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Fluid Structure Analysis of a reed valve / Analisi Fluido Struttura di una valvola a lamella


TVK Ricerca Interazione fluido-struttura


In questo studio si presenta un metodo generico per lo studio del moto di una trave a sbalzo immersa in un fluido. Il moto del fluido viene risolto mediante il solutore commerciale (FLUENT 6.2) mentre la struttura č risolta grazie ad un solutore dedicato basato sul metodo degli elementi finiti (FEM), sviluppato all'interno del Fluent come una user-defined function (UDF). Lo strumento gestisce travi di tipo arbitrario considerando l'effetto del contatto con pareti rigide o flessibili. Grazie al metodo proposto č possibile effettuare analisi accoppiate di interazione fluido struttura (FSI) nel solutore CFD generico. Utilizzando tale strumento č stato possibile studiare il moto della valvola a lamella di un motore a due tempi.

A general approach to study the motion of a cantilever beam interacting with a 2D fluid flow is presented. The fluid is solved by a general purpose commercial Computational Fluid Dynamic (CFD) package (FLUENT 6.2) while the structure is managed by means of a dedicated finite element method (FEM) solver, coded in FLUENT as a user-defined function (UDF). Arbitrary cantilever beams are handled by the model, and include non-linear effects, such as the contact between rigid and flexible walls. The proposed approach results to be very versatile because it allows to extend to Fluid Structure Interaction (FSI) study all the capability of the general CFD solver. Adopting the proposed approach, the study of a two stroke engine reed valve dynamic is presented.


Figure 1: Cantilever beam.

For a cantilever in planar motion (see Figure 1), the beam theory is applicable. Under the hypothesis of small displacements and considering a straight member, the dynamic problem is governed by the following equations:




where the external force contribution is given by fluid pressure force contributions at up and down wetted side of the beam.
In order to account for arbitrary external forces, including contact, a dedicated FEM solver was implemented for the solution of the structural dynamic problem.
Fluid pressure, computed by means of the CFD solver are exchanged with the FEM solver that modify the CFD solution because of the motion induced in its boundary. The coupling technique used is based on a weak approach, that means that the structural problem is solved after the fluid time step is computed. The coupling scheme is summarized in the diagram of figure 2.

Figure 2: Flow chart of the solution process

The computational mesh has to be prepared with an appropriate pre-processing tool for Fluent. No different mesh has to be added to the fluid mesh because the FEM mesh is automatically built by the FSI code from the boundary mesh of the fluid domain.
One practical application that needs an FSI approach is the working cycle of a reed valve. Reed valves are pressure-driven flow stoppers, used in systems such as two-stroke engines, compressors and shock absorbers. A typical reed valve configuration is shown in figure 3. The reed petal is made of flexible material and it is mounted between a base and a stopper. The base serves as a barrier for valve closure, while an upper stop, which can be curved, has a double function: it serves as a guide to prevent excessive opening of the valve, and it optimises the shape of the open channel from a fluid dynamics point of view.

Figure 3: A diagram of the reed valve, showing the flexible reed petal, base, and stopper

The overall gas dynamics and local fluid motion depend on the open area of the valve. The open area depends on the reed motion, which is driven by pressure loads produced by the fluid. Air suction in the crankcase is produced when the piston moves toward top dead centre (TDC). This gives rise to a pressure difference relative to the inlet manifold, which causes the reed valve to open. The actual behaviour is complicated by the valve dynamics, the inertial effects of the entering air, and a local vortex.
Baudille and Biancolini (2002) have studied non linear dynamics of this component lumping the effects produced by fluid motion in an equivalent viscous load. In the present work an FSI simulation was performed on the reed valve of the Yamaha RD350 engine that has been widely studied by Fleck et al. (1987). The main engine characteristics are reported in table 1. The reed petal analysed in this work is made of fibreglass and its characteristics are collected in table 2.




The reed valve has been modelled as a 2D problem, using a symmetry condition in the middle line of the valve, representing only a small part of the engine manifold. Actual pressure histories found in this work were applied as boundary conditions at the inlet and outlet boundaries of the CFD domain. The applied pressure difference is shown in figure 5.
The FSI model was used with the contact algorithm capability, in order to correctly manage the contact of the petal with its stoppers. The FLUENT standard k-epsilon was chosen as turbulence model for the fluid simulation.
Pressure and velocity fields are illustrated in figure 4 at various time steps during an engine cycle at 4770 RPM. In the images it is possible to notice the reed petal reaction to the pressure variation, the vortex formation downstream the valve and the contact algorithm prediction of the anticipated bouncing of the petal at closure and at full opening. The reed tip vertical displacement agrees very well with experimental data found in the work by Fleck et al. (1987), as shown in figure 5.

Figure 4: Flow fields in the reed valve at various times during an engine cycle at 4770 RPM


Figure 5: Tip displacement of the reed valve predicted by the FSI model and measured experimentally by Fleck et al. (1987); the measured pressure differential is also shown

The tool presented in this paper is currently under development by FSI-Project Team (http://www.torvergata-karting.it/article/articleview/72/1/16/) of Tor Vergata Karting (http://www.torvergata-karting.it/); a 3D study of the same problem is one of the ongoing activities (figure 6). The dynamic model for reed valve motion will became a part of the software suite of TVK-Project (http://www.tvk-project.com/).


Figure 6: CFD results for a constant tip opening condition


Further details can be found in the paper attached.

References

Fleck, R., Blair, G.P., Houston, R.A.R., 1987. An improved model for predicting reed valve behaviour in two-stroke cycle engines. SAE Paper no. 871654.

Baudille, R., Biancolini, M.E., 2002. Dynamic analysis of a two stroke engine reed valve. Proceedings of XXXI AIAS Conference, Parma, Italy.

Baudille, R., Biancolini, M.E., 2006, Modelling FSI Problems in FLUENT: a General Purpose Approach by Means of UDF Programming, FISITA Congress 2006 F2006M235.





Files allegati

Paper Fisita 2006
Reed 3d
FSI Makes Fluent More Flexible
Fluent News · Spring 2005




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