The Influence of Turbulence Memory on Idealized Tornado Simulations

2020 ◽  
Vol 148 (12) ◽  
pp. 4875-4892
Author(s):  
Aaron Wang ◽  
Ying Pan ◽  
Paul M. Markowski
Keyword(s):  
Boundary Condition ◽  
Shear Stress ◽  
Stress Component ◽  
Lower Boundary ◽  
Surface Friction ◽  
Surface Shear Stress ◽  
Normal Surface ◽  
Shear Stress Component ◽  
Surface Shear ◽  
Lower Boundary Condition

AbstractSurface friction contributes to tornado formation and maintenance by enhancing the convergence of angular momentum. The traditional lower boundary condition in atmospheric models typically assumes an instant equilibrium between the unresolved stress and the resolved shear. This assumption ignores the physics that turbulent motions are generated and dissipated at finite rates—in effect, turbulence has a memory through its lifetime. In this work, a modified lower boundary condition is proposed to account for the effect of turbulence memory. Specifically, when an air parcel moves along a curved trajectory, a normal surface-shear-stress component arises owing to turbulence memory. In the accompanying large-eddy simulation (LES) of idealized tornadoes, the normal surface-shear-stress component is a source of additional dynamic instability, which provides an extra pathway for the development of turbulent motions. The influence of turbulence memory on the intensity of quasi-steady-state tornadoes remains negligible as long as assumptions employed by the modified lower boundary condition hold over a relatively large fraction of the flow region of interest. However, tornadoes in a transient state may be especially sensitive to turbulence memory.Significance StatementFriction between the wind and the ground can influence atmospheric phenomena in important ways. For example, surface friction can be a significant source of rotation in some thunderstorms, and it can also help to intensify rotation when rotation is already present. Unfortunately, the representation of friction’s effects in atmospheric simulations is especially error-prone in phenomena characterized by rapid temporal evolution or strong spatial variations. Our work explores a new framework for representing friction to include the effect of the so-called turbulence memory. The approach is tested in idealized tornado simulations, but it may be applied to a wide range of atmospheric vortices.

scholarly journals An Improved Surface Boundary Condition for Large-Eddy Simulations Based on Monin–Obukhov Similarity Theory: Evaluation and Consequences for Grid Convergence in Neutral and Stable Conditions

2019 ◽  
Vol 174 (2) ◽  
pp. 297-325 ◽  
Author(s):  
Björn Maronga ◽  
Christoph Knigge ◽  
Siegfried Raasch
Keyword(s):  
Boundary Condition ◽  
Shear Stress ◽  
Similarity Theory ◽  
Surface Boundary ◽  
Surface Shear Stress ◽  
Surface Shear ◽  
Stable Conditions ◽  
Grid Convergence ◽  
Surface Boundary Condition ◽  
Les Model

Abstract Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves grid convergence of these quantities. The LES data indicate improved grid convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved grid convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.

Visualization of separation and reattachment in an incident shock-induced interaction

2021 ◽  
pp. 095441002098349
Author(s):  
Yun Jiao ◽  
Chengpeng Wang
Keyword(s):  
Numerical Simulation ◽  
Shock Wave ◽  
Shear Stress ◽  
Separation Bubble ◽  
Incident Shock ◽  
Incident Shock Wave ◽  
Bottom Wall ◽  
Surface Shear Stress ◽  
Surface Shear ◽  
Oil Flow

An experimental study is conducted on the qualitative visualization of the flow field in separation and reattachment flows induced by an incident shock interaction by several techniques including shear-sensitive liquid crystal coating (SSLCC), oil flow, schlieren, and numerical simulation. The incident shock wave is generated by a wedge in a Mach 2.7 duct flow, where the strength of the interaction is varied from weak to moderate by changing the angle of attack α of the wedge from 8° and 10° to 12°. The stagnation pressure upstream was set to approximately 607.9 kPa. The SSLCC technique was used to visualize the surface flow characteristics and analyze the surface shear stress fields induced by the initial incident shock wave over the bottom wall and sidewall experimentally which resolution is 3500 × 200 pixels, and the numerical simulation was also performed as the supplement for a clearer understanding to the flow field. As a result, surface shear stress over the bottom wall was visualized qualitatively by SSLCC images, and flow features such as separation/reattachment and the variations of position/size of separation bubble with wedge angle were successfully distinguished. Furthermore, analysis of shear stress trend over the bottom wall by a hue value curve indicated that the relative magnitude of shear stress increased significantly downstream of the separation bubble compared with that upstream. The variation trend of shear stress was consistent with the numerical simulation results, and the error of separation position was less than 2 mm. Finally, the three-dimensional schematic of incident shock-induced interaction has been achieved by qualitative summary by multiple techniques, including SSLCC, oil flow, schlieren, and numerical simulation.

Measurement of surface shear stress vectors using liquid crystal coatings

AIAA Journal ◽  
1994 ◽  
Vol 32 (8) ◽  
pp. 1576-1582 ◽  
Author(s):  
Daniel C. Reda ◽  
Joseph J. Muratore
Keyword(s):  
Shear Stress ◽  
Liquid Crystal ◽  
Surface Shear Stress ◽  
Surface Shear

Seismic interpretation theory for an elastic earth

1954 ◽  
Vol 224 (1156) ◽  
pp. 43-63 ◽  
Keyword(s):  
Shear Stress ◽  
Seismic Interpretation ◽  
Elastic Theory ◽  
Static Case ◽  
Surface Shear Stress ◽  
Elastic Parameters ◽  
Surface Shear ◽  
Special Cases ◽  
Present Solution ◽  
Ray Path

The seismic interpretation problem for an isotropic spherical earth is analyzed on the basis of elastic theory, under the assumption that the three independent elastic parameters are unknown continuous functions of the depth. It is shown that solutions for these functions may be obtained in the form of Taylor’s series. The problem is treated for three types of symmetrical excitation conditions on the free surface: (1) a shear source of type p rϕ only; (2) a pressure distribution with vanishing surface shear stress; (3) an excitation consisting of pressure in combination with surface shear stress of type p rθ . In each case the excitation functions are arbitrary functions of time. It is assumed that the associated components of surface displacement over the sphere are known from available observations, as functions of time. Thus, the complete information contained in seismic records is used in the proposed interpretation process, without need of selecting, identifying and assigning arrival times to specific events on the records. The two static elastic parameters may theoretically be determined from observations at a single frequency, including the frequency zero, or static case. The determination of the dynamic elastic parameter requires the use of at least two frequencies. Algebraic checks are obtained by comparing the general solutions with the corresponding results for two special cases in which the elastic parameters vary in a prescribed manner in the interior of the sphere. In both these cases treatment by the classical ray-path method of interpretation is excluded, because the wave velocity decreases with depth. Furthermore, the ray-path method (which is essentially a method of geometrical optics) would fail to distinguish between the two examples in any case, since the velocity function is the same in both, although the elastic parameters differ. In contrast to the valuable ray-path method, the analytical procedures in the present solution of the elastic problem are prohibitively cumbersome. Practical application of elastic theory to the direct interpretation of seismograms requires further development of the theory with probable utilization of modern high-speed computing methods.

Characterization of Thermally Induced Stress in IC Packages Using PiFETs Over a Temperature Range of −180°C to 80°C

2014 ◽  
Vol 2014 (1) ◽  
pp. 000500-000504 ◽  
Author(s):  
Francy J. Akkara ◽  
Uday S. Goteti ◽  
Richard C. Jaeger ◽  
Michael C. Hamilton ◽  
Michael J. Palmer ◽  
...  
Keyword(s):  
Shear Stress ◽  
Stress Component ◽  
Shear Stress Component ◽  
Thermally Induced ◽  
Temperature Changes ◽  
Temperature Range ◽  
Induced Stress ◽  
Highly Sensitive ◽  
Stress Components

In certain applications, IC packages may be exposed to extreme temperatures and knowledge of thermally induced stress aids the prediction of performance degradation or failure of the IC. In the devices that are used in extreme conditions, the stress is caused mainly by the mismatch in expansion of various materials triggered by the different coefficients of thermal expansion. This work performed in this study is conducted using NMOS current mirror circuits that are cycled through a wide temperature range of −180°C to 80°C. These circuits are highly sensitive to stress and provide well-localized measurements of shear stress. The sensors are fabricated in such a way that the effects of certain stress components are isolated. These sensors are also temperature compensated so that only the effect of mechanical stress components is observed and changes in device performance due to temperature changes are minimal. Current readings obtained from the sensors are used to extract the shear stress component. Finite element simulations, using expected materials performance parameter information were also performed for similar packages and these results are compared to the measured results.

Modified electromyography-assisted optimization approach for predicting lumbar spine loading while walking with backpack loads

2020 ◽  
Vol 234 (5) ◽  
pp. 527-533
Author(s):  
Simon SW Li ◽  
Daniel HK Chow
Keyword(s):  
Boundary Condition ◽  
Lumbar Spine ◽  
Maximal Voluntary Contraction ◽  
Lower Boundary ◽  
Voluntary Contraction ◽  
Optimization Approach ◽  
Compression Force ◽  
Lower Boundary Condition ◽  
Force Profile ◽  
Mean Square Difference

This study modified an electromyography-assisted optimization approach for predicting lumbar spine loading while walking with backpack loads. The modified-electromyography-assisted optimization approach eliminated the electromyography measurement at maximal voluntary contraction and adopted a linear electromyography–force relationship. Moreover, an optimal lower boundary condition for muscle gain was introduced to constrain the trunk muscle co-activation. Anthropometric information of 10 healthy young men as well as their kinematic, kinetic, and electromyography data obtained while walking with backpack loads were used as inputs in this study. A computational algorithm was used to find and analyse the sensitivity of the optimal lower boundary condition for achieving minimum deviation of the modified-electromyography-assisted optimization approach from the electromyography-assisted optimization approach for predicting lumbosacral joint compression force. Results validated that the modified-electromyography-assisted optimization approach (at optimal lower boundary condition of 0.92) predicted on average, a non-significant deviation in peak lumbosacral joint compression force of −18 N, a standard error of 9 N, and a root mean square difference in force profile of 73.8 N. The modified-electromyography-assisted optimization approach simplified the experimental process by eliminating the electromyography measurement at maximal voluntary contraction and provided comparable estimations for lumbosacral joint compression force that is also applicable to patients or individuals having difficulty in performing the maximal voluntary contraction activity.

scholarly journals Nonlinear Dynamic Characteristic Analysis of a Landing String in Deepwater Riserless Drilling

2018 ◽  
Vol 2018 ◽  
pp. 1-17
Author(s):  
Jun Liu ◽  
Hongliang Zhao ◽  
Simon X. Yang ◽  
Qingyou Liu ◽  
Guorong Wang
Keyword(s):  
Boundary Condition ◽  
Dynamic Model ◽  
Marine Environment ◽  
Lower Boundary ◽  
Dynamic Responses ◽  
Characteristic Analysis ◽  
Lower Boundary Condition ◽  
Response Characteristics ◽  
Proposed Model ◽  
Nonlinear Dynamic Characteristic

The landing string is an important component of deepwater riserless drilling systems. Determination of the dynamic characteristics of the landing string plays an essential role in its design for ensuring its safe operation. In this paper, a dynamic model is developed to investigate the dynamic response characteristics of a landing string, where a landing string in a marine environment is modeled as a flexible slender tube undergoing coupled transverse and axial motions. The heaving motion of the drilling platform is taken as the upper boundary condition and the motion of the drilling bit caused by the interaction between the rock and the bit as the lower boundary condition. A semiempirical Morison equation is used to simulate the effect of the load imposed by the marine environment. The dynamic model, which is nonlinearly coupled and multibody, is discretized by a finite element method and solved by the Newmark technique. Using the proposed model, the dynamic responses of the displacement, axial force, and moment in the landing string are investigated in detail to find out the influences of driving depth of surface catheter, platform motion, bit movement, and marine environment on the dynamical characteristics of the landing string.

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