Shinsuke Mochizuki

Direct total skin-friction measurement of a flat plate in zero-pressure-gradient boundary layers

The total skin friction on a flat plate is directly measured by using a towing tank up to Reynolds number ReL 107 (or Rθ 104). Plates of 3.3 and 4.3 m in length are towed in still water, balancing the vertical weight by small flotation devices, and their drag force is measured by a highly sensitive load cell. We have developed a new technique to correct wave-making resistance, pressure resistance and drag on a turbulence simulator. When the measured total drag is converted into local drag, it is found that the local frictional resistance is about 6% smaller than that given by the Karman–Schoenherr formula. But it is consistent with the values obtained by the floating element technique, oil film interferometry and asymptotic evaluations.

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

Numerical-based theoretical analysis on the decay of homogeneous turbulence affected by small strain based on constant and linear strain variations

This study clarifies the effects of small strain on the decay of homogeneous turbulence by focusing on the temporal profile of the small strain. Small strain is defined in such a way as to not affect the anisotropy of the homogeneous turbulence. We apply the framework of the standard k–e model to examine the effects of small strain. Constant and linearly varying small strains are studied. The effects of linearly varying small strain are found to be greater than those of constant strain. To discuss the results, we derive an analytical solution that describes the effects of the two types of small strain. Although the form of the analytical solutions is the same for constant and linearly varying strains, the coefficients used in the analytical solutions, for which an equation is also obtained, differ. The difference observed in the effects of the two types of small strain is thus caused by the difference in coefficient.

Analysis and Application of Decaying Turbulence with Initial Fractal Geometry

In this paper, we address high‐Schmidt‐number (Sc) scalar turbulent mixing that results from grid‐generated turbulence using the initial fractal geometry of the velocity pro‐ file. More specifically, as was proposed in our recent study, we adopt an initial flow field generated by a fractal grid and apply it to a water channel experiment based on a high‐Sc‐number scalar‐mixing layer in order to create grid‐generated turbulence, and thus solve our current research problem. The high‐Sc‐number scalar and velocity fields of the grid‐generated turbulence are then measured using planar laser‐induced fluo‐ rescence (PLIF) and particle image velocimetry (PIV), respectively. By means of fractal analysis, this study specifically addresses the turbulent mixing phenomena in which the fractal dimension of the mixing interface of an observed high‐Sc‐number scalar field is calculated. Additionally, we discuss the efficiency of using fractal grids as devices for enhancing high‐Sc‐number scalar turbulent mixing by observing turbulent intensities and dissipation by PIV.

Wall Similarity in Turbulent Boundary Layers Subjected to Weak Pressure Gradients

In this study, the effects of adverse and favorable pressure gradients on the law of the wall were investigated experimentally in turbulent boundary layers. The divergence and convergence of the mean streamline in the wall layer are considered to yield suitable velocity scales in the wall layer by integrating the equations of motion at finite Reynolds number.

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.