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Published on November 7, 2007

Author: Gavril

Source: authorstream.com

CFD Analysis To Observe the Effect of Baffles In Reducing Fluid Sloshing Inside a Tank :  CFD Analysis To Observe the Effect of Baffles In Reducing Fluid Sloshing Inside a Tank INTRODUCTION:  INTRODUCTION Sloshing phenomenon is likely to occur whenever a partially filled tank of liquid undergoes acceleration or deceleration Analysis of fluid sloshing involves identification of flow patterns inside the tank If the oscillations in the tank are allowed to continue they can create slosh forces that may have an adverse effect on the stability and structural integrity of the vehicle Sloshing Analysis is done using Computational Fluid Dynamics tools CFD-GEOM, CFD-ACE and CFD-VIEW OBJECTIVES:  OBJECTIVES To observe the effect of baffles in reducing fluid sloshing and thereby improving the stability of vehicles carrying fluids To determine the best configuration for the placement of baffles with different number of holes in them THEORY:  THEORY The basis of the method used in the CFD-ACE is the Volume-Of-Fluid (VOF) method The model can accommodate any two fluids that are incompressible and immiscible and where localized slip between the two is negligible (as in case of Water and Air) The distribution of the second fluid (water) in the computational grid is accounted for using a single scalar field variable, F, which specifies the fraction of the volume of each computational cell in the grid occupied by fluid Two (Water) Thus F takes the value 1 in cells which contain only fluid two (Water) and the value 0 in cells which contain only fluid 1 (Air) THEORY (Cont.):  THEORY (Cont.) In a flow field and an initial distribution F on a grid, the manner in which the volume fraction distribution F evolves is determined by solving the passive transport equation (1) together with the fundamental equations of conservation of mass and momentum ∂ F / ∂ t +▼•v F = 0 (1) The average value of any volume-specific quantity, Φ, in a computational cell can be computed from the value of F in accordance with Φ = F Φ2 + (1 – F ) Φ1 (2) To include the effect of density, ρ, equation (2) can be written as: Φ = [F ρ1 Φ2 + (1 – F ) ρ2 Φ1] /ρmix (3) THEORY (Cont.):  THEORY (Cont.) To determine the flux of fluid from one cell to the next cell PLIC Surface Reconstruction method is used (the liquid-gas interface is assumed to be planar and can take any orientation within the cell) The flux of the secondary fluid (water) for a given velocity can be determined by the back projection from the cell face By using back projection, the secondary flux is computed explicitly based on the old liquid surface orientation. and depends on the information from the upwind cell only CFD ANALYSIS:  CFD ANALYSIS CFD-ACE is used as a solver for the computational analysis Flow and Free Surface (VOF) modules of CFD-ACE are used for sloshing analysis Euler (1st order) scheme is applied to all the cases Automatic Transient option is selected as the evaluation method for calculating the time step This evaluation method allows specifying the start and ending time of the simulation The simulation is given an end time of one second with a target CFL of 0.2 to limit the advection of the surface relative to the grid CFD ANALYSIS (Cont.):  CFD ANALYSIS (Cont.) From the Implicit and Explicit time integration schemes, Explicit option is selected as it exhibits greater stability and better convergence, but slightly lower accuracy as compared to the Implicit scheme Initial condition of “All Fluid 1” i.e. Air is applied to the upper half tank while the lower half tank is applied an initial condition of “All Fluid 2” i.e. Water. Each case is given 200 iterations with a convergence criteria of 0.001 TWO DIMENSIONAL ANALYSIS:  TWO DIMENSIONAL ANALYSIS DESCRIPTION:  DESCRIPTION Fluid tank geometries (with different baffle configurations) are modeled and meshed in CFD-GEOM An acceleration of 5 m/s2 in the positive x direction is applied to the half filled tank Initially the analysis is done without placing any baffle plate inside the tank then baffle plates are added to observe their effect in reducing sloshing GEOMETRY:  GEOMETRY Length of Fluid Tank is taken as 1335 millimeters & height 440 millimeters There are 9 different cases considered, the difference is the placement of baffle plates and the number of holes in them CFD-RESULTS:  CFD-RESULTS Pressure Distribution and Density Variation inside the fuel tank after 0.5 sec and 1 sec from CFD are shown below FLUID TANK WITHOUT BAFFLES:  FLUID TANK WITHOUT BAFFLES FLUID TANK WITH 2 BAFFLES 4 HOLES:  FLUID TANK WITH 2 BAFFLES 4 HOLES FLUID TANK WITH 2 BAFFLES 6 HOLES:  FLUID TANK WITH 2 BAFFLES 6 HOLES FLUID TANK WITH 3 BAFFLES 4 HOLES (EQUIDIST):  FLUID TANK WITH 3 BAFFLES 4 HOLES (EQUIDIST) FLUID TANK WITH 4 BAFFLES 4 HOLES:  FLUID TANK WITH 4 BAFFLES 4 HOLES Slide19:  After analyzing the behavior of fluid inside the tank in the above cases a new configuration with three baffles and four holes is considered (X-Location of baffles from start of tank is 285, 626 and 967 millimeters respectively) Results for Pressure Distribution and Density Variation from CFD for this case are shown below FLUID TANK WITH 3 BAFFLES 4 HOLES (Final):  FLUID TANK WITH 3 BAFFLES 4 HOLES (Final) DISCUSSION OF RESULTS:  DISCUSSION OF RESULTS For each configuration the Max. Pressure after 0.5 Sec and 1 sec is shown in Table below From the Pressure Distribution, Density Variations and the Table above it is obvious that the results for the new configuration case are better as compared to other cases THREE DIMENSIONAL ANALYSIS:  THREE DIMENSIONAL ANALYSIS DESCRIPTION:  DESCRIPTION Fluid tank geometries are modeled in Pro-Engineer and meshed in CFD-GEOM An acceleration of 5 m/s2 in the positive x direction is applied to the half filled tank Baffle Plates (X-Location of baffles from start of tank is 285, 626 and 967 millimeters respectively) are added to observe their effect on the longitudinal fluid sloshing GEOMETRY:  GEOMETRY CFD-RESULTS:  CFD-RESULTS Pressure Distribution and Density Variation inside the fuel tank after 0.5 sec and 1 sec from CFD are shown below FLUID TANK WITHOUT BAFFLES:  FLUID TANK WITHOUT BAFFLES FLUID TANK WITH BAFFLES (Press Distribution):  FLUID TANK WITH BAFFLES (Press Distribution) FLUID TANK WITH BAFFLES (Dens Variation):  FLUID TANK WITH BAFFLES (Dens Variation) FURTHER WORK ON 3D SLOSHING:  FURTHER WORK ON 3D SLOSHING CONCLUSION:  CONCLUSION From the CFD Analysis of fluid tank it is clear that The placement of baffles and number of holes in them plays an important role in reducing sloshing Also the induction of baffle plates controls sudden change in the c.g, thereby improving the stability of vehicles THANK YOU:  THANK YOU

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