Density and velocity measurements in turbulent He-air boundary layers

Abstract

A turbulent boundary layer with large density variations has been generated by tangential injection of air or helium Into a boundary layer of air-helium mixture. Instrumentation based on thermoanemometry has been successfully developed for the investigation of this flow Analysis or the mean and fluctuating density fields shows that the flow is mainly governed by the ratio of the injection to the external velocity and that the density ratio plays a secondary role in the mixing processes. However, a sight enhancement of turbulent activity is observed when helium is injected.Keywords: Turbulent boundary layer, mixture of gas, flow with variable density, thermoanemometry. RésuméUne couche limite turbulente avec différences de densité importantes a été produite par injection tangentielle d'air ou d'hélium dans un mélange de ces deux constituants. L'instrumentation, basée sur la thermo-anémométrie, a été développée avec succès pour l'étude de ce type d'écoulement. L'analyse des résultats obtenus, à travers les champs moyen et fluctuant de densité, montre que l'écoulement est principalement régi par le rapport de l'injection à la vitesse externe, et que le rapport de densité joue un rôle secondaire dans les processus de mélange. Cependant, on observe un perfectionnement du point de vue de l'activité turbulente quand l'hélium est injecté. Mots clés:Couche limite turbulente, mélange de gaz, écoulement à densité variable, thermo-anémométrie.

Design of thermal-anemometry-based probes for the simultaneous measurement of velocity and gas concentration in turbulent flows

“…(ii) Probes with separation distances small enough that thermal interference effects are likely (s/d hot ≤ 10), which herein are referred to as interference probes, appear to be necessary in He/air mixtures. For most of these probes, s/d hot is approximately 1 or 2, but Harion et al [5] (and related works [6,[15][16][17][18]), designed a probe in which s/d hot = 10, and in which interference effects were considered to be of secondary importance due to the large diameter ratio of the sensing elements. Interference probes do not appear to be necessary in other gas mixtures, as evidenced by the work of McQuaid and Wright [9] and Sakai et al [19].…”

“…Then, in section 4, we present the details and results of an experimental investigation into the design of thermal-anemometry-based probes capable of making measurements in He/air mixtures. The present work specifically focuses on such probes since (i) helium is generally an attractive gas for studying flows of heterogeneous mixtures (as it is inert, non-toxic, and relatively inexpensive), and (ii) its relatively low density and Schmidt number makes it useful for studying variable-density flows (as is the case for many of the works cited in table 1 [8,[10][11][12][14][15][16][17][18]) or differential diffusion. Finally, the conclusions and future work are discussed in section 5.…”

The Influence of Variable Density on Turbulent Wall Jet: A Numerical Comparative Study

“…Adopting improved hot-wire and, laser Doppler anemometry, many studies in the decade of 1990s -2000s [7,[17][18][19][20][21], ended with the following conclusions: A transition region of about 20-30e long succeeded by a developed flow. Two main factors influence the flow; the velocity ratio r and less severely the density ratio S. Low values of S increase all of three elements: first, turbulence, especially in the transition region, which is consequently shorter in this case; second, velocity near the wall.…”

“…The use of Helium-Air specifically is going to reinforce the generation of large density differences even in low-speed flows. In the wall jet we observe two regions, one which is so close to the wall: an internal region which is similar to the boundary layer, and another which is far from the wall: an external region which shares similarities with the free shear flow, and which may be either motionless [1], or moving with velocity lower [2][3][4][5][6], or higher than that of the internal region [7].…”