An Experimental Investigation of an Accelerated Turbulent Boundary Layer

The experimentally determined response of the time-averaged turbulence of a $Re_{ heta}=4590$ boundary layer exposed to a large favorable pressure gradient is presented. The favorablepressure gradient is generated using a linear contraction with a contraction ratio of $9.5:1$ and is characterized by a peak acceleration parameter, $K_{max}=4.4 imes10^{-6}$. Threecomponents of velocity are measured using multiple X-wires and the wall shear stress is directly measured using oil film interferometry. The accelerated boundary layer is found to exhibit behavior typical of extit{relaminarization} for values of $K geq 3.0 imes 10^{-6}$. The onset of reverse transition is believed to coincide with the modification of the near-wall structure and its eventual propagation into the downstreamboundary layer. Modification of the normal stresses in the near-wall region, large-scale changes in the skewness and kurtosis of the velocity components, a change in integral length scales of the accelerated boundary layer, and a shift in the energy spectrum each lend support to this assertion. Finally, a conceptual model of the motions of the coherent structures in the accelerated turbulent boundary layer is shown to be consistent with the measured data.