Les analysis on cylinder cascade flow based on energy ratio coefficient

The flow field around the cylinder cascade is widely used to analyze the interaction of vortex shedding and the information on heat transfer. Large eddy simulation (LES) can be used to get the turbulent flow information in detail. The resolved largescale structures are determined by the size of the grid, and the turbulent vortex dissipation is modeled with a subgrid scale model. Whereas there is no accurate criterion to provide the subgrid scale with the physical meaning. Based on turbulent energy ratio coefficient and numerical simulation results with turbulent model, the subgrid was generated for the incompressible fluid flowing around a column of cylinder cascade with a gap-to-diameter ratio of 2. Smagorinsky-Lily (SM) model was applied to LES analysis. The turbulent flow information was compared with the experimental data by PIV. Two cases with different Reynolds numbers were studied. When the turbulent energy ratio coefficient reached to 30%40%, the turbulent dissipation could be captured by LES method with less grid number. The large scale vortex interaction behind the cylinder cascade was analyzed further. It is verified that LES method can be used for engineering based on the turbulent energy ratio coefficient with acceptable computational cost. INTRODUCTION Circular cylinder arrays are widely used in heat exchanger by the interaction of the vortex shedding. Short circular cylinder arrays, the span width is comparable to the cylinder diameter, are usually used in the confined spaces, such as the blade cooling system in the gas turbine. The cylinder span width is limited by the blade width in high pressure stages. The vortex shedding behind a cylinder is one of the classic problems in fluid dynamics. The large eddy simulation (LES) method is one of the options to study the flow recently, as there are the interactions of flow separation and the coherent structures in the turbulent flow. Breuer[1] found that the low-order upwind scheme could not obtain the recirculation zone and the separated angle, the high-order upwind scheme had strong numerical dissipation, but the second-order central-difference scheme was with high precision. He also suggested that the subgrid models had no significant effect on the numerical results. It might be the reason that Smagorinsky-Lily (SM) mode is used universally with the simple form and low computational cost. When there are more than one cylinder in the flow field, the flow structures vary with the gap-to-diameter ratio (ST/D) and Reynolds number (Re=UinD/). In the turbulent flow field with two circular cylinders arranged side by side, the flow structure is decided by ST/D. There is only one pair of vortex shedding behind the cylinders in the condition ST/D<1.2. It could be regarded as the flow field with single circular cylinder. Xu[2] found that the boundary layer between cylinders would not separate because of the wall effect, the boundary layer without the impact of the other cylinder would shed from the solid wall. The vortex shedding frequency was about half of that of the single cylinder. The gap flow swung between the two cylinders, forming a narrow and a broad vortex in the condition 1.2<ST/D<2.0. Sumner[3] pointed out that the flow deviation angle and the vortex shedding frequency between two cylinders reduced accordingly with the increment of ST/D. Brun[4] showed that the frequency of the alternating deflection was accelerated with the increase of Reynolds number. Two pairs of parallel vortex street would be observed with the same vortex shedding frequency in the condition 2.0<ST/D<6.0. Sumner[5] showed that the two vortex streets are not independent and the vortex shedding had time synchronization, vortex shedding was in the same direction reverse state in space. Two same phase vortex streets would merge into one vortex street spreading to the downstream eventually, and contraryphase vortex street remained to develop in time history. The flow field with more cylinders side by side, named as cylinder cascade here, is the common structure in engineering. There are more complicated interaction of the vortex mixing. Therefore, it is necessary to study it further by LES method to get enough turbulence information. According to the basic idea of LES, turbulent flow structures can be separated into the resolved scales and the subgrid scales. The resolved large-scale structures are determined by the size of the grid, and the turbulent vortex dissipation is modelled with a subgrid scale model. In order to achieve the accurate numerical simulation, proper subgrid model and the corresponding subgrid scale should be set up. Commonly, the subgrid scales in different turbulent flows were 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics