## 5. Results of models with 20% velocity gradientModels KM, KS, LM and LS break the axisymmetry very strongly
since they have as boundary condition a 20% velocity gradient over one
accretion radius. This asymmetry induces a strong clockwise vortex
around the accretor as can be seen in Fig. 7. The sign of the
velocity gradient has been chosen in such a way that upstream of the
accretor the higher velocities are on the negative
## 5.1. Moderately supersonic accretors, Mach 3The accretion rates of several quantities for models KS and KM can be found in Fig. 8. Contrary to model IM, model KM does not exhibit such strong fluctuations, thus it never even comes close (by a factor 3) to the analytically predicted value (-0.9) of Eq. (7), nor does it ever come close to the zero line. The large scale motion of the vortex around the accretor is fairly stable in time, which explains why the fluctuations relative to the mean of the specific angular momentum are smaller in model KM than in model IM.
The fluctuations of the mass accretion rate (left panels of Fig. 8 of model KM seems to increase with time indicating that this model is still evolving in time and has not yet reached a steady mean state. These panels should be compared to the analogous panels (left in Fig. 4 for the models IM and IS. One notices that both the mean and the amplitude of the fluctuations is larger in model KM than in model IM Although the mean mass accretion rate of model KS is lower than the mean in model IS this might be a transient effect, since toward the end of model IS the mass accretion rate is of the same magnitude as in model KS. Thus for the larger accretors (M-models) the vortex allows more mass to be accreted, while for the small accretors (S-models) the difference is negligible. Both models KM and KS have one difference in common compared to model IM and IS respectively: the unstable flow, which manifests itself e.g. via a fluctuating mass accretion rate, begins much faster for the models with a large gradient, models KM and KS. Also the large-scale fluctuations of the specific angular momentum appear at roughly in models KM and KS, while it takes until in models IM and IS. ## 5.2. Highly supersonic accretors, Mach 10The accretion rates of several quantities for the highly supersonic models LM and LS are shown in Fig. 9. The fluctuation amplitude of the mass accretion rate of model LM is larger than both the amplitudes of model JM (top left panel in Fig. 6) and of model FM (top left panel of Fig. 9 in Ruffert 1994), however the mean seems roughly equal. On the whole model LM looks more unstable and active than the models JM and FM with small or no gradients. The mass accretion rate of model LS does not seem to decline constantly during the first two time units as was the case for model JS.
The trend that the model with the smaller accretor (in this case
model LS compared to model LM) has the © European Southern Observatory (ESO) 1997 Online publication: July 8, 1998 |