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

We report on the experimental observation of fluid flow caused by propagation of femtosecond filaments in dry air. We find that the ionization of the medium deposits a non-negligible amount of heat, which creates vortices in a semi-confined glass cylinder. We confirm the influence of thermal gradients on vortex formation by the use of a heated wire in a similar configuration.

/pdf/vortices-in-the-wake-of-a-femtosecond-laser-filament-1sz9imgpji.pdf

Vortices in the wake of a femtosecond laser filament.

Using a two-dimensional airfoil, a disturbance was introduced into a plane mixing layer some distance
downstream of the splitter plate trailing edge. Results indicate that it is possible to induce very large changes in
the layer growth rate downstream of the disturbance location, while leaving the portion of the shear layer
between the splitter plate and the disturbance source essentially unaffected. Furthermore, the use of forcing for
modification of the mixing layer in the region upstream of the disturbance is demonstrated. It Is shown that two
different mechanisms are responsible for coupling such disturbances to the flow in the present forcing of
upstream and downstream regions.

/pdf/effects-of-a-downstream-disturbance-on-the-structure-of-a-1c8uykqh48.pdf

Effects of a downstream disturbance on the structure of a turbulent plane mixing layer

In this study, a simple vortex array model is employed to compute the unsteady velocity field in the wake of an airfoil undergoing harmonic as well as nonharmonic pitch oscillation. The parameters of the model are determined by minimizing the difference between the model predictions and existing single-component laser Doppler velocimetry measurements of the streamwise velocity in the near wake of an oscillating NACA 0012 airfoil, at a chord Reynolds number of 11,400. It is shown that the vortex array model is an effective tool for determination of the wake vortex parameters (circulation, core diameter, spacing, etc.) from pointwise measurements of only the streamwise component of the velocity. This is demonstrated through comparison of the vortex parameters estimated using the model with those obtained directly from whole-field measurements in a similar flow. In addition, the mean streamwise force acting on the airfoil is calculated using the model in conjunction with the integral momentum equation. Comparison of the model-predicted force coefficient and its dependence on the reduced frequency are found to be consistent with published experimental and computational data.

/pdf/finite-core-vortex-array-model-of-the-wake-of-a-periodically-s7heulh51f.pdf

Finite-Core Vortex Array Model of the Wake of a Periodically Pitching Airfoil

We present an investigation into the influence of upstream shear on the viscous flow around a steady two-dimensional (2-D) symmetric airfoil at zero angle of attack, and the corresponding loads. In this computational study, we consider the NACA 0012 airfoil at a chord Reynolds number in an approach flow with uniform positive shear with non-dimensional shear rate varying in the range 0.0–1.0. Results show that the lift force is negative, in the opposite direction to the prediction from Tsien’s inviscid theory for lift generation in the presence of positive shear. A hypothesis is presented to explain the observed sign of the lift force on the basis of the asymmetry in boundary layer development on the upper and lower surfaces of the airfoil, which creates an effective airfoil shape with negative camber. The resulting scaling of the viscous effect with shear rate and Reynolds number is provided. The location of the leading edge stagnation point moves increasingly farther back along the airfoil’s upper surface with increased shear rate, a behaviour consistent with a negatively cambered airfoil. Furthermore, the symmetry in the location of the boundary layer separation point on the airfoil’s upper and lower surfaces in uniform flow is broken under the imposed shear, and the wake vortical structures exhibit more asymmetry with increasing shear rate.

Lift on a steady 2-D symmetric airfoil in viscous uniform shear flow

A technique is proposed to correct the bias error associated with one-component molecular tagging velocimetry (1c-MTV). 1c-MTV is typically used to capture the velocity component normal to a single or multiple lines of tagged fluid molecules with very high spatial resolution. However, the measurements are affected by an inherent error, which arises from the presence of a velocity component parallel to the lines. In the present study, a Taylor-series-based approach is used to derive a general mathematical expression for this error. The derived expression, which is validated using simulated MTV measurements in laminar channel flow with suction/blowing, is used as the basis for introducing a correction method involving the acquisition of MTV images at multiple time delays. To examine the effectiveness of this ‘multi-time-delay’ approach for correcting 1c-MTV data, an experiment is conducted whereby a known bias error is deliberately imposed on measurements in laminar channel flow. The corrected measurements agree with the true velocity profile to better than 2%, thus validating the correction method.

/pdf/a-multi-time-delay-approach-for-correction-of-the-inherent-89nqhx8xa4.pdf

Manoochehr Koochesfahani

Papers

Vortices in the wake of a femtosecond laser filament.

Effects of a downstream disturbance on the structure of a turbulent plane mixing layer

Finite-Core Vortex Array Model of the Wake of a Periodically Pitching Airfoil

Lift on a steady 2-D symmetric airfoil in viscous uniform shear flow

A multi-time-delay approach for correction of the inherent error in single-component molecular tagging velocimetry