| The present study deals with (i) coherent structures within plane turbulent wakes generated by a porous body (a mesh strip of 60% solidity) and a solid body (a flat plate) and (ii) the uniform distortion of the coherent structures within the porous-body wake. Wind tunnel experiments and multi-point hot-wire measurements were carried out. Spectral analysis of velocity signals was used to obtain the basic characteristics of the flows. Pattern-recognition and conditional sampling techniques were employed to extract ensemble-averaged statistical properties of the coherent structures. A discrete-vortex numerical simulation was performed to gain insight into the formation region of the porous-body wake.;In a flow region very close to the mesh strip, the present results establish that the porous-body wake is characterized by the two thin shear layers at the edges of the wake and a turbulent buffer region in the core. In the near region of the flow, spanwise large-scale vortices, with vorticity axes parallel to the wake generator, and lateral coherent vortices, with vorticity axes roughly perpendicular to the spanwise structures and inclined in a lateral/streamwise direction, are found to exist. Pattern-recognition and spectral analysis data indicate that the spanwise vortices are essentially two-dimensional, quasi-periodic Kaman-like coherent structures, which are generated from the interaction and pairing of small-scale eddies in the shear layers. As the flow evolves downstream, these structures gradually become less periodic, and some of their energy is transferred to the lateral structures.;A fine-scale turbulence indicator was used to study the relation between the spanwise and lateral structures. From the results, it is inferred that, in the near wake region, the lateral structures are "rib-like" vortices which connect the spanwise vortices. In the far-wake region, the original spanwise structures develop "legs" which merge with the "ribs" so that the resulting coherent structures are essentially randomly distributed hairpin-like vortices.;A comparison of the data for the porous-body wake and solid-body wake shows clearly that, in the near regions, the entrainment processes for the two flows are significantly different. With respect to the porous-body wake, the existence of the buffer region in the core of the wake results in the entrainment occurring mainly along the edges (or interface) of the flow, and an "interface-layer" type of entrainment process is proposed for this wake. In contrast, with respect to the solid-body wake, engulfment of free-stream fluid by the spanwise coherent structures causes the entrainment to occur within the core region of the wake, so that an "engulfment" type of entrainment process characterizes this flow.;Finally, on the basis on comparative study of the uniformly distorted porous-body wake and the undistorted porous-body wake, it is found that, as a consequence of vortex stretching, the distortion enhances the strength and prolongs the life of the spanwise coherent structures within the distorted wake. |
