Manoochehr Koochesfahani

Vortical Patterns in the Wake of an Oscillating Airfoil

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

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.

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

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.

LASER-INDUCED FLUORESCENCE MEASUREMENTS OF MIXED FLUID CONCENTRATION IN A LIQUID PLANE SHEAR LAYER.

The processes of entrainment and mixing are investigated in a nonreacting, uniform density, liquid mixing layer. Laser-induced fluorescence diagnostics and high-speed, real-time digital image acquisitions techniques are combined to measure the probability density function of the composition field. Results show that the vortical structures in the mixing layer initially roll up with a large excess of high-speed fluid in the cores. It is found that the mixed fluid composition, above the mixing transition, is essentially uniform across the entire transverse extent of the layer and is asymmetric with a bias in favor of the high-speed fluid. Preliminary observations indicate that the composition of the mixed fluid is more uniform across a liquid shear layer than that in the gaseous layer. The important effect of the resolution capability of the measurement apparatus on the results are discussed and comparisons with recent theoretical calculations are presented.

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

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.

Particle streak velocity field measurements in a two­ dimensional mixing layer

Using digital image processing of particle streak photography, the streamwise and perpendicular components of the velocity field were investigated, in the mid‐span plane of a two‐dimensional mixing layer, with a 6:1 velocity ratio. The Reynolds number of the flow, based on the local vorticity thickness and the velocity difference across the layer, ranged from 1360 to 2520, in the plane of observation. The significant result of this experiment was that the region of vorticity bearing fluid is confined to a small fraction of the flow. A second finding, consistent with the small regions of concentrated vorticity, was the observation of instantaneous streamwise velocity reversal, in the laboratory frame, in small regions of the flow.

Hydroxyl tagging velocimetry in a supersonic flow over a cavity

Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 (M 2) flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7 x 7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow in the plane of the laser sheets. Velocity precision is limited by the error in finding the crossing location of the OH lines written by the excimer tag laser. With a signal-to-noise ratio of about 10 for the OH lines, the determination of the crossing location is expected to be accurate within +/- 0.1 pixels. Velocity precision within the freestream, where the turbulence is low, is consistent with this error. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the nonreacting M 2 flow with a wall cavity under low- and high-pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the M 2 nonreacting flow.

On wall-pressure sources associated with the unsteady separation in a vortex-ring/wall interaction

Molecular tagging velocimetery measurements from an earlier investigation are used to study the wall-pressure and the flow structures responsible for its generation in the flow field resulting from the impingement of an axisymmetric vortex ring on a flat wall. The velocity-field data are used to obtain the spatial distribution of the pressure sources, and the result is employed in conjunction with the solution to Poisson’s equation to yield the wall-pressure information. The outcome reveals a characteristic wall-pressure signature that is produced repeatedly whenever the primary vortex ring interacts with the wall to form vortex rings with opposite sense of vorticity. The details of the signature are analyzed and related to specific flow features and their mutual interaction at different phases of the generation/evolution cycle of the new rings. Finally, the flow mechanisms leading to the generation of substantial positive and negative wall pressure in the characteristic signature are clarified.

A "Flip" Experiment in a Chemically Reacting Turbulent Mixing Layer

An experimental investigation of entrainment and mixing in a reacting, uniform density, liquid plane shear layer has been carried out using laser-induced fluorescence diagnostics. Results indicate that the reactants mix on a molecular level and react at a composition that is nearly uniform across the transverse extent of the layer. The composition of the mixed fluid is found to be asymmetric with an excess of high-speed fluid, suggesting that entrainment into the shear layer is also asymmetric. These results are at variance with predictions of conventional models of turbulent transport and mixing.

Physical factors that trigger cholesterol crystallization leading to plaque rupture

BACKGROUND Triggers of plaque rupture have been elusive. Recently it was demonstrated that cholesterol expands when transforming from a liquid to a crystal state, disrupting overlying plaque. This study examined the effect of physical conditions including saturation, temperature, hydration, pH on cholesterol crystallization. METHODS Graduated cylinders were filled with varying amount of cholesterol powder (1, 2 and 3g) and dissolved in corn oil at 37 degrees C. Change in volume expansion (DeltaVE) and time to crystallization were measured for each saturation. The same was repeated while varying temperature (22-44 degrees C); hydration (1-3ml H(2)O); pH (5-8.4) and combination of saturation and temperature. Scanning electron microscopy was performed to evaluate crystal morphology and X-ray diffractometry to assess molecular structure of cholesterol. RESULTS Increasing saturation raised both DeltaVE (3g: 0.53+/-0.1ml vs. 1g: 0.14+/-0.02ml and 2g: 0.3+/-0.1ml; p<0.0001; p<0.01) and rate of change over 3min (3g: 60% vs. 1g: 14%). Crystal morphology was the same seen with crystals perforating human plaques. Temperature drop increased DeltaVE (44 degrees C: 0.05+/-0.01ml vs. 22 degrees C: 0.5+/-0.07ml; p<0.0001) and initiated earlier crystallization. Hydration resulted in greater DeltaVE (3ml: 0.7+/-0.07 vs. 0ml: 0.1+/-0.05; p<0.001) with corresponding changes in cholesterol molecular structure. Rising pH was associated with increased DeltaVE (1.3+/-0.03ml vs. 0.1+/-0.02ml; p<0.001). Combined increase in saturation and temperature had greater DeltaVE than expected from the sum of each alone. CONCLUSIONS Physical factors influenced both volume and rate of cholesterol crystallization. This suggests that local factors may play an important role in triggering plaque rupture. Combination of several factors may even be a more powerful trigger for acute cardiovascular events.