LAMINARISATION AND RE-TRANSITION OF A TURBULENT BOUNDARY-LAYER SUBJECTED TO FAVORABLE PRESSURE-GRADIENT

Experimental results are reported for the response of an initially tur bulent boundary layer (Re-theta approximate to 1700) to a favourable p ressure gradient with a peak value of K equivalent to (- upsilon/rho U -E(3)) dp/dx equal to 4.4 x 10(-6). In the near-wall region of the bou ndary layer (gamma/delta < 0.1) the turbulence intensity u' scales rou ghly with the free-stream velocity U-E until close to the location whe re K is a maximum whereas in the outer region u' remains essentially f rozen. Once the pressure gradient is relaxed, the turbulence level inc reases throughout the boundary layer until K falls to zero when the ne ar wall u' levels show a significant decrease. The intermittency gamma is the dearest indicator of a fundamental change in the turbulence st ructure: once K exceeds 3 x 10(-6), the value of gamma in the immediat e vicinity of the wall gamma(s) falls rapidly from unity, reaches zero at the location where K again falls below 3 x 10(-6) and then rises b ack to unity. Although gamma is practically zero throughout the bounda ry layer in the vicinity of gamma(s) = 0, the turbulence level remains high. The explanation for what appears to be a contradiction is that the turbulent frequencies are too low to induce turbulent mixing. The mean velocity profile changes shape abruptly where K exceeds 3 x 10(-6 ). Values for the skin friction coefficient, based upon hot-film measu rements, peak at the same location as K and fall to a minimum close to the location where K drops back to zero.