High-speed quantum dot tracking and velocimetry using evanescent wave illumination

Total internal reflection velocimetry (TIRV) is applied to measure the dynamics of 17 nm diameter, colloidal quantum dot (QD) tracer particles within 200 nm of a microchannel wall at shear rates in excess of 20,000 s⁻¹. QDs are quickly developing into viable tracer particles for measuring microscale fluid dynamics. However, the low emission intensities of QDs usually require long exposure and inter-frame times, which limit velocity resolution and compromise accuracy (due to their fast diffusion as a consequence of a small diameter). In this study, a two-stage, high-speed image intensifier and camera were integrated into an evanescent wave microscopy imaging system. This provided the necessary temporal resolution to image the fast diffusion dynamics of QDs in real-time (up to 10,000 fps), which allowed individual particles to be tracked continuously for extended periods of time. In addition to examining the trajectories of individual particles, ensemble-averaged tracking measurements reveal near-wall velocity distributions in high-speed microchannel flows (Re ∼ 10), where velocities on the order of 5 mm/s are measured within 200 nm of the microchannel wall. This data provides a robust confirmation of recent results demonstrating diffusion-induced bias error for near-wall velocimetry.     

Statistical particle tracking velocimetry using molecular and quantum dot tracer particles

We present a statistical approach to particle tracking velocimetry developed to treat the issues associated with nanometer-sized tracer particles such as fluorescent molecules and quantum dots (QDs) along with theory and experimental results. Extremely small tracers pose problems to traditional tracking methods due to high levels of thermal motion, high levels of intensified camera noise, high drop-in/drop-out rates and, in the case of QDs, fluorescence intermittency (“blinking”). The algorithm presented here compensates for these problems in a statistical manner and determines the physical velocity distributions from measured particle displacement distributions by statistically removing randomly distributed, non-physical tracking events. The algorithm is verified with both numerically simulated particle trackings and experiments using 54 nm diameter fluorescent dextran molecules and 6 and 16 nm diameter QDs.     

Simultaneous,ensemble-averaged measurement of near-wall temperature and velocity in steady micro-flows using single quantum dot tracking

We present results from a series of experiments demonstrating the use of single quantum dots (QDs) as simultaneous temperature and velocity probes at the micro-scale. The fluorescence intensity of QDs varies predictably with temperature due to changes in quantum efficiency. We use total internal reflection fluorescence microscopy to study the region within 200 nm of a fluid-solid interface. A two-color, time-averaged temperature sensing technique based on the ensemble intensity changes of single QDs as compared to a reference dye (rhodamine 110) is presented. Many single QD intensity measurements are used to build intensity distributions which can be mapped to fluid temperature. Simultaneously, we track the motion of individual QDs, building a distribution of particle displacements, where the mean displacement yields the local fluid velocity. We also show that the width of the displacement distribution (or the diffusion coefficient) captures the scaling of the temperature to viscosity ratio, which may allow for independent viscosity measurement.     

Particle velocity field measurements in a near-wall flow using evanescent wave illumination

Evanescent waves from the total internal reflection of a 488 nm argon-ion laser beam at a glass-water interface were used to measure velocity fields in creeping rotating Couette flow within 380 nm of the stationary solid surface. Images of fluorescent 300 and 500 nm diameter polystyrene and silica particles suspended in water recorded at 30 Hz were processed using cross-correlation particle image velocimetry to determine the two in-plane velocity components with an in-plane spatial resolution of 40Ꮀ µm over a 200 µm (h)쏦 µm (v) field of view. The results are in reasonable agreement with the exact solution for the corresponding single-phase Stokesian flow. These data are, to our knowledge, the first velocity field measurements with this small out-of-plane spatial resolution (in all cases less than 380 nm), and the first such measurements in this interfacial or near-wall region. This paper describes the novel experimental diagnostic technique used to obtain these results.     

Correction of hot-wire measurements in the near-wall region

 A detailed numerical investigation on the corrections needed by hot-wire velocity measurements in wall vicinity was performed. In the case of a perfectly conducting wall the computed results agreed well with available experimental data. At first, the numerical results for the case of an adiabatic wall contradicted previous experimental observations. However, a precise evaluation led to a better understanding of the entire problem. Received: 7 August 1998/Accepted: 7 December 1998     

Assessment of turbulent spectral bias in laser Doppler velocimetry

A critical evaluation is made of the spectral bias which occurs in the use of a laser Doppler velocimeter (LDV). In order to accommodate the randomly sampled LDV data, statistical treatments of particle arrival times are needed. This is modeled as a doubly stochastic Poisson process which includes the intensity function of the velocity field. Three processing algorithms are considered for spectral estimates: the sample and hold method (SH), the modified Shannon sampling technique (SR), and the direct transform (RG). Assessment is made of these for varying data densities (0.05 ≤ d.d ≤ 5) and turbulence levels (t.i.=30%, 100%). The effects of the values of the Reynolds stress coefficients and the transversal standard deviation on the spectral contents were examined. As an improved version of the spectral estimator, the utility of POCS (the projection onto convex sets) has been tested in the present study. This algorithm is found useful to be in the region when d.d. ? 3.     

Quantitative visualization of the near-wall structures in a turbulent pipe flow by image correlation velocimetry

Laser-induced fluorescent dye visualization and image correlation velocimetry were employed to delineate near-wall turbulent structures in a pipe flow. The sweeping and ejection events near the wall and the downstream evolution of a large-scale eddy structure rotating in a counter-clockwise direction were clearly reflected in the instantaneous fluctuating velocity fields. This eddy structure was found to form mostly in the logarithmic region and to dominate the flow structures there, while the ejection and sweeping events in the log layer were greatly influenced by the existence of the large-scale eddy structure. Received: 29 January 2001 / Accepted: 22 October 2001     

Methods of near-wall pressure-fluctuation measurements in the presence of vibration

Near-wall pressure fluctuations in turbulent flows are of considerable interest in many engineering applications. We shall concentrate on a number of specific questions related to the resolution of components of wall pressure spectra. Our emphasis shall be on outstanding problems of turbulent pressure fluctuations in the presence of vibration. A study on the interaction of a transducer with wall vibration resulting from near-wall turbulent flows has been performed. Three methods are described for the study of spectral components of turbulent surface pressure in conditions of flow-induced vibration: the method of separation of turbulent and vibration signals; the method of a vibration-proof turbulent pressure transducer; and a modified method of vibration suppression. A method of low-frequency acoustic-noise suppression is also suggested.     

Volumetric intake flow measurements of an IC engine using magnetic resonance velocimetry

Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of $9.21$  mm to simulate the induction flow at crank-angle $270^{\circ }$ bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., $y \ge -20$  mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications.     

Gaseous flow measurements in an internal combustion engine assembly using molecular tagging velocimetry

A molecular tagging velocimetry (MTV) system was applied for mapping in-cylinder flows in an internal combustion engine. The images were captured inside an optical engine assembly that reproduces operation of a 2.2 L four stroke gasoline engine. A recently developed algorithm to process MTV images is based on a fast-normalized spatial correlation approach implemented using MATLAB software. The code allows accurate detection of the MTV grid nodes displacements. It processes simultaneously velocity vector and circulation fields for individual cycles, and ensemble averages of those over a few hundred sequential cycles to obtain mean and standard deviation values. Then probability density functions are reconstructed to quantify cycle-to-cycle variability of the in-cylinder flow.     

Particle image velocimetry measurements in complex geometries

 One of the advantages of Particle Image Velocimetry (PIV) is its ability to determine the instantaneous flow field over two- or three-dimensional domains. Yet PIV has had limited application to complex flow passages because of the difficulty in replicating these geometries with optically transparent materials. In this work, we describe a method for overcoming this difficulty using rapid prototyping techniques. As an illustrative example, the technique has been used to characterize flow in a model of the human nasal cavity. Received: 12 January 1999/Accepted: 30 September 1999     

On the numerical near-wall corrections of single hot-wire measurements

The present paper numerically investigates the near-wall correction of velocity readings when using hot wires to measure the flows very close to walls. It is found that the near-wall correction is necessary not only for the conducting wall but also for the adiabatic wall. For an infinitely long 5-μm diameter hot wire, measurement error begins to appear at Y⁺ < 5 for an infinitely conducting wall and at Y⁺ < 2 for an adiabatic wall. In addition to the distance from wall, the wire diameter also exerts significant influence on the velocity measurements. However, provided the flow is two-dimensional (2-D), the effect of operating overheat ratio seems to be insignificant.     

Pulsed-wire measurements in the near-wall layer in a reattaching separated flow

Pulsed-wire velocity measurements have been made in the near-wall layer, including the viscous sublayer, beneath a separated flow. A method for correcting the error caused by fluctuations in velocity gradient is given, extending the work of Schober et al. (1998). The measurements show that the r.m.s. of the streamwise velocity fluctuations scale closely in accordance with an inner-layer scaling, where the velocity scale, , is based on the r.m.s. of the wall shear stress fluctuations (measured by means of a pulsed-wire shear stress probe), rather than the mean wall shear stress. The effects of velocity gradient are only significant beneath of 10 or less.List of symbols C Calibration constant - f Function representing mean velocity - h_f Height of fence above splitter plate surface - L Length scale of outer-layer structures - s Distance between pulsed and sensor wires - u r.m.s. of U - Velocity scale based on r.m.s of wall shear stress fluctuation - U Instantaneous velocity in x-direction - U_m Instantaneous measured velocity in x-direction - U_r Free-stream reference velocity - x Streamwise direction from separation point - y Distance from splitter plate surface, in normal direction - X_r Length of separation bubble - ₀ Thickness scale in oscillating layer - Blasius laminar boundary layer parameter - Density - Wall shear stress - r.m.s. of wall shear stress fluctuation - Frequency of oscillating layer - Kinematic viscosity - Overbar denotes time average     

Laser velocimetry measurements in a laboratory vaned diffuser

Laser velocimetry measurements were made within a laboratory radial vaned diffuser with three different blade configurations. Measurements were made through passages with four, six and eight blades installed at off design conditions. Also, in the eight blade diffuser measurements were made between the blade passage exit and diffuser exit so that the complete secondary flow could be defined. The flow was found to separate from the blades and form large separation zones. The separation zones consisted primarily of two vortices rotating in opposite directions. At the passage exit the separation region encompassed 23% of the circumferential area for the four blade diffuser, 45% for the six blade and 40% in the eight blade diffuser. Separation occurred at 23%, 27% and 50% from the leading edge of the blades for the 4, 6 and 8 bladed diffusers, indicating that more blades better controlled the separation. Turbulence intensities ranged from approximately 5% to 15% in the primary flow and reached a few hundred percent in the secondary flow within the separation regions.     

Experimental observation using particle image velocimetry of inertial waves in a rotating fluid

Inertial waves generated by a small oscillating disk in a rotating water filled cylinder are observed by means of a corotating particle image velocimetry system. The wave takes place in a stationary conical wavepacket, whose angle aperture depends on the oscillation frequency. Direct visualisation of the velocity and vorticity fields in a plane normal to the rotation axis are presented. The characteristic wavelength is found to be approximately equal to the disk diameter. The classical dispersion relation for plane waves is verified from the radial location of the wavepacket, and from the ellipticity of the projected velocity diagram. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

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Three-component multi-phase velocimetry measurements on a GDI spray using optically efficient fluorescent tracers

Two- and three-component multi-phase air/fuel measurements have been performed on a GDI injector. UV-excitable fluorescent tracers have been used to seed the gas phase, and the naturally occurring droplets in the fuel are the other phase. A high-pressure multi-hole GDI injector was mounted in a rig with a glass barrel to simulate the engine cylinder and provide optical access. Images were obtained under controlled conditions of fuel pressure and injection duration. Flow phase and pulse order have been determined from a single 3CCD colour camera. Suitable corrective processes have been adapted and implemented to account for crosstalk and chromatic aberrations so that the uncertainty of the velocity vectors produced is comparable to that of conventional PIV using 532 nm illumination. Multi-phase air/fuel vector maps have been produced. A second colour camera has been added to obtain stereo velocity measurements providing previously unavailable simultaneous information on the multi-phase (fuel/air) interaction with three velocity components.     

Simultaneous measurements of velocity and stress distributions in polyisobutylenes using laser-Doppler velocimetry and flow induced birefringence

An experimental device was set up for the synchronous measurement of velocities and stresses in polyisobutylenes using laser-Doppler velocimetry (LDV) and the two-colour flow-induced birefringence method (FIB). The materials investigated are three low molecular polyisobutylenes. Velocity (LDV) and stress (FIB) measurements are performed in the flow entrance region and inside a slit die with a contraction ratio of 1:10. The behaviour of the polyisobutylenes is Newtonian under the flow conditions applied. Therefore, the stresses inside the fluids can be calculated and compared to the stresses experimentally determined. A good agreement in shear and elongational flows was found between the calculated (LDV) and directly measured stresses (FIB). This result demonstrates the applicability of the experimental setup as an optical rheometer that can preferentially be used to measure elongational properties of low viscous fluids.