An introduction to heat transfer

One of the most challenging and time-consuming problems in experimental fluid mechanics is the measurement of the overall flow field properties, such as the velocity, vorticity, and pressure fields. Local measurements of the velocity field (i.e., at individual points) are now done routinely in many experiments using hot-wire (HW) or laser velocimetry (LV). However, many of the flow fields of current interest, such as coherent structures in shear flows or wake flows, are highly unsteady. HW or LV data of such flows are difficult to interpret, as both spatial and temporal information of the entire flow field are required and these methods are commonly limited to simultaneous measurements at only a few spatial locations.

Particle Image Velocimetry

Vortex formation in the wake of an oscillating cylinder

Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Karman street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion.

Oscillating foils of high propulsive efficiency

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

The superposition of Laguerre-Gaussian guided into and scattered by a particle in a rotated fluid allows the detection of the rotational of the fluid. The presented system allows for virtually real-time determination of vorticity characterization in a fluid. The system allows the direct measurements of fluid flow vorticity using a spatially shaped beam with a superposition of Laguerre-Gaussian modes that reports on the rotational Doppler shift from microparticles intersecting the beam focus.

Laser system for measuring fluid dynamics

We experimentally investigate the lift characteristics of a NACA-0012 airfoil which translates steadily across a non-uniform approach flow. Measurements are conducted in a wind tunnel at a chord Reynolds number of for a range of angles of attack (AoA) for both stationary and translating airfoils at various airfoil cross-stream velocities. The motion of the airfoil across the shear zone produces flow unsteadiness that is characterized by the product of the non-dimensional shear rate and the ratio of the airfoil to freestream velocity . The lift coefficient of the translating airfoil is compared against that of the stationary airfoil while accounting for the former’s motion using the uniform-freestream concepts of effective freestream velocity and effective AoA. The results show that the lift coefficient of the moving airfoil is higher than that of the stationary airfoil at positive AoA. The largest difference is found near stall, where the airfoil motion leads to early stall and a higher maximum lift coefficient. On the other hand, less discrepancy between the static and the moving airfoils is found at negative AoA. Overall, these results demonstrate that the flow around the moving airfoil at AoA near stall is not quasi-steady for the experimental conditions considered in the present work. This finding also holds for a companion study at a smaller Reynolds number of .

https://arc.aiaa.org/doi/pdf/10.2514/6.2019-0639.c1

Correction: Reynolds Number Effect on Lift Characteristics of an Airfoil Translating Across a Non-uniform Approach Flow

Temperature Measurement of Aqueous Ammonium Chloride Solution During Solidification Process Using Laser-Induced Fluorescence (LIF)

The flow structure and the molecular mixing field are investigated in a liquid-phase low Reynolds number forced wake in a confining channel using chemically reacting LIT. The absolute level of molecular mixing enhancement is quantified. Results show that the amount of chemical -product can be dramatically increased when high amplitude forcing is used. Mixing at mid-span is enhanced by about a factor of 40 over the unforced case. More importantly, the maximum level of mixing is nearly three times that in high Reynolds number liquid-phase mixing layers, and 60% higher than that in gas-phase turbulent shear layers.

Molecular mixing enhancement in the confined wake of a splitter plate

This work investigates the aerodynamics of a NACA-0012 airfoil moving at steady velocity and fixed geometric angle of attack across a uniform-shear flow at low chord Reynolds numbers. The airfoil translates across a uniform-positive-shear approach flow, with non-dimensional shear rate values ranging 0.40-0.66 and chord Reynolds numbers ranging 1.00×10-1.61×10, where the ratio of the airfoil velocity relative to the approach stream velocity ranges 0.040-0.066. Airfoils performing similar dynamic motion in uniform flow may be analyzed in a Galilean reference frame; however, no equivalent analysis exists for uniform-shear flows. The current results for an airfoil moving at steady velocity across a shear flow in a Galilean frame of reference show the lift coefficient, obtained by direct measurement, may be approximated by the lift coefficient on a stationary airfoil under the same flow conditions at negative and low-positive angles of attack. However, an asymmetry is observed where a significant difference occurs between the moving and the stationary airfoils at positive angles of attack approaching stall. This difference is associated with a change in the airfoil stall behavior due to movement across the shear zone.

https://arc.aiaa.org/doi/pdf/10.2514/6.2019-2157.c1

Manoochehr Koochesfahani

Papers

Laser system for measuring fluid dynamics

Correction: Reynolds Number Effect on Lift Characteristics of an Airfoil Translating Across a Non-uniform Approach Flow

Temperature Measurement of Aqueous Ammonium Chloride Solution During Solidification Process Using Laser-Induced Fluorescence (LIF)

Molecular mixing enhancement in the confined wake of a splitter plate

Correction: A Study of the Aerodynamics of a Low Reynolds Number Airfoil Translating Across a Uniform-Shear Approach Flow