The forced turbulent wall jet

Abstract

The effects of external two-dimensional excitation on the plane turbulent wall jet were investigated experimentally and theoretically. Measurements of the streamwise component of velocity were made throughout the flow field for a variety of imposed frequencies and amplitudes. The present data were always compared to the results generated in the absence of external excitation. Two methods of forcing were used: one global, imposed on the entire jet by pressure fluctuations in the settling chamber and one local, imposed on the shear layer by a small flap attached to the outer nozzle lip. The fully developed wall jet was shown to be insensitive to the method of excitation. Furthermore, external excitation has no appreciable effect on the rate of spread of the jet nor 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. External excitation enhanced the two-dimensionality and the periodicity of the coherent motion. Spectral analysis and flow visualization suggested that the large coherent structures in this flow might be identified with the most-amplified primary instability modes of the mean velocity profile. Detailed stability analysis confirmed this proposition though not at the same level of accuracy as it did in many free shear flows.

Fluidic Elements based on Coanda Effect

“…The theoretical results of this computing program were confirmed with good approximation with the Olson results and with the results obtained by more recent studies made by Katz (1992) [12 ] and other researchers.…”

Are breaking waves, bores, surges and jumps the same flow?

“…A key feature is the developing shear layer and the recirculation region above. The turbulent shear flow is somehow analogous to a wall jet although basic differences between ideal wall jet and hydraulic jumps include the effect of longitudinal pressure gradient P/x < 0, conservation of mass, and the natural forcing which is likely to enhance the two-dimensionality and periodicity of coherent structures (Katz et al, 1992). In the jump, the maximum velocity V max decays longitudinally as x -1/2 (Chanson and Brattberg 2000, Murzyn and Chanson 2009, Chanson 2010c.…”

Numerical Investigation of the Control of Separation from Curved and Blunt Trailing Edges Using DNS and LES

“…The results presented in [4], however, indicate that the unsteady coherent motion strongly influences the mean flow of the turbulent wall jet. In addition, if the coherent motion is a result of a local flow instability, then steady turbulent simulations are unlikely to capture the relevant physics correctly Therefore, in order to understand these physical mechanisms, an unsteady simulation spanning many eddy turnover times of the large coherent motion is necessary; at the same time, the simulation has to be accurate enough to account for the random turbulent motion.…”

“…4 For case 1 (Figure 22a) which has the lowest Reynolds number, Re N = 75, the flow spreads rapidly after exiting the nozzle and separates after a short distance of about 60° from the nozzle. In this case, the jet is not energetic enough to remain attached for very long.…”

“…From a hydrodynamic stability point of view, the shear layer region could be unstable to disturbances due to the inflection point in the mean velocity profile. This notion was investigated experimentally by Katz and coworkers [4], who have shown that large coherent structures do exist in the turbulent wall jet, and although these structures are visible mainly in the outer region of the wall jet, they profoundly change the whole flow field, as indicated by the significant change in the mean skin friction. More importantly, deliberately introducing these structures using periodic forcing results in predictable changes of the mean flow.…”