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d’Alembert’s Paradox: ν → 0 Is Not ν = 0 In 1752, Jean-le-Rond d’Alembert proved a result that still trips people up: An inviscid, incompressible, steady flow exerts zero drag on a body. No wake. No resistance. The equations let the fluid slip past as if the object weren’t there. The first animation shows exactly that world. The flow is ideal Euler flow. Streamlines bend around the body, accelerate, slow down, then recombine perfectly downstream. Nothing is left behind. The motion you see comes from pathlines moving through a steady velocity field, not from any evolving structure in the flow itself. The setup is the ideal fluid model: Euler (ν = 0) ρ(∂u/∂t + (u·∇)u) = −∇p ∇·u = 0 Assume steady flow and zero viscosity and the picture locks in. If the flow is also irrotational, ∇×u = 0, you can write u = ∇φ and the problem collapses to potential flow: ∇²φ = 0 u = ∇φ The force on the body comes entirely from pressure: F = −∮ p n dS D = F·eₓ Under these assumptions the pressure field is perfectly front–back symmetric, so the integral gives D = 0 That’s the paradox. Not a small correction. Zero. Now look at the second animation. This is the same geometry and the same inflow, but with viscosity turned on, even if it’s only a small amount. Navier–Stokes (ν > 0) ρ(∂u/∂t + (u·∇)u) = −∇p + μ∇²u ∇·u = 0 That extra term changes everything. A thin boundary layer forms near the surface. Separation becomes possible. Vorticity is generated and shed. A wake appears. Drag is no longer optional. The contrast is the point. Letting viscosity go to zero is not the same thing as setting it to zero. The inviscid limit deletes the mechanism that breaks time-reversal symmetry and allows energy dissipation. Once that mechanism is gone, wakes can’t exist, and drag vanishes by construction. #FluidDynamics #NavierStokes #EulerEquations #DAlembertParadox #BoundaryLayer #Physics

Mathelirium

27,053 次观看 • 5 个月前