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The present paper is the second part of a combined (experimental and computational) study on stall cells (SCs) on a rectangular wing. In the first part, tuft data were used in order to geometrically characterize a stabilized SC resulting from a localized spanwise disturbance introduced by a zigzag tape. Here, pressure measurements on the model and in the wake and aerodynamic polars at midspan are reported. The wing model had an aspect ratio value of 2, the Reynolds number was 10 6 and the range of angles of attack (a) was from À6 to 16. Experimental results confirm previous findings. Furthermore, two-dimensional and three-dimensional Reynolds Averaged Navier-Stokes RANS simulations are used in order to better understand the structure of SCs. 3D simulations reproduce the experimental data with a 3 delay in a and permit a qualitative analysis. It is found that the SC vortices start normal to the wing surface and extend downstream in the wake; the evolution of the SC vortices in the wake is in strong interaction with the separation line vortex and the trailing edge line vortex; as the SC vortex develops downstream in the wake, its centreline is contracted towards the SC centre; the wing wake is pushed upstream at the centre of the SC and downstream at the sides by the SC vortices; spanwise lift and drag distributions always attain their minimum at the SC centre.

The onset of stall cells (SCs) is experimentally investigated on a flattop loaded 18% thick airfoil optimized for use on wind turbine blades, exhibiting trailing edge separation. SCs are dynamic coherent vortical structures that appear on wings under separated flow conditions. Although SCs have been known for long, neither are their characteristics completely documented nor their generating mechanisms fully understood. The present investigation aims at providing additional information on the geometric characteristics in terms of width, length and occupied area. The relevant data are presented as functions of Reynolds (Re) number, angle of attack and aspect ratio (AR) of the model. In the tests reported, the dynamic character of SCs is suppressed by imposing a localized flow disturbance. For the specific airfoil and for the Re and AR range tested, it is found that: the angle of attack at which SCs are initially formed decreases linearly with Re number and independently of the AR; unlike two-dimensional separation, their chordwise length increases with Re; the SC area relative to the wing planform area (defined as the relative SC area) grows asymptotically with angle of attack and Re number reaching an upper bound, which is independent of the AR; at intermediate angles of attack, the SC relative area is higher for the lower AR wing; for a fixed increment in Re number, the growth of the SC relative area is independent of the initial Re number; at lower angles of attack, the actual SC area is independent of the wing span.

The mechanics of breathing is a fascinating and vital process. The lung has complexities and subtle heterogeneities in structure across length scales that influence mechanics and function. This study establishes an experimental pipeline for capturing alveolar deformations during a respiratory cycle using synchrotron radiation micro-computed tomography (SR-micro-CT). Rodent lungs were mechanically ventilated and imaged at various time points during the respiratory cycle. Pressure-Volume (P-V) characteristics were recorded to capture any changes in overall lung mechanical behaviour during the experiment. A sequence of tomograms was collected from the lungs within the intact thoracic cavity. Digital volume correlation (DVC) was used to compute the three-dimensional strain field at the alveolar level from the time sequence of reconstructed tomograms. Regional differences in ventilation were highlighted during the respiratory cycle, relating the local strains within the lung tissue to the global ventilation measurements. Strains locally reached approximately 150% compared to the averaged regional deformations of approximately 80–100%. Redistribution of air within the lungs was observed during cycling. Regions which were relatively poorly ventilated (low deformations compared to its neighbouring region) were deforming more uniformly at later stages of the experiment (consistent with its neighbouring region). Such heterogenous phenomena are common in everyday breathing. In pathological lungs, some of these non-uniformities in deformation behaviour can become exaggerated, leading to poor function or further damage. The technique presented can help characterize the multiscale biomechanical nature of a given pathology to improve patient management strategies, considering both the local and global lung mechanics.

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