5/48 7.1.1 Flow Visualization -Methods Way 1:Introduction of tracer particles: ●In Liquid Flows: en Colored Dyes; n Hydrogen()bubbles formed in water by applying electric pulses. ●In Gas Flows: Smoke: Helium(氦气)-filled“soap”bubbles; n Gas molecules made luminous()by an ionizing electric spark. Way 2:Detection of flow related changes in fluid optical properties. ●Qualitative Method-Shadowgraph(阴影图)and Schilieren(纹影仪)methods: light and dark patterns related to flow conditions are produced by the bending of light rays as they pass through a region of varying density. Quantitative Technique: Interferometer:light/dark pattems are formed by interference effects resulting from phase shifts between a reference beam and the measuring beam. Particle Image Velocimetry (PIV):the liquid or gas flow must be seeded with near- neutrally-buoyant particles;the velocity of these particles is what is actually measured. Other Methods:Tomographic Techniques,Molecular Tagging Velocimetry, Holographic Techniques,etc. 6/48 7.1.1 Flow Visualization -Shadowgraph ◆Shadowgraph For compressible flow (Mach Number >0.3),density varies with velocity sufficiently to produce measurable effects; In principle,we cannot directly see a difference in temperature,a different gas, or a shock wave in the transparent air; ●However,,all these disturbances refract(折射)light rays,so they can cast(投射) shadows. Collimated light bea is used as light source. "Edgerton"shadowgram of the firing of an AK-47 assault rifle Images and artwork by Gary S.Settles,Penn State Gas Dynamics Lab7.1.1 Flow Visualization - Methods ‹ Way 1: Introduction of tracer particles: z In Liquid Flows: Colored Dyes; Hydrogen(氢气) bubbles formed in water by applying electric pulses. z In Gas Flows: Smoke; Helium(氦气)-filled “soap” bubbles; Gas molecules made luminous(发光) by an ionizing electric spark. ‹ Way 2: Detection of flow related changes in fluid optical properties. z Qualitative Method - Shadowgraph(阴影图) and Schilieren(纹影仪) methods: light and dark patterns related to flow conditions are produced by the bending of light rays as they pass through a region of varying density. z Quantitative Technique: Interferometer: light/dark patterns are formed by interference effects resulting from phase shifts between a reference beam and the measuring beam. Particle Image Velocimetry (PIV): the liquid or gas flow must be seeded with near￾neutrally-buoyant particles; the velocity of these particles is what is actually measured. ‹ Other Methods: Tomographic Techniques, Molecular Tagging Velocimetry, Holographic Techniques, etc. 5/48 7.1.1 Flow Visualization - Shadowgraph "focused" shadowgram of a common firecracker explosion Sunlight shadowgram of a martini glass A prehistoric shadowgraphy "Edgerton" shadowgram of the firing of an AK-47 assault rifle Images and artwork by Gary S. Settles, Penn State Gas Dynamics Lab ‹ Shadowgraph z For compressible flow (Mach Number >0.3), density varies with velocity sufficiently to produce measurable effects; z In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air; z However, all these disturbances refract(折射) light rays, so they can cast (投射) shadows. z Collimated light beam is used as light source. 6/48