Wave Simulation And Propagation Characteristics Study Under Hypergravity Fields

The traditional small scale marine engineering experiments performed under normal gravity field always encounter one stubborn difficulty related to full scale prototype.Because of the scale effect,the self-weight stresses(or the pressure field)can not be reproduced properly by a small scale model under normal gravity field.Therefore,many phenomena can not be reproduced properly in a small-scale model.Nevertheless,the stubborn difficulty can be solved by centrifuge experiments.That is why the centrifuge experiments are frequently used for the study of engineering problems.However,at the present the centrifuge is designed only for the geotechnical engineering,the water wave problems can not be performed properly in a centrifuge.Few people study on the water wave problems systematically or deeply in hypergravity fields.In this study,the scaling laws,wave generation and propagation used for wave simulation in hypergravity fields are established and presented.And the wave field differences caused by the discrepancies between the hypergravity fields produced by a centrifuge and the ideal hypergravity field are studied in this paper.The following are the main contents:1.The wave scaling laws used for wave studying in hypergravity fields is established.The scaling laws for ideal fluid steady flow,ideal fluid unsteady flow,viscous fluid flow and compressible fluid flow is presented.The results show that the Froude number,the Strouhal number,the Reynolds number and the Euler number can be met at the same time in hypergravity fields.However,the condition is harsh when the Reynolds number is considered.And only for the small size fluid the Reynolds number can be considered at the same time.Therefore,in this paper the pressure similarity is considered.And the discrepancy caused by the viscosity is discussed in the paper.2.The numerical wave tanks used for wave studing in hypergravity fields is established.For this purpose,the wave geneartion theory and wave absorption method is present in this section.3.The regular wave and the solitary wave are simulated in ideal hypergravity fields based on the scaling laws presented in this study.The results show that the full scale fluid field can be reproduced properly by a scaled-down fluid field in an ideal hypergravity field.The slight discrepancies on the wave height can be offset by a well designed incident wave condition.And the pressure fields obtained from the simulations agree well in macroscale.However,because the viscosity similarity is ignored,wave attenuation may be more serious in small-scale model experiments.There are some differences on the velocity fields and dynamic fields on the study of solitary waves.4.The circle arc numerical wave tank and the wave generation theory used for circle arc numerical wave tank are presented in this paper in order to adjust the characteristics of the centrifugal acceleration field which is a radial field.And wave generation and propagation in circle arc numerical wave tanks which in hypergravity fields is studied,too.The results show that a small wave tank curvature may cause the wave height bigger.However,there are no significant discrepancies caused by the small curvature.Even in a drum-centrifuge with a rather short rotating arm(0.7m),the wave length to the rotating arm is 0.572,the value of the wave height increment is only 7%.And the wave height increment can be ignored when the ratio of the wave length to the rotating arm is bigger than the 0.12.5.The discrepancies on the wave length,the wave velocity and the static pressure field caused by the centrifugal acceleration which vary with the rotating arm(the depth of the model)are studied.The results show that the variations of the acceleration with depth can break the consistency of length scale and period scale.Therefore,in a specific experiment the wave period and the effective centrifuge radius should be adjusted to the specific experiment objective.6.The Coriolis acceleration is also discussed in this paper.For this purpose the vertical velocity obtained from the simulations performed in different levels of gravity fields are compared with the effective linear velocity of the centrifugal fields.The results show that the vertical velocity is very small.There are no significant discrepancies caused by Coriolis acceleration.Even in a drum-centrifuge with a rather short rotating ann under 100g hypergravity field,the influence of the Coriolis can be ignored,too.However,it should be noted that because the velocity scale is 1,the influence of the Coriolis effects may be more appreciable in a larger scale model.