We have re-examined the correlation of transition onset data with tunnel operating conditions by modeling transition onset location with two-dimensional response surfaces in terms of h_(res) and P_(res) variables. We observe a positive correlation of X_(tr) with h_(res) in both air and N_2 and a negative correlation of X_(tr) with P_(res). The parameter Re_(tr)~* exhibits a positive correlation with reservoir pressure in both gases. Controlling for variations in P_(res), no statistically significant dependence of Re~(tr)~* on h_(res) was found for either the present air or N_2 data or the historical air data in Adam and Hornung. We explore an alternative normalization of the transition onset location by the local laminar boundary layer thickness X_(tr)/δ_(.99) and find that this is essentially independent of the edge Mach number for a given gas type and pressure range (Figure 5). We examine the correlation of transition onset with the most amplified acoustic frequency in the boundary layer and find that lower frequencies correlate with smaller values of normalized transition distance X_(tr)/δ_(.99), suggesting that the frequency-dependent amplification of the tunnel noise may be responsible for the observed systematic variations in transition onset distance.
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