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### Let SDJT's damping damp out

On August 22, 2019, Mr Liu Aoxiang and Mr Liu Lingfeng made two presentations in one big window. Hereby the first presentation is reported, Ms Wei JQ. reports the second one separately. Mr Liu AX. presented his findings on adding water damping to the dynamical structural model of the submerged floating tunnel (SFT). He attempted three methods. Let them be named method A, B, and C. The approach of A is just adding one line of command in FEA software, B adding a special FE element that can work like an actuator with one end fixed, the other end linked to the nodes of beam elements; the beam elements stands for the structure of SFT’s main tube. The method C is a bit tricky, as Aoxiang said. First, let structure vibrate. Then, in the next time step of the dynamic calculation, he derives the velocity of nodes of the beam element, from which the reaction force of water damping in forms of concentrated load can be calculated using the well known formula F=CV. Subsequently, the reaction load can be imposed into the same nodes as said above. The procedure is exactly like an old saying what goes around comes around.
Aoxiang further elaborated his works of verifications. Members of SDJT asked many interesting questions. Via discussions, it became apparent that Aoxiang did make an elegant move by comparing the results of method B to that of method A, and the two matched. Since method A had already been verified in Mr Liu Lingfeng’s early work, method B is verified. To put it in another way, we can appreciate the simple logic, suppose the hand calculation from textbook is true, and hand calculation proves method A, method A proves B, then we can draw the conclusion that B is true too.
The unique advantage of method B is that it can impose three varied values water damping coefficient namely the transverse translation, the vertical translation, and the twisting along tunnel axis (marine architecture prefers to say sway, heave and rolling, which are exactly the same things) into the simulation of structural dynamic problems. Comparatively, method A can define only one damping coefficient.
Damping exists everywhere in our daily life and our works. In the imagined area named science frontier where no one ever stepped in, the overall environment can be seen as over-damped, the higher work value we want to create, the higher damping coefficient shall we confront, the faster we want to move, the higher damping coefficient shall we face up to. That is our current state for tackling the challenging SFT technological problems. Nevertheless, I trust we keep moving forward, and let the damping damp out.
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