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Turbulence in a channel flow with a fully transpired wall has been investigated experimentally. Two-dimensional particle image velocimetry is used to measure the instantaneous flowfields within the channel for two different porous surfaces: a 6.35-mm honeycomb and a 3.175-mm honeycomb. The 6.35-mm honeycomb creates a larger fluctuation of the injection velocity than the 3.175-mm honeycomb by a factor of 1.7. The boundary conditions on the porous surface are very important to the internal core flow evolution and flow pattern. For a coarse porous surface (the 6.35-mm honeycomb with higher perturbation level on the transpiration surface), the mean flow differs significantly from the classical laminar solution and computational results, and much more turbulent shear stress is indicated. However, with small pore size (the 3.175-mm honeycomb, with relatively lower perturbation level on the surface), the mean velocity profiles are very close to the analytical laminar solution for a considerable downstream length, and the turbulent shear stress is much smaller than in the first case. Therefore, profound modification of the flow structure can occur due to the effect of nonideal wall boundary conditions on the porous surface.

Vortex organization in the outer layer of a turbulent boundary layer overlying sparse, hemispherical roughness elements is explored with two-component particle-image velocimetry (PIV) in multiple streamwise–wall-normal measurement planes downstream and between elements. The presence of sparse roughness elements causes a shortening of the streamwise length scale in the near-wall region. These measurements confirm that vortex packets exist in the outer layer of flow over rough walls, but that their organization is altered, and this is interpreted as the underlying cause of the length-scale reduction. In particular, the elements shed vortices which appear to align in the near-wall region, but are distinct from the packets. Further, it is observed that ejection events triggered in the element wakes are more intense compared to the ejection events in smooth wall. We speculate that this may initiate a self-sustaining mechanism leading to the formation of hairpin packets as a much more effective instability compared to those typical of smooth-wall turbulence. [ABSTRACT FROM PUBLISHER]

Turbulence in a channel flow with a fully transpired wall has been investigated experimentally. Two-dimensional particle image velocimetry is used to measure the instantaneous flowfields within the channel for two different porous surfaces: a 6.35-mm honeycomb and a 3.175-mm honeycomb. The 6.35-mm honeycomb creates a larger fluctuation of the injection velocity than the 3.175-mm honeycomb by a factor of 1.7. The boundary conditions on the porous surface are very important to the internal core flow evolution and flow pattern. For a coarse porous surface (the 6.35-mm honeycomb with higher perturbation level on the transpiration surface), the mean flow differs significantly from the classical laminar solution and computational results, and much more turbulent shear stress is indicated. However, with small pore size (the 3.175-mm honeycomb, with relatively lower perturbation level on the surface), the mean velocity profiles are very close to the analytical laminar solution for a considerable downstream length, and the turbulent shear stress is much smaller than in the first case. Therefore, profound modification of the flow structure can occur due to the effect of nonideal wall boundary conditions on the porous surface. [ABSTRACT FROM AUTHOR]