FEA Simulation - Detailed Observations
The figure below (left side) provides a detailed picture of the location of these high stresses, which were shown to localise around the vertices of the leg support/lattice structure interface. The high stress concentration at these locations is as expected since the faces that made up this interface consisted of many sharp right angles. The figure below (right side) shows the contour of equivalent (von Mises) stress, regions at the base chamber of the tank with stress magnitudes almost approaching the yield stress of stainless steel occurred within 6s after the injection of the coolant. The continual subjection of such thermal induced stress may eventually compromise the structural integrity of this part of the tank.
Case Study - FSI Study on Cryogenic Tank
The main objectives of this study are as follows:
- Perform a CFD study on the fluid phase of the system (flow inside the cooling jacket) to obtain the transient temperatures induced by the fluid phase on the solid phase. To achieve this, a transient simulation will be performed to model the feeding of coolant into the tank jacket using ANSYS Fluent. As it starts from the bottom, this part of the tank will be subjected to -15⁰C first whereas the top part remains at room temperature. Hence, this simulation will provide the data to be used as the boundary conditions for the FEA simulation component. This will take into consideration the flow rate of coolant and room initial tank temperature.
- Perform a stress and displacement analysis on the solid phase (the tank) using the FEA method and the transient temperature results from the CFD study as inputs. To achieve this, a one-way coupling structural analysis using ANSYS Mechanical will be conducted by importing the transient temperature distributions impacting on various part of the tank, obtained from CFD post-processing. This part of the simulation will take into consideration all the structural physical properties of the tank.
As shown in the figure below (left side) the tank geometry was created using ANSYS Design Modeller. To render the geometry suitable for simulation, selected elements of the original geometry were defeatured to avoid low quality mesh elements for both the fluid and solid domains. These features were deemed to have minimal impact on the CFD and FEA relevant results. After the geometry was created, it was meshed using ANSYS Mesh as shown in the figure below (right side). Mesh generation is one of the most important steps to ensure a highly accurate and reliable result. To ensure the results are accurate and mesh independent, a mesh independence test was conducted on the geometrical model. Two meshes of differing cell densities were generated for each of the CFD and FEA simulations. Then simulations were ran on each of this mesh and the results compared to each other.
FSI ANALYSIS SPECIALISTS
Fluid-Structure Interaction (FSI) analysis is an advanced computational method used to investigate the interactions between fluid flow and solid structures. This analysis is vital in contexts where the dynamic behaviour of a fluid impacts a solid structure, or where the structural changes in a solid influence the fluid's behaviour. FSI analysis requires the simultaneous solution of the coupled equations that describe both fluid dynamics and structural mechanics, providing a detailed understanding of the intricate interactions between fluids and solids. As one of the leading FSI simulation consulting firms within Australia and Singapore, our FSI analysis specialists have successfully delivered many challenging projects. Fluid structure interaction analysis is one of our core simulation services.
[1] Key Aspects of FSI Analysis
Fluid Dynamics
In FSI analysis, a thorough understanding of fluid behaviour is crucial. Fluid behaviour is typically governed by the Navier-Stokes equations, which account for parameters such as velocity, pressure, density, and viscosity. The complexity of fluid dynamics in FSI scenarios, due to factors like turbulence, compressibility effects and variable viscosity, requires advanced numerical methods for precise solutions. Computational fluid dynamics (CFD) is widely used for this purpose, as it allows for detailed simulation and prediction of fluid flow behaviour under various conditions. CFD enables engineers to accurately model fluid forces acting on structures, which is a critical component of the overall FSI analysis.
Structural Mechanics
The structural mechanics aspect of FSI analysis focuses on the response of solid structures to external forces, such as those exerted by fluid pressures. This response is characterised by equations of elasticity or plasticity, describing how materials deform and respond under load. Finite element analysis (FEA) is the primary method used to solve these equations, providing a detailed approach to understanding structural behaviour. FEA facilitates the simulation of complex geometries and diverse material properties, enabling accurate predictions of structural deformation, stress distribution, and potential failure modes under fluid-structure interactions. The precision of FEA is vital for ensuring the integrity and safety of engineering designs in FSI applications.
Coupling
The interaction between the fluid and the structure, known as coupling, is the pivotal aspect of FSI analysis. This interaction involves the transfer of forces and motion across the interface separating the fluid from the structure, which can be classified as either strong or weak.
Strong coupling involves a two-way interaction where the deformation of the structure influences the fluid flow, and the fluid flow simultaneously affects the structural deformation. Both effects are solved simultaneously, requiring iterative convergence at each time step. Strong coupling is used in scenarios with significant interaction between fluid and structure, where changes in one domain can substantially affect the other. This approach is essential for accurately capturing phenomena where fluid forces lead to considerable structural deformation, which in turn alters the flow field, such as in flexible structures or cases with significant deformations or dynamic responses. In contrast,
Weak coupling involves to a one-way interaction where either the fluid forces affect the structure or the structural deformations affect the fluid, but not both simultaneously. The solution may not require iterative convergence between the fluid and structural solvers at each time step. Weak coupling is appropriate when the interaction between fluid and structure is less critical, such as when fluid forces cause minimal structural deformation or when the structural deformations do not significantly alter fluid flow.
[2] Coupling Methods
One-Way Coupling
This approach involves passing information in only one direction, either from CFD to FEA or vice versa. For example, fluid forces are applied to the structure, but the structural deformation does not affect the fluid flow. This method is simpler and is commonly used for weak coupling, but it may not capture all interactions accurately, especially in cases of strong coupling.
Two-Way Coupling
In a more comprehensive FSI analysis, information is exchanged iteratively between CFD and FEA solvers. The fluid forces affect the structure, and the structural response alters the fluid flow, requiring continuous updates and feedback between the two solvers. This method is necessary for capturing complex interactions accurately and is commonly used for strong coupling.
[3] Interaction between CFD and FEA
CFD to FEA
In this scenario, CFD analysis calculates fluid flow parameters, such as pressure and shear stress, at the surface of the structure. These fluid-induced forces act as boundary conditions for the structural analysis (FEA). For example, in the analysis of pipe flow, the behaviour of a pipe carrying high-pressure fluid is a common FSI scenario. The pressure distribution and flow dynamics from the fluid (CFD results) are applied to the inner surface of the pipe in the structural model to determine the deformation, stress and strain within the pipe wall (FEA). Understanding these interactions is crucial for assessing the structural integrity of the pipe, particularly in high-pressure applications, where excessive deformation or stress can lead to leaks, ruptures, or failure of the piping system.
FEA to CFD
Conversely, the deformation of the structure calculated from FEA can influence the fluid flow around it. Structural deformation can change the shape of the flow domain, affecting the fluid's velocity and pressure fields. For example, in the case of a flexible pipe under fluid pressure, the deformation of the pipe (FEA results) may alter the flow characteristics within the pipe. This change in flow can, in turn, affect the forces acting back on the structure, necessitating adjustments in the fluid flow analysis (CFD). This feedback loop is essential for accurately modelling scenarios where structural changes significantly influence fluid behaviour, ensuring that the interaction between the fluid and structure is comprehensively captured.
CFD Results - Temperature Distribution
The figure below (left side) shows our CFD simulations provided the transient temperature distribution inside the mixing tank fluid domain within the first 20s as propylene glycol at -15⁰C is injected into the mixing tank cooling jacket. At approximately around 20s post injection, propylene glycol in lower half of the tank reached subzero temperatures.
FEA Simulation- Deformation Visualisation
The figure below (right side) illustrates the displacement magnitudes and the deformation mode of the tank due to the thermal stresses induced upon it at various points in the simulation within 20s of coolant injection. As illustrated in the contours of total displacement, the displacement magnitudes were not significant, with a maximum displacement magnitude of approximately 0.9 mm located at the outlet pipe of the mixing tank cooling jacket. The orientation of the pipe displacement also infers that there was a relative (but small) displacement between the inner tank and outer shell since the temperatures of the inner tank dropped rapidly whilst the temperatures at outer shell remained at room temperature. This could potentially affect components that were attached to both the inner tank and outer shell such as the inlet and outlet pipes, leg supports and lattice structure. Despite such a phenomenon, as will be demonstrated, the stresses caused by the relative displacement of the inner tank and outer shell were within the yield stress of the material.
FEA Simulation - Stress Visualisation (von Mises)
The figure below shows the equivalent (von Mises) stress magnitudes of the tank due to thermal stresses induced upon it at various points within the first 20s of coolant injection. Shown for each point in time are: Temperature distribution of Propylene Glycol in the mixing tank cooling jacket (top left) and equivalent stress magnitudes at the Outer Shell (top right), Inner Tank (bottom left) and Inner Tank Cross Section (bottom right) of the mixing tank.
FSI Study - Conclusion
A fluid structure interaction (FSI) study was performed to ascertain whether the injection of propylene glycol coolant at -15⁰C into the Washed Oil Chiller tank cooling jacket will cause the tank to buckle under the severe thermal loads induced. From the simulation results, it was observed that whilst the tank would not buckle significantly under these extreme thermal loads, as evidenced by a maximum deformation of less than 1 mm, several areas such as the leg support/lattice structure interface and cooling jacket base chamber were identified to experience stress levels that exceeded the yield stress of the stainless steel tank construction material. Based on the results analysed and observed, the fabrication of this tank was not recommended. It was further recommended that a design review supported by the results of this study be conducted and have the highlighted problem areas addressed.
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