3 ms·

Shadowgraphy is a method of measuring distortion or flow in a fluid by shining a light through it and photographing the shadow. Very useful in studying heat, s

Shadowgraphy is a method of measuring distortion or flow in a fluid by shining a light through it and photographing the shadow. Very useful in studying heat, shockwaves, and turbulence. See: https://en.wikipedia.org/wiki/Shadowgraph https://en.wikipedia.org/wiki/Shadowgraph Penn State did a lot of pioneering work in and with this technique.

roymurdock 9y ago
In this case it looks like he puts objects in the way of the electric field, creating and measuring the resulting "ion shadows" through thermal and Schlieren photography: To further strengthen the role of surface charges as the key ingredient responsible for the RWI, we shield part of the oil surface from charge accumulation by creating an ion shadow. An ion shadow forms when an object blocks the path of the charged ions towards the surface of the oil. Even thin objects, like paper, are sufficient to produce a sharp shadow that inhibits charge accumulation on the oil surface. In the absence of surface charges the instability is locally inhibited, as evident from figure 11. Even metal objects produce the same effect when placed in the path of the charged ions. Using Schlieren photography (figure 12), we imaged the corona streamer which shows that the streamer and gas flow stay roughly the same size as the needle diameter, while the instability continues to occur at a larger radius. As you note, Schlieren photography "uses light from a single collimated source shining on, or from behind, a target object. Variations in refractive index caused by density gradients in the fluid distort the collimated light beam. This distortion creates a spatial variation in the intensity of the light, which can be visualised directly with a shadowgraph system." Basically the point charge is like the sun emitting light (ions) and if you put something in the way of the grounding plate, you get a shadow (local inhibition of the instability pattern). Check out the pics on the Schlieren photography page, they're cool: https://en.wikipedia.org/wiki/Schlieren_photography https://en.wikipedia.org/wiki/Schlieren_photography

roymurdock 9y ago

In this case it looks like he puts objects in the way of the electric field, creating and measuring the resulting "ion shadows" through thermal and Schlieren photography: To further strengthen the role of surface charges as the key ingredient responsible for the RWI, we shield part of the oil surface from charge accumulation by creating an ion shadow. An ion shadow forms when an object blocks the path of the charged ions towards the surface of the oil. Even thin objects, like paper, are sufficient to produce a sharp shadow that inhibits charge accumulation on the oil surface. In the absence of surface charges the instability is locally inhibited, as evident from figure 11. Even metal objects produce the same effect when placed in the path of the charged ions. Using Schlieren photography (figure 12), we imaged the corona streamer which shows that the streamer and gas flow stay roughly the same size as the needle diameter, while the instability continues to occur at a larger radius. As you note, Schlieren photography "uses light from a single collimated source shining on, or from behind, a target object. Variations in refractive index caused by density gradients in the fluid distort the collimated light beam. This distortion creates a spatial variation in the intensity of the light, which can be visualised directly with a shadowgraph system." Basically the point charge is like the sun emitting light (ions) and if you put something in the way of the grounding plate, you get a shadow (local inhibition of the instability pattern). Check out the pics on the Schlieren photography page, they're cool: https://en.wikipedia.org/wiki/Schlieren_photography https://en.wikipedia.org/wiki/Schlieren_photography