  This study seeks to characterise the breakdown of the steady 2D solution in
the flow around a 180-degree sharp bend to infinitesimal 3D disturbances using
a linear stability analysis. The stability analysis predicts that 3D transition
is via a synchronous instability of the steady flows. A highly accurate global
linear stability analysis of the flow was conducted with Reynolds number
$Re<1150$ and bend opening ratio (ratio of bend width to inlet height)
$0.2\leq\beta\leq5$. This range of $Re$ and $\beta$ captures both steady-state
2D flow solutions as well as the inception of unsteady 2D flow. For
$0.2\leq\beta\leq1$, the 2D base flow transitions from steady to unsteady at
higher Reynolds number as $\beta$ increases. The stability analysis shows that
at the onset of instability, the base flow becomes three-dimensionally unstable
in two different modes, namely spanwise oscillating mode for $\beta=0.2$, and
spanwise synchronous mode for $\beta \geq 0.3$. The critical Reynolds number
and the spanwise wavelength of perturbations increase as $\beta$ increases. For
$1<\beta\leq2$ both the critical Reynolds for onset of unsteadiness and the
spanwise wavelength decrease as $\beta$ increases. Finally, for $2<\beta\leq5$,
the critical Reynolds number and spanwise wavelength remain almost constant.
The linear stability analysis also shows that the base flow becomes unstable to
different 3D modes depending on the opening ratio. The modes are found to be
localised near the reattachment point of the first recirculation bubble.
