Fully rough submerged plane wall-jets

Measurements in a zero-pressure-gradient turbulent boundary layer over a mesh-screen rough wall indicate several differences, in both inner and outer regions, in comparison to a smooth-wall boundary layer. The mean velocity distribution indicates that, apart from the expected k-type roughness function shift in the inner region, the strength of the rough-wall outer region ‘wake’ is larger than on a smooth wall. Normalizing on the wall shear stress, there is a significant increase in the normal turbulence intensity and a moderate increase in the Reynolds shear stress over the rough wall. The longitudinal turbulence intensity distribution is essentially the same for both surfaces. Normalized contributions to the Reynolds shear stress from the second (Q2) and fourth (Q4) quadrants are greater over the rough wall. The data indicate that not only are Q2 and Q4 events stronger on the rough wall but their frequency of occurrence is nearly twice as large for the rough wall as for the smooth wall. Comparison between smooth- and rough-wall spectra of the normal velocity fluctuation suggests that the strength of the active motion may depend on the nature of the surface.

An experimental investigation has been made on the Reynolds stress field in the interaction process between boundary layer and longitudinal vortex. Two types longitudinal vortex pair, namely common-flow up and common-flow down, were introduced into developed turbulent boundary layer from the free stream. Effect of the extra rate of strain, ∂W/∂z and ∂V/∂y, was examined in the profiles of three turbulent intensity components and Reynolds shear stress profiles. The boundary layer along symmetrical plane is classified into “cross flow” in case of cfd and “identifiable streamwise vortices” in case of cfu. The extra rates of strain modify magnitude of turbulent intensity and Reynolds shear stress in the inner layer in case of cfd. Otherwise, close to the wall in the case of cfu, local adverse pressure gradient is deduced from the fact that magnitude of transverse divergence is grater than that of spanwise convergence.

Experimental data are presented describing the transonic turbulent separated flow generated by an axisymmetric flow model. The model consisted of a circular-arc bump affixed to a straight circular cylinder aligned with the flow direction. Measurements of the mean velocity, turbulence intensity, and Reynolds shear stress profiles were made in the separated flow. These data revealed the dramatic changes in the shear stress levels as the flow passed from the interaction through to reattachment. Information on the behavior of the turbulence reaction to the imposed pressure gradients, as presented in this investigation, will be required for the development of the turbulence models used in predicting nonequilibrium turbulent flow fields.

This paper presents an experimental investigation on the characteristics of the development of turbulent horseshoe vortex flow in an evolving (intermediate stages and equilibrium) scour hole at square cylinder measured by an acoustic Doppler velocimeter (ADV). As the primary objective was to study the turbulent flow characteristics of horseshoe vortex in an evolving scour hole, the flow zone downstream the cylinder was not considered. Experiments were conducted with a square cylinder of width 12 cm embedded in the bed of uniform sand of median diameter 0.81mm under the approaching flow having undisturbed flow depth (= 25 cm) greater than twice the width of the cylinder and the depth-averaged approaching flow velocity (= 35.7 cm/s) equaling approximately 0.95 times the critical velocity for the uniform bed sand. The ADV flow measurements were taken inside the intermediate scour holes (having depths of 0.25, 0.5 and 0.75 times the equilibrium scour depth) and the equilibrium scour hole (frozen by spraying glue). The contours of the time-averaged velocities, turbulence intensities and Reynolds stresses at different azimuthal planes (0?, 45? and 90?) are presented. The change of the characteristics of horseshoe vortex flow associated with a downflow from intermediate stages to equilibrium condition of scour hole is revealed through the vector plots of the flow field at different azimuthal planes. Also, the flow characteristics of the horseshoe vortex are analyzed from the point of view of similarity with the velocity and turbulence characteristic scales. The important observation is that the flow and the turbulence intensities in horseshoe vortex flow in a developing scour hole are reasonably similar.

The response of a fully developed turbulent pipe flow, in which air was flowing at Reynolds number of 6.2×104, to a pulsewise disturbance due to an attached ring-type manipulator with rectangular cross section, was investigated by measuring turbulent flow field. The results indicate that the response of turbulence flow field delayed as compared with that of mean flow field. Close to the wall, the values of turbulence intensities and Reynolds shear stress profiles definitely decrease being incidental to the decrease of wall shear stress. Whereas at the locus of a ring height all the turbulence quantities take a peak value based on the development of the separated shear layer. The separated shear layer accompanying with the separation and the reattachment process have a intermediate feature between the case of upstanding roughness and downward step. Present flow field disturbed by the ring-type manipulator is classified into six region determined from the mean flow and turbulence flow characteristics.

Near-ground turbulence of the Bora wind in summertime

Direct Numerical Simulations (DNS) have been carried out for a spatially fully developed turbulent channel flow with one smooth wall and one k-type rough-wall surface to study the influence of a square bar roughness on the behaviour of unsteady turbulent flow. The flow state under investigation is a uniform acceleration from an initially steady turbulent flow. Results are compared with simulations of flows in a smooth wall channel undergoing a similar acceleration, and with quasi-steady behaviour obtained using steady flow simulations. The turbulence in the wall region and the buffer layer of an accelerating flow with a rough wall shows strikingly different behaviour from that of corresponding flows with smooth walls. The characteristic long delays in the response of the streamwise turbulent velocity and the different behaviours of the three turbulent velocity components that are exhibited in smooth wall flows are not seen in the rough wall flow. This is attributed to different turbulent production mechanisms in the two types of flows. Important differences are also seen in the wall shear stress responses one wall is rough and when both walls are smooth.

Thermal patterns on the smooth and rough walls in turbulent flows

Measurement of the local skin friction coefficient and re-examination of the Reynolds-number-effect on the mean flow quantities were made in a zero-pressure gradient, smooth wall, turbulent boundary layer. In order to do this fairly, the two-dimensionality of the flow field was carefully adjusted. And, the wall shear stress τω was determined by direct measurement using a floating element device. The Reynolds number based on the momentum thickness Rθ=U1θ / ν ranges from 840 to 6 220. The present experimental results of cf propose a new empirical formula which well represents the present experimental data and confirms that the Karman constant χ=0.41, which implies a slope of the logarithmic linear layer, is independent of Rθ down to Rθ=860, whereas the additive constant varies with Rθ. The magnitude of the wake parameter takes an asymptotic value of 0.62 if the Reynolds number is sufficiently high. The effect of Rθ on all the turbulent intensities is evident even within the viscous sublayer. The constant stress layer does exist in the Reynolds shear stress profiles despite that the constant value varies with Rθ.

This paper presents experimental investigation of upstream roughness and Reynolds number effects on the recirculation region over a smooth forward facing step. The upstream rough wall was produced from 1.5 mm sand grains and the Reynolds number based on step height, Reh, was varied from 2040 to 9130 for both the upstream smooth and rough walls. For the smooth wall, the reattachment length increased monotonically with Reh to an asymptotic value of 2.2 step heights for Reh ≥ 6380. Upstream roughness reduced the reattachment length by 44% because of larger momentum deficit and higher turbulence level in the rough wall boundary layer. The mean velocities and Reynolds stresses were also reduced by roughness. The Reynolds shear stress and production of turbulent kinetic energy showed high negative values at the leading edge of the step indicating counter-gradient diffusion. The implications of these results for standard eddy viscosity models are discussed.

The present paper reports a numerical study of fully developed turbulent flow over a flat plate with a step change from a smooth to a rough surface. The Reynolds number based on momentum thickness for the smooth flow was Reθ=5950. The focus of the study was to investigate the capability of the Reynolds-averaged Navier–Stokes (RANS) equations to predict the internal boundary layer (IBL) created by the flow configuration. The numerical solution used a two-layer k−ε model to implement the effects of surface roughness on the turbulence and mean flow fields via the use of a hydrodynamic roughness length y0. The prediction for the mean velocity field revealed a development zone immediately downstream of the step in which the mean velocity profile included a lower region affected by the surface roughness below and an upper region with the characteristics of the smooth-wall boundary layer above. In this zone, both the turbulence kinetic energy and Reynolds shear stress profiles were characterized by a significant reduction in magnitude in the outer region of the flow that is unaffected by the rough surface. The turbulence kinetic energy profile was used to estimate the thickness of the IBL, and the resulting growth rate closely matched the experimental results. As such, the IBL is a promising test case for assessing the ability of RANS models to predict the discrete roughness configurations often encountered in industrial and environmental applications.

Abstract

This paper describes an experimental study carried out in a laboratory flume with a smooth surface to investigate the effect of a surface wave on unidirectional current. The measured velocity data were analyzed within the framework of the phase averaging for combined wave–current flow and verified by velocity equations based on the phase-averaged Prandtl momentum-transfer theory. The results highlight the changes induced on the mean velocity profile, turbulence intensity, and Reynolds shear stress in a plane of symmetry due to the superposition of surface waves of different frequencies. Modifications in the mean velocities, the turbulence intensities, and the Reynolds shear stresses with respect to the current-only flow are explored. As the frequency of the surface waves in unidirectional current changes, the results show variations in the mean flow and in the turbulence statistics that may affect the local sediment mobility in the coastal region.

Turbulence features in a wall-wake flow downstream of a wall-mounted vertical cylinder

CFD Simulations of Abnormal Flow in Horizontal Pipes for Offshore Hydrocarbon Transport