This paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner-outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions.

The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy ͑TKE͒ occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels.

We investigate the turbulent structure of shallow open channel flows where the flow depth is too small ͑compared with the roughness height͒ to form a logarithmic layer but large enough to develop an outer layer where the flow is not directly influenced by the roughness elements. Since the log layer is not present, the displacement height d, which defines the position of the zero plane, and the shear velocity u * cannot be found by fitting the velocity data to the log law. However, these parameters are still very important because they are used for scaling flow statistics for the outer and roughness layers. In this paper we propose an alternative procedure for evaluating d in laboratory conditions, where d is found from additional experiments with the fully developed log layer. We also point out the appropriate procedure for evaluating the shear velocity u * for flows with low submergence. These procedures are applied to our own laboratory flume experiments with uniform sphere roughness, where velocities were measured using Particle Image Velocimetry. Results were interpreted within the framework of the double-averaged Navier-Stokes equations and include mean velocities, turbulence intensities, Reynolds stresses, and form-induced normal and shear stresses. The data collapse well and show that in flows without a developed log layer the structure of turbulence in the outer layer remains similar to that of flows with a log layer. This means that even though the roughness layer in the experiments reported herein was sufficiently high to prevent the development of the log layer, influence of the bed roughness did not spread further up into the outer layer. Furthermore, the results show that flow statistics do not depend on relative submergence except for the form-induced stresses which increase when relative submergence decreases.

Abstract. Mountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micro-meteorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological boundary-layer model (Advanced Regional Prediction System) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modeled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances of about 4-6 m. Measurements suggest that mean wind velocities of about 5 m s −1 are required for advective heat transport to increase snow ablation over a long fetch distance of about 20 m. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value of approximately 0.6. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is low. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover.Correspondence to: R. Mott (mott@slf.ch) Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.

Purpose of ReviewThere are three technological parameters that play a key role on the performance of an ideal stent. These are its material, design and surface coating. This article highlights some fundamental developments that took place in these three areas of stent’s technology, in order to contribute to the identification of an ideal stent.Recent FindingsIn addition to technological developments concerning stent’s material, design and surface coating, the flow dynamic performance of stents has recently attracted increasing attention. Notably, it has been postulated that the local flow field in a stent is correlated with the deposition of crystals and microorganisms. These findings could potentially revolutionise future stent’s designs, and complement developments made on materials and coatings.SummaryThe most relevant changes in materials, designs and surface coatings of ureteric stents are reviewed in this article. These are described in the context of a specific cause of stent’s failure they aim to address, with a particular focus on encrustation and biofilm formation.