Hideo Osaka

The Effects of Splitter Plates on Turbulent Boundary Layer on a Long Flat Plate Near the Trailing Edge

An experimental investigation has been made on a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake centerline. The length of the splitter plate, I, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of the base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate without the splitter plate. A significant increase in the base pressure coefficient was achieved with the splitter plate (l/h≥1), fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, the mean velocity increased more than that in the upstream position in the case without the splitter plate. With the splitter plate, however, the base pressure rise made the mean velocity distribution more closely approach that of a fully developed turbulent boundary layer.

Effect of aspect ratios on potential core length for cruciform jet

Abstract This paper documents an experimental study of the mean flow and turbulence characteristics for three-dimensional turbulent free jets of air issuing into still air surroundings from cruciform nozzles with four aspect ratio values (AR = 3,5,12.5, and 50). The measured quantities comprise streamwise, spanwise, and lateral mean velocities obtained with a Pitot-static pressure tube and a five-hole pressure tube, and the turbulent intensity with both an X-type hot-wire probe and a linearized constant-temperature anemometer. The experimental results reveal the “saddleback” shape of both the streamwise mean velocity and turbulent intensity distributions in the near flowfield. Potential core lengths nondimensionalized by jet nozzle width d for the cruciform jets are larger than those for two-dimensional and axisymmetric jets. Furthermore, a relationship between the aspect ratio and the potential core length is examined and it is clearly shown that the potential core length could be significantly controlled by the variation of the magnitude of the inward secondary flow velocity on both the y and z axes for the cruciform jets.

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.

Flow Past a Permeable Manipulator Ring Placed in a Turbulent Pipe Flow

Turbulent pipe flow past a ring-type permeable manipulator was investigated by measuring the mean flow and turbulent flow fields. The permeable manipulator ring had a rectangular cross section and a height 0.14 times the pipe radius. The experiments were performed under four conditions of the open area ratio β of the permeable ring (β = 0.1, 0.2, 0.3 and 0.4) for Reynolds number of 6.2×104 . The results indicate that as the open-area ratio increased, the separated shear layer arising from the permeable ring top became weaker and the pressure loss was reduced by increasing fluid flow through the permeable ring. When β was less than 0.2, the velocity gradient was steeper over the permeable ring and in the shear layer near the reattachment region. When β was greater than 0.3, the width of the shear layer showed a relatively large augmentation and the back pressure in the separating region increases. Further, the response of the turbulent flow field to the permeable ring was delayed compared with that of the mean velocity field, and these differences increased with β. The turbulence intensities and Reynolds shear stress profiles near the reattachment point increased near the wall region as β increased, while those peak values that were taken at the locus of the manipulator ring height decreased as β increased.Copyright

LDV Measurement Near a Rough Surface for a Turbulent Boundary Layer

LDV(Laser Doppler Velocimeter) measurement has been made close to a rough surface beneath an equilibrium boundary layer. The experiments were conducted at the momentum thickness Reynolds number of 6,000 and the roughness Reynolds number of 150. The local skin friction coefficient can be represented as the sum of the mean flow momentum flux and the Reynolds shear stress at the top surface between the roughness elements. The moment centroid of the drag acting on the rough surface measured from the top of the roughness element might be represented as the integral length scale on both the mean flow momentum flux and Reynolds shear stress estimated in the cavity between the roughness elements. The integral length scales of the mean flow momentum flux and the Reynolds shear stress contribute 30% and 70% respectively to the moment centroid.

Non-Equilibrium and Equilibrium Boundary Layers without Pressure Gradient

The effect of the friction parameter ω, defined as the ratio of the friction velocity to the free stream velocity, has been investigated on mean velocity fields for non-equilibrium and equilibrium boundary layers developing under zero pressure gradient. The wall shear stress was measured by a drag balance using a floating element device with a zero displacement mechanism. For the equilibrium boundary layer, the local skin friction coefficient is independent of two parameters, both the streamwise distance and the Reynolds number, based on the momentum thickness, and the boundary layer thickness is proportional to the streamwise distance. On the other hand, for the non-equilibrium boundary layer, the local skin friction coefficient depends on the above two parameters. The wake parameters for both boundary layers approach constant values, which depend on the surface condition, for high Reynolds numbers. From analysis using both the momentum integral equation and Coles’s wake law, the wake parameter for the equilibrium boundary layer is uniquely expressed as a function of the friction parameter. However, for the non-equilibrium boundary layer, the wake parameter depends on the friction parameter as well as the growth rate of the boundary layer thickness.

Scaling Law of the Near Wall Flow Subjected to an Adverse Pressure Gradient

Detailed experiments have been conducted to investigate the effect of adverse pressure gradients on the log-law in turbulent boundary layers. The wall shear stress was measured by a direct measurement device and the scaling law of the mean velocity was discussed based on high-accuracy experimental data. Considering the significant contribution of the inertia term in the equations of motion, a new velocity scale is defined and a similarity law was obtained for the mean velocity profile subjected to an adverse pressure gradient.

Management of Two-dimensional Channel Flow with a Pair of Streamwise Vortices (Behaviour of Streamwise Vortices and Mean Velocity Field)

An experimental study was conducted to examine the management of a two-dimensional turbulent channel flow with a pair of streamwise vortices. A common-flow down type streamwise vortex pair, generated by a pair of half-delta wings mounted on the wall, was introduced into a fully developed turbulent channel flow. The half-delta wings were as high as the inner layer thickness of the channel flow. The mean velocity and Reynolds shear stress distributions were measured and various properties were obtained in order to find meanings of the vortex generator for management of the turbulent channel flow. The convective motion of the secondary current is responsible for most of the streamwise momentum transfer toward the wall in the interaction between the vortices and the shear layer. In the velocity profile averaged over the spanwise extent, the velocity is accelerated below the vortex center and decelerated above the vortex center. Deformation of the mean velocity profile remained at the remarkable downstream distance of 250 times the wing height, which corresponds to 50 times channel the half-width H.

The Effects of Splitter Plates on Turbulent Boundary Layer Developing on a Flat Plate near the Trailing Edge

An experimental investigation has been made for a turbulent boundary layer near the trailing edge on a long flat plate. The flow was controlled by an additional splitter plate fitted to the trailing edge along the wake center line. The length of the splitter plate, l, was varied from a half, to five times the trailing edge thickness, h. Measurements of base pressure behind the trailing edge and of mean velocity and pressure distribution in the turbulent boundary layer on the flat plate were made under the freestream zero-pressure gradient. The absolute value of base pressure coefficient of the long flat plate was considerably smaller than that of the short flat plate (bluff body) without splitter plate (l/h=0). A significant increase in the normalized base pressure coefficient (about 50% as same as short one) was achieved with the splitter plate (l/h≥1) fitted to the long flat plate. Within an inner layer in the turbulent boundary layer near the trailing edge, mean velocity increased than that in the upstream position, especially in the case without splitter plate. With splitter plate, however, the base pressure rise made mean velocity distribution come closer to that of fully developed turbulent boundary layer. In the case of l/h≥1, the affected range of acceleration decreased from 10 h upstream of the trailing edge without splitter plate to 5 h upstream.

Management of Stronger Wall Jet by a Streamwise Vortex with Periodic Perturbation (Analysis by Phase Averaging)

In a stronger wall jet managed by a streamwise vortex, effect of periodic perturbation on the interaction and evolution processes has been investigated in the equations derived from phase averaging and triple decomposition. Transverse and spanwise locations of vortex center have time variations associated with variation in strength of streamwise vortex. Normal Reynolds stress difference (V2-W2) and shear stress VW generated by periodic perturbation reduce decay rate of maximum vorticity and accelerate growth rate of spanwise vortex radius in terms of production terms in transport equation for streamwise vorticity. Prediction of Reynolds shear stress component VW by taking account of time variation of both vortex center and strength successfully explains experimental data of conventional time average stress of vw.