  A vapor bubble collapsing near a solid boundary in a liquid produces a liquid
jet that points toward the boundary. The direction of this jet has been studied
for boundaries such as flat planes and parallel walls enclosing a channel.
Extending these investigations to enclosed polygonal boundaries, we
experimentally measure jet direction for collapsing bubbles inside a square and
an equilateral triangular channel. Following the method of Tagawa and Peters
(Phys. Rev. Fluids 3, 081601, 2018) for predicting the jet direction in
corners, we model the bubble as a sink in a potential flow and demonstrate by
experiment that analytical solutions accurately predict jet direction within an
equilateral triangle and square. We further use the method to develop
predictions for several other polygons, specifically, a rectangle, an isosceles
right triangle, and a $30^{\circ}$-$60^{\circ}$-$90^{\circ}$ right triangle.
