Passive flight in density-stratified fluids

2018 ◽  
Vol 860 ◽  
pp. 200-223 ◽  
Author(s):  
Try Lam ◽  
Lionel Vincent ◽  
Eva Kanso
Keyword(s):  
Pure Water ◽  
Reynolds Numbers ◽  
Density Stratification ◽  
Space Vehicles ◽  
Stratified Fluids ◽  
Fluid Medium ◽  
Complex Interactions ◽  
Seed Dispersion ◽  
Marine Larvae ◽  
Radial Dispersion

Leaves falling in air and marine larvae settling in water are examples of unsteady descents due to complex interactions between gravitational and aerodynamic forces. Understanding passive flight is relevant to many branches of engineering and science, ranging from estimating the behaviour of re-entry space vehicles to analysing the biomechanics of seed dispersion. The motion of regularly shaped objects falling freely in homogenous fluids is relatively well understood. However, less is known about how density stratification of the fluid medium affects passive flight. In this paper, we experimentally investigate the descent of heavy discs in stably stratified fluids for Froude numbers of order 1 and Reynolds numbers of order 1000. We specifically consider fluttering descents, where the disc oscillates as it falls. In comparison with pure water and homogeneous saltwater fluid, we find that density stratification significantly enhances the radial dispersion of the disc, while simultaneously decreasing the vertical descent speed, fluttering amplitude and inclination angle of the disc during descent. We explain the physical mechanisms underlying these observations in the context of a quasi-steady force and torque model. These findings could have significant impact on the design of unpowered vehicles and on the understanding of geological and biological transport where density and temperature variations may occur.

scholarly journals Heat Transfer Enhancement and Hydrodynamic Characteristics of Nanofluid in Turbulent Flow Regime

2015 ◽  
Vol 2015 ◽  
pp. 1-6 ◽  
Author(s):  
Mohammad Nasiri-lohesara
Keyword(s):  
Heat Transfer ◽  
Numerical Study ◽  
Pure Water ◽  
Reynolds Numbers ◽  
Hydrodynamic Characteristics ◽  
Two Phase ◽  
Tube Heat Exchanger ◽  
Turbulent Flow Regime ◽  
Lower Slope ◽  
Good Agreement

Turbulent forced convection ofγ-Al2O3/water nanofluid in a concentric double tube heat exchanger has been investigated numerically using mixture two-phase model. Nanofluids are used as coolants flowing in the inner tube while hot pure water flows in outer tube. The studies are conducted for Reynolds numbers ranging from 20,000 to 50,000 and nanoparticle volume fractions of 2, 3, 4, and 6 percent. Results showed that nanofluid has no effects on fully developed length and average heat transfer coefficient enhances with lower slope than wall shear stress. Comparisons with experimental correlation in literature are conducted and good agreement with present numerical study is achieved.

On hydromagnetic waves in a stratified rotating incompressible fluid

1969 ◽  
Vol 39 (2) ◽  
pp. 283-287 ◽  
Author(s):  
R. Hide
Keyword(s):  
Incompressible Fluid ◽  
Angular Speed ◽  
Density Stratification ◽  
Stratified Fluids ◽  
Rotating Fluid ◽  
Rotating Fluids ◽  
Hydrodynamic Behaviour ◽  
Dispersion Relationship ◽  
Hydromagnetic Waves ◽  
Made In

The dispersion relationship for plane hydromagnetic waves in a stratified rotating fluid (α) indicates that the well-known analogy between rotating fluids and stratified fluids in regard to their hydrodynamic behaviour does not extend to magnetohydrodynamic behaviour, and (b) lends credence to a certain conjecture made in a previous paper, namely that effects due to density stratification can be neglected when considering the dispersion relationship for free hydromagnetic oscillations of the Earth's core if the Brunt—Väisälä frequency is much less than twice the angular speed of the Earth's rotation.

Pool Boiling Heat Transfer With Binary Mixture on Open Microchannel Surface

2013 ◽  
Author(s):  
Ankit Kalani ◽  
Satish G. Kandlikar
Keyword(s):  
Binary Mixture ◽  
Heat Dissipation ◽  
Surface Effect ◽  
Pure Water ◽  
Pool Boiling ◽  
Electronics Cooling ◽  
Working Fluid ◽  
Two Phase ◽  
Maximum Heat ◽  
Fluid Medium

Two-phase cooling is considered an attractive option for electronics cooling due to its ability to dissipate large quantities of heat. In recent years, pool boiling has shown tremendous ability in high heat dissipation applications. Researchers have used various fluid medium for pool boiling including water, alcohol, refrigerants, nanofluids and binary mixture. In the current work, binary mixture of water with ethanol was chosen as the working fluid. Plain copper chip was used as the boiling surface. Effect of various concentrations of binary mixture was investigated. A maximum heat flux of 1720 kW/m2 at a wall superheat of 28°C was recorded for 15% ethanol in water. It showed a 1.5 fold increase in CHF over pure water.

The stability of compressible swirling pipe flows with density stratification

2017 ◽  
Vol 823 ◽  
pp. 689-715 ◽  
Author(s):  
Navneet K. Yadav ◽  
Arnab Samanta
Keyword(s):  
Reynolds Number ◽  
Rotational Speed ◽  
Reynolds Numbers ◽  
Density Stratification ◽  
Unstable State ◽  
Density Profiles ◽  
Pipe Flows ◽  
Leading Role ◽  
Flow Transitions ◽  
The Stability

We investigate the spatial stability of compressible, viscous pipe flows with radius-dependent mean density profiles, subjected to solid body rotations. For a fixed Rossby number $\unicode[STIX]{x1D716}$ (inverse of the rotational speed), as the Reynolds number $Re$ is increased, the flow transitions from being stable to convectively unstable, usually leading to absolute instability. If flow compressibility is unimportant and $Re$ is held constant, there appears to be a maximum $Re$ below which the flow remains stable irrespective of any rotational speed, or a minimum azimuthal Reynolds number $Re_{\unicode[STIX]{x1D703}}$ $(=Re/\unicode[STIX]{x1D716})$ is required for any occurrence of absolute instabilities. Once compressible forces are significant, the effect of pressure–density coupling is found to be more severe below a critical $Re$, where as rotational speeds are raised, a stable flow almost directly transitions to an absolutely unstable state. This happens at a critical $Re_{\unicode[STIX]{x1D703}}$ which reduces with increased flow Mach number, pointing to compressibility aiding in the instability at these lower Reynolds numbers. However, at higher $Re$, above the critical value, the traditional stabilizing role of compressibility is recovered if mean density stratification exists, where the gradients of density play an equally important role, more so at the higher azimuthal modes. A total disturbance energy-based formulation is used to obtain mechanistic understanding at these stability states, where we find the entropic energy perturbations to dominate as the primary instability mechanism, in sharp contrast to the energy due to axial shear, known to play a leading role in incompressible swirling flows.

Vortex-induced instabilities and accelerated collapse due to inertial effects of density stratification

2010 ◽  
Vol 646 ◽  
pp. 415-439 ◽  
Author(s):  
HARISH N DIXIT ◽  
RAMA GOVINDARAJAN
Keyword(s):  
Reynolds Number ◽  
Stability Analysis ◽  
Reynolds Numbers ◽  
Passive Scalar ◽  
Density Stratification ◽  
Low Reynolds Numbers ◽  
Density Interface ◽  
Lower Reynolds Number ◽  
Number Flow ◽  
Good Agreement

A vortex placed at a density interface winds it into an ever-tighter spiral. We show that this results in a combination of a centrifugal Rayleigh–Taylor (CRT) instability and a spiral Kelvin–Helmholtz (SKH) type of instability. The SKH instability arises because the density interface is not exactly circular, and dominates at large times. Our analytical study of an inviscid idealized problem illustrates the origin and nature of the instabilities. In particular, the SKH is shown to grow slightly faster than exponentially. The predicted form lends itself for checking by a large computation. From a viscous stability analysis using a finite-cored vortex, it is found that the dominant azimuthal wavenumber is smaller for lower Reynolds number. At higher Reynolds numbers, disturbances subject to the combined CRT and SKH instabilities grow rapidly, on the inertial time scale, while the flow stabilizes at low Reynolds numbers. Our direct numerical simulations are in good agreement with these studies in the initial stages, after which nonlinearities take over. At Atwood numbers of 0.1 or more, and a Reynolds number of 6000 or greater, both stability analysis and simulations show a rapid destabilization. The result is an erosion of the core, and breakdown into a turbulence-like state. In studies at low Atwood numbers, the effect of density on the inertial terms is often ignored, and the density field behaves like a passive scalar in the absence of gravity. The present study shows that such treatment is unjustified in the vicinity of a vortex, even for small changes in density when the density stratification is across a thin layer. The study would have relevance to any high-Péclet-number flow where a vortex is in the vicinity of a density-stratified interface.

Entropy Generation Analysis for Nanofluid Flow in Microchannels

2010 ◽  
Vol 132 (12) ◽  
Author(s):  
Jie Li ◽  
Clement Kleinstreuer
Keyword(s):  
Reynolds Number ◽  
Entropy Generation ◽  
Thermal Effects ◽  
Pure Water ◽  
Reynolds Numbers ◽  
Computer Simulation Model ◽  
Number Range ◽  
Aspect Ratios ◽  
Reynolds Number Range ◽  
Steady Laminar

Employing a validated computer simulation model, entropy generation is analyzed in trapezoidal microchannels for steady laminar flow of pure water and CuO-water nanofluids. Focusing on microchannel heat sink applications, local and volumetric entropy rates caused by frictional and thermal effects are computed for different coolants, inlet temperatures, Reynolds numbers, and channel aspect ratios. It was found that there exists an optimal Reynolds number range to operate the system due to the characteristics of the two different entropy sources, both related to the inlet Reynolds number. Microchannels with high aspect ratios have a lower suitable operational Reynolds number range. The employment of nanofluids can further minimize entropy generation because of their superior thermal properties. Heat transfer induced entropy generation is dominant for typical microheating systems while frictional entropy generation becomes more and more important with the increase in fluid inlet velocity/Reynolds number.

scholarly journals The investigation of the average diameter of the SiO2 nanoparticles effect on the oil displacing efficiency from the pore in the rock formation using nanosuspension as a displacing agent

2022 ◽  
Vol 2150 (1) ◽  
pp. 012025
Author(s):  
A S Lobasov ◽  
A V Minakov
Keyword(s):  
Oil Recovery ◽  
Pure Water ◽  
Vertical Channel ◽  
Average Diameter ◽  
Sio2 Nanoparticles ◽  
Reynolds Numbers ◽  
Pore Channel ◽  
Computational Domain ◽  
Recovery Coefficient ◽  
Rock Formation

Abstract The numerical investigation of the nanofluid flow, which displaced the oil, in a microchannel was carried out. The effect of the average diameter of the SiO2 nanoparticles on the oil displacing efficiency by nanofluids for different sizes of microchannel at various Reynolds numbers was studied. A T-shaped microchannel with a vertical channel, called a pore channel, which imitated the pore in the rock formation was considered as a computational domain. The main flow channel width and height were 200 µm. The width and height of the pore channel were varied in the range from 100 µm to 800 µm. The Reynolds number varied from 0.1 to 100. The oil recovery coefficient, which is defined as the ratio of the displacing volume of oil from the pore to the volume of the pore was considered as the main studied characteristic. The nanofluid is considered a single-phase fluid with experimentally obtained properties. The mass concentration of SiO2 nanoparticles was 0.5%. The average diameters of nanoparticles were 5 nm, 18 nm, and 50 nm. It was found, that the oil recovery coefficient increased with a decrease in the average diameter of nanoparticles. It was obtained that the nanofluid can enhance the oil recovery several times compared to pure water.

scholarly journals Effect of Rotation and Hole Arrangement in Cold Bridge-Type Impingement Cooling Systems

2019 ◽  
Vol 4 (2) ◽  
pp. 13 ◽  
Author(s):  
Lorenzo Cocchi ◽  
Alessio Picchi ◽  
Bruno Facchini
Keyword(s):  
Heat Transfer ◽  
Film Cooling ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Impingement Cooling ◽  
Complex Interactions ◽  
Supply Condition ◽  
Rotation Numbers ◽  
Supply Channel ◽  
Bridge Type

Experimental activity has been performed to study different impingement cooling schemes in static and rotating conditions. Geometry replicates a leading-edge cold bridge system, including a radial supply channel and five rows of film-cooling and showerhead holes. Two impingement geometries have been studied, with different numbers of holes and diameters but with equal overall passage area. Reynolds numbers up to 13,800 and rotation numbers up to 0.002 have been investigated (based on an equivalent slot width). Tests have been performed using a novel implementation of transient heat transfer technique, which allows correct replication of the sign of buoyancy forces by flowing ambient temperature air into a preheated test article. Results show that complex interactions occur between the different features of the system, with a particularly strong effect of jet supply condition. Rotation further interacts with these phenomena, generally leading to a slight decrease in heat transfer.

Fluid Flow and Heat Transfer Characteristics of Ethylene Glycol and Water in Copper-Based Microchannel Devices

2009 ◽  
Author(s):  
Fanghua Mei ◽  
B. Lu ◽  
W. J. Meng ◽  
S. Guo
Keyword(s):  
Heat Transfer ◽  
Ethylene Glycol ◽  
High Performance ◽  
Heat Transfer Performance ◽  
Pure Water ◽  
High Heat Flux ◽  
Transfer Performance ◽  
Fluid Medium ◽  
Flow And Heat Transfer ◽  
Transfer Testing

Metal-based microchannel heat exchangers (MHEs) offer potential solutions to applications demanding high heat flux removal, such as cooling of high-performance microelectronic and energy-efficient lighting modules. Efficient fabrication of metal-based MHEs and quantitative flow and heat transfer measurements on them are critical for establishing the economic and technical feasibility of such devices. Adopting metal-based MHEs in many applications demands quantification of flow and heat transfer performance with application-relevant coolants, e.g. ethylene glycol (EG)/water mixtures rather than pure water. As a first step in this direction, we report here fabrication and assembly of all-Cu MHE prototypes, as well as results of flow and heat transfer testing using pure EG and pure water as the fluid medium. Results of heat transfer testing indicate sensitivity of overall heat transfer performance to entrance length effects, which in the case of pure EG, is significantly influenced by its physical properties under the testing condition.

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