In this work, the subgrid-scale (SGS) stress and the SGS heat flux of compressible isotropic turbulence are modeled by an artificial neural network (ANN) mixed model (ANNMM), which maintains both functional and structural performances. The functional form of the mixed model combining the gradient model and the Smagorinsky’s eddy viscosity model is imposed, and the ANN is used to calculate the model coefficients of the SGS anisotropy stress, SGS energy, and SGS heat flux. It is shown that the ANNMM can reconstruct the SGS terms more accurately than the gradient model in the a priori test. Specifically, the ANNMM almost recovers the average values of the SGS energy flux and SGS energy flux conditioned on the normalized filtered velocity divergence. In an a posteriori analysis, the ANNMM shows advantage over the dynamic Smagorinsky model (DSM) and dynamic mixed model (DMM) in the prediction of the spectra of velocity and temperature, which almost overlap with the filtered direct numerical simulation data, while the DSM and DMM suffer from the problem of the typical tilted spectral distribution. Besides, the ANNMM predicts the probability density functions of SGS energy flux much better than DSM and DMM. ANN with functional model forms can enlighten and deepen our understanding of large eddy simulation modeling.

The characteristics of molten pools provide valuable insight into the complexity of the metal additive manufacturing process, which has a significant influence on the quality of the parts built using this process. In our work, we develop a three-dimensional multiphysics finite element model of a selective laser melting (SLM) process on a Ti–6Al–4V alloy. The dynamic characteristics of the molten pool are studied by multiphysics simulation with consideration of phase transitions, recoil pressure, surface tension, and Marangoni effect. The results show the time-evolution of temperature distribution, flow field, and surface morphology of a single track during the SLM process. The recoil pressure caused by evaporation plays a significant role in molten pool dynamics and induces a depression at the head of the molten pool. As a result of the backward Marangoni flow, the material is shifted to the tail region and a vortex is generated. In addition, a protrusion is presented at the middle and start points of the scanned track, while a depression is formed at both sides and at the terminal point. The simulation and the experimental results on the surface morphology of the molten track during the SLM process are in good agreement. Furthermore, it is found that metal evaporation may take place not only on the surface of the molten pool but also inside it, due to the drop in pressure. This is a significant contributor to the formation of porosity in SLM parts.

For a non-contact, non-destructive quality evaluation, laser ultrasonic testing (LUT) has received increasing attention in complex manufacturing processes, such as additive manufacturing (AM). This work assessed the LUT method for the inspection of internal hole defects in additive manufactured Ti-6Al-4V part. A Q-switched pulsed laser was utilized to generate ultrasound waves on the top surface of a Ti-6Al-4V alloy part, and a laser Doppler vibrometer (LDV) was utilized to detect the ultrasound waves. Sub-millimeter (0.8 mm diameter) internal hole defect was successfully detected by using the established LUT system in pulse-echo mode. The method achieved a relatively high resolution, suggesting significant application prospects in the non-destructive evaluation of AM part. The relationship between the diameter of the hole defects and the amplitude of the laser-generated Rayleigh waves was studied. X-ray computed tomography (XCT) was conducted to validate the results obtained from the LUT system.

This article presents a reverse modeling of the headform when wearing a filtering facepiece respirator (FFR) and a computational fluid dynamics (CFD) simulation based on the modeling. The whole model containing the upper respiratory airway, headform, and FFR was directly recorded by computed tomography (CT) scanning, and a medical contrast medium was used to make the FFR “visible.” The FFR was normally worn by the subject during CT scanning so that the actual deformation of both the FFR and the face muscles during contact can be objectively conserved. The reverse modeling approach was introduced to rebuild the geometric model and convert it into a CFD solvable model. In this model, we conducted a transient numerical simulation of air flow containing carbon dioxide, thermal dynamics, and pressure and wall shear stress distribution in the respiratory system taking into consideration an individual wearing a FFR. The breathing cycle was described as a time-dependent profile of the air velocity through the respiratory airway. The result shows that wearing the N95 FFR results in CO₂ accumulation, an increase in temperature and pressure elevation inside the FFR cavity. The volume fraction of CO₂ reaches 1.2% after 7 breathing cycles and then is maintained at 3.04% on average. The wearers re-inhale excessive CO₂ in every breathing cycle from the FFR cavity. The air temperature in the FFR cavity increases rapidly at first and then stays close to the exhaled temperature. Compared to not wearing an FFR, wearers have to increase approximately 90 Pa more pressure to keep the same breathing flow rate of 30.54 L/min after wearing an FFR. The nasal vestibule bears more wall shear stress than any other area in the airway.