Numerical simulation of fluid-structure interaction phenomena

In this thesis, a new method for the investigation of aeroelastic phenomena for long-span bridges is proposed: the aerodynamic fields and the motion of structure are simulated simultaneously and in a coupled manner. The structure is represented as a bidimensional elastically suspended rigid body with two degrees of freedom whose natural frequencies correspond to those of the fundamental flexural and torsional modes of vibration of the structure. The aerodynamic fields are simulated by numerically integrating the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a finite volume scheme on moving grids which adapt to the structural motion. The URANS equations are completed by the turbulent closure relations which are expressed as a function of the turbulent kinetic energy, the turbulence frequency and the strain tensor according to the k- SST approach. The presented model is used in order to identify the critical flutter wind velocity of the Forth Road Bridge deck, and the numerical results are compared with those of an experimental campaign. For wind velocities equal or greater than the critical wind flutter velocity, the deck starts to oscillate increasingly. It is demonstrated that the reason for the onset of the torsional-branch coupled flutter lies in the fact that, within each of the first oscillation cycles, there is a portion of the cycle in which the energy supplied by the aerodynamic field to the deck motion is more than the energy extracted in the rest of the cycle. Then it is shown that the reason for the amplification of the instability resides in the drifting of large vortical formations along the deck surface. The numerical model is also used to test the effect, on the aeroelastic stability of the Forth Road Bridge deck, of the introduction of a couple of sloping barriers at the windward and leeward bridge deck edges. It is demonstrated that the aerodynamic modifications produced by the introduction of such barriers is effective in increasing the critical flutter velocity and mitigating the vibration amplitudes which develop during the flutter instability

Numerical integration of the contravariant integral form of the Navier–Stokes equations in time-dependent curvilinear coordinate systems for three-dimensional free surface flows

We propose a three-dimensional non-hydrostatic shock-capturing numerical model for the simulation of wave propagation, transformation and breaking, which is based on an original integral formulation of the contravariant Navier–Stokes equations, devoid of Christoffel symbols, in general time-dependent curvilinear coordinates. A coordinate transformation maps the time-varying irregular physical domain that reproduces the complex geometries of coastal regions to a fixed uniform computational one. The advancing of the solution is performed by a second-order accurate strong stability preserving Runge–Kutta fractional-step method in which, at every stage of the method, a predictor velocity field is obtained by the shock-capturing scheme and a corrector velocity field is added to the previous one, to produce a non-hydrostatic divergence-free velocity field and update the water depth. The corrector velocity field is obtained by numerically solving a Poisson equation, expressed in integral contravariant form, by a multigrid technique which uses a four-colour Zebra Gauss–Seidel line-by-line method as smoother. Several test cases are used to verify the dispersion and shock-capturing properties of the proposed model in time-dependent curvilinear grids

3D free surface flow simulations based on the integral form of the equations of motion

This work deals with a novel three-dimensional finite-volume non-hydrostatic shock-capturing model for the simulation of wave transformation processes and wave-structure interaction. The model is based on an integral formulation of the Navier-Stokes equations solved on a coordinate system in which the vertical coordinate is varying in time. A finite-volume shock-capturing numerical technique based on high order WENO reconstructions is adopted in order to discretize the fluid motion equations

Numerical investigation of the three-dimensional velocity fields induced by wave-structure interaction

Submerged shore-parallel breakwaters for coastal defence are a good compromise between the need to mitigate the effects of waves on the coast and the ambition to ensure the preservation of the landscape and water quality. In this work we simulate, in a fully three-dimensional form, the hydrodynamic effects induced by submerged breakwaters on incident wave trains with different wave height. The proposed three-dimensional non-hydrostatic finite-volume model is based on an integral form of the Navier-Stokes equations in σ-coordinates and is able to simulate the shocks in the numerical solution related to the wave breaking. The obtained numerical results show that the hydrodynamic phenomena produced by wave-structure interaction have features of three-dimensionality (undertow), that are locally important, and emphasize the need to use a non-hydrostatic fully-three-dimensional approach

Numerical simulation of the Rio Fucino dam-break flood

In this paper a dam-break flood model based on a contravariant integral form of the shallow water equations is presented. The equations of motion are numerically solved by means of a finite volume-finite difference numerical scheme that involves an exact Riemann solver and is based on a WENO reconstruction procedure. An original scheme for the simulation of the wet front progress on the dry bed is adopted. The proposed model is used to simulate the Rio Fucino dam-break and subsequent flood wave propagation, downstream of the Campotosto reservoir (Italy)

On the integral form of the motion equations for free surface flow

This work deals with a novel three-dimensional finite-volume non-hydrostatic shock-capturing model for the simulation of wave transformation processes and wave-structure interaction. The model is based on an integral formulation of the Navier-Stokes equations solved on a coordinate system in which the vertical coordinate is varying in time. A finite-volume shock-capturing numerical technique based on high order WENO reconstructions is adopted in order to discretize the fluid motion equations

A dam-break flood simulation model in curvilinear coordinates

A dam-break flood model based on a contravariant integral form of the shallow water equations is presented. The numerical integration of the equations of motion is carried out by means of a finite volumefinite difference numerical scheme that involves an exact Riemann solver and which is based on a high-order WENO reconstruction procedure. An original scheme for the simulation of the wet front progress on the dry bed is adopted. The proposed model capacity to correctly simulate the wet front progress velocity is tested by numerically reproducing the dry bed dam-break problem. The model is adopted for the real case study of the Rio Fucino lake-dam collapse and subsequent flood wave propagation, downstream of the Campotosto reservoir (Italy)

Numerical investigation of the three-dimensional velocity fields induced by wave-structure interaction

Submerged shore-parallel breakwaters for coastal defence are a good compromise between the need to mitigate the effects of waves on the coast and the ambition to ensure the preservation of the landscape and water quality. In this work we simulate, in a fully three-dimensional form, the hydrodynamic effects induced by submerged breakwaters on incident wave trains with different wave height. The proposed three-dimensional non-hydrostatic finite-volume model is based on an integral form of the Navier-Stokes equations in σ-coordinates and is able to simulate the shocks in the numerical solution related to the wave breaking. The obtained numerical results show that the hydrodynamic phenomena produced by wave-structure interaction have features of three-dimensionality (undertow), that are locally important, and emphasize the need to use a non-hydrostatic fully-three-dimensional approach

Modulation of spinal excitability following neuromuscular electrical stimulation superimposed to voluntary contraction

Purpose. Neuromuscular electrical stimulation (NMES) superimposed on voluntary muscle contraction has been recently shown as an innovative training modality within sport and rehabilitation, but its effects on the neuromuscular system are still unclear. The aim of this study was to investigate acute responses in spinal excitability, as measured by the Hoffmann (H) reflex, and in maximal voluntary contraction (MVIC) following NMES superimposed to voluntary isometric contractions (NMES+ISO) compared to passive NMES only and to voluntary isometric contractions only (ISO). Method. Fifteen young adults were required to maintain an ankle plantar-flexor torque of 20% MVC for 20 repetitions during each experimental condition (NMES+ISO, NMES and ISO). Surface electromyography was used to record peak-to-peak Hreflex and motor waves following percutaneous stimulation of the posterior tibial nerve in the dominant limb. An isokinetic dynamometer was used to assess maximal voluntary contraction output of the ankle plantar flexor muscles. Results. H-reflex amplitude was increased by 4.5% after the NMES+ISO condition (p < 0.05), while passive NMES and ISO conditions showed a decrease by 7.8% (p < 0.05) and no change in reflex responses, respectively. There was no change in amplitude of maximal motor wave and in MVIC torque during each experimental condition. Conclusion. The reported facilitation of spinal excitability following NMES+ISO could be due to a combination of greater motor neuronal and corticospinal excitability, thus suggesting that NMES superimposed onto isometric voluntary contractions may provide a more effective neuromuscular stimulus and, hence, training modality compared to NMES alone