Experimental investigation into localized instabilities of mixed Rayleigh-Bénard-Poiseuille convection

The stability of the Rayleigh-Bénard-Poiseuille flow in a channel with large transverse aspect ratio (ratio of width to vertical channel height) is studied experimentally. The onset of thermal convection in the form of ‘transverse rolls' (rolls with axes perpendicular to the Poiseuille flow direction) is determined in the Reynolds-Rayleigh number plane for two different working fluids: water and mineral oil with Prandtl numbers of approximately 6.5 and 450, respectively. By analysing experimental realizations of the system impulse response it is demonstrated that the observed onset of transverse rolls corresponds to their transition from convective to absolute instability. Finally, the system response to localized patches of supercriticality (in practice local ‘hot spots') is observed and compared with analytical and numerical results of Martinand, Carrière & Monkewitz (J. Fluid Mech., vol. 502, 2004, p. 175 and vol. 551, 2006, p. 275). The experimentally observed two-dimensional saturated global modes associated with these patches appear to be of the ‘steep' variety, analogous to the one-dimensional steep nonlinear modes of Pier, Huerre & Chomaz (Physica D, vol. 148, 2001, p. 49

Experimental investigation into localized instabilities of mixed Rayleigh-Bénard-Poiseuille convection

An experimental study of the stability of the Rayleigh-Bénard-Poiseuille flow was performed in a large transverse aspect ratio channel. The onset for the transverse thermo-convective rolls was determined as a function of the Reynolds number for two different fluids (water: Pr = 6.5 and mineral oil: Pr = 450). Then, the system impulse response was studied and a good agreement with theory was found for the convective/absolute instability transition. Finally the response of the system to localized heating was observed and compared with analytical and numerical results by Martinand, Carrière and Monkewitz (2004 & 2006): experimental thermo-convective global modes are found to correspond to the saturated "steep" variety constructed by Pier, Huerre and Chomaz (2001)

Experimental investigation into localized instabilities of mixed Rayleigh-Benard-Poiseuille convection

The stability of the Rayleigh-Benard-Poiseuille flow in a channel with large transverse aspect ratio (ratio of width to vertical channel height) is studied experimentally. The onset of thermal convection in the form of 'transverse rolls' (rolls with axes perpendicular to the Poiscuille flow direction) is determined in the Reynolds-Rayleigh number plane for two different working fluids: water and mineral oil with Prandtl numbers of approximately 6.5 and 450, respectively. By analysing experimental realizations of the system Impulse response it is demonstrated that the observed onset of transverse rolls corresponds to their transition from convective to absolute instability. Finally, the system response to localized patches of supercriticality (in practice local 'hot spots') is observed and compared with analytical and numerical results of Martinand, Carriere & Monkewitz (J. Fluid Mech., vol. 502, 2004, p. 175 and vol. 551, 2006, p. 275). The experimentally observed two-dimensional saturated global modes associated with these patches appear to be of the 'steep' variety, analogous to the one-dimensional steep nonlinear modes of Pier, Huerre & Chomaz (Physica D, vol. 148, 2001, p. 49)

Contactless rotational concentric microviscometer

We investigated the feasibility of a concentric rotational microviscometer which is actuated remotely. This type of sensor might find application in vivo as a passive implant. The heart of the sensor is a cylindrical rotor, actuated from a distance of up to 10 mm, that rotates in the viscous liquid. Its angular position is remotely measured using two Hall effect sensors. The advantage of our system is the simplicity of the principle and the miniaturization potential as well as the small sample volume required (5 mu L). In the low viscosity range, the limit of Taylor-Couette instable flow is reached. In this work we present both a theoretical and a practical approach to the limit of the Taylor-Couette flow that is observed at low viscosities (3-4 mPa s). Despite this effect, we show that viscosity measurements with this non-linear behavior are still possible. The observed sensor stability over 14 days is better than +/-0.5% for liquids ranging from 1.35 to 17.25 mPa s. (C) 2011 Elsevier B.V. All rights reserved