tokens: with other studies where an agreement with Shakura Sunyaev ( 1973 ) is reported in appropriate regimes. This is seen, for instance, in the limit of weak wind-driving in the models of Scepi et al. ( 2019 , see their Figure 2) or the outer regions of the semiglobal simulations of Hogg Reynolds ( 2018 ). We note that the level of alignment with the Shakura Sunyaev ( 1973 ) models may change with a more realistic treatment of the system. On one hand, radiative cooling (planned for our future work; see Section 5 ) will introduce nonlocal interactions that may break the locality assumption of α -models. On the other, accretion disks in astrophysical semidetached binaries are colder and geometrically thinner than those presented here (as discussed in Section 1.2 ), which alleviates some of the tension related to large H / R ratio, bringing true accretion disks closer to the Shakura Sunyaev ( 1973 ) solution. 4.3.1. Spiral Structure Drives Accretion at Very Low Mach Numbers As we discussed in Section 1.2 , numerical resources limit our global models to accretion disks that are hotter (of M in = 5, 10) than real astrophysical systems (e.g., CVs, where M in ≳ 50). While the Mach 5 model is unlikely to be realized in nature, it can provide a useful context for the Mach 10 data. With this in mind, we use Figures 7 and 9 to explain the mechanisms driving accretion in our hot-disk model Mach 5. As seen in Figure 9 , the body of the disk (solid brown line, Θ ≲ 2.5 H / R ) dominates the accretion rate for most of the grid. Its has a minimum at r ∼ 0.15, where its impact becomes equal to that of the “corona” (dashed brown line, Θ ∈ [2.5, 5] H / R ), the latter’s being maximal at r ∼ 0.15. This radius also corresponds to the transition from a two-peaked to a triangle-like vertical profile in Maxwell stress (Figure 7 ; see Section 4.2 ), a likely related observation. In Figure 9 we see that Reynolds stress dominates over Maxwell stress both within the main body and the “corona.” In the former, the difference is about a factor of 2–3, comparable with the MHD model of Ju et al. ( 2016 ). The radial velocity slice in Figure 6 associates this Reynolds-stress-driven accretion with the grand-design spiral structure, pointing to it as the main accretion driver in Mach 5. 4.4. Long-term Evolution of the Mach 10 Model Within our current computational limits on the Mach 10 simulation’s run time, the disk is not seen to reach a statistically steady state. However, as we are only able to study a single viscous timescale of the disk (see Section 4.1.1 ), it is entirely possible that such a state would be achieved if the model was allowed to continue. Still, the long timescale (∼10 P orb ) variability observed in our Mach 10 model may be of interest for understanding disk accretion. Not only is it potentially present during a change in disk accretion rate, but there is some indication that it may be recurring in nature (see Section 4.1.1 ). In this section, we attempt to
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