High Resolution Instrumentation for Flow Measurements

Abstract

Turbulent quantities are in general difficult to measure, mostly because of the wide range of length and time scales involved. Instrumentation without adequate spatial and temporal resolution can compromise the measurements and bias conclusions. To address this issue, a family of nano-scale, high resolution sensors have been developed utilizing semiconductor fabrication techniques, and validated with either available data or theoretical predictions.

A miniature cold-wire probe (the T-NSTAP) was designed to reduce low-frequency attenuation in measurements of temperature, and can minimize errors inherent in conventional cold-wires due to insufficient resolution. With a sensing length of only 200 μm and a roll-off frequency of more than 10 kHz in air, the T-NSTAPs successfully captured small scale information that is generally missed by filtering.

Elastic filament velocimetry (EFV) is a novel method of measuring flow velocity utilizing the deflection of a freestanding nanoribbon. The elongation of the nanoribbon under fluid forcing results in a measurable change in the electrical resistance, which can be correlated with flow velocity. This technique is versatile and can be used in any fluid, regardless of the fluid properties. Experimental results in both air and water displayed good agreement with theoretical predictions derived from nonlinear beam theory, and exhibited great potential for low velocity measurements.

A new method for humidity measurement using hot-wire heat transfer in low Péclet number regime is proposed. The new sensor, q-NSTAP, designed to be insensitive to velocity, has a sensing element width as small as 500 nm. The sensor has been operated with a custom circuit with high bandwidth. Preliminary results confirmed the feasibility of this new humidity measurement method.

A nano-scale crossed hot-wire (x-NSTAP) was created and characterized. The x-NSTAP has been deployed in the Princeton Superpipe and obtained very promising results with low spatial and temporal filtering. A novel combining method developed for the x-NSTAP can be generalized for simultaneous measurements of any quantities in a small measurement volume, enabling measurements that were extremely challenging or not possible prior to this development.