scholarly journals Linear stability analysis for monami in a submerged seagrass bed

2015 ◽  
Vol 786 ◽  
Author(s):  
Ravi Singh ◽  
M. M. Bandi ◽  
Amala Mahadevan ◽  
Shreyas Mandre
Keyword(s):  
Growth Rate ◽  
Steady Flow ◽  
Linear Instability ◽  
Flow Profile ◽  
Seagrass Beds ◽  
Seagrass Bed ◽  
Helmholtz Instability ◽  
Model Predictions ◽  
Mode 2 ◽  
Complex Growth Rate

The onset of monami – the synchronous waving of seagrass beds driven by a steady flow – is modelled as a linear instability of the flow. Unlike previous works, our model considers the drag exerted by the grass in establishing the steady flow profile, and in damping out perturbations to it. We find two distinct modes of instability, which we label modes 1 and 2. Mode 1 is closely related to Kelvin–Helmholtz instability modified by vegetation drag, whereas mode 2 is unrelated to Kelvin–Helmholtz instability and arises from an interaction between the flow in the vegetated and unvegetated layers. The vegetation damping, according to our model, leads to a finite threshold flow for both of these modes. Experimental observations for the onset and frequency of waving compare well with model predictions for the instability onset criteria and the imaginary part of the complex growth rate respectively, but experiments lie in a parameter regime where the two modes can not be distinguished.

Nonlinear Kelvin–Helmholtz instability in hydromagnetics

1979 ◽  
Vol 22 (1) ◽  
pp. 27-39 ◽  
Author(s):  
H. Nagano
Keyword(s):  
Growth Rate ◽  
Time Scales ◽  
Linear Instability ◽  
The Other ◽  
Multiple Time Scales ◽  
Multiple Time ◽  
Helmholtz Instability ◽  
Wavenumber Domain ◽  
The Difference

Nonlinear Kelvin–Helmholtz instability for an incompressible plasma under the influence of gravity is investigated theoretically, using the method of multiple time-scales. The effect of nonlinearity is to reduce the growth rate for the linear instability and to narrow the unstable wavenumber domain. There are two cases, in one of which the effect is strong and in the other slight. The difference between these cases is studied.

scholarly journals GROWTH RATE, COVER, AND SURVIVAL RATE (Enhalus acoroides) TRANSPLANTED IN MONOSPECIES AND MULTISPECIES

2016 ◽  
Vol 7 (2) ◽  
Author(s):  
Ilham Antariksa Tasabaramo ◽  
Mujizat Kawaroe ◽  
Rohani Ambo Rappe
Keyword(s):  
Growth Rate ◽  
Combined Treatment ◽  
Survival Rates ◽  
Coastal Communities ◽  
Percent Cover ◽  
Seagrass Beds ◽  
Seagrass Bed ◽  
Average Growth Rate ◽  
Average Growth ◽  
Associated Species

<p><em>One of seagrass dominant species found in Indonesia is <span style="text-decoration: underline;">Enhalus</span> <span style="text-decoration: underline;">acoroides</span>. This species can form single seagrass bed vegetation (monospecies vegetation) and mixed with others species (multispecies vegetation). Seagrass composition in coastal areas can be affected by surrounding associated species such as herbivorous fish and invertebrates. Human activities, especially in coastal communities, can negatively influence seagrass beds. Therefore, it is needed an effort to rehabilitate the affected seagrass. Transplantation technic is one effort for rehabilitation. The purposes of the research were to analyze growth rate, percent cover, and survival rates of seagrass <span style="text-decoration: underline;">E</span>. <span style="text-decoration: underline;">acoroides</span> transplanted as monospecies and multispecies. This research used a monospecies of <span style="text-decoration: underline;">E</span>. <span style="text-decoration: underline;">acoroides</span> and multispecies (2, 4, and 5 species) that combined to others species such as <span style="text-decoration: underline;">T</span>. <span style="text-decoration: underline;">hemprichii</span>, <span style="text-decoration: underline;">C</span>. <span style="text-decoration: underline;">rotundata</span>, <span style="text-decoration: underline;">H</span>. <span style="text-decoration: underline;">ovalis</span>, <span style="text-decoration: underline;">H</span>. <span style="text-decoration: underline;">uninervis</span>, and <span style="text-decoration: underline;">S</span>. <span style="text-decoration: underline;">isoetifolium</span>. Research results showed that the highest average growth rate of transplanted <span style="text-decoration: underline;">E.</span> <span style="text-decoration: underline;">acoroides</span> was found in monospecies treatment with 0.29 cm/day. The highest average cover changing was found on two combined species i.e., <span style="text-decoration: underline;">E</span>. <span style="text-decoration: underline;">acoroides</span> and <span style="text-decoration: underline;">C</span>. <span style="text-decoration: underline;">rotundata</span>, as high as 0.10% per day. The highest survival rates were found in 2 combined treatment i.e., <span style="text-decoration: underline;">E</span>. <span style="text-decoration: underline;">acoroides</span> and <span style="text-decoration: underline;">H</span>. <span style="text-decoration: underline;">ovalis</span>, and 5 combined species such as <span style="text-decoration: underline;">E</span>. <span style="text-decoration: underline;">acoroides</span>, <span style="text-decoration: underline;">S</span>. <span style="text-decoration: underline;">isoetifolium</span>, <span style="text-decoration: underline;">C</span>. <span style="text-decoration: underline;">rotundata</span>, <span style="text-decoration: underline;">H</span>. <span style="text-decoration: underline;">uninervis</span> dan <span style="text-decoration: underline;">H</span>. <span style="text-decoration: underline;">ovalis</span> with value 100 percent, respectively. </em></p><strong><em>Keywords:</em></strong><em> <span style="text-decoration: underline;">Enhalus</span> <span style="text-decoration: underline;">acoroides</span>, cover, growth, survival rates, seagrass, transplantation</em>

scholarly journals Application of a model of internal hydraulic jumps

2017 ◽  
Vol 834 ◽  
pp. 125-148 ◽  
Author(s):  
S. A. Thorpe ◽  
J. Malarkey ◽  
G. Voet ◽  
M. H. Alford ◽  
J. B. Girton ◽  
...  
Keyword(s):  
Hydraulic Jump ◽  
Mixing Layer ◽  
Strong Support ◽  
Deep Ocean ◽  
Hydraulic Jumps ◽  
Model Predictions ◽  
Mode 2 ◽  
Mediterranean Outflow ◽  
Unknown Structure ◽  
Internal Hydraulic Jump

A model devised by Thorpe & Li (J. Fluid Mech., vol. 758, 2014, pp. 94–120) that predicts the conditions in which stationary turbulent hydraulic jumps can occur in the flow of a continuously stratified layer over a horizontal rigid bottom is applied to, and its results compared with, observations made at several locations in the ocean. The model identifies two positions in the Samoan Passage at which hydraulic jumps should occur and where changes in the structure of the flow are indeed observed. The model predicts the amplitude of changes and the observed mode 2 form of the transitions. The predicted dissipation of turbulent kinetic energy is also consistent with observations. One location provides a particularly well-defined example of a persistent hydraulic jump. It takes the form of a 390 m thick and 3.7 km long mixing layer with frequent density inversions separated from the seabed by some 200 m of relatively rapidly moving dense water, thus revealing the previously unknown structure of an internal hydraulic jump in the deep ocean. Predictions in the Red Sea Outflow in the Gulf of Aden are relatively uncertain. Available data, and the model predictions, do not provide strong support for the existence of hydraulic jumps. In the Mediterranean Outflow, however, both model and data indicate the presence of a hydraulic jump.

Bounds for growth rate of a perturbation in thermohaline convection

1981 ◽  
Vol 378 (1773) ◽  
pp. 301-304 ◽  
Keyword(s):  
Growth Rate ◽  
Prandtl Number ◽  
Taylor Number ◽  
Governing Equations ◽  
Thermohaline Convection ◽  
Rayleigh Numbers ◽  
Complex Growth Rate

A new scheme of combining the governing equations of thermohaline convection is shown to lead to the following bounds for the complex growth rate p of an arbitrary oscillatory perturbation: | p | 2 < R s σ (Veronis thermohaline configuration), | p | 2 < – R σ (Stern thermohaline configuration), where R and R s are the thermal and the concentration Rayleigh numbers, and σ is the Prandtl number. The analysis is applicable to rotatory thermal and rotatory thermohaline convections for which the corresponding bounds are | p | 2 < T σ 2 (rotatory simple Bénard configuration), | p | 2 < max ( T σ 2 , R s σ) (rotatory Vernois thermohaline configuration), | p | 2 < max ( T σ 2 , – R σ) (rotatory Stern thermohaline configuration), where T is the Taylor number. The above results are valid for all combination of dynamically free and rigid boundaries.

Suppression of Baroclinic Instabilities in Buoyancy-Driven Flow over Sloping Bathymetry

2017 ◽  
Vol 47 (1) ◽  
pp. 49-68 ◽  
Author(s):  
Robert D. Hetland
Keyword(s):  
Growth Rate ◽  
Linear Instability ◽  
Linear Stability Theory ◽  
Buoyancy Frequency ◽  
Bottom Slope ◽  
Coriolis Parameter ◽  
Plume Front ◽  
Instability Theory ◽  
Instability Growth ◽  
Flow Configurations

AbstractBaroclinic instabilities are ubiquitous in many types of geostrophic flow; however, they are seldom observed in river plumes despite strong lateral density gradients within the plume front. Supported by results from a realistic numerical simulation of the Mississippi–Atchafalaya River plume, idealized numerical simulations of buoyancy-driven flow are used to investigate baroclinic instabilities in buoyancy-driven flow over a sloping bottom. The parameter space is defined by the slope Burger number S = Nf−1α, where N is the buoyancy frequency, f is the Coriolis parameter, and α is the bottom slope, and the Richardson number Ri = N2f2M−4, where M2 = |∇Hb| is the magnitude of the lateral buoyancy gradients. Instabilities only form in a subset of the simulations, with the criterion that SH ≡ SRi−1/2 = Uf−1W−1 = M2f−2α 0.2, where U is a horizontal velocity scale and SH is a new parameter named the horizontal slope Burger number. Suppression of instability formation for certain flow conditions contrasts linear stability theory, which predicts that all flow configurations will be subject to instabilities. The instability growth rate estimated in the nonlinear 3D model is proportional to ωImaxS−1/2, where ωImax is the dimensional growth rate predicted by linear instability theory, indicating that bottom slope inhibits instability growth beyond that predicted by linear theory. The constraint SH 0.2 implies a relationship between the inertial radius Li = Uf−1 and the plume width W. Instabilities may not form when 5Li > W; that is, the plume is too narrow for the eddies to fit.

scholarly journals Intra and inter annual variation in a seagrass meadow on the Caribbean coast of Costa Rica: 2009-2015

2018 ◽  
Vol 66 (3) ◽  
pp. 1149 ◽  
Author(s):  
Margarita Loría-Naranjo ◽  
Jimena Samper-Villarreal ◽  
Marylaura Sandoval-Siles ◽  
Jorge Cortés
Keyword(s):  
Costa Rica ◽  
Leaf Area ◽  
Turnover Rate ◽  
Seagrass Meadow ◽  
Seagrass Beds ◽  
Seagrass Bed ◽  
Monitoring Methods ◽  
Caribbean Coast ◽  
Area Index ◽  
The Caribbean

Seagrass beds are an important ecosystem on the Caribbean coast of Costa Rica. At Cahuita National Park (CNP) a seagrass bed at Perezoso has been monitored continually since 1999 within the CARICOMP program. Thalassia testudinum is the dominant seagrass species, in some cases mixed with Syringodium filiforme. The results from the 2009 to 2015 monitoring period are presented here, and contrasted with data before 2009. Total (above and below ground tissue) mean biomass of T. testudinum was higher (1 255.4 ± 146.0 gm-2) than biomass before 2009, with an increasing tendency. However, productivity (1.5±0.59 gm-2d-1) and turnover rate (4.3 ± 1.22 %d-1) were lower than previous monitoring periods. In this period, mean leaf area diminished considerably (4.9 ± 2.30 m2), but leaf area index (LAI) increased (1.9 ± 0.80 m2leafm-2) in comparison to prior monitoring. Productivity, density, turnover rate, LAI and biomass showed intra-annual variations; while mean biomass of T. testudinum did not vary significantly among years. No correlations were found between water salinity, temperature and clarity with seagrass measurements. However, most seagrass parameters were strongly correlated with precipitation. These results highlight the effect of external environmental agents acting on the ecosystem. CNP presents a long-term stable seagrass meadow. However, there are indirect signals, such as high biomass and above-ground biomass proportion, along with low productivity and LAI, which point to a nutrient increment in Perezoso’s seagrass bed. To continue protecting this seagrass bed, it is necessary to improve monitoring methods, and seagrass beds should be included in national conservation policies and monitoring programs.

scholarly journals Magnetic field generation in a jet-sheath plasma via the kinetic Kelvin-Helmholtz instability

2013 ◽  
Vol 31 (9) ◽  
pp. 1535-1541 ◽  
Author(s):  
K.-I. Nishikawa ◽  
P. Hardee ◽  
B. Zhang ◽  
I. Duţan ◽  
M. Medvedev ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Growth Rate ◽  
Magnetic Fields ◽  
Electric Field ◽  
Flow Direction ◽  
Transverse Magnetic ◽  
Velocity Shear ◽  
Helmholtz Instability ◽  
Magnetic Field Generation ◽  
Relativistic Jet

Abstract. We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic fields generated by kinetic shear (Kelvin–Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of the kinetic Kelvin–Helmholtz instability (KKHI) of our jet-sheath configuration is slightly different, even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field Ez, perpendicular to the flow boundary, and the magnetic field By, transverse to the flow direction. After the By component is excited, an induced electric field Ex, parallel to the flow direction, becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios mi/me = 1836 and mi/me = 20 are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case (γj = 1.5) is larger than for a relativistic jet case (γj = 15).

On the stability of an inviscid shear layer which is periodic in space and time

1967 ◽  
Vol 27 (4) ◽  
pp. 657-689 ◽  
Author(s):  
R. E. Kelly
Keyword(s):  
Growth Rate ◽  
Shear Layer ◽  
Finite Amplitude ◽  
Periodic Component ◽  
Periodic Flow ◽  
Flow Profile ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean ◽  
The Stability

In experiments concerning the instability of free shear layers, oscillations have been observed in the downstream flow which have a frequency exactly half that of the dominant oscillation closer to the origin of the layer. The present analysis indicates that the phenomenon is due to a secondary instability associated with the nearly periodic flow which arises from the finite-amplitude growth of the fundamental disturbance.At first, however, the stability of inviscid shear flows, consisting of a non-zero mean component, together with a component periodic in the direction of flow and with time, is investigated fairly generally. It is found that the periodic component can serve as a means by which waves with twice the wavelength of the periodic component can be reinforced. The dependence of the growth rate of the subharmonic wave upon the amplitude of the periodic component is found for the case when the mean flow profile is of the hyperbolic-tangent type. In order that the subharmonic growth rate may exceed that of the most unstable disturbance associated with the mean flow, the amplitude of the streamwise component of the periodic flow is required to be about 12 % of the mean velocity difference across the shear layer. This represents order-of-magnitude agreement with experiment.Other possibilities of interaction between disturbances and the periodic flow are discussed, and the concluding section contains a discussion of the interactions on the basis of the energy equation.

Effect of intermediate fluid layer on Kelvin–Helmholtz instability

2018 ◽  
Vol 96 (10) ◽  
pp. 1145-1154 ◽  
Author(s):  
Ying Zhang ◽  
Wenqiang Shang ◽  
Mengjun Yao ◽  
Boheng Dong ◽  
Peisheng Li
Keyword(s):  
Growth Rate ◽  
Stokes Equations ◽  
Fluid Layer ◽  
Immiscible Fluids ◽  
Navier Stokes ◽  
Helmholtz Instability ◽  
Flow Configuration ◽  
Front Tracking Method ◽  
Internal Disturbance ◽  
Over Time

Two-dimensional K-H (Kelvin–Helmholtz) instability of the three-component immiscible fluids with an intermediate fluid layer is numerically studied using the front-tracking method (FTM). The instability is governed by the Navier–Stokes equations and the conservation of mass equation for incompressible flow. A finite difference method is used to discretize the governing system. This study focuses on the influence of flow configuration, the thickness of intermediate fluid layer and the distribution of intermediate fluid layer on K-H instability. It is shown that the larger the initial horizontal velocity difference is, the faster the internal disturbance increases, and the characteristic form of K-H instability becomes more obvious for different flow configuration. It is also observed that the thickness of the intermediate fluid layer is negatively correlated to the billow height and the numerical growth rate. In addition, when the intermediate fluid layer is thicker than 0.4 times the disturbance wavelength, the billow height and the numerical growth rate for the K-H instability of the upper and lower interfaces change over time synchronously. The higher the initial height of the lower interface is, the greater the growth rate and billow height of the upper interface are. Besides, the upper and lower interfaces are rolled up synchronously over time when the intermediate fluid layer is symmetrically distributed with y = 0.5 in the fluid system.

Spatial stability analysis of subsonic corrugated jets

2019 ◽  
Vol 876 ◽  
pp. 766-791 ◽  
Author(s):  
F. C. Lajús ◽  
A. Sinha ◽  
A. V. G. Cavalieri ◽  
C. J. Deschamps ◽  
T. Colonius
Keyword(s):  
High Reynolds Number ◽  
Base Flow ◽  
Flow Profile ◽  
Azimuthal Direction ◽  
Instability Mechanism ◽  
Helmholtz Instability ◽  
Circular Jets ◽  
Unstable Solutions ◽  
High Reynolds ◽  
Axis Switching

The linear stability of high-Reynolds-number corrugated jets is investigated by solving the compressible Rayleigh equation linearized about the time-averaged flow field. A Floquet ansatz is used to account for periodicity of this base flow in the azimuthal direction. The origin of multiple unstable solutions, which are known to appear in these non-circular configurations, is traced through gradual perturbations of a parametrized base-flow profile. It is shown that all unstable modes are corrugated jet continuations of the classical Kelvin–Helmholtz modes of circular jets, highlighting that the same instability mechanism, modified by corrugations, leads to the growth of disturbances in such flows. It is found that under certain conditions the eigenvalues may form saddles in the complex plane and display axis switching in their eigenfunctions. A parametric study is also conducted to understand how penetration and number of corrugations impact stability. The effect of these geometric properties on growth rates and phase speeds of the multiple unstable modes is explored, and the results provide guidelines for the development of nozzle configurations that more effectively modify the Kelvin–Helmholtz instability.

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