As we know, if an airfoil oscillates in a fluid, pressure boundary condition on airfoil surface can be calculate by dp/dn=Du/Dt.n. if flow is viscous, acceleration of fluid on airfoil surface is aqual to accelaration of airfoil. how can we calculate acceleration of fluid on airfoil surface if flow is invicsid?

You asked this already..You are either prescribing the airfoil motion in which case you have a explicit expression for u(t) or you are doing a coupled simulation which you are computing u(t) using a set of structural equations of motion. Either way you have u(t)...

Since this is a potential flow, there is a concept called "Apparent mass" to calculate acceleration of the fluid due to a body in motion. This is basically calculating the change in kinetic energy of the fluid.

for further assistance, go through Mechanics of Fluids by Currie.

He said it was inviscid..not inviscid,incompressible and irrotational. Sounds to me like he's solving the Euler equations. This isn't rocket science..his/her airfoil is oscillating ie he/she has u(t) on the surface so he/she can differentiate analytically or numerically to get the acceleration.

Oh..yeah i think I misunderstood it as a potential flow. But apparent mass is not restricted to Rocket Science.Its very well used in Fluid flow.

I think your suggestion is valid.

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