I talked to Ana Bonaca (Harvard) and Lauren Anderson (Flatiron) about their projects in the morning. With Bonaca I discussed the computation of numerically stable derivatives with respect to parameters. This is not a trivial problem when the model (of which you are taking derivatives) is itself a simulation or computation. With Anderson we edited and prioritized the to-do list to finish writing the first draft of her paper.
At lunch time, Geoff Ryan (NYU) gave the CCPP brown-bag talk, about accretion modes for binary black holes. Because the black holes orbit in a cavity in the circum-binary accretion disk, and then are fed by a stream (from the inner edge of the cavity), there is an unavoidable creation of shocks, either in transient activity or in steady state. He analyzed the steady-state solution, and finds that the shocks drive accretion. It is a beautiful model for accretion that does not depend in any way on any kind of artificial or sub-grid viscosity.