Rough-wall Turbulent Boundary Layers Research Articles

Rough Wall Turbulent Boundary Layers in Shallow Open Channel Flow

An experimental study was undertaken to investigate the effects of roughness on the structure of turbulent boundary layers in open channels. The study was carried out using a laser Doppler anemometer in shallow flows for three different types of rough surface, as well as a hydraulically smooth surface. The flow Reynolds number based on the boundary layer momentum thickness ranged from 1400 to 4000. The boundary layer thickness was comparable with the depth of flow and the turbulence intensity in the channel flow varied from 2 to 4 percent. The defect profile was correlated using an approach which allowed both the skin friction and wake strength to vary. The wake parameter was observed to vary significantly with the type of surface roughness in contradiction to the “wall similarity” hypothesis. Wall roughness also led to higher turbulence levels in the outer region of the boundary layer. The profound effect of surface roughness on the outer region as well as the effect of channel turbulence on the main flow indicates a strong interaction, which must be accounted for in turbulence models. [S0098-2202(00)00803-8]

Effect of different surface roughnesses on a turbulent boundary layer

The classical treatment of rough wall turbulent boundary layers consists in determining the effect the roughness has on the mean velocity profile. This effect is usually described in terms of the roughness function D U+. The general implication is that different roughness geometries with the same D U+ will have similar turbulence characteristics, at least at a sufficient distance from the roughness elements. Measurements over two different surface geometries (a mesh roughness and spanwise circular rods regularly spaced in the streamwise direction) with nominally the same D U+ indicate significant differences in the Reynolds stresses, especially those involving the wall-normal velocity fluctuation, over the outer region. The differences are such that the Reynolds stress anisotropy is smaller over the mesh roughness than the rod roughness. The Reynolds stress anisotropy is largest for a smooth wall. The small-scale anisotropy and interniittency exhibit much smaller differences when the Taylor microscale Reynolds number and the Kolmogorov-normalized mean shear are nominally the same. There is nonetheless evidence that the small-scale structure over the three-dimensional mesh roughness conforms more closely with isotropy than that over the rod-roughened and smooth walls.

95/06415 Investigation of the effect of a smooth strip on rough-wall turbulent boundary layers

95/06415 Investigation of the effect of a smooth strip on rough-wall turbulent boundary layers

Investigation of the effect of a smooth strip on rough-wall turbulent boundary layers

A series of experiments was conducted to study the effects of a smooth strip on heat transfer and fluid dynamics in the turbulent boundary layer on an otherwise rough surface. The first 0.9m of the test section is rough, followed by 0.1-m smooth strip, and the remaining 1.4m is rough. The rough surface is composed of 1.27-mm diameter hemi-spheres spaced 2-diameters apart in staggered arrays. The experiments include measurements of Stanton number distributions as well as mean temperature, mean velocity, and turbulence intensity profiles. The results are compared with previously published data from experiments with a rough leading portion and a smooth final portion and from experiments on an all-rough surface. Over the smooth strip, Stanton number decreases by almost 45% relative to the all-rough value; however, the Stanton number distribution recovers typical rough-wall behavior in a short distance. The Stanton number measurements are compared with predictions using the discrete-element method. In general, the agreement is excellent. Mean velocity and turbulence intensity profiles show the flow downstream of the strip to rapidly attain rough-wall behavior in the near region, while requiring more distance to exhibit a complete rough-wall behavior.

Anisotropy of the Reynolds stresses in a turbulent boundary layer on a rough wall

The measured anisotropy invariants of the Reynolds stress tensor in a self-preserving rough wall turbulent boundary layer indicate that the anisotropy is significantly smaller than in a smooth wall layer.

The use of orthogonal X-wire arrays for structure investigation in a turbulent boundary layer

This note focuses on details of the experimental technique used to obtain simultaneous velocity measurements from two orthogonal arrays of X-wires and its potential for extracting information on various aspects of the organized motion. The experiments were carried out in a rough wall turbulent boundary layer in an attempt to obtain some insight into the 3-D nature of the large scale motion. Although information is only obtained in two orthogonal planes, it is shown that the technique can provide useful information about the 3-D nature of the large structures. Relative to large scale structure detections in the (x, y) plane, the most probable pattern in the (x, z) plane consists of counter-rotating vortical motions of approximately equal strength. Co-rotating patterns are strongly asymmetrical.

Scaling of the bursting period in turbulent rough wall boundary layers

Measurements in laboratory rough wall turbulent boundary layers have been made over a sufficient Reynolds number range for independence on the Reynolds number to be established adequately. The data have been analyzed with respect to the average period between ejections and sweeps, using methods previously applied to smooth wall flows. The results clearly show that the period is independent of Reynolds number when it is scaled on outer variables.

Roughness element shape effects on heat transfer and skin friction in rough-wall turbulent boundary layers

Stanton number and skin friction coefficient results are presented for zero pressure gradient incompressible turbulent boundary layer air flows over two rough surfacescomposed of 1.27 mm base diameter by 0.635 mm high truncated cones spaced in staggered arrays two ( L/d 0 = 2) and four diameters( L/d 0 = 4)

Coherent Structure of a d-Type Rough Wall Boundary Layer in a Transitionally Rough and a Fully Rough Regime.

The Reynolds number effect on the turbulent structure of a d-type rough wall turbulent boundary layer was studid experimentally in terms of the coherent structure. Mean bursting frequency, conditional averaged velocity signals and fractional contribution of the stress-generating motions were obtained with both the VITA method and the four-quadrant analysis, respectively. The effect of the three parameters, namely, bulk Reynolds number Rθ, roughness Reynolds number and friction parameter uτ/U1, were considered in the present study. Even scaled with the inner, outer and mixed variables, the nondimensional bursting frequency depends on the Reynolds number. The relative magnitude of fractional stress contribution of ejection to that of sweep increases with decreasing Reynolds number.

Turbulent drag reduction of a d-type rough wall boundary layer with longitudinal ribs placed with in the grooves. Reduction rate and time mean properties.

In order to control the turbulent energy production and transport processes due to the coherent vortices associated with the bursting phenomenon in a d-type rough wall turbulent boundary layer, longitudinal thin ribs were placed within the transverse grooves with a suitable spanwise spacing. Direct measurements of the local skin friction coefficient evidently show the effectiveness of drag reduction using the longitudinal ribs. Maximum drag reduction rates to d-type rough wall flow and to smooth wall flow are -10% and -3%, respectively. The drag reduction rate can be reasonably expressed in terms of the rib Reynolds number. Comparisons of some mean flow properties between the modified flow and the d-type rough wall flow provide evidence that the present passive control device reduces the turbulent energy production rate.

Rough-Wall Turbulent Boundary Layers

This review considers theoretical and experimental knowledge of rough-wall turbulent boundary layers, drawing from both laboratory and atmospheric data. The former apply mainly to the region above the roughness sublayer (in which the roughness has a direct dynamical influence) whereas the latter resolve the structure of the roughness sublayer in some detail. Topics considered include the drag properties of rough surfaces as functions of the roughness geometry, the mean and turbulent velocity fields above the roughness sublayer, the properties of the flow close to and within the roughness canopy, and the nature of the organized motion in rough-wall boundary layers. Overall, there is strong support for the hypothesis of wall similarity: At sufficiently high Reynolds numbers, rough-wall and smooth-wall boundary layers have the same turbulence structure above the roughness (or viscous) sublayer, scaling with height, boundary-layer thickness, and friction velocity.

Structure of rough-wall turbulent boundary layers

Recent experiments have shown that, in rough-wall turbulent boundary layers, drag varies systematically with the spanwise aspect ratio λz (span/height) of roughness elements. In this paper, the effect of λz on turbulence structure has been examined. Based on λz, the roughness in a transversely grooved surface (λz≫1) is the opposite extreme of model plant canopies (λz≪1) studied in wind tunnels, whereas sandgrain is an intermediate type [λz=O(1)]. Second-, third-, and fourth-order turbulence moments have been measured in turbulent boundary layers over transversely grooved and smooth surfaces and compared with available turbulence structure measurements over other types of surfaces. The near-wall turbulence structure is found to vary with λz. The instantaneous motions involved in the flux of shear stress near the wall in smooth and transversely grooved surfaces are opposite in sign to those in three-dimensional roughness. The former is explained in terms of hairpin vortices alone while the latter group is modeled to have an additional vortex, viz., the so-called necklace vortex which straddles a three-dimensional roughness element near its base.

Rough-wall turbulent boundary layers in the transition regime

An experimental investigation of turbulent boundary layers over two-dimensional spanwise groove and three-dimensional sandgrain roughnesses in the transition regime between hydraulically smooth and fully rough conditions is presented. It is found that a self-preserving state can be reached in boundary layers developing over both d-type groove and sandgrain roughnesses, and that the drag of a k-type rough wall can be reduced by lowering the spanwise aspect ratio of the roughness elements. The two roughness Reynolds numbers defining the boundaries of the transition regime of the k-type roughnesses are shown to decrease with increasing roughness-element spanwise aspect ratio, and the upper critical transition Reynolds number is shown to determine the roughness behavior in both the transition and fully rough regime.

Drag reducing outer-layer devices in rough wall turbulent boundary layers

The ability of outer-layer devices to reduce wall shear stress over a substantial streamwise distance in rough-wall turbulent boundary layers has been studied experimentally. The devices examined are a pair of thin flat ribbons placed in tandem as well as those having symmetric airfoil sections. The wall conditions examined are smooth, d- and k-type transverse-groove and sandgrain roughnesses. The wall drag is found to be reduced from the respective normal levels in all rough walls. All k-type rough walls exhibit a similar level of relative wall drag reduction which is also smaller than that in a smooth-wall. The d-type rough walls exhibit a transitional behaviour — the relative wall drag reduction drops from the smooth wall level to that of the k-type roughness with increasing roughness Reynolds number. However, the absolute reductions in the local wall shear stress are similar in both the rough and smooth walls. On the other hand, the relative reductions are lower in the rough walls because of a higher reference drag which is caused by the unique presence of a pressure component on which the devices are not as effective.

Turbulence Models for High-speed, Rough-Wall Boundary Layers

7 <5 Three distinct approaches have been investigated for the calculation of heat, momentum, and mass transfer in rough-wall turbulent boundary layers. Emphasis is placed on the high-speed flow regime. Of particular interest is the turbulent kinetic energy model, which has been modified to incorporate a two-phase flow concept such that the formulation allows explicit consideration of the effects of roughness shape, pattern, and density. The assessment consists of comparisons between numerical results and experimental data. These preliminary results suggest the following: 1) a local roughness-induced heating reduction is possible in high-speed flow; 2) the specification of nominal roughness height alone is not sufficient to define the rough-wall geometry characteristics; and 3) a strong Mach number effect on rough-wall heat transfer and shear stress distributions may exist. More accurate and complete data are needed in the high-speed regime to verify advanced numerical models of the type considered herein, which are required to adequately predict high-speed rough-wall boundary-layer behavior.

Conditional statistics of Reynolds stress in rough-wall and smooth-wall turbulent boundary layers

Quadrant analysis has been used to investigate the events contributing to the Reynolds shear stress in zero-pressure-gradient turbulent boundary layers over regularly arrayed rough surfaces of several different densities, and over a smooth surface. By partitioning the stress into ejections, sweeps, and inward and outward interactions, it is shown that sweeps account for most of the stress close to rough surfaces, and that the relative magnitude of the sweep component increases both with surface roughness and with proximity to the surface. The sweep-dominated region delineates a ‘roughness sublayer’ with a depth of up to several roughness element heights, in which the turbulence characteristics depend explicitly on the roughness. In the remainder of the inner (or constant-stress) layer, and in the outer layer, the flow obeys familiar similarity laws with respect to surface roughness.The difference ΔS0between the fractional contributions of sweeps and ejections to the stress is shown to be well related everywhere to the third moments of the streamwise and normal velocity fluctuations. Experimental proportionalities are established between the third moments and δS0, and are shown to agree with predictions made from cumulant-discard theory.The time scale for the passage of large coherent structures past a fixed point,T, is assumed proportional to the mean time between occurrences in a specified quadrant of an instantaneous stressu'w’at leastHtimes the local mean stressu'w’, whereHis a threshold level. For both the ejection and sweep quadrants and for any choice ofH, it is found thatTscales with the friction velocityu*and the boundary-layer thickness δ, such thatTu*/δ is invariant with change of surface roughness.

Closure to “Discussion of ‘An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers’” (1977, ASME J. Fluids Eng., 99, p. 510)

Closure to “Discussion of ‘An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers’” (1977, ASME J. Fluids Eng., 99, p. 510)

Discussion: “An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers” (Lee, B. E., and Soliman, B. F., 1977, ASME J. Fluids Eng., 99, pp. 503–509)

Discussion: “An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers” (Lee, B. E., and Soliman, B. F., 1977, ASME J. Fluids Eng., 99, pp. 503–509)

An Investigation of the Forces on Three Dimensional Bluff Bodies in Rough Wall Turbulent Boundary Layers

A study has been made of the influence of grouping parameters on the mean pressure distributions experienced by three dimensional bluff bodies immersed in a turbulent boundary layer. The range of variable parameters has included group density, group pattern and incident flow type and direction for a simple cuboid element form. The three flow regimes associated with increasing group density are reflected in both the mean drag forces acting on the body and their associated pressure distributions. A comparison of both pressure distributions and velocity profile parameters with established work on two dimensional bodies shows close agreement in identifying these flow regime changes. It is considered that the application of these results may enhance our understanding of some common flow phenomena, including turbulent flow over rough surfaces, building ventilation studies and environmental wind around buildings.