Example: Rotating Machinery (Fan / Pump)

Example: Rotating Machinery (Fan / Pump)#

This example demonstrates simulating a rotating fan or pump impeller using the Multiple Reference Frame (MRF) approach. MRF is a steady-state approximation that avoids the cost of a full transient simulation.

Objective#

  • Generate a multi-region mesh with rotating and stationary zones

  • Run an MRF simulation of a fan or pump

  • Analyze performance (pressure rise, mass flow, efficiency)

  • Visualize flow patterns through the rotor

Background: MRF Approach#

The Multiple Reference Frame method:

  1. Divides the domain into a rotating zone (containing the impeller) and a stationary zone (inlet/outlet ducts)

  2. Solves the flow equations in a rotating reference frame within the rotating zone (adding Coriolis and centrifugal source terms)

  3. Uses a frozen rotor interface to couple the rotating and stationary zones

This gives a steady-state approximation of the time-averaged rotating flow. It’s fast (single steady solve) but doesn’t capture blade-passing transient effects.

Note

MRF is best for design-point analysis and comparative studies. For detailed transient analysis (noise, vibration, blade-to-blade interaction), use transient sliding mesh (available on Pro tier and above).

Step 1: Create a Meshing Project#

  1. Dashboard → New Project → Meshing

  2. Name: “Fan Simulation”

  3. Upload your fan/pump geometry (STEP format)

Step 2: Domain Configuration#

  1. Select Rotating Machinery domain type

  2. Studio creates two zones:

    • Rotating zone — Cylindrical region around the impeller

    • Stationary zone — Surrounding duct/enclosure

Rotating Zone Setup#

Parameter

Description

Example

Rotation axis

Direction of rotation

Z-axis: [0, 0, 1]

Rotation origin

Center of rotation

[0, 0, 0]

RPM

Rotational speed

3000 RPM

Zone shape

Cylindrical region enclosing the rotor

Radius slightly larger than blade tips

Zone Sizing

The rotating zone should:

  • Fully enclose all rotating parts (blades, hub, shroud)

  • Have a small gap (5–10% of blade span) between the blade tips and the zone boundary

  • Extend axially to include the blade passages

Do not include stationary parts (duct walls, volute) in the rotating zone.

Step 3: Multi-Region Setup#

Regions#

Region

Type

Description

rotor

Rotating (MRF)

Contains the impeller blades

inlet_duct

Fluid (stationary)

Upstream duct or plenum

outlet_duct

Fluid (stationary)

Downstream duct or volute

Interfaces#

Interface

Between

Type

rotor_inlet

rotor ↔ inlet_duct

Frozen rotor

rotor_outlet

rotor ↔ outlet_duct

Frozen rotor

Step 4: Surface Naming#

Surface

Name

Condition

Duct entrance

inlet

Velocity inlet or total pressure

Duct exit

outlet

Pressure outlet

Blade surfaces

blades

No-slip wall (rotating)

Hub

hub

No-slip wall (rotating)

Shroud / duct wall

shroud

No-slip wall (stationary)

Step 5: Mesh Settings#

Parameter

Value

Target cell size

Blade span / 20

Min cell size

Blade span / 100

Refinement levels

10

BL layers

8

BL first layer height

Target y+ ≈ 30 for RANS

BL growth rate

1.2

Refinement Zones#

Zone

Location

Purpose

Blade passage

Between blades

Resolve blade-to-blade flow

Leading edges

Blade leading edges

Capture flow around blade entry

Tip gap

Between blade tips and shroud

Resolve tip leakage flow

Step 6: Generate Mesh#

Expect 5–20 million cells for a medium-resolution fan simulation. Check:

  • Quality in the rotating zone (blade passages should have uniform, fine cells)

  • Interface quality (cells should be similar size on both sides)

  • Boundary layer quality on blade surfaces

Step 7: Simulation Setup#

Setting

Value

Turbulence model

k-ω SST

Rotating zone RPM

3000 RPM (match your design speed)

Inlet velocity

Based on operating point (e.g., 5 m/s)

Outlet pressure

0 Pa (or set based on system curve)

Turbulence intensity

3%

Max iterations

1500

Algorithm

SIMPLE

Step 8: Results Analysis#

Performance Metrics#

  1. Total pressure rise — Measure total pressure at inlet and outlet:

    ΔP_total = P_total_outlet - P_total_inlet

  2. Mass flow rate — Integrate velocity across the inlet face

  3. Efficiency — Compare actual pressure rise to ideal:

    η = (ΔP_total × Q) / (τ × ω)

    Where Q is volume flow rate, τ is torque on blades, ω is angular velocity

Flow Through Blade Passages#

  1. Add a slice plane perpendicular to the rotation axis, at mid-blade height

  2. Color by velocity magnitude

  3. Look for:

    • Uniform flow through all passages (good)

    • Separation on blade suction side (may indicate stall)

    • High velocity near blade tips (tip effects)

Meridional View#

  1. Add a slice plane through the rotation axis (meridional plane)

  2. Color by axial velocity or pressure

  3. Shows how the flow develops from inlet through the rotor to the outlet

Blade Loading#

  1. Color blade surfaces by pressure

  2. The pressure side (concave) should show higher pressure

  3. The suction side (convex) should show lower pressure

  4. The pressure difference drives the torque and pressure rise

Tip Flow#

  1. Add a slice plane at the blade tip radius

  2. Color by velocity

  3. Check for tip leakage flow (flow from pressure to suction side through the tip gap)

  4. This is a key source of loss in unshrouded impellers

Design Point vs. Off-Design#

To map a performance curve:

  1. Run simulations at several flow rates (vary inlet velocity)

  2. Record pressure rise and efficiency at each point

  3. Plot ΔP vs. Q (fan curve) and η vs. Q (efficiency curve)

  4. Identify the best efficiency point (BEP)

Off-Design Caution

MRF becomes less accurate far from the design point, especially in stalled or highly separated conditions. If you need off-design performance mapping with high accuracy, consider transient sliding mesh simulations.

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