Accession Number : ADA267757


Title :   Transition Induced by Fixed and Freely Convecting Spherical Particles in Laminar Boundary Layers


Descriptive Note : Final rept.,


Corporate Author : PENNSYLVANIA STATE UNIV UNIVERSITY PARK APPLIED RESEARCH LAB


Personal Author(s) : Petrie, H L ; Morris, P J ; Bajwa, A R ; Vincent, D C


Full Text : https://apps.dtic.mil/dtic/tr/fulltext/u2/a267757.pdf


Report Date : Aug 1993


Pagination or Media Count : 111


Abstract : An experimental and analytical study of aspects of transition induced by disturbances from spherical particles in laminar boundary layers is discussed. The generation of turbulent wedges by fixed spherical particles in a laminar boundary layer on or near the surface of a flat plate is considered experimentally using flow visualization with fluorescent dye and laser Doppler velocimetry. Turbulent spots generated by freely convecting spherical particles that are released in the freestream to fall into a flat plate laminar boundary layer and impact the plate are also discussed. A combination of dye flow visualization and a video based particle tracking technique was used to study the convecting particle problem. Although the Reynolds number at the critical condition for turbulent wedge generation by fixed particles and turbulent spot generation by convecting particles are similar, transition in these two situations appears to be fundamentally different. The development of a turbulent wedge near the critical condition is a relatively gradual process. In contrast, turbulent spots form relatively quickly after the convecting particles enter the boundary layer and impact the plate. Turbulent wedge formation downstream of a fixed particle results from the destabilization of the near wall flow by the vortical structures shed into particle wake. This shedding process is dominated by periodically shed loop shaped hairpin vortices. Observation of subharmonic oscillations at 1/2 and 1/4 of this shedding frequency suggest that a chaotic route to turbulence by a series of period doubling bifurcations is possible.


Descriptors :   *FLOW VISUALIZATION , *BOUNDARY LAYER TRANSITION , *LAMINAR FLOW , *LAMINAR BOUNDARY LAYER , FREQUENCY , CONTRAST , IMPACT , CHAOS , TRACKING , PARTICLES , WAKE , TRANSITIONS , LASER VELOCIMETERS , BOUNDARY LAYER FLOW , REYNOLDS NUMBER , OSCILLATION , WEDGES , FLUORESCENT DYES , WALLS , PLATES , SURFACES , CONVECTION , TURBULENCE , TURBULENT FLOW , VORTICES


Subject Categories : Fluid Mechanics


Distribution Statement : APPROVED FOR PUBLIC RELEASE