Chiharu Fukushima

Effect of controlled longitudinal vortex arrays on the development of a turbulent boundary layer

Abstract Measurement was made on the mean flowfield to investigate the development of a turbulent boundary layer interacting with controlled longitudinal vortex arrays that were artifically generated from tiny airfoils arranged side by side in the free stream. Five arrangements varying in spacing between neighboring airfoils and distance of airfoil arrays from the test wall were studied. Within the boundary layer, there is spanwise transport of streamwise momentum due to pairs of counterrotating secondary flows. As a result, a spanwise periodic variation corresponding to the spanwise arrangement of airfoil elements arises in the mean flowfield and persists in the far-downstream region. The spacing between neighboring airfoils leads to a significant effect on the streamwise path of longitudinal vortices, which significantly affect the magnitude of boundary layer distortion. Even in the present three-dimensional flowfield, the streamwise velocity profiles collapse reasonably well on the standard logarithmic law of the wall. However, the wake region of logarithmic profiles is different at representative spanwise locations.

Experimental Study on the Flow behind a Swept Backward-Facing Step (1st Report, Mean Flow Field in a Whole Region)

In the present study, the flow over a swept backward-facing step, at a Reynolds number (based on the step height and the free stream velocity) of 8 000 for the swept angle of 30 degrees, was investigated experimentally. Measurements were made for the mean flow field such as the surface flow direction, forward flow fraction, and mean velocity components. The flow field was examined in the regions of separation, reattachment, and following recovery to the two-dimensional flow. Lateral flows induced by the swept step significantly contribute to the variation of mean velocity profiles in the separated region. Reattachment distance of the flow over the swept step becomes about 10% shorter than that of the two-dimensional one. The mean velocity profiles collapse reasonably well on the Johnstons triangular model in the recovery region, and the profiles near the wall also collapse on the three-dimensional logarithmic law proposed by Johnston.

Turbulent Boundary Layer Distorted by a Longitudinal Vortex Pair Produced by a Delta-Wing with an Attack Angle (Spanwise Variation of Mean Velocity Field)

An experimental investigation has been made on the interaction process between a boundary layer and longitudinal vortices. Comparable experimental conditions were set up for two types of vortex pair, namely, common-flow up (cfu) and common-flow down (cfd) and results obtained from two conditions were compared. The longitudinal vortex pair moves away from the wall in a faster rate in case of cfu. Spanwise spreading rate of two longitudinal vortex paths is larger in case of cfd. The decay of maximum longitudinal vorticity is slower in case of cfd. At the symmetrical plane between two vortices, the interaction between longitudinal vortex makes closely distributed mean velocity contour lines and higher skin-friction coefficient in case of cfd. Otherwise, in case of cfu the interaction makes widely distributed mean velocity contour lines and low skin-friction coefficient. Effect of extra rate of strain involved in momentum integral equation is applied to explain behaviors of boundary layer in the two cases.

Spanwise Variation of Turbulent Flowfield for the Turbulent Boundary Layer Interacting with Controlled Longitudinal Vortex Arrays.

A vortical structure within a turbulent boundary layer and spanwise variation of turbulent flowfields have been investigated for further understanding of the development of turbulent boundary layer interacting with the spanwise periodic longitudinal vortex arrays which is artificially generated in a free stream. One of the parameters, spanwise periodicity of longitudinal vortices L/S (L denotes the spanwise distance between neighboring airfoils, S airfoil span), rules both magnitude and streamwise path of secondary vortices caused within the turbulent boundary layer. As a result, whether the turbulent quantities show linear or nonlinear interaction is significantly dependent on the value of L/S. Therefore, in the case of L/S 1, turbulent quantities show a noticeable spanwise periodic variation corresponding to the arrangement of airfoil elements, and those spanwise periods are strictly maintained. Comparing the spanwise variations of turbulent flowfield and those of mean flowfield, mechanisms of spanwise variation of turbulent boundary layer are discussed.

Streamwise Variation of Mean Velocity Field for the Turbulent Boundary Layer interacting with Controlled Longitudinal Vortex Arrays.

Detailed mean flow measurements have been made in a planar turbulent boundary layer interacting with artificially generated trailing vortex arrays in a free stream. Two parameters, namely, spanwise periodicity of the longitudinal vortex arrays (L/S ; here L denotes the spanwise distance between neighboring airfoils, S airfoil span) and ratio of the longitudinal vortex scale b to boundary layer scale δ0, were independently varied for five cases. Within the boundary layer, there exists a spanwise transport of streamwise momentum due to pairs of counterrotating secondary flows. As a result, spanwise periodic variation corresponding to the spanwise spacing of airfoil elements arises in the mean flow properties, and is persistently maintained in the far downstream region. The value of L/S plays a dominant role under the distortion process in this flow field. In the case of L/Sg1, the spanwise periodic variations of mean flow profiles are significantly large, and these spanwise periodicities are persistently maintained in the streamwise direction. In the case of L/S<1, although the spanwise variations are small in magnitude, the spanwise periodicities of mean flow profiles change in the streamwise direction, while the effect of b/δ0 is caused as a local Reynolds number effect.