Steven T. Wereley

Spatio-temporal character of non-wavy and wavy Taylor-Couette flow

The stability of supercritical Couette flow has been studied extensively, but few measurements of the velocity field of flow have been made. Particle image velocimetry (PIV) was used to measure the axial and radial velocities in a meridional plane for non-wavy and wavy Taylor–Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder with fixed end conditions. The experimental results for the Taylor vortex flow indicate that as the inner cylinder Reynolds number increases, the vortices become stronger and the outflow between pairs of vortices becomes increasingly jet-like. Wavy vortex flow is characterized by azimuthally wavy deformation of the vortices both axially and radially. The axial motion of the vortex centres decreases monotonically with increasing Reynolds number, but the radial motion of the vortex centres has a maximum at a moderate Reynolds number above that required for transition. Significant transfer of fluid between neighbouring vortices occurs in a cyclic fashion at certain points along an azimuthal wave, so that while one vortex grows in size, the two adjacent vortices become smaller, and vice versa. At other points in the azimuthal wave, there is an azimuthally local net axial flow in which fluid winds around the vortices with a sense corresponding to the axial deformation of the wavy vortex tube. These measurements also confirm that the shift-and-reflect symmetry used in computational studies of wavy vortex flow is a valid approach.

The theory of diffraction-limited resolution in microparticle image velocimetry

A theory of diffraction-limited spot size is developed for infinity-corrected microscope optics. Previously reported formulae were originally derived using a single-lens system. In addition, the previously reported relationship between f-number and numerical aperture assumed a paraxial approximation and was limited to air-immersion lenses. Here, a new relationship between f-number and numerical aperture is developed, and is valid for all numerical apertures and all immersion media. In addition, a new theory is developed that estimates the effective numerical aperture of an oil-immersion lens when imaging into fluid of a lower refractive index, such as water. The results indicated that when imaging into water, high numerical aperture NA = 1.0 or 1.2 water-immersion lenses provide comparable and sometimes better diffraction-limited resolution than NA = 1.4 oil-immersion lenses. In addition, when imaging into water, water-immersion lenses may provide superior image quality, because they are corrected for aberrations resulting from the water/glass interface.

Velocity field for Taylor – Couette flow with an axial flow

The flow in the gap between an inner rotating cylinder concentric with an outer stationary cylinder with an imposed pressure-driven axial flow was studied experimentally using particle image velocimetry (PIV) in a meridional plane of the annulus. The radius ratio was η=0.83 and the aspect ratio was Γ=47. Velocity vector fields for nonwavy toroidal and helical vortices show the axial flow winding around vortices. When the axially averaged axial velocity profile is removed from the velocity field in a meridional plane, the velocity field looks much like it would with no imposed axial flow except that the vortices translate axially and the distortion of the azimuthal velocity contours in meridional plane related to the vortices is shifted axially by the axial flow. The velocity vector fields for wavy vortices also show axial flow winding around the vortices. Again, removing the axial velocity profile results in a flow that appears similar to that with no axial flow. The path of the vortices is generally axia...

Automated and temperature-controlled micro-PIV measurements enabling long-term-stable microchannel acoustophoresis characterization

We present a platform for micro particle image velocimetry (μPIV), capable of carrying out full-channel, temperature-controlled, long-term-stable, and automated μPIV-measurement of microchannel acoustophoresis with uncertainties below 5% and a spatial resolution in the order of 20 μm. A method to determine optimal μPIV-settings for obtaining high-quality results of the spatially inhomogeneous acoustophoretic velocity fields of large dynamical range is presented. In particular we study the dependence of the results on the μPIV interrogation window size and the number of repeated experiments. The μPIV-method was further verified by comparing it with our previously published particle tracking method. Using the μPIV platform we present a series of high-resolution measurements of the acoustophoretic velocity field as a function of the driving frequency, the driving voltage, and the resonator temperature. Finally, we establish a direct and consistent connection between the obtained acoustophoretic velocity fields, and continuous flow mode acoustophoresis, commonly used in applications.

Electrokinetic patterning of colloidal particles with optical landscapes

We demonstrate an opto-electrokinetic technique for non-invasive particle manipulation on the surface of a parallel-plate indium tin oxide (ITO) electrode that is biased with an alternating current (AC) signal and illuminated with near-infrared (1064 nm) optical landscapes. This technique can generate strong microfluidic vortices at higher AC frequencies (>100 kHz) and dynamically and rapidly aggregate and pattern particle groups at low frequencies (<100 kHz).

Design, fabrication and characterization of a conducting PDMS for microheaters and temperature sensors

In this paper, we present the design and fabrication procedure of a conducting polydimethylsiloxane (PDMS) and then evaluate its potential uses for heating and temperature sensing. The conducting PDMS was made up of a mixture of a PDMS prepolymer and metallic powder. Depending on purpose, i.e. heater or sensor, different weight ratios of the powder and geometric shapes were considered. Characterization of both the microheaters and the temperature sensors includes stability, repeatability, durability and time response. The results suggest that the microheater is feasible for constantly heating at a fixed temperature instead of running thermal cycles. The optimal heating range was estimated below 100 °C under the current setup and a power consumption of 210 ± 12 mW was needed for 92 °C. Hysteresis and time lag were observed in the temperature sensor. Accordingly, the sensor is recommended to be used for long-term monitoring instead of rapid temperature detections.

Phase-resolved flow field produced by a vibrating cantilever plate between two endplates

The flow field created by a vibrating cantilever plate was studied using phase-resolved particle image velocimetry measurements as well as a smoke visualization technique. The cantilever is 38 mm wide, 31 mm long, and is actuated by a piezoelectric material. It is immersed in initially quiescent air, i.e., no free stream velocity is imposed on the system. The cantilever’s vibration frequency in these experiments is set to 180 Hz—the fundamental natural frequency of cantilever. The flow is quite complicated in nature. During each vibration cycle a pair of counter-rotating vortices is generated. A high velocity region is formed between these two counter-rotating vortices in which the maximum velocity is nearly four times the maximum speed of the free end of the plate. Front and rear walls are installed at the lateral edges of the cantilever initially with the thought of making the flow quasi-two-dimensional. While a two-dimensional flow field is indeed formed near the cantilever tip, the flow downstream of ...

Inertial particle motion in a Taylor Couette rotating filter

In rotating filtration, which is based on supercritical cylindrical Couette flow with a rotating porous inner cylinder, the motion of particles in the suspension depends on both centrifugal sedimentation and transport due to the vortical motion of Taylor vortices. We have simulated the motion of dilute, rigid, spherical particles in Taylor Couette flow using computational particle tracking in an analytic velocity field for flow just above the transition to supercritical Taylor vortex flow. Neutrally buoyant particles follow fluid streamlines closely, but not exactly due to the curvature of the velocity field very near the particle. The motion of particles with a density greater than the fluid is primarily determined by the competition between the centrifugal sedimentation related to the primary cylindrical Couette flow and the secondary radial and axial transport of the Taylor vortex flow. As a result, particles that start near the outer edge of a vortex spiral inward toward a limit cycle orbit. Likewise,...

Experimental and Computational Investigation of Flow Development and Pressure Drop in a Rectangular Micro-channel

Flow development and pressure drop were investigated both experimentally and computationally for adiabatic single-phase water flow in a single 222 μm wide, 694 μm deep, and 12 cm long rectangular micro-channel at Reynolds numbers ranging from 196 to 2215. The velocity field was measured using a micro-particle image velocimetry system. A three-dimensional computational model was constructed which provided a detailed description of liquid velocity in both the developing and fully developed regions. At high Reynolds numbers, sharp entrance effects produced pronounced vortices in the inlet region that had a profound influence on flow development downstream. The computational model showed very good predictions of the measured velocity field and pressure drop. These findings prove the conventional Navier-Stokes equation accurately predicts liquid flow in micro-channels, and is therefore a powerful tool for the design and analysis of micro-channel heat sinks intended for electronic cooling.

Micro-resolution particle image velocimetry

During the past five years, significant progress has been made in the development and application of micron-resolution Particle Image Velocimetry (µPIV). Developments of the technique have extended typical spatial resolutions of PIV from order 1-mm to order 1-µm. These advances have been obtained as a result of novel improvements in instrument hardware and post processing software.