Introduction | Radiation Force Balances | Measurement | Additional Information | Further Reading

Streaming

Streaming is a force that is experienced in addition to the radiation force incident on a target. As the ultrasound wave propagates through the water there is a small amount of energy abosrbed in the water path. As the water itself will experience a radiation force this causes additional movement which is incident on the target (yellow arrows) in addition to the radiation force from the transducer (red arrows).

The magnitude of the streaming force is highly dependent on frequency due to the attenuation of ultrasound in the liquid. Due to the decrease in path length of the ultrasound, a higher frequency of operation corresponds to an increase in the magnitude of the streaming force. A simple way to reduce the effects of streaming is to use a membrane between the transducer and the target. The streaming currents are deflected at the membrane and only the radiation force is incident on the target. Reducing the separation between the transducer and the target is beneficial.

The time history below indicates the effect of streaming force. Three power measurement runs were completed on the NPL Primary Standard Radiation Force balance using an absorbing target. The reference source is operating at 26 MHz at an output of approximatley 50 mW. The output is measured at three transducer/target separations and all results have been corrected for attentuation. Close to the target the response of the balance to the incident force indicates a trend affected predominantly by the change in bouyancy from target heating. As the separation increases the response starts to deviate from a flat response with additional force at the start of the ON transition and more prominently by a delay in the settling time after the drive signal to the transducer is switched off.

Radiation Force (red) and the streaming force (yellow) incident on the target.

A visual indication of the effects of the streaming force.

References:

• Ultrasonic Exposimetry : Chapter 5 Part B 'Radiation Force and Force Balances' Author : K Beissner 1993
• Ultrasound in Medicine : Chapter 3 'Radiation Pressure and Acoustic Streaming' Author : F. A. Duck 1997