  The kinetic energy of the fluid shell in the cosmological first-order phase
transition is crucial for predicting the gravitational wave signals generated
by the sound wave mechanism. We propose a model-dependent method to calculate
the kinetic energy fraction by dividing the bubble-fluid system into three
distinct regions: the symmetric phase, the broken phase, and the bubble wall.
By solving the local equation of motion of the scalar field with a
phenomenological friction term, the bubble wall velocity and the boundary
conditions of the fluid equations of both phases can be derived simultaneously.
Then, for a given particle physics model, the fluid profiles of different
hydrodynamical modes and the corresponding kinetic energy fraction can be
obtained. Our method can also capture the temperature dependency of the sound
speed of the plasma. Compared with the conventional model-independent method,
our approach is based on an accurate equation of state derived directly from
the effective potential and takes into account the contribution of the bubble
wall to the energy-momentum tensor. Therefore, our method in-principle provides
a more consistent and accurate result, which is crucial for high-precision
calculations of the gravitational waves induced by the first-order phase
transition.
