# Reference: Michael M. Kostreva, # "Elasto-Hydrodynamic Lubrication: a Non-linear Complementarity # Problem", Int. Journal for Num. Methods in Fluids (4), 377-397 (1984). # The lubricant film gap and the pressure between two lubricated # elastic cylinders in line contact are calculated. When the # pressure is positive, Reynolds's equation must be satisfied; # when the pressure is 0, the surfaces must diverge. # The load (in pounds) is represented by alpha. The speed # (in rpm) of the cylinders is represented by lambda. A more # elaborate version of this model would involve a table of # (alpha, lambda) pairs. # In this variant, h is a "defined variable", which AMPL automatically # substitutes out: the solver does not see it. param N integer >= 1, default 100; # grid 0..N; # half-points go 1 .. N param pi := 4 * atan(1); param xa := -3; param xf > xa, := 2; param dx := (xf - xa) / N; param alpha := 2.832; param lambda := 6.057; param w {i in 0..N} := if i in {0,N} then 0.5 else 1; var k := 1.6; var p {i in 1..N} >= 0, := max(0, 1 - abs((xa +1 + i*dx)/2)); var h {j in 0..N} = ( xa + (j+.5)*dx)^2 + k + 1 + 1/pi * (sum {i in 0..N} w[i] * (i-j-0.5)*dx * log(abs(i-j-.5)*dx) * ((if i < N then p[i+1]) - (if i > 1 then p[i-1]) ) ) ; s.t. psum: ( 1 - dx*2/pi * sum {i in 1..N} w[i]*p[i] ) == 0; s.t. reynolds {i in 1..N}: p[i] >= 0 complements ( (lambda / dx) * (h[i]-h[i-1]) - (1/dx^2) * ( h[i]^3 * ((if i < N then p[i+1])-p[i]) / exp(alpha*((if i < N then p[i+1])+p[i])*.5) - h[i-1]^3 * (p[i]-(if i > 1 then p[i-1])) / exp(alpha*(p[i]+(if i > 1 then p[i-1]))*.5) ) ) >= 0;