Author(s): Kai-Uwe Graw; Joachim Lengricht

Linked Author(s): Joachim Lengricht, Kai-Uwe Graw

Keywords: No Keywords

Abstract: Three optical methods are applicable to velocity measurements beneath gravity waves.All three are connected with huge problems,as an instationary and non-periodic flow hasto be analysed with a high resolution per unit time.Velocity probes disturb the flow andmost of them cannot be used here.The small ultrasonic probes developed in the lastyears are easy to use,and the accuracy should be sufficient also for small scaleexperiments.The aim of this paper is a comparison of practical measurements with an IDv system andan ultrasonic probe.They were carried out in a small scale wave flume.The results showthat still both methods have drawbacks in this application.Sometimes the largest velocityis not measured with the IDv-as no particle is in the measuring volume at the specificmoment,sometimes the probe does not measure the maximum velocity signal-due tothe averaging over time and space.Very surprisingly,the first results with the probe were not satisfactory.Eventually thereason for this was found to be an undocumented change in the system hardware.Afterthe "repair",the probe resuts are in very good agreement with the IDv data,thedifferences are smaller than the errors expected.

DOI:

Year: 1993