  This paper exposes how to obtain a relation that have to be hold for all
free--divergence velocity fields that evolve according to Navier--Stokes
equations. However, checking the violation of this relation requires a huge
computational effort. To circumvent this problem it is proposed an additional
antsatz to free-divergent Navier--Stokes fields. This makes available six
degrees of freedom which can be tuned. When they are tuned adequately, it is
possible to find finite $L^2$ norms of the velocity field for volumes of
$\mathbb{R}^3$ and for $t\in[t_0,\infty)$. In particular, the kinetic energy of
the system is bounded when the field components $u_i$ are class $C^3$ functions
on $\mathbb{R}^3\times[t_0,\infty)$ that hold Dirichlet boundary conditions.
This additional relation lets us conclude that Navier--Stokes equations with
no-slip boundary conditions have not unique solution. Moreover, under a given
external force the kinetic energy can be computed exactly as a funtion of time.
