Quadrant Analysis of Reynolds Shear Stresses in a Turbulent Square-Duct Flow

Primary and secondary Reynolds shear stresses, -(u'v')^^- and -(v'w')^^-, have been measured with a two-component LDV in a fully developed turbulent square-duct flow. Quadrant analysis has been performed for these Reynolds stresses in order to demonstrate typical flow structures responsible for their generation. In the near wall region close to the wall bisector, the primary Reynolds stress is generated by streamwise vortices through both ejection and sweep motions in a similar manner to turbulent boundary layers and two-dimensional channel flows. In the near-wall region apart from the wall bisector, on the other hand, the contribution of a sweep motion to the primary Reynolds stress is significantly reduced. In the corner region, apart from the wall bisector, the secondary Reynolds shear stress is induced by fluid motions from one wall toward another wall via an antisymmetric streamwise vortex. These motions transport the streamwise momentum near one wall, which is lower than that close to the wall bisector because of the no-slip constraint by the wall, toward another wall. As a result, in the corner region relatively-low-speed fluids inrush toward the wall, which leads to the reduction of a sweep motion apart from the wall bisector.