Designing oil skimmers presents several challenges, including adapting to varying oil viscosities, changing sea states, and environmental conditions.
Skimmers must efficiently separate oil from water without clogging or losing effectiveness in waves, currents, or debris-filled environments. Durability, ease of deployment, and maintenance are also critical, especially during emergency responses. Ensuring high recovery rates with minimal water intake while remaining cost-effective adds to the complexity. Additionally, compatibility with different containment systems and adherence to environmental regulations must be considered in the design process.
Typical engineering solutions for oil skimmer design focus on optimizing separation efficiency and durability. Common types include weir, drum, disk, and belt skimmers, each tailored for specific oil types and conditions. Materials are selected for corrosion resistance and oil adherence, such as oleophilic surfaces that attract oil but repel water. Designs often incorporate adjustable intake levels and modular components for easy deployment and maintenance. Engineers may also integrate pumps and sensors for automated operation and recovery rate monitoring. Skimmers are often designed to be buoyant and stable, ensuring effectiveness in rough waters while minimizing environmental impact and water collection.
ANSYS CFX can be used to evaluate oil skimmer designs by simulating fluid dynamics and multiphase flow behavior. Engineers can model oil-water separation under various conditions, analyzing how oil interacts with skimmer components like drums, weirs, or belts. CFX enables detailed visualization of flow patterns, turbulence, and velocity fields, helping optimize skimmer geometry and placement. It also allows testing of different oil viscosities, wave actions, and environmental factors without physical prototypes. By simulating performance metrics such as oil recovery rate and water entrainment, ANSYS CFX helps refine designs for maximum efficiency, stability, and effectiveness in real-world conditions. The first step in CFX simulation is proper prediction of oil and water stratification.
Setting up oil slick and skimmer simulation with Ansys CFX in this discussion involves several steps. These steps include thought map, product map, and CFX case setup.
Thought Map: A thought map of modeling characteristics is generated to organize and represent ideas, concepts, or information in a structured way. The thought map below shows the objective of the simulation study and questions asked to address the objective. Each question is followed by a theory, action, and prediction to address each question. Results would also be added to the bottom of each branch as they are generated.
Product Maps: A product map of the oil slick and a weir skimmer is generated to list and categorize product features. A product map indicates some factors that correspond to theories/actions in the thought map.
CFX Simulation Mesh: The images below show the mesh of the model. The upper shows the entire domain and the lower shows detail of the oil layer. The number of cells across the oil layer is 9 and the mesh size in the horizontal direction yields a aspect ratio of 2.
CFX Simulation Setup: CFX models are setup to address the questions and geometry addressed by the thought map and the product map. Steady-state calculations are performed.
Materials: The following image collection shows example material specification. The density and the dynamic viscosity are the key material properties in these panels.
Domains - Basic Settings: The following image collection shows basic settings for each domain. Each domain uses the same Fluids; however, each fluid has a different material assignment. The continuous fluid morphology is used for each fluid along with the same reference pressure (1atm), buoyancy model, and buoyancy model specifications. In this case, the reference density uses the lightest fluid with a reference pressure location in the middle of the air domain.
Domains - Fluid Models: The following image collection shows example model specification of the Homogeneous Multiphase Model along with the Interface Compression Level Option. Fluid Specific Models use density difference fluid buoyancy model for all three fluids.
Domains - Fluid Pairs: The following image collection shows example surface tension coefficients for the fluid pairs. The continuum surface model option is used for all three pairs, and the fluid with higher density is chosen as the primary fluid.
Domains - Initialization: The following image collection shows example initialization strategies for the difference domains. Expressions for hydrostatic pressure are used for the water and for the oil domains. The Automatic With Value is selected with volume fraction of 1 to match the domain to the fluid.
Water Domain - Boundary Conditions: The following image collection shows water domain boundary condition settings. A zero relative pressure and backflow volume fraction of 100% water are used at the outflow opening to generate flow. A hydrostatic pressure in the form of an expression is used for the water inflow boundary where only water is allowed to enter. Symmetry and wall boundary conditions are used elsewhere. The expression is as follows: densitywater * 9.81 [m s^-2] * (oildepth - y) + densityoil * 9.81 [m s^-2] * oilthickness
Air Domain - Boundary Conditions: The following image collection shows air region boundary condition settings. A zero opening pressure is used at the openings to permit inflow of air only. Symmetry and wall boundary conditions are used elsewhere.
Oil Domain - Boundary Conditions: The following image collection group shows oil region boundary condition settings. A hydrostatic pressure in the form of an expression is used for the oil, and only oil is allowed to enter. Symmetry and wall boundary conditions are used elsewhere. The expression is as follows:
densityoil * 9.81 [m s^-2] * (0.0 [m] -y)
Solver Control: The following image collection show solver control settings. The high-resolution advection scheme is used, and the coupled volume fraction multiphase advanced option is used.
CFX Solver: All runs are performed in Double Precision.
Baseline:
An initial check on the results is done by looking at the absolute pressure. The hydrostatic pressure should have a distinct vertical gradient as shown below.
The following image shows volume fraction of oil when no outflow boundary is used. The oil layer is confined well to the expected oil layer. Negligible oil has been lost from the domain.
The following image shows volume fraction of oil with outflow. The oil layer is confined well to the expected oil layer. The outflow boundary has already begun to pull oil down into the weir skimmer. The oil layer is thinning further away from the skimmer.
Coarse Mesh: The image below shows the volume fraction of oil with a coarser mesh. The oil disappears from the domain even though the outlet is set to a wall.
Surface Tension: The image below shows a negative impact on volume fraction of oil when surface tension is deactivated. The oil diffuses more from the oil domain into the air and water domains.
Reference Density and Location: If reference density is set to the heaviest fluid (water) and the reference location is moved into the water domain, the solution diverges.
Interface Compression Level: The images below show the impact of interface compression on the oil layer. There is minor difference between the results, and level 1 or2 is preferred.
Setup Details: The following video steps through highlights of the setup using CFX.
ANSYS offers advanced capabilities for simulating oil skimmers which offer numerous benefits, including enhanced design optimization, improved reliability, and cost savings. By accurately predicting skimmer performance, manufacturers can design products that meet specific requirements more efficiently.
Ultimately, ANSYS CFX provides a comprehensive, virtual environment to evaluate oil layer movement.
Ansys CFX enables the evaluation of multiple design/input factors such as fluid properties, flow rates, and skimmer geometry. A design engineer can evaluate multiple design options to understand the flow behavior. Beyond CFX, ANSYS provides tools such as Fluent, LS-Dyna, DesignXplorer, OptiSLang, and Mechanical for further design parametrization and evaluation.
Ozen Engineering Inc. leverages it's extensive consulting expertise in CFD, FEA, thermal, optics, photonics, and electromagnetic simulations to achieve exceptional results across various engineering projects, addressing complex challenges like oil skimming.
We offer support, mentoring, and consulting services to enhance the performance and reliability of your oil skimming system. Trust our proven track record to accelerate projects, optimize performance, and deliver high-quality, cost-effective results for both new and existing systems. For more information, please visit https://ozeninc.com.