Wave number selection and large-scale-flow effects due to a radial ramp ofthe spacing in Rayleigh-Benard convection

We present experimental results for wave numbers q(s) selected in a thin ho rizontal fluid layer heated from below. The cylindrical sample had an inter ior section of uniform spacing d = d(o) for radii r < r(o) (Gamma(o) = r(o) /d(o) = 43) and a ramp d(r) for r > r(o). For Rayleigh numbers R-o > R-c = 1708 in the interior, straight or slightly curved rolls with an average [q( s)] = (q) over tilde(c) + alpha epsilon(o)(epsilon(o) = R-o/R-c - 1) and (q ) over tilde(c) < q(c) = 3.117 were selected, and q(s) varied on two length scales approximately equal to Gamma(o) and to four roll wavelengths. For e psilon less than or similar to 0.03 and epsilon less than or similar to 0.1 8 the pattern repeatedly formed defects.