Guangqiang Qian

Wind tunnel simulation of the three-dimensional airflow patterns around shrubs

[1] Dunes formed around vegetation such as shrubs are common in arid and semiarid regions and some coastal regions. Understanding their dynamics requires insights into the airflow patterns around shrubs. Thus, we analyzed the horizontal and vertical flow patterns around model shrubs using wind measurements obtained by particle image velocimetry in a scaled wind tunnel simulation. Flow patterns around shrubs were complicated by the presence of bleed flows and displaced flows. Below a critical shrub density (between 0.05 and 0.08), bleed flow is dominant. The flows separate in both horizontal and vertical planes when the shrub density equals or exceeds this critical density, resulting in reverse cells in the lee of shrubs. The horizontal reverse cells are characterized by two symmetrical, opposing reverse eddies, whereas the vertical reverse cells form a single eddy. The positions of the reverse cells shift in response to changes in shrub density, reflecting the interaction between bleed flow and reverse flow. The parameters of horizontal reverse cells are functions of shrub density and wind velocity. Parameters of vertical reverse cells change mainly with respect to shrub density and vary only slightly with wind velocity. The lengths of the horizontal and vertical reverse cells are correlated, reflecting interactions between them. Field experiments on dune morphology will be required to relate these airflow patterns to dune development.

The pressure-field characteristics around porous wind fences: results of a wind tunnel study

Using detailed measurements of the instantaneous velocity fields around fences with different heights and porosities, the pressure fields around the fences were calculated using the Reynolds equations. Based on the results of calculation, the relationships among the pressure field, fence porosity (i.e., the β coefficient), fence height, and free-stream wind velocity were examined. For all fences, a high-pressure region exists upwind of the fence, and a low-pressure region exists downwind of the fence, the change of mean pressure is gradually less to a long distance from the fence. The pressure value at the center of the low-pressure area downwind of the fence decreases with increasing fence height and free-stream wind velocity. The impact of the fence’s porosity on pressure is large, when β < 0.2, the mean pressure upwind of the fence is relatively small, and the center value of the low-pressure area downwind of the fence increases with increasing fence porosity; when β > 0.3, the mean pressure increases around the fence.

High-altitude aeolian research on the Tibetan Plateau

Aeolian processes and their role in desertification have been studied extensively at low elevations, but have been rarely studied at high elevations in areas such as the Tibetan Plateau, where aeolian processes were active in the geologic past and remain active today. In this review, we summarize research that improves our understanding of aeolian processes on the Tibetan Plateau, including the distribution, characteristics, and provenance of aeolian sediments; the history of aeolian activity; aeolian geomorphology; and wind-driven land degradation. Contemporary aeolian processes primarily occur in dry basins, in wide river valleys, on lakeshores, on mountain slopes, and on gravel pavements. Sediment characteristics suggest a local origin, and provide interesting contrasts with those of Chinas Loess Plateau. The history of aeolian activity and its paleoclimatic implications, reconstructed based on aeolian archives, is short (mostly since the Late Glacial) and shows wide regional differences. Aeolian geomorphology is simple and suggests short formation time. Wind-driven land degradation is less severe than previously thought, driven by different factors in different areas, and exhibited complex interactions with freeze–thaw processes. Aeolian research has been conducted within the general framework of aeolian science, but addresses issues specific to the Tibetan Plateau that arise due to the low air temperature, low air density, and the presence of a cryosphere. We propose six priorities for future research: aeolian physics, the effect of freeze-thaw cycles, comparisons with other areas, regional differences, effects of wind-driven land degradation, and integrated observation and monitoring.

Field observations of the vertical distribution of sand transport characteristics over fine, medium and coarse sand surfaces

The vertical distribution of sand transport characteristics is an important issue in aeolian research. Surface characteristics affect sand transport processes, but their effects are not yet fully understood. To provide more data on this subject, we observed sand transport in 16 field experiments above surfaces covered by fine, medium and coarse sand. The sand transport rate over relatively coarser-grained medium and coarse surfaces could be expressed as a Gaussian peak function: qz = a + b exp (-0.5[(|z – Ch|)/d]e), where qz is the measured sediment transport at height z above the bed and a, b, Ch, d, and e are regression coefficients. The measured sand transport flux peak values (Hh) were linearly related to Ch, and both values were significantly related to the mean surface grain size. However, for the relatively finer-grained medium and fine sand surfaces, the sediment transport could be expressed as an exponential function. The cumulative sand transport below 0.1 m was directly related to the mean surface grain size, and the relationship could be expressed as the following exponential function: Cz = f + g exp –Mz/i, where Cz is the cumulative sand transport at height z above the bed, Mz is the mean grain size and f, g, and i are regression coefficients. Above 0.1 m, there were no significant relationships between the cumulative sand transport and the mean surface grain size. The mean grain size decreased with increasing height below the peak height and then increased with increasing height. The surface grain size distribution and proportions of the particles in different grain size categories controlled the mean grain size as a function of height. The observed changes in the sand transport rate and grain size with height will provide support for sand disaster mitigation, numerical modelling and studies of dune formation. This article is protected by copyright. All rights reserved.

Pattern analysis of a linear dune field on the northern margin of Qarhan Salt Lake, northwestern China

In terms of formation mechanisms of linear dunes, there are open arguments for their widespread distribution and multi-morphological diversities. In order to clarify the formation mechanism of linear dunes of Qarhan Salt Lake, we used pattern analysis method to analyze the statistical characteristics and spatial variation of their pattern parameters. Except at the west-northwest margin, the pattern parameters showed regular spatial variation from the up-middle part towards the downwind end of the dune field. Based on the cumulative probability plots for inter-crest spacing and crest length, we divided the linear dunes into three groups, which corresponding to the three evolution stages of these dunes. The first group comprises erosional relics, with shorter crests, smaller inter-crest spacing and more random dune orientation. The second group comprises dunes whose sand supply is just sufficient to maintain stability and these dunes are approaching the net erosion stage. The crest length and inter-crest spacing of these dunes are much larger than those of the first group, and dune orientation is closer to the resultant drift direction (RDD). The last group comprises linear dunes that are still undergoing vertical accretion and longitudinal elongation, which follows the RDD of the modern wind regime. The presence of regular spatial variation of pattern parameters and a similar geometry with the vegetated linear dunes suggest that deposition and erosion coexist in the development and evolution of linear dunes of Qarhan Salt Lake, i.e. deposition predominates at the downwind end of linear dunes in the vertical accretion and longitudinal elongation stage, whereas erosion mainly occurs at the upwind end of linear dunes in the degradation stage. Therefore, the formation mechanism of linear dunes in Qarhan Salt Lake can be reasonably explained by the combination of depositional and erosional theories.

Grain-size characteristics of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China

In order to clarify the formation mechanism of linear dunes on the northern margin of Qarhan Salt Lake, northwestern China, we analyzed the grain-size and sorting parameters of the dune and interdune sands. The surface sands (0–30 mm) from the dune base to the crest of both flanks and interdune corridors were sampled along transects from upwind to downwind through the dune field. The results indicated that the grain-size distribution differed at different positions between and within the dunes. The frequency curve for dune sands mainly showed a bimodal distribution, while the interdune sediments showed a trimodal distribution. The grain size distribution of the linear dunes showed a finer crest pattern, i.e. the crests were composed of sands that were generally finer, better sorted than those of base sands. In addition, at the dune field scale, the dune crest sands were tending to become much finer but sorting became worse along the downwind transects. However, the grain-size parameters of sediments in the interdune corridors showed no clear pattern. The results demonstrated that the grain size and sorting parameters exhibited a systematic change not only at the individual scale but also at the dune field scale. Our results quantitatively estimate the limited role of cohesive sediments on the formation of linear dune under unidirectional wind regime. More attention should be paid to a long-term wind regime observation, internal sedimentary structures and their formation ages.

Airflow patterns upwind of obstacles and their significance for echo dune formation:A field measurement of the effects of the windward slope angle

The velocities in front of five topographic obstacles with windward slope angles between 45° and 80° were measured using ultrasonic anemometers in the field at the southeastern margin of the Tengger Desert. The characteristics of the horizontal and vertical velocities indicated that the airflow pattern was significantly influenced by the obstacle’s windward slope angle. The horizontal flow decelerated but the vertical flow accelerated upwind of the obstacles when the windward slope was less than 60°, but a reversed flow formed in front of steeper obstacles. The size of the area occupied by the reversed horizontal component increased with increasing windward slope, but the region of reversed vertical velocity became narrower and taller. Airflow turbulence was strengthened as the wind approached the obstacle, and low-frequency high-energy velocity fluctuations were observed upwind of the obstacle, and could affect the entrainment and saltation processes of sand particles. Turbulence intensities for both the horizontal and vertical components of the airflow increased with increasing windward slope and with decreasing distance from the obstacle. The deceleration and downward movement of airflow upwind of the obstacle would result in supersaturation of the air-sand flow, leading to deposition of moving sand, and these deposited grains would become the material basis for the formation of echo dunes. The reversed airflow would both determine the initial position of the echo dune and provide the necessary motive power to subsequently shape the echo dune.

An analysis of drag force on wind of shrubs simulated in a wind tunnel

Shrubs that exert drag force on the wind significantly affect the air–surface interaction in arid and semiarid regions, and some coastal regions where aeolian processes are active. An understanding of the drag force on wind of shrubs provides important information on the dynamics of shrubs in reducing wind erosion, and their geomorphological significances. Thus, we analyzed the drag force and drag coefficient for model shrubs using wind measurements obtained by particle image velocimetry in a scaled wind tunnel simulation. The drag force was found to be a function of wind velocity and shrub density. Both drag force and drag coefficient revealed a critical shrub density of around 0.08. The vertical drag force changed direction when the shrub density became greater or less than the critical density. The drag coefficient increased rapidly with shrub density when the density was below 0.08, but it increased very gently when the density was increased beyond the critical value. The results have an important theoretical significance for our understanding of the dynamics of vegetated dunes and the windbreak mechanism of shrubs.

Dune types and their distribution in the Kumtagh Sand Sea, northwestern China

Integrated regional studies are important tools for understanding dune geomorphology. As a part of an integrated regional study of dune geomorphology in the Kumtagh Sand Sea, northwestern China, we described the dune types and their distribution. In this paper, we propose a tentative dune classification scheme for these dunes based on previous schemes. We identified 11 main types of dunes, described their distribution, and estimated the areas they covered. Dunes generally became more complex moving from the north to the south of the study area, and from the edges to the inner areas of the sand sea. The wind regime also became more complex moving towards the south due to the physical blocking and thermodynamic effects of the mountains and the Qinghai-Tibetan Plateau, as well as the increasingly complex underlying topography. The combination of these factors appears to be responsible for the observed spatial variation of the dune types. Dune heights were estimated from 1:100,000 topographic maps and remote-sensing images. More than 80% of the dunes in the Kumtagh Sand Sea were less than 100 m tall, but with a maximum height exceeding 300 m. Dune height increased from north to south, with the tallest dunes occurring near the southern mountains. This general distribution of dune heights suggests that dune height was determined by a combination of sediment availability, airflow patterns, the blocking effect of mountains, and the underlying topography.

Mega-blowouts in Qinghai-Tibet Plateau: morphology, distribution and initiation: Mega-blowouts in Qinghai-Tibet Plateau

Blowouts are wind-eroded landforms that are widely distributed in the north-eastern part in Qinghai–Tibet Plateau (QTP), China. These blowouts are thought to form in response to climate change and/or human activity but little is known about their morphodynamics. Using field surveys, remote sensing and geographic information system (GIS) spatial analysis, the distribution and morphology of blowouts are analysed and their initiation considered. Results show the QTP mega-blowouts are some of the largest in the world. The orientations of the trough shaped blowouts are parallel with the prevailing wind, but the saucer and bowl-shaped blowouts are influenced by bi-directional transport. Whilst regional patterns of blowout shape and size were observed to reflect the extent of aeolian sediments and wind regimes, the relationship between the different morphological parameters showed consistency. During initial stages of development, the length to width ratios of blowouts increase rapidly with area but after they reach a mega size this relationship stabilizes as blowouts widen. Initial luminescence dating shows that blowouts appear to have initiated ~100 to 500 years ago, coinciding with the Little Ice Age (LIA) climate event when northwest winds are known to have intensified. Further work is required to confirm this initiation period and establish the significance of mega blowouts for landscape degradation and human activities.