J. Tihon

Calibration of electrodiffusion friction probes using a voltage-step transient

Voltage-step transient experiments are used to calibrate electrodiffusion (ED) friction probes. The approach is demonstrated on the probes in high-speed rotating impellers. This calibration has shown that shear rates as high as 106s-1 can be measured reliably using 0.5mm Pt working electrodes in ferro/ferricyanide aqueous solutions. The complete transient calibration experiment provides sufficient information about the dynamics of ED friction probes.

Effect of an external excitation on the flow structure in a circular impinging jet

The effect of an external excitation on circular impinging jet flow is studied experimentally. Velocity fields in moderate air flows (Re=10000) are investigated by hot-wire anemometry. The basic flow is excited by a small sinusoidal modulation of the nozzle exit velocity. The phase-averaging technique is used to study the behavior of vortex structures in the jet, specifically rolling-up, pairing, and interaction with the wall. The jet is found to be sensitive to excitation in the frequency range characterized by a Strouhal number, Ste=feD∕U, from 0.3 to 3. Different flow regimes are identified in the excited impinging jet: a periodic flow regime with the same frequency as the excitation, a regime with a frequency corresponding to a subharmonic of the excitation frequency, a regime alternating between these two frequencies, and two border regimes with more complicated behavior. The low-frequency excitation leads to the formation of vortices, which are larger than those occurring in an unexcited jet. Conseq...

Stability and rheology of dilute TiO2-water nanofluids.

The apparent wall slip (AWS) effect, accompanying the flow of colloidal dispersions in confined geometries, can be an important factor for the applications of nanofluids in heat transfer and microfluidics. In this study, a series of dilute TiO2 aqueous dispersions were prepared and tested for the possible presence of the AWS effect by means of a novel viscometric technique. The nanofluids, prepared from TiO2 rutile or anatase nanopowders by ultrasonic dispersing in water, were stabilized by adjusting the pH to the maximum zeta potential. The resulting stable nanofluid samples were dilute, below 0.7 vol.%. All the samples manifest Newtonian behavior with the fluidities almost unaffected by the presence of the dispersed phase. No case of important slip contribution was detected: the Navier slip coefficient of approximately 2 mm Pa-1 s-1 would affect the apparent fluidity data in a 100-μm gap by less than 1%.

Electrodiffusion detection of the near-wall flow reversal in liquid films at the regime of solitary waves

The liquid film flow down an oscillating plate was used as a suitable flow configuration to study the dynamic behaviour of electrodiffusion friction probes at large fluctuations. The two-segment probe was flush mounted into the plate wall to measure the fluctuating wall shear rate. The hydrodynamics of the experiment made it possible to adjust both the steady and oscillatory component of the wall shear rate through the operation parameters (flow rate, plate inclination, amplitude and frequency of wall oscillations). The approximate model of the probe dynamic response based on the similarity of concentration profiles at the probe surface was verified. This simple model proved to be able to calculate the instantaneous wall shear rate from the measured current signal even at large flow fluctuations. The analysis of the probe dynamic behaviour under reversing flow conditions provided a new method of the detection of short-time flow reversal. Finally, this method was successfully applied to confirm the existence of a small backflow region located in front of the large solitary waves, which were excited on the surface of a liquid film flowing down an inclined stationary plate.

Linear oscillations of a supported bubble or drop

Shape oscillations of a spherical bubble or drop, for which part of its interface is fixed due to contact with a solid support, are studied analytically using variational methods. Linear oscillations and irrotational flow are assumed. The present analysis is parallel to those of Strani and Sabetta [“Free vibrations of a drop in partial contact with a solid support,” J. Fluid Mech. 141, 233–247 (1984)]10.1017/S0022112084000811; and Bostwick and Steen [“Capillary oscillations of a constrained liquid drop,” Phys. Fluids 21, 032108 (2009)]10.1063/1.3103344 but is also able to determine the response of bubbles or drops to movements imposed on their supports or to variations of their volumes. The analysis leads to equations of motion with a simple structure, from which the eigenmodes and frequency response to periodic forcing are easily determined.

Analysis of wave modes in liquid film falling down a vertical oscillating plate

Abstract The stability of periodic two-dimensional waves on a liquid film falling down a vertical oscillating plate was studied experimentally using the aqueous solutions of carboxy-methylcellulose. A set of capacitance probes was used to measure the instantaneous film thickness. Artificial flow fluctuations induced by the plate oscillations caused distinct intensification of the wavy flow. We use the methods of phase space reconstruction and deterministic chaos analysis to show that the dynamics can be understood within the framework of deterministic chaos. We find that regions of relatively ordered and more complex chaotic dynamics alternate as the frequency of oscillations is varied. There are particular oscillating frequencies at which the system is close to a three-frequency quasiperiodic motion with the spectrum of Lyapunov exponents of type (0,0,0, −, …), elsewhere the system is either in a simple chaotic mode with the spectrum of type (+, 0, 0, −, …) or in a hyperchaotic mode with spectrum of type (+, +, 0, −, …).

AWS Viscometry—Principles and Applications

Quantitative identification of Apparent wall slip (AWS) effect, accompanying the flow of colloidal dispersions in confined geometries, requires a specific modification of common viscometric methods. Essential distinction between common and AWS viscometry is outlined—including theoretical background, calibration, data acquisition, and downstream data treatment—and demonstrated for a class of rotational viscometers.