Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Fig gure 6.18: Comp parison of Expe erimental and Num merical Results of f Norm malized RMS Va alues of Ax xial Velocity at z 1 . Chapter 6 do not distinguish between b turbuulence and unnsteadiness (GGyllenram et aal., 2008). The main reason r is RAANS assumptiion of all turrbulence beinng stochast tic and averag ging thus remmoves all unstteady motion (Leschziner MM. A., 2010 0). This (ensem mble averaginng) leads the ssolution to bee determined bby initial and a boundary conditions annd one can gett same solutioon repeatedly bby keeping g the same com mputer and innitial and boundary conditions (Gyllenram et al., 2008).The 2 UR RANS simulattion conducteed by (Spencer et al., 20009) could only o resolve th he ensemble oor phase averraged coherennce unsteadineess with sm mall fluctuating amplitude. Compa ared to eddy viscosity v basedd URANS, the RSM based simulations in several cases e.g. (Hir rai et al., 19888) (Benim et al., 2005) andd (Lübcke et aal., 2001) etc. e gives bett ter results. Thhe main reasson is a betteer treatment of Reynold ds stresses by y solving a ttransport equuation for eacch componennt. Howeve er, for a better understandiing and predicction of the fllow, turbulence modelin ng approache es are requireed where, connsidering the computationnal costs, a part of the tu urbulence specctrum is resolvved and the reest are modeleed (Hanjal lic K., 2005). Such S modelingg approaches include LES aand hybrid LEES RANS etc. e In the current c simula ation cases, a steady state cconverged soluution could not be achieved with RN NG k andd RSM. This inndicated that the problem to be inhe erently unsteady in naturre and led too run URANNS simulationns. Figure 6.18 shows a comparison of normalizeed root meann square (RMS) values of o axial veloc city for experiimental data and the simuulation cases at position n z . 1 It can be b seen that th he URANS simmulations of aall the current cases show aan unstead dy behavior. The RMS vaalues are almmost of the same order as experim mental data but b profiles are not in good agreemment with thhe 162 Numerical Modeling
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Chapter 6 experimental data. Addressing the aforementioned issues of properly defining the inlet boundary conditions may also improve the RANS/URANS results to some extent, however, the major reason may simply lie in the inadequateness of the RANS/URANS based methodology in this particular case. This indicates that using Hybrid RANS-LES or purely LES approach will probably give better simulation results by resolving a portion of the turbulence scales. Thus performing a grid independence study by increasing the computational mesh size in the current simulation cases may not significantly improve the simulation results compared to what already have been obtained using URANS. Due to very limited number of Ansys ® FLUENT licenses available at MEK-DTU, Hybrid RANS-LES or LES models could not be applied. The URANS simulations with a computational gird containing the guide vanes and for cases with partially closed cylinder intake port are currently under progress. 163 Numerical Modeling
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DTU Mechanical Engineering Section
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Experimental and Numerical Study of Swirling ... - Solid Mechanics

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