The Effects of External Excitation on the Reynolds-Averaged Quantities in a Turbulent Wall Jet
The effects of external, two-dimensional excitation on the plane turbulent wall jet were investigated experimentally. Measurements of the streamwise component of velocity were made throughout the flow field for a variety of Reynolds numbers, imposed frequencies, and amplitudes. The present data were always compared to the results generated in the absence of external excitation. Although the bulk of the unexcited flow is self-similar, it depends on the momentum flux at the nozzle and on the viscosity and density of the fluid and not on the width of the nozzle, which was commonly used to reduce the data. These similarity scales were used to check the consistency of the skin friction measurements, which are otherwise determined with considerable difficulty. It was shown that external excitation has no appreciable effect on the rate of spread of the jet or on the decay of its maximum velocity. In fact, the mean velocity distribution did not appear to be altered by the external excitation in any obvious manner. The flow near the surface, however (i.e., for 0 < Y+ < 100), was profoundly different from the unforced flow, indicating a reduction in wall stress exceeding at times 30%. The production of turbulent energy near the surface was also reduced, lowering the intensities of the velocity fluctuations. It was also shown that the inviscid, logarithmic portion of the “law of the wall” cannot be valid in either the forced or the unforced flows, because the Reynolds stress decreases rapidly beyond the distance at which the viscous stress becomes vanishingly small. This casts a doubt on the existence of a “constant stress layer” in the wall jet.
Unable to display preview. Download preview PDF.
- Katz, Y., Nishri, B., and Wygnanski, I., 1989, The delay of turbulent boundary-layer separation by oscillatory active control, AIAA Paper 89–0975.Google Scholar
- Kline, S. J., Reynolds, W. C., Schraub, F. A., and Runstadler, P. W., 1967, The structure of turbulent boundary layers, J. Fluid Mech., 50:133.Google Scholar
- Narasimha, R., Narayan, K. Y., and Parthasarathy, S. P., 1973, Parametric analysis of turbulent wall jets in still air, Aeronautical J., 77: 335.Google Scholar
- Newman, B. G., 1961, The deflexion of plane jets by adjacent boundaries-Coanda effect, in: “Boundary Layer and Flow Control,” Pergamon, London.Google Scholar
- Ozarapoglu, V., 1973, Measurements in incompressible turbulent flows, Doctor of Sciences Thesis, Laval University. Quebec City.Google Scholar
- Tailland, A. and Mathieu, J., 1967, Jet parietal, J. de Mecanique, 6: 1.Google Scholar
- Willmarth, W. W., 1975a, Pressure fluctuations beneath turbulent boundary layers, Annual Review Fluid Mech., 7: 13.Google Scholar
- Willmarth, W. W., 1975b, Structure of turbulence in boundary layers, in: “Advances in Applied Mechanics,” Academic Press, New York.Google Scholar
- Wygnanski, I. and Petersen, R. A., 1987, Coherent motion in excited free shear layers, AIAA J., 25: 201.Google Scholar