Fix some $f\in H^1(\partial (0,1)^d)$.
Let $\eta\ge 0$ and for each such $\eta$ consider the solution $u^{\eta}$ to the solution PDE $$ \begin{cases} \Delta u & = u_t - \eta u_{tt} \mbox{ on } (0,1)^d \\ u& =f \mbox{ on } \partial (0,1)^d. \end{cases} $$

Are there any stability bounds for $$ \|u^{\eta}-u^0\|_{L^2(0,1)^d}\le \mbox{ some function of }\eta \mbox{ and of } \|f\|_{H^2(\partial (0,1)^d)}? $$