Logarithmic-layer turbulence: A view from the wall

The observability of the flow away from the wall in turbulent channels is studied using noiseless, although potentially incomplete, wall measurements. Reconstructions of the velocities are generated using linear stochastic estimation. All the velocities are well reconstructed in the buffer layer, but only relatively large "wall-attached" eddies are observable farther from the wall. In particular, large-scale structures that account for approximately 50% of the total kinetic energy and of the tangential Reynolds stress are accurately captured up to y/h less than or similar to 0.2. It is argued that this should allow the use of wall sensors for the detection (and potential control) of the large eddies populating the logarithmic region, and that it suggests a natural quantitative definition for "wall-attached" eddies.