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Why Sloshing Simulation?

Sloshing has many applications in the automotive,aerospace, and ship building industries. In theautomotive industry sloshing behavior in fueltanks is investigated as part of an overall NVHstudy to design cars that run smoother and quieter. Sloshing in the fuel tank typically occursduring acceleration and deceleration (braking) ofthe car. The objective is to effectively reducenoise levels caused by fluid motion inside thefuel tank by designing baffles and separators tocontrol the sloshing flow pattern. In addition, different materials such as steel, aluminum,molded plastics, and composites are examinedfor fuel tank design, to reduce weight and cost,and to provide structural integrity at higher stress levels.

In the aerospace industry, sloshing behavior playsan important role in the stability and control of anaircraft. The fuel has a significant contribution tothe aircraft's total mass. Since the center of gravityof a structure changes due to fuel depletion andflow pattern during aircraft maneuvers, the designand arrangement of baffles inside the fuel tanksbecome a design challenge to ensure aircraftstability. A second sloshing application in the aerospace industry is the liquid oxygen tanksdesign for rockets and spacecrafts undergoingthe booster ascend loads.

In ships, sloshing loads can cause cracks andweld-line failure in sheet metals of cargo shipcompartments. Therefore, fluid flow is a major

design consideration in hull design when oiltankers, cargo ships, and cruise ships sailthrough rough seas.

Since the sloshing behavior depends on theshape, the location, the number of baffles and separators inside a tank, testing is a very tediousand expensive task when studying the numerousdesign scenarios. According to some industryobservers, the cost of tooling for fuel tanksranges from $100K to $200K per prototype. Forthis reason, designers and engineers have longbeen searching for an advanced, yet user-friendly simulation tool to tackle the many complexities of the sloshing phenomenon. Overthe years, MSC.Dytran has been recognized asa powerful solution for sloshing applications.

Why MSC.Dytran?

MSC.Dytran is uniquely qualified to simulate fullycoupled fluid - structure interaction. Based onthe advanced Lagrangian and Eulerian technologies, MSC.Dytran offers several techniques to model fluid-structure interaction byeither the specialized Arbitrary Lagrange-Euler(ALE) coupling or the unique General Couplingalgorithm. The advantage of the GeneralCoupling method is that it dramatically reducesmodeling time by defining the entire structure,such as a complicated fuel tank, as the interaction surface. Next, the fluid (Eulerian)mesh, which is used to simulate the fluid flowbehavior, can be automatically generatedand defined to interact with the structure.

By using this method, a sloshing simulationwhich, until recently seemed to be an impossible modeling effort, now becomes an ordinary task.

In addition, advanced Eulerian technology inMSC.Dytran can handle multi-material interactions with up to five different materialsin one element. This is important in sloshing applications since fluid, air, and gases can simultaneously share the same finite volume. One distinctive advantage of MSC.Dytran overconventional CFD codes is its capability to notonly simulate sloshing behavior but also tomodel the structure as a deformable body. Byhaving the tankage interact with fluid, one caneasily detect the stresses, deformations, andmaterial failures in the structure due to the sloshing pressures of the fluid. MSC.Dytranoffers a comprehensive element and materiallibrary to model a wide range of fluids and structures.

Case Study - Horie Metal FuelTank SimulationHorie Metal Co. Ltd., is a leading manufacturerof fuel tanks world-wide. To design their nextgeneration of automobile fuel tanks (Figure 1),MSC.Dytran was used to reduce the noise levelscaused by fuel sloshing during braking speeds.To optimally design and place separators inside asteel tank, a model was initially built without separators to predict the sloshing behavior offuel during braking. Because stresses and deformation of the structure were not requiredinformation for the initial simulation, the fuel tankwas modeled as a rigid body structure. Rigidbody modeling has a great benefit in drastically reducing the required analysis computationaltime. The rigid fuel tank was positioned inside a“virtual” Eulerian box constructed with solid elements (see Figures 2 -3). Fluid and air materialproperties were defined for the Eulerian elementsat the appropriate fuel level to initialize the partially filled tank . Next, a sinusoidal velocityfield (see Figure 4) was applied to the tank to simulate deceleration of the vehicle during braking.

Figure 1 - Horie Metal Fuel Tank Prototype

Figure 2 - Rigid Fuel tank

Figure 3 - Rigid Tank inside Euler Box

Figure 4 - Velocity Field

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To verify the simulation results, Horie Metal Co.conducted an experiment by building a transparent fuel tank and filling it with taintedwater. For safety reasons water was usedinstead of fuel.

Figure 5 - Fuel Tank Test Setup

The partially filled tank was locked in place on atest fixture as shown in Figure 5 and was drivenforward and backward with a hydraulic piston tosimulate driving conditions. The experimentalresults were recorded by high-speed camerasand installed pressure gauges on the tank walls.The simulation results were in good correlationwith the test data in terms of identifying criticalareas and sloshing patterns. The experiment andthe simulation were both indicative that the fluidflow had long wavelength and periods. Once thesloshing behavior was determined, separatorswere designed and placed inside the fuel tank todampen sloshing (Figure 6). The separatorswere connected to the tank with a series ofspotwelds. Both the separators and the tankwere again modeled as a rigid structure whileproviding clues to fluid flow inside the tank.

Figure 6 - Fuel Tank Separators (Prototype andmodel)

In this case, the results of the simulation showedthat the sloshing wave length and time periodhad become much shorter and were sporadicdue to interaction with the separator walls(Figure 7-14).

Without Separators With Separators

Figure 11 - time = 0.0 sec.Figure 7 - time = 0.0 sec.

Figure 8 - time = 0.1 sec. Figure 12 - time = 0.1 sec.

Figure 9 - time = 0.2 sec. Figure 13 - time = 0.2 sec.

Figure 10 - time = 0.3 sec. Figure 14 - time = 0.3 sec.

To investigate noise levels, it's necessary tomodel the tank as a deformable body with thestructural elements.The reason is that there is aclose correlation between noise levels and pressure peaks exerted on fuel tank walls.

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Therefore by monitoring the pressure peaks onthe tank walls during the simulation, similar toreading the pressure gauges in the experiment,engineers can easily identify the areas wherenoises are induced under the sloshing loads. Inorder to investigate the pressure profile on astructure, the rigid fuel tank and separators wereredefined with steel properties. To connect theseparator to the tank, a special 2D spotweld element was used in MSC.Dytran to correctlydefine the connectivity between the two structures.The General Coupling technique was applied todefine the fluid-structure interaction. In this case,the fuel tank and the separators were modeledas a deformable structure, which also acts as theinteraction (coupling) surface. The fluid and gasregion, which was modeled by Eulerian elements,consists of water and air inside the tank. Unlikethe conventional Arbitrary Lagrange-Euler (ALE)method where the nodes of the Euler andLagrangian elements have to be coincident, inGeneral Coupling they can be arbitrarily located,which makes the modeling effort much simpler,especially in case of geometrically complex fueltanks.

Figure 15 - Location of Pressure gauge

Figure 16 - Pressure Peak without Separators

Figure 17 - Pressure Peak with Separators

Figures 15-17 show the location of pressuregauges and typical pressure profiles with, andwithout separators at a critical location on thefuel tank during braking speeds. As shown, thepressure magnitudes are much higher withoutthe separators as opposed to when separatorsare placed inside the fuel tank. A comparison ofthe results indicated that without separators, highpressure peaks occurred after the tank stopped,while with the presence of separators, peakpressures occurred during the deceleration period. This is in agreement with the sloshing wavelengths and periods in the previous analyses.

To design smoother and quieter cars, it is critical to reduce noise levels in all systems andcar components; the sloshing noise inside fueltanks, which is a potential source of this problem, can be effectively controlled by reducing pressure levels in the tank walls toacceptable industry standard levels. For this purpose, MSC.Dytran is a proven simulation toolwith unparalleled capabilities which is essentialfor a sloshing application to offer a comprehensivesolution to reduce costs and time to market.

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Brochure designed by Marlene Hoctor

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