.. _Gas: Gas disk ======== The gas disk is implemented according to :cite:p:`Morishima2010`. It supports gas drag, Type I migration and drag enhancement for small particles. The parameters for the gas disk can be set with the following arguments: In the :ref:`param.dat` file - :literal:`Use gas disk` - :literal:`Use gas disk potential` - :literal:`Use gas disk enhancement` - :literal:`Use gas disk drag` - :literal:`Use gas disk tidal dampening` - :literal:`Gas dTau_diss` - :literal:`Gas disk inner edge`: - :literal:`Gas disk outer edge`: - :literal:`Gas disk grid outer edge`: - :literal:`Gas disk grid dr`: - :literal:`Gas Sigma_10` - :literal:`Gas alpha` - :literal:`Gas beta` - :literal:`Gas Mgiant` - :literal:`Gas file name` In the :ref:`define.h` file - :literal:`def_Gasnz_g`: Number of cells in z direction for gas grid - :literal:`def_Gasnz_p`: Number of cells in z direction for particle grid - :literal:`def_h_1`: scale height at 1AU for c = 1km/s - :literal:`def_M_Enhance`: factor for enhancement - :literal:`def_Mass_pl`: factor for enhancement - :literal:`def_fMass_min`: factor for enhancement - :literal:`def_Gas_cd`: numerical gas drag coefficient Gas disk structure ------------------ The gas disk structure is implemented as a uniform disk in space, which decays exponentially in time (:cite:p:`Morishima2010`). .. math:: :label: eq_Gas1 \Sigma_{gas}(r,t) = \Sigma{gas,0} \left( \frac{r}{1 AU} \right)^{-\alpha} \exp \left( - \frac{t}{\tau_{decay}} \right), with the gas surface density at 1 AU :math:`\Sigma{gas,0}`, the dissipation time :math:`\tau_{decay}` and the power law exponent :math:`\alpha`. The scale height of the gas disk is set to .. math:: :label: eq_Gas2 h(r) = h_0 \frac{r}{1 AU} \left( \frac{r}{1 AU} \right)^{\beta}, with the scale height at 1AU :math:`h_0`. Gas disk physical range in r ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The range of the gas disk can be set with the :literal:`Gas disk inner edge`: and :literal:`Gas disk outer edge`: parameters. Outside of these boundaries, the gas disk density is 0. Gas disk grid ^^^^^^^^^^^^^ In order to calculate the gas disk gravitational effect on the particles, the gas disk gravitational force in r and z is tabulated and stored in a gas disk grid. While the tabulated values of the grid respect the entire gas disk, ranging from the inner edge to the outer edge, the gas disk itself can have a smaller range in r. This is especially useful, when the gas disk extends a broader range than the particles. Therefore, the outer range of the gas disk grid can be set by a different parameter :literal:`Gas disk grid outer edge`:. The inner edge of the gas disk grid corresponds to the physical inner edge of the disk, :literal:`Gas disk inner edge`: The spacing of the gas disk grid in r can be set with the parameter :literal:`Gas disk grid dr`. When a particle is located outside of the gas disk grid, then the effect of the gas disk potential on the particles is not applied by using the tabulated values, but with a simpler approach according to Ward(1981). Gas drag force -------------- The gas drag force is enabled with the :literal:`Use gas disk drag` parameter. The gas drag force is implemented as .. math:: :label: eq_Gas3 \mathbf{F}_{drag} = - \frac{1}{2m} c_D \pi r^2 \rho_{gas} | \mathbf{v}_{rel} | \mathbf{v}_{rel}, with the radius of the particle :math:`r` and the mass of the particle :math:`m`. The numerical coefficient :math:`c_D` is set to 2 (:cite:p:`Morishima2010`). The value of :math:`c_D` can be changed in the :ref:`define.h` file. Gas disk file ------------- When a gas disk file name is specified in the :literal:`Gas file name` parameter. Then the gas disk structure is red from this file. The file must contain the following columns:: time0 r Sigma h time1 r Sigma h . . . with: - time in years. - r the distance from the cell to the star in AU. - Sigma, the surface density at the cell location r,in in g/:math:`\text{cm}^3`. - h, the gas disk scale height at the cell location r, in AU.