  Water's density maximum at 4C makes it well suited to study internal gravity
wave excitation by convection: an increasing temperature profile is unstable to
convection below 4C, but stably stratified above 4C. We present numerical
simulations of a water-like fluid near its density maximum in a two dimensional
domain. We successfully model the damping of waves in the simulations using
linear theory, provided we do not take the weak damping limit typically used in
the literature. In order to isolate the physical mechanism exciting internal
waves, we use the novel spectral code Dedalus to run several simplified model
simulations of our more detailed simulation. We use data from the full
simulation as source terms in two simplified models of internal wave excitation
by convection: bulk excitation by convective Reynolds stresses, and interface
forcing via the mechanical oscillator effect. We find excellent agreement
between the waves generated in the full simulation and the simplified
simulation implementing the bulk excitation mechanism. The interface forcing
simulations over excite high frequency waves because they assume the excitation
is by the "impulsive" penetration of plumes, which spreads energy to high
frequencies. However, we find the real excitation is instead by the "sweeping"
motion of plumes parallel to the interface. Our results imply that the bulk
excitation mechanism is a very accurate heuristic for internal wave generation
by convection.
