B3.24

Fluid Dynamics II
Part II, 2004

Using the Milne-Thompson circle theorem, or otherwise, write down the complex potential ww describing inviscid incompressible two-dimensional flow past a circular cylinder of radius aa centred on the origin, with circulation κ\kappa and uniform velocity (U,V)(U, V) in the far field.

Hence, or otherwise, find an expression for the velocity field if the cylinder is replaced by a flat plate of length 4a4 a, centred on the origin and aligned with the xx-axis. Evaluate the velocity field on the two sides of the plate and confirm that the normal velocity is zero.

Explain the significance of the Kutta condition, and determine the value of the circulation that satisfies the Kutta condition when U>0U>0.

With this value of the circulation, calculate the difference in pressure between the upper and lower sides of the plate at position x(2ax2a)x(-2 a \leq x \leq 2 a). Comment briefly on the value of the pressure at the leading edge and the force that this would produce if the plate had a small non-zero thickness.

Determine the force on the plate, explaining carefully the direction in which it acts.

[The Blasius formula FxiFy=iρ2C(dwdz)2dzF_{x}-i F_{y}=\frac{i \rho}{2} \oint_{C}\left(\frac{d w}{d z}\right)^{2} d z, where CC is a closed contour lying just outside the body, may be used without proof.]