Equation Icon  ∂2ξ + 2vj∇ ∂ξ + (vj∇ )2ξ + ∇ δΦ + ξj∇ ∇ Φ − (∇ ξj)∇ ˜μ + ∇ Δ ˜μ = 0. (235 ) t i j ti j i i i j i j i