I think the whole point of ultrasonic is it's not intrusive.
An intrusive one would be like keeping the remote velcro'd to the TV.

I think they work utilising either the dopler effect or by looking at the time taken for a pulse to travel between two points, the sound pulse will be carried along with the liquid so will from A to B quicker if the liquid is flowing in that direction.
Del

There are "in-line" ultrasonic flow meters where the transducers are integrated into a spool piece that is part of the pipeline, as opposed to the 'clamp-on' style that is not part of the pipe, but comes in contact with the pipe externally.

Many of the major instrument manufacturers have in-line ultrasonics.

Siemens model is here:

http://www.automation.siemens.com/w1/process-instrumentation-inline-flow-14482.htm

Ultrasonic is used for non-intrusive flow measurements; though inline meters are available but the measuring technique is still non-intrusive.

The basic principle is using Ultrasonic waves for flow measurement; either by using "Doppler Effect" or by using the " Transit time Method". No physical interaction between the transducers and the process.

All ultrasonic flow meters are intrusive in the sense that the pipe containing the stream being monitored is transparent to ultra sound. What is being measured is the effect of the interaction between ultrasound and the flowing stream.

Yes there is. Normally this type is used for Flare flow measurement where high turndown ratio is required but the medium is gas with pressure is less than 4 bar where clamp on sensor unable to work.