Shear Stress Characteristics Research Articles

Numerical simulation of wall shear stress downstream of a headcut

Headcut erosion is a very common process occurring at the head of rills, crop furrows and ephemeral gullies. A numerical model of the headcut flow was built to study the wall shear stress characteristics downstream of the scour hole, which is very important in headcut erosion prediction models. The calculated nappe parameters are very close to experimental data and/or equation results. The calculated flow field, velocity centreline of the free jet part and the locations of the maximum velocity in the scour hole are also close to the experimental data. It was found that the upstream flow rate and downstream water level can affect both the maximum value and the distribution of the wall shear stress. Taking into account the affecting factors and referencing the form of the existing relations on the maximum wall shear stress caused by the impinging jet, the relation of the maximum wall shear stress occurring downstream of the headcut is obtained. The wall shear stress distribution can be described by exponential functions. The new prediction equation of the maximum wall shear stress can be used in existing headcut erosion prediction models for calculating equilibrium scour hole depth or scour hole deepening rate.

Characteristics of shear stress based on magnetorheological fluid flexible fixture during milling of the thin-walled part

With light weight and good overall structure, thin-walled part has been widely used in various fields like aerospace. However, it has low stiffness in the cutting process. Traditional fixture clamping workpiece has a long positioning and adjustment cycle and is likely to deform, making it difficult to control machining efficiency and quality. In this study, a mechanical-magnetorheological fluid (MRF) composite flexible clamping method was proposed, and a theoretical mathematical model of the shear stress of MRF was constructed. The real value and the theoretical value of shear stress show consistent upward and downward trends. Based on the prediction model, the coupling relations between magnetic field intensity and thickness and position of workpiece were studied. Results demonstrate that workpiece thickness has less significant influence on shear stress compared with workpiece position. Combined with the experimental results, the smaller the workpiece thickness and the closer to the magnetic field, the larger the shear stress. The shear stress of the workpiece 1 mm thick is 21.5% higher than that of the workpiece with the thickness of 8 mm at position of 10 mm. Finally, the thin-walled part was used to demonstrate MRF clamping. Compared with the traditional clamping method, the proposed method can decrease the cutting force by 14.3%. Moreover, the residual stress σx and σy decrease by 37.0% and 43.9%, the flatness value decreases by 55.6%, and the roughness Ra, Rz and Rq decrease by 55.6%, 53.2%, and 56.7%. The quality of the thin-walled part was effectively controlled.

Research on the Effect of Dip Angle on Shear Stress on Normal Fault Plane and Water Inrush in Floor Strata During Mining Activities

This paper presents an investigation on the characteristics of shear stress on normal fault plane by employing the mechanical model and the characteristics of water inrush from floor strata during mining activities by employing numerical model. The shear stress formula on fault plane is obtained by using the solution of a homogeneous half-plane affected by normally distributed forces in the theory of elasticity. The mechanical model shows the variation of shear stress on fault plane with different dip angle and fault reactivation is analyzed according to the shear stress. Based on these, the fracture distribution and seepage distribution on floor strata are numerically simulated by using the RFPA2D-Flow software, and the effect of dip angle on water inrush from floor strata is discussed. These results indicate that the smaller value of dip angle is easier to be reactivated and to produce connecting fractures between floor strata and normal fault comparing with the larger value of dip angle, which provide significant insights into remaining coal pillar with similar geological conditions.

MHD Double‐diffusive thermosolutal Marangoni convection non‐Newtonian Casson fluid flow over a permeable stretchable sheet

AbstractIn the present paper, we have discussed the thermosolutal Marangoni force acting on the electrically conducting Casson fluid flow over a permeable horizontal stretching surface. It is presumed that the condition at the interfaces is influenced by the surface tension, which is proportional to the temperature and concentration profiles. At the interface, both concentration and temperature are heated in such a way that they are quadratic functions in Furthermore, we have introduced the magnetic field in the transverse direction of the fluid flow along with heat generation/absorption, thermal radiation, viscous dissipation, and first‐order chemical effect with heat and mass flux into the present system. Similarity transformations have been used to convert the system of the nonlinear partial differential equations into a system of nonlinear ordinary differential equations (ODEs). The reduced ODEs are then solved using the MATLAB program bvp4c, which is based on the fourth‐order Runge‐Kutta and shooting method. The impact of various pertinent flow parameters on the flow field, temperature, and species concentration has been studied through graphs. To know the characteristics of shear stress, heat and mass rate near the boundary, numerical values of them are also calculated and given in the tabular form. The results show that the momentum boundary layer's thickness is getting thicker with an increase in solutal surface tension ratio, while its opposite trends have been observed in the thermal boundary layer region, this is due to the Marangoni effect. This Marangoni effect is very much important in the field of melting metals, crystal growth, welding, and electron beam.

Characteristics of Wall Shear Stress and Pressure of Intracranial Atherosclerosis Analyzed by a Computational Fluid Dynamics Model: A Pilot Study.

Background: Although wall shear stress (WSS) and pressure play important roles in plaque vulnerability, characteristics of the two indices in intracranial atherosclerosis (ICAS) have not been fully investigated yet. This study aimed to elucidate this issue by means of establishing a non-invasive computational fluid dynamics method with time-of-flight magnetic resonance angiography (TOF-MRA) of the whole cerebral artery.Materials and Methods: Subjects with symptomatic ICAS in the middle cerebral artery domain were enrolled, excluding those with concomitant internal carotid artery stenosis. Based on patient-specific TOF-MRA images for three-dimensional (3D) meshes and arterial blood pressure with patient-specific carotid artery ultrasonography for inlet boundary conditions, patients' three-dimensional hemodynamics were modeled by a finite element method governed by Navier-Stokes equations.Results: Among the 55 atherosclerotic lesions analyzed by this TOF-MRA based computational fluid dynamics model, the maximum WSS (WSSmax) was most frequently detected at the apex points and the upper half of the upstream sections of the lesions, whereas the maximum pressure was most often located at the lower half of the upstream sections. As the percent stenosis increases, the relative value of WSSmax and pressure drop increased with significantly increasing steep beyond 50% stenosis. Moreover, WSSmax was found to linearly correlate with pressure drop in ICAS.Conclusions: This study on ICAS revealed certain trends of longitudinal distribution of WSS and pressure and the influences of percent stenosis on cerebral hemodynamics, as well as the correlations between WSS and pressure drop. It represents a step forward in applying computational flow simulation techniques in studying ICAS and stroke, in a patient-specific manner.

Investigation of fluctuating characteristics of wall shear stress in supersonic flow

The characteristics of the flow structure and wall shear stress (WSS) in a hydrogen-fueled scramjet were investigated. A microelectromechanical system-based sensor was used for measuring the WSS. The flow structure was found to be stable in both nonreacting and reacting flows. The expansion waves near the cavity step that occurred in a nonreacting flow were transformed to oblique shock waves owing to hydrogen combustion, and the cavity shear layer was lifted into the core flow. The flame was in the cavity shear layer and the wall boundary layer near the top wall. The time-averaged value (TAV) of the WSS in the isolator was about 825.6 Pa, and the flow was turbulent. The intensity of WSS fluctuations was about 21.5%, and the probability density function of the WSS was negatively skewed in a nonreacting flow. The TAV of the WSS decreased from 370 Pa to 269.5 Pa in the combustor after combustion occurred. This was because the velocity gradient was decreased in the combustion flow. The intensity of WSS fluctuations was increased by the hydrogen combustion.

Effects of local hemodynamics and plaque characteristics on neointimal response following bioresorbable scaffolds implantation in coronary bifurcations.

Heterogeneous neointimal response has been observed after implantation of all generations of coronary stents. Our aim was assessing local factors of shear stress (SS) and plaque characteristics in neointimal response after implantation of bioresorbable scaffolds (BRS) in bifurcations. Ten patients from the BIFSORB pilot study were analysed. Follow-up optical frequency domain imaging (OFDI) was performed at 1 month and 2 years. Coronary lumen and BRS structure were reconstructed by fusion of OFDI and angiography and were used for subsequent flow simulation. Plaque arc degree and SS were quantified using post-procedural OFDI data and were matched with follow-up OFDI using anatomical landmarks. Strut-level and segment-level analysis were performed for 1-month and 2-year follow-up respectively. A total of 444 struts (54 jailing struts) were included at 1-month follow-up. Time-average SS (TASS) was significantly lower for covered struts than for uncovered struts in non-bifurcation segments (TASS: 1.81 ± 1.87 vs. 3.88 ± 3.72Pa, p < 0.001). The trend remained the same for jailing struts, although statistically insignificant (TASS: 10.85 ± 13.12 vs. 13.64 ± 14.48Pa, p = 0.328). For 2-year follow-up, a total of 66 sub-regions were analysed. Neointimal hyperplasia area (NTA) was negatively correlated with TASS in core-segments (ρ = - 0.389, p = 0.037) and positively correlated with plaque arc degree in non-core segments (ρ = 0.387, p = 0.018). Slightly stronger correlations with NTA were observed when combining TASS and plaque arc degree in both core segments (ρ = - 0.412, p = 0.026) and non-core segments (ρ = - 0.395, p = 0.015). Hemodynamic microenvironment and baseline plaque characteristics may regulate neointimal response after BRS implantation in bifurcation. These findings underline the combined role of plaque characteristics and local hemodynamics in vessel healing after stent implantation.

Numerical analysis of punch shear failure and stress characteristics of three-dimensional braided composite with different braiding angles

This paper presents a multi-scale finite element model to calculate the stress field and analyze the punch shear failure of three-dimensional braided composite at high strain rates. The multi-scale model was established based on real braided structure taking the surface and corner braiding yarns into consideration. Constitutive material modeling and failure criterions were introduced into the model. Three braiding angles of 25°, 35°, and 45° were applied to reveal the relations between failure states and braided structure. The results showed that the punch shear failure states and stress distribution were greatly dependent on the strain rates and braiding angles. Nonuniform stress propagation resulted in shear bands and different formation paths were observed on the composite with different braiding angles. The ultimate failure of braided composite was determined by comprehensive action of compressive and tensile stress. In addition, the progressive damage of typical braiding yarns in different conditions was obtained from the modeling simulation. The three-dimensional braided composites with different braiding angles showed unique failure morphology. It was closely determined by the complex braided structures.

The 3-D Morphology Evolution of Spur Dike Scour under Clear-Water Scour Conditions

By changing the alignment angle of spur dike, this study focused on the evolution of scour hole morphology in three alignments under clear-water scour conditions, including the 3-D structure of the scour hole, 2-D profile morphological evolution process and the evolution characteristics of the local bed shear stress. The results show that the plane area and volume of the scour hole both are power functions over time, which is similar to the evolution characteristics of scour depth. Local scour includes three stages: The initial stage, development stage and balance stage. The local bed shear stress presents successively: τb &gt; τc, τb = τc and τb &lt; τc. Based on this characteristic, the evolution mechanism between scour hole morphology and the local bed shear stress is further clarified. Furthermore, although the alignment of the spur dike significantly affects the longitudinal and vertical dimension erosion rates of the scour hole, the scour hole morphology is not only relatively constant but also has a specific proportion, and the evolution process is orderly in the whole process of evolution. To the scouring equilibrium state, the length of the scour hole on the upstream and downstream of the spur dike is approximately in line with the golden section feature. The related results provide technical support for scour parameter design and scour hole protection of spur dike in flood period.

Bed shear stress and turbulence characteristics under unsteady flows

A series of laboratory experiments were conducted to investigate the turbulence and bed shear stress characteristics under solitary waves and dam break flows. The Laser Doppler velocimetry (LDV) systems were used to measure the velocity profile very near the bottom. A method was developed to find the wave-induced bed shear stress over a smooth bed from the measured velocities within or near the viscous sub-layer. A unified 2D numerical model was developed to simulate the entire flow field from bottom to free surface. The model solves the Reynolds Averaged Navier-Stokes (RANS) equations, with the BSL k-ω turbulence closure model that can capture the turbulence inside and outside of boundary layer. The numerical results were compared to the experimental data and available theories in terms of free surface displacement, mean velocity, turbulence intensity, and bed shear stress. Discussions were made on the effects of the turbulence characteristics on changing the patterns of mean velocity profile and bed shear stress. Further efforts were made to revisit the existing boundary layer theories, trying to quantifying the role of free stream velocity and acceleration as well as eddy viscosity in modifying the response properties of the bed shear stress under unsteady flows. A formula was proposed to calculate the instantaneous bed shear using the free stream velocity and acceleration. The results using the present formula are compared to those from the earlier works for solitary wave, dam break wave, and saw-tooth wave.

Determination of the bond–slip relationship of fully grouted rockbolts

The bond–slip relationship of fully grouted rockbolts with long encapsulation lengths is critical to the bolt axial performances. However, how to properly determine its profile still remains a challenge. It is proposed that the pullout tests of short encapsulated rockbolts could be used to estimate the bond–slip relationships of long rockbolts under the same conditions. This method is based on two assumptions: (1) The bond–slip relationship simply calculated from the load–displacement curve of a short rockbolt could represent its interfacial shear stress characteristics and (2) bolts with different embedment lengths have the same bond–slip relationship when subjected to the same conditions. Pullout tests were carried out on instrumented rockbolts with short embedment lengths to verify the first assumption, and pullout tests on rockbolts with various embedment lengths were numerically modeled to validate the second assumption. It was found that the shear stresses were not uniformly distributed along the bolt–grout interface; the load–displacement curve of a short rockbolt could still be used to derive the bond–slip relationship of a bolt. The bond–slip relationships computed from short grouted rockbolts tend to underestimate the interfacial shear bond stresses of longer bolts.

Characteristics of the wall shear stress in pulsating wall-bounded turbulent flows

A pulsating turbulent pipe flow has been investigated experimentally using hot-film anemometry and particle image velocimetry. Particularly, a ‘paradoxical phenomenon’ that is known to occur for a range of forcing frequencies and time-averaged Reynolds numbers has been investigated in detail. The paradoxical phenomenon is that the oscillating component of the wall shear stress exhibits a smaller amplitude in a turbulent flow compared to in a laminar flow exposed to the same oscillation in the pressure gradient. In here, the phenomenon is explained by splitting the response of the wall shear stress into one contribution resulting from the imposed pressure gradient τ∼p, and a second contribution resulting from the oscillating Reynolds shear stress, τ∼t. At the conditions of maximum reduction of the wall shear stress amplitude, τ∼p and τ∼t are nearly 136° out of phase. The contributions are thus interfering destructively, this being the ultimate reason for the reduced amplitude. It is also shown that the level of reduction is dependent on the imposed forcing amplitude, this in turn residing from a dependence of the time-development of the oscillating Reynolds shear stress on the forcing amplitude.

The ejector loop reactor: Application for microbial fermentation and comparison with a stirred-tank bioreactor.

Ejector loop reactors (ELR) are successfully used in industrial chemical processes for gas/liquid reactions. They achieve higher mass transfer rates compared to the stirred-tank reactor (STR) at comparable specific power input. Insufficient oxygen transport and shear stress induced growth inhibition are limiting parameters during microbial fermentation. Due to its better mass transfer characteristics, the ELR was expected to have beneficial effects on biomass and recombinant protein production. One concern, however, was whether the ELR's shear stress characteristics would have a negative effect. This study evaluated the suitability of using the Buss-Loop® Reactor (BLR), one of the most advanced ELR technologies, as a bioreactor. The well-studied STR was used as a reference. A lab scale BLR was adapted for microbial fermentation. Mass transfer rates and specific power inputs were within the same order of magnitude in the ELR and the reference STR. Maximum values of 207 and 205 h-1 at power inputs of 6.9 and 9.7 W/L were measured in the ELR and STR, respectively. During batch fermentation of Escherichia coli K12 MG1655, maximum cell densities were higher in the ELR (OD600 of 22) than in the STR (OD600 of 18). Green fluorescence protein (GFP) production with pGS1 was comparable; however, more GFP was released into the media in the ELR. This indicates higher cell disruption compared to the STR. Despite this drawback of the first prototype, our work clearly demonstrates the potential of the ELR as a system for microbial fermentations.

Effect of slag and bentonite on shear strength parameters of sandy soil

A series of direct shear tests were implemented on three different types of specimens (i.e., clean Perth sand, sand containing 10, 20 and 30% bentonite, sand containing 1, 3 and 5% slag, and sand containing 10, 20 and 30% bentonite with increasing percentages of added slag (1%, 3% and 5%). This paper focuses on the shear stress characteristics of clean sand and sand mixtures. The samples were tested under different three normal stresses (100, 150 and 200 kPa) and three curing periods of no curing time, 7 and 14 days. It was observed that the shear stresses of clean sand and mixtures were increased with increasing normal stresses. In addition, the use of slag has improved the shear strength of the sand-slag mixtures; the shear stresses rose from 128.642 kPa in the clean sand at normal stress of 200 kPa to 146.89 kPa, 154 kPa and 161.14 kPa when sand was mixed with 1%, 3% and 5% slag respectively and tested at the same normal stress. Internal friction angle increased from 32.74 in the clean sand to 34.87, 37.12 and 39.4 when sand was mixed with 1%, 3% and 5% slag respectively and tested at 100, 150, and 200 kPa normal stresses. The cohesion of sand-bentonite mixtures increased from 3.34 kPa in 10% bentonite to 22.9 kPa, 70.6 kPa when sand was mixed with 20% and 30% bentonite respectively. All the mixtures of clean sand, different bentonite and slag contents showed different behaviour; some mixtures exhibited shear stress more than clean sand whereas others showed less than clean sand. The internal friction angle increased, and cohesion decreased with increasing curing time.

Role of Low Endothelial Shear Stress and Plaque Characteristics in the Prediction of Nonculprit Major Adverse Cardiac Events: The PROSPECT Study

ObjectivesThis study sought to determine whether low endothelial shear stress (ESS) adds independent prognostication for future major adverse cardiac events (MACE) in coronary lesions in patients with high-risk acute coronary syndrome (ACS) from the United States and Europe. BackgroundLow ESS is a proinflammatory, proatherogenic stimulus associated with coronary plaque development, progression, and destabilization in human-like animal models and in humans. Previous natural history studies including baseline ESS characterization investigated low-risk patients. MethodsIn the PROSPECT (Providing Regional Observations to Study Predictors of Events in the Coronary Tree) study, 697 patients with ACS underwent 3-vessel intracoronary imaging. Independent predictors of MACE attributable to untreated nonculprit (nc) coronary lesions during 3.4-year follow-up were large plaque burden (PB), small minimum lumen area (MLA), and thin-cap fibroatheroma (TCFA) morphology. In this analysis, baseline ESS of nc lesions leading to new MACE (nc-MACE lesions) and randomly selected control nc lesions without MACE (nc-non-MACE lesions) were calculated. A propensity score for ESS was constructed for each lesion, and the relationship between ESS and subsequent nc-MACE was examined. ResultsA total of 145 lesions were analyzed in 97 patients: 23 nc-MACE lesions (13 TCFAs, 10 thick-cap fibroatheromas [ThCFAs]), and 122 nc-non-MACE lesions (63 TCFAs, 59 ThCFAs). Low local ESS (<1.3 Pa) was strongly associated with subsequent nc-MACE compared with physiological/high ESS (≥1.3 Pa) (23 of 101 [22.8%]) versus (0 of 44 [0%]). In propensity-adjusted Cox regression, low ESS was strongly associated with MACE (hazard ratio: 4.34; 95% confidence interval: 1.89 to 10.00; p < 0.001). Categorizing plaques by anatomic risk (high risk: ≥2 high-risk characteristics PB ≥70%, MLA ≤4 mm2, or TCFA), high anatomic risk, and low ESS were prognostically synergistic: 3-year nc-MACE rates were 52.1% versus 14.4% versus 0.0% in high-anatomic risk/low-ESS, low-anatomic risk/low-ESS, and physiological/high-ESS lesions, respectively (p < 0.0001). No lesion without low ESS led to nc-MACE during follow-up, regardless of PB, MLA, or lesion phenotype at baseline. ConclusionsLocal low ESS provides incremental risk stratification of untreated coronary lesions in high-risk patients, beyond measures of PB, MLA, and morphology.

Intracoronary Shear Stress and CT Characteristics of Vulnerable Coronary Plaques

Abstract Vulnerable coronary plaques are associated with a significant risk for rupture, and the ability to detect their characteristic features is of extreme importance, as timely detection of rupture-prone plaques could lead to the appropriate initiation of adequate therapeutic measures and prevent the evolution to an acute coronary event. The most common features of vulnerability in coronary plaques are represented by the presence of low density atheroma, a thin fibrous cap, spotty calcifications, and positive remodeling. However, there is still a huge amount of information to be learned about the role of local forces, represented by the shear stress, on the plaque vulnerability. This clinical update aims to present the most recent advances in the field of knowledge regarding the relation between shear stress and plaque vulnerability, starting from the hypothesis that shear stress significantly correlates with the CT features of plaque vulnerability and can represent a new marker of vulnerability in coronary artery plaques.

P3468Coronary microvascular endothelial dysfunction is associated with low endothelial shear stress and high-risk coronary plaque characteristics in patients with early coronary atherosclerosis

P3468Coronary microvascular endothelial dysfunction is associated with low endothelial shear stress and high-risk coronary plaque characteristics in patients with early coronary atherosclerosis

Magnetite micropolar nanofluid non-aligned MHD flow with mixed convection

The magnetite micropolar nanofluid ( Fe3O4 /water) oblique flow in the presence of mixed convection and magnetic field is considered in the present investigation. Magnetite nanoparticles are added to water in order to examine the temperature and velocity characteristics of the flow. Appropriate transformations are employed to obtain the governing equations. Numerical solutions are attained by the Range-Kutta-Fehlberg integration scheme with the shooting method. Characteristics of flow velocity profiles, temperature distribution, micro-rotation, shear stress and heat flux are remarkably influenced by magnetic parameter, magnetite nanoparticles volume fraction and mixed convection parameter. The obtained results indicate that the shear stress at the wall decreases but the local heat flux increases with increase in the nanoparticles volume fraction. Moreover, an increase in the magnetic field strength consequently enhances the shear stress at the surface but decreases the local heat transfer rate at the surface.

A prediction model and numerical simulation of the location of the longwall face during the highest possible failure period of gob gas ventholes

The failure of gob gas ventholes (GGVs) is the main cause of shortening the highly efficient drainage period of GGVs. Therefore, in order to improve the stability of GGVs, it is of great significance to investigate the location of a longwall face when a GGV is in the highest possible failure period. Based on the probability integral method, this paper obtained a modified equation for calculating the amount of horizontal movement of an overlying stratum during longwall mining, and in this equation, the influence of the distance between the overlying stratum and the coal seam on the subsidence coefficient of the stratum was taken into account. In addition, this paper established a prediction model of the location of the working face during the highest possible failure period of GGVs. Furthermore, taking Zhangbei coal mine as an example, this paper investigated the distribution of the maximal amount of relative horizontal movement (RHM) of the contiguous strata along a venthole and explored the changing characteristics of the maximal amount of RHM of the overlying contiguous strata as the distance of the working face passing GGV increases. In addition, this paper also researched the distribution of shear stress exerted on casing along a venthole during coal mining and the changing characteristics of shear stress as working face advances using numerical simulations. The results showed that when the longwall face passed the venthole by approximately 30–70 m, GGVs were in the highest possible failure period, which is the early stage of longwall face passing GGV. Zhangbei and Xieqiao coal mines in Huainan mining area of China carried out the engineering application of extracting depresssurized gas with GGVs. The measured failure data of ventholes 11418-1, 11418-2, 1242(1)-2 and 1242(1)-3 validated the prediction.

Organic Additives Used as Binders and Pore-Forming Elements of TIO&lt;sub&gt;2&lt;/sub&gt; Ceramics

When starches / proteins are added to the ceramic matrix, they originate pores whose morphology and sizes are varied. However, ceramic final properties are influenced by the interaction kinetics amongst particulate materials. This work was designed to analyze the interactions between proteins and disaccharides and their influence on the properties of TiO2 ceramics. Compositions containing protein and protein-sucrose were added to TiO2 powder. Rheological parameters, shear stress and viscosity were studied in order to understand the protein behavior. Furthermore, sintered ceramics were characterized as for porosity, roughness and image analysis by light microscopy. The dispersions produced with sucrose-protein showed lower viscosity and shear stress values and characteristics of a Newtonian fluid. The samples, from compositions which used 40% solids (80 wt% protein and 20 wt%), showed a higher apparent porosity (23.28%) and larger pore sizes (4947.07 μm2).