Flow over a cylinder

At the end of this you will have a two-dimensional transient run of flow past a circular cylinder, with a shed wake you can watch in the viewport and a lift coefficient that oscillates in the Monitors tab.

This is the first tutorial because the answer checks itself. Above a Reynolds number of a few tens the wake behind a cylinder sheds alternately, and lift oscillates about zero while drag stays roughly steady. If your wake sits still and symmetric, something is wrong, and you do not need a plot to see it.

Runs on the free tier. The analysis type is Incompressible, which the free tier allows, and a two-dimensional mesh comfortably fits the 250,000-cell cap — it is one cell thick, so it costs roughly a hundredth of the equivalent 3-D mesh.

Before you start

Read Your first simulation. This tutorial assumes you know what the setup tree is and where the bottom dock lives.

You need to supply one file. The application imports geometry; it does not draw it, and the only body it ships is the streamlined hull behind Demo body. Export a cylinder from whatever CAD you use, as STL or OBJ, extruded along x. The extruded length does not matter much — the 2-D mesh collapses that direction to a single cell — so make it comparable to the diameter and centre it near the origin.

1. Create the case

From the home screen, press New case. The Create Simulation dialog opens on the physics drill-down.

Choose Single Phase, then Low Speed, then Transient (PIMPLE), pressing Next to go down each level. The right-hand panel says what each choice covers before you commit to it, and Next on the last one takes you to a review page listing what the case will derive and what you still have to set.

The wizard derives pimpleFoam. Shedding is unsteady, so a steady solver is the wrong tool here — it will grind towards a symmetric answer that does not exist. There is also a Transient (PISO) leaf; PIMPLE is used here because the adjustable time step in Simulation control is built around it.

Give the simulation a name under NAME on the review page, then press Create simulation. Inside an existing case the same button reads Add simulation.

2. Geometry

Select Geometry and press Import geometry. Pick your cylinder file.

Check Units matches what the file is in — geometry is rescaled to metres on import, and a cylinder exported in millimetres and left as one solves a body a thousand times too big.

Then read the Geometry info block. Size should be the cylinder you expect. Watertight should say yes; if it says open edges, snappyHexMesh will still mesh it but the surface has holes, and holes are where a mesh leaks into the inside of a body.

3. Mesh

Select Mesh.

Leave Mesher on snappyHexMesh (hex-dominant) — there is a body to wrap a mesh around.

Under Dimensionality, turn on Two-dimensional and set Thin direction to x, the axis you extruded along. A frontAndBack patch appears in the tree under Boundary conditions, held at Empty (2D). That patch is not a boundary condition; it is the statement that the direction is not solved.

Leave Flow direction at its default, −Z. It must not be the thin direction, and here it is not.

Leave Auto wind tunnel on. It sizes the background block from the geometry bounding box in multiples of body length, and the five pads — Ahead, Wake, Sides, Above, Below — are those multiples. The wake pad is the largest by default, which is right: everything interesting happens downstream.

Now look at the estimate block near Generate. It reads Estimate: … M cells, and if it exceeds the budget it says so in amber.

4. Refinements

Select Refinements.

The one setting that matters on the free tier is Max cells. It defaults to 2,000,000, which is eight times the free cap, so the Problems tab will refuse the run before you ever reach it. Set Max cells to 250000 or below.

Then bring the estimate down until it fits. Two levers, in order of effect:

Raise Base cell size under Mesh — or press Suggest from domain there to get a size scaled to the box. Then reduce Level max here if you are still over. The → base … surface … block in this pane shows both cell sizes and the running estimate as you type.

Watch the Problems tab as you go. The cell-cap error names the estimate and the limit, so you can tell whether you are close or nowhere near.

5. Boundary layers

Select Boundary layers.

Leave Target y+ at 30 and Wall treatment — under Models — at Wall functions. Wall functions want y+ between 30 and 300, and the pane says so; resolving the boundary layer instead would need y+ near 1, and a mesh that fine will not fit in 250,000 cells even in two dimensions.

The block at the top reports the Reynolds number the case is at, the first-layer height that y+ implies, and the total stack thickness. If the amber warning appears saying the stack is as thick as the surface cell, reduce Layers or Expansion until it clears — a layer stack thicker than the cell it sits inside is a mesh that will not build cleanly.

6. Generate the mesh, then check it

Press Generate mesh. Watch the Log tab.

When it finishes, select Quality. Seven rows, each with the threshold it is judged against:

• Negative volumes must be 0. Anything else means the mesh is broken and no solver will produce sense from it.
• Max non-orthogonality under 70°. Between 60 and 70 you get a recommendation with an Apply to numerics button, which sets the snGrad scheme and the non-orthogonal correctors for you. Take it.
• Layer coverage (body) above 70%. Below that there is an Apply to mesh settings button, which relaxes the layer controls; re-mesh afterwards.

7. Models, Materials, Initial conditions

Models should already read Incompressible, Transient, PIMPLE, with Model at k-omega SST. Leave it. The Derived block at the bottom shows the solver and the field set the case will write.

Materials defaults to Air (incompressible). The derived block shows the Reynolds number this fluid gives at the reference velocity and length — worth a glance, because it is the number that decides whether you get shedding at all.

Initial conditions defaults to Uniform with Track the inlet on, so the domain starts at the inlet velocity rather than at rest. Leave it. A transient case started from rest spends its first physical seconds establishing the flow, and the pane warns you when that is what you have asked for.

8. Boundary conditions

Select Boundary conditions. Five patches, already assigned:

Patch	Type
inlet	Velocity inlet	10 m/s
outlet	Pressure outlet	0 Pa gauge
body	Wall	the cylinder
farfield	Far field	the sides
frontAndBack	Empty (2D)	added by the 2-D toggle

You do not need to change any of them. Select inlet and look at the This writes block at the bottom of the pane: one boundary condition entry per field, generated from the single choice Velocity inlet. That fan-out is the thing the application is doing for you.

9. Result control

Select Result control and press + Force coefficients.

Set Patch to body. Leave References from the case on — it takes the frontal area, the model length and the inlet speed from the geometry and the boundary conditions rather than making you retype them.

This is the item that makes the answer checkable. Without it you will have a picture; with it you will have Cl against time, which is what says the wake is shedding.

Add + y+ as well. It writes y+ on the walls so you can confirm afterwards that the wall treatment was being asked for something it can do.

10. Simulation control

Select Simulation control.

End time and Time step define the run; the block underneath tells you how many time steps that is at the starting Δt and how many times it will write. Adjust time step to Courant number is on by default with Max Courant at 5, so the step is set by the flow rather than by your guess.

You need enough end time for several shedding cycles — one cycle is roughly the time the flow takes to travel a few diameters. Start with the default, watch the lift monitor, and extend it if the oscillation has not established.

On the free tier Cores is clamped to 1. The Problems tab says so as a warning, not an error: the run goes ahead, on one core, and takes longer.

11. Run

Select Runs and press Start run. The top bar’s Run button does the same thing and opens the bottom dock for you — on Residuals if the case is clean, on Problems if it is not.

What to watch, in the bottom dock:

• Residuals should fall within each time step and stay bounded across them. A transient run does not converge to a flat line the way a steady one does.
• Monitors is where the answer appears. Cd settles to a roughly constant value; Cl starts near zero, then grows into a regular oscillation about zero. That oscillation is the shedding.
• Log shows the Courant number. If it is being clipped hard every step, the time step is fighting the flow.

If Cl stays flat at zero for the whole run, the wake has not tripped. Give it more end time first; a perfectly symmetric mesh and a perfectly symmetric initial field can take a while to fall off the knife edge.

12. Post-processing

Select Post-processing and press Show results.

The fastest route to the picture is under Ready-made views: press Wake and vortices, which builds a surface, a vorticity threshold named “Vortex cores”, and streamlines. Step through the written times with the time controls and the shed vortices march downstream.

If you would rather build it yourself, + Slice through the middle coloured by velocity is the single most useful view.

13. Report

Select Report, tick the sections you want, and press Export PDF (A4). On the free tier it carries a watermark; everything in it is real.

What you should see

A wake that sheds. Vortices leaving alternate sides of the cylinder and travelling downstream in two staggered rows, and a lift coefficient oscillating about zero at a steady frequency and amplitude. Drag oscillates too, at twice the lift frequency and much more weakly.

A symmetric, stationary wake means one of three things: the run has not gone on long enough, the Reynolds number is too low for shedding, or the case is being solved steady. Check Models shows Transient before you look anywhere else.

What to try next

• Mesh independence study uses this exact case and asks whether the answer you just got is a property of the flow or of your mesh. It is the natural second step and the one that turns a picture into a result.
• Flow in a pipe for the internal-flow half of the workflow, where there is no body and the mesh is just the domain.
• External aerodynamics for the same workflow in three dimensions, with refinement regions and a drag split.