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It is known that stretching and intensification of a hairpin vortex by mean shear play an important role to create a hairpin vortex packet, which generates the large Reynolds shear stress associated with skin-friction drag in wall-bounded turbulent flows. In order to suppress the mean shear at the wall for high efficient drag reduction (DR), in the present study, we explore an active flow control concept using streamwise shear control (SSC) at the wall. The longitudinal control surface is periodically spanwise-arranged with no-control surface while varying the structural spacing, and an amplitude parameter for imposing the strength of the actuating streamwise velocity at the wall is introduced to further enhance the skin-friction DR. Significant DR is observed with an increase in the two parameters with an accompanying reduction of the Reynolds stresses and vorticity fluctuations, although a further increase in the parameters amplifies the turbulence activity in the near-wall region. In order to study the direct relationship between turbulent vortical structures and DR under the SSC, temporal evolution with initial eddies extracted by conditional averages for Reynolds-stress-maximizing Q2 events are examined. It is shown that the generation of new vortices is dramatically inhibited with an increase in the parameters throughout the flow, causing fewer vortices to be generated under the control. However, when the structural spacing is sufficiently large, the generation of new vortex is not suppressed over the no-control surface in the near-wall region, resulting in an increase of the second- and fourth-quadrant Reynolds shear stresses. Although strong actuating velocity intensifies the near-wall turbulence, the increase in the turbulence activity is attributed to the generation of counter-clockwise near-wall vortices by the increased vortex transport. [ABSTRACT FROM AUTHOR]

Results of an experimental investigation of the skin friction for a range of rough surfaces in fully-developed, turbulent channel flow are presented. The fifteen surfaces were generated by grit blasting with various types and sizes of media. The Reynolds number ( Re m ) for the experiments based on the bulk mean velocity and the channel height spanned from 10,000–300,000. The shape of the roughness function ( ΔU + ) for all of these surfaces shows self-similarity with an effective roughness height ( k eff + ) indicating that a scale does exist to collapse results throughout the entire roughness function. The roughness function displays inflectional behavior in the transitionally-rough regime in a similar manner to the results of Nikuradse for uniform sand. The equivalent sand grain roughness height can be predicted using k rms and the skewness ( Sk ) of the probability density function of the roughness amplitude. A long-wavelength filtering procedure based on the Taylor microscale ( λ ) of the roughness is presented, and it is recommended that roughness profiles be filtered at a scale of ∼ 100 λ . [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) of a turbulent channel flow with the slip boundary condition was performed to investigate the spatial characteristics of large-scale motions (LSMs) in a drag-reducing flow and the influence of LSMs on drag reduction. The slip boundary condition was applied in the streamwise direction. DNS of a channel flow with the no-slip boundary condition was also conducted for comparison. The overall mean skin-friction coefficient of the flow with the slip boundary condition was reduced by 35% relative to the flow with the no-slip condition at the same bulk Reynolds number of 10,333. The streamwise and spanwise length scales of the LSMs in the outer region are larger with the slip boundary condition than with the no-slip condition due to a large population of long LSMs. These LSMs influence the near-wall region via roll-cell motions and footprints. Although the number of total streaks decreases, a portion of the footprints increases due to the influence of the outer LSMs. The streamwise vorticities around the low-speed footprints are attenuated more than those around the high-speed footprints. The LSMs and the near-wall footprints contribute 45% of the total reduction in the skin friction coefficient, with 25% of this contribution due to low-speed LSMs. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flow over the regularly distributed triangular ribs was investigated. [•] Simultaneous measurement of flow resistance and velocity profile was performed. [•] Flow resistance was measured directly and velocity profile was measured by LDV. [•] Flow resistance was estimated by the information of the velocity field. [•] Estimated flow resistance has good agreement with the measured flow resistance. [Copyright &y& Elsevier]

Abstract: Understanding the relationship between a surface’s topography and its hydraulic resistance is an important, yet elusive, goal in fluids engineering. Particularly poorly understood are the flow conditions at which a given surface will begin to show the effects of roughness in the form of increased wall shear stress above that of the hydraulically smooth wall. This phenomenon is the focus of the present study. The results from a small scale fully-developed turbulent channel flow facility are presented for a hydraulically smooth wall and three types of rough surfaces (a sandpaper surface, two ship bottom paints and painted surfaces smoothed by sanding). Experiments were conducted over a Reynolds number (ReH ) range of 5800–64,000 based on the channel height and the bulk mean velocity. The onset of roughness effects occurs for the sandpaper surface at ∼1 or ∼5. This value of and the shape of the roughness function in the transitionally rough regime agree rather well with the results of for uniform sand. The frictional resistance of the two painted surfaces agree within experimental uncertainty despite a factor of two difference in krms . The onset of roughness effects occurs at a maximum peak to trough roughness height, , of ∼10. Painted surfaces sanded with progressively finer sandpaper (60–220 grit) show roughness effects for ∼0.5–0.7 or ∼9, while the 400 grit sandpaper displays hydraulically smooth conditions over the entire Reynolds number range. The roughness scale that best predicts the onset of roughess effects is kt , indicating that the largest roughness features have the most influence in determining when a surface ceases to be hydraulically smooth. It is also of note that the roughness functions for the marine paint and painted-sanded surfaces do not exhibit either Nikuradse or Colebrook-type behavior. [Copyright &y& Elsevier]

Abstract: A model of a bubble train flow accompanied with mass transfer in a long capillary tube is developed. In contrast to models presented in literature, our modeling approach accounts for expansion of gas bubbles and flow velocity increase along the channel due to the pressure drop caused by friction losses. The model performance is illustrated by a number of computational examples. The distributions of the bubble velocity and the volumetric mass transfer coefficient along the channels of different diameters are presented. The deviation of the dissolved gas concentration from the saturation concentration along the channel is used as a characteristic of closeness of a fluid system to the phase equilibrium. The calculations explicitly demonstrate that the deviations of gas–liquid mixture flows from equilibrium in long capillary channels of small diameters are small. The effect of the channel diameter, the channel length, and the bubble nucleation frequency on the deviation of the system from equilibrium is also studied. [Copyright &y& Elsevier]

Abstract: This paper describes the design of a floating element friction balance which is based upon a commercially available micro force balance. The balance has a perfectly linear calibration function and was successfully applied to rough wall flows in a channel and a diffusor. Extrapolation of the turbulent shear stress measured by two component LDA to the wall matched very well the shear stress measured using the friction balance. Also, the wall shear stress obtained from the balance in the fully developed channel flow agreed with the stress that could be derived from the pressure gradient to within 3%. [Copyright &y& Elsevier]

Abstract: Three-dimensional elliptic governing equations were solved to investigate laminar mixed convection of a nanofluid consists of water and Al2O3, buoyancy-affected and heat transfer of a curved tube. Simultaneous effects of the buoyancy force, centrifugal force and nanoparticles concentration on the fluid flow developing and heat transfer along the pipe is investigated in this paper. The nanoparticles concentration does not have any significant effect on the secondary flow, while the axial velocity, Nusselt number, skin friction factor as well as fluid temperature have been affected considerably. In this paper, some important new results are obtained. Firstly, for a given flow rate; nanoparticles concentration have positive effects on the axial velocity and skin friction factor. Secondly, buoyancy force has negative effect on the Nusselt number and skin friction factor. [Copyright &y& Elsevier]