Helmholtz instability

Helmholtz instabilities may cause drops to be formed by roll waves erupting from the film into the vapor core. 

An improved CHF model for low-quality flow The Weisman-Pei model was later improved by employing a mechanistic CHF model developed by Lee and Mu-dawwar (1988) based on the Helmholtz instability at the microlayer-vapor interface as a trigger condition for microlayer dryout (Fig. 5.21). The CHF can be expressed by the following equation due to the energy conservation of the microlayer (Lin et al. 1989) ... 

Kelvin, defined, 24 434-435 Kelvin, Lord, 24 433 Kelvin equation, 9 113 19 182 Kelvin-Helmholtz instability, 11 762-763, 765, 772... 

As described previously, in the atomization sub-model, 232 droplet parcels are injected with a size equal to the nozzle exit diameter. The subsequent breakups of the parcels and the resultant droplets are calculated with a breakup model that assumes that droplet breakup times and sizes are proportional to wave growth rates and wavelengths obtained from the liquid jet stability analysis. Other breakup mechanisms considered in the sub-model include the Kelvin-Helmholtz instability, Rayleigh-Taylor instability, 206 and boundary layer stripping mechanisms. The TAB model 310 is also included for modeling liquid breakup. 

The first simulations of the collapsar scenario have been performed using 2D Newtonian, hydrodynamics (MacFadyen Woosley 1999) exploring the collapse of helium cores of more than 10 M . In their 2D simulation MacFadyen Woosley found the jet to be collimated by the stellar material into opening angles of a few degrees and to transverse the star within 10 s. The accretion process was estimated to occur for a few tens of seconds. In such a model variability in the lightcurve could result for example from (magneto-) hydrodynamic instabilities in the accretion disk that would translate into a modulation of the neutrino emission/annihilation processes or via Kelvin-Helmholtz instabilities at the interface between the jet and the stellar mantle. 

Steady skirt lengths increase with Re (B3, H5, W2). Wairegi (W2) and Bhaga (B3) also reported skirt lengths which increased with time. The length of steady skirts is controlled by a balance of viscous and capillary forces at the rim of the skirt (B3), whereas the length of wavy skirts appears to be determined by growth of Helmholtz instability waves (H5). 

To test whether the Kelvin-Helmholtz instability, induced by convection, is capable of... 

However numerical simulations of early supernova-driven winds fail to find any evidence for substantial gas ejection from luminous ( L ) galaxies. One can ask what is wrong with the hydrodynamic simulations Certainly, the simulations lack adequate resolution. Rayleigh-Taylor instabilities enhance wind porosity and Kelvin-Helmholtz instabilities enhance wind loading of the cold interstellar medium. Both effects are certain to occur and will enhance the wind efficacity. Yet another omission is that one cannot yet resolve the motions of massive stars before they explode. This means that energy quenching is problematic and the current results are inconclusive for typical massive galaxies. 

Another role of the surfactant is to initiate interfacial instability, e.g., by creating turbulence and Raykleigh and Kelvin-Helmholtz instabilities. Turbulence eddies tend to disrupt the interface since they create local pressures. Interfacial instabilities may also occur for cylindrical threads of disperse phase during emulsification. Such cylinders undergo deformation and become unstable under certain conditions. The presence of surfactants will accelerate these instabilities as a result of the interfacial tension gradient. 

Presence of the imaginary part with negative sign implies temporal instability for all wave lengths. Also, to be noted that since the group velocity and phase speed in y-direction is identically zero, therefore the Kelvin-Helmholtz instability for pure shear always will lead to two-dimensional instability. 

Figure 11 Phenomena connected with drop breakup (a) deformation (splitting) of droplets, (b) deformation into lenticular shape, (c) development and separation of the boundary layer, (d) velocity distribution near the separation point of the boundary layer, (e) Kelvin Helmholtz instability, and (f) Rayleigh Taylor instability, pi < P2-...

The proposed approach will also represent an advance in multiphase fluid flow numerics. The numerical treatment of the interface will use the highly accurate approach of Nallapati [13]. The free surface will be a true interface, maintained coincidentally with cell faces on a deforming unstructured mesh. This approach allows the possibility of capturing the rapid velocity change near the interface that may play a pivotal role in spray behavior. This gradient controls the Kelvin-Helmholtz instability. 

The Kelvin-Helmholtz instability is thought to be a main driver in the early steps of atomization. This instability is driven by the lift forces on the interface because of the relative velocity between the two phases. When a numerical method smears velocities across the interface with insufficient resolution, the consequence may be a retardation of the predicted instability growth. Since the Kelvin-Helmholtz instability is so important, the numerical method should be capable of capturing this effect accurately. 

It is anticipated that the results of calculations will show the governing mechanisms of primary atomization. They will indicate the relative importance of turbulence, the Kelvin-Helmholtz instability, the Rayleigh-Taylor instability, the initial perturbation level (attributable to cavitation or oscillations in fuel injection equipment), and other phenomena. The quantitative detail of the simulations will provide information and inspiration for the construction of a new generation of spray models. The proposed code can be used for other kinds of simulations, including wall impingement, liquid film flow, and impinging injections. 

Schmidt There is not much data available. Perhaps we will use Kelvin-Helmholtz instability. 

Eventually, the velocity of vapor in the jets becomes so large that the jets themselves become unstable near the interface as a result of Helmholtz instability (of wavelength XH) as shown in Fig. 15.63). The breakup of the jets destroys the efficient vapor-removal mechanism, increases vapor accumulation at the interface, and leads to liquid starvation at the surface and to the critical heat flux phenomenon. If jet breakup occurs at a vapor velocity UH within the jets, the critical heat flux q"m is given by... 

Figure 10. One frame from the calculation of the evolution of a Kelvin-Helmholtz instability at the surface of a round jet of air into an air background. Cylindrical symmetry -was used and the axis of symmetry is. the left-hand bo mdary. The inflow speed is about 10 cm/s, and the outer co-flow is about 10 cm/s.

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