The impingement of lift jets beneath a short/vertical takeoff aircraft on the ground results in the formation of a wall jet that flows radially from the impinging point along the ground surface. The interaction of the wall jet with the freestream results in the formation of a highly curved flow (ground vortex) far away from the impinging jet that has profound influences on the flow development. This Note reports a study of the highly curved flow resulting from the interaction of such a wall jet with a boundary layer counterflow. The influence of the impinging jet on the curved flow region is eliminated by producing the wall jet independently by means of a two-dimensional experimental configuration. The wall jet originates from a narrow slot mounted on the ground plane of a conventional wind tunnel, and it impinges with the adversely directed boundary layer flow above the wind tunnel ground plane. This interaction yields a highly curved flow which can be studied for different velocity rations between the wall jet and the wind tunnel base flow. The velocity field was measured in the spanwise midplane of the configuration with a laser Doppler velocimeter (LDV) comprising two lasers of different wavelengths (633 and 532 nm, respectively). Sensitivity to the flow direction was provided by frequency shifting from a Bragg cell. Visualization studies were performed using direct digital photography and a smoke generator to produce tracer particles. The experiments were carried out for wall jet and boundary-layer velocities of 6 and 3.48 m/s, respectively. The photographs show, in agreement with LDV measurements, that after impinging the boundary layer the wall jet is strongly deflected backward at an average angle of 36 deg. In the impingement zone, both wall jet and boundary layer flow are directed toward the wall giving rise to an extremely complex flow, which includes a small secondary vortex flow on the boundary-layer side near the ground plane that seems to entrain fluid from the lower part of the wall jet. The flow is characterized by intense velocity fluctuations with peaks that cannot be identified or correlated with mean velocity gradients clearly. The measured velocity probability distribution of the turbulent normal stresses are nearly Gaussian, suggesting the absence of discrete frequency oscillations. The turbulence is anisotropic in that the ratio of horizontal turbulent normal stresses to vertical turbulent normal stresses is about 3 in the boundary layer, wall jet, and impingement zone, whereas in the deflected upwash flow region, this ratio is about 0.7. In the upwash deflected region, the relation between turbulent shear stress and the shear strain is consistent with a turbulent viscosity hypothesis. However, in the region where the boundary layer starts to be deflected upward due to the influence of the small ground vortex, and near the stagnation point, large effects of flow distortion on the turbulence structure are expected.


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    Title :

    Laser doppler measurements of a highly curved flow


    Additional title:

    Laser-Doppler-Messung hochgradig gekrümmter Strömungsverläufe


    Contributors:

    Published in:

    AIAA Journal ; 43 , 12 ; 2654-2657


    Publication date :

    2005


    Size :

    4 Seiten, 6 Bilder, 1 Tabelle, 11 Quellen




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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