Figure 2: Analogy between displacements of flow lines and smectic layers.
From: Mapping two-dimensional polar active fluids to two-dimensional soap and one-dimensional sandblasting
In the case of 2D incompressible polar active fluids, the field h(r) is the vertical displacement of the flow lines (that is, the solid lines) from the set of parallel lines (that is, the dotted lines) along 
that would occur in the absence of fluctuations. For a defect-free 2D smectic, it likewise gives the vertical displacement of the smectic layers (that is, the solid lines) from their reference positions (that is, the dotted lines) at zero temperature.
