The Phenomenon of Bubbles Negative Relative Velocity in Vertical Bubbly Jets

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Jamel Chahed ◽  
Aroua Aouadi ◽  
Mariem Rezig ◽  
Ghazi Bellakhal
Keyword(s):  
Slip Velocity ◽  
Added Mass ◽  
Mass Force ◽  
Relative Slip ◽  
Velocity Inversion ◽  
Injection Zone ◽  
Bubbly Jet ◽  
New Formulation ◽  
Interfacial Force ◽  
Bubbly Jets

Many experiments demonstrate that the bubble relative (slip) velocities in vertical turbulent sheared bubbly flows are significantly lower than those in quiescent infinite fluid. Moreover, vertical bubbly jet experiments performed by Sun and Faeth (1986, “Structure of Turbulent Bubbly Jets-1. Methods and Centerline Properties,” Int. J. Multiphase Flow, 12(1), pp. 99–114) indicate that bubble slip velocities have negative values in the high sheared zone near the injector. The present analysis shows that the phenomenon of the slip velocity inversion is associated with the effect of the turbulent part of the interfacial force. A new formulation of the turbulent contribution of the added mass force is proposed. This formulation is analyzed using the vertical bubbly jet experimental data. The results provide evidence that the turbulent contribution of the added mass force is at the origin of the slip velocity reduction and could explain the appearance of the negative values observed in bubbly jet experiments. As a whole, the turbulent contribution of the added mass force which comprises two terms (a nonlinear turbulent term and a convective acceleration term associated to the drift velocity) opposes the action of the gravity and their effect may be high enough to produce negative slip velocities. Taken separately, the two turbulent terms cannot explain the reversal and the reduction of slip through the entire section in the near injection zone of the bubbly jet. The combined effect of the two turbulent terms makes it possible to reproduce slip velocity profiles as observed in the near injection zone.

A Double Birkhoff Wake Oscillator for the Modeling of Vortex-Induced Vibration

2011 ◽  
Author(s):  
R. H. M. Ogink
Keyword(s):  
Free Vibration ◽  
Added Mass ◽  
Mass Force ◽  
Near Wake ◽  
Vibration Measurements ◽  
Vortex Induced Vibration ◽  
Fluid Forces ◽  
Wake Oscillator ◽  
Model Equations ◽  
Far Wake

A double Birkhoff wake oscillator for the modeling of vortex-induced vibration is presented in which the oscillating variables are assumed to be associated with the boundary layer/near wake and the far wake. The fluid forces are assumed to consist of a potential added mass force and a force due to vortex shedding. In the limit of vanishing incoming flow velocity, the model equations reduce to a form similar to the Morison equation. The results of the double wake oscillator have been compared with forced vibration measurements and free vibration measurements over a range of mass and damping ratios. The model is capable of describing the most important trends in both the forced and free vibration experiments. Specifically, the double wake oscillator is able to model both the upper and lower branch of free vibration.

An Investigation to Added Mass Force on the Wing of Insect Inspired from a Rigid Sphere Model

2012 ◽  
Vol 476-478 ◽  
pp. 2485-2488
Author(s):  
Mei Jun Hu ◽  
Xing Yao Yan ◽  
Jin Yao Yan
Keyword(s):  
Insect Flight ◽  
Aerodynamic Force ◽  
Mass Effect ◽  
Added Mass ◽  
Rigid Sphere ◽  
Force Model ◽  
Mass Force ◽  
Real Time Control ◽  
Wing Model ◽  
Force Peak

There is a force peak at the beginning of each stroke during the insect flight, this force peak contributes a lot to the total aerodynamic force. To build a man made insect inspired man-made micro aero vehicle, this force need to be considered in the aero force model, and this model should as simple as possible in order to be used in feedback real-time control. Here we presented a simplified model to take the medium added mass effect of the wing into account. Simulated results show a high force peak at the beginning of each stroke and are quite similar to the measured forces on the physical wing model which were carried out by Dickinson et.al.

On the unsteady inviscid force on cylinders and spheres in subcritical compressible flow

2008 ◽  
Vol 366 (1873) ◽  
pp. 2161-2175 ◽  
Author(s):  
M Parmar ◽  
A Haselbacher ◽  
S Balachandar
Keyword(s):  
Mach Number ◽  
Compressible Flow ◽  
Propagation Speed ◽  
Added Mass ◽  
The Body ◽  
Mass Force ◽  
Unsteady Force ◽  
Mass Coefficient ◽  
Added Mass Coefficient ◽  
Cylinders And Spheres

The unsteady inviscid force on cylinders and spheres in subcritical compressible flow is investigated. In the limit of incompressible flow, the unsteady inviscid force on a cylinder or sphere is the so-called added-mass force that is proportional to the product of the mass displaced by the body and the instantaneous acceleration. In compressible flow, the finite acoustic propagation speed means that the unsteady inviscid force arising from an instantaneously applied constant acceleration develops gradually and reaches steady values only for non-dimensional times c ∞ t / R ≳10, where c ∞ is the freestream speed of sound and R is the radius of the cylinder or sphere. In this limit, an effective added-mass coefficient may be defined. The main conclusion of our study is that the freestream Mach number has a pronounced effect on both the peak value of the unsteady force and the effective added-mass coefficient. At a freestream Mach number of 0.5, the effective added-mass coefficient is about twice as large as the incompressible value for the sphere. Coupled with an impulsive acceleration, the unsteady inviscid force in compressible flow can be more than four times larger than that predicted from incompressible theory. Furthermore, the effect of the ratio of specific heats on the unsteady force becomes more pronounced as the Mach number increases.

scholarly journals Decomposition of the forces on a body moving in an incompressible fluid

2019 ◽  
Vol 881 ◽  
pp. 1097-1122
Author(s):  
W. R. Graham
Keyword(s):  
Velocity Field ◽  
Pressure Field ◽  
Rotational Velocity ◽  
Inviscid Flow ◽  
Added Mass ◽  
The Body ◽  
Mass Force ◽  
Natural Approach ◽  
Fluid Forces ◽  
Point Pressure

In analysing fluid forces on a moving body, a natural approach is to seek a component due to viscosity and an ‘inviscid’ remainder. It is also attractive to decompose the velocity field into irrotational and rotational parts, and apportion the force resultants accordingly. The ‘irrotational’ resultants can then be identified as classical ‘added mass’, but the remaining, ‘rotational’, resultants appear not to be consistent with the physical interpretation of the rotational velocity field (as that arising from the fluid vorticity with the body stationary). The alternative presented here splits the inviscid resultants into components that are unquestionably due to independent aspects of the problem: ‘convective’ and ‘accelerative’. The former are associated with the pressure field that would arise in an inviscid flow with (instantaneously) the same velocities as the real one, and with the body’s velocity parameters – angular and translational – unchanging. The latter correspond to the pressure generated when the body accelerates from rest in quiescent fluid with its given rates of change of angular and translational velocity. They are reminiscent of the added-mass force resultants, but are simpler, and closer to the standard rigid-body inertia formulae, than the developed expressions for added-mass force and moment. Finally, the force resultants due to viscosity also include a contribution from pressure. Its presence is necessary in order to satisfy the equations governing the pressure field, and it has previously been recognised in the context of ‘excess’ stagnation-point pressure. However, its existence does not yet seem to be widely appreciated.

scholarly journals Effect of interference on fretting wear of planetary frame axle hole

2020 ◽  
Vol 12 (5) ◽  
pp. 168781402091922
Author(s):  
Zhipeng Lyu ◽  
Fuyuan Wang ◽  
Sizhu Zhou ◽  
Si Liu
Keyword(s):  
Slip Velocity ◽  
Dominant Role ◽  
Axial Direction ◽  
Characteristic Parameter ◽  
Middle Part ◽  
Fretting Wear ◽  
Wear Depth ◽  
Relative Slip ◽  
Finite Element Software ◽  
Distribution Rule

In order to study fretting wear damage law of planetary frame axle hole, the distribution of normal stress and relative sliding velocity at axle hole was obtained by finite element software, and a method of extracting fretting wear characteristic parameter data was put forward and verified. According to the modified model of fretting wear depth calculation, the wear depth of each step at axle hole was calculated, and the influence of interference on wear depth was analyzed. The results show that the stress distribution obtained by this method corresponds to the values of each node in the Workbench stress nephogram at that time and has the same distribution rule, which shows that the method is correct. The stress concentration near the inner part of the axle hole of the planetary frame is obvious. Along the circumferential and axial direction of the shaft hole, the relative slip velocity of both ends is larger, and the relative slip velocity of the middle part is smaller. Average wear in both axial and circumferential directions increases with the increase in interference, while wear in the axial direction plays a dominant role in the whole meshing process.

Solution of Singularity Points in Strip Rolling by Rigid Plasticity Finite Element Method

2010 ◽  
Vol 97-101 ◽  
pp. 219-226
Author(s):  
Chang Sheng Li ◽  
Rui Bin Mei ◽  
Xiang Hua Liu
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Slip Velocity ◽  
Velocity Model ◽  
Iteration Step ◽  
Parabolic Model ◽  
Strip Rolling ◽  
Relative Slip ◽  
Singularity Point ◽  
Element Method

The concept of the first and the second singularity point was introduced in the paper. The singularity points would lead to the iteration divergence in the solution of rolling by rigid plasticity finite element method. Double velocity model and parabolic model of relative slip velocity were proposed for solution of the first and the second singularity point respectively. The influence of the models to improve the effects of singularity point on calculating time and iteration step was discussed according to the practical strip rolling condition. The results showed that for the element numbers from 200 to 2000, the iteration step and total calculating time was reduced about 8~67% by the double velocity model in the same condition compared to normal model for the solution of first singularity point. The iteration step and calculating time was reduced about 15~61% with the parabolic model of relative slip velocity. The double velocity model and parabolic model of relative slip velocity could be used to improve the convergence and increase efficiency of the solution in strip rolling by rigid plasticity finite element method.

scholarly journals A Paradox in Sliding Contact Problems With Friction

2003 ◽  
Vol 72 (3) ◽  
pp. 450-452 ◽  
Author(s):  
G. G. Adams ◽  
J. R. Barber ◽  
M. Ciavarella ◽  
J. R. Rice
Keyword(s):  
Half Plane ◽  
Slip Velocity ◽  
Finite Strain ◽  
Applied Pressure ◽  
Strain Analysis ◽  
Contact Problems ◽  
Rigid Punch ◽  
Flat Punch ◽  
Relative Slip ◽  
Velocity Changes

In problems involving the relative sliding to two bodies, the frictional force is taken to oppose the direction of the local relative slip velocity. For a rigid flat punch sliding over a half-plane at any speed, it is shown that the velocities of the half-plane particles near the edges of the punch seem to grow without limit in the same direction as the punch motion. Thus the local relative slip velocity changes sign. This phenomenon leads to a paradox in friction, in the sense that the assumed direction of sliding used for Coulomb friction is opposite that of the resulting slip velocity in the region sufficiently close to each of the edges of the punch. This paradox is not restricted to the case of a rigid punch, as it is due to the deformations in the half-plane over which the pressure is moving. It would therefore occur for any punch shape and elastic constants (including an elastic wedge) for which the applied pressure, moving along the free surface of the half-plane, is singular. The paradox is resolved by using a finite strain analysis of the kinematics for the rigid punch problem and it is expected that finite strain theory would resolve the paradox for a more general contact problem.

Lift, drag and added mass of a hemispherical bubble sliding and growing on a wall in a viscous linear shear flow

2008 ◽  
Vol 366 (1873) ◽  
pp. 2233-2248 ◽  
Author(s):  
Dominique Legendre ◽  
Catherine Colin ◽  
Typhaine Coquard
Keyword(s):  
Shear Flow ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Added Mass ◽  
Unsteady Motion ◽  
Navier Stokes ◽  
Mass Force ◽  
Navier Stokes Equations ◽  
Wide Range ◽  
Linear Shear

The three-dimensional flow around a hemispherical bubble sliding and growing on a wall in a viscous linear shear flow is studied numerically by solving the full Navier–Stokes equations in a boundary-fitted domain. The main goal of the present study is to provide a complete description of the forces experienced by the bubble (drag, lift and added mass) over a wide range of sliding and shear Reynolds numbers (0.01≤ Re b , Re α ≤2000) and shear rate (0≤ Sr ≤5). The drag and lift forces are computed successively for the following situations: an immobile bubble in a linear shear flow; a bubble sliding on the wall in a fluid at rest; and a bubble sliding in a linear shear flow. The added-mass force is studied by considering an unsteady motion relative to the wall or a time-dependent radius.

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