Understanding Turbulence in Vibration Analysis
In vibration analysis, turbulence refers to the chaotic, random and unstable flow of a fluid — liquid or gas — through a machine such as a pump, fan or turbine. This erratic flow creates pressure fluctuations that act as a forcing function, inducing a low-frequency, random vibration in the machine’s structure. Unlike the discrete, periodic forces produced by unbalance or misalignment, the vibration from turbulence does not occur at a single sharp frequency. Instead it appears as a broad “hump” of non-synchronous energy in the Wigo wa FFT — and recognising that signature is the key to diagnosing it correctly.
1. Definition: What is Turbulence?
Turbulence is fundamentally a flow phenomenon rather than a mechanical defect. When a fluid moves smoothly along its intended path the pressure it exerts on blades, vanes and casings is steady; when that flow breaks up into eddies and swirls, the pressure becomes a rapidly varying, statistically random load. The machine structure responds to this random forcing exactly as it responds to any other excitation — by vibrating — but because the force itself has no fixed period, the resulting vibration has no fixed frequency either. This places turbulence in the broader family of flow-induced excitation alongside hydraulic forces in pumps and aerodynamic forces in fans and blowers, and it is closely related to the concept of flow turbulence as a vibration source.
2. Characteristics of Turbulence Vibration
- Frequency: a low-frequency phenomenon, typically below 10–20 Hz and well below the running speed of the machine.
- Broadband nature: it does not produce a sharp, distinct peak. Instead it raises the noise floor in the low-frequency region of the spectrum, often described as a “random hump” or “haystack.”
- Random and non-periodic: the vibration is not steady — amplitude and phase fluctuate constantly and randomly. In the time waveform it appears as a chaotic, non-repeating signal with no clean repeating pattern.
- Direction: the vibration is typically radial and can be present in both the horizontal and vertical directions.
Because the energy is spread across a band rather than concentrated in a line, the overall vibration level can rise noticeably even though no single spectral peak looks alarming — a pattern worth keeping in mind when reviewing trended overall readings.
3. Common Causes of Turbulence
Turbulence is a hydraulic or aerodynamic issue caused by disruptions to the smooth, designed flow of the fluid. Common causes include:
- Operating away from the Best Efficiency Point (BEP): pumps and fans are designed to run most efficiently and smoothly at a specific point on their performance curve. Operating significantly above or below the BEP flow rate forces the fluid to move inefficiently, generating turbulence — and at very low flow this can shade into recirculation, an internal back-flow that is itself a recognised source of low-frequency energy.
- Obstructions in the flow path: anything that obstructs or disrupts the fluid’s path can cause turbulence, including poorly designed piping (such as a sharp bend immediately before a pump’s suction inlet), partially closed valves, clogged strainers, or foreign objects.
- Air entrainment or cavitation: air bubbles in a liquid (entrainment), or the formation and collapse of vapour bubbles (cavitation), create highly turbulent and impulsive conditions that generate significant random vibration.
- Poor sump or inlet design: in pumps, a badly designed sump can create vortices that draw air and turbulence straight into the suction.
4. Diagnosis and Differentiation
The key to diagnosing turbulence is its random, broadband and low-frequency nature. An experienced analyst can often spot it from the “unsteady” and beating-like feel of the vibration on the machine itself. It is important, though, to differentiate turbulence from other low-frequency issues that can look superficially similar:
- Mechanical looseness: looseness also creates broadband noise, but it is usually marked by a raised noise floor across the entire spectrum together with distinct harmonics of running speed — harmonics that are absent in pure turbulence.
- Oil whirl: this is a distinct sub-synchronous peak at roughly 0.4–0.48×, not a broad hump of random energy.
- Rubbing: a rub can generate a wide range of frequencies, but it typically includes many high-frequency harmonics and sub-harmonics, and its time waveform may show truncated or clipped peaks.
A frequency-based fault chart such as the Vibration Source Identifier can help confirm which of these signatures you are looking at, and where cavitation is suspected a Pump Cavitation Frequency Estimator narrows it down further.
5. Correcting Turbulence
Since turbulence is a process-related issue rather than a mechanical fault, the cure normally lies in correcting the operational or system-design problem — not in working on the rotor. Typical remedies include adjusting the operating point of the pump or fan back toward its BEP, opening throttled valves, cleaning strainers, or modifying the piping to remove a flow disturbance near the inlet. The diagnostic role of the vibration instrument here is to confirm that the broadband energy genuinely originates from flow and not from a rotating-component defect. A portable two-channel analyser such as the Balancet-1A makes that distinction straightforward in the field: by capturing the spectrum and time waveform at each bearing, it lets you confirm there is no dominant synchronous peak and no residual unbalance driving the vibration — pointing the investigation toward the process rather than the machine, and preventing the common mistake of trying to balance a problem that balancing cannot fix.
