2010 Theodore Yaotsu Wu, "A Review on Fish Swimming and Bird/Insect Flight" arxiv
Here, γ₀(ξ, t) is the vortex bound to S_b in the wakeless ”quasi-steady” flow past S_b which can be determined by steady airfoil theory for the original prescribed Uₙ(ξ, t), with time t frozen to serve merely as a parameter for the moment.