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

http://absimage.aps.org/image/DFD16/MWS_DFD16-2016-000784.pdf

Experiments on a Steady Low Reynolds Number Airfoil in a Shear Flow

In this study we investigate the ability of a simple, vortex-array model to represent the unsteady velocity field in the wake of an airfoil undergoing harmonic as well as non-harmonic pitch oscillation. To this end, the model predictions are compared against existing experimental data of the streamwise velocity in the near wake of an oscillating NACA 0012 airfoil at a chord Reynolds number of 11,000 and a range of reduced frequencies extending up to approximately 10. The results show that the model predictions are in agreement with the experimental observations. Moreover, the model yields the transverse velocity profile, which is not available from the experimental data used here. This enables computation of the mean streamwise force acting on the airfoil from application of 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.

http://enu.kz/repository/2009/AIAA-2009-393.pdf

A vortex array model of the unsteady wake of a pitching airfoil

Development of a Transverse Low-Turbulence Gust Generator in a Wind Tunnel

Experimental Evaluation of a Micro Optical Shear Stress Sensor

: Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 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 7x7 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. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the non-reacting Mach 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 Mach 2 non-reacting flow.

Manoochehr Koochesfahani

Papers

Experiments on a Steady Low Reynolds Number Airfoil in a Shear Flow

A vortex array model of the unsteady wake of a pitching airfoil

Development of a Transverse Low-Turbulence Gust Generator in a Wind Tunnel

Experimental Evaluation of a Micro Optical Shear Stress Sensor

Hydroxyl tagging velocimetry in a mach 2 flow with a wall cavity (postprint)