Wake frequency calculation theory

Vortex shedding, or von Karman vortex street, as it is sometimes known, has been around for many years. It is a phenomenon in fluid mechanics for a repeating pattern of swirling vortices caused by the unsteady separation of liquids or gases over bluff bodies.

It is vortex shedding that causes singing in the telephone wires, or causes flags to flutter, as the vortices are shed from the flagpole.

When a Thermowell − used in industrial temperature measurement to provide isolation between the Temperature Sensor (PRT or Thermocouple) and process being measured - is placed in a pipe and fluid or gas flows down the line, vortices are shed from the thermowell. The frequency of the shedding is related to the Reynolds number, flow velocity, viscosity and the dimensions of the Thermowell.

Due to a high number of failures, a method of evaluating whether the Thermowell was ‘fit for purpose’ was established by Murdock way back in the 1950s. He calculated that the shedding or wake frequency should not exceed 0.8 times the natural frequency of the Thermowell.

When the fluid passes over the Thermowell it sets the Thermowell into oscillation. Initially it was thought that it only moved transverse to the flow, but, with further experiments, it was proved that the Thermowell also moved in line with the flow. Unfortunately, the frequencies at which these two movements took place were quite different. As a result, the original ratio of <0.8 (wake frequency/natural frequency) is still valid, but a new limit of <0.4 is used to account for the ‘in-line’ frequency.

In 2010 a new version of the ASME 19.3 standard was published, incorporating the new 0.4 ratio and also taking into account the process connection, the shielded length of Thermowell (usually contained within a stand-off), as well as working pressure. This new standard encompasses many features not included in the previous standard and ensures that the thermowell is suitable for its function.

If you would like us to carry out a Wake Frequency Calculation or are just looking for further information on the Temperature Sensors and Thermowells manufactured by H&b Sensors, we would be happy to hear hear from you. Please give us a call on +44(0)1243 866866 or email us at sales@hbsensors.co.uk.