Research was undertaken with the purpose of determining the effect of the unit Reynolds number on boundary-layer transition under conditions where disturbances associated with wind tunnel flows would not be present. The location of boundary-layer transition was determined from shadowgrams of nominally sharp, 4° and 10°s emi-angle cones in an aeroballistic range at freestream Mach numbers of 23 and 5.0 and unit Reynolds numbers of 0.3 x 10 6 to 8 x 10 6 per in. Owing to constant and equal freestream and cone skirt temperatures, the average ratio of cone wall-to-adiabatic recovery temperature was 0.52 at Mach 2.3 and 0.19 at Mach 5.0. Features of free-flight experimentation that may be suspected of influencing boundary-layer transition were investigated. These included 1) oscillatory motion and finite angles of attack, 2) surface roughness, 3) vibration of the model, and 4) non-uniform (hot-tip) surface temperature. There was no evidence that any of these conditions influenced the major results. The data show local Reynolds number of transition increasing with unit Reynolds number for both Mach numbers. A siren was used to elevate the fluctuating sound pressure ratio by a factor of 200, but that produced no measurable effect on transition locations. Nomenclature c r = phase velocity of disturbance = velocity of wave propagation in freestream direction / = frequency of vibration of cone; also "function of" k = roughness depth profilometer reading M = Mach number N = number of cycles of vibration p --pressure p -fluctuating sound pressure amplitude Re = Reynolds number s = distance measured along surface from stagnation point T = temperature U = velocity a = total angle of attack a p = angle of attack in plane of photograph /? = characteristic angular frequency of disturbance spectrum y = ratio of specific heats 5 = boundary-layer thickness 6 = characteristic orientation of disturbance spectrum 9 C = cone semi-apex angle / = characteristic wavelength of disturbance spectrum fj, = absolute viscosity v = kinematic viscosity -n/p p = mass density (f > = orientation of a cone meridian relative to the windward stagnation line where 1fa= 0 a) = exponent in the approximation \i oc T C) Subscripts aw = adiabatic wall j = at station of temperature discontinuity o = total, e.g., total temperature; also designates a = 0 t = transition w = cone wall 6 = local flow parameter at outer edge of boundary layer oo = freestream a = denotes a ^ 0
