Helical instabilities

Bubble instability is one of the complications of this process. Only recently did this matter receive theoretical attention. As pointed out by Jung and Hyun (28), there are three characteristic bubble instabilities axisymmetric draw resonance, helical instability, and metastability where the bubble alternates between steady states, and the freeze line moves from one position to another. Using linear stability analysis, Cain and Denn (62) showed that multiple steady state solutions are possible for the same set of conditions, as pointed out earlier. However, in order to study the dynamic or time-dependent changes of the process, transient solutions are needed. This was recently achieved by Hyun et al. (65), who succeeded in quite accurately simulating the experimentally observed draw resonance (28). 

Draw resonance Helical instability Frost line oscillation Bubble sag... 

On the other hand, when a strong longitudinal field superimposed, the terms of eq. (26) which contain (B V) and B div cannot even be neglected at long wave lengths X. Then there are helical modes of for which the terms of eq. (26) nearly cancel, thus making the electromagnetic induction effect from quite small. As a result pressure-driven flute-like helical instabilities develop which cannot be suppressed by the helical field Bp + By + Bz. This is the situation predicted by the Mercier criterion 26-28. 

Here, TUR = (thickness reduction)/BHi andPg = 2B in terms of the nomenclature used in Section 10.4 TUR and BUR are the takeup ratio and blowup ratio, respectively. The region marked U is unstable according to the linear theory. Only results for BUR > 1 are of practical interest. We see that both draw resonance and helical instabilities can occur, with the latter comprising a large part of the parameter space for dimensionless pressure B less than about 0.25 (pg < 0.50). This is consistent with what is seen experimentally, although the comparisons are qualitative. 

Figure 1 Film blowing instabilities la) Draw resonance lb) Helical instability Ic) Freeze line height instability Id) Bubble sag le) Bubble tear If) Bubble flutter.

In addition, the large helical gear used for the bullgear creates an axial oscillation or thmsting that contributes to instability within the compressor. This axial movement is transmitted throughout the machine-train. 

Screw compressors have two rotors with interlocking lobes and act as positive-displacement compressors (see Figure 44.11). This type of compressor is designed for baseload, or steady state, operation and is subject to extreme instability should either the inlet or discharge conditions change. Two helical gears mounted on the outboard ends of the male and female shafts synchronize the two rotor lobes. 

The gear set should be monitored for axial thrusting. Because of the compressor s sensitivity to process instability, the gears are subjected to extreme variations in induced axial loading. Coupled with the helical gear s normal tendency to thrust, the change in axial vibration is an early indicator of incipient problems. 

Fig. 3. Solubility of silk proteins in solution as a function of time. Low solubility corresponds to protein aggregation. The fast and slow aggregations are observed in vitro (Dicko et al., 2004a), whereas the stable helical conformation (storage structure) is observed in vivo (Dicko et al., 2004b,d). This illustrates the inherent instability of silk protein in solution and shows the /(-sheet polymorph structure as the most stable form. In other words, the spiders actively control and modulate the unavoidable silk protein aggregation prior to fiber formation.

Chu (Ref c) attempted, but did not succeed, to solve the problem of the origin of oscillations. Shchelkin (Ref d) postulated that instability of a deton wave occurs if the induction time is doubled by the drop in temperature of unburned gas. An alternative theory of spinning detonation has been proposed by Predvoditelev (Ref e), according to which helical (low is assumed to occur in the tube. 

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]. 

Experimental data and consideration of the crystal structure of rhodopsin suggest that intramolecular interactions stabilize the inactive conformation of GPCRs. Removal or rearrangement of these constraining interactions results in receptor activation as a result of movements of the TM helices, which are then relayed to the G protein-interacting intracellular loops. In the activated conformation, receptors display structural instability and enhanced conformational flexibility, as evidenced by the thermolability of constitutively active mutants (75). 

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