Understanding the Rayleigh instability in humping phenomenon during laser powder bed fusion process

The periodic undulation of a molten track’s height profile in laser-based powder bed fusion of metals(PBF-LB/M)is a commonly observed phenomena that can cause defects and building failure during the manufacturing process.However a quantitative analysis of such instabilities has not been fully established and so here we used Rayleigh–Plateau theory to determine the stability of a single molten track in PBF-LB/M and tested it with various processing conditions by changing laser power and beam shape.The analysis discovered that normalized enthalpy,which relates to energy input density,determines whether a molten track is initially unstable and if so,the growth rate for the instability.Additionally,whether the growth rate ultimately yields significant undulation depends on the melt duration,estimated by dwell time in our experiment.

Suppressing of humping bead using an external magnetic field in high-speed gas metal arc welding

During high-speed gas metal arc welding （GMAW）, the backward flowing molten jet with high momentum in the weld pool is considered to be responsible for the occurrence of humping bead. To suppress humping bead, an electromagnetic device is developed and coupled with the welding system. By adjusting the conditions of external magnetic field, forward electromagnetic force is obtained to reduce the momentum of the backward flow of molten metal in weld pool. Consequently, the humping bead can be suppressed by adjusting the external magnetic field. Bead-on-plate welding experiment was conducted on mild steel plates, and the influence of magnetic flux density on the arc deflection angle and weld bead quality is investigated. It is found that external magnetic field can remarkably adjust the momentum of backward flow jet and significantly improve the quality of weld bead.

Effect of welding current and speed on occurrence of humping bead in high-speed GMAW

The developed mathematical model of humping formation mechanism in high-speed gas metal arc welding （GMAW） is used to analyze the effects of welding current and welding speed on the occurrence of humping bead. It considers both the momentum and heat content of backward flowing molten jet inside weld pool. Three-dimensional geometry of weld pool, the spacing between two adjacent humps and hump height along humping weld bead are calculated under different levels of welding current and welding speed. It shows that wire feeding rate, power intensity and the moment of backward flowing molten jet are the major factors on humping bead formation.

Influence of backward flowing molten jet on humping bead formation during high-speed GMA welding

Considering the influence of backward flowing molten jet observed by experiments, a new pool surface deformation formula and droplets heat content model are used to investigate the humping formation mechanism during high-speed gas metal arc （GMA） welding. Three-dimensional geometry of the humping bead is numerically simulated only if some extra force and heat acted at the rear part of weld pool are taken into account in the model. It has proved that both the momentum and heat content of backward flowing molten jet must be appropriately treated to quantitatively analyze the physical mechanism of the humping phenomenon.

Effect of speed humps on instantaneous traffic emissions in a microscopic model with limited deceleration capacity

As a common transportation facility, speed humps can control the speed of vehicles on special road sections to reduce traffic risks. At the same time, they also cause instantaneous traffic emissions. Based on the classic instantaneous traffic emission model and the limited deceleration capacity microscopic traffic flow model with slow-to-start rules, this paper has investigated the impact of speed humps on traffic flow and the instantaneous emissions of vehicle pollutants in a single lane situation. The numerical simulation results have shown that speed humps have significant effects on traffic flow and traffic emissions. In a free-flow region, the increase of speed humps leads to the continuous rise of CO_(2), NO_(X) and PM emissions. Within some density ranges, one finds that these pollutant emissions can evolve into some higher values under some random seeds. Under other random seeds, they can evolve into some lower values. In a wide moving jam region, the emission values of these pollutants sometimes appear as continuous or intermittent phenomenon. Compared to the refined Na Sch model, the present model has lower instantaneous emissions such as CO_(2), NO_(X) and PM and higher volatile organic components(VOC) emissions. Compared to the limited deceleration capacity model without slow-to-start rules, the present model also has lower instantaneous emissions such as CO_(2), NO_(X) and PM and higher VOC emissions in a wide moving jam region. These results can also be confirmed or explained by the statistical values of vehicle velocity and acceleration.

Numerical simulation of humping phenomenon in high speed gas metal arc welding

It is of great significance to obtain a thorough understanding of the physical mechanisms responsible for humping bead phenomenon in high speed gas metal arc welding （GMAW） in order to raise welding efficiency. Experiments were conducted to observe the weld pool behaviors in high speed GMAW, and it was found that both the severely deformed weld pool surface and strong backward flowing play a dominant role in humping bead formation. In this study, a mathematical model is developed to quantitatively analyze the forming mechanism of humping beads for high speed GMAW through considering both the momentum and heat content distribution of the backward flowing molten metal inside the weld pool. The transient development of temperature profiles in the weld pool with severe deformation demonstrates the humping bead forming process under some welding conditions. The predicted and measured humping bead dimensions are in agreement.

Effect of droplet heat content distribution on humping formation in high speed GMAW

The momentum of strong backward flowing melt jet and the thermal action from transferred droplets are two dominating factors affecting the formation of humping bead in high speed gas metal arc welding （GMAW）. Appropriate describing the influ- ence of the distribution mode of droplet heat content in the weld pool is essential to understand the physical mechanism of humping bead formation. Based on the exper- imental results, four kinds of droplet heat content distribution modes are proposed and employed to calculate the transient evolution of the temperature field and weld pool during high speed GMAW process. Through making comparison of predicted and measured weld bead dimensions, a suitable and adaptive distribution mode of droplet heat content is found, i.e., droplet heat content is distributed in bottom layer of gouging region at the front of weld pool, and is averagely distributed in the whole layer at the rear of weld pool. The proposed mode is also validated by experimental observation of the weld pool images and measured by geometric dimensions of the weld bead.

Experimental determination of the critical welding speed in high speed MAG welding

In high speed MAG welding process, some weld formation defects may be encountered. To get good weld quality, the critical welding speed beyond which humping or undercutting weld bead can occur must be known for different conditions. In this research, high speed MAG welding tests were carried out to check out the effects of different factors on the critical welding speed. Through observing the weld bead profiles and the macrographs of the transverse sections of MAG welds, the occurrence tendency of humping weld was analyzed, and the values of critical welding speed were determined under different levels of welding current or voltage, and the effect of shielding gas compositions on the critical welding speed was also investigated.

Extraction of mode shapes of beam-like structures from the dynamic response of a moving mass

This paper proposes an approach to extract the mode shapes of beam-like structures from the dynamic response of a moving mass. When a mass passes through a beam containing several artificially installed humps, its vertical acceleration can be recorded. After applying fast Fourier transformation to the dynamic response, one can extract the mode shapes of the beam. The surface roughness was neglected compared to the humps and its adverse effect on the extraction was reduced. The passing mass performs as both “exciter” and “massage receiver”;therefore, this method requires only one single accelerometer, making it more convenient and time saving in practice. Moreover, to estimate the possible error in extracting mode shapes, a wavenumber domain filtering technique is used to reconstruct the general profiles of mode shapes. Experimental validation of this approach in laboratory scale was conducted. The experimental results show that the proposed method performs well in extracting lower order mode shapes. It should also be noted that the passing mass can not have a very high velocity (e.g. 80 mm/s), otherwise the mass may jump and separate from the beam, and the proposed method may fail to identify mode shapes.

Entropy Production Analysis for Hump Characteristics of a Pump Turbine Model

The hump characteristic is one of the main problems for the stable operation of pump turbines in pump mode.However,traditional methods cannot reflect directly the energy dissipation in the hump region.In this paper,3D simulations are carried out using the SST k-ω turbulence model in pump mode under different guide vane openings.The numerical results agree with the experimental data.The entropy production theory is introduced to determine the flow losses in the whole passage,based on the numerical simulation.The variation of entropy production under different guide vane openings is presented.The results show that entropy production appears to be a wave,with peaks under different guide vane openings,which correspond to wave troughs in the external characteristic curves.Entropy production mainly happens in the runner,guide vanes and stay vanes for a pump turbine in pump mode.Finally,entropy production rate distribution in the runner,guide vanes and stay vanes is analyzed for four points under the 18 mm guide vane opening in the hump region.The analysis indicates that the losses of the runner and guide vanes lead to hump characteristics.In addition,the losses mainly occur in the runner inlet near the band and on the suction surface of the blades.In the guide vanes and stay vanes,the losses come from pressure surface of the guide vanes and the wake effects of the vanes.A new insight-entropy production analysis is carried out in this paper in order to find the causes of hump characteristics in a pump turbine,and it could provide some basic theoretical guidance for the loss analysis of hydraulic machinery.

Experimental Investigation of Flow Separation Control Using Dielectric Barrier Discharge Plasma Actuators

Influence of plasma actuators as a flow separation control device was investigated experimentally. Hump model was used to demonstrate the effect of plasma actuators on external flow separation, while for internal flow separation a set of compressor cascade was adopted. In order to investigate the modification of the flow structure by the plasma actuator, the flow field was examined non-intrusively by particle image velocimetry measurements in the hump model experiment and by a hot film probe in the compressor cascade experiment. The results showed that the plasma actuator could be effective in controlling the flow separation both over the hump and in the compressor cascade when the incoming velocity was low. As the incoming velocity increased, the plasma actuator was less effective. It is urgent to enhance the intensity of the plasma actuator for its better application. Methods to increase the intensity of plasma actuator were also studied.

CHARACTERIZATIONS OF SOME MATRIX CLASSES OF QUASI-HOMOGENEOUS OPERATORS

In this paper, we give a general characterization of a class of matrices of quasi-homogeneous operators between topological vector spaces.

A method for optimization of time intervals between rolling cuts on sorting humps

Sorting humps are the main technical means providing the breaking- and making-up of freight trains. Automation of sorting process by implementing microprocessor control systems is the main direction ofincreasing hump yard capacity, enhancement of safety in trains breaking-up, improvement of working conditions on hump yards. However, for wider introduction of such systems at the humps, it is necessary to reduce their cost. One of the main tasks of hump control systems is the determination of the optimal cars exit speeds from the retarders. Its solution is significantly complicated because of the uncertainty of cars rolling resistance coefficient, wind velocity and direction, retarder control errors. The paper is aimed at solving the problem of searching for such cars exit speeds from the retarders that provide the longest time intervals between them at the switches. Implementation of this method will make it possible to improve the quality of sorting process as a whole by improving the software of microprocessor control systems for automatic humping, rather than by increasing their technical facilities, which as a result will reduce the cost of their construction and operation. The research results, when solving the problem of searching for cars exit speeds from the retarders, allow us to move from searching for the maximum of a nonlinear non-smooth function of two variables to solution of an equation with one variable, which significantlyaccelerates and simplifies the solution to the problem.

Hump nosed viper bite in Sri Lanka-descriptive observational study of 1543 cases

Objective:To identify the clinical manifestations of hump nosed viper envenomation and to recognize the available treatment methods to prevent complications.Methods:Descriptive observational study involving a series of 1 543 patients admitted with hump nosed viper bite to 5 major hospitals in Sri Lanka was conducted.Data collection was done consecutively during February 1990 and February 2008.Except Hypnale,identification of the biting snake was made by the corresponding author after visual examination of the dead or live snakes,which were brought to hospital.Results:Sixty seven(4.34%) patients developed systemic effects and two (0.1%) patients died due to effects of envenomation or complications of treatment.Systemic effects varied from coagulopathy,nephropathy to some neurological manifestations.Fifty nine (3.8%) patients had only coagulopathy and they received either,intravenous isotonic saline to ensure adequate urine out put i.e.0.5 mL/kg /hour or 15 mL/kg of fresh frozen plasma(FFP). None of the patients that had coagulopathy developed renal failure.Contamination of the sample by mildly venomous species of Hypnale may have contributed to the low incidence of systemic complications.Conclusions:It is likely that early hydration with normal saline or FFP can prevent acute renal failure.FFP showed a tendency for early correction of coagulopathy.Role of FFP in hump nosed viper envenomation is worth studying in randomized double blind controlled clinical trials.

Positive gate bias stress-induced hump-effect in elevated-metal metal-oxide thin film transistors

Under the action of a positive gate bias stress, a hump in the subthreshold region of the transfer characteristic is observed for the amorphous indium-gallium-zinc oxide thin film transistor, which adopts an elevated-metal metal-oxide structure. As stress time goes by, both the on-state current and the hump shift towards the negative gate-voltage direction. The humps occur at almost the same current levels for devices with different channel widths, which is attributed to the parasitic transistors located at the channel width edges. Therefore, we propose that the positive charges trapped at the back-channel interface cause the negative shift, and the origin of the hump is considered as being due to more positive charges trapped at the edges along the channel width direction. On the other hand, the hump-effect becomes more significant in a short channel device （L=2 μm）. It is proposed that the diffusion of oxygen vacancies takes place from the high concentration source/drain region to the intrinsic channel region.

The chest X ray in pulmonary embolism: Westermark sign, Hampton's Hump and Palla's sign. What's the difference?

Pulmonary embolism (PE), with the incidence of about 60 per 100000 annually, can be a life-threatening disease if it is not treated promptly. It has been estimated that some 10% of PE patients die within the first hour of the event. Untreated PE has a mortality of about 30%. PE is a condition that is treatable if suspected and diagnosed early. The chest radiograph is still the first investigation that is ordered in patients presenting with cardiorespiratory symptoms or symptoms suggestive of PE. The CXR is also helpful in identifying or excluding other conditions or diagnoses. Thus, knowing and understanding some of the more specific CXR signs can be useful. We suggest that physicians to be aware of and utilize CXR findings such as Palla's sign, Westermark sign and Hamptons hump to help with the diagnosis of PE and to exclude other conditions that can mimic venous thrombo-embolism. Even if these signs are not common, their presence, even in an unsuspected patient without a high pretest probability of PE, should prompt further investigations such as a D-dimer test, lung scintigraphy or computed tomography pulmonary angiography as required.

On Gliding Hump Properties of Matrix Domains

In this note, we establish several results concerning the gliding hump properties of matrix domains. In order to discuss F-WGHP, we introduce the UAK-property and find that this sort of property has close relationship with F-WGHP. In the course of discussing F-WGHP and WGHP of （C0）cn, we discuss the F-WGHP and WGHP of the almost-null sequence space f0.

Hump formation mechanism in gas-jet-assisted keyhole laser welding of HR-2 stainless steel

Gas-jet-assisted keyhole laser welding offers the possibility of a breakthrough in the limitations of penetration depth in laser welding,which currently suffers from equipment restrictions.A gas jet of sufficient intensity to assist the keyhole should be used to obtain suppressed plasma,a deepened keyhole,and increased penetration depth.However,an excessively strong gas jet gives rise to humps.The incident angle of the keyhole-assisted gas jet is 60°,with a nozzle ahead of the laser beam.A series of experiments were carried out with different welding velocities and gas parameters by using HR-2 hydrogen-resistant stainless steel and a slab CO2 continuous-wave laser welding machine.The weld profiles can be categorized into four types,welds of traditional laser welding,welds of enhanced laser welding,undercut welds,and humping welds with increased gas pressure.A high-speed camera was employed in the experiments to monitor the formation of humps under an excessively strong gas jet.The results of analysis show that hump formation can be divided into six stages.Its main driving force is the intense turbulence of gas jet.There are two main reasons for hump formation：premature solidification of the molten pool caused by the large temperature gradient between the front and rear parts of the molten pool,and the emergence of a thin layer liquid bridge with one-directional flow under the enhanced cooling effect of excessively strong gas.

The role of endothelial biomarkers in predicting damp-heat syndrome in diabetic kidney disease

Objective:To explore the role of endothelial biomarkers in predicting damp-heat syndrome in diabetic kidney disease(DKD).Methods:A total of 183 patients with DKD were divided into 3 groups:the early DKD group,established DKD group,and advanced DKD group.All patients were classified according to traditional Chinese medicine(TCM)syndrome type,and clinical indexes were collected for statistical analysis.Results:A total of 183 DKD patients were included in this study.Fibroblast growth factor 23(FGF23),chitinase-3-like protein 1(CHI3L1),endocan,tumor necrosis factor receptor 1(TNFR1),secretory leukocyte protease inhibitor(SLPI),and vascular endothelial growth factor A(VEGF-A)were increased in advanced DKD.FGF23,CHI3L1,endocan,SLPI,and TNFR1 showed a negative correlation with estimated glomerular filtration rate(eGFR),while they had a positive correlation with 24 h urine protein.After adjusting for age,gender,diabetes duration,body mass index(BMI),hemoglobin,glucose,uric acid,24 h urine protein,cholesterol,triglyceride,low-density lipoprotein,and hemoglobin A1c(HbA1c),the multiple regression analysis showed that FGF23,endocan,TNFR1,and SLPI significantly correlated with eGFR.Conclusions:FGF23,endocan,TNFR1,and SLPI are elevated in advanced DKD compared with early stage,and they may take part in the pathogenesis and progression of DKD.Our study provides useful biomarkers for predicting the appearance of damp-heat syndrome,including FGF23,endocan,TNFR1,and SLPI.

Augmentation of literature review of COVID-19 radiology

We suggest an augmentation of the excellent comprehensive review article titled“Comprehensive literature review on the radiographic findings,imaging modalities,and the role of radiology in the coronavirus disease 2019(COVID-19)pandemic”under the following categories:(1)“Inclusion of additional radiological features,related to pulmonary infarcts and to COVID-19 pneumonia”;(2)“Amplified discussion of cardiovascular COVID-19 manifestations and the role of cardiac magnetic resonance imaging in monitoring and prognosis”;(3)“Imaging findings related to fluorodeoxyglucose positron emission tomography,optical,thermal and other imaging modalities/devices,including‘intelligent edge’and other remote monitoring devices”;(4)“Artificial intelligence in COVID-19 imaging”;(5)“Additional annotations to the radiological images in the manuscript to illustrate the additional signs discussed”;and(6)“A minor correction to a passage on pulmonary destruction”.