How to Calculate for Damping Coefficients

General Discussion
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I am running some simulations on various ROVs including the BlueROV2 in MATLAB and in Gazebo. The model I am basing my approach on is Fossen’s Marine Craft model.

For this model to work, one of the properties I need is the linear and quadratic damping coefficients for each of the six (6) degrees of freedom of the ROVs (i.e. surge, sway, heave, roll, pitch, yaw). For quadratic damping, I am using the drag equation below:

F_d = \frac{1}{2}\rho c_d A u_{x}^{2} \\ where: \ X_{uu} = \frac{1}{2}\rho c_d A

This is done for each degree of freedom. I am currently experimenting with using the default C_d value for cubes and cuboids (i.e. 1.05), and deriving a C_d using SolidWorks Flow simulation.

For linear damping however, I am unable to find any equation or method of deriving its drag coefficient. The closest thing I have seen from research is using Stokes Drag equation for tiny spherical objects at low speeds, which ROVs are not.

I want to know how the linear drag coefficients are determined for the BlueROV2 and other ROVs. I am okay with either using an approximation, CFD with SolidWorks Flow Simulation, or experimentation. Also correct me if I am wrong on the calculation of quadratic drag.