Technology / Technical guide
Wind Turbine Tower Disturbance & Cyclic Blade Loading
The tower can disturb the flow a rotating blade encounters. Upstream influence and a downstream tower wake describe different situations. A periodically encountered disturbance can produce changing section forces even under otherwise steady background wind.
The blade’s path turns a spatial disturbance into a time history.
Define rotor/tower position and a particular section trajectory first. Geometry and aerodynamic response then connect the flow pattern to loading frequency and waveform.
01 / Layout
Upwind and downwind describe relative positions.
An upwind rotor lies on the incoming-flow side of the tower; a downwind rotor lies behind it for the stated aligned flow.
An upwind rotor lies on the incoming-flow side of the tower; a downwind rotor lies behind it for the stated aligned flow. These are arrangement definitions, not statements about a particular MARSTON model.
Tower interaction also depends on the rotor-plane offset, tower geometry and actual inflow direction. The terms alone do not provide a local velocity field or establish which disturbance a section encounters.
02 / Upstream effect
A disturbance can occur before the tower.
Flow around a body changes its nearby velocity field, including upstream regions.
Flow around a body changes its nearby velocity field, including upstream regions. AeroDyn distinguishes a potential-flow representation of tower influence from a downstream shadow representation. The two should not be treated as the same profile with opposite labels.
A potential-flow model is an idealisation; it does not resolve every viscous wake property. Its predicted local velocity belongs to the geometry, assumptions and coordinates represented by that calculation.
03 / Downstream shadow
A tower wake creates a different local flow.
Behind a tower, the represented flow can contain a velocity deficit and fluctuations associated with its wake.
Behind a tower, the represented flow can contain a velocity deficit and fluctuations associated with its wake. A blade passing through that region can encounter conditions different from the background flow.
That shadow is distinct from the rotor’s own downstream wake. The rotor-wake guide describes a separate field, while both can matter in a model incorporating their interaction.
04 / Periodicity
Section passage and aggregate loading differ.
For a simple stationary disturbance, one represented blade section repeats its path once per revolution.
For a simple stationary disturbance, one represented blade section repeats its path once per revolution. Its passage frequency is n/60 Hz when n is rpm. A hypothetical 60 rpm corresponds to one path repetition per second.
If B identical blades pass evenly, some aggregate contributions may have a B-times repetition component. This is conditional on symmetry and the quantity being combined; it is not a declared MARSTON blade count or the only possible frequency.
05 / Force history
A frequency does not determine amplitude.
The local flow change modifies relative velocity and angle of attack, so the section force response depends on aerodynamic state.
The local flow change modifies relative velocity and angle of attack, so the section force response depends on aerodynamic state. Two disturbances with the same repetition frequency can have different magnitudes, durations and resulting force histories.
Unsteady separation can add history dependence. The stall guide explains why a rapidly changing encounter need not follow a steady coefficient curve exactly.
06 / Structural response
Cyclic forcing is not a vibration prediction.
The supporting structure responds according to its modes, damping and constraints.
The supporting structure responds according to its modes, damping and constraints. Identifying a periodic aerodynamic input does not establish the resulting deflection, resonance or fatigue outcome.
No actual MARSTON rotor placement, tower-shadow amplitude, frequency margin or cyclic-load value is assigned here. The page explains flow-to-loading causality without an installation change, operating limit or test procedure.
Continue the review with Small Wind Turbine Wakes: Deficit, Mixing & Recovery.
Continue the review with Wind Blade Aerodynamic Stall & Flow Separation.
Continue the review with Wind Blade Mode Shapes & Natural Frequencies.
General background: OpenFAST: Distinct tower influence and tower-shadow models · NREL: Aerodynamic tower disturbance and model conditions. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Layout | Rotor position relative to tower and inflow | Upwind/downwind are geometric definitions. |
| Potential influence | Idealised nearby body-flow disturbance | Not a resolved viscous wake. |
| Tower shadow | Represented downstream tower-wake effect | Different from the rotor’s own wake. |
| n/60 | Section-path repetition frequency, Hz | Does not supply force amplitude. |
| Structural response | Modes, damping and boundary conditions | Needed beyond the forcing frequency. |
Engineering questions.
Can a tower influence an upwind rotor?
Nearby flow can be disturbed upstream; the applicable model and geometry determine its representation.
Is tower shadow the rotor wake?
No. They refer to disturbances associated with different parts of the system.
Does 60 rpm apply to a MARSTON model?
No. It is a hypothetical frequency conversion.
Is B-times repetition always the only load frequency?
No. The aggregate quantity and symmetry determine which components appear.
Does a periodic load prove resonance?
No. The structural modes, damping and forcing need their own analysis.
Does this identify an actual rotor layout?
No product upwind/downwind configuration is declared here.
Technical enquiry
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