Sliding mesh

Bakker, A., Laroche, R.D., Wang, M.H. and Calabrese, R.V., 1997. Sliding mesh simulation of laminar flow in stirred reactors. Transactions of the Institution of Chemical Engineers, 75, 42M4. 

Simulation of rotating flows sliding mesh/multiple reference frames... 

To simulate turbulent flows, Reynolds-averaged Navier-Stokes (RANS) equations form the basis for most codes. Several turbulence models are usually provided. A new turbulence model may also usually be incorporated via user-defined routines. Recently, many of the commercial CFD codes have announced the inclusion of large eddy simulation (LES) capabilities. Considering the importance of rotating equipment used in reactor engineering applications, the ability to handle multiple reference frames or sliding meshes is important. Most leading commercial CFD codes provide... 

FIGURE 10.3 Approaches to modeling flow in stirred reactors, (a) Black box approach, (b) sliding mesh approach, (c) multiple reference frame or inner-outer approach, (d) snapshot approach. 

Ranade, V.V, Tayaliya, Y. and Choudhury, D. (1997), Modeling of flow in stirred vessels comparison of snapshot, multiple reference frame and sliding mesh approaches. Presented at 16th NAME Meeting, Williamsberg, June 22-21. 

Domain decomposition with sliding meshes. This strategy is very popular in the finite volume literature and implemented in finite volume-based software (Fluent , CFX , and Star-CD ). The idea is to decompose the flow domain into several concentric cylindrical meshes and allow slipping of the meshes between the partitions. The continuity of the solution at the mesh interface is imposed by... 

Perng, C.Y. Murthy, J. A sliding-mesh technique for simulation of flow in mixing tranks. ASME 93-WA-HT-33 1993, Old SRN049. 

Basara et al [3] simulated single- and two-phase turbulent flows in stirred vessels equipped with six- and four blade Rushton-t3q)e turbines using the sliding mesh impeller method. To describe turbulence in the liquid phase a standard k-e model was used for single phase calculations and an extended k-e model was employed for the two-phase simulations. These simulations were performed in transient mode with 1 (ms) time steps. The whole calculation contains 3900 time steps, which means approximately 4s of real time and 17 complete rotations of the impeller. One such simulation took 13 days of CPU time using an Intel single processor with 2.6 GHz). The flow patter predictions were compared with experimental data and fair agreement was obtained. It was stated that the standard k-e model over-predicted the... 

Murthy JY, Mathur SR, Choudhury D (1994) CFD Simulation of Flows in Stirred Tank Reactors Using a Sliding Mesh Technique. ICHEME Symposium Series, No. 136, pp. 341-348. Eight European Conference on Mixing, Cambridge (ISBN 0852953291)... 

The third standard for modeling rotating impellers is the sliding mesh approach (Rai 1985). Here, a grid is attached to the impeller that does not extend much beyond the outer radius of the impeller. This is the most computationally intensive of the three standard techniques, but also the most accurate. In principle, it fully captures the effect of the agitator on the flow. 

At present, the simulation of flow in stirred tanks requires particular attention to accurate treatment of the impeller. The rotating impeller is difficult to simulate directly in the context of a stationary CFD domain. Even with the introduction of sliding mesh techniques which allow the impeller to rotate in a fixed tank, and thus reproduce the trailing vortices behind the impeller blades [13,23]), only 5 to 10 rotations of the impeller have been reported [13]). Laroche reported that 16 sliding mesh steps, for 90° of tank simulation, took over 10 hours on a Cray [23]. Since the (time varying) bulk flows of interest take of the order of 50 rotations to become established, and the process results of interest may span 10,000 rotations (60 rpm for 3 hours on an industrial scale), this approach is still impractical for the typical user. 

In this section, two methods of modeling the impeller have been discussed general mathematical models and use of experimental data, The newer sliding mesh techniques were reviewed earlier in this chapter. A final alternative which has been suggested is to model the impeller as a source of momentum. It is not clear, however, how this method should be applied, and successful simulations have never, to the knowledge of the author, been published. 

It has been demonstrated that accurate representation of the impeller is central to obtaining accurate CFD simulations. Four alternatives are available for impeller modeling. The sliding mesh technique requires no measurements or assumptions but consumes large amounts of CPU time. Impeller modeling based on generalized mathematical models assumes that there is no interaction between the impeller and... 

Where new models are being developed, a qualitatively reasonable result is the first requirement, and convergence criteria are typically looser. Luo et al., who reported an early sliding mesh simulation in a short note, used the flow pattern became cyclically repeatable as their convergence criterion [13]. Gosman et al., who report early multiphase simulations, simply required that residuals in the equations solved become smaller than a prescribed tolerance. [9]... 

Mathur, S. and J. Y. Murthy, Computation of the Flows in Mixing Tanks using Unstructured Sliding Meshes, paper 186h, A.l.Ch.E. Annual Meeting, San Francisco, November 13-18, 1994. 

Preprosessing and grid generation was done with the commercial Fluent Gambit 1.3. The CFD code Fluent 5.5 was used in the simulation. Standard k-e turbulence model and standard wall functions were used. Multiple reference frame method was used in all simulations instead of the computationally slower sliding mesh method. Simulations were done in one phase (t). 

If a sliding mesh simulation is used, 20 or more rotations of the impeller are needed for convergence (Jaworski et al., 1998). 

Coy, D., R. LaRoche, and S. Kresta (1996). Use of sliding mesh simulations to predict circulation patterns in stirred tanks, presented at AIChE Annual Meeting, Chicago, Nov. 10-15. 

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