Ecoulements turbulents pulses en presence d'un gradient de pression adverse.

Velocity and wall shear stress measurements in periodic turbulent flows with adverse pressure gradients in a conical diffuser (air tunnel) and in a diverging 2D channel (water tunnel) with a total divergence angle of 6 are reported. The measurement techniques included: hot-wire/film anemometry, particle image and laser-Doppler velocimetry. The method of phase-averaging was used to separate the time-mean, oscillating and turbulent components of various quantities. The influence of 3 different pressure gradients was investigated. The imposed oscillation period in the conical diffuser flow was 0,9s and the amplitudes were 0,1 and 0,4times the inlet centerline velocity. 2 periods, 8s and 48 s were imposed to the flow in the diverging water channel. Wall shear stress measurements were made for 3 amplitudes and 6 periods. The time-mean quantities, except the mean wall shear stress, were only slightly affected by the imposed oscillations whatever their period and amplitude were. At long periods, the oscillating flow is strongly influenced by the turbulence whereas at short periods, it differs little from the viscous oscillating Stokes flow. The modulations of the streamwise and cross-stream turbulent intensities and of the Reynolds shear stress become weaker near the wall at higher frequency. No significant influence of imposed amplitude was detected. In these cases of adverse pressure gradient, the turbulent activity is enhanced with respect to that observed in parallel channel flow, but the changes are not dramatic.Available from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi