Julia_influenceMatrix

function influence_coeff(surf::TwoDSurf,curfield::TwoDFlowField,coll_loc,n,dt)
    # With the surface and flowfield, determine the influence matrix "a"
    a = zeros(surf.ndiv,surf.ndiv)
    a[end,:] .= 1. # for wake portion
    vc = surf.bv[1].vc

    if size(curfield.tev,1) == 0 # calculate TEV placement location
        xloc = surf.bv[end].x + 0.5*surf.kinem.u*dt*cos(surf.kinem.alpha)
        zloc = surf.bv[end].z + 0.5*surf.kinem.u*dt*sin(surf.kinem.alpha)
    else
        xloc = surf.bv[end].x + (1. /3.)*(curfield.tev[end].x - surf.bv[end].x)
        zloc = surf.bv[end].z + (1. /3.)*(curfield.tev[end].z - surf.bv[end].z)
    end

    for i = 1:surf.ndiv-1, j = 1:surf.ndiv
        # i is test location, j is vortex source
        t_x = coll_loc[i,1]
        t_z = coll_loc[i,2]

        if j < surf.ndiv # Bound vortices (a_ij)
            src = surf.bv[j]
            xdist = src.x .- t_x
            zdist = src.z .- t_z
        else # Wake vorticy (a_iw)
            xdist = xloc .- t_x
            zdist = zloc .- t_z
        end

        distsq = xdist.*xdist + zdist.*zdist
        u = -zdist./(2*pi*sqrt.(vc*vc*vc*vc .+ distsq.*distsq))
        w = xdist./(2*pi*sqrt.(vc*vc*vc*vc .+ distsq.*distsq))

        a[i,j] = u*n[i,1] + w*n[i,2]
    end
    return a
end