Hydrodynamic resistance of rafting units of new design

The article presents the results of experimental data analysis of the logs rafting units’ unsteady motion under the shallow waters effect. A float unit composed of four small volume bundles (up to 5 cubic meters) linked by the means of the fastening logs and flexible couplings has been proposed for rafting in shallow rivers. As the result of experimental research the mathematical model of hydrodynamic resistance was obtained. The timber transport units (TTU) included from 1 to 6 raft sections. The study was performed with a 1 : 20-scaled model. The length-to-width ratio of the TTU varied from 1 to 6. Froude scaling was performed over the range from 0.16 to 0.46 in case of uniform motion of the TTU. The experiments in unsteady motion conditions were performed using Froude scaling and Ho-criterion. The experiments were performed within the Reynolds number range of 3.1∙104…5.5∙105. The models were towed at speeds of 0.1…0.3 m/sec. Quadratic dependences of the water resistance force to the velocity of the uniform motion of the TTU models have been derived. The water resistance in the case of the TTU non-stationary motion is represented as the latter one for the uniform motion plus additional resistance arising during non-stationary motion. The values taken into account as the aggregated nonstationarity coefficient included: entrained water, body of water in the TTU cavities and additional water resistance. Dependences of the nonstationarity coefficient to the relative motion speed for all TTU models at the depth / draft ratios of 7.0; 5.0; 4.0; 2.7; 1.6 have been derived. The results obtained allow the researchers to determine the TTU acceleration time and starting course. Some additional tasks become solvable. For instance, necessary acceleration force for attainment of a TTU predetermined motion speed can be found.