Engineering topics

Johnston, James P.

Publications and source records attributed to Johnston, James P..

4 records

NASA NTRS1991

Evolution of three-dimensionality in stable and unstable curved mixing layers

Results are presented of an experimental study tracing the effects of mild streamwise curvature (delta/R is less than 5 percent) on the 3D structure of a two-stream mixing layer at high Reynolds number (about 27,000). Measurements of the mean and fluctuating velocities were obtained on fine cross-plane grids at 11 streamwise locations using a rotatable cross-wide probe. Well-organized streamwise vorticity was generated in both cases through the braid instability. Although the vortex structures decayed with streamwise distance in both cases, the rate of decay for the unstable case was lower. As a result, the unstable layer exhibited noticeable spanwise variations in the mean velocity and Reynolds stresses. Both cases achieved linear growth, but the rate of growth for the unstable case was higher. The far-field spanwise-averaged peak Reynolds stresses were significantly higher for the destabilized case compared to the stabilized case, which exhibited levels comparable to those of a straight case.

Plesniak, Michael W.↗

NASA NTRS1990

Experimental Study of a Three-Dimensional Shear-Driven Turbulent Boundary Layer with Streamwise Adverse Pressure Gradient

The effects of a strong adverse pressure gradient on a three-dimensional turbulent boundary layer are studied in an axisymmetric spinning cylinder geometry. Velocity measurements made with a three-component laser Doppler velocimeter include all three mean flow components, all six Reynolds stress components, and all ten triple-product correlations. Reynolds stress diminishes as the flow becomes three-dimensional. Lower levels of shear stress were seen to persist under adverse pressure gradient conditions. This low level of stress was seen to roughly correlate with the magnitude of cross-flow (relative to free stream flow) for this experiment as well as most of the other experiments in the literature. Variations in pressure gradient do not appear to alter this correlation. For this reason, it is hypothesized that a three-dimensional boundary layer is more prone to separate than a two-dimensional boundary layer, although it could not be directly shown here. None of the computations performed with either a Prandtl mixing length, k-epsilon, or a Launder-Reece-Rodi full Reynolds-stress model were able to predict the reduction in Reynolds stress.

Driver, David M.↗

NASA NTRS1989

The relaxation of a turbulent boundary layer in an adverse pressure gradient

Reattached turbulent boundary layer relaxation downstream of a wall fence is investigated. An adverse pressure gradient is imposed upon it which is adjusted to bring the boundary layer into equilibrium. The pressure gradient is adjusted so as to bring the Clauser parameter G down to a value of about 11.4 and then maintain it constant. In the region from the reattachment point to 2 or 3 reattachment lengths downstream, the boundary layer recovers from the initial major effects of reattachment. Farther downstream, where G is constant, the pressure-gradient parameter changes very slowly and profiles of non-dimensionalized eddy viscosity appear self-similar. However, pressure gradient and eddy viscosity are both roughly twice as large as expected on the basis of previous equilibrium turbulent boundary layer studies.

Cutler, Andrew D.↗

NASA NTRS1989

Three-dimensional boundary layer flow with streamwise adverse pressure gradient

The present study examines the effects of a strong adverse pressure gradient on a 3D turbulent boundary layer in an axisymmetric spinning cylinder geometry. Velocity measurements made with a three-component laser Doppler velocimeter include all three mean flow components, all six Reynolds stress components, and all ten triple-product correlations. Total Reynolds shear stress diminishes as the flow becomes 3D. Lower levels of shear stress were found to persist under adverse pressure gradient conditions. This low stress level was observed to roughly correlate with the magnitude of the crossflow. Variations in the pressure gradient do not alter this correlation. It is inferred that a 3D boundary layer is more prone to separate than a 2D boundary layer.

Driver, David M.↗