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Fu Y.B.,Keele University | Fu Y.B.,Tianjin University | Xie Y.X.,Tianjin University | Xie Y.X.,Tianjin Key Laboratory of Modern Engineering Mechanics
Journal of the Mechanics and Physics of Solids | Year: 2014

It is well-known that for most spherical rubber balloons the pressure versus volume curve associated with uniform inflation is N-shaped (the pressure increases rapidly to a maximum, falls to a minimum, and subsequently increases monotonically), and that somewhere along the descending branch of this curve the spherical shape may bifurcate into a pear shape through localized thinning near one of the poles. The bifurcation is associated with the (uniform) surface tension reaching a maximum. It is previously known that whenever a pear-shaped configuration becomes possible, it has lower energy than the co-existing spherical configuration, but the stability of the pear-shaped configuration itself is unknown. With the use of the energy stability criterion, it is shown in this paper that the pear-shaped configuration is unstable under pressure control, but stable under mass control. Our calculations are carried out using the Ogden material model as an example, but it is expected that the qualitative stability results should also be valid for other material models that predict a similar N-shaped behavior for uniform inflation. © 2014 Elsevier Ltd.

Fu Y.B.,Keele University | Xie Y.X.,Tianjin University | Xie Y.X.,Tianjin Key Laboratory of Modern Engineering Mechanics
International Journal of Engineering Science | Year: 2010

We study the stability of localized bulging solutions in an inflated hyperelastic membrane tube using an energy stability criterion. We first use this criterion to confirm a previously known result obtained using a dynamic stability criterion, namely that under pressure control all such bulging solutions are unstable. It is then shown that, under volume control, the solutions in the early stages of bulging are unstable whereas those in the later stages of bulging are stable. To be more precise, it is found that the unstable solutions correspond exactly to the snap-back section of the associated pressure-volume diagram. © 2010 Elsevier Ltd. All rights reserved.

Tang Z.,Tianjin University | Jiang N.,Tianjin University | Jiang N.,Tianjin Key Laboratory of Modern Engineering Mechanics
Science China: Physics, Mechanics and Astronomy | Year: 2012

We present dynamic mode decomposition (DMD) for studying the hairpin vortices generated by hemisphere protuberance measured by two-dimensional (2D) time-resolved (TR) particle image velocimetry (PIV) in a water channel. The hairpins dynamic information is extracted by identifying their dominant frequencies and associated spatial structures. For this quasi-periodic data system, the resulting main Dynamic modes illustrate the different spatial structures associated with the wake vortex region and the near-wall region. By comparisons with proper orthogonal decomposition (POD), it can be concluded that the dynamic mode concentrates on a certain frequency component more effectively than the mode determined by POD. During the analysis, DMD has proven itself a robust and reliable algorithm to extract spatial-temporal coherent structures. © Science China Press and Springer-Verlag Berlin Heidelberg 2011.

Hong S.C.,Tianjin University | Hong S.C.,Tianjin Key Laboratory of Modern Engineering Mechanics
Science China: Physics, Mechanics and Astronomy | Year: 2012

The transition criterion in the improved eN method is that transition would occur whenever the velocity amplitude of disturbance reaches 1%-2% of the free stream velocity, while in the conventional eN method, the N factor is an empirical factor. In this paper the reliability of this key assumption in the improved eN method is checked by results of transition prediction by using the Parabolized Stability Equations (PSE). Transition locations of an incompressible boundary layer and a hypersonic boundary layer at Mach number 6 on a flat plate are predicted by both the improved eN method and the PSE method. Results from both methods agree fairly well with each other, implying that the transition criterion proposed in the improved eN method is reliable. © Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Yang S.,Tianjin University | Jiang N.,Tianjin University | Jiang N.,CAS Institute of Mechanics | Jiang N.,Tianjin Key Laboratory of Modern Engineering Mechanics
Science China: Physics, Mechanics and Astronomy | Year: 2012

In this paper, we calculated the spatial local-averaged velocity strains along the streamwise direction at four spatial scales according to the concept of spatial local-averaged velocity structure function by using the three-dimensional three-component database of time series of velocity vector field in the turbulent boundary layer measured by tomographic time-resolved particle image velocimetry. An improved quadrant splitting method, based on the spatial local-averaged velocity strains together with a new conditional sampling phase average technique, was introduced as a criterion to detect the coherent structure topology. Furthermore, we used them to detect and extract the spatial topologies of fluctuating velocity and fluctuating vorticity whose center is a strong second-quadrant event (Q2) or a fourth-quadrant event (Q4). Results illustrate that a closer similarity of the multi-scale coherent structures is present in the wall-normal direction, compared to the one in the other two directions. The relationship among such topological coherent structures and Reynolds stress bursting events, as well as the fluctuating vorticity was discussed. When other burst events are surveyed (the first-quadrant event Q1 and the third-quadrant event Q3), a fascinating bursting period circularly occurs: Q4-S-Q2-Q3-Q2-Q1-Q4-S-Q2-Q3-Q2-Q1 in the center of such topological structures along the streamwise direction. In addition, the probability of the Q2 bursting event occurrence is slightly higher than that of the Q4 event occurrence. The spatial instable singularity that almost simultaneously appears together with typical Q2 or Q4 events has been observed, which is the main character of the mutual induction mechanism and vortex auto-generation mechanism explaining how the turbulence is produced and maintained. © Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Cao X.,Tianjin University | Liu J.,Tianjin University | Jiang N.,Tianjin University | Jiang N.,Tianjin Key Laboratory of Modern Engineering Mechanics | And 2 more authors.
Energy and Buildings | Year: 2014

Quantifying the airflow field in building room or vehicle cabin is crucial for creating a thermal comfortable and healthy indoor environment. Airflow field measurement can provide quantitative information of indoor air distribution and local air velocity around occupants or passengers, which has strong relationship with the ventilation effectiveness, the pollutant transportation and the energy conservation in a building or a vehicle. Specifically, PIV has gradually became the most popular and promising technique for airflow field measurement in indoor environment during the last decade. This paper firstly gave an overview of the typical PIV technologies used in indoor environment and the state-of-the-art applications of PIV in measuring the indoor airflow fields. The overview shows that the quantitative and detailed turbulent flow information obtained by PIV is critical for analyzing turbulent properties and validating numerical simulations. Specifically, the authors focused on the pros and cons of PIV measurement and gave the typical parameters of PIV used in indoor airflow field measurements. Generally, the researchers should pay more attention to the selection of appropriate PIV system parameters according to their specific research needs. The accuracy of PIV measurement and the limitations of measurement systems using PIV were also discussed. © 2013 Elsevier B.V. All rights reserved.

Yao S.,Tianjin University | Guo Y.,Tianjin University | Jiang N.,Tianjin University | Jiang N.,Tianjin Key Laboratory of Modern Engineering Mechanics | Liu J.,Tianjin University
Building and Environment | Year: 2015

The impinging jet concept has been proposed as a new ventilation strategy for use in office and industrial buildings. To improve the performance of impinging jet ventilation system to create a better thermal comfort environment, quantifying the detailed information of the flow behavior of impinging jet should be an essential prerequisite. This study reports an experimental study of an isothermal turbulent jet impinging normally on a flat surface in a closed cabin. The air jet issued from a round pipe with an inner diameter D. The distance between the pipe exit and the flat impingement plate was 9D. The Reynolds number, based on the jet centerline velocity at pipe exit and the pipe inner diameter, was 10,338. Measurements were performed in the free- and wall-jets using cross hot-wire anemometry, mean velocity, turbulence intensity, and power spectrum results being presented. In addition, a multi-scale analysis technique based on empirical mode decomposition was used to analyze and gain deeper insight into the multi-scale characteristics of coherent structures of the jet impingement on flat surface. © 2014 Elsevier Ltd.

Zhang X.,Tianjin University | Chen J.,Tianjin University | Chen J.,Tianjin Key Laboratory of Modern Engineering Mechanics | Wang Z.,Tianjin University | And 2 more authors.
Optics and Lasers in Engineering | Year: 2012

High accuracy measurement of the large rotation is a crucial issue in engineering. In this paper, the displacement hypothesis of quadrilateral element in finite element method (FEM) is chosen for digital image correlation (DIC) technique, and NewtonRaphson iterative method is selected to solve the partial differential equation to measure the displacement field. Because the ring template possesses rotation invariance and translation invariance, ring template matching technique is presented to estimate the integer pixel displacements of nodes which could make the initial value of the iterative method close to the real value. Simulated images are used to verify the performance of the improved DIC technique which shows that this method has a low request to the speckle pattern and the measurement range of the rotational angle is from 0° to 360°with high accuracy. Furthermore, the improved DIC technique is used to measure the large deflection deformation of the polyurethane specimen and the measured deformation is in accordance with the actual deformation of the specimen. All the results show that the improved DIC technique could be used to measure the large rotation and the application scope of DIC technique is extended. © 2012 Elsevier Ltd. All rights reserved.

Quan B.,Tianjin University | Nan J.,Tianjin Key Laboratory of Modern Engineering Mechanics
Advanced Materials Research | Year: 2013

Tomographic particle image velocimetry (Tomo-PIV) is a newly developed technique for three-component three-dimensional (3C-3D) velocity measurement based on the tomographic reconstruction of a 3D volume light intensity field from multiple two-dimensional projections. A simplification of 3D tomographic reconstruction model, which reduced from a 3D volume with 2D images to a 2D slice with 1D lines, simplify this 3D reconstruction into a problem of 2D plane reconstruction by means of optical tomography, is applied in this paper. The principles and details of the tomographic algorithm are discussed, as well as the study of ART and MART reconstruction algorithm is carried out by means of computer-simulated image reconstruction procedure. The three-dimensional volume particle field is reconstructed by MART reconstruction algorithm base on the simplified 3D reconstruction model which made a high reconstruction quality Q=81.37% prove that the way of simplification by MART reconstruction is feasible, so it could be applied in reconstruction of 3D particle field in tomographic particle image velocimetry system. © (2013) Trans Tech Publications, Switzerland.

Su C.,Tianjin University | Su C.,Tianjin Key Laboratory of Modern Engineering Mechanics
Science China: Physics, Mechanics and Astronomy | Year: 2014

Up to now, the most widely used method for transition prediction is the one based on linear stability theory. When it is applied to three-dimensional boundary layers, one has to choose the direction, or path, along which the growth rate of the disturbance is to be integrated. The direction given by using saddle point method in the theory of complex variable function is seen as mathematically most reasonable. However, unlike the saddle point method applied to water waves, here its physical meaning is not so obvious, as the frequency and wave number may be complex. And on some occasions, in advancing the integration of the growth rate of the disturbance, up to a certain location, one may not be able to continue the integration, because the condition for specifying the direction set by the saddle point method can no longer be satisfied on the basis of continuously varying wave number. In this paper, these two problems are discussed, and suggestions for how to do transition prediction under the latter condition are provided. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg.

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