FLOW VISUALIZATION

FLOW VISUALIZATION

Methods are of importance not only in giving a clear qualitative picture of the flow phenomena, but also in many cases in yielding quantitative information .the methods usually depend either on the reflection or scattering of light by small solid or liquid particles introduced into the stream, or on the natural changes of refractive index which accompany the density changes of a compressible fluid ,or on the refraction of light by filaments or particles of of fliud of a different refractive index introduced into the flow.recently a series of methods for the indication of the position of the transition from laminar to turbulent flow in the boundary layer of a body have been developed and these depend on the physical or chemical behavior of a deposit placed on the surface of the body.Physical principles involved n the visualization of fluid motion

the first group of methods of fluid motion visualization, small particles of solid or liquid are introduced to the air stream and observed by reflected or scattered light .It is necessary to assume that such particles are of sufficiently small inertia to follow the local direction of fluid motion ,and successfully light not to be sensibly influenced by gravity .typical examples of these methods are the use of smoke filaments and the ultra microscope .In the latter method of microscope (of normal power ) is used to observe the motion of very small (ultra-microscopic) particles occurring naturally in the fluid stream and visualize by the refraction and scattering of intense illumination .An analogies method of visualisation depends on the method of fine threads of silk or cotton to indicate the general direction and steadiness of the flow close to their points of attachments.This method is usually termed the wool tuft technique , and is particular in usefull in the visualization of the flow close to the surface of a body The second group of methods depends on changes of refractive index from that of the working fliud may be introuduced , or a filament of the working fluid itself may be heated continiously by a hot wire or intermittently by a spark discharge .this group includes methods based on the changes in the refractive index which accompany the natural changes in density in the field of flow .These latter methods are essentially applicable only to compressible flow and are especially suitable at high mach number It is important to remember , however ,that they do not show directly either the stream lines ,filament lines ,or particle paths , and are essential methods for the visualization of the density field .

For the observation of the flow in the boundary layer it is possible to use several of the methods outlined above or a number of spealised techniques described below SMOKE TECHNIQUES

Smoke consists of suspensions of smaller solid or liquid particles in a transparent gas, and is usually observed by the scattering and reflection of light by these particles. The principal requirements of a smoke for wind tunnel work are

a) That it shall not disturb the flow in the tunnel (e.g. By the formation of deposits on the surface of a body exposed to the air stream) nor form deposits which led to blockage of the pipe work used for ducting the smoke in to the tunnel