The position vector of a particle is $\overrightarrow{\mathrm{r}}=(\mathrm{a} \cos \mathrm{\omega t})\hat{\mathrm{i}}+(\mathrm{a} \sin \mathrm{\omega t})\hat{\mathrm{j}}$ .The velocity of the particle is :

$\overrightarrow{\mathrm{v}}=\frac{\mathrm{d}\overrightarrow{\mathrm{r}}}{\mathrm{dt}}=(-\mathrm{a\omega } \sin \mathrm{\omega t})\hat{\mathrm{i}}+(\mathrm{a\omega } \cos \mathrm{\omega t})\hat{\mathrm{j}}$
Here $\overrightarrow{\mathrm{v}}. \overrightarrow{\mathrm{r}} = 0$, so velocity of the particle is perpendicular to position vector.