Acoustic Measurement Of Suspended Sediments

The acoustic measurement of suspended sediments,based on the theory of acoustic backscattering,is a fundamental technique for measuring suspended sediments.Compared with the traditional technique,it has the advantages of real-time and non-intrusive measurement,high resolution and profile measurement.However,there are still problems that deserve further study,such as the difficulty in measuring the radius,the bad robustness of inversion algorithm and the unclear effects of bubbles in the water.In order to investigate a more effective method of measurement,the thesis studied several key problems of the acoustic measurement of suspended sediments in the aspects of principle,algorithm,simulation,system and experiment.The present thesis includes the following contents.(1)A layer-by-layer iteration algorithm was studied.Based on the principle of layered measuring region,the algorithm calculated the concentration and radius layer-by-layer.The results show that this algorithm can successfully solve the implicit equation caused by the acoustic particle scattering attenuation.Because the measuring error would accumulate during the algorithm,for the cases of large measuring error,this algorithm would lead to large inversion error of sediments radius and concentration.(2)In order to solve the bad robustness of layer-by-layer iteration algorithm,the thesis studied the explicit inversion algorithm.When the inversion equation was transferred into operator equation,one directly calculated the concentration profile through matrix manipulation.The simulations show that when the measuring error gets larger,compared with the implicit layer-by-layer iteration algorithm,the explicit inversion algorithm has better performance in inversing the concentration.(3)To calculate the sediments radius and concentration simultaneously,multi-frequency inversion algorithm was studied.In the simulation,one used acoustic wave with different frequency to measure the same region,the result shows that by solving the simultaneous equations of these measuring frequency,the sediments radius and concentration can be both calculated.(4)In order to further improve the robustness of the inversion algorithm when the measuring error gets larger,the thesis proposed the concept of optimum frequency,and applied the regularization method into the concentration inversion.The simulation results indicate that the inversion error can be reduced if the frequency can be properly decided.At the same time,one successfully reduced the measuring error by transferring the inversion equation with measuring error into a well-conditioned equation.Optimum frequency and regularization can both improve the robustness of the inversion algorithm.(5)Further,the thesis also set up a multi-frequency acoustic measurement system of sediments for lab,and carried out several lab experiments to verified some conclusions in the thesis.The experiment results show that in the condition of low sediments concentration and small measuring distance,for flow with uniform sediments concentration,the square of received voltage is linear to the sediments concentration,which is consistent with the conclusion in the thesis.