جستجوی مقالات مرتبط با کلیدواژه « relative roughness » در نشریات گروه « آب و خاک »

The transfer of solid particles in fluid flow has been a common phenomenon in nature. The threshold of movement of sediments from the physical dimension is mixed with both deterministic and random characteristics. Certainty in the movement threshold is caused by the average flow rate, while its randomness is caused by the turbulence of the flow. The initial movement of sediment is the beginning of scouring process in natural streams. The sedimentation threshold is the condition where the destabilizing forces equal the stabilizing forces and the sediment particle is on the verge of experiencing primary motion. Two common sedimentation threshold parameters, Shields parameter and moving number, have been studied by different researchers. The initial movement of bed sediments is an important component for various researches such as sustainable waterway design, river management, sediment transport and sedimentation prevention. The beginning of the movement of sedimentary particles is called the threshold of movement and the conditions in which the particles are on the threshold of movement are called threshold or critical conditions. In low-speed flows, the substrate particles do not have any movement and are fixed in their place, but with the increase in the flow speed, the substrate particles start to move. This movement is initially in the form of up and down of the particle without transfer, but gradually the movement The particles start downstream.

In the current research, the criterion of the threshold of particle movement is the beginning of non-stop movement of particles that are set in motion in the model and move along the flume to the downstream side of the flume as the threshold of particle movement. During the test, the flow rate of the model and the upstream water level are controlled several times so that there may be no change. From dividing the flow rate, leading to the movement threshold by the flow section, the critical speed or the movement threshold speed is determined. To find the threshold speed of movement of a sediment sample in a specific slope of the channel, each experiment is repeated several times and the results are averaged. After finishing the test, the remaining sediment on the metal plate is removed. Some of the sediments have also entered the tank, and after collecting the sediments, they are dried and prepared for the next stage of tests. After the completion of each test, the slope of the channel is changed so that the main section is placed in another slope. Then all the steps mentioned above are repeated again. It seems that the Shields diagram is the reference point for all the research done in the field of sediment transport. Due to the fact that no research has been done on the effect of temperature on the Shields diagram, the present study aims to investigate the sediment movement threshold using the effect of temperature on the Shields parameter and shear stress in a laboratory model with three slopes of 0.005, 00.1 and 0.05 at four temperatures of 8, 12, 20, and 40 degrees Celsius and three flow rates. In the current research, the criterion of the threshold of particle movement is the beginning of non-stop movement of particles that are set in motion in the model and move along the flume to the downstream side of the flume as the threshold of particle movement.

In this research, using the physical model and dimensional analysis, the condition of movement threshold in open channels with rectangular section was investigated. The results showed that with the increase of relative roughness (ds/y) regardless of the temperature variable, the stability number of the particle at the movement threshold (SN) decreases and the influence of the slope on these two parameters is not noticeable. Also, in the condition of constant slope and relative roughness (ds/y), with increasing temperature, the value of particle stability at the movement threshold (SN) decreases. Changes in particle stability number at the threshold of movement (SN) have an upward trend with respect to temperature, and with increasing temperature, the number of particle stability at the threshold of movement (SN) increases. In addition, the changes of the relative roughness parameter (ds/y) with respect to temperature are associated with a downward trend, and with the increase in temperature, the relative roughness parameter (ds/y) decreases. In addition, as the slope increases, the particle stability parameter decreases. This result can be extended to different seasons of the year so that the threshold of sediment movement and sedimentation in seasonal and permanent rivers is lower than in cold seasons in hot seasons. Comparing the results of the current research with the results of other researches, there is a very small difference between the results of this research and the results of other researchers. The main reasons for these differences are the different definitions of movement threshold among researchers, the different flow conditions and test conditions.

Predicting the water level in non-prismatic compound channels is necessary to determine the boundaries of floodplains in the range of converging or diverging and also to determine the stage-discharge curves of river flow during floods. In this study, using laboratory data taken from three compound channels with divergent and convergent floodplains at different angles of divergence and convergence, depth ratios and different roughness ratios, the relationships for estimating the water level in such sections have been investigated. The maximum relative error of the proposed relationships for depth ratios is less than 0.4 in divergence and convergence conditions is 13.29% and in depth ratios is greater than 0.4 and in divergence and convergence floodplains is about 1.98%. The results show that the water surface level along a diverging and converging range in non-prismatic compound channel with different divergence angles respectively increases and decreases with increasing depth ratios. Also, in this experimental conditions, the water level in the divergence (convergence) range of floodplain is affected by factors such as depth ratio and relative distance and follows less than relative roughness and divergence (convergence) angle.

Density current is produced due to a density contrast with the ambient fluid. This density difference can result from dissolved solids, suspended materials, temperature, etc. Density jumps significantly influence the characteristics of the gravity currents and the ambient fluid (e.g., velocities and concentrations). In this paper, the density jump is studied analytically by considering the bed roughness and entrainment ratio. For both smooth and roughened beds, a generalized relationship was obtained for estimating the conjugate depth ratio as a function of the upstream densimetric Froude number, the entrainment ratio, and the relative roughness. Moreover, various equations for calculating the maximum possible value of the relative roughness, the minimum possible value of the prejump densimetric Froude number, and the maximum possible value of the relative roughness were proposed. The minimum possible value of the conjugate depth ratio was determined. It was found that both the conjugate depth ratio and the length of the roller zone decrease as the bed roughness increases. Moreover, as the entrainment ratio increases, the ratio may also decrease, or may initially increase and then decrease. Finally, a relationship was obtained for estimating the minimum possible value of the obstacle height as a function of the conjugate depth ratio.

In this research the effect of roughness for different bed slopes on the control of sedimentary density current in subcritical conditions was investigated. The results showed that for each bed slope increasing the roughness height had considerable effect on reducing the load discharge of density current front (from 40% to 70% for concentration of 10 g L-1 and 35% to 55% for concentration of 20 g L-1). Also the amount of water entrainment rate reduced with increasing the roughness height (from 50% to 60% for concentration of 10 g L-1 and 48% to 60% for concentration of 20 g L-1). Finally, an equation for predicting water entrainment as a function of Richardson Number and relative roughness was presented. Sensitivity analysis of the proposed equation in this research showed that the effect of Richardson number on estimation of the water entrainment is much more than the effect of relative roughness on it.

Density current is produced because of a density contrast with the ambient fluid. This density difference can result from dissolved solids, suspended materials, temperature, etc. Density jumps significantly influence the quality characteristics of the gravity currents and the ambient fluid (e.g., lakes and reservoirs). In this research, using dimensional analysis and incomplete self-similarity method, these jumps were studied and a new equation was obtained for calculating the sequent depth ratio. This equation was validated using experimental results. Therefore, these experiments were carried out on both smooth and rough beds. The results indicate that the new relationship is more accurate than the classical equation; it was shown that the effects of both the entrainment ratio and the bed roughness are not negligible, even for their small amount.

Instantaneous velocity fluctuations are very important insuspended sediment load transportation. In this study experiments were performed toinvestigate the effects of artificial bed roughness on the instantaneous velocity fluctuations of saline density currents. Conic and cylindrical shapes of roughness with three heights were used. Velocity profiles were measured with an acoustic profiler velocity meter. Velocity and concentrationprofiles were measured in 3 and 4 cross-sections, respectively. The results show that increasing inrelative roughness height has a little impact on instantaneous velocity fluctuations in the entrainment region of the body of current. Also results of theinstantaneous velocity fluctuations profile on the rough bed show that turbulence intensity at the boundary of the ambient fluid and body of the current for cylindrical and conic roughness were 48 and 32 percent of shear velocity, respectively.