No abstract

The fluid mechanics and heat transfer characteristics of film cooling are three-dimensional and highly complex. To understand this problem better, an experimental study was conducted in a low-speed wind tunnel on a row of six rectangular jets injected at 90 deg to the crossflow (mainstream flow). The jet-to-crossflow velocity ratios (blowing ratios) examined were 0.5, 1.0, and 1.5, and the jet spacing-to-jet width ratio was 3.0. No significant temperature difference between jet and crossflow air was introduced. Mean velocities and six flow stresses were measured using a three-component laser-Doppler velocimeter operating in coincidence mode. Seeding of both jet and cross-stream air was achieved with a commercially available smoke generator. Flow statistics are reported in the form of vector plots, contours, and x-y graphs, showing velocity, turbulence intensity, and Reynolds stresses. To complement the detailed measurements, flow visualization was accomplished by transmitting the laser beam through a cylindrical lens, thereby generating a narrow, intense sheet of light. Jet air only was seeded with smoke, which was illuminated in the plane of the light sheet. Therefore, it was possible to record on video tape the trajectory and penetration of the jets in the crossflow. Selected still images from the recordings are presented. Numerical simulations of the observed flow field were made by using a multigrid, segmented, k–ε CFD code. Special near-wall treatment included a nonisotropic formulation for the effective viscosity, a low-Re model for k, and an algebraic model for the length scale. Comparisons between the measured and computed velocities show good agreement for the nonuniform mean flow at the jet exit plane. Velocities and stresses on the jet centerline downstream of the orifice are less well predicted, probably because of inadequate turbulence modeling, while values off the centerline match those of the experiments much more closely.

Measurements are reported for the separted reattaching flow around a long rectangular plate placed at zero incidence in a low-turbulence stream. This laboratory configuration, chosen for its geometric simplicity, exhibits all of the important features of two-dimensional flow separation with reattachment. Conventional hot-wire anemometry, pulsed-wire anemometry and pulsed-wire surface shear stress probes were used to measure the mean and fluctuating flow field at a Reynolds number, based on plate thickness, of 5 × 104. The separated shear layer appears to behave like a conventional mixing layer over the first half of the separation bubble, where it exhibits an approximately constant growth rate and a linear variation of characteristic frequencies and integral timescales. The characteristics of the shear layer in the second half of the bubble are radically altered by the unsteady reattachment process. Much higher turbulent intensities and lower growth rates are encountered there, and, in agreement with other reattaching flow studies, a low frequency motion can be detected.

The turbulentflow obout a two-dimensional blunt rectangular section is used os o test case to examine the performonce of seven neorwoll turbulence models. The Jint two models ore one-equotion low Reynolds number (Re) models requiring o fine grid neorwoll treatment. The other neorwoll turbulence models considered ore based on wollfunctions t h bridge with a single cell the thermally importnnt neorwoll region. Standard woll functions bosed on the local equilibrium assumption, woll functions using o two-and three-layer oppmach to evoluote locol variations of turbulence quadies in the k equation, ond extension of the two-ond three-layer treatments to the c eqvation o n considered. m e numerieol predictions ore obtoined using o variant of the k+ model incorpomtr'ng o curvoture correction. The governing eguoiions ore discretized using oJinite-volume formulolion employing the bounded-skew hybrid differencing scheme. The solutions ore obtained using a two-pass procedure, devised to allow for the correct use of the woll functions. CompuWns ore pegormed for Re in the mnge 20,000-75.000. The various neorwoll models ore assessed by compPMng resulting Nusselt number distributions and selected fluid dynamic results with ov&le experimenlol o h . A three-layer model of the woll region, applied to the k ond e equafions, gives good ognement with the data; the standard w d function treatment is not sotisfnclry ot oll for this flow.

No abstract