Preload State Research Articles

Determination of Effective Load Shedding Schemes in Electric Power Plants

The ability of power system to survive the transition from preloading state to the gradual increase in load and thereafter reach an acceptable operational condition is an indication of transient stability of the system. The study analyzed load shedding scheme through the use of empirical measurement tools and load-flow simulation techniques. It was geared towards determining effective load shedding strategies to reduce unnecessary overload in order to achieve dynamic stability of the electric power network in the Export Free Trade Zone, Calabar, Nigeria. From the tests and the measurements taken, it was observed that the real and reactive powers from the generator and the mechanical power from the turbine engine were stable when the load shedding controller was switched on, as compared to when it was off. The engine speed, the bus-bar frequency and the output voltage of the generator stabilized within a shorter time (about 8 seconds) when the controller was switched on than when it was on the off condition. Also, there were noticeable fluctuations in the speed of the remaining two generators. It became stable at about 12 seconds after the loss. The variations were 0.3 per cent of the nominal speed value. The excitation voltage fluctuated from 1.2 (pu) to 4.5 (pu) when the bus voltage dipped as a result of additional load. It then came down and stabilized at 1.8 (pu) after few swings. This confirmed that the stability of power system is much enhanced when load shedding controllers are effectively configured on the network.

Post-Tensioning Steel Rod System for Flexural Strengthening in Damaged Reinforced Concrete (RC) Beams

In this study, a post-tensioning method using externally unbonded steel rods was applied to pre-damaged reinforced concrete beams for flexural strengthening. Nine simply-supported beams, three reference beams and six post-tensioned beams, were subjected to three-point bending. The design parameters observed in this study were the amount of tension reinforcements (3-D19, 4-D19, and 2-D22 + 2-D25; “D” indicates the nominal diameter of the rebar) and the diameters of the external rod (φ22 mm and φ28 mm). A V-shaped profile with a deviator at the bottom of the mid-span was applied to the pre-damaged beams, and a post-tensioning force was added to overcome the low load resistance and deflection already incurred in the pre-loading state. The post-tensioning force caused by tightening the nuts at the anchorage corresponded to a strain of 2000 με in the external rods; this value was approximately equal to the strain caused by torque that two adults can apply conveniently. The post-tensioning system increased the load-carrying capacity and flexural stiffness by approximately 40–112% and 28–73%, respectively, when compared with the control beams. However, the external rods did not yield in the post-tensioned beam with larger steel reinforcements and external steel rods. The external rod with the larger diameter increased the flexural strength more effectively.

Monitoring concept for structural integration of PZT-fiber arrays in metal sheets: a numerical and experimental study

ABSTRACTThe technique joining by forming allows the structural integration of piezoceramic fibers into locally microstructured metal sheets without any elastic interlayers. A high-volume production of the joining partners causes in statistical deviations from the nominal dimensions. A numerical simulation on geometric process sensitivity shows that the deviations have a high significant influence on the resulting fiber stresses after the joining by forming operation and demonstrate the necessity of a monitoring concept. On this basis, the electromechanical behavior of piezoceramic array transducers is investigated experimentally before, during and after the joining process. The piezoceramic array transducer consists of an arrangement of five electrical interconnected piezoceramic fibers. The findings show that the impedance spectrum depends on the fiber stresses and can be used for in-process monitoring during the joining process. Based on the impedance values the preload state of the interconnected piezoceramic fibers can be specifically controlled and a fiber overload.

Denervated aortic smooth muscle tension and cellular membrane current in variations of calcium ion concentration in iso-osmotic solution

Objective To study the influence of high calcium ion environment on the membrane current of denervated aortic smooth muscles in iso-osmotic condition, as well as study the myogenic spontaneous vasomotion of the aortic smooth muscles under different pre-load state and intervention of calcium-channel blockers. Methods Smooth muscle samples were taken from the aortic smooth muscle layer of Kunming mice. The samples were fixed at two-ends in relaxed state and infiltrated in Ringer's solution. After stabilization, a glass microelectrode was adsorbed on the smooth muscle membrane to form gigaohm-seal. The current state of the membrane was observed under the physiological osmotic pressure. Then, the calcium ions concentration in the Ringer's solution was increased from 0.9 mol/L to 1.2 mol/L, and the immediate change of membrane current was observed after changing of osmotic pressure. Finally, the membrane potential change in iso-osmotic condition was observed when the pre-load was increased to 1 g, and the spontaneous vasomotion curves of the samples were recorded. Results In the relaxation state, the membrane current of the smooth muscle was increased significantly, i.e. (10.25±1.34) pA vs. (24.91±3.27) pA (P<0.05), when the calcium ion concentration was increased from 0.9 mol/L to 1.2 mol/L in iso-osmotic condition. When the pre-load was increased, the variation amplitude of the membrane current was increased in iso-osmotic condition, i.e. (10.25±1.34) pA to (15.33±4.33) pA (P<0.05) for the lower calcium ion concentration, and (24.91±3.27) pA to (33.31±7.25) pA (P<0.05) for the higher calcium ion concentration. If only increasing the pre-load, the myogenic spontaneous vasomotion could be increased by 175% for the condition of lower calcium ion concentration (0.9 mol/L). When the calcium ion concentration was increased to 1.2 mol/L, the spontaneous vasomotion frequency of the specimen was increased, and the range of the vasomotion could be further increased by 40%. After pretreatment with the calcium-channel blocker (0.5 g/L nitrendipine), the membrane current and myogenic spontaneous vasomotion were significantly decreased in the higher calcium ion environment, which indicated that the membrane potential and spontaneous vasomotion was dominated by calcium ions. Conclusions The increase of extracellular calcium ion concentration not only can improve the excitability of the aorta smooth muscle cells, but also can cause significant spontaneous vasomotion, and can improve the compliance of the smooth muscle tissue in pre-load variation. The calcium-channel blockers can inhibit calcium-dominated transmembrane currents, reduce myogenic spontaneous vasomotion, and enhance the stiffness of smooth muscle, which may have negative effects on windkessel vessel function. Key words: Denervation; Quick stretching; Overloading; Spontaneous vasomotion; Membrane current

Effect of bearing preload on the dynamic characteristics of the spindle-bearing system

Angular contact ball bearings have been extensively utilized in machine tool spindles and the bearing preload plays a significant role on spindle stiffness, rotating precision, heat generation and service life span. With the development of high speed and high precision machining, especially for high speed grinding and milling, both heavy preload at low speed and light preload at high speed must been abled in series in a single machine tool spindle. In order to investigate the effect of the bearing preload on the performance of the spindle, we developed a spindle test rig. In this paper. The effect of bearing preload on the dynamic characteristics of the spindle-bearing system was investigated. The working principle of fixed position preloading method of the spindle-bearing system are introduced. For the spindle that utilizing fixed position preloading method, the theory of assembly dimensional chain was applied to analyze the preloading state of the spindle-bearing system in static conditions. The equivalent parametric identification model was established. The finite element dynamic analysis model of the spindle assembly was developed by taking the advantage of the spring-damper elements to simulate the bearing supports. Finite element analysis (FEA) was conducted to evaluate the effect of the preload on the dynamic characteristics of the spindle-bearing system. This paper not only provides guidance on how to correctly assembling high speed and high precision spindle, but also lay a foundation for the investigation of thermal-mechanical-dynamic characteristic of high speed spindle-bearing system.

On the cyclic stability of the thermomechanical behavior of NiTi shape memory cylindrical actuators

The cyclic performance of cylindrical SMA actuators are investigated, taken into account the individual effects of the (1) selected SMA material, (2) geometric configuration of the actuator, and (3) prior mechanical load path taking to a targeted stressed preload state before starting thermal cycles. The “ordinary”, commercially-available, binary Ni49.9Ti50.1 and a developmental “high temperature” ternary Ni50.3Ti29.7Hf20 alloys are used for comparison. The responses of these alloys are characterized using a multimechanism SMA model. Cylindrical actuator geometries, ranging from thin tubular configuration, where there is more uniformity in stress distribution, to a solid section, having significant non-homogeneities in stress, are utilized. Both proportional and non-proportional loading conditions are examined under limited stress levels and thermal cycles. Based on the results obtained for the alternative actuator designs, the conclusions are: (i) high actuation strokes are produced by Ni49.9Ti50.1 actuators, (ii) the Ni50.3Ti29.7Hf20 actuators show relatively more dimensionally stable actuation performance, (iii) there is a marked difference in the degree of mechanical load-path dependency on cyclic behavior between the Ni49.9Ti50.1 and Ni50.3Ti29.7Hf20 actuators, (iv) the cyclic response of the Ni50.3Ti29.7Hf20 is far less sensitive to the actuator geometry in comparison to the Ni49.9Ti50.1, and (v) differences in actuator performance are connected to the intrinsic transformation behavior of the selected shape memory alloys.

The Assessment of the Right Heart Failure Syndrome.

The right heart failure (RHF) syndrome is a pathophysiologically complex state commonly associated with dysfunction of the right ventricle (RV). The normal RV is suited for its purposes of distributing venous blood to the low-resistance pulmonary circulation. Myriad stresses imposed upon it, though, can ultimately result in its failure, with the threat of cardiovascular collapse being the most dreaded outcome. Decreased cardiac output with increased central venous pressures are hemodynamic hallmarks of this highly morbid condition. Proper management of RHF is predicated on the accurate assessment of the key hemodynamic and clinical components signaling the syndrome that is the result of the failing RV. Appropriate use of diagnostic tools is paramount for understanding the key components of RV function: the preload state of the RV, its contractility, and the afterload burden placed on it. In making these assessments, it remains crucial to understand the limitations of these tools when managing RHF in the intensive care unit. An understanding of each of these components allows for the understanding of the physiology and the clinical presentation which can guide the use of therapies appropriately tailored to manage the condition.

On the effect of preload and pre-stretch on hemodynamic simulations: an integrative approach.

In this work, we address the simulation of three-dimensional arterial blood flow and its effect on the stress state of arterial walls. The novel contribution is the unprecedented combination of several modeling techniques to account for (1) the fact that known configurations for the arterial wall are in a preloaded state, (2) the compliance of the vessel segments, (3) proper boundary data over the non-physical interfaces resulting from the isolation of an arterial district from the rest of the arterial tree, (4) the presence of surrounding tissues in which the vessel is embedded and (5) residual stress state due to pre-stretch. Firstly, we formulate both the forward mechanical problem when the reference (zero-load) configuration is assumed to be known and, the preload problem arising when the known domain is a configuration at equilibrium with a certain load state (typically due to internal pressure and tethering forces). Then, two additional complexities are faced: the fluid-structure interaction problem that follows when the compliant vessels are coupled with the blood flow, and the introduction of non-physical boundaries coming from the artificial isolation of the arterial district from the original vessel. This, in turn, posses the problem of coupling dimensionally heterogeneous models to incorporate the effect of upstream and downstream systemic impedances. Additionally, a viscoelastic support on the external surface of the vessel is also incorporated. Two examples are presented to quantify in a physiologically consistent scenario the differences in simulation results when either considering or not the preload state of arterial walls. These computational simulations shed light on the validity of simplifying hypotheses in most hemodynamic models.

MODAL ANALYSIS AND DAMPING MEASUREMENT OF THE HEAD ARM ASSEMBLY OF A SMALL FORM FACTOR HARD DISK DRIVE

As non-traditional applications of hard disk drives (HDDs) emerge, the interest in the effects of shock and vibration on small form factor (SFF) drives has come into currency due to the increasingly hostile environments encountered in the usage of the portable computer as well as the application in consumer devices. In this paper, the dynamic characteristics of an SFF drive were investigated using both experimental and numerical techniques, including modal analysis and damping measurement of the head arm assembly (HAA) of the drive. A finite element (FE) model of the HAA was created to perform numerical analysis. The FE model was verified and modified according to numerical results and experimental results. It is found that numerical results of the HAA in it free state and those in its preloading state coincide well with those of experiments, and/or those by other researchers.

Experiencia clínica de monitorización hemodinámica del shock mediante el sistema PiCCO ®

Purpose To evaluate the PiCCO ® hemodynamics monitor in terms of clinical usefulness in children with shock. Methods Prospective multicenter analytical study in children aged from one month to 18 years with shock admitted to five pediatric intensive care units. Measurements were made before and after three interventions: a) volume load; b) increases in vasoactive drugs; c) dosage changes of drugs that could lessen vascular resistance. Recorded parameters included thermodilution data, along with the usual hemodynamic parameters. Results A total of 120 measurements were performed on 35 patients: mean age 36 (2.6–156) months, mean weight 15 (5.8–72) kg. Shock etiology was septic in 37% of cases, cardiogenic in 26%, hypovolemic in 20% and neurogenic in 17%. No procedure related complication was noticed. Twenty-two volume challenges in 17 patients were registered. Volume load induced a significant intrathoracic blood volume index (ITBI) increase from 501(235–763) to 584 (418–810) ml/m 2, cardiac index (CI) 4.04 (2.58–6.25) to 4.48 (2.86–8.71) l min-1 m 2, and mean blood pressure from 74 (53–99) to 87 (59–112) mmHg. CI changes correlated with ITBI increase (r=0.678, p=0.001). 13 interventions to increase vasomotor tone were associated with an increase in contractility of 18% in systemic vascular resistance index (SVRI). Conclusions Hemodynamic monitoring with the PiCCO ® system is feasible and seems safe in children with shock. PiCCO ® derived parameters could add clinically important information to assess preload state and its modifications with therapy.

Impact behaviour of preloaded glass/polyester woven plates

In this work, the effect of a biaxial preload in the behaviour of glass/polyester woven-laminate plates subjected to high-velocity transversal impact was studied. For this, an analytic model based on energy considerations that include the presence of an in-plane preload was used. The results of the analytic model for the biaxial preload state were compared with those found for a non-preload plate, the difference between them being minimal for the pre-stressed level reached in the tests (31% of the static UTS). Therefore, numerical simulations were made in order to study the effect of the preload in greater detail; furthermore, experimental tests were conducted, validating the analytic and numerical model. In general, the two methods revealed minimal differences between the values of the ballistic limit and those of the residual velocity.

Pulse pressure and stroke volume variations during severe haemorrhage in ventilated dogs

Similarly to systolic pressure variation (SPV), pulse pressure variation (PPV) and stroke volume variation (SVV) derived from arterial pulse contour analysis have been shown to reflect fluid responsiveness in ventilated patients. However, unlike the SPV, both PPV and SVV have not been validated during extreme hypovolaemia. The aim of the present study was to examine whether these newly introduced variables respond to gradual hypovolaemia like the SPV by increasing gradually with each step of the haemorrhage even during extreme hypovolaemia. SPV, SVV and PPV were measured in 8 dogs following initial volume loading (10% of the estimated blood volume administered as colloid solution), 5 steps of graded haemorrhage, each consisting of 10% of the estimated blood volume, followed by retransfusion of the shed blood. The correlations of the SVV, SPV and PPV to the stroke volume (SV) throughout the study were -0.89, -0.91 and -0.91, respectively. Correlations of the CVP and the global end-diastolic volume (GEDV) of the heart chambers to the SV were 0.79 and 0.95, respectively. The SPV correlated significantly with both the PPV and the SVV (r=0.97 and 0.93 respectively). However, the PPV increased by more than 400% at 50% haemorrhage compared with increases of 200% and 120% for the SVV and %SPV, respectively. This study demonstrates that the present algorithm used for the calculation of the SVV and the formula used to calculate the PPV, perform well over a wide range of preload states including severe hypovolaemia. However, the PPV changes more than the SPV and SVV. This may be due to the changing relation of the SV to the pulse pressure when the filling of the aorta is greatly decreased.

補強土橋脚模型の鉛直圧縮特性に及ぼす繰返し・荷重保持載荷履歴の効果

To evaluate the effects of preloading and prestressing on deformation characteristics by cyclic loading for a long term performance of reinforced soil structures, a series of cyclic and creep tests were performed on small models. No distinct difference was found in the increasing rate with time of residual strain between a cyclic loading test and a constant pressure creep test at a preloading state. The amount of residual strain by cyclic loading at an unloaded state (i. e., prestressed state) decreased by applying in advance long-term constant pressure or cyclic loading at the preloaded condition. Applying cyclic loading at the preloading state was slightly more effective. When swelling was allowed to take place at a prestress state. the deformation during the subsequent cyclic loading test became larger.

Contribution of humoral systems to the short-term variability of blood pressure after severe hemorrhage.

The aim of this study was to generate hemorrhage-triggered fluctuations in blood pressure (BP) at low frequency (LF, < 0.2 Hz) in conscious rats and investigate with spectral analysis the relative roles of hemorrhage-activated catecholamines, the renin-angiotensin system (RAS), and arginine vasopressin (AVP) on the generation of these fluctuations. The individual contribution of these factors was assessed using a combination of the selective antagonists, prazosin, losartan, and Manning compound (AVPX). At rest, systolic BP (SBP) LF fluctuations were slightly increased by losartan. The mid-frequency (MF; 0.2-0.6 Hz) oscillations of SBP and diastolic BP (DBP) were decreased by prazosin alone or associated with AVPX or losartan. The high-frequency (HF; respiratory) oscillations of SBP were increased by prazosin, prazosin plus losartan, and prazosin plus AVPX. After severe hemorrhage (20 ml/kg), the spontaneous BP recovery was characterized by the occurrence of slow fluctuations of SBP and DBP, centered approximately 0.065 Hz, and by increases of MF (89%) oscillations of SBP. The HF component of SBP variability tended to be increased by blood loss. The occurrence of the SBP LF fluctuations was prevented when alpha 1-adrenergic activity was blocked by prazosin. These oscillations were always present, despite inhibition of angiotensin II, and were increased after inhibition of the AVP activity. Pretreatment with the specific inhibitors used in these studies favored the amplifying effect of hemorrhage on HF fluctuations while they prevented the postbleeding increase in MF oscillations. In conclusion, the present results show an association between the dependence of the postbleeding blood pressure level on catecholamines and the occurrence of slow fluctuations of BP. The buffering role of AVP suggests the establishment of a hierarchy between humoral systems in the genesis of the LF oscillations of BP, with the slow oscillations being generated by the main pressor system and being dampened by the other systems. The postbleeding rise in the MF component of SBP variability could be considered a reflection of the activations of both the sympathetic vasomotor drive and the RAS. The postbleeding increase in the HF component of BP variability was dampened by the activation of the humoral systems. These effects may reflect the low preload state due to hypovolemia.

Defibrillation efficacy with endocardial electrodes is influenced by reductions in cardiac preload.

Little is known about the effects of cardiac preload and cardiac geometry on defibrillation efficacy with endocardial electrodes. We studied nine pigs implanted with an endocardial lead system in the normal and reduced preload state. In the reduced preload state, a balloon catheter was inflated in the inferior vena cava (IVC) for 20 seconds prior to the induction of ventricular fibrillation (VF). Complete occlusion of the IVC and reductions in preload were confirmed by observing deformation of the contrast-filled balloon, a reduction in cardiac size by fluoroscopy, and reductions in ventricular pressures. Biphasic shocks were delivered after 10 seconds of VF using a recursive up-down protocol. VF was induced 20 times for each preload state, and the 50% effective doses (ED50) for energy, current, and voltage were estimated by averaging all shocks for that state. At reduced preloads, energy decreased from 12.1 +/- 3.0 J (+/- SD) to 10.5 +/- 2.9 J (p < 0.01), voltage decreased from 415 +/- 51 V to 390 +/- 51 V (p < 0.05), and current decreased from 8.6 +/- 1.5 A to 7.6 +/- 1.5 A (p < 0.01), while impedance rose from 49.2 +/- 3.8 omega to 52.8 +/- 4.4 omega (p < 0.001). We conclude that reducing cardiac preload and cardiac size significantly lowers ED50 defibrillation energy, current, and voltage. This outcome may be caused directly by the decrease in blood volume as evidenced by increased impedance and/or may be due to changes in heart geometry and stretch.

Effects of graded hemorrhage on short‐term variability of blood pressure in conscious rats

A controlled graded bleeding was performed in conscious rats with 15 min intervals between two withdrawals, in order to induce a 25% blood loss without hypotension. Heart rate (HR) was unaffected as well. The spectral profiles of systolic blood pressure (SBP) analyzed on 51.2 s segments exhibited increases in the high frequency (HR, respiratory) component. This increase paralleled the blood loss with a rise ranging from 20% for a 1 mL/kg hemorrhage to 90% for a 11 mL/kg removal. These changes were associated with increases in the mid-frequency (MF, Mayer waves) component of SBP variability. These latter rises were between 30 and 40% of the control value. Breathing frequency (BF) and blood gases were unaltered following hemorrhage. A shift of fluid occurred during the 3 h session as reflected by the significant hemodilution. Rats were also bled after pretreatment with prazosin (1 mg/kg) or with an association of prazosin (1 mg/kg) and losartan (10 mg/kg). These treatments increased HR. A marked fall in SBP occurred with the double blockade. Hemorrhage determined a relative bradycardia together with the SBP decrease (reversible shock) after prazosin and losartan treatment. Prazosin determined opposite changes in MF (-33%) and HF (+58%) SBP components. A further decrease in the MF SBP component was observed following the double blockade. Spectral profiles following hemorrhage were unchanged compared to the prehemorrhage blocked levels. Therefore graded nonhypotensive, ie, normotensive hemorrhage in rats, was associated with progressive increases in the respiratory SBP variations, estimated from the SBP spectrum. This sensitive index could reflect the low preload state due to hypovolemia. The hemorrhage-induced MF SBP component increase could reflect an increased sympathetic vasomotor drive, prevented with prazosin, as a reflex adjustment to hypovolemia. Renin activation could also contribute to the genesis of MF waves and its role in maintaining BP following hemorrhage was exemplified with alpha 1-adrenoceptor blockade.

Midazolam Malate as an Intravenous Induction Agent for Open Heart Surgery

Midazolm, a water-soluble benzodiazpine that is shorter-acting, more potent, and less irri-tating toveins than diazepam, has been suggested for use for inducation of anesthesia. The cardiovascular effect of an induction dose(0.2~03mg/kg) of midazolam in ASA class 111cardiac surgical patients(N=15) were compared in a double-blind fashion with a similar group og patients(N=15) receiving thiopental(5.0mg/kg). The patients were premedicated by triflupromazine, pethidine, hydroxyzine, atropine and diazepam. The results were summarized as follows. 1) The thiopental group were more decreased in blood pressure and increase in heart rates than midazolam group. 2) Spontaneous eye closing time and loss of eyelash reflexe were observed. But those were a poor sign of adequate inducation because of heavy preanesthetic sedation. 3) The changes of blood pressure and puls rate after induction and intubation were almost, depends on preload state before inducation. From the above results, midazolam was sufficient as an induction agent for open heart cases. But a hypovolemic patients and completely beta blocked patiens should be used cautiously because they may be developed severe hypotention and tachycardia

Behavior of rat stomach kallikrein-like substance in experimental gastric ulceration occurrence

Fibrinolysis, coagulation, and kinin system relate to pathogenesis of gastric ulcer. Serum kinin has complicated backgrounds and its changes are not in accordance with pathogenesis of gastric ulcer. The relationships between tissue-fibrinolysis and tissue-kallikrein were studied in tissue-plasminogen activator and tissue-kallikrein.Recently, kallikrein has been found in stomach tissue. Synthetic substrate, Prolyl-Phenyl-Arginyl-Naphtyl-Ester, was used to determine its activity. It was studied whether its value reflects clearly tissue-kallikrein or not.Satisfactory results were obtained in the basic study. Then an experiment of stress ulcer due to water immersion was performed with DONRYU rat. In the study on a relationship between tissue-plasminogen activator and tissue-kallikrein during the term from preloading state to the development of ulcer, tissue-kallikrein was reduced and reduction of tissue-plasminogen activator was also observed.