A pulsed LED system for schlieren flow visualisation

Abstract

Schlieren flow visualisation can be performed using light emitting diodes (LEDs) operated with a strong pulse of current (typically a few amps can be passed through the LED for a number of microseconds). Commercial systems are available to produce such pulses, but they are prohibitively expensive. This report provides details of a low cost pulsed current circuit which can be used to drive LEDs in flow visualisation systems. The aim of this work is to improve the accessibility of schlieren and shadowgraph flow visualisation in laboratories with significant financial constraints. The LED drive circuit provides a 12 A (maximum) current pulse with a selectable duration of between about 1.5 and 38 microseconds. The optical performance of two LEDs operated with the high current pulses produced by this circuit is also discussed. A sample of the visualisation results obtained with a hot air jet is also presented to demonstrate the success of the schlieren system.