The atomization characteristics of sheets formed by both laminar and turbulent impinging jets were experimentally studied as a function of flow and injector geometric parameters. In particular, sheet breakup length along the sheet centerline, distance between adjacent waves apparent on the sheet, and drop-size distributions were measured over a Weber number range between 350-6600 and a Reynolds number range between 2.8 x 10 3 to 2.6 x 10 4 . A linear stability-based model was used to determine the most unstable wave number and the corresponding growth rate factor on two-dimensional thinning inviscid and viscous sheets. These wave characteristics were used to predict both the sheet breakup length and the resulting drop sizes. A second model, applicable for a low Weber number regime, in which sheet disintegration is controlled by stationary antisymmetric waves, was used to predict the shape of the sheet formed by two impinging liquid jets. The linear stabilitybased theory predictions of breakup length did not agree in trend or magnitude with experimental measurements. However, for Weber numbers less than 350, the measured breakup length for laminar impinging jets was within 50% of that predicted by the stationary antisymmetric wave-based model. Finally, drop-size predictions based on linear stability theory agreed in trend, but not in magnitude, with the measured drop sizes. The contrast between the sheet atomization characteristics of laminar vs turbulent impinging jets suggest that the initial conditions of the impinging jets significantly influence the sheet breakup mechanism. Also, the comparison between experimental results and theoretical predictions indicates that the impact wave generation process at the jet impingement point needs to be incorporated in the theoretical models for sheet atomization. Nomenclature d = diameter F = thickness distribution h = sheet thickness k = wave number L = length of injection element / = length r = radial distance from impingement point Re = Reynolds number, Ujdjv/, based on liquid properties, jet velocity, and orifice diameter Re x = Reynolds number, U s hlvi, based on liquid properties, sheet velocity, and sheet thickness 5 = ratio of gas density to liquid density t = time U = velocity W = maximum width of sheet We = Weber number, piUjdJcr, based on liquid properties, jet velocity, and orifice diameter We s = Weber number, p,£/;/z/cr, based on liquid properties, sheet velocity, and sheet thickness x = axial distance from impingement point y = coordinate perpendicular to x in the plane of the sheet a = fan inclination angle ft = complex growth rate factor, p r + //3, 77 = disturbance amplitude 6 = impingement half-angle A = wavelength ju = dynamic viscosity v = kinematic viscosity TT = pi, 3.14159 p = density a = surface tension = angular coordinate on sheet Subscripts b D e § i i L I m nd 0 r s sw 10 30 = breakup = drop = edge = gas = imaginary = jet = ligament = liquid = maximum = nondimensional = orifice or initial = real = sheet = surface wave = arithmetic = volume

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Pulsed ultrasound propagating in water was used at megahertz carrier frequencies (nominally 10-50MHz) to reflect and scatter from rough surfaces in the same way as light. We have considered noncontact ultrasonic techniques as complementary to optical techniques in several ways: (a) for specificapplications such as wet surfaces, (b) for rougher surfaces with average roughness, R(a) ≥ 0.1 µm, and (c) for (simultaneous) profilometry by time-of-flight measurements. Stylus and ultrasonic data are compared. An example of application to the manufacturing environment is for on-line, real-time sensor feedback and process control in the cutting or grinding of metals and ceramics.

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