Laminarisation and re-transition of a turbulent boundary layer subjected to favourable pressure gradient

 Experimental results are reported for the response of an initially turbulent boundary layer (Reθ≈1700) to a favourable pressure gradient with a peak value of K≡(−υ/ρU3E) dp/dx equal to 4.4×10-6. In the near-wall region of the boundary layer (y/δ<0.1) the turbulence intensity u′ scales roughly with the free-stream velocity UE until close to the location where K is a maximum whereas in the outer region u′ remains essentially frozen. Once the pressure gradient is relaxed, the turbulence level increases throughout the boundary layer until K falls to zero when the near wall u′ levels show a significant decrease. The intermittency γ is the clearest indicator of a fundamental change in the turbulence structure: once K exceeds 3×10-6, the value of γ in the immediate vicinity of the wall γs falls rapidly from unity, reaches zero at the location where K again falls below 3×10-6 and then rises back to unity. Although γ is practically zero throughout the boundary layer in the vicinity of γs=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 3×10-6. Values for the skin friction coefficient, based upon hot-film measurements, peak at the same location as K and fall to a minimum close to the location where K drops back to zero.