Dimensional Simulation Research Articles

Nonlinear incompressible shear wave models in hyperelasticity and viscoelasticity frameworks, with applications to Love waves

General equations describing shear displacements in incompressible hyperelastic materials, holding for an arbitrary form of strain energy density function, are presented and applied to the description of nonlinear Love-type waves propagating on an interface between materials with different mechanical properties. The model is valid for a broad class of hyper-viscoelastic materials. For the Murnaghan constitutive model, shear wave equations contain cubic and quintic differential polynomial terms, including viscoelasticity contributions in terms of dispersion terms that include mixed derivatives uxxt of the material displacement. Full (2+1)-dimensional numerical simulations of waves propagating in the bulk of a two-layered solid are undertaken and analysed with respect to the source position and mechanical properties of the layers. Interfacial nonlinear Love waves and free upper surface shear waves are tracked; it is demonstrated that in the fully nonlinear case, the variable wave speed of interface and surface waves generally satisfies the linear Love wave existence condition c1<v<c2, while tending to the larger material wave speed c1 or c2 for large times.

Three dimensional time-variation simulator for water flooding reservoir based on “effective water flux”

Long-term water flooding leads to changes in pore throat structure, resulting in alterations in macroscopic reservoir petrophysical parameters. However, commercial numerical simulation software does not have this capability. Ignoring variations in physical parameters during the formulation of development plans and numerical simulations can lead to significant prediction errors, which severely impacts oil field recovery. This paper, based on an analysis of effective flow rate and waterflood intensity, proposes a new erosion degree characterization parameter: Effective water flux, to represent the time-varying patterns of physical parameters. It is embedded into a black oil model to develop a time-variation simulator, whose accuracy and stability in both black oil and time-variation models are validated through comparison with the commercial numerical simulation software CMG. The study further explores the effects of different parameter variations on the development process. It was found that increases in permeability and oil viscosity exacerbate heterogeneity and reduce displacement efficiency, while decreases in residual oil saturation and water phase permeability under residual oil saturation enhance water flooding efficiency. In complex models, the effects of variations in different parameters intertwine, collectively influencing development outcomes. This paper advances the development of time-variation numerical simulation technology.

Local heating at the running crack tip in bcc iron according to molecular dynamics

Abstract This study presents estimates of a possible temperature rise at the crack tip from three dimensional (3D) atomistic simulations of fracture via molecular dynamics (MD) technique. Simulations start from an initial temperature of 0 K. The pre-existing edge crack was loaded in tension mode I. Crack initiation in MD is accompanied by surface dislocation emissions and later by a cross slip of the emitted dislocations into other slip systems. This leads both to stress waves radiation and to local heating in the plastic zone created by these dislocations. The local heating at the crack tip in the surface layers reaches a level of 480–500 K at some atoms, but an average temperature in the plastic zone is lower and depends on a chosen crack tip zone size. In the bulk crystal, where no dislocation emission (i.e. no plastic zone) has been realized, no significant heating is observed at the crack tip. MD results at the free sample surface comply with experimental data for ferritic steels with a pre-existing notch, loaded (quasi-statically) in mode I, as well as with some continuum predictions.

Parallelization of Three Dimensional Cardiac Simulation on GPU.

The simulation of electrophysiological cardiac models plays an important role in facilitating the investigation of cardiac behavior under various conditions. However, these simulations often require a lot of computational resources. To address this challenge, this study introduced a method for speeding up three-dimensional cardiac simulations using GPU parallelization. A series of optimizations was introduced, encompassing various aspects such as data storage, algorithmic enhancements, and data transfer. The experimental results reveal that the optimized GPU parallel simulations achieve an approximate 50-fold acceleration compared with their CPU serial program. This investigation substantiates the considerable potential of GPUs in advancing the field of cardiac electrophysiology simulations.

Three dimensional simulations of FRC beams and panels with explicit definition of fibres-concrete interaction

High performance concrete (HPC) is a quite novel material which has been rapidly developed in the last few decades. It exhibits superior mechanical properties and durability comparing to normal concrete. HPC can achieve also superior tensile performance if strong fibres (steel or carbon) are implemented in the matrix. Thus, there exist the unabated interest in studying how the addition of different types of fibres modifies the behaviour of HPC. Nowadays, a standard numerical approaches to model the behaviour of fibre reinforced concrete (FRC) are carried out by means of the smeared or discrete crack modelling of homogenous media with appropriately changed stress-strain relationships. The objective of this paper is to develop a new and efficient mesoscale modelling approach for steel fibre reinforced high-performance concrete. The main idea of presented approach is to assume the fully 3D modelling with taking into account explicitly the distribution and orientation of the steel fibres. As a benchmark, results obtained from experimental campaign on beams and panels made from high-performance concrete with steel fibres of different sizes and dosages were taken. Results of numerical simulations were directly compared with experimental outcomes in order to validate and calibrate FE-model and to introduce the efficient numerical modelling tool.

Energy and exergy performance improvement of coupled PV–TEG module by using different shaped nano-enhanced cooling channels

In this study, impacts of using different cooling channels (L, T and U-shaped) on the energy and exergy performances of combined PV/TEG (photovoltaic/thermoelectric generator) unit are numerically assessed by using FEM based three dimensional high fidelity computational simulations. Cooling performance is boosted by using ternary nanofluid in the channels. The study is conducted for a range of Reynolds numbers (Re, between 50 and 500), nanofluid cooling inlet temperatures (between 15 °C and 23 °C), and nanoparticle loading amounts (between 0 and 3%). Evaluations are done on average PV-cell temperature, PV/TEG output powers, exergy efficiency, improve potential (IP), and sustainability index (SI). U-shaped and T-shaped channels are found to offer the greatest and worst cooling performances among various cooling channels. When comparing the lowest and maximum Re cases, the average temperature drops for PV cells are 3.54 °C for T channel and 2.55 °C for U channel. Exergy efficiency is determined to be 14.2% with T-channel at Re=50 and 14.6% with U-channel at Re=500. Taking into consideration different channels, an increase in the inlet temperature from 15 °C to 23 °C leads to an average rise in cell temperature of 6 °C. The IP rises with coolant inlet temperature while using T-channel. SI values fall as input temperature increases, although values between 1.164 and 1.175 may be achieved by using different cooling channels. Using nanofluid with higher loading results in higher exergy efficiency when comparing exergetic performances, with U-channel design exhibiting the highest performance. For the range of parameters considered, the coolant inlet temperature at the channel entry has the highest effect on PV cell temperature reduction; 8.5 °C of temperature reduction is feasible. By employing U-channel cooling, the maximum value of Re, the highest loading of nanoparticles, and the lowest inlet temperature yield the best exergy efficiency. The best and worst situations’ respective exergy efficiencies are determined to be 15% and 14.1%. Utilizing T-channel cooling with water at a flow flow rate and a higher input temperature is the scenario with the biggest potential for exergy improvement; the value is 2.9. When different operational parameters are varied, the SI falls between 1.176 to 1.16.

The influence of membrane fiber arrangement on gas exchange in blood oxygenators: A combined numerical and experimental analysis

Oxygenators take over lung function in the event of severe lung injuries by exchanging gas into the blood stream using hollow fiber membrane mats. A central limitation of oxygenators is their large foreign surface area and priming volume, which cause blood damage.Thus, gas exchange efficiency needs to be improved through an understanding of the interaction between fiber arrangement and blood flow directions. This has only been investigated in two dimensional or simplified non-realistic fiber bundles. The aim of this study was to quantify gas exchange in realistic 3D fiber bundles.We performed three dimensional micro scale CFD simulations of different realistic fiber arrangements and flow directions that we validated in corresponding test oxygenators using porcine blood.Fiber-configurations influence gas transfer by factor 2, with highest transfer rates for blood flowing in circumferential direction, and lowest for longitudinal direction in wound oxygenators. The CFD model correlates with the experiments with an R2 = 0.88.For the first time, the configuration of realistic 3D fiber bundles was proven to strongly influence gas transfer in oxygenators, in simulations and experiments. The CFD model serves for investigation of further configurations and to derive transfer coefficients for full scale oxygenators.

Micromagnetic simulation of the magnetization-controlled critical current in a S–(S/F)–S superconducting switch

In this Letter, we provide three dimensional micromagnetic simulations describing the nonvolatile magnetization control of the critical current of a superconductor–proximity-modified superconductor–superconductor junction by initializing and training its magnetization state in an external magnetic field, the experimental demonstration of which had been reported earlier. In the present work, we develop a microscopic explanation for the observed general behavior of the reduced critical current Ic in states of high magnetization M. We are able to reproduce the non-monotonous behavior of Ic(M) and can clearly correlate the discrete jumps in Ic(M) with flips of single or few magnetic domains in granular cobalt. We show that both the three-dimensional modeling and the grain size distribution are important to replicate the experimental observations.

Enhanced quasi-three-dimensional transient simulation technique incorporating component volume effects for aero engine

Current transient analysis predominantly relies on zero-dimensional/one-dimensional tools, proficient at capturing aerothermodynamic variations across critical engine stations but insufficient for analyzing the internal flow field evolution during transients. Addressing this gap, the study presents an enhanced quasi-three dimensional (quasi-3D) transient simulation technique that integrates component volume effects, offering a significant leap from the preceding quasi-3D transient simulation method based on quasi-steady assumption. By embedding the component volume effects on density, momentum, and energy within the physical temporal dimension of the Navier-Stokes equations, the refined quasi-3D transient model achieves a closer representation of physical phenomena. Validation against a single-shaft turbofan engine’s experimental data confirms the model’s accuracy. Average errors for key performance indicators, including shaft speed, thrust, mass flow rate, and critical component exit temperature and pressure, remain below 0.41%, 5.69%, 2.55%, 3.18% and 0.67%, respectively. Crucially, the model exposes a discernible temporal lag in the compressor outlet pressure and temperature response due to volume effects—previously unquantified in quasi-3D transient simulations. And further exploration of the meridional flow field emphasizes the consequential role of volumes in transient flow field evolution. Incorporating volume effects within quasi-3D transient simulations enhances engine modeling and is pivotal for precise transient analysis in engine design and optimization.

Deciphering MAP kinase pathway in Cicer arietinum upon Helicoverpa armigera -infestation

The activation of Mitogen-Activated Protein Kinase (MAPK) pathway is recognized as a key regulator of the plant defense mechanisms in response to insect attacks. The MAPK pathway comprises three main components: MAP Kinase Kinase Kinase (MAPKKK), MAP Kinase Kinase (MAPKK) and MAP Kinase (MAPK). The MAPKKK is the first kinase in the MAPK cascade, which typically responds to external stimuli and activates downstream component, MAPKK, which, in turn, activates the final component, MAPK, which further plays a key role in regulating gene expression of defense-associated transcription factors. Elucidation of complete MAPK pathway and its components has been investigated in the model plants in the past, however, it has not been attempted in leguminous plant, chickpea, which is attacked vigorously by Helicoverpa armigera, causing huge loss in crop production. Therefore, the study aims to decipher and elucidate MAP kinase pathway activated during chickpea-H. armigera interaction. MAPKKK, MAPKK, MAPK are a family of genes and specific genes from each components, were identified and selected for further analysis based on their expression patterns in response to H. armigera-infestation. Further, three dimensional structure prediction, validation, docking and simulation of the selected proteins from each component of the pathway, were executed and specific components from chickpea were predicted. The probable pathway forecasted is MEKK4/Raf22 (MAPKKK)-MAPKK(4)10-MAPK12/6). This article provides an overview of the molecular mechanisms involved in plant's response to pest attacks, unravelling the components of MAPK pathway in chickpea induced during herbivory. Further validation of this pathway by in vitro/in vivo approaches is highly recommended.

A general drag coefficient model for a spherical particle incorporating rarefaction and particle-to-gas temperature ratio effects

A concept and model of two critical Reynolds numbers Rep,cr1 and Rep,cr2 corresponding respectively to onsets of drag crisis and recovery are proposed. A drag model at limits of zero particle Mach and Knudsen numbers is constructed. On this basis, we develop a general drag coefficient model applicable for a spherical particle traveling in a gas by using a large number of available data derived from the previous experiments, direct numerical simulations and direct simulation Monte-Carlo methods. The scope of applicability of the proposed drag model covers all flow regimes relative to the particle characterized by particle Reynolds and Mach (or Knudsen) numbers and different particle-to-gas temperature ratios. Its comparison with two latest general drag models shows the significantly smaller relative error. Furthermore, quasi-one dimensional simulations against two supersonic nozzle gas-particle flow experiments are conducted with an in-house code to validate its accuracy in comparison with the two drag models.

Transient numerical simulation to investigate pressure fluctuation spectrum and deficit in the near wake of a horizontal axis wind turbine

Wind energy becomes one of the promised solutions as alternatives of using fossil power energy in the upcoming decades. As pressure fluctuation and flow deficit in the near wake play a crucial role in predicting the turbine performance, the novelty in the current study focuses on a three dimensional (3 D) transient numerical simulation to investigate near wake characteristics of NREL phase IV wind turbine. Pressure fluctuation spectrum and wake deficit at four sections downstream the turbine are the main parameters studied as well as the flow streams and iso-surface characteristics. The mentioned parameters are studied at four sections: (0.25 D), (0.5 D), (0.75 D), and (1 D) of turbine diameter. The obtained results are compared with corresponding data from the NREL Phase IV experiments for validation purposes. Results show that the highest-pressure fluctuation spectrum would be at the nearest wake region, and, then it decreases periodically. Hub pressure spectrum density is much lower than that density for the turbine blade. The wake deficit profile shows that the deficit is developing with time at a rate equal to (0.115 rate per sec). In addition, it was found that the wake is recovered just (1 d) of the hub diameter while the blade wake deficit becomes the dominant after (0.75 D) of turbine diameter to cover the whole wake region. It was also found that the downstream distances should be extended more to resolve the whole wake recovery. The comparison with the tests measurements shows agreement between both the numerical and experimental works such that the error doesn't pass 4.3 %.

Investigation of hydraulic characteristics of Pelton turbine with casing structure using a moving particle simulation method

Abstract This paper reports the initial results of three dimensional simulation of the jet/bucket interactions in the vertical Pelton turbine with different casing structures. The casing is an important part of the Pelton turbine, and a reasonable casing structure can reduce the interaction between the jet and bucket, so as to improve the hydraulic efficiency of the turbine. Previous research has little numerical simulation analysis on the casing. In this paper, a Moving Particle Simulation(MPS) method without grid generation is adopted to compare and analyze the hydraulic characteristics of the runner under the regular hexagonal and circular casing structures. The simulation results show that the hydraulic efficiency of the runner under the hexagonal casing structure is better, and the runner time average torque is improved by 3.09%, which compared with the circular casing. The distribution of free surface flows in the hexagonal casing is analysis, and the obvious interference phenomenon between jets can be observed, which has an important guiding role in the design of the casing structure of Pelton turbine in the future.

Two dimensional simulations to study the relationship between settling velocity and flexibility of a particle

In various industrial and real-life scenarios, sedimentation, whether involving flexible fibres, permeable structures, or a combination of both, plays a pivotal role. Its impact spans from influencing paper properties to waste water treatment and microorganism transport dynamics. Understanding sedimentation is crucial for optimizing processes like flocculation, organic matter removal, and particulate material management. Settling velocity, a key metric, is vital in designing instruments and formulating optimization strategies across environmental engineering and sediment transport. Despite extensive research on settling velocity correlations with viscosity, structure density, and permeability, the relationship with structural flexibility remains unexplored. This study employs the Immersed Boundary (IB) method, utilizing a MATLAB code to numerically investigate the correlation between settling velocity and the flexibility of settling structures, addressing a gap in prior research. The results demonstrate a robust correlation between settling velocity and flexibility, supported by high R-squared values (ranging from 0.9979 to 1) for exponential fits across all discussed cases. The R-squared value, a statistical measure assessing model accuracy, reinforces the superiority of the exponential fit in describing the settling velocity-flexibility relationship. To confirm the optimal fit, we conducted fitting attempts with various curve types using MATLAB, encompassing polynomial, Fourier, and smooth spline curves for both impermeable and permeable structures. The exponential curve consistently emerged as the most fitting model in this context.In our recent research, we conducted a sensitivity analysis focusing on the time-step to validate the robustness of our findings. The investigation encompassed both impermeable and permeable scenarios for the structures under study. The time-step was systematically varied across a specified range, revealing a notable outcome: the results demonstrated a consistent independence from the chosen time-step values.

Synthetically impacts of the topography and typhoon periphery on the atmospheric boundary layer structure and special regional pollution pattern of O3 in North China Plain

Westerly warm advection from the Loess Plateau to the over layer of North China Plain (NCP) suppresses the mixed layer during the daytime in summer, having already be confirmed as an important formation mechanism of regional O3 pollution all over NCP. However, when NCP was simultaneously affected by the westerly warm advection from the Loess Plateau and the typhoon periphery airflow from the Western Pacific tropical cyclone system, a rare pollution pattern of surface O3 was detected, characterized by a very low ozone pool in Beijing (LOP-BJ) contrast to much higher O3 in surrounding areas. To explore the causes and deep formation mechanism to this special pollution phenomenon, regional pollution characteristics of O3, characteristics of the atmospheric boundary layer (ABL) structure and vertical airflows affected by different weather systems were analyzed based on the analysis of observations and three dimensional simulations. It was found that the synthetic impacts of the topography and the typhoon peripheral flow promoted the stabilization of the ABL and introduced a nearly isolated air mass in terms of both thermodynamic and dynamic structures within ABL in Beijing region. In the stable and isolated ABL air masses, the significant lower ozone near the ground was primarily attributed to the strong loss by NO titration consumption from local excessive emissions and extremely weak transport contributions from surrounding areas. The conclusion was also confirmed by the integrated process rate (IPR) analysis results of contributions from chemistry tendency (CHEM) and total transport tendency (TRAN) to surface ozone. Our work insight the understanding of the formation mechanism of regional ozone pollution and improve the forecasting capabilities under comprehensive weather conditions over NCP.

Optimization of simulation model adaptability and CFD of particles of Geldart‐B in a gas–solid fluidized bed for silicone monomer synthesis

AbstractThe flow characteristics of gas–solid in fluidized bed reactors constrain or promote the reaction process by affecting gas–solid mixing homogeneity, heat transfer timeliness, and fluidization stability. To improve the production efficiency of silicone monomers, this work shows a comparative analysis of the effects of geometric modelling in different dimensions, flow state, and the interphase exchange coefficient on the characteristics of particles of Geldart‐B in a gas–solid fluidized bed for silicone monomer synthesis based on two‐fluid models (TFM). The results of different dimensional simulations show that the third dimension plays an important role in the bubble behaviour and the particle volume fraction spatial distribution within the fluidization zone, and the three‐dimensional simulation results are closer to the experimental data, and the maximum average error of the simulation results is 4.52% lower than that of the two‐dimensional simulation results. Meanwhile, the flow model has a greater influence on the bubble behaviour during the flow process, whereas the interphase force model has little effect on the flow state of small‐diameter particles. However, for large‐diameter particles, the interphase force model is closely related to the bubble behaviour and the direction and size of the vortex, and the lift model prediction of bubble breakage and coalescence behaviours are higher than the experimental results from the power spectrum results. The inlet velocity has a greater effect on the formation of vortices near the wall; with the same inlet velocity, the more dispersed the particle distribution in the bed, and the more uniform the gas–solid mixing.

MUST dome design based on dome seeing quantitative evaluation

ABSTRACT The Multiplexed Survey Telescope (MUST) project is led by Tsinghua University, which entrusted the European Industrial Engineering (EIE) GROUP with the design, manufacture, and assembly of the dome. Located on Saishiteng Mountain in Qinghai Province, MUST benefits from exceptional atmospheric seeing conditions etc. Local dome seeing may be comparable to atmospheric seeing and requires careful consideration. This research, based on numerical simulations, focuses on refining the dome structure and temperature regulation strategies to achieve optimal dome seeing. The existing simulations only consider nighttime dome seeing and overlook the impact of daytime dome heating on nighttime conditions. The Computational Fluid Dynamics (CFD) part mostly relies on a steady-state k − ε model, which cannot simulate transient processes or capture optical turbulence. In this study, a comprehensive multiphysics field coupling simulation was conducted, encompassing radiation heat transfer, fluid heat transfer, CFD, and ray tracing. Simulations include both daytime and nighttime scenarios, taking into account the daytime heating of the dome due to solar irradiation, as well as dome seeing under natural ventilation at night. The CFD utilizes the large eddy simulation model, enabling three-dimensional transient simulation and the simulation of optical turbulence. Ultimately, the broadening of the point spread function was statistically analysed after a certain integration time, facilitating a quantitative evaluation of dome seeing. This numerical simulation approach is closer to real world conditions, improving simulation accuracy and addressing the shortcomings of existing simulations. Some qualitative conclusions are consistent with practical engineering experience. In the end, the dome seeing was successfully regulated to 0.21 arcsec, meeting the observational requirements.

Nonplanar effects in simulations of laser-driven ejecta microjet experiments

Recent experiments of laser-driven ejecta microjets performed at OMEGA 60 reveal tortuous jets whereby the jets appear to deviate from their initial trajectory as they travel in vacuum. To understand these data, we perform two dimensional numerical simulations, considering different target thicknesses, pressures, and models of the drive conditions. In particular, modeling the finite laser spot size appears essential in reproducing qualitatively the non-planar shock observed in the experiment. Simulations capture jet deflection by accounting for a slight misalignment of the laser pointing with respect to the groove axis along with spatial variation of the laser pulse intensity. The principal physical mechanism appears to be that lateral momentum is imparted by release waves arising from the non-planar drive. The induced off-axis velocity is small in comparison to the jet axial velocity but integrates into a pronounced deflection over the course of the experiment. The analysis of jet axial and lateral mass distributions is found to be reproduced reasonably by the simulations. Simulated radiographs are in qualitative agreement with the experiments, though their differences point to potential shortcomings in modeling strictly three-dimensional experiments using two-dimensional hydrodynamic simulations. The simple analysis is able to explain part of the observed discrepancy in simulated vs experimental jet masses.

Thermal enhancement of microchannel heat sink using pin-fin configurations and geometric optimization

The microchannel heat sink (MCHS) is a robust cooling technique that ensures the efficiency and reliability of compact electronic devices by dissipating a large amount of heat because of its high surface area-to-volume ratio. This study proposes a novel modification of the pin-fins geometry in MCHS, and geometric optimization using response surface methodology (RSM) to build a low thermal resistant MCHS with enhanced heat transfer efficiency with low-pressure drop. Three dimensional numerical simulations using ANSYS FLUENT 2021 R2 are performed on three pin-fins configurations, i.e., MC-BW (pins mounted transversely to the bottom wall), MC-SW (pins mounted transversely to the side wall), and MC-Mixed (pins mounted transversely to the bottom and side wall). The thermal and flow characteristics are investigated using a laminar conjugate heat transfer model at Reynolds numbers 100–1000. Results show that introducing pin-fins significantly enhances heat dissipation as it continuously breaks the boundary layer and generates flow separation downstream of the pin-fins, which enhances fluid mixing and increases heat transfer augmentation inside MCHS. Among different configurations, the MC-Mixed gives the highest improvement of 50% in the convective heat transfer coefficient at Re = 1000. The highest thermal enhancement factor of η = 1.4 is obtained for the MC-Mixed configuration at Re = 600. For the base wall pin fin configuration RSM yields optimized values of 2.50 mm, 0.25 mm, and 0.045 mm for transverse pitch, longitudinal pitch, and diameter of pin respectively, and for the mixed pin fin configuration it gives 1.0 mm, 0.150 mm, 0.035 mm and 1.250 mm values for transverse pitch, longitudinal pitch, diameter of pin and pitch of side wall pins respectively for the maximum heat transfer and minimum pressure drop.

The Effect of Preoperative Three Dimensional Modeling and Simulation on Outcome of Intracranial Aneursym Surgery.

Three-dimensional (3D) printing in vascular surgery is trending and is useful for the visualisation of intracranial aneurysms for both surgeons and trainees. The 3D models give the surgeon time to practice before hand and plan the surgery accordingly. The aim of this study was to examine the effect of preoperative planning with 3D printing models of aneurysms in terms of surgical time and patient outcomes. Forty patients were prospectively enrolled in this study and divided into two groups : groups I and II. In group I, only the angiograms were studied before surgery. Solid 3D modelling was performed only for group II before the operation and was studied accordingly. All surgeries were performed by the same senior vascular neurosurgeon. Demographic data, surgical data, both preoperative and postoperative modified Rankin scale (mRS) scores, and Glasgow outcome scores (GOS) were evaluated. The average time of surgery was shorter in group II, and the difference was statistically significant between the two groups (p<0.001). However, no major differences were found for the GOS, hospitalisation time, or mRS. This study is the first prospective study of the utility of 3D aneurysm models. We show that 3D models are useful in surgery preparation. In the near future, these models will be used widely to educate trainees and pre-plan surgical options for senior surgeons.