Intrinsic features of flow past three square prisms in side-by-side arrangement

2017 ◽  
Vol 826 ◽  
pp. 996-1033 ◽  
Author(s):  
Qinmin Zheng ◽  
Md. Mahbub Alam
Keyword(s):  
Beat Frequency ◽  
Phase Relationship ◽  
Streamwise Velocity ◽  
Viscous Force ◽  
Phase Lag ◽  
Force Coefficient ◽  
Fluid Forces ◽  
Gap Flow ◽  
Base Bleed ◽  
Primary Frequency

An investigation on the flow around three side-by-side square prisms can provide a better understanding of complicated flow physics associated with multiple, closely spaced structures in which more than one gap flow is involved. In this paper, the flow around three side-by-side square prisms at a Reynolds number $Re=150$ is studied systematically at $L/W=1.1{-}9.0$, where $L$ is the prism centre-to-centre spacing and $W$ is the prism width. Five distinct flow structures and their ranges are identified, viz. base-bleed flow ($L/W<1.4$), flip-flopping flow $(1.4<L/W<2.1)$, symmetrically biased beat flow $(2.1<L/W<2.6)$, non-biased beat flow $(2.6<L/W<7.25)$ and weak interaction flow $(7.25<L/W<9.0)$. Physical aspects of each flow regime, such as vortex structures, vortex dynamics, gap-flow behaviours, shedding frequencies and fluid forces, are discussed in detail. A secondary (beat) frequency other than the Strouhal frequency (primary frequency) is observed in the symmetrically biased and non-biased beat flows, associated with the beat-like modulation in $C_{L}$-peak or amplitude, where $C_{L}$ is the lift force coefficient. Here we reveal the generic and intrinsic origin of the secondary frequency, establishing its connections with the phase lag between the two shear-layer sheddings from the two sides of a gap. When the two sheddings are in phase, no viscous force acts at the interface (i.e. at the centreline of the gap) of the two sheddings, resulting in the largest fluctuations in streamwise momentum, streamwise velocity and pressure; the maximum $C_{L}$ amplitude thus features the in-phase shedding. Conversely, when the two sheddings are antiphase, a viscous force exists at the interface of the two sheddings and restricts the momentum fluctuation through the gap, yielding a minimum $C_{L}$ amplitude. When the phase relationship between the two sheddings changes from in phase to antiphase, the extra viscous force acting at the interface becomes larger and causes the $C_{L}$ amplitude to change from a maximum to a minimum.

Vibrations of a square cylinder submerged in a wake

2018 ◽  
Vol 853 ◽  
pp. 301-332 ◽  
Author(s):  
Rajesh Bhatt ◽  
Md. Mahbub Alam
Keyword(s):  
Phase Relationship ◽  
Reynolds Numbers ◽  
Vibration Response ◽  
Stream Velocity ◽  
Phase Lag ◽  
Structure Modification ◽  
Square Cylinder ◽  
Gap Flow ◽  
Different Characteristics ◽  
Work Done

A numerical investigation is conducted on the flow around and vibration response of an elastic square cylinder (side width $D$) in the wake of a stationary cylinder at Reynolds numbers of $Re=100$ and 200 based on $D$ and the free-stream velocity. The downstream cylinder, referred to as the wake cylinder, is allowed to vibrate in the transverse direction only. The reduced velocity $U_{r}$ is varied from 1 to 30. Cylinder centre-to-centre spacing ratios of $L^{\ast }(=L/D)=2$ and 6 are considered. Simulations are also conducted for a single isolated cylinder, and the results are compared with those for the wake cylinder. The focus is given to vibration response, frequency response, fluctuating lift force, phase relationship between the lift and displacement, work done and the flow structure modification during the cylinder vibration. The results reveal that the dependence of the Strouhal number $St$ on $U_{r}$ can distinguish different branches more appropriately than that of the vibration amplitude on $U_{r}$. The vibration response of the single cylinder at $Re=100$ is characterized by the initial, lower and desynchronization branches. On the other hand, that at $Re=200$ undergoes initial, lower and galloping branches. The galloping involves the characteristics of both the initial and the lower branches or the initial and the desynchronization branches depending on $U_{r}$. For the wake cylinder, the gap flow has a significant impact on the vibration response, leading to (i) the absence of galloping at either $Re$ and $L^{\ast }$, (ii) the presence of an upper branch at $Re=200$, $L^{\ast }=6$ and (iii) an initial branch of different characteristics at $Re=100$, $L^{\ast }=6$. The different facets are discussed in terms of wake structures, work done and phase lag between lift and displacement.

scholarly journals Phase relationships between Antarctic and Greenland climate records

2002 ◽  
Vol 35 ◽  
pp. 451-456 ◽  
Author(s):  
Eric J. Steig ◽  
Richard B. Alley
Keyword(s):  
Southern Hemisphere ◽  
Climate Models ◽  
Phase Relationship ◽  
Ice Cores ◽  
Phase Lag ◽  
The North ◽  
Climate Records ◽  
Climate Time Series ◽  
Different Characteristics ◽  
Phase Relationships

AbstractComparison of climate records from Antarctic and Greenland ice cores shows that the two regions respond asynchronously during millennial-scale climate changes. the apparent out-of-phase relationship between the records has been described as a climate ``seesaw’’ in which cooling in the Northern Hemisphere is balanced by warming in the Southern Hemisphere. the same relationship has also been attributed to the initiation of climate-change events in the Southern Hemisphere, rather than the North Atlantic as is conventionally assumed. A simple statistical approach−band-pass filtering combined with lag–correlation tests−used to examine the phase relationships in more detail shows that neither an anti-phase nor a phase-lag relationship adequately describes the observations. Whereas Antarctic and Greenland climate records do exhibit approximate anti-phase behavior about 50% of the time, they are generally in phase during cooling. A phase lead of Southern Hemisphere climate of 1000–1600 years is statistically indistinguishable from a lag of 400–800 years, whether for Dansgaard–Oeschger, Heinrich or longer-duration events. the ``seesaw’’ or ``Southern lead’’ appearance of the data arises from the fundamentally different characteristics of the climate time series, most importantly the absence of rapid warming events in Antarctica comparable to those in Greenland. to be consistent with the observations, climate models will need to capture these characteristics, in addition to reproducing the correct phase relationships.

scholarly journals Reconfiguration of Network Hub Structure after Propofol-induced Unconsciousness

2013 ◽  
Vol 119 (6) ◽  
pp. 1347-1359 ◽  
Author(s):  
Heonsoo Lee ◽  
George A. Mashour ◽  
Gyu-Jeong Noh ◽  
Seunghwan Kim ◽  
UnCheol Lee
Keyword(s):  
General Anesthesia ◽  
Brain Networks ◽  
Phase Relationship ◽  
Clustering Coefficient ◽  
Phase Lag ◽  
Hub Location ◽  
Functional Brain ◽  
Functional Changes ◽  
States Of Consciousness ◽  
Functional Brain Network

Abstract Introduction: General anesthesia induces unconsciousness along with functional changes in brain networks. Considering the essential role of hub structures for efficient information transmission, the authors hypothesized that anesthetics have an effect on the hub structure of functional brain networks. Methods: Graph theoretical network analysis was carried out to study the network properties of 21-channel electroencephalogram data from 10 human volunteers anesthetized on two occasions. The functional brain network was defined by Phase Lag Index, a coherence measure, for three states: wakefulness, loss of consciousness induced by the anesthetic propofol, and recovery of consciousness. The hub nodes were determined by the largest centralities. The correlation between the altered hub organization and the phase relationship between electroencephalographic channels was investigated. Results: Topology rather than connection strength of functional networks correlated with states of consciousness. The average path length, clustering coefficient, and modularity significantly increased after administration of propofol, which disrupted long-range connections. In particular, the strength of hub nodes significantly decreased. The primary hub location shifted from the parietal to frontal region, in association with propofol-induced unconsciousness. The phase lead of frontal to parietal regions in the α frequency band (8–13 Hz) observed during wakefulness reversed direction after propofol and returned during recovery. Conclusions: Propofol reconfigures network hub structure in the brain and reverses the phase relationship between frontal and parietal regions. Changes in network topology are more closely associated with states of consciousness than connectivity and may be the primary mechanism for the observed loss of frontal to parietal feedback during general anesthesia.

Bed forms in alluvial channels

1966 ◽  
Vol 26 (3) ◽  
pp. 507-514 ◽  
Author(s):  
A. J. Raudkivi
Keyword(s):  
Surface Waves ◽  
Relative Phase ◽  
Phase Relationship ◽  
Wave Formation ◽  
Phase Lag ◽  
Alluvial Channels ◽  
Sediment Movement ◽  
Bed Forms ◽  
Relationship Of ◽  
Good Agreement

A solution is offered for the relative phase relationship of bed, depth and surface waves observed in alluvial channels, and is found to be in good agreement with observation in laboratory flumes. The solution does not depend on an assumption of a phase lag between the velocity and sediment movement. The emphasis of the paper is on the mechanics of bed-wave formation.

scholarly journals Local transport of passive scalar released from a point source in a turbulent boundary layer

2018 ◽  
Vol 846 ◽  
pp. 292-317 ◽  
Author(s):  
K. M. Talluru ◽  
J. Philip ◽  
K. A. Chauhan
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Phase Relationship ◽  
Passive Scalar ◽  
Streamwise Velocity ◽  
Concentration Fluctuations ◽  
Normal Velocity ◽  
Velocity Fluctuations

Simultaneous measurements of streamwise velocity ($\tilde{U}$) and concentration ($\tilde{C}$) for a horizontal plume released at eight different vertical locations within a turbulent boundary layer are discussed in this paper. These are supplemented by limited simultaneous three-component velocity and concentration measurements. Results of the integral time scale ($\unicode[STIX]{x1D70F}_{c}$) of concentration fluctuations across the width of the plume are presented here for the first time. It is found that$\unicode[STIX]{x1D70F}_{c}$has two distinct peaks: one closer to the plume centreline and the other at a vertical distance of plume half-width above the centreline. The time-averaged streamwise concentration flux is found to be positive and negative, respectively, below and above the plume centreline. This behaviour is a resultant of wall-normal velocity fluctuations ($w$) and Reynolds shear stress ($\overline{uw}$). Confirmation of these observations is found in the results of joint probability density functions of$u$(streamwise velocity fluctuations) and$\tilde{C}$as well as that of$w$and$\tilde{C}$. Results of cross-correlation coefficient show that high- and low-momentum regions have a distinctive role in the transport of passive scalar. Above the plume centreline, low-speed structures have a lead over the meandering plume, while high-momentum regions are seen to lag behind the plume below its centreline. Further examination of the phase relationship between time-varying$u$and$c$(concentration fluctuations) via cross-spectrum analysis is consistent with this observation. Based on these observations, a phenomenological model is presented for the relative arrangement of a passive scalar plume with respect to large-scale velocity structures in the flow.

Numerical Simulation of Flows Driven by a Torsionally Oscillating Lid in a Square Cavity

1992 ◽  
Vol 114 (2) ◽  
pp. 143-151 ◽  
Author(s):  
Reima Iwatsu ◽  
Jae Min Hyun ◽  
Kunio Kuwahara
Keyword(s):  
Thin Layer ◽  
Numerical Solutions ◽  
Stokes Equations ◽  
Fluid Motion ◽  
Numerical Data ◽  
Navier Stokes ◽  
Physical Parameters ◽  
Phase Lag ◽  
Two Dimensional ◽  
Force Coefficient

Numerical studies were made of the flow of a viscous fluid in a two-dimensional square container. The flows are driven by the top sliding wall, which executes sinusoidal oscillations. Numerical solutions were acquired by solving the time-dependent, two-dimensional incompressible Navier-Stokes equations. Results are presented for wide ranges of two principal physical parameters, i.e., Re, the Reynolds number and ω′, the nondimensional frequency of the lid oscillation. Comprehensive details of the flow-structure are presented. When ω′ is small, the flow bears qualitative similarity to the well-documented steady driven-cavity flow. The flow in the bulk of cavity region is affected by the motion of the sliding upper lid. On the contrary, when ω′ is large, the fluid motion tends to be confined within a thin layer near the oscillating lid. In this case, the flow displays the characteristic features of a thin-layer flow. When ω′ is intermediate, ω′ ~ O(1), the effect of the side walls is pronounced; the flow pattern reveals significant changes between the low-Re and high-Re limits. Streamline plots are constructed for different parameter spaces. Physically informative interpretations are proposed which help gain physical insight into the dynamics. The behavior of the force coefficient Cf has been examined. The magnitude and phase lag of Cf are determined by elaborate post-processings of the numerical data. By utilizing the wealth of the computational results, characterizations of Cf as functions of Re and ω′ are attempted. These are in qualitative consistency with the theoretical predictions for the limiting parameter values.

scholarly journals Decoding the Relationships between Body Shape, Tail Beat Frequency, and Stability for Swimming Fish

Fluids ◽  
2020 ◽  
Vol 5 (4) ◽  
pp. 215
Author(s):  
Alexander P. Hoover ◽  
Eric Tytell
Keyword(s):  
Driving Forces ◽  
Swimming Performance ◽  
Beat Frequency ◽  
Immersed Boundary ◽  
Fluid Forces ◽  
Fish Model ◽  
Fluid Environment ◽  
Boundary Model ◽  
The Stability ◽  
Driving Torque

As fish swim through a fluid environment, they must actively use their fins in concert to stabilize their motion and have a robust form of locomotion. However, there is little knowledge of how these forces act on the fish body. In this study, we employ a 3D immersed boundary model to decode the relationship between roll, pitch, and yaw of the fish body and the driving forces acting on flexible fish bodies. Using bluegill sunfish as our representative geometry, we first examine the role of an actuating torque on the stability of the fish model, with a torque applied at the head of the unconstrained fish body. The resulting kinematics is a product of the passive elasticity, fluid forces, and driving torque. We then examine a constrained model to understand the role that fin geometry, body elasticity, and frequency play on the range of corrective forces acting on the fish. We find non-monotonic behavior with respect to frequency, suggesting that the effective flexibility of the fins play an important role in the swimming performance.

scholarly journals Effect of Gap Flow on the Characteristics of Flow-Around and Flow-Induced Vibration for Two Circular Cylinders with Roughness Strips

2019 ◽  
Vol 9 (17) ◽  
pp. 3587 ◽  
Author(s):  
Zuo-Mei Yang ◽  
Lin Ding ◽  
Qian-Yun Ye ◽  
Lin Yang ◽  
Li Zhang
Keyword(s):  
Strouhal Number ◽  
Circular Cylinders ◽  
Vibration Frequencies ◽  
Flow Induced Vibration ◽  
Force Coefficient ◽  
Wake Structure ◽  
Vortex Pattern ◽  
Gap Flow ◽  
Relationship Of ◽  
Main Factor

In order to understand the gap flow between two cylinders, the characteristics of flow around two stationary cylinders and the flow-induced vibration of two staggered cylinders with roughness strips are numerically studied. The lift–drag responses, Strouhal number (St) and wake structure of two stationary cylinders in tandem, as well as the vibration response and vortex pattern of two oscillating staggered cylinders are analyzed. The results indicate that the spacing dc of two stationary cylinders at which the gap flow can be observed is different for different Re, and dc is 3D when Re = 2000 and dc = 2.5D at Re = 6000~14,000. When the distance d = dc, the force coefficient and St of two cylinders increase sharply. For the two oscillating staggered cylinders, there is a critical reduced velocity Uc* = 7, which makes the amplitude magnitude relationship of the two cylinders change. With the change of the reduced velocity, the vibration frequencies of the two cylinders are consistent. When the staggered distance increases, the frequency difference of the two cylinders decreases. At the same inflow velocity, with the increase of staggered distance, a gap flow is formed between the two cylinders. When T > 0.6D and U* < 8, the gap flow becomes the main factor affecting the vibration of the two cylinders, which can be divided into the dominant region of gap flow.

Tides in a System of Connected Estuaries

2011 ◽  
Vol 41 (5) ◽  
pp. 946-959 ◽  
Author(s):  
Amy F. Waterhouse ◽  
Arnoldo Valle-Levinson ◽  
Clinton D. Winant
Keyword(s):  
Spatial Structure ◽  
Analytical Model ◽  
East Coast ◽  
Phase Relationship ◽  
Phase Lag ◽  
Local Phase ◽  
Tidal Amplitude ◽  
Maximum Friction ◽  
Tidal Velocity ◽  
Water Elevation

Abstract The spatial structure of tidal amplitude and phase in a simplified system of connected estuaries, an idealized version of Florida’s Intracoastal Waterway, is analyzed with a linear analytical model. This model includes friction, the earth’s rotation, and variable bathymetry. It is driven at the connection with the ocean by a co-oscillating tide. Model results compare well with observations of pressure and currents in a section of the Intracoastal Waterway on the east coast of Florida. The comparison suggests that the waterway is highly frictional, causing the amplitude of the water elevation and tidal velocity to decrease away from the inlets to a minimum in the middle of the waterway. The local phase relationship between velocity and water elevation changed nonlinearly from 90° with no friction to 45° with maximum friction. In moderately to highly frictional basins, the phase lag was consistently less than 45°.

scholarly journals On the wake-induced vibration of tandem circular cylinders: the vortex interaction excitation mechanism

2010 ◽  
Vol 661 ◽  
pp. 365-401 ◽  
Author(s):  
G. R. S. ASSI ◽  
P. W. BEARMAN ◽  
J. R. MENEGHINI
Keyword(s):  
The Body ◽  
Circular Cylinders ◽  
Positive Energy ◽  
Phase Lag ◽  
Vortex Interaction ◽  
Excitation Mechanism ◽  
Fluid Force ◽  
Fluid Forces ◽  
Coherent Vortices ◽  
Steady Shear Flow

The mechanism of wake-induced vibrations (WIV) of a pair of cylinders in a tandem arrangement is investigated by experiments. A typical WIV response is characterized by a build-up of amplitude persisting to high reduced velocities; this is different from a typical vortex-induced vibration (VIV) response, which occurs in a limited resonance range. We suggest that WIV of the downstream cylinder is excited by the unsteady vortex–structure interactions between the body and the upstream wake. Coherent vortices interfering with the downstream cylinder induce fluctuations in the fluid force that are not synchronized with the motion. A favourable phase lag between the displacement and the fluid force guarantees that a positive energy transfer from the flow to the structure sustains the oscillations. If the unsteady vortices are removed from the wake of the upstream body then WIV will not be excited. An experiment performed in a steady shear flow turned out to be central to the understanding of the origin of the fluid forces acting on the downstream cylinder.

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