 displac thick displac thick altern theoret treatment 'displac thick' gener influenc boundari layer wake flow set two-dimension three-dimension laminar turbul incompress flow call method 'flow reduct' 'equival sourc' 'veloc comparison' princip express displac thick three-dimension flow written orthogon coordin posit surfac choos veloc potenti extern flow coordin constant extern-flow streamlin dy distanc distanc surfac compon veloc take boundari layer 