Wave propagation in flexible tubes

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Wave dissipation was previously investigated intensively in the frequency domain, in which the dissipation of waves is described as attenuation of pressure pulse decay with respect to the frequency or harmonics. In this thesis, wave dissipation, including decay of pressure pulse, peak of wave intensity and wave energy, is investigated in the time domain using wave intensity analysis (WIA). Wave intensity analysis benefits to this research in several aspects including: 1) WIA allows for wave dissipation investigated in the time domain; 2) WIA does not make any assumptions about the tube's wall non-linearity and the analysis takes into account the effects of the vessel's wall viscoelastic properties, convective, frictional effects and fluid viscosity; 3) WIA offers a technique (separation) to study wave dissipation in one direction whilst taking into account the effect of reflections from the opposite direction; 4) The physical meaning of wave intensity provides a convenient method to study the dissipation of energy carried by the waves along flexible tubes. In this research, it is found that the degree of dissipation in flexible tube were not only affected by the mechanical properties of the wall property and viscosity of liquid but also by the other factors including initial pressure and pumping speed of piston as well as direction of wave in relation to direction of flow. Also an new technique to separate waves into forward and backward directions only using diameter and velocity might potentially be used to separate the waves in both directions non-invasively based on the non-invasive measurement of diameter (wall movement) available

Wave propagation in flexible tubes

Wave dissipation was previously investigated intensively in the frequency domain, in which the dissipation of waves is described as attenuation of pressure pulse decay with respect to the frequency or harmonics. In this thesis, wave dissipation, including decay of pressure pulse, peak of wave intensity and wave energy, is investigated in the time domain using wave intensity analysis (WIA). Wave intensity analysis benefits to this research in several aspects including: 1) WIA allows for wave dissipation investigated in the time domain; 2) WIA does not make any assumptions about the tube's wall non-linearity and the analysis takes into account the effects of the vessel's wall viscoelastic properties, convective, frictional effects and fluid viscosity; 3) WIA offers a technique (separation) to study wave dissipation in one direction whilst taking into account the effect of reflections from the opposite direction; 4) The physical meaning of wave intensity provides a convenient method to study the dissipation of energy carried by the waves along flexible tubes. In this research, it is found that the degree of dissipation in flexible tube were not only affected by the mechanical properties of the wall property and viscosity of liquid but also by the other factors including initial pressure and pumping speed of piston as well as direction of wave in relation to direction of flow. Also an new technique to separate waves into forward and backward directions only using diameter and velocity might potentially be used to separate the waves in both directions non-invasively based on the non-invasive measurement of diameter (wall movement) available.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Optimization of the control performance of a novel 3/2 water proportional directional valve with a special position following servo mechanism

The 3/2 water proportional directional valve (PDV) is an important hydraulic component to ensure a precise, low impact, and safety control of hydraulic powered roof support, which has been a technical challenge facing to coal mining industry for many years. To address this issue, a new 3/2 water PDV is developed. The valve includes a novel three-core follow-up servo mechanism consisting of three key components: the pilot inlet valve, the pilot outlet valve, and the main inlet spool. This mechanism enables precise and reliable opening, and closing capacity of the valve. The prototype is manufactured and the performance test is conducted to improve the control performance of the new water PDV. During the test, two significant discoveries are made. First, it is determined that the control chamber of the main inlet spool must be in the predischarge state at the initial time to ensure adequate closing capacity of the new valve during the returning process. Second, it is found that the continuous small-step control mode can better cope with the friction and is more effective in achieving quick and smooth following characteristics of the three valve cores in the opening process, compared with the ramp control mode

Haemodynamic mechanism of formation and distribution of coronary atherosclerosis: A lesion-specific model

© IMechE 2020. Coronary arterial disease, as the most devastated cardiovascular disease, is caused by the atherosclerosis in the coronary arteries, which blocks the blood flow to the heart, resulting in the deficient supply of oxygen and nutrition to the heart, and eventually leading to heart failure. To date, haemodynamic mechanisms for atherosclerosis development are not fully understood although it is believed that the haemodynamic disturbance at the region of the arterial bifurcation, particular, bifurcation angle, plays an important role in the atherosclerosis development. In this study, two types of computational fluid dynamics models, lesion-specific and idealized models, combined with the computer tomography imaging techniques, are used to explore the mechanism of formation and distribution of the atherosclerosis around the bifurcation of left coronary artery and its association with the bifurcation angle. The lesion-specific model is used to characterize the effect of personalized features on the haemodynamic performance, while the idealized model is focusing on the effect of single factor, bifurcation angle, on the haemodynamic performance. The simulated results from both types of the models, combined with the clinical observation, revealed that the three key areas around the bifurcations are prone to formation of the atherosclerosis. Unlike the idealized models, lesion-specific modelling results did not show the significant correlation between the wall shear stress and bifurcation angle, although the mean value of the wall shear stress in smaller bifurcation angles (less than 90°) is higher than that with larger bifurcation angles (greater than 90°). In conclusion, lesion-specific computational fluid dynamics modelling is an efficient and convenient way to predict the haemodynamic performance around the bifurcation region, allowing the comprehensive information for the clinicians to predict the atherosclerosis development. The idealized models, which only focus on single parameter, may not provide the sufficient and reliable information for the clinical application. A novel multi-parameters modelling technique, therefore, is suggested to be developed in future, allowing the effects of many parameters on the haemodynamic performance to be evaluated

Experimental Verification of the Elastic Formula for the Aspirated Length of a Single Cell Considering the Size and Compressibility of Cell During Micropipette Aspiration

In this study, an aspiration system for elastic spheres was developed to verify the approximate elastic formula for the aspirated length of a single solid-like cell undergoing micropipette aspiration (MPA), which was obtained in our previous study by theoretical analysis and numerical simulation. Using this system, foam silicone rubber spheres with different diameters and mechanical properties were aspirated in a manner similar to the MPA of single cells. Comparisons between the approximate elastic formula and aspiration experiments of spheres indicated that the predictions of the formula agreed with the experimental results. Additionally, combined with the MPA data of rabbit chondrocytes, differences in terms of the elastic parameters derived from the half-space model, incompressible sphere model, and compressible sphere model were explored. The results demonstrated that the parameter ξ (ξ = R/a, where R is the radius of the cell and a is the inner radius of the micropipette) and Poisson’s ratio significantly influenced the determination of the elastic modulus and bulk modulus of the cell. This work developed for the first time an aspiration system of elastic spheres to study the elastic responses of the MPA of a single cell and provided new evidence supporting the use of the approximate elastic formula to determine cellular elastic parameters from the MPA data

Numerical modelling of multilayered coatings : latest developments and applications

The remarkable mechanical properties of multilayered coatings, such as super-hardness, excellent resistance against cracking, low wear-rate and high thermal-stability, are due to their unique interfacial structures and deformation mechanisms at the nanometer scale. The multilayered coating process itself is a typical multi-scale phenomenon. The modelling of multilayered coatings has become an important topic in research recently, largely due to the recent progress that has been made in the numerical modelling of materials and structures at different length-scales as well as the improved effectiveness achieved in linking such progress in numerical modelling to enable multi-scale modelling. In this paper, numerical modelling for the analysis of multilayered coatings at individual length-scales: Continuum, Molecular and Nano-scale, is reviewed, along with that at multi-scale modelling. Examples are presented showing numerical models obtained using: the Finite Element Method (FEM), Molecular Dynamics (MD), First-principles calculations and Multi-scale modelling, are presented. Their relative limitations are discussed and challenges to their future work highlighted

Load bearing capacity investigation and coating failure mechanism for coated spur gears

Spur gears are the most common type of gears for industry, due to its simple structures and low costs of manufacture. Under the complex loading conditions, failures can easily occur in the form of de-bonding, pitting, spalling or crushing of coating structures. Failure may originate from initiation of cracks, and its growth and propagation, however, basic failure mechanism is still not clear. In order to investigate the failure mechanism of coating structure failure for the spur gears, this paper presents some understandings about the coating damage at the teeth flank of a spur gear, based on a novel Finite Element simulation-procedures. This modeling procedure was developed based on several modeling approaches including: parameterized FE modeling, Cohesive-Zone Model and sub-modeling technique. The numerical model of spur gear was based on 42CrMo4 steel with PVD coating deposited as TiN/CrN multilayer structures. It was found that greater load bearing capacity exist for spur gears with the coating of nitride states deposited on 42CrMo4 steel

Investigation of the initiation and propagation of cracks in the coated surface of spur gear: An application of irreversible cohesive zone model

Spur gears are the most common type of gears for industry, due to its simple structures and low costs of manufacture. Under the action of rolling contact fatigue, the main mode of the coated teeth failure is surface or subsurface originated micro-pitting, leading to the debonding of coating from substrate, fracture in the coating and even subsurface failure. Failure may originate from initiation of cracks, and its growth and propagation, however, basic failure mechanism is still not clear. In this paper, a comprehensive finite element modeling procedure including submodelling techniques and irreversible cohesive zone modelling techniques (CZM) is developed to investigate the failure mechanisms under the rolling contact fatigue loading. The details of the localized stresses distribution will be simulated using the submodelling technique. Irreversible CZM will be used to simulate the initiation and propagation of the cracks in the coated surface of the spur gears under the cyclic contact loading

Investigation into initiation and propagation of cracks in the coated surfaces of spur gears with submodelling and irreversible cohesive-zone modelling techniques

Spur gears are one of the commonly used transmission parts in industry, due to its simplicity in structures and low cost in manufacturing. Due to a performance requirement, a spur gear may be coated with a specific coating arrangement. Under working conditions, the coating on the teeth may be damaged due to contact fatigue, in the forms such as micro-pitting and/or delamination. The failure mechanism of the coated surface under the gearing contact loading has been investigated intensively through experiment. A comprehensive computational model, which could be used to investigate the propagation of cracks in the coated surfaces, is still lacking. In the research reported in this paper, several finite element modelling techniques, including that for submodelling and irreversible cohesive zone modelling (CZM), have been developed to investigate the failure mechanisms of the coated surfaces of gears under the gearing contact fatigue loading. These techniques not only allow the localized stresses distribution and deformation in the interested locations in the coating and the substrate to be investigated in detail but also enable visual observation on the development of fatigue damages in the coating

Quantifying adhesion energy of mechanical coatings at atomistic scale

Coatings of transition metal compounds find widespread technological applications where adhesion is known to influence or control functionality. Here, we, by first-principles calculations, propose a new way to assess adhesion in coatings and apply it to analyze the TiN coating. We find that the calculated adhesion energies of both the (1 1 1) and (0 0 1) orientations are small under no residual stress, yet increase linearly once the stress is imposed, suggesting that the residual stress is key to affecting adhesion. The strengthened adhesion is found to be attributed to the stress-induced shrinkage of neighbouring bonds, which results in stronger interactions between bonds in TiN coatings. Further finite elements simulation (FEM) based on calculated adhesion energy reproduces well the initial cracking process observed in nano-indentation experiments, thereby validating the application of this approach in quantifying adhesion energy of surface coating systems