Shear-banding instabilities

Jan K.G. Dhont,Kyongok Kang,M.P.Lettinga,W. J. Briels 한국유변학회 2010 Korea-Australia rheology journal Vol.22 No.4

Gradient-banding and vorticity-banding instabilities, as well as a shear-induced instability due to shear-gradient induced mass transport will be discussed. Various scenarios that underly these instabilities are addressed and simple constitutive relations that allow for a (semi-) quantitative analysis are proposed. A relatively simple constitutive equation that has been proposed some time ago is reviewed, which captures a number of the experimentally observed gradient-banding phenomena. This constitutive equation is based on the usual formal expansion of the stress tensor with respect to gradients in the flow velocity, but now including the second order term. The second order term is necessary to describe the relatively large spatial gradients within the interface between the two bands. The resulting simple constitutive equation is shown to give rise to stationary gradient-banded states, where the shear rates within the bands are constant, it describes stress selection under controlled rate conditions and explains why banding can not occur under controlled stress conditions. The simple constitutive equation does not include coupling to concentration, which may give rise to banding also under controlled stress conditions. Two examples of mechanisms that lead to the strong shear thinning that is necessary for gradient banding are discussed: (i) transient forces due to entanglements in polymer systems, and (ii) critical slowing down. The latter mechanism is shown to be important for a worm-like micellar system. The mechanism that leads to vorticity banding is still under debate. Vorticity banding of fd-virus suspensions within the two-phase isotropic-nematic coexistence will be discussed. Experiments on the kinetics of banding and particle-tracking experiments lead to a recently proposed mechanism for the vorticity-banding instability, where the instability is identified as an elastic instability similar to the polymer-Weissenberg effect. The role of polymer chains in the classic Weissenberg effect is now played by inhomogeneities formed during the initial stages of phase separation. For other systems than fd-virus suspensions that exhibit vorticity banding, the inhomogeneities general have a different origin, like in weakly aggregated colloids and worm-like micellar systems where the inhomogeneities are the colloidal aggregates and the worms, respectively. An instability that has been discovered some time ago, which is an instability due to shear-gradient induced mass transport is also discussed. The coupling between shear-gradients and mass transport has been formally introduced through a shear-rate dependent chemical potential, of which the microscopic origin was not explained. It will be shown that the microscopic origin of this coupling is related to the shear-induced distortion of the pair-correlation function. Contrary to the stationary gradient-banded and vorticity-banded state, it is not yet known what the stationary state is when this shear-concentration-coupling instability occurs.

Structure and Vorticity of the Current Observed Across the Western Channel of the Korea Strait in September of 1987-1989

Byun, Sang-Kyung,Kaneko, Arata Korea Institute of Ocean Science Technology 1999 Ocean and Polar Research Vol.21 No.2

With sectional data obtained in September of 1987, 1988 and 1989 by quadrireciprocal ADCP measurement and CTD cast, the current structure, volume transport and vorticity in the Western Channel of the Korea Strait were studied. The characteristics of Tsushima Current water persisted throughout the summer especially in the homogeneous water of temperature $14-16^{\circ}C$ located at the depth of 50-100m below seasonal termocline. Thickness and velocity of the homogeneous layer are about 10-170m and 20-60cm/s. and the relative vorticity for this layer is shown to be nearly constant and it is smaller than the planetary vorticity. Potential vorticity of $2.70-7.10{\times}10^{-6}m^{-1}s^{-1}$ is found to be dependent mainly on planetary rather than on the relative vorticities. The Tsushima Current water represented by the homogeneous layer R14-16^{\circ}C$ may keep the potential vorticity at the area of strong current in the Strait. The ADCP current structure is similar to geostrophic current and the core of the current with the speed of 30-50cm/s is situated in the middle layer over the deep trough. With large tidal fluctuation the volume transport has mean value of 1.17sv which was about 40% larger than that of geostrophic calculation.

A combined vortex and panel method for numerical simulations of viscous flows: a comparative study of a vortex particle method and a finite volume method

Kim, Kwang-Soo,Lee, Seung-Jae,Suh, Jung-Chun Wiley 2005 International journal for numerical methods in flu Vol.49 No.10

<P>This paper describes and compares two vorticity-based integral approaches for the solution of the incompressible Navier–Stokes equations. Either a Lagrangian vortex particle method or an Eulerian finite volume scheme is implemented to solve the vorticity transport equation with a vorticity boundary condition. The Biot–Savart integral is used to compute the velocity field from a vorticity distribution over a fluid domain. The vorticity boundary condition is improved by the use of an iteration scheme connected with the well-established panel method. In the early stages of development of flows around an impulsively started circular cylinder, and past an impulsively started foil with varying angles of attack, the computational results obtained by the Lagrangian vortex method are compared with those obtained by the Eulerian finite volume method. The comparison is performed separately for the pressure fields as well. The results obtained by the two methods are in good agreement, and give a better understanding of the vorticity-based methods. Copyright © 2005 John Wiley & Sons, Ltd.</P>

Catalina Eddy as Revealed by the Historical Downscaling of Reanalysis

Masao Kanamitsu,Elena Yulaeva,Haiqin Li,홍성유 한국기상학회 2013 Asia-Pacific Journal of Atmospheric Sciences Vol.49 No.4

Climatological properties, dynamical and thermodynamical characteristics of the Catalina Eddy are examined from the 61 years NCEP/NCAR Reanalysis downscaled to hourly 10 km resolution. The eddy is identified as a mesoscale cyclonic circulation confined to the Southern California Bight. Pattern correlation of wind direction against the canonical Catalina Eddy is used to extract cases from the downscaled analysis. Validation against published cases and various observations confirmed that the downscaled analysis accurately reproduces Catalina Eddy events. A composite analysis of the initiation phase of the eddy indicates that no apparent large-scale cyclonic/anti-cyclonic large-scale forcing is associated with the eddy formation or decay. The source of the vorticity is located at the coast of the Santa Barbara Channel. It is generated by the convergence of the wind system crossing over the San Rafael Mountains and the large-scale northwesterly flow associated with the subtropical high. This vorticity is advected towards the southeast by the northwesterly flow, which contributes to the formation of the streak of positive vorticity. At 6 hours prior to the mature stage, there is an explosive generation of positive vorticity along the coast, coincident with the phase change of the sea breeze circulation (wind turning from onshore to offshore), resulting in the convergence all along the California coast. The generation of vorticity due to convergence along the coast together with the advection of vorticity from the north resulted in the formation of southerly flow along the coast, forming the Catalina Eddy. The importance of diurnal variation and the lack of large-scale forcing are new findings, which are in sharp contrast to prior studies. These differences are due to the inclusion of many short-lived eddy events detected in our study which have not been included in other studies.

수치모의를 이용한 전단면 식생 수로에서의 와도 생성 분석

강형식(Kang Hyeongsik) 대한토목학회 2010 대한토목학회논문집 B Vol.30 No.2B

본 연구에서는 수치모의를 통하여 전단면 식생 수로에서 와도의 생성을 분석하였다. 지배방정식에서 난류 폐합을 위해 레이놀즈응력모형을 이용하였다. 거친 하상-매끄러운 측벽 및 매끄러운 하상-거친 측벽을 갖는 개수로 흐름을 수치모의하여 서로 다른 형태의 이차흐름 구조가 형성되는 것을 확인하였다. 즉, 거친 하상 조건에서는 지유수면 이차흐름의 규모가 감소되고, 거친 측벽 조건에서는 자유수면 이차흐름의 구조가 더 커지는 것으로 나타났다. 또한 전단면 식생 수로를 수치모의하여 수심 크기의 바닥 이차흐름이 형성되고, 식생 밀도가 증가함에 따라 자유수면 이차흐름이 점차 사라지는 것을 확인하였다. 또한 이차흐름 생성에 중요한 역할을 하는 난류의 비등방성 및 레이놀즈응력 분포를 식생밀도에 따라 살펴보았다. 한편, 와도 방정식을 분석한 결과, 비식생 수로의 경우 벽 및 수면 경계 근처에서는 난류 비등방성에 의한 생성항이, 경계와 떨어진 곳에서는 레이놀즈응력에 의한 생성항이 와도 생성에 중요한 역할을 히는 것으로 나타났다. 그러나 식생 수로에서는 이러한 특성이 사라지는 것으로 확인되었다. 또한 비식생 수로에서는 바닥과 수면에서의 와도 생성이 강하게 발생되지만, 식생 수로에서는 바닥과 식생 높이에서 와도 생성이 크게 발생되는 것으로 나타났다. This paper presents a numerical investigation of vorticity generation in fully vegetated open-channel flows. The Reynolds stress model is used for the turbulence closure. Open-channel flows with rough bed-smooth sidewalls and smooth bed-rough sidewalls are simulated. The computed vectors show that in channel flows with rough bed and rough sidewalls, the free-surface secondary currents become relatively smaller and larger, respectively, compared with that of plain channel flows. Also, open-channel flows over vegetation are simulated. The computed bottom vortex occupies the entire water depth, while the free-surface vortex is reduced. The contours of turbulent anisotropy and Reynolds stress are presented with different density of vegetation. The budget analysis of vorticity equation is carried out to investigate the generation mechanism of secondary currents. The results of the budget analysis show that in plain open-channel flow, the production by anisotropy is important in the vicinity of the wall and free-surface boundaries, and the production by Reynolds stress is important in the region away from the boundaries. However, this rule is not effective in vegetated channel flows. Also, in plain channel flows, the vorticity is generated mainly in the vicinity of the free-surface and the bottom, while in vegetated channel flows, the regions of the bottom and vegetation height are important to generate the vorticity.

Vortex Dynamics in a Josephson Junction Array by Using the Time-dependent Vorticity-vorticity Correlation Function

이기호 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.56 No.5

We have studied numerically the motion of vortices in an underdamped Josephson junction array,especially in the presence of a magnetic field of 1/5 flux quantum per plaquette. Such a magnetic field density provides a square vortex lattice that has a lattice constant of √5a, where a is the plaquette length of the array. For a ratchet array, we use both alternate critical currents and alternate inter-capacitances along the direction of vortex flow. To analyze the dynamics of several vortices around a mid row of the array, we use the time-dependent vorticity-vorticity correlation function as well as the time-dependent vorticity. The vortices on a fractional Shapiro n/5-step are found to move through n plaquettes transversely to the direction of the applied current during a single ac cycle.

선박평형수 살균처리기 내 고정 혼합기 형상에 따른 유체의 혼합특성에 관한 수치해석적 연구

정은익(Eun-Ik Jeong),박상진(Sang-Jin Park),김태화(Tae-Wha Kim),서정세(Jeong-Se Suh) 한국기계가공학회 2023 한국기계가공학회지 Vol.22 No.5

Ballast water refers to seawater loaded into ballast tanks to balance ships. To prevent marine pollution, the sterilization of ballast water in ships is essential. The methods for sterilizing ballast water include chemical and electrolytic treatments, as well as other methods. The purpose of this study was to improve the efficiency of the electric sterilization method by varying the cross-junction and twisting number of the static mixer elements to investigate their effects on pressure drop and mixing. As a result, when the twisting number of static mixers was high, the shape of the cross-junction static mixer elements yielded a higher pressure drop than that of the continuously bonded elements but with a lower vorticity. When the twisting number of the static mixer was low, there was no significant change in the pressure drop or vorticity, regardless of whether the elements were crossed. Because it is believed that the pressure drop will have a greater impact than vorticity on the mixing of seawater within the pipe, a static mixer with a high twisting number and cross-junction elements is considered the most suitable shape.

부유식방파제의 수리특성 및 와 상호작용에 관한 실험적 연구

윤재선,손혁준,전시영,조용식,Yoon. Jae-Seon,Son. Hyok-Jun,Chun. Si-Young,Cho. Yong-Sik 한국방재학회 2010 한국방재학회논문집 Vol.10 No.6

본 연구에서는 LDV시스템을 이용하여 부유식 방파제 주변의 유동장의 변화를 수리모형실험을 통해 분석하였다. LDV시스템은 유체의 유속을 측정하기 위해 다양한 수리모형실험에서 폭넓게 사용되어왔다. LDV시스템을 이용하면 측정시간이 오래 걸리는 단점이 있지만, 측정한 자료를 가공하지 않고 정확한 유속장을 제시할 수 있다는 장점이 있다. 방파제의 형상과 흘수를 달리하여 수리모형실험을 수행하였으며, 유동장과 와류의 상관관계를 분석하였다. 또한, 연직 2차원 Navier-Stokes 방정식 모형을 이용하여 수치모형실험을 수행하였으며, 수치모형실험 결과와 수리모형실험 결과를 비교분석하였다. In this study, laboratory experiments are conducted to investigate flow-fields around floating breakwaters by using the LDV(Laser Doppler Velocimetry) system. The LDV system is a well-known equipment to measure fluid particle velocities in laboratory experiments. Although the system requires great efforts and enormous time for measurements, it can provide precise velocity fields comparing to other available equipments. Various types of drafts and shapes for breakwaters are employed in laboratory experiments to analyze a relation between flow-fields and vorticity. A series of numerical experiments are also carried out by using a two-dimensional Navier-Stokes equations model. Numerically predicted results are compared with laboratory measurements.

The MODIS ice surface temperature product as an indicator of sea ice minimum over the Arctic Ocean

Kang, D.,Im, J.,Lee, M.I.,Quackenbush, L.J. American Elsevier Pub. Co 2014 Remote sensing of environment Vol.152 No.-

This study examines the relationship between sea ice extent and ice surface temperature (IST) between 2000 and 2013 using daily IST products from the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. The empirical prediction of September sea ice extent using its trend and two climate variables-IST and wind vorticity-exhibits a statistically significant relationship (R=0.97) with a time lag, where IST maximum in summer (June-July) corresponds to the sea ice extent minimum in September. This suggests that IST may serve as an indicator of the basin-wide heat energy accumulated in the Arctic by solar radiation and large-scale atmospheric heat transport from lower latitudes. The process of inducing higher IST is related to the change of atmospheric circulation over the Arctic. Averaged IST and 850hPa relative vorticity of the polar region show a significant negative correlation (-0.57) in boreal summer (June-August), suggesting a weakening of the polar vortex in the case of warmer-than-normal IST conditions. Weakening of the polar vortex is accompanied by above-normal surface pressure. Minimum sea ice extent in September was successfully predicted by both multiple linear regression and machine learning support vector regression using preceding summer IST and wind vorticity along with the trend of sea ice extent (R<SUP>2</SUP>~0.95, cross validation RMSE of 3-4x10<SUP>5</SUP>km<SUP>2</SUP>, and relative cross validation RMSE of 5-8%).

Digital Design of DWPT Technique on FPGA for Power System Harmonics Estimation

Baraskar Savita,Tiwari Vinay K. 대한전기학회 2022 Journal of Electrical Engineering & Technology Vol.17 No.6

Real-time detection and quick amplitude estimation of power system harmonics are signifi cant issues among various power quality indices in the electrical power distribution system. In this paper digital design of the discrete wavelet packet transform (DWPT) technique is presented for accurate and fast amplitude estimation of power system harmonics in real-time applications. The DWPT technique provides a uniform frequency bandwidth of the power supply waveform at the output end. Consequently, the input signal's frequency spectrum is evenly divided into eight equal frequency bands that give accurate amplitude estimation of the harmonics by the proposed technique. Generally, the DWPT technique gives the output in the form of coeffi cients, so the proposed digital architecture incorporates arrangements for giving root mean square values directly. The DWPT based wavelet decomposition tree has been developed with the help of MAC-based wavelet fi lters that gives an advantage in saving the number of multipliers compared with the polyphase structure on the FPGA board. Also, the downsampling operation performed at each decomposition level reduces the wavelet coeffi cients by half which minimizes the computations involved in it. The digital design of the proposed DWPT technique has been implemented on the Xilinx Artix-7 FPGA AC-701 board with Xilinx System Generator (XSG)/Vivado design suite 18.3. Thereafter, standard experimental test signals have been generated on the MATLAB/Simulink software platform to validate the robustness and accuracy of the proposed digital hardware under various experimental test conditions, proving their applicability in real-time estimation of power system harmonics.