[alex_03]

Hi everyone,

I’m running into an issue with under-predicted drag when using a wall-function approach in ANSYS Fluent for a free-surface flow. The case is a partially submerged vertical cylinder, velocities 0.5–3.0 m/s, VOF + k–ω SST.

For the purpose of testing my model, I ran all simulations mentioned below at 1m/s.
When I fully resolve the viscous sublayer (low y⁺ mesh, y⁺ ≈ 1–4, PRESTO! pressure discretisation), the drag is close to expected (~21 N). However, I can’t use this approach for all velocities because it would either require an excessive number of cells (beyond my available computational resources) or very high aspect ratio cells in the near-wall region, which cause free-surface stability issues in VOF simulations.

With the wall function approach (target y⁺ ≈ 30-50), drag drops significantly under the same conditions. Using the same settings as the fully resolved case and only changing the inflation layer to achieve y⁺ ≈ 50 gave a drag of ~9.5 N. After being advised to switch the pressure discretisation from PRESTO! to Modified Body Force Weighted, the drag increased to ~15.5 N (better but still too low compared to the ~21 N baseline). The main loss is in pressure drag; viscous drag changes only slightly and makes up only a small portion (> 4%) of the total drag.

I've tried my best to summarise the main geometry, mesh and Fluent settings below.
Geometry
The model consists of a rectangular fluid domain containing a partially submerged vertical cylinder of 100mm diameter. The domain is split into two parts:

• The main rectangular region.
• A 400mm diameter cylindrical sub-domain placed coaxially around the cylinder for finer near-cylinder mesh control.

Bodies of Influence (BOIs):

• Cylindrical BOI overlapping the structured fluid sub-domain for near-cylinder sizing.
• Rectangular wake BOI extending downstream of the cylinder.
• Upstream free-surface BOI, extended slightly downstream where deformation is minimal.
• Downstream free-surface BOI, extended slightly upstream to capture upstream deformation.

Meshing

• Structured hexahedral mesh in the cylindrical sub-domain using MultiZone.
• Inflation: first layer height = 0.005 m (y⁺ ≈ 50 at 1 m/s), growth rate = 1.1, 8 layers.
• Edge sizing: 150 vertical divisions in the structured mesh region.
• BOI element sizes: Cylinder = 0.01 m; Wake = 0.02 m; Downstream = 0.02 m; Upstream = 0.04 m.
• Mesh quality: Min element quality = 0.244, Avg = 0.835; Max aspect ratio = 9.92; Avg = 1.84; Max skewness = 0.838; Avg = 0.201; Min orthogonal quality = 0.162; Avg = 0.801.

Fluent

• Mesh converted to polyhedral in unstructured regions.
• Double precision enabled.
• Models: VOF (implicit formulation, implicit body force, interfacial anti-diffusion enabled), k–ω SST turbulence.
• Boundary conditions: Top/bottom/side walls = no-slip; inlet = velocity inlet; outlet = pressure outlet.
• Methods: PISO pressure–velocity coupling; pressure discretisation = Modified Body Force Weighted (after initial PRESTO! tests); volume fraction = Compressive; momentum, TKE, and ω = Second Order Upwind; transient formulation = Second-order implicit.

I have been struggling with this issue for well over a month now and am pretty much stuck on what else I can try. Any help would be greatly appreciated.