Experiments on low-Reynolds-number turbulent flow through a square duct

Abstract

Previous experimental studies on turbulent square duct flow have focused mainly on high Reynolds numbers for which a turbulence-induced eight-vortex secondary flow pattern exists in the cross-sectional plane. More recently, direct numerical simulations (DNS) have revealed that the flow field at Reynolds numbers close to transition can be very different; the flow in this 'marginally turbulent' regime alternating between two states characterised by four vortices. In this study, we experimentally investigate the onset criteria for transition to turbulence in square ducts. In so doing, we highlight the potential importance of Coriolis effects on this process for low-Ekman-number flows. We also present experimental data on the mean flow properties and turbulence statistics in both marginally and fully turbulent flow at relatively low Reynolds numbers using laser Doppler velocimetry. Results for both flow categories show good agreement with DNS. The switching of the flow field between two flow states at marginally turbulent Reynolds numbers is confirmed by bimodal probability density functions of streamwise velocity at certain distances from the wall as well as joint probability density functions of streamwise and wall normal velocities which feature two peaks highlighting the two states.

Turbulent duct flow with polymers

“…This feature was explained with the relation between coherent structures and secondary flow: if the dimension of the cross-section, in wall units, is below the one needed to accommodate a complete minimal turbulent cycle on all four walls (Jimenez & Moin 1991), then just two facing walls can alternatively give rise to a complete turbulent regeneration mechanism, while the other two faces remain in a relative quiescent state. Such 4-vortex states at marginal Reynolds number have also been observed experimentally (Owolabi et al 2016). Pinelli et al (2010) studied turbulent duct flows at higher Reynolds numbers, where the length of the edge of the square cross-section, expressed in viscous units, is larger, therefore allowing for the simultaneous presence of multiple pairs of high-and low-velocity streaks.…”

Turbulent drag reduction by polymer additives in parallel-shear flows

“…The experimental rigs used in this study (figure 1) have similar arrangements to those employed in previous research in our laboratory (Dennis & Sogaro 2014;Owolabi, Poole & Dennis 2016;Whalley et al 2017), with slight modifications. For the pipe flow studies, a 23 m long pipe consisting of a series of borosilicate glass tubes with internal diameter (2R) of 100 mm was employed.…”

Simulación directa de turbulencia de flujo secundario en ductos con forzado no homogéneo

“…Estas investigaciones presentan cálculos por DNS de flujo turbulento en un ducto de sección cuadrada para valores de Re τ igual a 150 y 180, respectivamente. Además se contrastaron los resultados numéricos con observaciones experimentales obtenidas por Owolabi [17]. La Tabla 1 presenta valores característicos del problema para los casos estudiados y para las referencias utilizadas en la comparación.…”

“…En la Figura 3 se presenta la comparación entre los valores de flujo medio en la línea vertical z=h ¼ 1 de la sección obtenidos y los presentados en [9] y [17]. Los valores se encuentran normalizados con el valor de velocidad de corte local en la pared inferior (u τ;l ).…”

“…Entre los trabajos que reportan experimentos con este tipo de flujo se encuentran los de Brundrett [13], Launder [14] y Melling [15] para números de Reynolds altos (Re b > 35000; donde Re ¼ u b h=ν, donde u b es la velocidad media, h es el semiancho del ducto y ν es la viscosidad cinemática). Experimentos para números de Reynolds bajos fueron realizados por Kawahara [16] (Re b ¼ 3535) y Owolabi [17] (Re b ¼ 1203 y 2230).…”