When viscosity is small but not negligible, boundary layers form. Batchelor's explanation of boundary layer theory and flow separation is widely considered one of the best ever written. How to Effectively Study Batchelor’s Fluid Dynamics
The final chapters on viscous flow and turbulence are where the book earns its legend. Batchelor introduces the concept of the Reynolds number not as a dimensionless group, but as a bifurcation parameter. His discussion of the laminar boundary layer is brief but profound. And then, the final chapter on turbulence—stopping precisely at the Kolmogorov spectrum—is a cruel cliffhanger. He gives you the tools, but refuses to hand you the solution. an introduction to fluid dynamics batchelor pdf
