Introduction: Almost all industrial, man-made flows are turbulent. Almost all naturally occurring flows on earth, in oceans, and atmosphere are turbulent. Therefore, the accurate measurement and calculation of turbulent flows has wide ranging application and significance.
In general, turbulent motion is 3-D, vortical, and diffusive making the governing Navier-Stokes equation (above) very hard (or impossible) to solve in most real applications, thus the need to measure flow.
Early turbulence research has been complemented by experimental methods that included pressure measurements and by the point measurement technique of Hot Wire Anemometry (HWA). Particular difficulties in using these intrusive methods include, reversing flows, vortices, and highly turbulent flows. In addition, intrusive probes are subject to non-linearity (require calibration), sensitivity to multi-variable effects (temperature, humidity, etc.), and breakage among other problems.
Spatial Resolution: High spatial resolution is a must for any advanced flow diagnostic tool. In particular, the spatial resolution of a sensorshould be small compared to the flow scale, or eddy size, of interest. For turbulent flows, accurate measurement of turbulence requiresthat scales as small as 2 to 3 times the Kolmogorov scale isresolved. Typical CTA sensors are a few microns in diameter, and afew millimeters in length, providing sufficiently high spatialresolution for most applications. Their small size and fast responsemake them the diagnostic of choice for turbulence measurements.
Temporal Resolution: Due to the high gain amplifiers incorporated into theWheatstone Bridge, CTA systems offer a very high frequencyresponse, reaching into hundreds kHz range. This makes CTA an ideal instrument for the measurement of spectral content in mostflows. A CTA sensor provides an analog signal, which is sampled using A/D converters at the appropriate rate obeying the NY Quistsampling criterion.
Intrusive and Non-Intrusive Measurement Techniques: Most emphasis in recent times has been in the development ofnon-intrusive flow measurement techniques, for measuring vector, as well as, scalar quantities in the flow. These techniques have beenmostly optically-based, but when fluid opaqueness prohibits access, then other techniques are available. A quick overview of several ofthese non-intrusive measurement techniques is given in the next few sections for completeness. More extensive discussion on thesetechniques can be found in the references cited.
Particle Tracking Velocimetry (PTV) and Laser Speckle Velocimetry (LSV): Just like PIV, PTV and LSV measure instantaneous flow fieldsby recording images of suspended seeding particles in flows at successive instants in time. An important difference among the three techniques comes from the typical seeding densities that can bedealt with by each technique. PTV is appropriate with “low” seedingdensity experiments, PIV with “medium” seeding density and LSV with “high” seeding density. The issue of flow seeding is discussed later in the paper.
Implementation of the Constant Temperature Anemometer: The velocity is measured by its cooling effect on a heatedsensor. A feedback loop in the electronics keeps the sensortemperature constant under all flow conditions. The voltage dropacross the sensor thus becomes a direct measure of the power dissipated by the sensor. The anemometer output thereforerepresents the instantaneous velocity in the flow. Sensors arenormally thin wires with diameters down to a few micrometers. Thesmall thermal inertia of the sensor in combination with very highservo-loop amplification makes it possible for the CTA to followflow fluctuations up to several hundred kHz. Shows a basic diagramof a Constant Temperature Anemometer.
First choice for applications in air flows with turbulence intensities up to 5-10%. They have the highest frequency response. Repairable.
Gold-plated wires: For applications in airflows with turbulence intensities up to 20- 25%. Repairable.