HELICITY FLUCTUATIONS AND TURBULENT ENERGY-PRODUCTION IN ROTATING ANDNONROTATING PIPES

In this paper finite-difference second-order accurate direct simulatio ns have been used to investigate how the helicity density fluctuations change when a turbulent pipe rotates about its axis. In this case the rotation axis is parallel to the near wall vortical structures which play a fundamental role on the wall friction and turbulence production . The helicity density is the trace of the tensor gamma(ij)'=(upsilon( i)'omega(j)') whose elements form the components of v' x omega'. When the momentum equations are written in rotational form the turbulence e nergy production term splits into two parts, one related to the convec tion of the large scales and the other related to the energy cascade t o the small scales. From data of direct simulations the changes of the turbulent production term profile with the rotation have been explain ed by the pdf of the v' x omega' components. The links between the cha nges on the pdf of the v' x omega' and the modifications of the vortic al structures have been also investigated. The joint pdf of the dissip ation with the helicity density has shown that the dissipation is high ly correlated with regions of very low helicity density in the non-rot ating pipe. When the pipe rotates the helicity density increases and t he dissipation decreases. Since a drag reduction is one of the results of the background rotation, in this paper, it has been speculated tha t the alignment between velocity and vorticity could be a common featu re in drag reducing flows. (C) 1997 American Institute of Physics.