Service overview
Fluid flow, heat transfer, multiphase, aerodynamic and turbulence simulation. This service is part of that division and is scoped individually around what you need to learn, decide or publish.
Many industrial and environmental flows involve more than one phase: bubbles rising in a reactor, droplets in a spray, sand carried by a river, particles in a fluidised bed. The phases interact through drag, lift, heat and mass transfer, and surface tension at interfaces. Modelling these interactions is among the harder problems in computational fluid dynamics because the physics occurs at scales much smaller than the equipment.
No single model is right for all multiphase flows. Interface-tracking methods resolve the shape of each bubble or droplet but are limited to modest numbers. Eulerian approaches treat each phase as an interpenetrating continuum and scale to large systems but rely on closure models for the interactions. Lagrangian approaches follow individual particles or parcels. Choosing among them, and understanding the assumptions of the chosen model, is the core of credible multiphase simulation.
If you plan to publish the outcome, see manuscript & publication support, for example computational & simulation manuscripts or original research article writing.
What this service covers
Problems this service can address
- Selecting between interface-capturing and dispersed-phase approaches
- Closure models whose parameters are poorly known
- Numerical diffusion smearing interfaces
Methods and analytical approaches
- Volume-of-fluid and Eulerian-Eulerian approaches
- Discrete-particle and Lagrangian tracking
- Interfacial force and closure model review
- Mesh and time-step sensitivity for interface problems
Applications
- Bubble columns and reactors
- Sprays and droplet transport
- Sediment and particle transport
Relevant engineering disciplines
Scope, inputs and deliverables
Scope of work
- Definition of the physics, geometry and quantities of interest
- Model set-up with verification and, where data allow, validation
- Parametric or sensitivity studies agreed at scoping
- Results and documentation suitable for reports or publication
- Feasibility check on: phases and properties
Typical inputs required from you
- Phases and properties
- Flow regime and scale
- Measurements such as hold-up or droplet size
Deliverables
- Model-selection rationale
- Phase-distribution and flow-field results
- Validation comparison and limitations
Highly specialised requests are first checked for feasibility from this information. Please do not send confidential data through the public form; describe it first and agree secure transfer afterwards.
Project workflow and method
- 01Enquiry and requirement assessment
- 02Technical scoping and proposal
- 03Execution
- 04Quality review and delivery, with an agreed round of revisions
Read the full project workflow.
In depth: how the work is done
Identifying the flow regime and the right model class
Gas-liquid flows range from dispersed bubbly flow to slug and annular regimes, with the interfacial structure changing the physics. Particle-laden flows differ according to volume fraction and particle size: dilute flows allow one-way or two-way coupled tracking, while dense flows require particle-particle interaction models. Dimensionless groups such as Weber, Eötvös, Stokes and Morton numbers guide expectations about deformation, breakup and particle response. Regime identification narrows the model options and sets realistic expectations for accuracy.
Volume-of-fluid and interface-tracking approaches
Volume-of-fluid and related methods solve a single set of momentum equations and track the interface through a volume fraction field. They suit free surfaces, large bubbles, droplet impact and stratified flow, and they require fine meshes and small time steps near interfaces to limit numerical smearing and to represent surface tension accurately. Parasitic currents, which are spurious velocities near interfaces, can contaminate results and must be monitored. Interface sharpening and curvature estimation methods affect accuracy.
Eulerian-Eulerian and Lagrangian approaches
Eulerian-Eulerian models solve conservation equations for each phase with exchange terms for interfacial forces. They are efficient for systems with many bubbles or particles, but results depend on closures for drag, lift, virtual mass, turbulent dispersion and bubble or particle size distributions. Lagrangian tracking follows individual particles under forces from the carrier fluid, supports polydispersity naturally and can include collisions in discrete-element variants. Computational cost grows with the number of tracked entities, so parcel approaches are often used.
Closures, mass transfer and population balance
Interfacial force closures encode much of the physics and carry significant uncertainty. Where bubble or droplet size changes through breakup, coalescence or mass transfer, population-balance models track the size distribution. Mass and heat transfer across interfaces introduce further correlations. We review which closures are appropriate for the regime, assess their applicability range and test sensitivity of results to the most uncertain ones, since this often dominates the overall uncertainty.
Verification and validation for multiphase problems
Mesh and time-step sensitivity is more demanding for interface problems, and the number of cells across a bubble or droplet is a key resolution measure. Validation uses data such as gas holdup, interfacial area, bubble size distribution, particle velocities or sedimentation rates. Because such measurements are difficult and uncertain, we state the uncertainty and avoid claiming close agreement from a single comparison. The report emphasises the conditions under which the model has been tested.
Quality control and good practice
Quality-control approach
- Verification: mesh, time-step and solver sensitivity are examined before results are reported.
- Validation: results are compared with measurements or reference solutions where these exist, and the absence of such data is stated.
- Boundary conditions and material or fluid properties are recorded with their sources.
- Sensitivity to the least certain inputs is shown or explained.
Common pitfalls this service helps avoid
- Using default closure models without checking. Drag and lift models are valid for particular regimes, and misapplied defaults can change results qualitatively.
- Under-resolved interfaces. Too few cells across a bubble or droplet cause smearing and inaccurate surface tension effects.
- Ignoring polydispersity. Assuming a single bubble or particle size can mislead when the real distribution is broad.
- Validating on a single measurement. Agreement on one integral quantity does not prove that the flow structure is right.
What good reporting looks like
A multiphase report states the regime and the reasoning for the chosen model class, the governing equations, closure models with their ranges of validity, mesh and time-step details, numerical stabilisation, boundary conditions, results with sensitivity to the key closures and numerics, validation against data with uncertainty, and limitations. It states clearly when multiphase predictions are qualitative rather than quantitative and what additional measurements would improve confidence.
An illustrative scenario
Hypothetical example written for explanation. It is not a client case, a result or a testimonial.
Imagine a researcher studying gas dispersion in a stirred tank. A Eulerian-Eulerian model with a population-balance closure is set up, and the predicted gas holdup is compared with measurements at two stirrer speeds. Agreement is acceptable at the lower speed but poorer at the higher one, where bubble breakup is more intense. A sensitivity study shows the result depends strongly on the breakup model. The report presents both outcomes, identifies the closure as the dominant uncertainty and suggests bubble size measurements that would improve calibration.
Questions to settle before you begin
- Which flow regime do I expect, and how dense is the dispersed phase?
- Do I need interface shape or only averaged behaviour?
- Which closures matter most, and what data can calibrate them?
- How many cells can I afford across a bubble or droplet?
- What measurement can validate the predictions?
Key terms
- Volume of fluid
- A method that tracks interfaces using the fraction of each phase in a cell.
- Eulerian-Eulerian model
- A model treating each phase as an interpenetrating continuum.
- Lagrangian tracking
- Following individual particles or droplets through the flow.
- Population balance
- An equation describing the evolution of a size distribution.
- Weber number
- A dimensionless ratio of inertial to surface tension forces.
Working together on Computational Fluid Dynamics (CFD) projects
CFD engagements start with a flow-physics review: expected regime, quantities of interest, available geometry and the measurements that could validate results. A coarse pilot simulation can check set-up and cost before the full study. We ask for geometry in a standard exchange format or dimensioned drawings, fluid properties, boundary conditions and any test data. Reports include mesh and model sensitivity so that you can see how much the answer depends on numerical choices. Results are presented as engineering quantities with units, and limits of validity are stated explicitly.
Frequently asked questions
Are multiphase simulations predictive without calibration?
Often they need calibration or reference data for closure parameters. We state clearly which results are calibrated and which are predicted.
Can you simulate bubble columns and spray systems?
Yes, within feasibility limits. The model choice and expected accuracy are discussed at scoping.
How long do multiphase simulations take?
They are usually more expensive than single-phase CFD, especially for transient interface problems. A pilot case provides an estimate.
Do you need experimental data?
Experimental data greatly improve credibility, particularly for closure calibration. Without them, results are explicitly labelled as uncalibrated.
Can particles be included with heat transfer or reactions?
Possibly, depending on the model. Complexity grows quickly, so the scope should be agreed carefully.
How is this service scoped and priced?
After your enquiry we review what you need, ask for missing information and send a written scope and quotation. Price depends on complexity, deliverables and timeline. See the project workflow.