unstable mode
Recently Published Documents


TOTAL DOCUMENTS

143
(FIVE YEARS 24)

H-INDEX

25
(FIVE YEARS 2)

2022 ◽  
Vol 10 (1) ◽  
pp. 46
Author(s):  
Malek Abid ◽  
Christian Kharif ◽  
Hung-Chu Hsu ◽  
Yang-Yih Chen

The theory of surface wave generation, in viscous flows, is modified by replacing the linear-logarithmic shear velocity profile, in the air, with a model which links smoothly the linear and logarithmic layers through the buffer layer. This profile includes the effects of air flow turbulence using a damped mixing-length model. In the water, an exponential shear velocity profile is used. It is shown that this modified and coupled shear-velocity profile gives a better agreement with experimental data than the coupled linear-logarithmic, non smooth profile, (in the air)–exponential profile (in the water), widely used in the literature. We also give new insights on retrograde modes that are Doppler shifted by the surface velocity at the air-sea interface, namely on the threshold value of the surface current for the occurrence of a second unstable mode.


2021 ◽  
Vol 932 ◽  
Author(s):  
Samuel D. Tomlinson ◽  
Demetrios T. Papageorgiou

It is known that an increased flow rate can be achieved in channel flows when smooth walls are replaced by superhydrophobic surfaces. This reduces friction and increases the flux for a given driving force. Applications include thermal management in microelectronics, where a competition between convective and conductive resistance must be accounted for in order to evaluate any advantages of these surfaces. Of particular interest is the hydrodynamic stability of the underlying basic flows, something that has been largely overlooked in the literature, but is of key relevance to applications that typically base design on steady states or apparent-slip models that approximate them. We consider the global stability problem in the case where the longitudinal grooves are periodic in the spanwise direction. The flow is driven along the grooves by either the motion of a smooth upper lid or a constant pressure gradient. In the case of smooth walls, the former problem (plane Couette flow) is linearly stable at all Reynolds numbers whereas the latter (plane Poiseuille flow) becomes unstable above a relatively large Reynolds number. When grooves are present our work shows that additional instabilities arise in both cases, with critical Reynolds numbers small enough to be achievable in applications. Generally, for lid-driven flows one unstable mode is found that becomes neutral as the Reynolds number increases, indicating that the flows are inviscidly stable. For pressure-driven flows, two modes can coexist and exchange stability depending on the channel height and slip fraction. The first mode remains unstable as the Reynolds number increases and corresponds to an unstable mode of the two-dimensional Rayleigh equation, while the second mode becomes neutrally stable at infinite Reynolds numbers. Comparisons of critical Reynolds numbers with the experimental observations for pressure-driven flows of Daniello et al. (Phys. Fluids, vol. 21, issue 8, 2009, p. 085103) are encouraging.


2021 ◽  
Vol 923 (2) ◽  
pp. 271
Author(s):  
C. S. Ng ◽  
A. Bhattacharjee

Abstract We consider the spectrum of eigenmodes in a stellar system dominated by gravitational forces in the limit of zero collisions. We show analytically and numerically using the Lenard–Bernstein collision operator that the Landau modes, which are not true eigenmodes in a strictly collisionless system (except for the Jeans unstable mode), become part of the true eigenmode spectrum in the limit of zero collisions. Under these conditions, the continuous spectrum of true eigenmodes in a collisionless system, also known as the Case–van Kampen modes, is eliminated. Furthermore, because the background distribution function in a weakly collisional system can exhibit significant deviations from a Maxwellian distribution function over long times, we show that the spectrum of Landau modes can change drastically even in the presence of slight deviations from a Maxwellian, primarily through the appearance of weakly damped modes that may be otherwise heavily damped for a Maxwellian distribution. Our results provide important insights for developing statistical theories to describe thermal fluctuations in a stellar system, which are currently a subject of great interest for N-body simulations as well as observations of gravitational systems.


2021 ◽  
Vol 9 ◽  
Author(s):  
Natanael Karjanto

This article discusses a limiting behavior of breather solutions of the focusing nonlinear Schrödinger equation. These breathers belong to the family of solitons on a non-vanishing and constant background, where the continuous-wave envelope serves as a pedestal. The rational Peregrine soliton acts as a limiting behavior of the other two breather solitons, i.e., the Kuznetsov-Ma breather and Akhmediev soliton. Albeit with a phase shift, the latter becomes a nonlinear extension of the homoclinic orbit waveform corresponding to an unstable mode in the modulational instability phenomenon. All breathers are prototypes for rogue waves in nonlinear and dispersive media. We present a rigorous proof using the ϵ-δ argument and show the corresponding visualization for this limiting behavior.


2021 ◽  
Author(s):  
Manas Madasseri Payyappalli ◽  
A. M. Pradeep

Abstract In this experimental study, we investigate the fundamental behaviour of a low speed contra-rotating fan and describes the reasons leading to the instabilities in the fan at low mass flow rates. A contra-rotating fan is a possible alternative to conventional fans and has potential aerodynamic advantages. This study identifies certain features that are unique to a contra-rotating configuration. Rotor-1 and rotor-2 behaves differently at low mass flow rates. Though rotor-1 is stable up to low mass flow rates, rotor-2 enters into an unstable mode of operation at mass flow rates close to the design mass flow rate. The critical region where the instability arise in rotor-1 is its tip and in rotor-2 is its hub. The instability is also found to change the structure as it propagates along the annulus. It is identified that the presence of rotor-2 downstream of rotor-1 under-loads rotor-1 and thus significantly affects the loading on rotor-1. The instability arises due to the tip-leakage vortex at high frequencies and due to modal waves at low frequencies. The study thus identifies the major regions of the rotors which are the sources of instabilities and also identifies the process of transition to instability in the contra-rotating fan.


2021 ◽  
Vol 78 (5) ◽  
pp. 1411-1428
Author(s):  
Tsz-Kin Lai ◽  
Eric A. Hendricks ◽  
M. K. Yau ◽  
Konstantinos Menelaou

AbstractIntense tropical cyclones (TCs) often experience secondary eyewall formations and the ensuing eyewall replacement cycles. Better understanding of the underlying dynamics is crucial to make improvements to the TC intensity and structure forecasting. Radar imagery of some double-eyewall TCs and a real-case simulation study indicated that the barotropic instability (BI) across the moat (aka type-2 BI) may play a role in inner eyewall decay. A three-dimensional numerical study accompanying this paper pointed out that type-2 BI is able to withdraw the inner eyewall absolute angular momentum (AAM) and increase the outer eyewall AAM through the eddy radial transport of eddy AAM. This paper explores the reason why the eddy radial transport of eddy AAM is intrinsically nonzero. Linear and nonlinear shallow water experiments are performed and they produce expected evolutions under type-2 BI. It will be shown that only nonlinear experiments have changes in AAM over the inner and outer eyewalls, and the changes solely originate from the eddy radial transport of eddy AAM. This result highlights the importance of nonlinearity of type-2 BI. Based on the distribution of vorticity perturbations and the balanced-waves arguments, it will be demonstrated that the nonzero eddy radial transport of eddy AAM is an essential outcome from the intrinsic interaction between the mutually growing vortex Rossby waves across the moat under type-2 BI. The analyses of the most unstable mode support the findings and will further attribute the inner eyewall decay and outer eyewall intensification to the divergence and convergence of the eddy angular momentum flux, respectively.


Author(s):  
Jose Luis Blázquez-Salcedo ◽  
Carlos A. R. Herdeiro ◽  
Sarah Kahlen ◽  
Jutta Kunz ◽  
Alexandre M. Pombo ◽  
...  

AbstractEinstein–Maxwell-scalar models allow for different classes of black hole solutions, depending on the non-minimal coupling function $$f(\phi )$$ f ( ϕ ) employed, between the scalar field and the Maxwell invariant. Here, we address the linear mode stability of the black hole solutions obtained recently for a quartic coupling function, $$f(\phi )=1+\alpha \phi ^4$$ f ( ϕ ) = 1 + α ϕ 4 (Blázquez-Salcedo et al. in Phys. Lett. B 806:135493, 2020). Besides the bald Reissner–Nordström solutions, this coupling allows for two branches of scalarized black holes, termed cold and hot, respectively. For these three branches of black holes we calculate the spectrum of quasinormal modes. It consists of polar scalar-led modes, polar and axial electromagnetic-led modes, and polar and axial gravitational-led modes. We demonstrate that the only unstable mode present is the radial scalar-led mode of the cold branch. Consequently, the bald Reissner–Nordström branch and the hot scalarized branch are both mode-stable. The non-trivial scalar field in the scalarized background solutions leads to the breaking of the degeneracy between axial and polar modes present for Reissner–Nordström solutions. This isospectrality is only slightly broken on the cold branch, but it is strongly broken on the hot branch.


2021 ◽  
Vol 345 ◽  
pp. 00009
Author(s):  
Marián Hocko ◽  
Samer Al-Rabeei

This paper analyses unstable mode of a free gas turbine of turboshaft helicopter engine TV3-117. The analysis is focused on the conditions of this phenomenon and the possibilities of its solution in a turboshaft helicopter engine and an industrial turbocharger engine with a free gas turbine. Knowing the causes of the unstable mode of operation of a free gas turbine will allow helicopter pilots to prevent accidents and increase the level of flight safety.


2020 ◽  
Vol 2020 (8) ◽  
Author(s):  
Elias Kiritsis ◽  
Francesco Nitti ◽  
Edwan Préau

Abstract Holographic CFTs and holographic RG flows on space-time manifolds which are d-dimensional products of spheres are investigated. On the gravity side, this corresponds to Einstein-dilaton gravity on an asymptotically AdSd+1 geometry, foliated by a product of spheres. We focus on holographic theories on S2× S2, we show that the only regular five-dimensional bulk geometries have an IR endpoint where one of the sphere shrinks to zero size, while the other remains finite. In the Z2-symmetric limit, where the two spheres have the same UV radii, we show the existence of a infinite discrete set of regular solutions, satisfying an Efimov-like discrete scaling. The Z2-symmetric solution in which both spheres shrink to zero at the endpoint is singular, whereas the solution with lowest free energy is regular and breaks Z2 symmetry spontaneously. We explain this phenomenon analytically by identifying an unstable mode in the bulk around the would-be Z2-symmetric solution. The space of theories have two branches that are connected by a conifold transition in the bulk, which is regular and correspond to a quantum first order transition. Our results also imply that AdS5 does not admit a regular slicing by S2× S2.


Sign in / Sign up

Export Citation Format

Share Document