Understanding Aerodynamic Forces

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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Aerodynamic forces are the forces that moving air or gas exerts on the rotating and stationary components of fans, blowers, compressors, and turbines. They arise from pressure differentials across blade surfaces, from momentum changes in the flowing gas, and from the continuous interaction between the fluid and the structure it flows over. These forces span both steady components — thrust and radial loads — and unsteady ones, such as pulsations at blade passing frequency and the random buffeting of turbulence. Together they produce vibration, load bearings and casings, and in some cases drive self-excited instabilities that can destroy a machine.

Aerodynamic forces are the gas-phase counterpart of the hydraulic forces found in pumps, but with three important differences: gas is compressible, its density varies strongly with pressure and temperature, and it couples acoustically with the machine and its ductwork. That acoustic coupling can create resonances and instabilities that simply do not exist in an incompressible liquid system, which is why fan and compressor problems often look quite different from pump problems on the spectrum.

1. Types of Aerodynamic Forces

1. Thrust forces

These are axial forces produced by pressure acting on the blade surfaces:

• Centrifugal fans: the pressure differential creates thrust directed toward the inlet.
• Axial fans: the reaction to accelerating the air produces an axial force.
• Turbines: gas expansion across the blading creates a large thrust.
• Magnitude: roughly proportional to the pressure rise and the flow rate.
• Effect: it loads the thrust bearing and produces axial vibration.

2. Radial forces

These are sideways forces created by a non-uniform pressure distribution around the rotor. They take two distinct forms.

Steady radial force:

• Caused by asymmetric pressure in the housing or ductwork.
• Varies with the operating point, i.e. the flow rate.
• Reaches a minimum at the design point.
• Creates bearing loading and a 1× vibration component.

Rotating radial force:

• Arises when the impeller or rotor carries an asymmetric aerodynamic load.
• The force rotates with the rotor.
• It creates a 1× vibration that looks just like unbalance.
• It can add vectorially to genuine mechanical unbalance, which is why a fan can appear to “go out of balance” purely because its operating point changed.

3. Blade passing pulsations

These are periodic pressure pulses at the rate at which blades pass a fixed point:

• Frequency: number of blades × RPM / 60 — a value our Blade Pass Frequency Calculator returns directly.
• Cause: each blade disturbs the flow field and emits a pressure pulse.
• Interaction: it occurs between the rotating blades and stationary struts, vanes, or the housing tongue.
• Amplitude: depends on the blade-to-stator clearance and the flow conditions.
• Effect: it is the primary source of tonal noise and vibration in fans and compressors.

4. Turbulence-induced forces

• Random forces: generated by turbulent eddies and flow separation.
• Broadband spectrum: the energy is spread across a wide frequency range rather than concentrated in tones.
• Flow dependent: they grow with Reynolds number and with off-design operation.
• Fatigue concern: this random loading contributes to component fatigue over time.

5. Unstable-flow forces

Rotating stall:

• A region of localised flow separation that rotates around the annulus.
• Appears at a sub-synchronous frequency, roughly 0.2–0.8× rotor speed.
• Creates severe unsteady forces.
• Common at low flow in compressors.

Surge:

• A system-wide flow oscillation, with the flow reversing forward and backward.
• A very low frequency, roughly 0.5–10 Hz.
• Extremely high force amplitudes.
• It can destroy a compressor if it is allowed to persist.

2. Vibration from Aerodynamic Sources

Blade passing frequency (BPF)

• The dominant aerodynamic vibration component.
• Its amplitude varies with the operating point.
• It is higher at off-design conditions.
• It can excite a structural or blade resonance.

Low-frequency pulsations

• Originating from recirculation, stall, or surge.
• Often severe in amplitude — they can exceed the 1× vibration.
• They indicate operation far from the design point.
• They call for a change in operating conditions, not a mechanical repair.

Broadband vibration

• Produced by turbulence and flow noise.
• Elevated in high-velocity regions.
• Increases with flow rate and turbulence intensity.
• Less worrying than tonal components, but a useful indicator of flow quality.

3. Coupling with Mechanical Effects

Aerodynamic–mechanical interaction

• Aerodynamic forces deflect the rotor.
• That deflection changes the running clearances, which in turn changes the aerodynamic forces.
• This feedback can create a coupled instability.
• A classic example is aerodynamic forces in seals contributing to rotor instability — closely related to the steam whirl seen in turbines.

Aerodynamic damping

• Air resistance generally provides damping for structural vibration.
• That effect is usually positive, i.e. stabilising.
• But under certain flow conditions it can become negative and destabilising.
• It is an important consideration in the rotor dynamics of turbomachinery.

4. Design Considerations

Minimising the forces

• Optimise blade angles and spacing.
• Use diffusers or a vaneless space to reduce pulsations.
• Design for a wide, stable operating range.
• Choose a blade count that avoids acoustic resonances.

Structural design

• Size the bearings for the aerodynamic loads on top of the mechanical loads.
• Make the shaft stiff enough to limit deflection under aerodynamic force.
• Separate the blade natural frequencies from the excitation sources.
• Design the casing and structure for the pressure-pulsation loads.

5. Operating Strategies and Field Measurement

Optimal operating point

• Operate near the design point for the lowest aerodynamic forces.
• Avoid very low flow, which invites recirculation and stall.
• Avoid very high flow, which raises velocity and turbulence.
• Use variable speed to hold the optimal point as demand changes — the affinity laws describe how flow, head, and power scale with speed.

Avoiding instabilities

• Stay to the right of the surge line in compressors.
• Implement anti-surge control.
• Monitor for the onset of stall.
• Provide minimum-flow protection for both fans and compressors.

In the field, the practical challenge is telling an aerodynamic problem apart from a mechanical one, because both can raise the 1× or BPF peaks. A portable two-channel analyser such as the Balanset-1A helps draw that line: by capturing the spectrum and the 1× amplitude and phase at several operating points, an engineer can see whether a peak tracks running speed and stays fixed with load — pointing to mechanical unbalance — or swells and shifts as the flow changes, pointing to an aerodynamic source. Where the 1× component proves to be true mechanical unbalance, the same instrument balances the fan or impeller in place, so the aerodynamic contribution can then be addressed on its own terms.

Aerodynamic forces are, in the end, fundamental to the operation and reliability of every air-moving and gas-handling machine. Understanding how these forces change with operating conditions, recognising their distinct vibration signatures, and both designing and operating equipment to keep the unsteady components small — chiefly by running near the design point — is what delivers reliable, efficient service from fans, blowers, compressors, and turbines across industry. Recognising the related fan defects and impeller defects that aerodynamic loading can accelerate completes the diagnostic picture.

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