Investigation of the flow inside an urban canopy immersed into an atmospheric boundary layer using laser Doppler anemometry

Laser Doppler anemometry (LDA) is used to investigate the flow inside an idealized urban canopy consisting of a staggered array of cubes with a 25% density immersed into an atmospheric boundary layer with a Reynolds number of δ+=32,300. The boundary layer thickness to cube height ratio (δ/h=22.7) is large enough to be representative of atmospheric surface layer in neutral conditions. The LDA measurements give access to pointwise time-resolved data at several positions inside the canopy (z=h/4, h/2, and h). Synchronized hot-wire measurements above the canopy (inertial region and roughness sublayer) are also realized to get access to interactions between the different flow regions. The wall-normal mean velocity profile and Reynolds stresses show a good agreement with available data in the literature, although some differences are observed on the standard deviation of the spanwise component. A detailed spectral and integral time scale analysis inside the canopy is then carried out. No clear footprint of a periodic vortex shedding on the sides of the cubes could be identified on the power spectra, owing to the multiple cube-to-cube interactions occuring within a canopy with a building density in the wake interference regime. Results also suggest that interactions between the most energetics scales of the boundary layer and those related to the cube canopy take place, leading to a broadening of the energy peak in the spectra within the canopy. This is confirmed by the analysis of coherence results between the flow inside and above the canopy. It is shown that linear interactions mechanisms are significant, but reduced compared to smooth-wall boundary-layer flow. To our knowledge, this is the first time such results are shown on the dynamics of the flow inside an urban canopy

Application of Laser Doppler Anemometry to estimate turbulent power spectra inside an urban canopy

International audienceIn the present contribution, we propose to make use of Laser Doppler Anemometry to investigate the dynamics of the flow inside an urban canopy immersed into a thick turbulent boundary layer. Previous studies dedicated to such flows (Castro et al. 2006; Reynolds and Castro 2008) have focused the core of their analysis on the roughness sublayer region or on the inertial region. In contrast, the region below the canopy interface have received little attention so far, and only mean velocity or Reynolds stresses data can be found in the literature. Indeed, getting access to the dynamics of the flow inside the canopy represents a real experimental challenge: the flow is turbulent, 3-dimensionnal, and due to the tight clearance, only non-intrusive laser-based techniques such as LDA or PIV can be used. In this contribution, for the first time, a spectral analysis of the flow is carried out inside the canopy region. This is made possible thanks to very thorough LDA measurements as well as the use of advanced post-processing algorithm to compute temporal power spectra. Indeed, LDA measurement yields non-equidistantly spaced data points, and standard FFT algorithms cannot be used. A detailed comparison of various algorithms is undertaken, and a slight improvement is proposed to reduce the impact of measurement noise. Finally, the power spectra inside the canopy are analyzed and the main implications on the flow dynamics are discussed

Influence of the plan density of a cube-based canopy on the structure of the lower atmospheric boundary layer

An experimental investigation of the structure of the boundary-layer developing over an urban-like rough wall is performed considering wall configurations that represent three idealised urban terrains. Both stereoscopic particle image velocimetry and two-point hot-wire data are investigated through one- and two-dimensionnal spectral analysis. It is shown that the flow characteristics within the inertial layer appear to be independent from the wall configuration. This region is populated with coherent structures of the same type as those found in smooth-wall flows, namely very large-scale motions and large-scale motions, the later showing self-similar features. These coherent structures, whose footprint is mainly visible on the streamwise and spanwise velocity components, also appear to be present in the roughness sublayer. Within the roughness sublayer, the influence of the canopy density shows in the characteristics of the wall-normal velocity component w. The most energetic scales in the auto-spectra of w and in the co-spectra between u and w are indeed of the same order of magnitude as the roughness obstacle size and decrease with increasing density. Instead of varying progressively with the packing density, they appear to be constrained by the presence of the canopy for the densest investigated configurations consistently with the change of the flow regime reported in the literature

Spatial modulations of kinetic energy in the roughness sublayer

High-Reynolds-number experiments are conducted in the roughness sublayer of a turbulent boundary-layer developing over a cubical canopy. Stereoscopic particle image velocimetry is performed in a wall-parallel plane to evidence a high degree of spatial modulation of the small-scale turbulence around the footprint of large-scale motions, despite the suppression of the inner layer by the high roughness elements. Both Fourier and wavelets analyses show that the near-wall cycle observed in smooth-wall-bounded flows is severely disrupted by the canopy, whose wake in the roughness sublayer generates a new range of scales, closer to that of the outer-layer large-scale motions. This restricts significantly scale separation, hence a diagnostic method is developed to divide carefully and rationally the fluctuating velocity fields into large- and small-scale components. Our analysis across all turbulent kinetic energy terms sheds light on the spatial imprint of the modulation mechanism, revealing a very different signature on each velocity component. The roughness sublayer shows a preferential arrangement of the modulated scales similar to what is observed in the outer-layer of smooth-wall-bounded flows – small-scale turbulence is enhanced near the front of high momentum regions and damped at the front of low-momentum regions. More importantly, accessing spanwise correlations reveals that modulation intensifies the most along the flanks of the large-scale motions

Horizon effects for surface waves in wave channels and circular jumps

Surface waves in classical fluids experience a rich array of black/white hole horizon effects. The dispersion relation depends on the characteristics of the fluid (in our case, water and silicon oil) as well as on the fluid depth and the wavelength regime. In some cases, it can be tuned to obtain a relativistic regime plus high-frequency dispersive effects. We discuss two types of ongoing analogue white-hole experiments: deep water waves propagating against a counter-current in a wave channel and shallow waves on a circular hydraulic jump.Comment: 4 pages, 2 figs. To appear in: Proceedings of the Spanish Relativity Meeting (ERE2010

Toward the development of a predictive model of the roughness sublayer flow

The non-linear interactions between large-scale momentum regions and small-scale structures induced by the presence of the roughness have been studied in boundary layers consisting of staggered cube arrays with plan area packing density of 6.25%, 25% or 44.4%. The measurements, consisting of hot-wire anemometry, were conducted at two Reynolds numbers in each of the canopy configurations. The canopy configuration is shown to have a significant influence on all parameters of the predictive model close to the roughness elements which is a result of the characteristics of the small-scale structures induced by the presence of the cubes. Several tests of the predictive model have been undertaken, demonstrating the good capability of the model to reproduce accurately spectra and statistics up to the 4th order. The model must be however calibrated for each type of canopy flow regime

Structure of high Reynolds number boundary layers over cube canopies

The influence of a cube-based canopy on coherent structures of the flow was investigated in a high Reynolds number boundary layer (thickness δ ∼ 30 000 wall units). Wind tunnel experiments were conducted considering wall configurations that represent three idealised urban terrains. Stereoscopic Particle Image Velocimetry was employed using a large field of view in a streamwise-spanwise plane combined to two-point hot-wire measurements. The analysis of the flow within the inertial layer highlights the independence of its characteristics from the wall configuration. The population of coherent structures is in agreement with that of smooth-wall boundary layers, i.e. consisting of large and very-large scale motions, sweeps and ejections, as well as smaller-scale vortical structures. The characteristics of vortices appear to be independent of the roughness configuration while their spatial distribution is closely linked to large meandering motions of the boundary layer. The canopy geometry only significantly impacts the wall-normal exchanges within the roughness sublayer. Bi-dimensional spectral analysis demonstrates that wall-normal velocity fluctuations are constrained by the presence of the canopy for the densest investigated configurations. This threshold in plan density above which large scales from the overlying boundary layer can penetrate the roughness sublayer is consistent with the change of the flow regime reported in the literature and constitutes a major difference with flows over vegetation canopies

Assessment of inner–outer interactions in the urban boundary layer using a predictive model

Urban-type rough-wall boundary layers developing over staggered cube arrays with plan area packing density, λ p , of 6.25%, 25% or 44.4% have been studied at two Reynolds numbers within a wind tunnel using hot-wire anemometry (HWA). A fixed HWA probe is used to capture the outer-layer flow while a second moving probe is used to capture the inner-layer flow at 13 wall-normal positions between 1.25h and 4h where h is the height of the roughness elements. The synchronized two-point HWA measurements are used to extract the near-canopy large-scale signal using spectral linear stochastic estimation and a predictive model is calibrated in each of the six measurement configurations. Analysis of the predictive model coefficients demonstrates that the canopy geometry has a significant influence on both the superposition and amplitude modulation. The universal signal, the signal that exists in the absence of any large-scale influence, is also modified as a result of local canopy geometry suggesting that although the non-linear interactions within urban-type rough-wall boundary layers can be modelled using the predictive model as proposed by Mathis et al. (2011a), the model must be however calibrated for each type of canopy flow regime. The Reynolds number does not significantly affect any of the model coefficients, at least over the limited range of Reynolds numbers studied here. Finally, the predictive model is validated using a prediction of the near-canopy signal at a higher Reynolds number and a prediction using reference signals measured in different canopy geometries to run the model. Statistics up to the 4 th order and spectra are accurately reproduced demonstrating the capability of the predictive model in an urban-type rough-wall boundary layer

Differential contribution of APP metabolites to early cognitive deficits in a TgCRND8 mouse model of Alzheimer’s disease

International audienceAlzheimer's disease (AD) is a neurodegenerative pathology commonly characterized by a progressive and irreversible deterioration of cognitive functions, especially memory. Although the etiology of AD remains unknown , a consensus has emerged on the amyloid hypothesis, which posits that increased production of soluble amyloid b (Ab) peptide induces neuronal network dysfunctions and cognitive deficits. However, the relative failures of Ab-centric therapeutics suggest that the amyloid hypothesis is incomplete and/or that the treatments were given too late in the course of AD, when neuronal damages were already too extensive. Hence, it is striking to see that very few studies have extensively characterized, from anatomy to behavior, the alterations associated with pre-amyloid stages in mouse models of AD amyloid pathology. To fulfill this gap, we examined memory capacities as well as hippocampal network anatomy and dynamics in young adult pre-plaque TgCRND8 mice when hippocampal Ab levels are still low. We showed that TgCRND8 mice present alterations in hippocampal inhibitory networks and g oscillations at this stage. Further, these mice exhibited deficits only in a subset of hippocampal-dependent memory tasks, which are all affected at later stages. Last, using a pharmacological approach, we showed that some of these early memory deficits were Ab-independent. Our results could partly explain the limited efficacy of Ab-directed treatments and favor multitherapy approaches for early symptomatic treatment for AD