Experimental Investigations On The Local Horizontal Forces Of The Vertical Cylinder In Internal Waves

Ocean internal wave occurs in the density stratification of the ocean.For thethermal radiation of the sun and seawater vertical density distribution and otherreasons, a stable density stratification exist in the seawater. The disturbance occurredin seawater pycnocline layer may stimulate the internal wave in the sea.Due to the large internal wave amplitude and the long wavelength,it containshuge energy. When the internal wave occurs, the flow caused by the wave is indifferent direction in the supernatant and lower flow, forming a strong shear flow.This strong force acting on the subsea engineering structures, will be a potentialthreat to its construction, installation and safe operation. And the distribution ofinternal wave is staggered set in the vertical distribution of positive and negative,resulting in periodic vibration of subsea structure, accelerating the fatiguedamage.Slender cylinder is composed of the basic building blocks of marinestructures.Firstly,estimate the force by the internal wave made on the model in the wavetank of the laboratory by the use of combined mKdv and Morrison equations.According to a rough estimate of the internal wave force, choose the right type ofload cells and the corresponding sensor range. As the load cells worksunderwater,effected by buoyancy,viscous effect of water and interference from otherfactors, sensor calibration performed in the air can not be a true reflection of therelationship between the voltage signal and the force on the cylinder model.Consequently the load cells calibration need to be performed in same conditions to theexperiment. On this basis, design the system of load cells calibration and the wavehorizontal force measurement system.The cylinders in internal wave field are tested tostudy the level of force and its influencing factor.Then, design the experiment conditions,and discuss the impact of internal waveamplitude and the depth and diameter of the cylinder model to its horizontal forces,totally twenty-seven kinds of conditions, select three load cells.Afterthat,calibrate the load cells underwater.The obtained data were processed and therelationship between the load cells output voltage signal and the force were obtainedby fitting the data.Finally, the experiment that tests the horizontal force on cylinder in the internalwave field are performed. Due to the experimental data obtained by the test apparatusmay be noise vibration and electromagnetic interference, the test data is analyzed intime domain after de-noising processing. Affects of the three variables on the force ofthe cylinder are obtained. Cylinders forces in internal wave field in differentconditions are analyzed to obtain the distribution of the force along the horizontalcylinder model and the relation between the horizontal force and both the cylinderdiameter and the wave amplitude.Then through the Fourier transform, the dynamicresponse spectrum of the cylinder under the internal wave field is obtained, found thatin addition to the frequency of internal waves along the flow there is a frequencycomponent at twice the frequency, the response to the cylinder in the masterfrequency is larger.