Solving the World’s Toughest CFD Problems

Immersed Boundary Method

The Immersed Boundary Method (IBM) improves the accuracy of flow simulations around complex solid geometry in FLOW-3D products. Built on the FAVOR™ method, which represents solid geometry on a regular Cartesian grid using area and volume fractions, IBM helps reduce pressure oscillations and numerical instabilities that can arise from flux loss near complex flow geometries. Introduced in v12.0, IBM uses a ghost-cell approach to enforce wall boundary conditions directly near solid surfaces. Ghost nodes are placed within solid regions and assigned velocities that satisfy the wall boundary condition. This allows velocity gradients near boundaries to be calculated directly using standard finite difference schemes. By eliminating the source of the flux loss, rather than correcting for it with pressure afterward as in the standard FAVOR™ method, IBM provides more accurate pressure fields, force predictions, energy loss calculations and improved stability for flows involving complex solid boundaries in FLOW-3D products.

Discharge of water from a 1m-diameter circular orifice using the Immersed Boundary Method, showing the pressure field alongside live plots of water surface elevation and volume flow rate over time.

When to Use the Immersed Boundary Method

IBM is most effective for advection-dominated flows where accurate force, drag, lift, and pressure loss prediction around solid bodies is the primary concern. It works with 3D mesh blocks and hybrid 3D/shallow water mesh blocks, but not with shallow-water-only meshes. IBM is not compatible with the Detailed Cutcell Representation. As a general guide: use IBM for advection-dominated problems involving force and pressure predictions around solid bodies, and use the Detailed Cutcell Representation for viscous boundary layer problems where wall shear stress accuracy is the priority

Applications

The Immersed Boundary Method improves solution accuracy for engineering problems involving flow around or through solid bodies in FLOW-3D products:

External hydrodynamics and aerodynamics for accurate drag, lift, and pressure distribution prediction on vessel hulls, vehicle bodies, and other bluff or streamlined shapes
Flow through orifices, contractions, and valves for improved discharge coefficient and energy loss calculations in hydraulic and industrial flow systems
Internal flows with complex geometry for pipe networks, manifolds, and flow passages where pressure drop and flow distribution accuracy are important
Flow past bluff bodies and obstacles for improved force predictions on bridge piers, offshore structures, and other geometries subject to hydrodynamic loading
Hydraulic structures and water control for more accurate discharge and head loss calculations in gates, weirs, and culverts in water and environmental engineering applications

In this simulation, using FLOW-3D‘s Immersed Boundary Method, the total resistance on the ship hull (neglecting air resistance) is made of the sum of the pressure and viscous forces acting on the submerged part of the ship. The variation in the pressure contours, as shown in the animation, indicates the creation of the waves on the sides of the hull and the wake development behind the hull. From an energy dissipation point of view, the energy used in the creation of the waves and the energy lost in the wake also contribute to the total resistance.