scholarly journals Scour at Bridge Foundations in Supercritical Flows: An Analysis of Knowledge Gaps

Water ◽  
2019 ◽  
Vol 11 (8) ◽  
pp. 1656 ◽  
Author(s):  
Oscar Link ◽  
Emmanuel Mignot ◽  
Sebastien Roux ◽  
Benoit Camenen ◽  
Cristián Escauriaza ◽  
...  
Keyword(s):  
Horseshoe Vortex ◽  
Flow Structures ◽  
Shear Stresses ◽  
Knowledge Gaps ◽  
Bridge Foundations ◽  
Measuring Techniques ◽  
Order Of Magnitude ◽  
Wall Jet Flow ◽  
Supercritical Flows ◽  
Analysis Of Knowledge

The scour at bridge foundations caused by supercritical flows is reviewed and knowledge gaps are analyzed focusing on the flow and scour patterns, available measuring techniques for the laboratory and field, and physical and advanced numerical modeling techniques. Evidence suggests that the scour depth caused by supercritical flows is much smaller than expected, by an order of magnitude compared to that found in subcritical flows, although the reasons for this behavior remain still unclear. Important questions on the interaction of the horseshoe vortex with the detached hydraulic-jump and the wall-jet flow observed in supercritical flows arise, e.g., does the interaction between the flow structures enhance or debilitate the bed shear stresses caused by the horseshoe vortex? What is the effect of the Froude number of the incoming flow on the flow structures around the foundation and on the scour process? Recommendations are provided to develop and adapt research methods used in the subcritical flow regime for the study of more challenging supercritical flow cases.

Experimental and Numerical Investigation of Secondary Flow Structures in an Annular Low Pressure Turbine Cascade Under Periodic Wake Impact—Part 2: Numerical Results

2019 ◽  
Vol 141 (2) ◽  
Author(s):  
Benjamin Winhart ◽  
Martin Sinkwitz ◽  
Andreas Schramm ◽  
David Engelmann ◽  
Francesca di Mare ◽  
...  
Keyword(s):  
Secondary Flow ◽  
Direct Interaction ◽  
Numerical Data ◽  
Horseshoe Vortex ◽  
Low Pressure ◽  
Navier Stokes ◽  
Flow Structures ◽  
Pressure Loss Coefficient ◽  
Low Pressure Turbine ◽  
Numerical Predictions

In this work, we present the results of the numerical investigations of periodic wake–secondary flow interaction carried out on a low pressure turbine (LPT) equipped with modified T106-profile blades. The numerical predictions obtained by means of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations using a k-ω-model have been compared with measurements conducted in the same configuration and showed a good agreement. Based on the verified numerical data, the Q-criterion has been employed to characterize the secondary flow structures and accurately identify their origin. An analysis of the fundamental wake kinematics and the unsteady vortex migration revealed dominant interaction mechanisms such as the circumferential fluctuation of the pressure side horseshoe vortex (HSV) and its direct interaction with the passage vortex (PV) and the concentrated shed vortex (CSV). Finally, a correlation with the total pressure loss coefficient is provided and a link to the incoming wake structures is given.

Influence of turbulent shear stresses on the numerical blood damage prediction in a ventricular assist device

2019 ◽  
Vol 42 (12) ◽  
pp. 735-747 ◽  
Author(s):  
Benjamin Torner ◽  
Lucas Konnigk ◽  
Frank-Hendrik Wurm
Keyword(s):  
Shear Stress ◽  
Equivalent Stress ◽  
Volumetric Analysis ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
Ventricular Assist ◽  
Damage Prediction ◽  
Blood Damage ◽  
Damage Models ◽  
Order Of Magnitude

The blood damage prediction in rotary blood pumps is an important procedure to evaluate the hemocompatibility of such systems. Blood damage is caused by shear stresses to the blood cells and their exposure times. The total impact of an equivalent shear stress can only be taken into account when turbulent stresses are included in the blood damage prediction. The aim of this article was to analyze the influence of the turbulent stresses on the damage prediction in a rotary blood pump’s flow. Therefore, the flow in a research blood pump was computed using large eddy simulations. A highly turbulence-resolving setup was used in order to directly resolve most of the computed stresses. The simulations were performed at the design point and an operation point with lower flow rate. Blood damage was predicted using three damage models (volumetric analysis of exceeded stress thresholds, hemolysis transport equation, and hemolysis approximation via volume integral) and two shear stress definitions (with and without turbulent stresses). For both simulations, turbulent stresses are the dominant stresses away from the walls. Here, they act in a range between 9 and 50 Pa. Nonetheless, the mean stresses in the proximity of the walls reach levels, which are one order of magnitude higher. Due to this, the turbulent stresses have a small impact on the results of the hemolysis prediction. Yet, turbulent stresses should be included in the damage prediction, since they belong to the total equivalent stress definition and could impact the damage on proteins or platelets.

Stress estimates from structural studies in some mantle peridotites

1978 ◽  
Vol 288 (1350) ◽  
pp. 49-57 ◽  
Keyword(s):  
State Of The Art ◽  
Rock Deformation ◽  
Flow Structures ◽  
Mantle Flow ◽  
Structural Studies ◽  
Experimental Calibration ◽  
Mantle Peridotites ◽  
Order Of Magnitude ◽  
The One ◽  
Deviatoric Stresses

Deviatoric stresses acting in rock deformation can be estimated by measuring parameters connected with the dislocation microstructure, after an experimental calibration. In olivine, the available structural geopiezometers are based on dislocation curvature, dislocation density, sub-boundary size and recrystallized grain (neoblast) size. Their application in the case of olivine bearing rocks (peridotites) deformed in mantle conditions is critically assessed. The most reliable geopiezometer is the one based on olivine neoblast size. It yields values in the range of 1 kbar (1 kbar = 10 8 Pa) and over in the case of the sheared nodules in kimberlites, of 0.3- 0.5 kbar in the case of basalt nodules and in the Lanzo massif, although locally the stress can be much higher. These values are compared with those ascribed to mantle flow by various independent methods and which tend to indicate a lower deviatoric stress for asthenosphere flow (10-100bar). In the state of the art, the disagreement between this stress estimate and those in basalt nodules and in massifs, which is nearly one order of magnitude, can be explained by the various uncertainties in the estimate, thus leaving room for the possibility that the flow structures in these peridotites do represent asthenosphere conditions.

A combined 2-D/1-D magma ascent model of explosive volcanic eruptions

2019 ◽  
Vol 219 (3) ◽  
pp. 1818-1835
Author(s):  
Hélène Massol
Keyword(s):  
Volume Fraction ◽  
Experimental Studies ◽  
Volcanic Eruptions ◽  
Gas Content ◽  
Magma Ascent ◽  
Explosive Eruptions ◽  
Shear Stresses ◽  
Order Of Magnitude ◽  
Gas Volume Fraction ◽  
Gas Volume

SUMMARY Explosive eruptions involve the fragmentation of magma that changes the flow regime from laminar to turbulent within the volcanic conduit during ascent. If the gas volume fraction is high, magma fragments and the eruption style is explosive, but if not, the magma flows effusively out of the vent. Gas escape processes depend on how the magma can rupture, and recent experimental studies measured rupture stress thresholds of the order of a few megapascals. It is thus critical to model the gas content and state of stress evolution in the flowing magma within the conduit. We present a new self-consistent model of an explosive eruption from the magma chamber to the surface, based on a critical gas volume fraction. Our model allows to explore irregular geometries below the fragmentation level (2-D). We first compare our model with classical 1-D models of explosive eruptions and find that in the case of straight conduits and fragmented flows, 1-D models are accurate enough to model the gas pressure and vertical velocity distribution in the conduit. However, in the case of an irregular conduit shape at depth, 2-D models are necessary. Despite a certain conduit radius visible at the surface, very different stress fields within the flow could be present depending upon the position and shape of any conduit irregularities. Stresses of the order of more than 1 MPa can be attained in some locations. High tensile stresses are located at the centre of the conduit, while high shear stresses are located at the conduit walls leading to several potential rupture locations. Due to the interplay between the velocity field and decompression rate, similar conduit radius visible at the surface might also lead to very different fragmentation depths with a difference of more than 1500 m between an enlarged conduit shape at some depth and a straight conduit. At depth, different conduit sizes might lead to the same order of magnitude for the mass flux, depending on the conduit geometry.

scholarly journals Predicting the roughness length of turbulent flows over landscapes with multi-scale microtopography

2016 ◽  
Vol 4 (2) ◽  
pp. 391-405 ◽  
Author(s):  
Jon D. Pelletier ◽  
Jason P. Field
Keyword(s):  
Turbulent Flows ◽  
Roughness Length ◽  
Velocity Profiles ◽  
Cfd Modeling ◽  
Shear Stresses ◽  
Law Of The Wall ◽  
Multi Scale ◽  
Rough Form ◽  
Order Of Magnitude ◽  
The One

Abstract. The fully rough form of the law of the wall is commonly used to quantify velocity profiles and associated bed shear stresses in fluvial, aeolian, and coastal environments. A key parameter in this law is the roughness length, z0. Here we propose a predictive formula for z0 that uses the amplitude and slope of each wavelength of microtopography within a discrete-Fourier-transform-based approach. Computational fluid dynamics (CFD) modeling is used to quantify the effective z0 value of sinusoidal microtopography as a function of the amplitude and slope. The effective z0 value of landscapes with multi-scale roughness is then given by the sum of contributions from each Fourier mode of the microtopography. Predictions of the equation are tested against z0 values measured in  ∼ 105 wind-velocity profiles from southwestern US playa surfaces. Our equation is capable of predicting z0 values to 50 % accuracy, on average, across a 4 order of magnitude range. We also use our results to provide an alternative formula that, while somewhat less accurate than the one obtained from a full multi-scale analysis, has an advantage of being simpler and easier to apply.

Critical Reynolds Number Estimates by Thermo-Dynamic (Stochastic) Methods

1974 ◽  
Vol 96 (3) ◽  
pp. 788-794
Author(s):  
P. Hrycak ◽  
M. J. Levy
Keyword(s):  
Reynolds Number ◽  
Critical Reynolds Number ◽  
Stochastic Methods ◽  
Shear Stresses ◽  
Statistical Probability ◽  
Average Shear ◽  
Order Of Magnitude ◽  
Dynamic Stochastic ◽  
Magnitude Estimates

Methods based on fundamental thermodynamic principles and the notion of statistical probability have been used to estimate the point of instability and the lower critical Reynolds number for a round pipe and an infinite channel. It is also shown that order of magnitude estimates of the ratio of the average shear stresses for each regime allow one to draw definite conclusions about the lower and the upper critical Reynolds number in a variety of geometries.

Horizontal Versus Vertical Annealing of Silicon Wafers at High Temperatures

2007 ◽  
Vol 131-133 ◽  
pp. 413-418 ◽  
Author(s):  
G. Kissinger ◽  
A. Fischer ◽  
G. Ritter ◽  
V.D. Akhmetov ◽  
Martin Kittler
Keyword(s):  
Displacement Vector ◽  
Complex Loading ◽  
Fem Analysis ◽  
Silicon Wafers ◽  
Elastic Behavior ◽  
Shear Stresses ◽  
3D Fem ◽  
Induced Stress ◽  
Order Of Magnitude ◽  
Von Mises

The gravitational induced shear stresses in 200 mm silicon wafers supported in verticaltype or horizontal-type furnace were calculated using 3D-FEM analysis of the displacement vector assuming linear elastic behavior of the anisotropic material. For comparison of the two complex loading cases and for relating the effect of gravitational constraints to the mechanical strength of the wafers, the invariant von Mises shear stress τM was chosen. The computed maximum values of τM demonstrate that the gravitational induced stress for vertical processing is approximately one order of magnitude less than the gravitational induced stress for horizontal processing. The experimental results obtained from processing of 200mm wafers with different oxygen concentration in horizontal and vertical boats at 1200°C are in an excellent agreement with the theoretical simulations.

scholarly journals Experimental Characterization of the Flow Field around Oblong Bridge Piers

Fluids ◽  
2021 ◽  
Vol 6 (11) ◽  
pp. 370
Author(s):  
Ana Margarida Bento ◽  
Teresa Viseu ◽  
João Pedro Pêgo ◽  
Lúcia Couto
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Bridge Pier ◽  
Bridge Piers ◽  
Shear Stresses ◽  
Velocity Measurements ◽  
Bridge Foundations ◽  
Scour Hole ◽  
Movable Bed

The prediction of scour evolution at bridge foundations is of utmost importance for engineering design and infrastructures’ safety. The complexity of the scouring inherent flow field is the result of separation and generation of multiple vortices and further magnified due to the dynamic interaction between the flow and the movable bed throughout the development of a scour hole. In experimental environments, the current approaches for scour characterization rely mainly on measurements of the evolution of movable beds rather than on flow field characterization. This paper investigates the turbulent flow field around oblong bridge pier models in a well-controlled laboratory environment, for understanding the mechanisms of flow responsible for current-induced scour. This study was based on an experimental campaign planned for velocity measurements of the flow around oblong bridge pier models, of different widths, carried out in a large-scale tilting flume. Measurements of stream-wise, cross-wise and vertical velocity distributions, as well as of the Reynolds shear stresses, were performed at both the flat and eroded bed stages of scouring development with a high-resolution acoustic velocimeter. The time-averaged values of velocity and shear stress are larger in the presence of a developed scour hole than in the corresponding flat bed configuration.

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