A new method for measuring turbulent heat fluxes using PIV and fast-response cold-wires

Abstract

In a laboratory setting, heat fluxes are generally acquired using a combination of hot-and cold-wire anemometry, where the probes are placed in close proximity to each other and sampled simultaneously to obtain fluctuations in velocity and temperature. Hot-wires, however, are sensitive both to velocity and to temperature, and so, if the flow has mean velocity and temperature gradients, as in a boundary layer, extensive calibrations are required. In addition, if the wall-normal heat flux wθ were needed, as is often the case, crossed wires would also be required, adding further complexity (w is the wall-normal velocity fluctuation, θ is the local temperature fluctuation, and the overline signifies an ensemble average). In addition, fluctuating temperature measurements obtained using cold-wires often suffer from significant frequency response limitations due to the thermal inertia of the probe and conduction to the prongs (Smits et al. 1978;Smits and Perry 1981;Arwatz et al. 2013).A limited number of previous studies have employed laser Doppler anemometry (LDA) in combination with a cold-wire to measure the heat fluxes at a single point while avoiding complex calibration requirements (see, for example, Thole and Bogard 1994;Heist and Castro 1998). Here, we outline a new method to obtain turbulent heat flux data using a combination of particle image velocimetry (PIV) and a nanoscale cold-wire specifically designed for improved frequency response (Fan et al. 2015). In its simplest form, this method gives the wall-normal (wθ) and streamwise turbulent heat fluxes (uθ) at a single point, using the velocities measured immediately adjacent to the cold-wire probe, with one sample per PIV image, as with previous LDA measurements. Crucially, however, this new method also provides spatial velocity information and thus a direct measure of the streamwise coherence of the velocity and temperature fields, so that the streamwise integral length scales can be Abstract A new method for measuring turbulent heat fluxes using a combination of particle image velocimetry and a nanoscale fast-response cold-wire is tested by examining a rough-wall turbulent boundary layer subject to weakly stable stratification. The method has the advantages of simple calibration and setup, as well as providing spatial correlations of velocity and temperature and their associated integral length scales. The accuracy of using Taylor's hypothesis when employing a large field of view is investigated. Heat flux, velocity-temperature correlation coefficients and turbulent Prandtl number profiles, as well as spatial velocity and temperature correlations, are presented.

A local sensor for joint temperature and velocity measurements in turbulent flows

“…One traditionnal method consists in setting up velocity measurement and temperature measurement, independantly of each other, e.g. fast cold wires combined with either Laser-Doppler-Anemometry [21], or Particle-Image-Velocimetry [22]. However, achieving accurate synchronisation and ensuring that the measurement points precisely match is not straightforward.…”

Batchelor Prize Lecture: Measurements in wall-bounded turbulence

“…In a further demonstration of the capabilities of T-NSTAP, Williams et al. (2015) made measurements of temperature in a rough-wall turbulent boundary layer subject to weakly stable stratification at the same time as measuring two components of velocity using PIV to determine heat fluxes. The T-NSTAP flat frequency response exceeded 300 Hz, so that no compensation of any kind was necessary, permitting accurate measurements of the temperature fluctuations, heat fluxes and temperature dissipation spectra.…”

“…A compensating network was therefore introduced by Dryden & Kuethe (1929) where the network generates a zero in the frequency response which is then tuned to match the pole response of the wire. Subsequent refinements of this concept have extended the frequency response by more than two orders of magnitude, and such constant current systems are still used in some supersonic flow applications and for the measurement of temperature fluctuations (see, for example, Smits, Perry & Hoffmann 1978;Bestion, Gaviglio & Bonnet 1983;Barre et al 1993;Williams, Van Buren & Smits 2015). In current practice, digital compensation has become a natural alternative to analogue networks (Briassulis et al 1995).…”