Fluid simulation
About flow studies
A flow study calculates the flow of a fluid around a part (external flow) or through a part (internal flow). The solver uses the lattice-Boltzmann method on a grid of cells.
Beta. The flow solver is in Beta (advanced). It is validated for internal laminar flow and for external flow up to a Reynolds number of about 1,000. Above that range, the drag is 2 to 3 times too high and the lift changes with the grid. Use high-Reynolds-number drag and lift values as indications only.
The "Beta" label is a label of the flow solver. It is not the release status of Meridian.
Flow studies are for parts only. You cannot make a flow study of an assembly.
Make a flow study
- Open the part.
- On the Simulation tab, click New Flow Study.
- Click Study Settings and set the flow.
- For an internal flow, set the openings (inlets and outlets).
- For an external flow, set the aerodynamic values.
- Click Run.
The start page has a guided tour for a flow study.
The flow solver can use the graphics processor. Refer to GPU compute.
External flow and aerodynamics
Study Settings has an Aerodynamics section with these items:
| Item | What it does |
|---|---|
| Up axis | Sets which axis points up. |
| Angle of attack | Turns the flow in the pitch plane. |
| Yaw | Turns the flow sideways. |
| Reference area and reference length | Sets the values that Meridian uses to calculate the coefficients. |
| Refinement to the box walls | Extends the fine grid levels to the walls of the box. Use it for a body near a ground or a symmetry plane. |
| Angle-of-attack sweep | Runs a set of angles one after the other. The result gives Cl, Cd, Cm and L/D for each angle. |
| Time-mean Cp window | Calculates the mean pressure coefficient over a time window. |
| Ground | Adds a ground. The ground can be fixed or moving. |
Limits:
- You cannot use a ground with an angle-of-attack sweep. Run one study for each angle.
- With a ground, the angle of attack turns the bodies, not the free stream.
Grid refinement
The refinement levels (0 to 3) add finer grid levels near the bodies. Each level has cells of half the size.
External flow results
A flow study gives:
- The force on the bodies, with its pressure part and its viscous part.
- The drag and the lift.
- The y+ of the first cells at the walls.
- The convergence state: converged, not converged or unstable.
The wind tunnel view
The wind tunnel view shows an external flow study as in a wind tunnel. To open it, select Simulation > Results > Wind Tunnel.
The view shows:
- Smoke lines that start upstream of the bodies.
- Flow lines on the surfaces.
- The pressure coefficient (Cp) on the surfaces.
- Pressure callouts: the stagnation point (largest Cp), the suction point (smallest Cp) and the base (the mean of the rear faces).
- Force arrows.
You can set the line density to Low, Medium or High. The pressures are gauge pressures, relative to the free stream.
To read Cp and the gauge pressure at a point on a body, use Probe.
The wind tunnel view is for external flow only.
Internal flow
An internal flow goes through pipes, ducts, channels or manifolds.
Where the fluid is
The results show the fluid region:
- Cavity of the part. The fluid is in the cavity of the part, between the inlet and outlet faces. Lids close the holes of these faces. The part is the wall.
- Volume of the body. The body is a model of the fluid. Its surface is the wall.
Openings
Each opening has a name, a kind, its faces and a value.
| Kind | Value |
|---|---|
| Velocity inlet | Speed in m/s |
| Mass flow inlet | Mass flow in kg/s |
| Pressure inlet | Gauge pressure in Pa |
| Pressure outlet | Gauge pressure in Pa |
Grid size
The mesh presets give 8 (coarse), 12 (medium) or 20 (fine) cells across the smallest opening. With fewer than 6 cells across, the result has a resolution warning.
Internal flow results
- The pressure drop from the inlets to the outlets, in Pa.
- The mass flow (kg/s) and the volume flow (m³/s) through the inlets.
- For each opening: the mass flow, the volume flow and the mean pressure.
- The mass balance error. The results panel shows OK below 1 % and a warning above 1 %.
All internal flow benchmarks are laminar. Turbulent internal flow is not validated.
Heat transfer with flow
A flow study can calculate forced-convection heat transfer. The heat is a passive value in the flow. The flow does not include buoyancy.
Results with heat transfer:
| Result | What it shows |
|---|---|
| Fluid Temperature | The fluid temperature on the slice. |
| Wall Temperature | The wall temperature on the part. |
| Heat Transfer Coefficient | The heat transfer coefficient on the part. |
A thermal study can use the wall heat of a flow study with the Heat From Flow condition.
Flow results
| Result | What it shows |
|---|---|
| Flow Pressure | The pressure on the part and on the slice. |
| Flow Speed | The flow speed on the slice. |
| Pressure Coefficient | The pressure coefficient on the part. |
| Wall Shear | The wall shear stress on the part. |
| Wall Y+ | The wall y+ of a study with a wall model. The wall model needs y+ from 30 to 300. |
| Wind Tunnel | The wind tunnel view. |
Good practice
- Put at least 32 cells across the diameter of a sphere. Put at least 128 cells along the chord of a wing section.
- If the mean y+ is more than about 300, the drag is too high. Make the grid finer.
- Compare lift values only between runs with the same refinement levels and the same refinement box.
- Use the flow results as indicative values above a Reynolds number of about 1,000.
For the benchmark data, refer to Validation.