Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.

                                                                                                                                            —Albert Einstein

Actuator Development and Optical Diagnostics

The microjet development laboratory facilitates the study of the fluid dynamics of microflows, particularly supersonic microjets. To date, supersonic microjets as small as 50 microns with exit velocities in the range of over 500 m/s have been examined. Microjets are being studied due to their many potential applications, which included heating and cooling of electronics and other devices, dust removal and as actuators for active control of the large scale supersonic impinging jets, which occur in STOVL (Short Takeoff and Vertical Landing) aircraft.

Properties that have been studied include: flow visualization of both the free and impinging microjet, surface pressure, skin friction coefficient, and heat transfer. Various experiment diagnostics have applied to the study of microjets. These diagnostics include the use of a specialized micro-Schlieren system (in-line and z-type) and oil film interferometry. This research has been Research Sponsors the the Air Force Office of Scientific Research (AFOSR) and NASA, Langley.

Laser-based Micro-schlieren

Excluding the light source, the micro-schlieren system is similar to that developed by Phalnikar. Initially the light passes through a rectangular slit. A 25.4 mm diameter lens with a focal length of 55 mm is used to collimate the light, which passes through the flow region. Another 25.4 mm lens with focal length 55 mm is used to focus light again. At this focal point a knife edge is used to increase the sensitivity of the schlieren system. In the previous setup, a gradient filter was used by Phalnikar to reduce the effects of diffraction. Lastly, a 28-20 mm variable camera lens was used to magnify the test section. The camera used was a Kodak Megaplus, with a resolution of 1018 x 1008 pixels.

The light source for the schlieren system was based off of the work of Volpe and Settles, and Beutner et al. The light source is formed from laser-induced breakdown in argon, which creates a white light source with a pulse duration of ~10ns. The laser used to generate this breakdown was a New Wave Gemini PIV laser. A high pressure tank was used to supply argon at a rate of 4 L/min onto the spark, which was found to increase the light intensity of the spark 3-4 times, and decrease the jitter of the spark in the direction of the laser. Although, the argon assisted in reducing the jitter, it was still present. Therefore, the knife edge had to be used parallel to the laser beam direction. The light from the spark is then focused using a 50 mm camera lens f1.4 onto the slit of the micro-schlieren system. Figure 2 is an image of the laser-based micro-schlieren setup.