Structure-Fluid Interaction Analysis of an Existing Water Tank

An analytical study was completed investigating the cause and consequences of significant vibrations resulting from the operation of a hot water storage tank. Deformations and strain readings of the tank wall were measured during the operation and used to calibrate and validate results from the analytical model. The steel storage tank is supported on a concrete foundation. The diameter of the tank is 18 meters and the height is 28 meters. The shell wall thickness varies in steps as plate segments reduce in thickness from 16.2mm at the bottom to 6.7 mm near tank mid height and above. A 64 inch diameter pipe at the top of the tank supplies the inflow water. Three 42 inch and two 36 inch diameter discharge nozzles are located near the bottom of the tank. The tank wall has been observed to be “breathing” at low water levels. These vibrations reduce as the tank water level increases. In order to evaluate the deformations and stress level of the tank, optical measurement of the deformations and strain gauge readings for inflow rates of 4500, 6000 and 8500 m3/hr were performed. The results were then used within a calculation to check for fatigue failure. To study the dynamic behavior of the hot water tank, a set of FE (Finite Element)-CFD (Computational Fluid Dynamics) models were prepared at flow rates of 4500 to 12500 m3/hr. Standard shell elements were used to model the tank shell and both content and inflow water were modeled using the SPH (Smooth Particles Hydraulics) elements available in LS-DYNA®. Interaction between the structure and fluid was defined using contact scenarios. These models were calibrated using the results of strain gauge readings and optical measurements. The size of the elements, geometry and physical properties were decided after numerous test analyses results were adjusted for consideration of certain variables. The final model represented the most critical arrangement considered. Results of the analyses were within an acceptable range of the readings from the strain gauges. Scale factors were used to calibrate the FE results for better compliance. Field measurements and results from the analysis demonstrate stress levels and displacement increase with higher inflow rates.