field measurements of the flux and speed of wind blown sand a field experiment was conducted to measure the flux and speed of wind blown sand under known conditions in a natural setting the experiment run at pismo beach california involved a tract 100 m long parallel with the wind by 20 m wide the site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary layer temperature probes to determine atmospheric stability and wind vanes to determine wind direction from these measurements wind friction speeds were derived for each experimental run in order to measure sand saltation flux a trench 3 m long by 10 m wide transverse to the wind direction by 0 5 m deep was placed at the downwind end of the tract and lined with 168 collector bins forming an egg box pattern the mass of particles collected in each bin was determined for four experimental runs in order to assess various sand trap systems used in previous experiments 12 leatherman traps one fryberger trap and one array of ames traps were deployed to collect particles concurrently with the trench collection particle velocities were determined from analysis of high speed 3000 and 5000 frames per second motion pictures and from a particle velocimeter sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined fluxes for each run were calculated using various previously published expressions and then compared with the flux derived from the trench collection results show that bagnold s 1941 model and white s 1979 equation most closely agree with values derived from the trench comparison of the various collector systems shows that the leatherman and ames traps most closely agree with the flux derived from the trench although these systems tended to under collect particles particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories results show that particle velocities from the velocimeter are in the range 0 5 7 0 m s 1 compared to a wind friction velocity of 0 32 0 43 m s 1 and a wind velocity of 2 7 3 9 m s 1 at the height of the particle measurements descending particles tended to exceed the speeds of ascending particles by 0 5 m s 1

near bed mass flux profiles in aeolian sand transport high resolution measurements in a wind tunnel vertical profiles of the streamwise mass flux of blown sand in the near bed 17 mm region are analysed from high resolution measurements made using an optical sensor in a wind tunnel this analysis is complemented by detailed measurements of mass flux and mean velocity profiles throughout the boundary layer depth 0 17 m using passive chambered sand traps of small dimensions and armoured thermal anemometers respectively the data permit a preliminary analysis of the relations between the observed forms of the profiles of near bed fluid stress and horizontal mass flux within a carefully conditioned boundary layer profiles of mass flux density are found to be characterized by three regions of differing gradient with transitions at about 2 mm and 19 mm above the bed the exponential decay of mass flux with height is confirmed for elevations above 19 mm and when plotted as a function of u 2 g a parameter of mean vertical trajectory height in saltation the gradient of mass flux in this region scales with the wake corrected friction velocity u where u 0 30 m s 1 a separate near bed region of more intense transport below 19 mm is identified which carries 80 per cent of the total mass flux this region is evident in some previous field and wind tunnel data but not in profiles simulated by numerical models ventilated passive sand traps underestimate mass flux in this region by 37 per cent at slow or moderate wind speeds a third significant region below 2 mm is observed these regions are likely to be related to grain populations in successive saltation low energy ejections and intermittent bed contact respectively optical measurements reveal locally high grain concentrations at some elevations below 5 mm these heights scale with transport rate mass flux gradient and wind speed copyright 1999 john wiley sons ltd

grain size and transport characteristics of non uniform sand in aeolian saltation abstract size frequency distributions of sediment particles in a wind tunnel containing a bed of non uniform sand are investigated by re interpreting existing experimental data using particle size analysis each particle sample is classified into one of eight groups according to its size grading the analysis reveals that the modal shape of the particle size frequency distributions of the saltating sand at different elevations or longitudinal distances is similar to that of the mixed sand in the bed once the boundary layer is fully developed the standard deviation of the grain size frequency distribution increases with increasing elevation above the bed then stays constant whereas its skewness decreases the mean grain size decays exponentially with elevation the aeolian sand mass flux is determined for each size grading at different vertical and horizontal measurement locations the vertical profile of aeolian horizontal mass flux depends on the size grading the distribution of the sand transport rate according to the mean grain size in each grading fits the normal distribution a parameter w i is defined to reflect the likelihood of saltation for sand particles of the i th size grading and the mean sand size corresponding to the maximum value of w i is found to be 0 2 mm in addition wind velocity strongly influences the magnitudes of the particle size distribution and the sand mass flux distribution in both vertical and longitudinal directions

reconstructing the vertical distribution of the aeolian saltation mass flux based on the probability distribution of lift off velocity the probability distribution of lift off velocity of the saltating grains is a bridge to linking microscopic and macroscopic research of aeolian sand transport the lift off parameters of saltating grains i e the horizontal and vertical lift off velocities resultant lift off velocity and lift off angle in a wind tunnel are measured by using a phase doppler particle analyzer pdpa the experimental results show that the probability distribution of horizontal lift off velocity of saltating particles on a bed surface is a normal function and that of vertical lift off velocity is an exponential function the probability distribution of resultant lift off velocity of saltating grains can be expressed as a log normal function and that of lift off angle complies with an exponential function a numerical model for the vertical distribution of aeolian mass flux based on the probability distribution of lift off velocity is established the simulation gives a sand mass flux distribution which is consistent with the field data of namikas namikas s l 2003 field measurement and numerical modelling of acolian mass flux distributions on a sandy beach sedimentology 50 303 326 therefore these findings are helpful to further understand the probability characteristics of lift off grains in aeolian sand transport c 2007 elsevier b v all rights reserved

analysis of velocity profile measurements from wind tunnel experiments with saltation abstract investigations of wind field modification due to the presence of saltating sediments have relied heavily on wind tunnels which are known to impose geometric constraints on full boundary layer development there remains great uncertainty as to which portion of the vertical wind speed profile to analyze when deriving estimates of shear velocity or surface roughness length because the lower sections are modified to varying degree by saltation whereas the upper segments may be altered by artificially induced wake like effects thus it is not obvious which of several alternative velocity profile parameterizations e g law of the wall velocity defect law wake law should be employed under such circumstances a series of experimental wind tunnel runs was conducted across a range of wind speed using fine and coarse grained sand to collect high quality fine resolution data within and above the saltation layer using thermal anemometry and ruggedized probes after each run the rippled bottom was fixed with fine mist and the experiment repeated without saltation the measured wind speed profiles were analyzed using six different approaches to derive estimates of shear velocity and roughness length the results were compared to parameter estimates derived directly from sediment transport rate measurements and on this basis it is suggested that one of the six approaches is more robust than the others specifically the best estimate of shear velocity during saltation is provided by the logarithmic law applied to the profile data within about 0 05 m of the bottom despite the fact that this near surface region is where profile modification by saltating sediments is most pronounced uncertainty remains as to whether this conclusion can be generalized to field situations because progressive downwind adjustments in the interrelationship between the saltation layer and the wind field are anticipated in wind tunnels thereby confounding most analyses based on equilibrium assumptions

the status of research on the development and characteristics of mass flux density profiles above wind eroded sediments a literature review the mass flux density profile above wind eroded sediments mfdp plays an important role in defining the characteristics and impacts of wind sand flows and therefore has great theoretical and practical significance as a result it has been extensively studied from the perspectives of geography mechanics physics and engineering this paper reviews the research work that has been conducted since bagnold s pioneering efforts summarizes the research achievements discusses the problems that have not yet been solved and proposes future research directions this field of research has progressed from field observations to theoretical analyses and numerical simulations accompanied by increasing sophistication in the research and the research scope includes land with mobile sands wind eroded farmland gravel gobis and degraded grasslands many mathematical functions have been proposed to describe mfdp including exponential power modified exponential modified power and polynomial functions this variation results from differences in the study conditions and in the parameters included in a given model and on differences between wind tunnel experiments and field observations however there are the limitations of both wind tunnel experiments and theoretical derivations and the key parameters of the profile lack a clear physical meaning which limits the practical application of these functions future research must aim to solve these problems

the saltations of different sized particles in aeolian sand transport abstract wind tunnel experiments were performed in order to investigate the effect of mixing on the aeolian transport of sands with different grain sizes two types of sand with different grain size distributions and an equal mass binary mixture of these sands were used comparing the gradients of their measured mass flux profiles and some published profiles for mixed sand transport with those for nearly uniform sand transport it was found that for both of these types of sand bed the negative gradient of the mass flux profile on a log linear plot varies with the mass averaged grain size of the sand bed according to the same power law hence it can be deduced that during the aeolian transport of mixed sand beds the mean vertical ejection speeds of different sized grains are nearly identical to that for the transport of monospecific sized sand with the same mass averaged grain size theoretical analysis was undertaken to explore the characteristics of ejections of different sized grains during the aeolian transport of sand with mixed size grains it is proposed that the mean ejection angles and the mean ejection speeds for sand grains of different sizes are nearly identical and are equivalent to that for monospecific sized sand with the same mass averaged grain size it was also evident that the ratio between the transport rate of each grain size group expressed as the fraction in the whole transport rate and its mass fraction in the mixed size sand is a combined consequence of both the wind effect on the mean saltation distance of different sized grains and the effect of the mass fraction of each grain size group in the original mixture on its ejection potential responding to an impact of a saltating sand grain

grain size characteristics of sediment transported during dust storms abstract the grain size characteristics of sediment transported by wind in creep saltation and suspension during 15 dust storms in the slims river valley yukon territory were investigated grain size analysis of the transported sediment indicated that the suspended sediment is significantly finer and better sorted than the sediment transported in either surface creep or saltation the data also show that during dust storms the mean particle size of the suspended sediment decreases as a power function of height the grain size distributions of the sediment transported in suspension and surface creep are characterized by strong positive skewness which results from the selective removal of surface particles small enough to be transported by the wind at a given velocity in contrast the grai size distribution of the saltation samples are negatively skewed which is thought to result from the selective removal of the finer particles into suspension from those particles initially lifted from the surface into the air stream thus the proportion of sediment initially entrained into the air stream which returns to the surface in saltation will lack the fine grains removed in suspension and will therefore tend to be less positively or negatively skewed

equations for the near surface mass flux density profile of wind blown sediments the erosion of sediment by wind and the resulting mass flux density profile is thought to be described by a mathematical function that bears information on the mechanisms responsible for the movement of individual particles by the wind and such functions have been studied extensively in this study several functions are evaluated that have been proposed to describe the variation in mass flux density with height of wind blown sediment with the flux containing a mixture of particles in suspension and saltation based on detailed field data at four land types in the minqin area of north western china where severe wind erosion occurs high resolution mass flux density measurements at 50 heights collected at 20 mm intervals to a height of 1 m above the surface were obtained using vertically segmented samplers three kinds of functions fit the measured flux density profiles reasonably well but a three parameter modified exponential function is preferred because it contains fewer coefficients to be defined and provides a reasonably good fit to the measured mass flux density profiles this and previous conclusions suggest that the decay with height of mass flux density of sediments dominated by saltation particles as in the present study tends to follow a modified exponential function law but a modified power function law replaces the modified exponential function law when the height extends to a level high enough to be dominated by suspension particles copyright 2011 john wiley sons ltd

saltating particles over flat beds measurements of ejection and impact velocities trajectory lengths and maximum rise heights of sand grains median diameters 118 and 188 m in saltation over a flat sand bed in a wind tunnel have been obtained from the digitization of multiple image photo graphs the mean angle of ejection is found to be about 30 o from the horizontal rather than 90 o with mean vertical ejection velocity of about 2u where u is the friction velocity trajectories of saltating grains have been computed using the measurements of the initial ejection velocities and the mean velocity profile of the air flow

convection in groundwater below an evaporating salt lake 1 onset of instability convective groundwater motion can be generated in aquifers beneath and adjacent to saline lakes such motions strongly control the subsurface distribution of salts the formation of saline minerals and the distribution of contaminants that may be disposed of in the saline lake in particular schemes to use saline lakes as evaporation basins for irrigation waste waters must consider the groundwater dynamics associated with convection the convection is driven by the evaporative concentration of salts at the land surface leading to an unstable distribution of density however the evaporative groundwater discharge can dynamically stabilize this saline boundary layer and inhibit convection in this work we investigate the nature and onset of convection for a range of boundary conditions typically found in saline lakes processes involved in the accumulation of salt at the surface of a groundwater discharge zone and in the gravitational instability of the near surface groundwater are considered results of theoretical stability analysis are applied through laboratory and numerical experimentation to the more complex geometries typically found in the field these comparisons indicate that for large saline lakes the stability of the saline boundary layer can be parameterized by traditional rayleigh criteria in which the aquifer permeability and the evaporation rate from the lake bed are principal controlling factors for typical saline lake environments convection will dominate in sediments whose permeability exceeds approximately 10 14 m2 below this threshold permeability the boundary layer should be stabilized by the evaporative flux resulting in the accumulation of salts and evaporites at the land surface these results provide insight into the predictive behavior of groundwater dynamics and solute distributions in many natural systems

numerical simulation of dust lifting within dust devils simulation of an intense vortex abstract based on an advanced dust devil scale large eddy simulation les model the atmosphere flow of a modeled dust devil in a quasi steady state was first simulated to illustrate the characteristics of the gas phase field in the mature stage including the prediction of the lower pressure and higher temperature in the vortex core the dust lifting physics is examined in two aspects through the experimental data analysis it is verified again that the horizontal swirling wind can only make solid particles saltate along the ground surface based on a lagrangian reference frame the tracks of dust grains with different density material and diameter are calculated to show the effect of dust particles entrained by the vertical swirling wind field the movement of solid particles depends on the interactions between the aloft dust particles and the airflow field of dust devils in which the drag and the centrifugal force component on the horizontal plane are the key force components there is the trend o

flow dynamics and sediment transport over a reversing barchan changli china abstract the flow and sediment transport over a reversing 6 9 m high barchan dune is examined on the changli gold coast in hebei province china wind velocity profiles are non logarithmic exhibit rapid accelerations up slope and display near surface jets at the dune crest and downwind lee slope from the windward lower slope to dune crest the wind speed at 5 cm height increased by 55 the windward slope is a slipface and therefore has a significantly greater gradient than most normal windward slopes and the magnitude of speed up is significantly greater speed up ratio of 5 25 the majority of sediment was transported in the 0 4 cm height above the bed range maximum total sediment flux occurred at the dune crest and was greater than the total flux from three other sites combined the leeward mid slope sedimentation rate is strongly influenced by the sediment plume streaming upwards and downwind from the actively reversing dune crest dunes in the process of reversing display quite different wind flow and sedimentation patterns than when the dune is in normal flow conditions

sidewall effects of a wind tunnel on wind velocity and mass flux in aeolian sand transport abstract the sidewall effects of a wind tunnel on aeolian sand transport were investigated experimentally a wind tunnel was used to conduct the experiments with a given channel height of 120 cm and varying widths b of 40 60 80 100 and 120 cm both vertical profiles of wind velocity and sand mass flux were measured at different locations across the test section the results show that the wind velocity with saltation first increases and then decreases to a minimum from the sidewall to the central line of the wind tunnel the discrepancy among wind velocities at different locations of the transverse section decreases with decreasing tunnel width the wind friction velocity across the wind tunnel floor with the exception of the region closest to the sidewalls does not deviate strongly in wide wind tunnels from that along the central line whereas it does vary in narrow tunnels the sand mass fluxes with the exception of some near bed regions are larger along the central line of the wind tunnel than they are at the quarter width location from the sidewall unlikely previously reported results the dimensionless sand transport rate qg u 3 where q is the total sand transport rate g is the gravitational acceleration constant is the air density and u is the wind friction velocity first decreases and then increases with the dimensionless friction velocity u u t where u t is the threshold friction velocity the above differences may be attributed to the sidewall effects of the wind tunnel a dimensionless parameter f b u gb 1 2 is defined to reflect the sidewall effects on aeolian sand transport the flows with f b of 0 33 or less may be free from the sidewall effects of the wind tunnel and can ensure accurate saltation tunnel simulation

aerodynamic roughness lengths over movable beds comparison of wind tunnel and field data 1 we evaluate the increase in apparent roughness length during aeolian saltation using a data set comprising 291 wind profiles measured in wind tunnels or in the field we tested the charnock 1955 modified charnock sherman 1992 and raupach 1991 relationships for the entire data set and for segregated wind tunnel and field data sets we assessed the abilities of the models to predict enhanced roughness and investigated scaling differences between wind tunnels and field environments for the entire data set the charnock relationship worked best with an r2 of 0 25 for the segregated data sets the charnock relationship also worked best with an average r2 of 0 675 as opposed to 0 66 for the modified charnock and 0 61 for the raupach model for conditions with shear velocity 1 0 ms 1 the comparable average values of r2 are 0 54 0 625 and 0 645 respectively there was an order of magnitude difference in the roughness length responses for wind tunnel and field data for each model evaluated convincing evidence of a fundamental scaling constraint in the wind tunnel data the scaling differences are attributed to conceptual and bias errors that include the use of the law of the wall versus the wake defect law or differences in turbulence characteristics especially regarding the role of large coherent structures raupach s 1991 a was found to be an extremely volatile constant making the modified charnock relationship most suitable for predicting aerodynamic roughness lengths over movable sand beds

an analytic expression for wind velocity profile within the saltation layer the vertical wind velocity profile within the saltation layer is investigated theoretically new equations for the fluid shear stress distribution in the saltation layer and the velocity profile affected by saltation are derived the dependence of the velocity profile on the threshold shear velocity the vertical variations of sand grain velocity the vertical sand mass flux distribution and the fluid shear stress within the saltation are demonstrated velocity profiles with or without bagnold s focus are predicted these velocity profiles are not very sensitive to the vertical distribution of sand mass flux predicted velocity profiles are compared with profiles measured in the wind tunnel and satisfactory agreement has been achieved

sampling efficiency of vertical array aeolian sand traps abstract previous investigations have indicated that the sampling efficiency of aeolian sand traps is low and varies greatly in the near bed region outside this region the efficiency tends to be consistently higher for all types of trap an evaluation was carried out to compare the sampling efficiency of different types of aeolian sand trap based on the comparison of the actual and the measured sand mass flux profiles with emphasis on the single tube vertical array trap conventional array trap and step like array trap a simple formula is proposed to express the actual vertical profile of sand mass flux which has been validated with the unique data obtained with an isokinetic trap by sedimentology 45 1998 789 using the experimental data collected by the present authors and those by other investigators sampling efficiencies of three types of trap are examined in terms of the frequency distribution of all the samples for the single tube traps the sampling efficiency varies from 65 to 95 with a mode at 75 for both the conventional array and step like array traps sampling efficiencies range from 15 to 85 with the modal frequencies at 35 and 75 respectively this review seems to suggest that the peak frequency with higher sampling efficiency coincides with the maximum sand grain reynolds number

a new method for precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field summary we present a new method which provides precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field assuming a 2 d magnetic structure the intensity of spatial differential vectors isdv exhibits a peak at the boundary of magnetic structures the strike of the magnetic boundary is obtained using the fact that the magnetic field is zero in the direction parallel to the boundary we have also applied this method to 3 d magnetic structures our method requires only the relative changes of the three components which are more accurately measured than their actual magnitudes we have applied this method to data from the southern juan de fuca ridge obtained on the r v hakuho maru expedition in 1988 kh 88 3 the results are summarized in magnetic boundary strike diagrams mbsd these are maps that show the position strike and character of the magnetic boundaries mbsd provides valuable information for understanding the magnetic structure of oceanic crust such as magnetic lineations pseudo faults fracture zones and the central anomaly magnetic high camh therefore relatively few vector magnetic profiles provide information similar to extensive total intensity geomagnetic surveys

a steady state model of wind blown sand transport a model for steady state eolian saltation is presented in which computation of sand grain trajectories in the wind and of the wind velocity profile modified by saltating grain drag forces is combined with experimental data on grain bed collisions in an iterative simulation scheme which resembles the time development of natural saltation qualitative features of eolian saltation are reproduced in the simulations including a height in the saltating layer at which the wind velocity remains roughly constant with changing free stream friction velocity and an approximate separation of the saltating population into successively hopping grains and reptating or creeping grains the numerical solutions give rise to more general arguments in support of the conclusions that fluid entrainment of grains is unimportant in steady state saltation that the mass flux profile obeys roughly an inverse power law dependence on height and that the vertically integrated mass flux dependence on the free stream friction velocity

on the efficiency of vertical array aeolian field traps the efficiency of three vertical array field traps for measuring the distribution of aeolian mass transport with height was compared with measurements with an isokinetic trap the vertical traps seem to work adequately efficiency 80 at heights greater than 15 mm above the bed closer to the bed however the open array trap catches about 70 the two continuous arrays only about 50 because most of the transport takes place close to the surface the overall efficiency of the field traps ranges from 50 to 70 based on these wind tunnel tests we have developed a low single compartment wedge shaped trap its shape in combination with flow straighteners at the entrance minimizes stagnation effects so that the efficiency is 80 our data indicate that with the wedge trap used in combination with the vertical array aberdeen trap we can sample mass transport in the field during periods with unidirectional winds with an overall efficiency of the order of 80

aeolian particle flux profiles and transport unsteadiness vertical profiles of aeolian sediment flux are commonly modeled as an exponential decay of particle mass transport with height above the surface data from field and wind tunnel studies provide empirical support for this parameterization although a large degree of variation in the precise shape of the vertical flux profile has been reported this paper explores the potential influence of wind unsteadiness and time varying intensity of transport on the geometry slope curvature of aeolian particle flux profiles field evidence from a complex foredune environment demonstrates that i the time series of wind and sediment particle flux are often extremely variable with periods of intense transport referred to herein as sediment flurries separated by periods of weak or no transport ii sediment flurries contribute the majority of transport in a minority of the time iii the structure of a flurry includes a ramp up phase lasting a few seconds a core phase lasting a few seconds to many tens of seconds and a ramp down phase lasting a few seconds during which the system relaxes to a background low intensity transport state and iv conditional averaging of flux profiles for flurry and nonflurry periods reveals differences between the geometry of the mean profiles and hence the transport states that produce them these results caution against the indiscriminate reliance on regression statistics derived from time averaged sediment flux profiles especially those with significant flurry and nonflurry periods when calibrating or assessing the validity of steady state models of aeolian saltation

vertical profiles of aeolian sand mass flux abstract vertical profiles of the horizontal mass flux of blown sand are investigated experimentally using a passive vertical array in a wind tunnel considering lower sampling efficiency of the sand trap in the near bed region this investigation is complemented by the measurements of the longitudinal profiles of mass flux made using a horizontal sand trap the experiments were conducted with two test sands and five different stream velocities in the upper part of the vertical profile the measured data exhibit an exponential decay distribution with a positive deviation occurring in the near bed region the measured longitudinal profiles are similar to the measured vertical profiles linking both profiles and the modes of sand transport it is possible that saltating sand grains give rise to the well known exponential decay distribution of mass flux and that creeping and reptating grains force a deviation from it a simple equation applicable for both the vertical and the longitudinal sand mass flux variations is introduced and the parameters are estimated from experimental data

aeolian vertical mass flux profiles above dry and moist sandy beach surfaces abstract the vertical distribution of aeolian mass flux was investigated in a natural beach environment field experiments conducted on the beach of the eba barrier southern baltic coast poland measured the sand transport rate and the vertical mass flux distribution above dry rippled sand and a moist flat sandy surface the experiments were intended to show the changes in the vertical distribution of sand with changing wind speed all the data represent saturated flux conditions sand transport was measured using 0 5 m high vertically segmented passive sand traps while the wind speed and direction were monitored at 1 m elevation the obtained dataset comprises 65 measurements on dry surfaces and 51 measurements on moist sandy surfaces the sand transport rate above the moist surface was up to 90 higher than above the dry surface for wind speeds of 7 11 m s but higher velocities gave smaller differences between the surfaces the saltation layer was thicker above the moist surface than above the dry surface all the vertical sand flux profiles are best described by exponential decay functions analysis of the normalised flux profiles grouped by wind velocity shows that the fitted curves are less inclined for moist surfaces than dry surfaces moreover the regression coefficients depict a marked trend in which the intercept decreases and the slope increases with increasing wind speed this indicates that more sand is transported at higher elevations above the bed and less at lower elevations the proportion of total transport seems to be independent of wind speed at elevations of approximately 35 mm and 50 mm above the dry and moist surfaces respectively differences between the measured and exponential fit values of mass flux are particularly distinct close to the bed where the exponential fit either over or under predicts the measured values over predictions occur in weaker winds up to 6 7 m s whereas under predictions become more pronounced as the wind becomes stronger and when the layer in which the under prediction occurs thickens the under predictions are particularly obvious above the moist surface it is not clear whether this phenomenon is due to technique measurement error or to another source the proportion of creep is often considered to be responsible for the difference from the exponential fit which represents the saltation mode of aeolian transport this proportion is difficult to determine because the mass flux measured by the lowermost chamber of the sand trap used in the experiments represents sand transported in both saltation and creep or and reptation

a numerical study of particle motion and two phase interaction in aeolian sand transport using a coupled large eddy simulation discrete element method wind blown sand movement considered as a particle laden two phase flow was simulated by a new numerical code developed in the present study the discrete element method was employed to model the contact force between sand particles large eddy simulation was used to solve the turbulent atmospheric boundary layer motions of sand particles were traced in the lagrangian frame within the near surface region of the atmospheric boundary layer interparticle collisions will significantly alter the velocity of sand the sand phase is quite dense in this region and its feedback force on fluid motion cannot be ignored by considering the interparticle collision and two phase interaction four way coupling was achieved in the numerical code profiles of sand velocity from the simulations were in good agreement with experimental measurements the mass flux shows an exponential decay and is comparable to reported experimental and field measurements the turbulence intensities and shear stress of sand particles were estimated from particle root mean square velocities distributions of slip velocity and feedback force were analysed to reveal the interactions between sand particles and the continuous fluid phase

characterizing the height profile of the flux of wind eroded sediment wind erosion causes severe environmental problems such as aeolian desertification and dust storms in arid and semiarid regions reliable prediction of the height profile of the wind eroded sediment flux is crucial for estimation of transport rates verification of computer models understanding of particle modified wind flows and control of drifting sand this study defined the basic height profile for the flux of wind eroded sediment and the coefficients that characterize its equation nine grain size populations of natural sand at different wind velocities were tested in a wind tunnel to measure the flux of sediment at different heights the resulting flux profiles resemble a golf club with a small back turn where the flux increases with increasing height within 20 mm above the surface if the small back turns are neglected the flux profiles can be expressed by an exponential decay function q text r z a text e bz r where qr z is the dimensionless relative flux of sediment at height z which follows the exponential decay law proposed by previous researchers for aeolian saltation three coefficients a creep proportion a relative decay rate and an average saltation height are proposed to characterize the height profile coefficients a and b in the above equation represent the creep proportion and relative decay rate as a function of height respectively coefficient a varies widely depending on grain size and wind velocity but averages 0 09 it is suggested that the grain size and wind velocity must be specified when discussing creep proportion coefficients a and b are nearly linearly correlated and decrease as grain size and wind velocity increase the average saltation height the average height sediment particles can reach was a function of grain size and wind velocity and was well correlated with coefficients a and b

controlling windblown sand problems by an artificial gravel surface a case study over the gobi surface of the mogao grottoes abstract in arid regions gravel surfaces are usually known for protecting the soil from wind erosion trapping the sand and dust carried by the wind thereby forming a sink of sediment however gravel surfaces with a suitable gravel size and coverage density can also act as the source of sediment under unsaturated wind flow a dynamically equilibrated gravel surface can be built to prevent potential windblown sand problems in the dunhuang mogao grottoes in northwestern china unlike previous studies that suggested constructing a high coverage 50 or more gravel surface on the gobi land on top of the mogao grottoes a much lower value of 30 is recommended this kind of artificial gravel surface can protect the underlying sand materials from wind erosion and trap the invading westerly windblown sand at the same time the deposited sand over the artificial gravel surface can be transported to the western megadunes by the easterly wind under the multidirectional wind regime the gravel surface can remain from sand burial over the long term

wind tunnel experimental investigation of sand velocity in aeolian sand transport sand velocity in aeolian sand transport was measured using the laser doppler technique of pdpa phase doppler particle analyzer in a wind tunnel the sand velocity profile probability distribution of particle velocity particle velocity fluctuation and particle turbulence were analyzed in detail the experimental results verified that the sand horizontal velocity profile can be expressed by a logarithmic function above 0 01 in while a deviation occurs below 0 01 m the mean vertical velocity of grains generally ranges from 0 2 m s to 0 2 m s and is downward at the lower height upward at the higher height the probability distributions of the horizontal velocity of ascending and descending particles have a typical peak and are right skewed at a height of 4 turn in the lower part of saltation layer the vertical profile of the horizontal rms velocity fluctuation of particles shows a single peak the horizontal rms velocity fluctuation of sand particles is generally larger than the vertical rms velocity fluctuation the rms velocity fluctuations of grains in both horizontal and vertical directions increase with wind velocity the particle turbulence intensity decreases with height the present investigation is helpful in understanding the sand movement mechanism in windblown sand transport and also provides a reference for the study of blowing sand velocity c 2007 elsevier b v all rights reserved