ClearAll["Global`*"]
X = 0.0105;
Y = 1.0823 10^-4;
Z = 0.0241;
B = 2;
\[Eta] = 0.4;
f[\[Xi]_, \[Alpha]w_] :=
Piecewise[{{B/(
2 \[Eta]) ((X + Y \[Alpha]w^2 )/(Z \[Alpha]w) -
Tan[\[Xi]]), -(\[Pi]/
2) < \[Xi] < -ArcTan[(X + Y \[Alpha]w)^2/(
Z \[Alpha]w)]}, {B/ \[Eta] (X + Y \[Alpha]w^2 )/(
Z \[Alpha]w), -ArcTan[(X + Y \[Alpha]w)^2/(
Z \[Alpha]w)] < \[Xi] < 0}}] /; 0 < \[Alpha]w < 20
Plot3D[f[\[Xi], \[Alpha]w], {\[Xi], -(\[Pi]/2), 0}, {\[Alpha]w, 0,
20}]