The Experts below are selected from a list of 297 Experts worldwide ranked by ideXlab platform
Liang Sun - One of the best experts on this subject based on the ideXlab platform.
-
General Instability Criteria For Stably Stratified Inviscid Flow
2010Co-Authors: Liang SunAbstract:The stability of stably stratified flow was investigated by analyzing the Taylor-Goldstein equation theoretically, where a sufficient and nearly necessary condition for instability was obtained. According to the analysis, the stable stratification has a destabilization mechanism, and the flow is always unstable given a modified Richardson number Ris ≥ 1. Besides, the unstable perturbation must be long-wave scale. This result extends the Rayleigh’s, Fjortoft’s, Sun’s and Arnol’d’s criteria for the inviscid Homogenous Fluid, but contradicts the well-known Miles’s and Howard’s theorems. It is argued here that the transform F = φ/(U − c)n will lead to contradictions with the results derived from the Taylor-Goldstein equation, and that such transform might be useful for Orr-Sommerfeld equation in viscous flows.
-
General Temporal Instability Criteria For Stably Stratified Inviscid Flow
arXiv: Fluid Dynamics, 2010Co-Authors: Liang SunAbstract:The temporal instability of stably stratified flow was investigated by analyzing the Taylor-Goldstein equation theoretically. According to this analysis, the stable stratification $N^2\geq0$ has a destabilization mechanism, and the flow instability is due to the competition of the kinetic energy with the potential energy, which is dominated by the total Froude number $Fr_t^2$. Globally, $Fr_t^2 \leq 1$ implies that the total kinetic energy is smaller than the total potential energy. So the potential energy might transfer to the kinetic energy after being disturbed, and the flow becomes unstable. On the other hand, when the potential energy is smaller than the kinetic energy ($Fr_t^2>1$), the flow is stable because no potential energy could transfer to the kinetic energy. The flow is more stable with the velocity profile $U'/U'''>0$ than that with $U'/U''' 1/4$. These results extend the Rayleigh's, Fj{\o}rtoft's, Sun's and Arnol'd's criteria for the inviscid Homogenous Fluid, but they contradict the well-known Miles-Howard theorem. It is argued here that the transform $F=\phi/(U-c)^n$ is not suitable for temporal stability problem, and that it will lead to contradictions with the results derived from the Taylor-Goldstein equation. However, such transform might be useful for the study of the Orr-Sommerfeld equation in viscous flows.
Adrian Constantin - One of the best experts on this subject based on the ideXlab platform.
-
Mean Velocities in a Stokes Wave
Archive for Rational Mechanics and Analysis, 2012Co-Authors: Adrian ConstantinAbstract:We prove that in a periodic travelling wave propagating at the surface of an inviscid Homogenous Fluid in irrotational flow over a flat bed, the horizontal mean velocity exceeds the depth-averaged velocity in the frame of reference in which the wave is stationary. The long-standing conjectural nature of this fundamental issue was due to its undecidability within the framework of linear wave theory.
Ramin K. Rahmani - One of the best experts on this subject based on the ideXlab platform.
-
A Numerical Study of the Thermal Performance of Two Stationary Insert Designs in Internal Compressible Flows
Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer, 2009Co-Authors: Emad Y. Tanbour, Ramin K. RahmaniAbstract:Enhancement of the natural and forced convection heat transfer has been the subject of numerous academic and industrial studies. Air blenders, mechanical agitators, and static mixers have been developed to increase the forced convection heat transfer rate in compressible and incompressible flows. Stationary inserts can be efficiently employed as heat transfer enhancement devices in the natural convection systems. Generally, a stationary heat transfer enhancement insert consists of a number of equal motionless segments, placed inside of a pipe in order to control flowing Fluid streams. These devices have low maintenance and operating costs, low space requirements and no moving parts. A range of designs exists for a wide range of specific applications. The shape of the elements determines the character of the Fluid motion and thus determines thermal effectiveness of the insert. There are several key parameters that may be considered in the design procedure of a heat transfer enhancement insert, which lead to significant differences in the performance of various designs. An ideal insert, for natural conventional heat transfer in compressible flow applications, provides a higher rate of heat transfer and a thermally Homogenous Fluid with minimized pressure drop and required space. To choose an insert for a given application or in order to design a new insert, besides experimentation, it is possible to use Computational Fluid Dynamics to study the insert performance. This paper presents the outcomes of the numerical studies on industrial stationary heat transfer enhancement inserts and illustrates how a heat transfer enhancement insert can improve the heat transfer in buoyancy driven compressible flows. Using different measuring tools, thermal performance of two different inserts (twisted and helix) are studied. It is shown that the helix design leads to a higher rate of heat transfer, while causes a lower pressure drop in the flowfield, suggesting the insert effectiveness is higher for the helix design, compared to a twisted plate.Copyright © 2009 by ASME
-
Enhancement of Natural Convection Heat Transfer Rate in Internal Compressible Flows by Inserting Stationary Inserts
Heat Transfer: Volume 3, 2008Co-Authors: Emad Y. Tanbour, Ramin K. RahmaniAbstract:Enhancement of the natural and forced convection heat transfer has been the subject of numerous academic and industrial studies. Air blenders, mechanical agitators, and static mixers have been developed to increase the forced convection heat transfer rate in compressible and incompressible flows. Stationary inserts can be efficiently employed as heat transfer enhancement device in the natural convection systems with compressible flow. Generally, a stationary heat transfer enhancement insert consists of a number of equal motionless units, placed on the inside of a pipe or channel in order to control flowing Fluid streams. These devices have low maintenance and operating costs, low space requirements and no moving parts. A range of designs exists for a wide range of specific applications. The shape of the elements determines the character of the Fluid motion and thus determines thermal effectiveness of the insert. There are several key parameters that may be considered in the design procedure of a heat transfer enhancement insert, which lead to significant differences in the performance of various designs. An ideal insert for natural conventional heat transfer of compressible flow applications provides a higher rate of heat transfer and a thermally Homogenous Fluid with minimized pressure drop and required space. To choose an insert for a given application or in order to design a new insert, besides experimentation, it is possible to use computational Fluid dynamics (CFD) tools to study insert performance. This paper presents the outcomes of the numerical studies by the authors on an industrial stationary heat transfer enhancement insert and illustrates how a heat transfer enhancement insert can improve the heat transfer in a buoyancy driven compressible flow. The numerical predictions were validated using experimental data. Using different measuring tools, the global performance of the insert and the impact of the geometrical parameters are studied in order to choose the most effective design for a given application.© 2008 ASME
-
Experimental Study of Natural Convection Heat Transfer in a Vertical Pipe With Stationary Inserts
Heat Transfer: Volume 2, 2008Co-Authors: Emad Y. Tanbour, Ramin K. RahmaniAbstract:Enhancement of natural and forced convectional heat transfer rate has been the subject of several academic and industrial studies. Air blenders, mechanical agitators, and static mixers have been developed to increase the forced convectional heat transfer rate in compressible and incompressible flows. Stationary devices can be efficiently employed as heat transfer enhancement tool in the natural convection systems with compressible flow. Generally, a stationary heat transfer enhancement insert consists of a number of equal motionless units, placed on the inside of a pipe or channel in order to control flowing Fluid streams. These devices have low maintenance and operating costs, low space requirements and no moving parts. A range of designs exist for a wide range of specific applications. The shape of the elements determines the character of the Fluid motion and thus determines thermal effectiveness of the heat transfer enhancement insert. There are several key parameters that may be considered in the design procedure of a heat transfer enhancement insert, which lead to significant differences in the performance of various designs. An ideal heat transfer enhancement insert for natural conventional heat transfer of compressible flow applications provides a higher rate of heat transfer and a thermally Homogenous Fluid with minimized pressure drop and required space. This paper presents the outcomes of the experimental studies by the authors on two industrial stationary inserts and illustrates how a heat transfer enhancement insert can improve the heat transfer in a buoyancy driven compressible flow. Using different measuring tools, the global performance of the inserts are studied in order to choose the most effective design.© 2008 ASME
-
A Numerical Study on Enhancement of Heat Exchanger Performance by Inserting Static Mixer Elements
Volume 3: Design and Analysis, 2006Co-Authors: Ramin K. Rahmani, Anahita Ayasoufi, Theo G. KeithAbstract:In chemical processing industries, heating, cooling and other thermal processing of viscous Fluids are an integral part of the unit operations. Static mixers are often used in continuous mixing, heat transfer, and chemical reactions applications. Generally, a static mixer consists of a number of equal stationary units, placed on the inside of a pipe or channel in order to promote mixing of flowing Fluid streams. These mixers have low maintenance and operating costs, low space requirements, and no moving parts. A range of designs exists for a wide range of specific applications. The shape of the elements determines the character of the Fluid motion and thus determines the effectiveness of the mixer. There are several key parameters in the design procedure of a static mixer. An ideal static mixer for heat transfer applications provides a higher rate of heat transfer and thermally Homogenous Fluid with low pressure drop and similar traveling history for all Fluid elements. To choose a static mixer for a given application or in order to design a new static mixer, besides experimentation, it is possible to use powerful computational Fluid dynamics (CFD) tools to study the performance of static mixers. This paper illustrates how static mixer can improve the performance of heat exchangers. Using different measuring tools, the global performance and costs of two popular commercial static mixers are studied in order to choose the most effective design for thermal applications.Copyright © 2006 by ASME
Paul Linden - One of the best experts on this subject based on the ideXlab platform.
-
Free-surface effects on the spin-up of Fluid in a rotating cylinder
Journal of Fluid Mechanics, 1991Co-Authors: James O'donnell, Paul LindenAbstract:We present a theory for the decay of the relative motion of a Homogenous Fluid with a free surface in a rotating cylindrical tank with a flat bottom, induced by an abrupt change in the angular velocity. We then describe a set of laboratory experiments designed to test the predictions of the theory. At low rates of rotation the dynamics of the adjustment is well understood and measurements have verified the established theoretical results that the motion decays exponentially, with a timescale proportional to the rotation period divided by the square root of the Ekman number, and that the relative vorticity remains independent of radius. A t higher rotation rates, however, the curvature and motion of the free surface complicate the dynamics, and have hindered the development of a more general theory. Both the theoretical predictions and the experiments show that at high rotation rates the decay of the relative vorticity is independent of radius and exponential in time, but with a decay timescale, re, that increases linearly with the rotational Froude number F, i.e. 7, = 1 +&F. An analysis of the vorticity dynamics during spinup indicates that, near the centre of the tank, this simple behaviour is the result of vigorous competition between the rate of vortex line stretching by Ekman-layer pumping and surface deformation. Near the boundary, these mechanisms cooperate, but are partially offset by the stretching produced by the secondary radial circulation.
Jeffrey R. Koseff - One of the best experts on this subject based on the ideXlab platform.
-
The viscous decay of progressive interfacial waves
Physics of Fluids, 2006Co-Authors: Cary D. Troy, Jeffrey R. KoseffAbstract:The viscous damping of progressive, two-layer interfacial waves is examined theoretically and experimentally. Traditional water wave theory is modified to derive the damping rates associated with interfacial wave propagation in a rectangular channel. The individual wave damping contributions are considered from the bottom, side, and interfacial boundary layers, as well as the damping associated with the wave-induced velocities within the Homogenous Fluid layers. These results show that for most laboratory-scale experiments, sidewall friction plays the dominant role in wave damping. Laboratory experiments are conducted to verify the damping rates for progressive two-layer internal waves in a rectangular channel. Experiments are conducted on both monochromatic and polychromatic wave trains. The results of these experiments are in good agreement with the derived damping rates, but show poorer agreement for large-amplitude waves when the sidewall boundary layers become turbulent. More work is necessary to qua...