Perturbation by and recovery from bend curvature of a fully developed turbulent pipe flow

An experimental investigation was carried out to study the perturbation by and recovery from bend curvature of a fully developed turbulent pipe flow. The pipe Reynolds number and the pipe-to-bend radius ratio were 50 000 and 0.077, respectively. Hot-wire techniques were used to measure the three components of mean velocity and the six components of the Reynolds stress along a horizontal as well as along a normal plane in streamwise locations extending from 18 diameters upstream to 18 diameters downstream of the U-bend. Results show that a Dean-type secondary motion is established in the bend and is confirmed by a numerical modeling of the flow based on a two-equation closure. Evidence of a second cell is obtained within the bend and again verified by numerical results and previous total pressure measurements. Therefore it is postulated that the pressure field is also responsible for the second cell. The second cell disappears at about one diameter downstream of the curved bend. Turbulent kinetic energy is greatly enhanced by additional production due to extra strain rates created by bend curvature. Since viscous dissipation is the only way in which this extra energy can be destroyed, recovery from bend curvature takes a long distance.