THE EFFECTS OF A FAVORABLE PRESSURE-GRADIENT AND OF THE REYNOLDS-NUMBER ON AN INCOMPRESSIBLE AXISYMMETRICAL TURBULENT BOUNDARY-LAYER - PART1 - THE TURBULENT BOUNDARY-LAYER

The effects of a favourable pressure gradient (K less than or equal to 4 x 10(-6)) and of the Reynolds number (862 less than or equal to Re- delta 2 less than or equal to 5800) on the mean and fluctuating quanti ties of four turbulent boundary layers were studied experimentally and are presented in this paper and a companion paper (Part 2). The measu rements consist of extensive hot-wire and skin-friction data, The form er comprise mean and fluctuating velocities, their correlations and sp ectra, the latter wall-shear stress measurements obtained by four diff erent techniques which allow testing of calibrations in both laminar-l ike and turbulent flows for the first time. The measurements provide c omplete data sets, obtained in an axisymmetric test section, which can serve as test cases as specified by the 1981 Stanford conference. Two different types of accelerated boundary layers were investigated and are described: in this paper (Part 1) the fully turbulent, accelerated boundary layer (sometimes denoted laminarescent) with approximately l ocal equilibrium between the production and dissipation of the turbule nt energy and with relaxation to a zero pressure gradient flow (cases 1 and 3); and in Part 2 the strongly accelerated boundary layer with ' inactive' turbulence, laminar-like mean flow behaviour (relaminarized) , and reversion to the turbulent state (cases 2 and 4). In all four ca ses the standard logarithmic law fails but there is no single parametr ic criterion which denotes the beginning or the end of this breakdown. However, it can be demonstrated that the departure of the mean-veloci ty profile is accompanied by characteristic changes of turbulent quant ities, such as the maxima of the Reynolds stresses or the fluctuating value of the skin friction. The boundary layers described here are mai ntained in the laminarescent state just up to the beginning of relamin arization and then relaxed to the turbulent state in a zero pressure g radient. The relaxation of the turbulence structure occurs much faster than in an adverse pressure gradient. In the accelerating boundary la yer absolute values of the Reynolds stresses remain more or less const ant in the outer region of the boundary layer in accordance with the r esults of Blackwelder & Kovasznay (1972), and rise both in the vincini ty of the wall in conjunction with the rising wall shear stress and in the centre region of the boundary layer with the increase of producti on.