Flow disturbances

When the wind encounters objects in its path such as an isolated structure, the flow usually is strongly perturbed and a turbulent wake is formed in the vicinity of the structure, especially downwind of it. If the structure is semistreamlined in shape, the flow may move around it with little disturbance. Since most structures have edges and corners, generation of a turbulent wake is quite common. Figure 17-23 shows schematically the flow in the vicinity of a cubic structure. The disturbed flow consists of a cavity... 

Disturbance flows on the zone boundary should be avoided because of the undesired return flow from the uncontrolled zone to the controlled zone. 

The propulsor, in a steady situation, has to provide the thrust equivalent to the resistance of the ship. The prediction of this balance between ship resistance and propeller thrust is a complex process because the water flow in the stern region of the ship interacts with the flow through the propeller. The ship resistance may be modified by the action of the propeller, while the propeller efficiency usually is influenced by the disturbed flow regime near the stern of the ship. 

Knowledge of spatial variations in bubble, drop, and crystal sizes is often desired or required, but extremely hard to obtain experimentally. Intrusive measuring and sampling probes may disturb flow and process locally. Taking samples may affect the sizes in the sampling procedure, samples may experience... 

It will be later proposed that an ideal blood-contacting surface would be one on which endothelial cells could adhere and grow normally to create a complete endothelium endowed with the physiological functions of such tissue. But the correct expression of such physiological functions requires normal blood flow conditions. And the correlation of thrombosis with regions of disturbed flow suggests that shear stress may alter the production of endothelial cell-derived products and directly affect endothelial cell function. 

S.) Disturbed flow primarily, this occurs intralobularly as a result of various disorders (e.g. DIC, intrasinus-oidal fibrin precipitation, centrilobular increase in... 

Figure 3. Direct-gate with vortex-formation and disturbed flow-front.

A turbulent flow is characterised by velocity fluctuations which overlap the main flow. The disturbed flow is basically three-dimensional and unsteady. At sufficiently high Reynolds numbers, the boundary layer is also no longer laminar but turbulent, such that the velocities, temperatures and concentrations all vary locally at a fixed position, as Fig. 3.14 shows for a velocity component wt. At every position it can be formed as the sum of a time-mean value (T here is the integration time)... 

The interfacial momentum transfer terms for the disturbed flow are approximated by the steady drag force ... 

Figure 7-3. A schematic representation of the proof that a spherical particle cannot undergo lateral migration in either 2-D or axisymmetric Poiseuille flow if the disturbance flow is a creeping flow. In (a) we suppose that the undisturbed flow moves from left to right and the sphere migrates inward with velocity u. Then, in the creeping-flow limit, if direction of the undisturbed flow is reversed, the signs of all velocities including that of the sphere would also have to be reversed, as shown in (b). Because the problems (a) and (b) are identical other than the direction of the flow through the channel or tube, we conclude that = 0.

In terms of velocity components, this contribution to the disturbance flow is... 

Now, we have expressed the general streamfunction, (7-149), and the disturbance flow contribution in (7-150) and (7-151), in terms of spherical coordinates. However, we have not yet specified a body shape. Thus the linear decrease of the disturbance flow with distance from the body must clearly represent a property of creeping-flows that has nothing to do with specific coordinate systems. Indeed, this is the case, and the velocity field (7-151) plays a very special and fundamental role in creeping-flow theory. It is commonly known as the Stokeslet velocity field and represents the motion induced in a fluid at Re = 0 by a point force at the origin (expressed here in spherical coordinates).17 We shall see later that the Stokeslet solution plays an important role in many aspects of creeping-flow theory. 

A second straightforward example, solved previously by other means, is Stokes original problem of uniform flow past a stationary sphere. To apply the methods of the preceding subsection to this problem, it is convenient to transform to the disturbance flow problem,... 

The boundary condition at the sphere surface for the disturbance flow,... 

Now, we can consider two possible consequences of the disturbance flow that is induced in the vicinity of each particle by the motion of the other particle. If the particle velocity is assumed to be fixed, in the form of Eq. (8-248), there will be a change in the hydrodynamic force acting on the body [that is, the external force required for producing the specified motion, (8-248), will be changed]. If, on the other hand, the external (and thus the hydro-dynamic) force is held constant, the velocity of the particles will change, and (8-248) is then just the first approximation to the particle velocity. 

Following the precedent of previous sections, dimensionless velocities are now indicated without primes. The caret continues to indicate that the quantity in question is a disturbance flow variable. Finally, we denote the dimensionless (ur, ue, ip) as (it, v,in). The result is... 

To solve these disturbance flow equations, we must apply boundary conditions. These are... 

The two boundaries at z = 0 and d can be either rigid walls or a free surface. In fact, we do not need to be so restrictive in our description of the problem. One or both of the boundaries may also be at infinity. The function U(z ) can be considered as an arbitrary function of z. The equations governing U7 and the linear disturbance flow are the dimensional Navier Stokes equations, (12-1) and (12-2), but, in this case, with the viscous terms neglected. 

Before considering the question of stability, it is useful to note that the general 3D disturbance flow problem, defined by (12-306)-(12-308), can be transformed to a form that is mathematically identical to the special case of a 2D disturbance with ay = v = 0. To motivate this transformation, we first combine Eqs. (12-307a) and (12-307b) by multiplying (12-307b) by av/ax and adding. The result is... 

The local heat-transfer coefficient near the tube entrance is somewhat higher than these values because of the disturbed flow conditions. The above values are reached after about 40 diameters into the tube. 

It must be remembered, however, that the above methods for calculating cellular flow patterns are valid only for infinitesimal amplitudes, because linear theory (which requires that disturbance flows grow exponentially without limit) cannot by itself predict the final steady flows which are established. On the other hand, flows of macroscopic amplitudes would require consideration of the nonlinear terms in the equations of motion and perhaps the variation of fluid properties with temperature. Still, researchers have persisted in... 

The role of these small molecules in the aortic valve is a relatively new frontier. miRNAs have been studied in the interstitial cells and two miRNAs have been found to play a role in the valve calcification process. MiRNA-30b has been found to be an inhibitor of osteoblast differentiation and its expression is reduced in stenotic valves compared to healthy valves [137]. Another mlRNA also studied in aortic valvular interstitial cells has been miRNA-141, this mlRNA reduces TGF-P activation, BMP-2 signaling and alkaline phosphatase activity and its expression is attenuated in bicuspid valves compared to tricuspid healthy valves [138]. Of note, several miRNAs have recently been reported to react specifically to differential mechanical shear stress (shear-sensitive miRNAs), indicating their potential importance in disturbed flow-mediated conditions such as atherosclerosis and aortic valve calcification. 

Nam, D., et al. A model of disturbed flow-induced atherosclerosis in mouse carotid artery by partial ligation and a simple method of RNA isolation from carotid endothelium. J. Vis. Exp.(40) (2010)... 

Convection and diffusion of reactants, products, cofactors and inhibitors Disturbed flow and turbulence Blood Chemistry-related Effects Coagulation status Antithrombotic and other therapies Contrast media Other Variables... 

Finally, there are devices that are completely different, such as the Multilayer Flow Modulator (M.F.M) from Cardiatis, that does not incorporate any fabric, or membrane at all, but relies on thrombosis of the sac due to disturbed flow through the stents. In theory, this would allow an extremely low profile, as well as the ability to cover branched portions of the aorta without concern for disrupting flow to essential organs such as the intestines or kidneys. However, while the theoretical concept has merit, the cUnical performance has yet to be demonstrated. 

One consequence of the disturbed flow is an increased dissipation of the fluid s kinetic energy as heat. The power dissipated per unit volume in a small region is equal to the viscosity r) times the square of the shear rate, 7. ... 

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