Surface Layer Flow Research Articles

A Systematic Investigation of the Applicability of Taylor’s Hypothesis in an Idealized Surface Layer

Taylor’s Frozen Turbulence Hypothesis (TH) is a critical assumption in turbulent theory and practice which allows time series of point measurements of turbulent variables to be translated to the spatial domain via the mean wind. Using a 3D array of fibre-optic distributed temperature sensing in the atmospheric surface layer over an idealized desert site we present a systematic investigation of the applicability of Taylor’s Hypothesis to atmospheric surface layer flows over a variety of conditions: unstable, near-neutral, and stable atmospheric stabilities; and multiple measurement heights between the surface and 3 m above ground level. Both spatially integrated and spatially scale-dependent eddy velocities are investigated by means of time-lagged streamwise two-point correlations and compared to the mean Eulerian wind. We find that eddies travel slower than predicted by TH at small spatial separations, as predicted by TH at separations typically between 5 and 16 m, and faster than predicted by TH at larger spatial separations. In unstable atmospheric conditions the spatial separation at which eddy velocity is larger than Eulerian velocity decreases with height.

Automated large-scale and terrain-induced turbulence modulation of atmospheric surface layer flows in a large wind tunnel

Automated large-scale and terrain-induced turbulence modulation of atmospheric surface layer flows in a large wind tunnel

Representing the effects of building height variability on urban canopy flow

AbstractWe conducted large‐eddy simulations over 98 urban arrays with varying building densities and height distributions. Compared with uniform‐height urban arrays, the influence of height variability on urban flow is pronounced and acts differently in two idealized urban configurations: the low buildings induce higher wind speed and stronger turbulence over staggered arrays but act inversely over aligned building configurations. The flow motions around tall buildings generate strong dispersive fluxes, which are sometimes of similar magnitude to the turbulent momentum flux and responsible for a persistent isolated roughness flow pattern in the upper canopy regardless of the urban density. Tall buildings further contribute disproportionately to the form drag of the urban surface, reaching up to 3.9 times the form drag induced by buildings of height equal to the average building height, in dense layouts. The flow inflection points—that is, the largest wind‐speed gradient that defines the aerodynamic interface between the urban canopy flow and the surface layer flow above—are found to be displaced to the maximum building height if less than 25% of buildings are below the mean building height. These findings provide critical insight for the development of urban canopy models, where the impacts of height variability on flow are often linked to the vertical variation in urban density alone. To address this deficiency, we provide a case study that considers the drag amplification due to the impact of vertical urban structures in the urban canopy model, enabling high‐resolution regional climate models to reproduce urban air flows better.

Sensitivity of gyre-scale marine connectivity estimates to fine-scale circulation

Abstract. We investigated the connectivity properties of an idealized western boundary current system separating two ocean gyres, where the flow is characterized by a well-defined mean circulation as well as energetic fine-scale features (i.e., mesoscale and submesoscale currents). We used a time-evolving 3D flow field from a high-resolution (HR-3D) ocean model of this system. In order to evaluate the role of the fine scales in connectivity estimates, we computed Lagrangian trajectories in three different ways: using the HR-3D flow, using the same flow but filtered on a coarse-resolution grid (CR-3D), and using the surface layer flow only (HR-SL). We examined connectivity between the two gyres along the western boundary current and across it by using and comparing different metrics, such as minimum and averaged values of transit time between 16 key sites, arrival depths, and probability density functions of transit times. We find that when the fine-scale flow is resolved, the numerical particles connect pairs of sites faster (between 100 to 300 d) than when it is absent. This is particularly true for sites that are along and near the jets separating the two gyres. Moreover, the connectivity is facilitated when 3D instead of surface currents are resolved. Finally, our results emphasize that ocean connectivity is 3D and not 2D and that assessing connectivity properties using climatologies or low-resolution velocity fields yields strongly biased estimates.

Scale‐Dependent Estimability of Turbulent Flux in the Unstable Surface Layer for Land Surface Modeling

AbstractSurface flux estimation is essential to land surface modeling in earth system models. In practice, parameterizations of surface turbulent fluxes are almost all based on the similarity theory. That is, the grid or subgrid mean surface‐layer flow is assumed at equilibrium with the underlying earth surface, and therefore some empirical relations can be used to estimate surface fluxes. In this paper, scale‐dependent estimability of turbulent flux in the unstable surface layer is systematically investigated based on high‐resolution large‐eddy simulation data over a flat and homogeneous domain, representing a typical land surface modeling grid. It is found that turbulent flow in the unstable surface layer inherently fluctuates over a wide range of scales. This kind of fluctuation affects the steady‐state relations between mean atmospheric quantities and underlying earth surface, and hence affects the estimability of surface fluxes. Sensitivity tests show that the relative root mean square error of the estimated surface friction velocity for a subdomain generally increases as the subdomain becomes smaller. The error can be as high as 35% as the subdomain size decreases to the order of the surface layer height. To achieve an error of 10% for all cases, the subdomain size should be at least on the order of the boundary layer height. These findings imply that estimability‐based strategies may be needed for representing subgrid heterogeneity for surface flux estimation in land surface modeling.

Surface layer response to heterogeneous tree canopy distributions: roughness regime regulates secondary flow polarity

Large-eddy simulation was used to model turbulent atmospheric surface layer (ASL) flow over canopies composed of streamwise-aligned rows of synthetic trees of height, $h$ , and systematically arranged to quantify the response to variable streamwise spacing, $\delta _1$ , and spanwise spacing, $\delta _2$ , between adjacent trees. The response to spanwise and streamwise heterogeneity has, indeed, been the topic of a sustained research effort: the former resulting in formation of Reynolds-averaged counter-rotating secondary cells, the latter associated with the $k$ - and $d$ -type response. No study has addressed the confluence of both, and results herein show secondary flow polarity reversal across ‘critical’ values of $\delta _1$ and $\delta _2$ . For $\delta _2/\delta \lesssim 1$ and $\gtrsim 2$ , where $\delta$ is the flow depth, the counter-rotating secondary cells are aligned such that upwelling and downwelling, respectively, occurs above the elements. The streamwise spacing $\delta _1$ regulates this transition, with secondary cell reversal occurring first for the largest $k$ -type cases, as elevated turbulence production within the canopy necessitates entrainment of fluid from aloft. The results are interpreted through the lens of a benchmark prognostic closure for effective aerodynamic roughness, $z_{0,{Eff.}} = \alpha \sigma _h$ , where $\alpha$ is a proportionality constant and $\sigma _h$ is height root mean square. We report $\alpha \approx 10^{-1}$ , the value reported over many decades for a broad range of rough surfaces, for $k$ -type cases at small $\delta _2$ , whereas the transition to $d$ -type arrangements necessitates larger $\delta _2$ . Though preliminary, results highlight the non-trivial response to variation of streamwise and spanwise spacing.

Modelling study on estuarine circulation and its effect on the turbidity maximum zone in the Yalu River Estuary, China

The along-channel, lateral estuarine circulation and the related baroclinic effects on the turbidity maximum zone (TMZ) of the Yalu River Estuary (YRE) were explored in our study based on the Finite Volume Coastal Ocean Model (FVCOM). The results show that along-channel estuarine flow in YRE had a two-layer-structure with seaward flow in the surface layer and landward flow in the bottom layer. The along-channel bottom landward flow can transport sediment to the upper estuary and influence the location of the TMZ, which was mainly induced by asymmetric tidal mixing. Then, for the variation in cross-channel circulation (lateral circulation), the TMZ moved westwards after reclamation, due to the decreased eastward residual flow resulting from weakened eastward barotropic and baroclinic pressure gradient forcing. This study qualitatively demonstrated the baroclinic effects and human-induced impacts on estuarine subtidal flow and sedimentation. Although site-specific, these findings have wider implications for sediment transport in other medium-scale estuaries.

An investigation of particles effects on wall-normal velocity fluctuations in sand-laden atmospheric surface layer flows

Based on the high-quality observational data in the Qingtu Lake Observation Array (QLOA), the difference in the energy distribution, the scale of the coherent structures, and the amplitude modulation effect of the wall-normal velocity fluctuations between particle-free and particle-laden flow in the atmospheric surface layer are analyzed. The results show that the presence of particles enhanced the wall-normal turbulence intensity, especially the increase at the top of the logarithmic region is more significant though the particle mass loading decreases with the wall-normal distance. A further insight indicates that the increase in the length scale of the wall-normal fluctuating velocity coherent structure by particles is more significant further from the wall, which is supported by the premultiplied energy spectra and the two-point correlation. This leads to a drastic increase in kinetic energy of the large-scale coherent structures by the particle away from the wall and thus results in increased amplitude modulation effects of large-scale wall-normal velocity fluctuations onto small-scales.

Visibility network analysis of large-scale intermittency in convective surface layer turbulence

Large-scale intermittency is a widely observed phenomenon in convective surface layer turbulence that induces non-Gaussian temperature statistics, while such a signature is not observed for velocity signals. Although approaches based on probability density functions have been used so far, those are not able to explain to what extent the signals’ temporal structure impacts the statistical characteristics of the velocity and temperature fluctuations. To tackle this issue, a visibility network analysis is carried out on a field-experimental dataset from a convective atmospheric surface layer flow. Through surrogate data and network-based measures, we demonstrate that the temperature intermittency is related to strong nonlinear dependencies in the temperature signals. Conversely, a competition between linear and nonlinear effects tends to inhibit the temperature-like intermittency behaviour in streamwise and vertical velocities. Based on present findings, new research avenues are likely to be opened up in studying large-scale intermittency in convective turbulence.

Initial time dependence of wind- and density-driven Lagrangian residual velocity in a tide-dominated bay

The nonlinear effect of the summer southeast wind and density on the 3D structures of the full Lagrangian residual velocity (LRV) was quantified for a generally nonlinear system, using Jiaozhou Bay (JZB), China as the test site. In the tidally energetic JZB, the basic patterns of the wind- and density-driven full LRVs were found to be consistent with semi-analytical solutions but highly dependent on initial times. The wind-driven full LRVs at different tidal phases flowed similarly downwind over shoals and upwind in the deep region; however, the main branches could migrate across nearly half of the bay. A density-driven, clockwise flow was dominant in the western inner bay at low tide, but it almost disappeared at high tide. The effect of density generally enhanced the outward flow in the surface layer and inward flow in the bottom layer. Along-trajectory integrated momentum balances indicated that viscosity was the main factor responsible for the time dependence of the wind-driven full LRVs, while viscosity, barotropic and baroclinic pressure gradients were the main drivers of the intra-tidal variations in the density-driven full LRVs. Generally, the summer wind and density had opposing effects, although their influence was weaker than that of the tide and could not change the patterns of the tide-driven full LRVs. When analysing the effects of wind and density on the coastal circulation in JZB, both the 3D structures and the possibility of a high initial tidal phase dependency should be considered.

Study of the thermal regime of a reservoir on the Qinghai-Tibetan Plateau, China.

The Qinghai-Tibetan Plateau region has unique meteorological characteristics, with low air temperature, low air pressure, low humidity, little precipitation, and strong diurnal variation. A two-dimensional hydrodynamic CE-QUAL-W2 model was configured for the Pangduo Reservoir to better understand the thermal structure and diurnal variation inside the reservoir under the local climate and hydrological conditions on the Qinghai-Tibetan Plateau. Observation data were used to verify the model, and the results showed that the average error of the 6 profile measured monthly from August to December 2016 was 0.1°C, and the root-mean-square error (RMSE) was 0.173°C. The water temperature from August 2016 to September 2017 was simulated by inputting measured data as model inputs. The results revealed that the reservoir of the Qinghai-Tibetan Plateau was a typical dimictic reservoir and the water mixed vertically at the end of March and the end of October. During the heating period, thermal stratification occurred, with strong diurnal variation in the epilimnion. The mean variance of the diurnal water temperature was 0.10 within a 5 m water depth but 0.04 in the whole water column. The mixing mode of inflow changed from undercurrent, horizontal-invaded flow and surface layer flow in one day. In winter, the diurnal variation was weak due to the thermal protection of the ice cover, while the mean variance of diurnal water temperature was 0.00 within both 5 m and the whole water column. Compared to reservoirs in areas with low altitude but the same latitude, significant differences occurred between the temperature structure of the low-altitude reservoir and the Pangduo Reservoir (P<0.01). The Pangduo Reservoir presented a shorter stratification period and weaker stratification stability, and the annual average SI value was 26.4 kg/m2, which was only 7.5% that of the low-altitude reservoir. The seasonal changes in the net heat flux received by the surface layers determined the seasonal cycle of stratification and mixing in reservoirs. This study provided a scientific understanding of the thermal changes in stratified reservoirs under the special geographical and meteorological conditions on the Qinghai-Tibetan Plateau. Moreover, this model can serve as a reference for adaptive management of similar dimictic reservoirs in cold and high-altitude areas.

An offline prediction of nanoscale ripples propagation under ion irradiation: A correlation between ripples velocity and surface erosion rate

An offline measurement of surface ripples propagation has been proposed using sequential ion beam irradiation approach. First, the ripple patterns were grown on Si (100) using 100 keV Ar+ at a fluence of 9 × 1017 ions/cm2 and then subsequent second stage irradiation was performed on partially masked rippled surface to study the ripple's propagation and surface erosion. Offline atomic force microscopy (AFM) study shows that with increase in fluence from 3 × 1017 ions/cm2 to 7 × 1017 ions/cm2, in second stage irradiation, the ripples are found to propagate along the projection of the ion beam's incident direction onto the surface. This observation of ripples propagation contradicts several existing models for ion-beam-induced ripple development. Furthermore, surface erosion rate due to sputtering was measured using stylus profilometer. A prediction of relation “v ~0.75 vo” between ripple's propagation velocity (v) and surface erosion rate (vo) shows the importance of Navier-Stokes viscous flow in near surface layer for the evolution of ripple patterns under ion beam irradiation (IBI).

Revisiting the role of intermittent heat transport towards Reynolds stress anisotropy in convective turbulence

Abstract

Nearshore response to cold air outbreaks in the Gulf of Mexico

The nearshore response to synoptic meteorological patterns called Cold Air Outbreaks (CAOs) was studied with a cross-shelf mooring array located 7 km west of an estuarine outflow in the Northern Gulf of Mexico. The array consisted of 6 stations that spanned the nearshore measuring waves, currents, temperature, and salinity for a two-week period in December 2013. CAOs are prevalent during winter and are characterized by the passage of atmospheric cold fronts with offshore winds known to cool coastal waters. The prefrontal phases of CAOs coincided with downwelling favorable winds and westward flow that introduced a river plume into the nearshore. Offshore winds during the high pressure phase formed a two-layer circulation with offshore flow in the surface layer and onshore flow beneath, which was moderated by a river plume. The downwind advection of the river plume tended to enhance surface transport until the plume was flushed out of the nearshore, which formed adverse density gradients that reduced surface transport through vertical mixing. When the plume was not fully flushed out of the inner shelf, surface transport remained enhanced during the high pressure phase due to stratification that limited the formation of adverse density gradients. When the plume was initially shore detached at the onset of the high pressure phase, offshore flow was not enhanced by the downwind advection of the plume due to the existence of initial adverse density gradients. These results highlight the effects of nearshore river plumes in regulating inner shelf circulation during cold fronts.

Similarity functions and a new [formula omitted] closure for predicting stratified atmospheric surface layer flows in complex terrain

Most of the k−ε closures for modeling stratified surface layer are derived from the classical similarity functions and might fail in complex terrain due to limitations of the classical similarity functions. Despite the classical similarity functions to limit the flux Richardson number Rf, we present new similarity functions estimated from field measurement in full range of Rf and propose a k−ε model using the new similarity functions to improve predictions of stratified surface layer flows in complex terrain. Measurements show that the classical similarity functions are partially valid in complex terrain and the wind shear under strongly stable conditions is constrained at large Rf. According to numerical studies in two areas of complex terrain, models using the classical similarity functions and the new similarity functions both present good predictions of convective airflows in complex terrain. The new similarity functions are shown to significantly improve the k−ε model in predicting stably stratified airflows in complex terrain by constraining the wind shear at large Rf, while the classical similarity functions without limiting the wind shear lead to significantly misestimating the wind speedup factor under stable conditions. Using the proposed model to predict flows in wind farm could benefit wind resource estimation and wind power forecasting.

Wall-attached and wall-detached eddies in wall-bounded turbulent flows

According to Townsend’s attached eddy hypothesis (AEH), a boundary layer flow is comprised of wall-attached eddies, but to extract the part of the flow whose statistical behaviours are well described by the AEH is not at all straightforward. The objective of this work is to extract the part of the flow that can be described by the AEH, and study the statistical behaviours of the other part, which cannot be described by the AEH. In this process, two types of eddies are identified in addition to the Kolmogorov-scale eddies, i.e. wall-attached eddies and wall-detached eddies. The statistical behaviours of the wall-attached eddies are shown to be very well described by the AEH, i.e. the eddies are wall-attached, self-similar and, importantly, Gaussian, whereas the wall-detached eddies cannot be modelled by the AEH. Specifically, a decomposition scheme is proposed following Townsend (The Structure of Turbulent Shear Flow, Cambridge University Press, 1976). We apply our decomposition scheme to three different flows, i.e. channel, boundary layer and atmospheric surface layer flows. The results are similar with only quantitative differences, suggesting possible universality in both the wall-attached eddies and the wall-detached eddies.

Three‐Dimensional Representation of Large‐Scale Structures Based on Observations in Atmospheric Surface Layers

AbstractLarge amounts of synchronous multipoint measurement data were acquired from long‐term observations in high‐Reynolds‐number atmospheric surface layers (Reτ∼O(106)) at the Qingtu Lake observation array site. Based on these experimental data, the general shapes of three‐dimensional (3‐D) large‐scale coherent structures are constructed coupled with the symmetry of two‐point streamwise velocity correlations in the spanwise direction, where the spanwise symmetry is confirmed to be applicable in the atmospheric surface layer flow by experimental verification. The full extent of the 3‐D structure is much larger than a 2‐D slice at the condition point and increases significantly with the reference height in the logarithmic region. Moreover, the streamwise length and the spanwise width of the horizontal section are found to vary systematically with the relative height to the condition point. These variations are parametrized and can subsequently be used to infer the horizontal section at different relative heights. Then, a plausible empirical model is proposed by taking into consideration the streamwise shift caused by the inclination angle, and the 3‐D structure can be predicted by this model using the primary flow parameters and location information. This study contributes to a better understanding of dominant structures in the outer region of wall turbulence at high Reynolds numbers.

Analysis of water flow in an asphalt pavement surface layer with different thicknesses and different permeability coefficients

The deleterious effect of water is a major factor that causes early degradation in hot mix asphalt (HMA) materials. The coefficient of permeability (k) is a parameter used to characterise HMA hydraulic transport properties. This study numerically evaluated the influence of the spatial distribution of k on the flow pattern within an asphalt surface layer with different thicknesses. Initially samples with different heights were prepared in the lab and cut in three parts and the air voids (AV) content and k values in each part were determined. From the experimental relationship between AV and k numerical simulations were conducted to characterise the transient flow inside an asphalt surface layer divided into sublayers in order to represent typical AV vertical distributions observed in the field. The results show that the heterogeneous distribution of k within the HMA layer could produce regions that are more susceptible to moisture damage due to the accumulation of water.

Representation of the Reynolds stress tensor through quadrant analysis for a near-neutral atmospheric surface layer flow

To disseminate the role of the eddy motions on the anisotropic states of the Reynolds stress tensor ($$\varvec{B}$$), we devise a novel methodology based on the quadrant analysis, where the distributions of the invariants of $$\varvec{B}$$ are studied on a $$u^{\prime }$$–$$w^{\prime }$$ quadrant plane, with $$u^{\prime }$$ and $$w^{\prime }$$ being the turbulent fluctuations in the streamwise and vertical velocities. We apply this methodology to a near-neutral atmospheric surface layer (ASL) flow, derived from a field experiment dataset having multi-level turbulence measurements. The results show that in a near-neutral ASL flow, the anisotropic states of $$\varvec{B}$$ are determined by the distribution of the streamwise and cross-stream velocity variances ($$\sigma _{u}^2$$ and $$\sigma _{v}^2$$) on the $$u^{\prime }$$–$$w^{\prime }$$ quadrant plane. By studying the contour maps of the invariants of $$\varvec{B}$$ on the $$u^{\prime }$$–$$w^{\prime }$$ quadrant plane, we discover three distinct zones with elliptical boundaries in the $$u^{\prime }$$–$$w^{\prime }$$ plane, across which the anisotropic states of the eddy motions evolve. We find that the eddy motions which occur We also notice that the distinction between these three zones in the $$u^{\prime }$$–$$w^{\prime }$$ plane is prominent at the lowest measurement level, but becomes progressively indistinguishable as the measurement height increases.

Amplitude modulation between multi-scale turbulent motions in high-Reynolds-number atmospheric surface layers

Long-term measurements were performed at the Qingtu Lake Observation Array site to obtain high-Reynolds-number atmospheric surface layer flow data ($Re_{\unicode[STIX]{x1D70F}}\sim O(10^{6})$). Based on the selected high-quality data in the near-neutral surface layer, the amplitude modulation between multi-scale turbulent motions is investigated under various Reynolds number conditions. The results show that the amplitude modulation effect may exist in specific motions rather than at all length scales of motion. The most energetic motions with scales larger than the wavelength of the lower wavenumber peak in the energy spectra play a vital role in the amplitude modulation effect; the small scales shorter than the wavelength of the higher wavenumber peak are strongly modulated, whereas the motions with scales ranging between these two peaks neither contribute significantly to the amplitude modulation effect nor are strongly modulated. Based on these results, a method of decomposing the fluctuating velocity is proposed to accurately estimate the degree of amplitude modulation. The corresponding amplitude modulation coefficient is much larger than that estimated by establishing a nominal cutoff wavelength; moreover, it increases log-linearly with the Reynolds number. An empirical model is proposed to parametrize the variation of the amplitude modulation coefficient with the Reynolds number and the wall-normal distance. This study contributes to a better understanding of the interaction between multi-scale turbulent motions and the results may be used to validate and improve existing numerical models of high-Reynolds-number wall turbulence.