Single Time Step in IB Method
•Given Xni evaluate force Fni on boundary marker i due to other material points for all i=1, Nparticles  in material (Lagrangian).
•Spread Fni  to Eulerian grid using Delta-function [for each layer if multiple layers]. Gives fn, the force on the fluid due to the boundary/material points.
•Solve forced Navier-Stokes equations to update velocity field to time level n+1, thus giving un+1
• at all Eulerian grid points.
•Gather Eulerian velocity field values surrounding a given particle using Delta-function. Gives Un+1i .
•Use No-Slip condition to push boundary points to new values, so Xn+1i  emerges.
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