretical model seems to be a good guideline in a low frequency range, while an empirical model is relatively good guideline in a high frequency range. Based on the whole collected data, a new correlation modified from Pettigrew’s correlation is suggested.

Experiment of Chaotic Vibration of Loosely Supported Tube Rows in Cross-Flow

A tube array supported by baffle plates in cross-flow may be subjected to fluidelastic instability in the tube-support-plate-inactive mode. An experimental study is presented to characterize the tube motion. Three series of tests were performed to measure tube displacements as a function of flow velocity for differing clearances. The motion was examined by root-mean-square values of tube displacements, power spectral densities, phase planes, Poincare´ maps, and Lyapunov exponents. The experimental data agree reasonably well with an analytical model based on the unsteady flow theory.

Chaotic Vibrations of Nonlinearly Supported Tubes in Crossflow

Chaotic vibrations associated with the fluidelastic instability of nonlinearly supported tubes in a crossflow is studied theoretically on the basis of the unsteady-flow theory and a bilinear mathematical model. Effective tools, including phase portraits, power spectral density, Poincare maps, Lyapunov exponent, fractal dimension, and bifurcation diagrams, are utilized to distinguish periodic and chaotic motions when the tubes vibrate in the instability region. The results show periodic and chaotic motions in the region corresponding to fluid-damping-controlled instability. Nonlinear supports, with symmetric or asymmetric gaps, significantly affect the distribution of periodic, quasi-periodic, and chaotic motions of a tube exposed to various flow velocities in the instability region of the tube-support-plate-inactive mode.

A Theory for Fluidelastic Instability of Tube-Support-Plate-Inactive Modes

Fluidelastic instability of loosely supported tubes, vibrating in a tube support plate (TSP)-inactive mode, is suspected to be one of the main causes of tube failure in some operating steam generators and heat exchangers. This paper presents a mathematical model for fluidelastic instability of loosely supported tubes exposed to nonuniform cross flow. The model incorporates all motion-dependent fluid forces based on the unsteady-flow theory. In the unstable region associated with a TSP-inactive mode, tube motion can be described by two linear models: TSP-inactive mode when tubes do not strike the TSP, and TSP-active mode when tubes do strike the TSP. The bilinear model (consisting of these linear models) presented here simulates the characteristics of fluidelastic instability of loosely supported tubes in stable and unstable regions associated with TSP-inactive modes. Analytical results obtained with the model are compared with published experimental data; they agree reasonably well. The prediction procedure presented for the fluidelastic instability response of loosely supported tubes is applicable to the stable and unstable regions of the TSP-inactive mode.

Dynamic Characteristics of Heat Exchanger Tubes Vibrating in a Tube Support Plate Inactive Mode

Tubes in shell-and-tube heat exchangers, including nuclear plant steam generators, derive their support from longitudinally positioned tube support plates (TSPs). Typically, there is a clearance between the tube and TSP hole. Depending on design and fabrication tolerances, the tube may or may not contact all of the TSPs. Non-contact results in an inactive TSP which can lead to detrimental flow-induced tube vibrations under certain conditions dependent on the resulting tube-TSP interaction dynamics and the fluid excitation forces. The purpose of this study is to investigate the tube-TSP interaction dynamics. Results of an experimental study of damping and natural frequency as functions of tube-TSP diametral clearance and TSP thickness are reported. Calculated values of damping ratio and frequency of a tube vibrating within an inactive TSP are also presented together with a comparison of calculated and experimental quantities.