Enhanced external counterpulsation(EECP)is able to treat myocardial ischemia,which is usually caused by coronary artery stenosis.However,the underlying mechanisms regarding why this technique is effective in treating ...Enhanced external counterpulsation(EECP)is able to treat myocardial ischemia,which is usually caused by coronary artery stenosis.However,the underlying mechanisms regarding why this technique is effective in treating myocardial ischemia remains unclear and there is no patient-specific counterpulsation mode for different rates of coronary artery stenosis in clinic.This study sought to investigate the hemodynamic effect of varied coronary artery stenosis rates when using EECP and the necessity of adopting targeted counterpulsation mode to consider different rates of coronary artery stenosis.Three 3-dimensional(3D)coronary models with different stenosis rates,including 55%(Model 1),65%(Model 2),and 75%(Model 3),were generated,then coupled with a 0-dimensional(0D)lumped parametric model of the blood circulatory system.EECP was applied to the 0D/3D coupled models to study the hemodynamic response of the coronary artery.Under the same counterpulsation mode,the ratio of diastolic blood pressure to systolic blood pressure of 3 models during counterpulsation was 1.4,and the cardiac output and coronary artery flow rate increased significantly.The low wall shear stress(WSS)and high oscillatory shear index(OSI)areas were mainly located at the posterior end of the stenosis and coronary artery bifurcation.Moreover,with an increase in the rate of coronary artery stenosis,the increased percentage of flow rate through the coronary artery stenosis and area-averaged WSS decreased.The geometric multiscale model in this study can be used to effectively simulate the hemodynamic characteristics of cardiovascular system following the application of EECP.Local precise hemodynamic effect of the coronary artery stenosis can be observed.It was found from the hemodynamic factors that the coronary artery with lower stenosis rate more likely led to better vascular endothelial remodeling.Thus,it is necessary to adopt patient-specific counterpulsation mode accounting for different condition of coronary artery stenosis.展开更多
In coronary artery bypass grafting(CABG),graft’s poor instant patency may lead to an abnormal hemodynamic environment in anastomosis,which could further cause graft failure after the surgery.This paper investigates t...In coronary artery bypass grafting(CABG),graft’s poor instant patency may lead to an abnormal hemodynamic environment in anastomosis,which could further cause graft failure after the surgery.This paper investigates the graft hemodynamics with different instant patency,and explores its effect on graft postoperative efficiency.Six CABG 0D/3D coupling multi-scale models which used left internal mammary artery(LIMA)and saphenous vein(SVG)as grafts were constructed.Different types of grafts were examined in the models,including normal grafts,grafts with competitive flow and grafts with anastomotic stenosis.Simulation results indicated that comparing with SVG grafts,there was a greater difference between normal LIMA graft and non-patent LIMA graft.Also,the backflow occurred even in LIMA systolic flow.The wall shear stress(WSS)in the graft of the competitive flow LIMA model had an appreciable decrease comparing with the normal graft.In addition,the WSS in the stenosis region of the anastomotic stenosis LIMA model was much higher than its adjacent regions.In contrast,the WSS distributions in the SVG models were much smoother than in the LIMA models.For oscillatory shear index(OSI),there was little difference between normal LIMA and SVG.But when the graft had competitive flow or anastomotic stenosis,much higher OSI occurred in some regions in LIMA than SVG.There are significant differences in hemodynamics between normal grafts and non-patent grafts both in LIMA and SVG.The hemodynamic environment in a normal LIMA is better than that in a normal SVG.However,in the situation of the two types of non-patent grafts,the hemodynamics of SVG is better than LIMA.展开更多
The choked back pressure characteristic of the steam turbine unit with air cooled condenser is very different with the unit with wet cooling technology, and the understanding of the choked back pressure performance ch...The choked back pressure characteristic of the steam turbine unit with air cooled condenser is very different with the unit with wet cooling technology, and the understanding of the choked back pressure performance change with operation load is important to guide the economic operation of the unit. One simplified Variable Operation Condition Analysis Method was put forward for calculation of the unit output-turbine back pressure characteristics. Based on this method, the choked back pressure for each operation load can be determined. An example was given for a super-critical, regenerative single-shaft, 2-casing with 2-exhaust steam turbine generation unit with air cooled condenser. The calculation result was provided and compared with the result of the unit with wet cooling technology.展开更多
To analyze the unit performance degradation for top heater out-of-service operation condition, two calculation methods were introduced, that was the model based calculation method and the simplified variable operation...To analyze the unit performance degradation for top heater out-of-service operation condition, two calculation methods were introduced, that was the model based calculation method and the simplified variable operation condition analysis method. A 600MW sub-critical steam turbine unit with air cooled condenser was analyzed using the two introduced methods. It is shown that calculation data got can reflect the redistribution effect on steam flows of the turbine extractions and turbine internals due to the top heater out-of-service, and similar and close calculation results were got from the two methods, and both results were lower than result calculated using the commonly used equivalent enthalpy drop method.展开更多
At present,solid-state laser-driven lighting fabricated with blue laser diode(LD)and phosphor has been greatly developed in high-power lighting applications.The major obstacle is the luminescence saturation threshold ...At present,solid-state laser-driven lighting fabricated with blue laser diode(LD)and phosphor has been greatly developed in high-power lighting applications.The major obstacle is the luminescence saturation threshold of the fluorescence conversion material(YAG:Ce).In this work,we successfully prepared a series of high-quality and robust YAG:Ce single crystals.With high quantum efficiency(QE=92%),regular crystal structure and high thermal conductivity,the single-crystal has excellent ability to deal with high radiation flux and thermal accumulation generated by high power.Matching the single-crystal with the laser,the luminous efficiency up to 274 lm/W and saturation threshold of 12 W/mm^(2) were obtained.In addition,the influence of Ce doping concentration and sample processing conditions on the indicators in laser-driven white lighting was systematically studied.The results are of great significance and are believed to promote the development of solid-state laser-driven lighting.展开更多
This paper explores an analytical model for Elastic Ring Squeeze Film Damper(ERSFD) with thin-walled ring and turbulent-jet orifices, and uncovers its Oil Film Pressure Performance(OFPP). Firstly, the ring deformation...This paper explores an analytical model for Elastic Ring Squeeze Film Damper(ERSFD) with thin-walled ring and turbulent-jet orifices, and uncovers its Oil Film Pressure Performance(OFPP). Firstly, the ring deformation is addressed by using the Fourier series expansion approach and the orifice outflow rate is characterized with the Prandtl boundary layer theory. Secondly, applying finite difference scheme, the influence of elastic ring flexibility, orifice diameter, and attitude angle on the OFPP is analyzed. Finally, Outer chamber pressure was measured experimentally at different rotor speeds. The results indicate that the outer chamber pressure coats an individual load-carrying region and spreads symmetrically pertaining to the attitude angle. Its amplitude drops as the elastic ring flexibility decreases but boosts with the reduction of the orifice diameter.For inner chamber pressure, the orifice diameter effects a similar trend to the outer cavity, but exhibits more stable distribution regarding the attitude angle. Minimizing the elastic ring flexibility causes an increase in amplitude. The model is validated by the test results giving that the outer chamber pressure shifts synchronously and periodically with the variation of the attitude angle,while the pressure amplitude increases slightly at higher rotor speeds.展开更多
In this paper, the authors investigate the optimal conversion rate at which land use is irreversibly converted from biodiversity conservation to agricultural production. This problem is formulated as a stochastic cont...In this paper, the authors investigate the optimal conversion rate at which land use is irreversibly converted from biodiversity conservation to agricultural production. This problem is formulated as a stochastic control model, then transformed into a HJB equation involving free boundary. Since the state equation has singularity, it is difficult to directly derive the boundary value condition for the HJB equation. They provide a new method to overcome the difficulty via constructing another auxiliary stochastic control problem,and impose a proper boundary value condition. Moreover, they establish the existence and uniqueness of the viscosity solution of the HJB equation. Finally, they propose a stable numerical method for the HJB equation involving free boundary, and show some numerical results.展开更多
Distraction osteogenesis(DO)is a mechanobiological process of regenerating bone tissue by tension stress.DO is used clinically to lengthen bones or to treat critical size bone defects.Although DO provides satisfactory...Distraction osteogenesis(DO)is a mechanobiological process of regenerating bone tissue by tension stress.DO is used clinically to lengthen bones or to treat critical size bone defects.Although DO provides satisfactory results in many cases,the prolonged period of treatment remains a major challenge that needs to be overcome.Various attempts have been devoted to accelerating bone regeneration during DO.One common approach is manipulation of the applied mechanical loading by altering distraction strategies.In this article,we reviewed relevant in vivo animal studies exploring the effects of changing mechanical environments,by varying distraction parameters(e.g.,rate and frequency)or adding compression loading,on bone regeneration in DO.We further presented how the mechanically-regulated bone regeneration process during DO could be simulated by in silico models incorporating mechano-regulatory tissue differentiation rules.A comprehensive review of those in vivo and in silico studies may not solely provide important references for development of improved DO protocols in clinic,but also promote a deeper understanding of the mechanobiological mechanism of bone regeneration.展开更多
Trabecular bone is natural material with heterogeneous tissue properties.The effect of tissue heterogeneity on the micromechanical behavior of trabecular bone is commonly evaluated by microCT-based finite element(micr...Trabecular bone is natural material with heterogeneous tissue properties.The effect of tissue heterogeneity on the micromechanical behavior of trabecular bone is commonly evaluated by microCT-based finite element(microFE)analysis.Results from prior work remain inconclusive and lack of experimental validation.To address these issues,we combined microFE analysis with mechanical testing and microCT-based digital volume correlation(DVC),as a validation for the microFE approach.Porcine trabecular specimens were tested in compression as sequential microCT scans were taken.DVC was performed to extract“realistic”boundary conditions that were applied to microFE models,and to measure microstructural deformation and strain of the trabecular specimens.Heterogeneous and homogeneous microFE models of each trabecular specimen were created and compared with the experimentally measured microstructural displacement and strains.Results showed strong correlations between DVC-measured and microFE-predicted trabecular displacement and strain fields(R^(2)>0.9,p<0.05),regardless of heterogeneous or homogeneous material assignments.The heterogeneous and homogeneous models predicted similar magnitudes for maximum or minimum principal strains(R^(2)=1,p<0.05).However,incorporation of tissue heterogeneity decreased more than 16.5%in the overall stress level of the trabecular tissues.Regardless,very strong correlations were found between the heterogeneous and homogeneous model-predicted principal strains or stresses.These results together suggest that tissue heterogeneity may have little effect on microFE modeling of typical elastic displacement and strains in the trabecular bone,suggesting that homogeneous material models might be sufficient to predict general trabecular micromechanics.展开更多
基金This research was supported by National Natural Science Foundation of China(11772016,11472022,11702008)Key Project of Science and Technology of Beijing Municipal Education Commission and Support Plan for High-level Faculties in Beijing Municipal Universities(CIT&TCD201804011).
文摘Enhanced external counterpulsation(EECP)is able to treat myocardial ischemia,which is usually caused by coronary artery stenosis.However,the underlying mechanisms regarding why this technique is effective in treating myocardial ischemia remains unclear and there is no patient-specific counterpulsation mode for different rates of coronary artery stenosis in clinic.This study sought to investigate the hemodynamic effect of varied coronary artery stenosis rates when using EECP and the necessity of adopting targeted counterpulsation mode to consider different rates of coronary artery stenosis.Three 3-dimensional(3D)coronary models with different stenosis rates,including 55%(Model 1),65%(Model 2),and 75%(Model 3),were generated,then coupled with a 0-dimensional(0D)lumped parametric model of the blood circulatory system.EECP was applied to the 0D/3D coupled models to study the hemodynamic response of the coronary artery.Under the same counterpulsation mode,the ratio of diastolic blood pressure to systolic blood pressure of 3 models during counterpulsation was 1.4,and the cardiac output and coronary artery flow rate increased significantly.The low wall shear stress(WSS)and high oscillatory shear index(OSI)areas were mainly located at the posterior end of the stenosis and coronary artery bifurcation.Moreover,with an increase in the rate of coronary artery stenosis,the increased percentage of flow rate through the coronary artery stenosis and area-averaged WSS decreased.The geometric multiscale model in this study can be used to effectively simulate the hemodynamic characteristics of cardiovascular system following the application of EECP.Local precise hemodynamic effect of the coronary artery stenosis can be observed.It was found from the hemodynamic factors that the coronary artery with lower stenosis rate more likely led to better vascular endothelial remodeling.Thus,it is necessary to adopt patient-specific counterpulsation mode accounting for different condition of coronary artery stenosis.
基金This work is supported by National Natural Science Foundation of China(11832003,11772016,11472022,11702008).
文摘In coronary artery bypass grafting(CABG),graft’s poor instant patency may lead to an abnormal hemodynamic environment in anastomosis,which could further cause graft failure after the surgery.This paper investigates the graft hemodynamics with different instant patency,and explores its effect on graft postoperative efficiency.Six CABG 0D/3D coupling multi-scale models which used left internal mammary artery(LIMA)and saphenous vein(SVG)as grafts were constructed.Different types of grafts were examined in the models,including normal grafts,grafts with competitive flow and grafts with anastomotic stenosis.Simulation results indicated that comparing with SVG grafts,there was a greater difference between normal LIMA graft and non-patent LIMA graft.Also,the backflow occurred even in LIMA systolic flow.The wall shear stress(WSS)in the graft of the competitive flow LIMA model had an appreciable decrease comparing with the normal graft.In addition,the WSS in the stenosis region of the anastomotic stenosis LIMA model was much higher than its adjacent regions.In contrast,the WSS distributions in the SVG models were much smoother than in the LIMA models.For oscillatory shear index(OSI),there was little difference between normal LIMA and SVG.But when the graft had competitive flow or anastomotic stenosis,much higher OSI occurred in some regions in LIMA than SVG.There are significant differences in hemodynamics between normal grafts and non-patent grafts both in LIMA and SVG.The hemodynamic environment in a normal LIMA is better than that in a normal SVG.However,in the situation of the two types of non-patent grafts,the hemodynamics of SVG is better than LIMA.
文摘The choked back pressure characteristic of the steam turbine unit with air cooled condenser is very different with the unit with wet cooling technology, and the understanding of the choked back pressure performance change with operation load is important to guide the economic operation of the unit. One simplified Variable Operation Condition Analysis Method was put forward for calculation of the unit output-turbine back pressure characteristics. Based on this method, the choked back pressure for each operation load can be determined. An example was given for a super-critical, regenerative single-shaft, 2-casing with 2-exhaust steam turbine generation unit with air cooled condenser. The calculation result was provided and compared with the result of the unit with wet cooling technology.
文摘To analyze the unit performance degradation for top heater out-of-service operation condition, two calculation methods were introduced, that was the model based calculation method and the simplified variable operation condition analysis method. A 600MW sub-critical steam turbine unit with air cooled condenser was analyzed using the two introduced methods. It is shown that calculation data got can reflect the redistribution effect on steam flows of the turbine extractions and turbine internals due to the top heater out-of-service, and similar and close calculation results were got from the two methods, and both results were lower than result calculated using the commonly used equivalent enthalpy drop method.
基金Project supported by the National Natural Science Foundation of China(51872207,51672192)the Key Research and Development Project in Zhejiang Province(2021C01024)。
文摘At present,solid-state laser-driven lighting fabricated with blue laser diode(LD)and phosphor has been greatly developed in high-power lighting applications.The major obstacle is the luminescence saturation threshold of the fluorescence conversion material(YAG:Ce).In this work,we successfully prepared a series of high-quality and robust YAG:Ce single crystals.With high quantum efficiency(QE=92%),regular crystal structure and high thermal conductivity,the single-crystal has excellent ability to deal with high radiation flux and thermal accumulation generated by high power.Matching the single-crystal with the laser,the luminous efficiency up to 274 lm/W and saturation threshold of 12 W/mm^(2) were obtained.In addition,the influence of Ce doping concentration and sample processing conditions on the indicators in laser-driven white lighting was systematically studied.The results are of great significance and are believed to promote the development of solid-state laser-driven lighting.
基金supported by the National Natural Science Foundation of China(No.52005158)。
文摘This paper explores an analytical model for Elastic Ring Squeeze Film Damper(ERSFD) with thin-walled ring and turbulent-jet orifices, and uncovers its Oil Film Pressure Performance(OFPP). Firstly, the ring deformation is addressed by using the Fourier series expansion approach and the orifice outflow rate is characterized with the Prandtl boundary layer theory. Secondly, applying finite difference scheme, the influence of elastic ring flexibility, orifice diameter, and attitude angle on the OFPP is analyzed. Finally, Outer chamber pressure was measured experimentally at different rotor speeds. The results indicate that the outer chamber pressure coats an individual load-carrying region and spreads symmetrically pertaining to the attitude angle. Its amplitude drops as the elastic ring flexibility decreases but boosts with the reduction of the orifice diameter.For inner chamber pressure, the orifice diameter effects a similar trend to the outer cavity, but exhibits more stable distribution regarding the attitude angle. Minimizing the elastic ring flexibility causes an increase in amplitude. The model is validated by the test results giving that the outer chamber pressure shifts synchronously and periodically with the variation of the attitude angle,while the pressure amplitude increases slightly at higher rotor speeds.
基金This work was supported by the National Natural Science Foundation of China(Nos.11771158,11801091)the Guangdong Basic and Applied Basic Research Foundation(No.2019A1515011338)the Guangzhou Natural Science Found(No.201904010189)。
文摘In this paper, the authors investigate the optimal conversion rate at which land use is irreversibly converted from biodiversity conservation to agricultural production. This problem is formulated as a stochastic control model, then transformed into a HJB equation involving free boundary. Since the state equation has singularity, it is difficult to directly derive the boundary value condition for the HJB equation. They provide a new method to overcome the difficulty via constructing another auxiliary stochastic control problem,and impose a proper boundary value condition. Moreover, they establish the existence and uniqueness of the viscosity solution of the HJB equation. Finally, they propose a stable numerical method for the HJB equation involving free boundary, and show some numerical results.
基金This study was supported by the National Natural Science Foundation of China[grant numbers 11702008,11832003]the Beijing Natural Science Foundation[grant numbers 7202003]the Beijing Municipal Education Commission Research Program[grant numbers KM202010005035].
文摘Distraction osteogenesis(DO)is a mechanobiological process of regenerating bone tissue by tension stress.DO is used clinically to lengthen bones or to treat critical size bone defects.Although DO provides satisfactory results in many cases,the prolonged period of treatment remains a major challenge that needs to be overcome.Various attempts have been devoted to accelerating bone regeneration during DO.One common approach is manipulation of the applied mechanical loading by altering distraction strategies.In this article,we reviewed relevant in vivo animal studies exploring the effects of changing mechanical environments,by varying distraction parameters(e.g.,rate and frequency)or adding compression loading,on bone regeneration in DO.We further presented how the mechanically-regulated bone regeneration process during DO could be simulated by in silico models incorporating mechano-regulatory tissue differentiation rules.A comprehensive review of those in vivo and in silico studies may not solely provide important references for development of improved DO protocols in clinic,but also promote a deeper understanding of the mechanobiological mechanism of bone regeneration.
基金This work was supported by National Natural Science Foundation of China[grant numbers 11702008,11832003]Beijing Natural Science Foundation of China[grant numbers 7202003]Beijing Municipal Education Commission Research Program[grant numbers KM202010005035].
文摘Trabecular bone is natural material with heterogeneous tissue properties.The effect of tissue heterogeneity on the micromechanical behavior of trabecular bone is commonly evaluated by microCT-based finite element(microFE)analysis.Results from prior work remain inconclusive and lack of experimental validation.To address these issues,we combined microFE analysis with mechanical testing and microCT-based digital volume correlation(DVC),as a validation for the microFE approach.Porcine trabecular specimens were tested in compression as sequential microCT scans were taken.DVC was performed to extract“realistic”boundary conditions that were applied to microFE models,and to measure microstructural deformation and strain of the trabecular specimens.Heterogeneous and homogeneous microFE models of each trabecular specimen were created and compared with the experimentally measured microstructural displacement and strains.Results showed strong correlations between DVC-measured and microFE-predicted trabecular displacement and strain fields(R^(2)>0.9,p<0.05),regardless of heterogeneous or homogeneous material assignments.The heterogeneous and homogeneous models predicted similar magnitudes for maximum or minimum principal strains(R^(2)=1,p<0.05).However,incorporation of tissue heterogeneity decreased more than 16.5%in the overall stress level of the trabecular tissues.Regardless,very strong correlations were found between the heterogeneous and homogeneous model-predicted principal strains or stresses.These results together suggest that tissue heterogeneity may have little effect on microFE modeling of typical elastic displacement and strains in the trabecular bone,suggesting that homogeneous material models might be sufficient to predict general trabecular micromechanics.