Manoochehr Koochesfahani

Bio: Manoochehr Koochesfahani is an academic researcher from Michigan State University. The author has contributed to research in topics: Molecular tagging velocimetry & Reynolds number. The author has an hindex of 32, co-authored 116 publications receiving 3699 citations. Previous affiliations of Manoochehr Koochesfahani include California Institute of Technology.

Vortical patterns in the wake of an oscillating airfoil

Abstract: The vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes are studied in a low speed water channel. it is shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude and also the shape of the oscillation waveform. An important observation in this study has been the existence of an axial flow along the cores of the wake vortices. Estimates of the magnitude of the axial flow suggest a linear dependence on the oscillation frequency and amplitude.

Mixing and chemical reactions in a turbulent liquid mixing layer

Abstract: An experimental investigation of entrainment and mixing in reacting and non-reacting turbulent mixing layers at large Schmidt number is presented. In non-reacting cases, a passive scalar is used to measure the probability density function (p.d.f.) of the composition field. Chemically reacting experiments employ a diffusion-limited acid–base reaction to directly measure the extent of molecular mixing. The measurements make use of laser-induced fluorescence diagnostics and high-speed, real-time digital image-acquisition techniques. Our results show that the vortical structures in the mixing layer initially roll-up with a large excess of fluid from the high-speed stream entrapped in the cores. During the mixing transition, not only does the amount of mixed fluid increase, but its composition also changes. It is found that the range of compositions of the mixed fluid, above the mixing transition and also throughout the transition region, is essentially uniform across the entire transverse extent of the layer. Our measurements indicate that the probability of finding unmixed fluid in the centre of the layer, above the mixing transition, can be as high as 0.45. In addition, the mean concentration of mixed fluid across the layer is found to be approximately constant at a value corresponding to the entrainment ratio. Comparisons with gas-phase data show that the normalized amount of chemical product formed in the liquid layer, at high Reynolds number, is 50% less than the corresponding quantity measured in the gas-phase case. We therefore conclude that Schmidt number plays a role in turbulent mixing of high-Reynolds-number flows.

MTV measurements of the vortical field in the wake of an airfoil oscillating at high reduced frequency

Abstract: We present an experimental investigation of the flow structure and vorticity field in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. Molecular tagging velocimetry is used to quantify the characteristics of the vortex array (circulation, peak vorticity, core size, spatial arrangement) and its downstream evolution over the first chord length as a function of reduced frequency. The measured mean and fluctuating velocity fields are used to estimate the mean force on the airfoil and explore the connection between flow structure and thrust generation.Results show that strong concentrated vortices form very rapidly within the first wavelength of oscillation and exhibit interesting dynamics that depend on oscillation frequency. With increasing reduced frequency the transverse alignment of the vortex array changes from an orientation corresponding to velocity deficit (wake profile) to one with velocity excess (reverse Karman street with jet profile). It is found, however, that the switch in the vortex array orientation does not coincide with the condition for crossover from drag to thrust. The mean force is estimated from a more complete control volume analysis, which takes into account the streamwise velocity fluctuations and the pressure term. Results clearly show that neglecting these terms can lead to a large overestimation of the mean force in strongly fluctuating velocity fields that are characteristic of airfoils executing highly unsteady motions. Our measurements show a decrease in the peak vorticity, as the vortices convect downstream, by an amount that is more than can be attributed to viscous diffusion. It is found that the presence of small levels of axial velocity gradients within the vortex cores, levels that can be difficult to measure experimentally, can lead to a measurable decrease in the peak vorticity even at the centre of the flow facility in a flow that is expected to be primarily two-dimensional.

A spatial correlation technique for estimating velocity fields using molecular tagging velocimetry (MTV)

Abstract: A direct spatial image correlation technique is presented for estimating the Lagrangian displacement vector from image pairs based on molecular tagging diagnostics. The procedure provides significant improvement in measurement accuracy compared to existing approaches for molecular tagging velocimetry (MTV) analysis. Furthermore, this technique is of more general utility in that it is able to accommodate other laser tagging patterns besides the usual grid arrangement. Simulations are performed to determine the effects of many experimental and processing issues on the sub-pixel accuracy of displacement estimates. The results provide guidelines for optimizing the implementation of MTV. Experimental data in support of this processing technique are provided.

Molecular tagging velocimetry and other novel applications of a new phosphorescent supramolecule

Abstract: The development and applications of a new class of water-soluble compounds suitable for molecular tagging diagnostics are described. These molecular complexes are formed by mixing a lumophore, an appropriate alcohol, and cyclodextrin. Using 1-BrNp as the lumophore, cyclohexanol is determined to be the most effective overall among the alcohols for which data are currently available. Information is provided for the design of experiments based on these complexes along with a less complex method for generating the grid patterns typically used for velocimetry. Implementation of a two-detector system is described which, in combination with a spatial correlation technique for determining velocities, relaxes the requirement that the initial tagging pattern be known a priori, eliminates errors in velocity estimates caused by variations in the grid pattern during an experiment, and makes it possible to study flows with non-uniform mixtures. This detection and analysis combination also solves one of the problems associated with using caged fluorescein to study high-speed flows. In addition to the traditional implementation for velocimetry, novel applications for studying the Lagrangian evolution of both reacting and non-reacting interfaces and obtaining combined passive scalar/velocity measurements are demonstrated.

Particle Image Velocimetry

Abstract: 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.

Oscillating foils of high propulsive efficiency

Abstract: 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.

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

Abstract: 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.