The velocity components determined from derivatives of stream function ψ (or velocity potential Φ) are linear in stream function.
As a consequence, sum of two stream functions, ψ1 and ψ2, is also a stream function and describes a velocity field which is vector sum of the individual fields.
For example, considering two plane flow combinatin
If,  | |
and,  |
then taking 
for the combination
for the combination | |
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where,  | |
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