Impact–entrainment relationship in a saltating cloud

Abstract

The problem of impact-entrainment relationship is one of the central issues in understanding saltation, a primary aeolian transport mode. By using particle dynamic analyser measurement technology the movement of saltating particles at the very near-surface level (1 mm above the bed) was detected. The impacting and entrained particles in the same impact-entrainment process were identified and the speeds, angle with respect to the horizontal, and energy of the impacting and entrained sand cloud were analysed. It was revealed that both the speed and angle of impacting and entrained particles vary widely. The probability distribution of the speed of impacting and entrained particles in the saltating cloud is best described by a Weibull distribution function. The mean impact speed is generally greater than the mean lift-off speed except for the 0Ð1-0Ð2 mm sand whose entrainment is significantly influenced by air drag. Both the impact and lift-off angles range from 0°to 180°. The mean lift-off angles range from 39°to 94°while the mean impact angles range from 40°to 78°, much greater than those previously reported. The greater mean lift-off and especially the mean impact angles are attributed to mid-air collisions at the very low height, which are difficult to detect by conventional high-speed photography and are generally ignored in the existing theoretical simulation models. The proportion of backward-impacting particles also evidences the mid-air collisions. The impact energy is generally greater than the entrainment energy except for the 0Ð1-0Ð2 mm sand. There exists a reasonably good correlation of the mean speed, angle and energy between the impacting and entrained cloud in the impact-entrainment process. The results presented in this paper deserve to be considered in modelling saltation.

Reconstructing the vertical distribution of the aeolian saltation mass flux based on the probability distribution of lift-off velocity

“…Nalpanis et al (1993) found that the probability distributions of resultant lift-off velocity and angle are similar to a log-normal distribution. The experiment of Dong et al (2002a) described the velocity distribution of entrained particles as a Weibull function, the probability of resultant lift-off velocity decreases with increasing resultant lift-off velocity, but the probability distribution of lift-off angles is complex and cannot be expressed by a simple function. In the numerical model of Namikas (2003), both gamma and exponential launch velocity distributions are studied.…”

“…White and Schulz, 1977;Willetts and Rice, 1986;Anderson and Hallet, 1986;Werner and Haff, 1988;Anderson and Haff, 1988;Sørensen, 1991;Haff and Anderson, 1993;Nalpanis et al, 1993;Rice et al, 1995Rice et al, , 1996Dong et al, 2002a;Namikas, 2003). If the probability distributions of the horizontal and vertical lift-off velocities or the resultant lift-off velocity and angle of saltating grains are known, the proportion of the number of grains with a given resultant lift-off velocity and lift-off angle will be acquired, then the macroscopic statistic of particles can be deduced from the representative trajectories of saltating grains.…”

“…Nalpanis et al (1993) found that the probability distribution of resultant lift-off velocity and angle is similar to the log-normal distribution. Dong et al (2002a) gave experimental results showing that the probability distribution of the velocity of entrained particles is best described by a Weibull function. Namikas (2003) used the gamma and exponential launch velocity distributions in the numerical model.…”

Numerical investigation of particle velocity distributions in aeolian sand transport

“…Willetts and Rice, 1986;Anderson and Haff, 1991;Nalpanis et al, 1993;Greeley et al, 1996;Zou et al, 2001;Dong et al, 2002;Namikas, 2003;Dong et al, 2004;Huang et al, 2006;Cheng et al, 2006). Haff (1988, 1991) used a Gaussian distribution for the rebound velocity of particles and an exponential function for the ejected velocity.…”

Wind tunnel experimental investigation of sand velocity in aeolian sand transport

“…However, it is difficult to deduce the sand velocity from the photographic images at very low heights where the grains are too crowded (e.g., Zou et al, 2001). Similarly, the selection of particles is inevitably subjective, and the number of detected particles is usually very limited and insufficient for detailed analysis of particle velocity in a blowing sand cloud (Dong et al, 2002a). The analysis of particle mean velocity, probability distribution of particle velocity, particle velocity fluctuation and particle turbulence requires measuring a sufficient number of sand particles.…”