[1] In this work, we numerically simulated the saltating particles in a turbulent boundary layer over flat bed and sand ripples. By using natural sand grains in a wind tunnel, we obtained the initial conditions for the simulation and also verified the correctness of the numerical model. We carefully analyzed the numerically simulated saltating particle movement over the two sand beds, and we found the following. (1) The aeolian sand transport is a dynamic equilibrium process on both sand beds, and it took longer to reach equilibration on the sand ripples than on the flat bed. (2) According to the mass flux profile at the trough of the sand ripples, there is a maximum mass flux at about 4 cm height in the leeward section. The mass flux increases with height below 4 cm and decreases with height above 4 cm. (3) The wind profile near the surface is modified by saltating particles on the two different sand beds, and the flow field characteristics of the sand ripples are more complex than that of the flat bed.Citation: Tong, D., and N. Huang (2012), Numerical simulation of saltating particles in atmospheric boundary layer over flat bed and sand ripples,
Straw checkerboard barrier (SCB) is the most representative antidesertification measure and plays a significant role in antidesertification projects. Large-eddy simulation and discrete-particle tracing were used to numerically simulate the wind sand movement inside the straw checkerboard barrier (SCB), study the movement characteristics of sand particles, find the transverse velocities of sand particles and flow field, and obtain the contour of the transverse velocity of coupled wind field within the SCB. The results showed that 1) compared with that at the inlet of the SCB, the sand transport rate inside the SCB greatly decreases and the speed of sand grain movement also evidently drops, indicating that the SCB has very good sand movement preventing and fixing function; 2) within the SCB there exists a series of unevenly distributed eddies of wind sand flow, their strength decreases gradually with increasing the transverse distance; 3) affected by eddies or reflux, sand particles carried by the wind sand flow have to drop forward and backward the two interior walls inside the SCB, respectively, forming a v-shaped sand trough; 4) the sand transport rate gradually decreases with increasing number of SCBs, which reveals that the capacity of the wind field to transport sand particles decreases. This research is of significance in sandstorm and land desertification control.
The stall and surge directly impact on the safety and reliability of compressors. The spike-type and modal-type stall inception exist in compressors. At present, few studies pay attention to the stall inception of centrifugal compressor, such as the formation reason for the stall inception and the action by the volute tongue on the stall precursor. This paper investigated the stall characteristics of a high-speed small-flow centrifugal compressor and illustrated the relationship between the volute tongue and the location of stall inception. In addition, the mechanism of stall inception was also clarified. Both the analysis of initial flow structures and the comparison of the frequency spectrum characteristics at different monitoring points show that the spike-type stall occurs at about 115° circumferential position in this centrifugal compressor. The nonaxisymmetric geometry structure of the volute leads to the uneven circumferential pressure distribution. The blockade effect of the volute tongue results in high static pressure area near the volute tongue. The disturbance caused by high static pressure adversely propagates into the diffuser, resulting in the static pressure peak value at different radii. As the pressure peaks adversely migrate to the impeller inlet and induce the leading edge spillover near the corresponding blade, the spike-type stall occurs. Therefore, the volute tongue both induces the stall inception and determines the circumferential position of the stall inception at the centrifugal compressor inlet.
In this paper, the cross flow in several internal and external volutes of a small centrifugal compressor is examined to highlight its effects on compressor performance. Different levels of the loss associated with shear stress related to the vortices in the volute cross section are shown to be the reason behind the diverse performance of internal volutes and external volutes. A numerical simulation is carried out to substantiate this and to further study this effect of cross sectional shape of volutes.
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