Takatsugu Kameda

Experimental Investigation of a Turbulent Boundary Layer Subjected to an Adverse Pressure Gradient

Experiments were performed to investigate the effect of an adverse pressure gradient on the mean velocity and turbulent intensity profiles for an equilibrium boundary layer. The equilibrium boundary layer, which makes self-similar profiles, was constructed using a power law distribution of free stream velocity. The exponent of the law was adjusted to −0.188. The wall shear stress was measured with a drag balance by a floating element. The investigation of the law of the wall and the similarity of the streamwise turbulent intensity profile was made using both a friction velocity and new proposed velocity scale. The velocity scale is derived from the boundary layer equation. The mean velocity gradient profile normalized with the height and the new velocity scale exists the region where the value is almost constant. The turbulent intensity profiles normalized with the friction velocity strongly depend on the nondimensional pressure gradient near the wall. However, by mean of the local velocity scale, the profiles might be achieved to be similar with that of a zero pressure gradient.Copyright

Streamwise normal Reynolds stress variations of fully developed turbulent pipe flow responds to rough wall disturbances

Fully developed pipe flow is disturbed by rough wall section and measurements are carried out in the downstream of rough wall section. Emphasis is placed on the response of the flow to different types (d- and k-type) of rough wall. Larger turbulent bore of k-type flow is found into still increase while providing longer rough wall section and d-type flow also seemly to increase but comparatively small in magnitude. The response function of flow changes according to rough wall length and internal boundary layer thickness but weakly depends on type of roughness. The edge of internal boundary layer is examined by pointing out the intersection of disturbed and undisturbed of normal Reynolds stress curves. Compare to the other estimation methods of internal boundary layer thickness, thicker and almost same trend of development are found in present method.

Effects of Rough Wall in Fully Developed Turbulent Pipe Flow

Hot wire measurement is carried in fully developed turbulent pipe flow which introducing the rough wall sections (containing d- and k-type roughness alternatively) and emphasis on the statistical properties of turbulence for each flows. Sufficient pipe length is provided to ensure for fully recovery after disturbed by rough wall. On comparison, the major differences between d- and k-type rough wall effect could be found in early response region (x/D = 0.1 to 4). The violent ejection from the k-type roughness is found to the large effect to the main flow which turns into larger additional turbulent energy production. The changing of wall friction could increase the local shear stress which leads to the formation of stress bore but depends on the amount of changes. This stress bore is found to propagate from the vicinity of the wall to the pipe center which does not depend on type or length of roughness. The effectiveness of rough wall can also be found in the power spectra of streamwise component. The energy containing region agrees to both undisturbed and disturbed flow but shifting in power spectra appears which primarily depends on strength of disturbances.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.

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.

An Experimental Study af a Self-Preserving Boundary Layer with a Power-Law Variation of Free-Stream Velocity

A careful experimental investigation on the scaling law of the turbulent boundary layers was made at a self-preserving condition adjusting free-stream velocity in a power-law variation of x m The similarity of mean velocity profile was examined with the wall shear stress value determined by newly designed drag balance measurement device. The logarithmic velocity profile does hold universality with the Karman constant 0.41 and additive constant C=5.0 under a mild adverse pressure gradient.

Near-Wall Turbulent Structure for the Boundary Layer Developing Over a K-Type Rough Wall in Low-Reynolds-Number

Measurements of the mean and the turbulent velocities have performed in the vicinity of a roughness element for the turbulent boundary layer developed over a k-type rough wall, which consists of two-dimensional square ribs (10mm × 10mm × 400mm) arrayed with pitch ratio of 4. The Reynolds-number Rθ and k+ based on momentum thickness θ and roughness height k are about 500 and 50 respectively, and relative roughness height k / δ = 0.156. LDV (Laser Doppler Velocitimetry) that it is possible to measure reverse flow was applied to investigate the momentum-exchange process in the open-face (yT = 0(mm)) over cavities. The variation of the mean and turbulent quantities profiles at yT = 0(mm) depend on the evolution of a free-shear layer formed behind roughness elements and the behavior of eddies in the cavity. Pressure drag coefficient acting on the roughness element in the local skin friction coefficient is 88% in the present k-type rough wall. The momentum exchange for the mean flow and turbulence contribute 6% and 56% respectively to the local skin friction coefficient.Copyright