scholarly journals Towards theoretical modeling of the sand dunes motion

2017 ◽  
pp. 17-23
Author(s):  
O.I. Gerasymov ◽  
I.S. Andrianova
Keyword(s):  
Sand Dunes ◽  
Reynolds Numbers ◽  
Short Review ◽  
Governing Equations ◽  
Viscous Forces ◽  
Robust Model ◽  
Fold Reduction ◽  
Frictional Dissipation ◽  
Suspended Dust ◽  
Simple Product

The transport of sand by wind is a potent erosion force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This article presents a short review of the physics of wind-driven sand. Specifically, we review the physics of saltation, the formation and development of sand dunes and ripples. We also discuss some classes of the governing equations which describe the physics of wind-driven sand and dune formation. We describe selected types of dunes and conditions under which they occur, and also some features of dunes as well as processes that they are involved in. We show that the normalized dunes height collapses using a simple product of the Froude and Reynolds numbers. This would obscure the effects of frictional dissipation, which clearly plays an important role in all mentioned upper process. Ignoring friction, one can construct a simple energy balance between the kinetic energy of the impacting and the potential energy of the dunes, where we assume the dunes thickness is proportional to ds. This produces the following scaling. In other words, was one to increase the grain diameter ds by a factor of 10 ~ i.e., reduce Re by 100! for the same impact conditions, then the frictionless flow would predict a 10-fold reduction in , whereas the experiments suggest a 100-fold reduction. This shows clearly that viscous forces play a role in the granular dunes formation (and their relevant dynamics), as well as gravity and inertia. These circumstances move us to conclude   the vide range of (non-dissipative) hydrodynamic approaches to describe dunes formation and their dynamics just as a robust model approaches.

Correlated Compressible and Incompressible Channel Flows

1997 ◽  
Vol 119 (4) ◽  
pp. 911-915 ◽  
Author(s):  
C. Crnojevic´ ◽  
V. D. Djordjevic´
Keyword(s):  
Cross Section ◽  
High Reynolds Number ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Isothermal Flow ◽  
Governing Equations ◽  
Adiabatic Flow ◽  
High Reynolds Numbers ◽  
High Reynolds ◽  
Reynolds Number Flows

Compressible flow in channels of slowly varying cross section at moderately high Reynolds numbers is treated in the paper by employing some Stewartson-type transformations that convert the problem into an incompressible one. Both adiabatic flow and isothermal flow are considered, and a Poiseuille-type incompressible solution is mapped onto compressible plane in order to generate some exact solutions of the compressible governing equations. The results show striking effects that viscosity may have upon the flow characteristics in this case, in comparison with more conventional high Reynolds number flows.

Spray Cloud Formation Over Marine Vessels by a Water Breakup Model

2017 ◽  
Author(s):  
Saeed R. Dehghani ◽  
Greg F. Naterer ◽  
Yuri S. Muzychka
Keyword(s):  
Numerical Solutions ◽  
Reynolds Numbers ◽  
Cloud Formation ◽  
Sea Spray ◽  
Governing Equations ◽  
Fishing Vessel ◽  
Wave Impact ◽  
Droplet Sizes ◽  
Marine Vessels ◽  
Breakup Model

Water breakup affects the variety of droplet sizes and velocities in a cloud of spray resulting from a sea wave striking a vessel bow. The Weber and Reynolds numbers of droplets are the main parameters for water breakup phenomena. “Stripping breakup” is a faster phenomenon than “bag breakup” and occurs at higher velocities and with larger diameters of droplets. A water breakup model employs droplet trajectories to develop a predictive model for the extent of spray cloud. The governing equations of breakup and trajectories of droplets are solved numerically. Stripping breakup is found as the major phenomenon in the process of the formation of wave-impact sea spray. Bag breakup acts as a complementary phenomenon to the stripping breakup. The extent of the spray as well as wet heights, for a Mediumsized Fishing Vessel (MFV), are obtained by numerical solutions. The results show that bag breakup occurs at higher heights. In addition, there is no breakup when droplets move over the deck.

Film spreading from a miscible drop on a deep liquid layer

2017 ◽  
Vol 829 ◽  
pp. 304-327 ◽  
Author(s):  
Raj Dandekar ◽  
Anurag Pant ◽  
Baburaj A. Puthenveettil
Keyword(s):  
Surface Tension ◽  
Water Layer ◽  
Reynolds Numbers ◽  
Capillary Waves ◽  
Water Mixture ◽  
Inertial Forces ◽  
Viscous Forces ◽  
Dimensionless Velocity ◽  
Deep Water Layer ◽  
Film Spreading

We study the spreading of a film from ethanol–water droplets of radii $0.9~\text{mm}<r_{d}<1.1~\text{mm}$ on the surface of a deep water layer for various concentrations of ethanol in the drop. Since the drop is lighter ($\unicode[STIX]{x1D709}=\unicode[STIX]{x1D70C}_{l}/\unicode[STIX]{x1D70C}_{d}>1.03$), it stays at the surface of the water layer during the spreading of the film from the drop; the film is more viscous than the underlying water layer since $\unicode[STIX]{x1D712}=\unicode[STIX]{x1D707}_{l}/\unicode[STIX]{x1D707}_{d}>0.38$. Inertial forces are not dominant in the spreading since the Reynolds numbers based on the film thickness $h_{f}$ are in the range $0.02<Re_{f}<1.4$. The spreading is surface-tension-driven since the film capillary numbers are in the range $0.0005<Ca_{f}<0.0069$ and the drop Bond numbers are in the range $0.19<Bo_{d}<0.56$. We observe that, when the drop is brought in contact with the water surface, capillary waves propagate from the point of contact, followed by a radially expanding, thin circular film of ethanol–water mixture. The film develops instabilities at some radius to form outward-moving fingers at its periphery while it is still expanding, till the expansion stops at a larger radius. The film then retracts, during which time the remaining major part of the drop, which stays at the centre of the expanding film, thins and develops holes and eventually mixes completely with water. The radius of the expanding front of the film scales as $r_{f}\sim t^{1/4}$ and shows a dependence on the concentration of ethanol in the drop as well as on $r_{d}$, and is independent of the layer height $h_{l}$. Using a balance of surface tension and viscous forces within the film, along with a model for the fraction of the drop that forms the thin film, we obtain an expression for the dimensionless film radius $r_{f}^{\ast }=r_{f}/r_{d}$, in the form $fr_{f}^{\ast }={t_{\unicode[STIX]{x1D707}d}^{\ast }}^{1/4}$, where $t_{\unicode[STIX]{x1D707}d}^{\ast }=t/t_{\unicode[STIX]{x1D707}d}$, with the time scale $t_{\unicode[STIX]{x1D707}d}=\unicode[STIX]{x1D707}_{d}r_{d}/\unicode[STIX]{x0394}\unicode[STIX]{x1D70E}$ and $f$ is a function of $Bo_{d}$. Similarly, we show that the dimensionless velocity of film spreading, $Ca_{d}=u_{f}\unicode[STIX]{x1D707}_{d}/\unicode[STIX]{x0394}\unicode[STIX]{x1D70E}$, scales as $4f^{4}Ca_{d}={r_{f}^{\ast }}^{-3}$.

Improving Adhesion Density and Coverage Uniformity of Antibody-Antigen Binding on a Sensor Surface Using U-Type, T-Type and W-Type Microfluidic Devices

2011 ◽  
Author(s):  
Chia-Che Wu ◽  
Ping-Kuo Tseng ◽  
Ching-Hsiu Tsai
Keyword(s):  
Quantum Dots ◽  
Microfluidic Channel ◽  
Microfluidic Devices ◽  
Reynolds Numbers ◽  
Yellow Mosaic Virus ◽  
Fluorescent Intensity ◽  
Low Reynolds Numbers ◽  
Viscous Forces ◽  
Drag Forces ◽  
Low Reynolds

Usually microorganisms, molecules, or viruses in the fluidic environment are at very low Reynolds numbers because of tiny diameters. At very low Reynolds numbers, viscous forces of molecules and viruses will dominate. Those micro- or nanoparticles will stop moving immediately when flows cease and drag forces disappear, those phenomena were discovered by the fluorescent particle experiment. Of course, molecules and viruses are still subject to Brownian motion and move randomly. In order to increase the adhesion density of micro- and nanoparticles on sensor’s surface, designs of the flow movements in microfluidic channel is proposed. Adhesion density of linker 11-mercaptoundecanoic acid (MUA) and Turnip yellow mosaic virus (TYMV) with specific quantum dots were measured by confocal microscope. Fluorescent intensity and coverage of quantum dots are used to identify the adhesion density quantitatively. Results show that TYMV and MUA layers disperse randomly by dipping method. Fluorescent intensity of quantum dots; i.e. relative to the amount of MUA and TYMV; were 2.67A.U. and 19.13A.U., respectively, in W-type microfluidic devices to contrast just 1.00A.U. and 1.00A.U., respectively, by dipping method. Coverage of MUA and TYMV were 80∼90% and 70∼90%, respectively, in W-type microfluidic channel to contrast just 20∼50% and 0∼10%, respectively, by dipping method.

scholarly journals Influence of wall waviness on friction and pressure drop in channels

1981 ◽  
Vol 4 (4) ◽  
pp. 805-818 ◽  
Author(s):  
K. Vajravelu ◽  
Ali H. Nayfeh
Keyword(s):  
Pressure Drop ◽  
Flow Behavior ◽  
Reynolds Numbers ◽  
Wavy Wall ◽  
Governing Equations ◽  
Short Waves ◽  
Gradient Parameter ◽  
Arbitrary Amplitude ◽  
Wavy Channels ◽  
Good Agreement

An attention has been given to investigate the flow behavior of an incompressible viscous fluid confined in horizontal wavy channels and set in motion due to the movement of the upper wall and the pressure differences. The governing equations have been solved analytically as well as numerically subject to the relevant boundary conditions by assuming that the solution consists of two parts: a mean part and a disturbance or perturbed part. For small and moderate Reynolds numbers, the analytical solution for the perturbed part has been found to be in good agreement with the numerical one. The effects of Reynolds number, the pressure gradient parameter, and the undulation wavenumber on friction and pressure drop are found to be quite significant. In addition to the flow behavior for both long and short waves and for large Reynolds numbers, the effect of the wall waviness on friction and pressure drop has been examined for any arbitrary amplitude of the wavy wall.

Dragonfly flight. I. Gliding flight and steady-state aerodynamic forces.

1997 ◽  
Vol 200 (3) ◽  
pp. 543-556 ◽  
Author(s):  
JM Wakeling ◽  
CP Ellington
Keyword(s):  
Steady State ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
The Body ◽  
Flat Plates ◽  
Viscous Forces ◽  
Drag Forces ◽  
Aerodynamic Properties ◽  
Gliding Flight ◽  
Lift And Drag

The free gliding flight of the dragonfly Sympetrum sanguineum was filmed in a large flight enclosure. Reconstruction of the glide paths showed the flights to involve accelerations. Where the acceleration could be considered constant, the lift and drag forces acting on the dragonfly were calculated. The maximum lift coefficient (CL) recorded from these glides was 0.93; however, this is not necessarily the maximum possible from the wings. Lift and drag forces were additionally measured from isolated wings and bodies of S. sanguineum and the damselfly Calopteryx splendens in a steady air flow at Reynolds numbers of 700-2400 for the wings and 2500-15 000 for the bodies. The maximum lift coefficients (CL,max) were 1.07 for S. sanguineum and 1.15 for C. splendens, which are greater than those recorded for all other insects except the locust. The drag coefficient at zero angle of attack ranged between 0.07 and 0.14, being little more than the Blassius value predicted for flat plates. Dragonfly wings thus show exceptional steady-state aerodynamic properties in comparison with the wings of other insects. A resolved-flow model was tested on the body drag data. The parasite drag is significantly affected by viscous forces normal to the longitudinal body axis. The linear dependence of drag on velocity must thus be included in models to predict the parasite drag on dragonflies at non-zero body angles.

scholarly journals Kazantsev model in non-helical 2.5-dimensional flows

2016 ◽  
Vol 806 ◽  
pp. 627-648 ◽  
Author(s):  
K. Seshasayanan ◽  
A. Alexakis
Keyword(s):  
Turbulent Flows ◽  
Three Dimensional ◽  
Analytical Calculation ◽  
Reynolds Numbers ◽  
Two Dimensions ◽  
Two Dimensional ◽  
Control Parameters ◽  
Governing Equations ◽  
Field Correlation ◽  
Good Agreement

We study the dynamo instability for a Kazantsev–Kraichnan flow with three velocity components that depend only on two dimensions $\boldsymbol{u}=(u(x,y,t),v(x,y,t),w(x,y,t))$ often referred to as 2.5-dimensional (2.5-D) flow. Within the Kazantsev–Kraichnan framework we derive the governing equations for the second-order magnetic field correlation function and examine the growth rate of the dynamo instability as a function of the control parameters of the system. In particular we investigate the dynamo behaviour for large magnetic Reynolds numbers $Rm$ and flows close to being two-dimensional and show that these two limiting procedures do not commute. The energy spectra of the unstable modes are derived analytically and lead to power-law behaviour that differs from the three-dimensional and two-dimensional cases. The results of our analytical calculation are compared with the results of numerical simulations of dynamos driven by prescribed fluctuating flows as well as freely evolving turbulent flows, showing good agreement.

scholarly journals Cu-Al2O3 Water Hybrid Nanofluid Transport in a Periodic Structure

Processes ◽  
2020 ◽  
Vol 8 (3) ◽  
pp. 285
Author(s):  
Aiman Alshare ◽  
Wael Al-Kouz ◽  
Waqar Khan
Keyword(s):  
Periodic Structure ◽  
Volume Fraction ◽  
Reynolds Numbers ◽  
Hybrid Nanofluid ◽  
Nanoparticle Volume Fraction ◽  
Pumping Power ◽  
Computational Investigation ◽  
Governing Equations ◽  
The Cost ◽  
Volume Method

The present work is a computational investigation of nanofluid and hybrid nanofluid transport in a periodic structure. The governing equations for this work along with the appropriate boundary conditions are solved using the finite-volume method. The simulations are carried out using five wavy amplitudes of the channel shape for a range of Reynolds numbers from 102 to103. It is found that increasing the amplitude and increasing the nanoparticle volume fraction achieve enhancement of the heat transfer at the cost of increased pumping power. Correlations for the friction factor and the Nusselt number for both fluid types are provided.

Determining the Optimum Arrangement of Micromixers in a Microchannel Filled with CuO-Water Nanofluid via Minimizing Entropy Generation

2017 ◽  
Vol 378 ◽  
pp. 39-58 ◽  
Author(s):  
Ahmad Ababaei ◽  
Mahmoud Abbaszadeh ◽  
Ali Akbar Abbasian Arani
Keyword(s):  
Boundary Conditions ◽  
Parallel Plate ◽  
Slip Velocity ◽  
Temperature Jump ◽  
Volume Fraction ◽  
Reynolds Numbers ◽  
Governing Equations ◽  
Simpler Algorithm ◽  
Optimum Arrangement ◽  
Volume Method

In this study, the flow of CuO-water nanofluid in a parallel-plate microchannel in the presence of several micromixers is examined to find optimum arrangements of the micromixers. The governing equations, which are accompanied with the slip velocity and temperature jump boundary conditions, are solved by the Finite Volume Method and SIMPLER algorithm. The study is conducted for the Reynolds numbers in the range of 10 ≤ Re ≤ 100, Knudsen numbers ranging of 0 ≤ Kn ≤ 0.1 and volume fraction of nanoparticles ranging of 0 ≤ ϕ ≤ 4%. The results show that the optimum arrangements of the micromixers belong to cases in which the heights of micromixers are smaller, the distance between them is lower, and their numbers are more.

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