Slip effect

Torsional vibrations are due to the stick-slip effect of the stabilizers in deviated boreholes. They can be seen at surface as large torque oscillations with a period of 3 to 10 s. Figure 4-308 shows a near-bit stabilizer in a deviated borehole. The stick-slip effect increases with WOB and RPM. 

Figure 4-308. Stick-slip effect due to near-bit fraction in a deviated borehole. (Courtesy Anadrill [113].,)...

Over the twentieth century, the mbber industry has developed special rheometers, essentially factory floor instmments either for checking process regularity or for quality control purposes, for instance, the well-known Mooney rheometer (1931), the oscillating disk rheometer (1962), and the rotorless rheometer (1976). All those instmments basically perform simple drag flow measurements but they share a common feature During the test, the sample is maintained in a closed cavity, under pressure, a practice intuitively considered essential for avoiding any wall slip effects. Indeed it has... 

Baudry, j., Chariaix, E., Tonck, A., Mazuyer, D., Experimental evidence for a large slip effect at a nonwetting fluid-solid interface, Langmuir 17 (2001) 5232-5236. 

In the case of constant interfacial-area-stirred cells, although zone A is certainly an indication that the process is controlled by diffusional processes, the opposite is not true for zone B. In fact, in spite of the increased stirring rate, it may happen that the thickness of the diffusion films never decreases below a sufficiently low value to make diffusion so fast that it can be completely neglected relative to the rate of the chemical reactions. This effect, sometimes called slip effect, depends on the specific hydrodynamic conditions of the apparatus in which the extraction takes place and simulates a kinetic regime. 

Figure 4. Schematic drawing of patterns to detect slip effects in parallel plate geometry.

Figure 7. Detection of slip from Bagley plots, 1 No slip, 2 Slip effects present.

A general conclusion that can be made after analyzing the flow behavior of these diverse materials is that wall-slip is affected by pressure. The onset of slip occurs at a certain applied stress (slip velocity must be zero at zero flow) and slip effects are lost above a certain higher stress. It is possible that the onset and the disappearance stress levels for wall-slip could be material specific for a given flow surface conditions. However, little work has been reported using wall-slip as a material characterization parameter. 

To date, most wall-slip studies have concentrated on either eliminating or quantifying slip effects in laboratory rheometry. Slip effects have been traditionally treated as flow anomalies, which make the measurement of material properties a challenging task. There is less evidence on how to utilize this flow anomaly to industry s advantage. A comprehensive understanding of the slip phenomenon as it relates to food texture and food processing is of crucial importance to the food industry, where the goal is to produce quality products at reduced cost... 

The semisolid consistency and the occurrence of slip effects during flow could limit the use of the rotary clamp method, the conveigent flow method or even Sridhar s extensional rheometer for a large number of food products such as mayonnaises and salad dressings. A novel rheometer (15) based on the concept of filament stretching has been developed by Plucinski et al (Fig. 10) to measure the extensional properties... 

The filament stretching device could play a new role in rheometry by providing a new direction for studying substances that exhibit severe slip effects during conventional rotational rheometry. 

U. Yilmazer and D.M. Kalyon, Slip Effects in Capillary and Parallel Disk Torsional Flows of Highly Filled Suspensions, J. Rheol., 33 1197-1212 (1989). 

J. Plucinski, R.K. Gupta and S. Chakrabarti, Slip Effects in the Rotational Flow of Food Emulsions (to be published). 

Various nonequilibrium and slip flow models have been proposed as improvements on the homogeneous equilibrium flow model. See, for example, Henry and Fauske (Trans. ASME J. Heat Transfer, 179-187 [May 1971]). Nonequilibrium and slip effects both increase... 

P 21] The mixing of gaseous methanol and oxygen was simulated. The equations applied for the calculation were based on the Navier-Stokes (pressure and velocity) and the species convection-diffusion equation [57]. As the diffusivity value for the binary gas mixture 2.8 x 10 m2 s 1 was taken. The flow was laminar in all cases adiabatic conditions were applied at the domain boundaries. Compressibility and slip effects were taken into account The inlet temperature was set to 400 K. The total number of cells was —17 000 in all cases. 

During extrusion of polymer melts with high throughputs, the elastic melt properties can also lead to elastic instabilities which can result in surface distortions of the extrudate. One example are wavy distortions also described as sharkskin. Depending on the polymer, this can also lead to helical extrudate structures (stick-slip effect) or to very irregular extrudate structures (melt fracture) at even higher throughput rates [10]. 

---