Stress Fluctuations Research Articles

Numerical analysis of wall shear stress in a rotating drum based on two fluid model

A better understanding of the interaction between particle and the wall in rotating drums is of great interest. Two Fluid Model (TFM) coupled with kinetic theory of granular flow (KTGF) is used for investigating the wall shear stress of particle phase in a rotating drum. The distribution characteristic of wall shear stress and the effects of operation parameters and contact parameters on wall shear stress are analyzed. The results show that except the fluctuation of wall shear stress with time, its distribution is basically fixed which corresponds to the stable granular flow. The wall shear stress increases with increasing rotational speed and fill level while the degree of increase gradually decreases. With increasing of specularity coefficient which is defined as the average fraction of relative tangential momentum transferred in a particle-boundary collision, the position of shoulder region of granular flow changed a little while the position of toe region is relatively fixed. The distribution of wall shear stress has no essential difference and only a slight increase of its average value with increasing angle of friction with a constant specularity coefficient. Fig. 2. The distribution of wall shear stress of the rotating drum at different times at 65 rpm. • Wall shear stress was studied using TFM coupled with KTGF. • The effect of operation parameters on wall shear stress is analyzed. • The effect of contact parameters on wall shear stress is analyzed.

Influence of shear zone thickness and strike-slip faulting on tectonic tremor in the Nankai Trough, southwest Japan

To investigate the mechanism of tectonic tremor and the factors that influence its occurrence and fluctuations, we use seismic reflection data to map in detail a shear zone associated with the plate boundary fault in the Nankai accretionary prism, southwest Japan. The shear zone can be identified from its low seismic velocity and low reflection amplitude, which may signify the presence of high pore pressure, around the transition boundary from frontal outer wedge to inner wedge. A detailed comparison of the mapped shear zone and the distribution of tectonic tremor demonstrate that tremor tended to occur in the thick part of the shear zone, where high pore fluid pressure may weaken faults and thereby facilitate slow earthquakes. Furthermore tremors align on the oblique strike-slip faults that caused variation of the shear zone thickness as well as fluctuation of stress and pore pressure. The oblique strike-slip faults that control the thickness of the shear zone and tremor distributions are interpreted as features formed by the collision of an accretionary prism against a landward tectonic backstop.

Numerical study on thermal stress fluctuation caused by coaxial-jet flow in lower head of central measurement column

In the region affected by thermal striping, the temperature fluctuation will cause alternating thermal stress in the components of nuclear reactor systems. It may lead to fatigue damage accumulation and final failure of the structure. In order to obtain the thermal stress, a simplified model of lower head of the central measurement column is built up based on the coaxial-jet flow model, and a unidirectional fluid-structure interaction simulation is conducted in the present study. The characteristics of transient thermal stress on the surface and cross-sections of the metal plate at different depths along the normal direction are numerically studied. In addition, the effects of the inlet velocity and temperature difference on the thermal stress are analyzed. The feasibility and reliability of the proposed calculation model and method are further verified by experimental data in literatures. The distribution feature of the thermal stress is similar to that of temperature. There is more than one principal frequency of thermal stress and the maximum frequency is 8 Hz. Along the depth direction, the distribution of thermal stress becomes uniform, and the thermal stress fluctuation decreases fast at first and then slows down. From the surface to the depth of 1.5 mm, peak-to-peak value decreases by 81.34% and fluctuation frequency drops to less than 5 Hz. The average thermal stress decreases in the form of slow-fast-slow with the increase of flow velocity and increases linearly with the increase of temperature difference. A high flow velocity will significantly increase the principal frequency of thermal stress fluctuation, which rises from 8 Hz to 25 Hz. A large temperature difference will significantly increase the intensity of thermal stress fluctuation.

Numerical Study to Promote the Residual Stresses Development during ISF Process with Improvement in Two Point Incremental Die Forming

Metastable austenitic stainless steel (MASS) has been the material of choice for the fabrication of disc springs employing incremental sheet forming (ISF) processes due to its high creep, fatigue, and chemical resistance, as well as its good surface quality. Previous research has shown that the presence of martensite enhances the formation of beneficial compressive residual stresses. However, if the ISF is accelerated to improve efficiency, the rise in temperature during ISF operation suppresses the deformation that causes martensite transition (DIMT). In essence, the cooling channel shapes are developed with numerical assistance such that its impact on residual stress induction is low. Variation in ISF process parameters, such as tool diameter, tool step-down, and contact force, as well as variation in cooling channel size, are used to construct the computational analysis. To analyze the finally produced residual stresses in the disc spring, the non-linear isotropic/kinematic hardening combined with the TRIP formulation is simulated. According to the comparison, the channel size must be between 0.8 and 1.2 mm in radius to minimize residual stress fluctuation. Additionally, when moving across the die with cooling channels, the force-controlled ISF produces more consistent results. Based on the numerical findings, it is conceivable to greatly enhance the ISF process speed and dissipate process heat by cooling the sheet on sides, allowing residual stresses and martensite content to be adjusted in a stable manner. As a result, the ISF process may be greatly expedited, making it more appealing for industrial applications.

Glyphosate exposure deteriorates oocyte meiotic maturation via induction of organelle dysfunctions in pigs

BackgroundRecently, defects in mammalian oocytes maturation induced by environmental pollution results in the decreasing animal reproduction. Animal exposed to glyphosate is largely unavoidable because glyphosate is one of the most widely used herbicide worldwide due to its high-efficiency and broad-spectrum effects, which causes glyphosate an environmental contaminant found in soil, water and food. During the last few years, the growing and wider use of glyphosate has raised great concerns about its effects of reproductive toxicity. In this study, using porcine models, we investigated effects of glyphosate on organelle functions during oocyte meiosis.ResultsThe results showed glyphosate exposure disrupted porcine oocyte maturation. Expression levels of cumulus expansion-related genes were interfered, further indicating the meiotic defects. The damaging effects were mediated by destruction of mitochondrial distribution and functions, which induced ROS accumulation and oxidative stress, also indicated by the decreased mRNA expression of related antioxidant enzyme genes. We also found an interference of endoplasmic reticulum (ER) distribution, disturbance of Ca2+ homeostasis, as well as fluctuation of ER stress, showing with the reduced ER stress-related mRNA or protein expression, which could indicate the dysfunction of ER for protein processing and signal transduction in glyphosate-exposed oocytes. Moreover, glyphosate exposure induced the disruption of lysosome function for autophagy, showing with the decrease of LAMP2 expression and autophagy-related genes mRNA expression. Additionally, our data showed the distribution of Golgi apparatus and the functions of ribosome were disturbed after glyphosate exposure, which might affect protein synthesis and transport.ConclusionsCollectively, our study showed that exposed to glyphosate could affect animal reproduction by compromising the quality of oocytes through its wide toxic effects on organelle functions.

FFT-based investigation of the shear stress distribution in face-centered cubic polycrystals

The onset of nonlinear effects in metals, such as plasticity and damage, is strongly influenced by the heterogeneous stress distribution at the grain level. This work is devoted to studying the local stress distribution in fcc polycrystals using FFT-based solvers. In particular, we focus on the distribution of shear stresses resolved in the slip systems as the critical driving force for plastic deformations. Specific grain orientations with respect to load direction are investigated in the linear elastic regime and at incipient plastic deformations based on a large ensemble of microstructures. The elastic anisotropy of the single crystal is found to have a crucial influence on the scatter of the stress distribution, whereas the Young’s modulus in the respective crystal direction governs the mean stress in the grain. It is further demonstrated that, for higher anisotropy, the shear stresses deviate from the normal distribution and are better approximated by a log-normal fit. Comparing the full-field simulations to the Maximum Entropy Method (MEM), reveals that the MEM provides an excellent prediction up to the second statistical moment in the linear elastic range. In a study on the spatial distribution of shear stresses, the grain boundary is identified as a region of pronounced stress fluctuations and as the starting point of yielding during the elastic–plastic transition.

A comparison of Newtonian and non-Newtonian pulsatile blood rheology in carotid bifurcation through fluid–solid interaction hemodynamic assessment based on experimental data

Endothelial cells play a crucial role in the arterial homeostasis. In addition to physiological risk factors, abnormal levels of hemodynamic parameters induced by the pulsatile flow contribute to atherosclerotic plaque formation and development. In this study, we used an experimental setup to study the hemodynamics of Newtonian and non-Newtonian blood flow on a deformable model of human carotid bifurcation. The flow/pressure pulses of the experimental model were fed into a fluid–structure interaction numerical model, and respective hemodynamic parameters were obtained and compared between the two flow regimes. Results revealed noticeable differences among the two flow regimes when the pulsatile nature of blood flow and pressure were considered, with more distinct differences near junction sites. Velocity profiles of the non-Newtonian model were more flattened with higher back flow during the diastole. The shear stress waves as well as shear-dependent parameters, such as oscillatory shear index, relative residence time, and vorticity, as well as wall stress and strain, also indicated significant differences among the two models. Regardless of flow regime, results showed a good agreement with clinical outcomes in human carotid bifurcation, especially the carotid sinus. Near the bifurcation, marked fluctuations of shear stress are evident. Around the junction site, wall pulsation experienced variations up to five times of the normal pulse span. The quantified hemodynamic parameters obtained from proposed accurate model of carotid bifurcation may help to achieve technological solutions to adjust the out of biological ranges of these parameters, and avoid atheroma formation or treat the diseased artery.

Micro/nano-mechanical behaviors of individual FCC, BCC and FCC/BCC interphase in a high-entropy alloy

• Micro-pillar compression tests were carried on a multiple high-entropy alloy. • Deformation behaviors of FCC, BCC and their interphase pillars were analyzed. • The strain bursts weakened with increasing strain in interphase pillar. • Interphase pillar exhibited better mechanical properties compared with FCC and BCC. • Revealed the relation between small scale deformation and interphase strengthening. Here, a systematic investigation was made on the interphase strengthening effects induced superior mechanical performances of multiphase high-entropy alloys (HEAs) at micro/nano-scale, compared with single phase HEAs. A pillar compression test under a scanning electron microscope (SEM) was performed on the individual face centered cubic (FCC), body centered cubic (BCC), and mixed-phases with different diameters in a Fe 24 Co 25 Ni 24 Cr 23 Al 4 HEA using focused ion beam (FIB) milling and a nanoindenter equipped with a flat punch. The stress-strain response of pillar underneath the indenter was selected to explore the diameter/phase-dependent size effect, the periodically fluctuation of local stress, and strain hardening. It was revealed that the pillars at the interphase exhibited significantly higher strength, compared with the FCC and BCC pillars. An experiment also verified the coincident mechanical size effects independent with the type of phases. The stress responses in the mixed-phase pillars manifested as a distinct transition from the dramatic drop to the minor fluctuation during the post-yield stages with the increasing strain, indicating the propagation of Al-Ni enriched solid solution phase (BCC1) under compression. Except the BCC1 phase, numerous dislocations were observed in the post-deformed pillars, particularly serving as the major source to enhance the strain hardening of BCC pillars.

Compressibility effect on interaction of shock wave and turbulent boundary layer

The compressibility effect when an oblique shock wave impinges on a turbulent boundary layer was analyzed with a direct numerical simulation using Helmholtz decomposition. The turbulent intensity near the impinging region is significantly enhanced by the interaction of the shock wave and boundary layer. In particular, the interaction behavior can enhance the dilatational components of the Reynolds stress and velocity fluctuations. This also enhances the negative correlation between the dilatational components of streamwise and normal velocity fluctuations. The dilatation fluctuation in the impinging region increases significantly. Moreover, in the impinging region, the dilatational components of the production term and transport term contribute to the production and transport terms of the kinetic energy equation mainly in the vicinity of the interface. This simulation shows that the interaction behavior of an oblique shock wave and turbulent boundary layer can enhance flow compressibility significantly. In the interaction region, the turbulent intensity of the flow field is stronger than those upstream and downstream. This study provides a theoretical basis for the improvement of other simulation methods and turbulence modeling for the interaction of an oblique shock wave and turbulent boundary layer.

Seismic Responses and Damage Mechanisms of Deep-Buried Underground Powerhouse in Layered Rock Mass

Layered rock mass generally has adverse effects on the seismic stability of deep-buried underground powerhouses. Aiming at the complexity of seismic wave field in deep-buried underground powerhouses, the obliquely input methods of P and SV waves considering the incident direction and multi-incident surfaces are constructed in this study, respectively. It can convert the seismic wave input problem into the problem of solving equivalent nodal force acting on the artificial boundaries. Based on the characteristics of dynamic interaction between interlayers in layered rock mass, an explicit dynamic contact force method considering the seismic deterioration effect and bond-slip characteristics of interface is also presented to simulate various contact states such as bond, separation, and sliding. The combined application of the above methods to analyze the seismic response of underground powerhouse at Azad Pattan hydropower station shows that they could accurately simulate the seismic damage evolution process of deep-buried underground powerhouses in layered rock mass. The numerical results indicate that the obliquely incident seismic waves contribute to a larger seismic reaction of underground powerhouses, which largely lies in the amplitudes of the displacement and stress fluctuations. After considering the dynamic contact, the obvious seismic deterioration effect and interlaminar dislocation displacement occur between soft and hard rock, and the sidewalls suffer more severe deterioration degree than the top arch. The seismic damage area of lining structure is mainly distributed in the place where the soft rock strata pass through and the upper structure with larger free surface. Additionally, two major damage modes of underground powerhouses in layered rock mass, namely, damage due to structural deformation and interlaminar dislocation, are derived from the numerical results and can be reasonably explained by the corresponding damage mechanisms.

Quantification of Recruit Training Demands and Subjective Wellbeing during Basic Military Training.

Purpose: Assess and describe the physical demands and changes in subjective wellbeing of recruits completing the 12 week Australian Army Basic Military Training (BMT) course. Methods: Thirty-five recruits (24.8 ± 6.8 y; 177.4 ± 10.1 cm, 75.6 ± 14.7 kg) consented to daily activity monitoring and weekly measures of subjective wellbeing (Multi-component Training Distress Scale, MTDS). The physical demands of training were assessed via wrist worn activity monitors (Actigraph GT9X accelerometer). Physical fitness changes were assessed by push-ups, sit-ups and multi-stage shuttle run in weeks 2 and 8. Results: All objective and subjective measures significantly changed (p < 0.05) across the 12 week BMT course. In parallel, there was a significant improvement in measures of physical fitness from weeks 2 to 8 (p < 0.001). The greatest disturbance to subjective wellbeing occurred during week 10, which was a period of field training. Weeks 6 and 12 provided opportunities for recovery as reflected by improved wellbeing. Conclusions: The physical demands of training varied across the Australian Army 12 week BMT course and reflected the intended periodization of workload and recovery. Physical fitness improved from week 2 to 8, indicating a positive training response to BMT. Consistent with findings in sport, wellbeing measures were sensitive to fluctuations in training stress and appear to have utility for individual management of personnel in the military training environment.

Wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence, part I: One-point statistics

This two-part study investigates the effects of Mach number and wall temperature on the statistics of wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence. In the first part, we focus on their one-point statistics, including the root mean square (r.m.s.), skewness factor (third-order moment), flatness factor (fourth-order moment), and their correlations. By exploiting the direct numerical simulation databases, we found that the r.m.s. of the streamwise wall shear stress and pressure, the skewness factor of all the flow quantities considered, and the flatness factor of streamwise wall shear stress monotonically vary with the friction Mach number (Mτ), while for the rest, the wall heat flux and global temperature parameters should be taken into account as well for a monotonic trend of variation. The correlation coefficients between wall shear stress, pressure, and heat flux fluctuations increase with the Mach number Mτ, suggesting the underlying interactions between dynamic and thermodynamic processes. The distributions of spectra and probability density functions indicate that the increased correlation is induced by the highly intermittent traveling wave packets among the streaky structures, as reflected by the “double-peak” feature of the spectra that gradually emerges with the increasing compressibility effects. The probability density distribution also manifests the alteration of the occurrence of extreme events caused by these structures. By accordingly decomposing the fluctuations with cutoff filtering, it is found that the root mean squares of streamwise wall shear stress and heat flux fluctuations related to the streaky structures are Mach number-independent, while those related to the traveling wave packets monotonically increase with the friction Mach number.

Wall shear stress, pressure and heat flux fluctuations in compressible wall-bounded turbulence. II. Spectra, correlation and nonlinear interactions

Wall shear stress, pressure, and heat flux are of significant importance in engineering applications. In this two-part study, we investigate the compressibility effects on wall shear stress, pressure, and heat flux fluctuations in compressible wall-bounded turbulence by exploiting direct numerical simulation databases. In Paper I, we primarily deal with the one-point statistics, whereas in this second part, we report the effects of compressibility on the frequency spectra, wavenumber-frequency spectra of these flow quantities, and the two-point cross-correlations between them. It is found that the scaling laws of the spectra at low and high frequencies are retained as those of incompressible flows, whereas the spectra intensities at mid frequencies increase with the enhancement of compressibility effects, which is identified to be related to the ever-predominating traveling wave packets. These wave packets are convected downstream at the same velocity of 0.87Ub as that of pressure fluctuations, higher than that of the streaky structures 0.65Ub (Ub the bulk velocity), and enhance the space and time cross correlation between wall shear stress, pressure, and heat flux fluctuations. By extracting the envelopes of the traveling wave packets and inspecting the time and space correlations between the envelopes and the streaky structures, we found that the emergence of traveling wave packets comes later than the streaky structures, both in time and space. Based on these observations, we provide a depiction of the physical processes regarding the formation and evolution of the traveling wave packets.

Wet foam flow: A suitable method for improving surface hygiene in the food industry

In the food industry, the cleaning of contaminated surfaces requires new strategies to be adopted which can provide greater cleaning efficiency with minimal energy and water consumption. The use of wet foams was proposed to clean stainless steel surfaces contaminated by droplets containing Bacillus spores. Methods such as polarography, conductimetry and foam structure identification were used. Foam flow conditions with varying wall shear stresses and bubble sizes were observed to measure their impacts on surface contaminant removal kinetics. Compared to conventional cleaning-in-place method, foam flow more effectively removed both hydrophilic and hydrophobic spores. The combination of high shear stress and small bubble sizes (<0.2 mm) showed promise for improving the cleaning efficiency of the foam. Mechanisms such as fluctuation in local stresses, or in the liquid film thickness between the bubbles and the steel wall induced by bubble passage, foam imbibition and liquid film drainage were then investigated.

A new wave phenomenon at the frictional interface

Frictional motion is intrinsically complex, resulting from the interactions between driving forces, interfacial nonlinearity and instabilities, material inertia, and bulk geometry contacts. In the present paper, a two-dimensional interface friction model with a single micro-bulge is established, and the wave phenomenon at the friction interface is numerically simulated and analyzed by the finite element method. The simulation results show that in the micro-process of loading, there are significant stress fluctuations and fine wave structure characteristics at the frictional interface. A new interesting wave phenomenon occurs when the incident stress wave has not yet propagated to the micro-bulge position, a micro-stress disturbance has been generated from the interface and propagated to the substrate by a longitudinal wave. This wave phenomenon implies that the overall response of the interface precedes the local wave response of the matrix. More comparative simulations and analyses proved that the overall gravity micro-adjustment on the interface might be responsible for such new waves. This study reveals a new wave phenomenson in the early stage of interface friction, which predicts earthquake in advance.

Consistency between the attached-eddy model and the inner–outer interaction model: a study of streamwise wall-shear stress fluctuations in a turbulent channel flow

The inner–outer interaction model (Marusic et al., Science, vol. 329, 2010, pp. 193–196) and the attached-eddy model (Townsend, Cambridge University Press, 1976) are two fundamental models describing the multiscale turbulence interactions and the organization of energy-containing motions in the logarithmic region of high-Reynolds-number wall-bounded turbulence, respectively. In this paper, by coupling the additive description with the attached-eddy model, the generation process of streamwise wall-shear fluctuations, resulting from wall-attached eddies, is portrayed. Then, by resorting to the inner–outer interaction model, the streamwise wall-shear stress fluctuations generated by attached eddies in a turbulent channel flow are isolated. Direct comparison between the statistics from these two models demonstrates that they are consistent with and complement each other. Meanwhile, we further show that the superpositions of attached eddies follow an additive process strictly by verifying the validity of the strong and extended self-similarity. Moreover, we propose a Gaussian model to characterize the instantaneous distribution of streamwise wall-shear stress, resulting from the attached-eddy superpositions. These findings are important for developing an advanced reduced-order wall model.

Wrinkling suppression in thin membranes using designed geometrical features

The rapidly increasing use of thin membranes; especially in the fabrication of flexible electronics, has motivated investigating the wrinkling behavior of these membranes. Wrinkling has an undesirable effect since wrinkles degrade the surface accuracy of structures incorporating thin membranes. This work introduces and evaluates an approach based on including optimized geometric features in thin membranes to suppress their wrinkling behavior. The ability of the optimized geometric features to suppress wrinkling in a 25 μm thick polyimide membrane subjected to uniaxial stretching is investigated computationally and experimentally in this work. A finite element model is developed to predict the wrinkling behavior of the membrane. An experimental setup equipped with 3D digital image correlation system is used to determine the wrinkles pattern, amplitude, and wavelength. Symmetric circular holes are introduced in the membrane to redistribute the stress field, eliminate the fluctuation in the minor principal stress in the membrane, and suppress wrinkles. The location and size of the circular holes are determined by coupling the Non-Linear Programming by Quadratic Largrangian technique and finite element simulations. An optimal design is obtained and the experimental results show that the optimally designed holes effectively suppressed the wrinkling behavior.

Changes in the Bacterial Community Associated With Experimental Symbiont Loss in the Mucus Layer of Cassiopea xamachana Jellyfish

Cassiopea xamachana is a model system for studies in animal symbiosis with algal symbionts. This medusa is also associated with a microbial community that can impact its health, but this community has not been thoroughly studied. Shifts in the bacterial community following the loss of symbionts involving stress, environmental changes, or seasonal fluctuations can be complex, as the role of symbionts in structuring this community is not well established. To understand the interplay among microbial associates with this host, we explored the experimental diminishing of algal symbionts, and the influence of seasonal fluctuations over the structure of the bacterial community, through 16S rRNA gene high-throughput sequencing. Results showed that Gammaproteobacteria, Bacteroidia, and Alphaproteobacteria were dominant in all the mucus samples at the beginning of the experiments. However, after 28 days, bleached medusas showed a marked increase in Gammaproteobacteria, specifically in the genus Vibrio, as evidenced by Linear Discriminant Analysis of Effect Size (LEfSe). Seasons also resulted in shifts of the bacterial community, although bacterial genera were distinct from those found in bleached medusas, suggesting temporal associations with the host. According to PERMANOVA analysis, seasonal fluctuations affected the dominant bacterial members (p = 0.07), but symbiont presence was a more significant driver (p=0.001). We found the bacterial community of C. xamachana is like that of other jellyfish and corals, which furthers the interest in this animal as a study model. Defining relevant bacterial genera can help us understand the functional role of the holobiont members that assemble and maintain a healthy microbial community. Also, studies in other regions where C. xamachana distributes can help us define a core bacterial community for this medusa.

Fluctuation in the fluid and tectonic pressures in the South Almora Thrust Zone (SATZ), Kumaun Lesser Himalaya; paleoseismic implications

This study uses veins, striated faults, and fluid inclusions (FIs) for deciphering the tectonic stresses and the role of fluids in the evolution of the South Almora Thrust Zone (SATZ). The paleostress analysis implies that the SATZ bears a record of four different stress states. The syntectonic fluids preserved in the fluid inclusions in the quartz veins are predominantly aqueous-carbonic saline with a minor amount of methane detected in Raman spectroscopy. FIs showing the variable and uniform degrees of fill were entrapped as the immiscible and miscible aqueous-carbonic fluids, respectively. Microthermometry on FIs implies a considerable fluctuation in fluid pressure during the evolution of the SATZ. The absolute paleostress magnitude, estimated by the 3-D Mohr circle reconstruction, substantiates fluctuation in the ambient stress, indicating the role of tectonic stress. The implosion and explosion textures in fluid inclusions, fluid pressure fluctuations, and tectonic stress fluctuations are attributed to multiple seismic events that occurred at a depth of 7–13 km during the Late Oligocene-Early Miocene (21–25 Ma).

Shear Stress and b-value Fluctuations in a Hierarchical Rate-and-State Asperity Model

Shear Stress and b-value Fluctuations in a Hierarchical Rate-and-State Asperity Model