摘要 :
Linear stability of Couette flow modified by transpiration applied at the lower wall is considered. It is shown that transpiration can induce flow instability resulting in the appearance of streamwise-vortex-like structures. It is...
展开
Linear stability of Couette flow modified by transpiration applied at the lower wall is considered. It is shown that transpiration can induce flow instability resulting in the appearance of streamwise-vortex-like structures. It is argued that the instability is driven by centrifugal forces associated with streamline curvature. The conditions leading to the onset of the instability depend on the amplitude and wavelength of the transpiration and can be expressed in terms of the critical Reynolds number. The global critical conditions describing the minimum critical Reynolds number required for the onset of the instability for the specified amplitude of the transpiration regardless of its wavelength are also given. The threshold amplitude required for the onset varies approximately as similar toRe(-1.15) for large Re, where the Reynolds number used is based on the velocity difference between the walls and the channel half-width. The existence of a global threshold, below which the instability cannot occur regardless of the amplitude of the transpiration, has been demonstrated. This threshold corresponds approximately to Re = 84. [References: 57]
收起
摘要 :
Linear stability of flow in a diverging-converging channel is considered. The flow may develop under either the fixed mass or the fixed pressure gradient constraint. Both cases are considered. It is shown that under certain condit...
展开
Linear stability of flow in a diverging-converging channel is considered. The flow may develop under either the fixed mass or the fixed pressure gradient constraint. Both cases are considered. It is shown that under certain conditions the divergence-convergence of the channel leads to the formation of a secondary flow in the form of streamwise vortices. It is argued that the instability is driven by centrifugal effect. The instability has two modes and conditions leading to their onset have been identified. These conditions depend on the amplitude and the length of the channel diverging-converging section and can be expressed in terms of a critical Reynolds number. The global critical conditions describing the minimum critical Reynolds number required to create the instability for the specified amplitude of the variations of the channel opening are also given. It is shown that the flow developed under the fixed mass constraint is slightly more unstable than the flow developed under the fixed pressure constraint. This difference increases with an increase of the amplitude of the channel divergence-convergence. [References: 42]
收起
摘要 :
Marangoni convection in a cavity with differentially heated sidewalls has been investigated. The analysis includes the complete effects of interface deformation. The results determined for large Blot and zero Marangoni (zero Prand...
展开
Marangoni convection in a cavity with differentially heated sidewalls has been investigated. The analysis includes the complete effects of interface deformation. The results determined for large Blot and zero Marangoni (zero Prandtl) numbers show that steady convection may exist for Reynolds numbers Re larger than, and for capillary numbers Ca and cavity lengths L smaller than, certain critical values. The main factor limiting the existence of steady convection involves the interface becoming tangential to the hot wall at the contact point (tangency condition). Unsteady analysis shows that the tangency condition defines the limit point for the system; its violation is most likely to lead to the formation of a dry spot at the hot wall. The critical values of RE, Ca, and L are mutually dependent and change with the heating rate (they reach a minimum for instantaneous heating). For a certain range of parameters, multiple (i.e. steady and oscillatory) states are possible. The oscillatory state has a form consisting of the steady mode with a simple harmonic sloshing motion superposed on it. A reduction in the heating rate permits heating of the liquid without triggering the oscillatory state. Transition between the steady and the oscillatory states involves a nonlinear instability process. [References: 17]
收起
摘要 :
Linear stability of wall-bounded shear layers modified by distributed suction has been considered. Wall suction was introduced in order to simulate distributed surface roughness. In all cases studied, i.e. Poiseuille and Couette f...
展开
Linear stability of wall-bounded shear layers modified by distributed suction has been considered. Wall suction was introduced in order to simulate distributed surface roughness. In all cases studied, i.e. Poiseuille and Couette flows and Blasius boundary layer, wall suction was able to induce a new type of instability characterized by the appearance of streamwise vortices. Results of calculations show that a linear model of suction-induced flow modifications provides a sufficiently accurate representation of the basic state. The effects of an arbitrary suction distribution can, therefore, be assessed by decomposing this distribution into Fourier series and carrying out stability analysis on a mode-by-mode basis, i.e. once and for ever. [References: 25]
收起
摘要 :
Linear stability analysis of flow in a channel bounded by wavy walls is considered. It is shown that wall waviness gives rise to an instability that manifests itself through generation of streamwise vortices. The available results...
展开
Linear stability analysis of flow in a channel bounded by wavy walls is considered. It is shown that wall waviness gives rise to an instability that manifests itself through generation of streamwise vortices. The available results suggest that the critical stability criteria based on the Reynolds number based on the amplitude of the waviness can be formulated. [References: 19]
收起
摘要 :
Marangoni convection in a cavity subject to point (concentrated) heating has been investigated. The analysis includes the complete effects of the interface deformation. The results determined for large Blot and zero Marangoni (zer...
展开
Marangoni convection in a cavity subject to point (concentrated) heating has been investigated. The analysis includes the complete effects of the interface deformation. The results determined for large Blot and zero Marangoni (zero Prandtl) numbers show that steady convection may exist only for a limited range of Reynolds numbers Re (bounded from above and from below), and for capillary numbers Ca and cavity lengths L smaller than certain critical values. The main factor limiting the existence of steady convection involves the interface approaching the bottom of the cavity. Unsteady analysis shows that when the conditions guaranteeing the existence of steady convection are not met, an interface rupture process sets in leading, eventually, to the formation of a dryout at the bottom of the cavity. The initial stages of the rupture process are characterized by a rapidly accelerating growth of the interface deformation. The critical values of Re, Ca and L, which guarantee the existence of steady convection, are mutually dependent and change with the heating rate; they reach a minimum for instantaneous heating. Too rapid heating produces an oscillatory transient which always decays in the range of parameters studied. [References: 6]
收起
摘要 :
We consider a fourth-order, compact finite-difference method for the Navier-Stokes equations using the streamfunction-vorticity formulation. Various algebraic boundary formulas for vorticity are investigated including new implicit...
展开
We consider a fourth-order, compact finite-difference method for the Navier-Stokes equations using the streamfunction-vorticity formulation. Various algebraic boundary formulas for vorticity are investigated including new implicit formulas of the third and fourth order. An algorithm for determination of pressure from a suitable Poisson equation is given. Results of various tests show that the error of the algorithm is proportional to Re-2 . h(4). Domain decomposition coupled with multiprocessing was investigated as a method for acceleration of computations. It is shown that the acceleration approaches the theoretical maximum. (C) 1995 Academic Press, Inc. [References: 23]
收起
摘要 :
Marangoni convection in a cavity subject to two types of heating, i.e., heating through the sidewalls and heating by a point source from above, has been investigated. The assumed wetting conditions permit motion of the interface a...
展开
Marangoni convection in a cavity subject to two types of heating, i.e., heating through the sidewalls and heating by a point source from above, has been investigated. The assumed wetting conditions permit motion of the interface along the sidewalls subject to a constant contact angle constraint. The analysis considers complete interface deformation effects. The results determined for large Biot and zero Marangoni numbers show the existence of limit points beyond which steady, continuous interface cannot exist. The limit points define the maximum capillary number Ca and the maximum cavity length L permitted. The permitted values of the Reynolds number may be bounded from below and from above depending on the values of other transport parameters and on the form of the external heating. Interface approaching bottom of the cavity leading, most likely, to the formation of a dry spot, represents the main factor limiting the existence of steady convection. The topology of the flow field is similar to the case when the wetting conditions result in the fixed location of the contact points while the topology of the interface is qualitatively different. The change of the contact conditions from the fixed contact points to the fixed contact angles results in a significant reduction of the range of parameters that guarantee the existence of a continuous interface. (C) 2003 American Institute of Physics. [References: 23]
收起
摘要 :
Linear stability of Couette flow over a wavy wall is considered. It is shown that centrifugal effects may create an instability that leads to the formation of streamwise vortices. The conditions leading to the onset of the instabi...
展开
Linear stability of Couette flow over a wavy wall is considered. It is shown that centrifugal effects may create an instability that leads to the formation of streamwise vortices. The conditions leading to the onset of the instability depend on the amplitude and the wavelength of the waviness and can be expressed in terms of the critical Reynolds number. The global critical conditions describing the minimum critical Reynolds number and the associated wave number of wall waviness required to create the instability for the specified amplitude of the waviness are also given. (C) 2002 American Institute of Physics. [References: 29]
收起
摘要 :
The effects of small wall vibrations in the form of traveling waves on the asymptotic stability of a channel flow are analyzed. It is shown that vibrations may affect flow instability only if they consist of modes with wavelength ...
展开
The effects of small wall vibrations in the form of traveling waves on the asymptotic stability of a channel flow are analyzed. It is shown that vibrations may affect flow instability only if they consist of modes with wavelength and frequency close to those of the neutral Tollmien-Schlichting (TS) waves. Vibrations in resonance with the neutral TS waves produce instability that amplifies linearly in time. It is shown that vibrations detuned with the neutral TS waves may lead to an initial transient growth linear in time. It is further demonstrated that this growth may lead to flow destabilization through nonlinear effects, resulting in a significant reduction in the critical Reynolds number. The wealth of possible flow responses for various forms of detuning, including modulated wall vibrations, is also discussed. (C) 2002 American Institute of Physics. [References: 18]
收起
原文获取