A turbulent boundary layer over a two-dimensional rough wall (original) (raw)
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Near wall organization of a turbulent boundary layer over a two- dimensional rough wall
Particle Image Velocimetry (PIV) measurements and Planar Laser Induced Fluorescence (PLIF) visualizations have been made in a turbulent boundary layer over a rough wall. The roughness consists of square bars placed transversely to the flow at a pitch to height ratio of 11. The near wall region flow is dominated by coherent structures associated with the shear layers which originate at the downstream edge of a roughness element and reattach on the wall upstream of the subsequent element. Shear layer vortices shed downstream a roughness elements impact on the consecutive element producing intermittent form drag and strong turbulence production.
Turbulent flow and organized motions over a two-dimensional rough wall
Particle image velocimetry measurements at relatively low Reynolds number in a turbulent boundary layer over a two-dimensional roughened surface are presented and compared to those for a smooth wall. The main goal of the study was to give a contribution to the understanding of the behavior of the turbulent organized motions, when the influence of the wall roughness on the turbulent structures may be expected to be noticeable, i.e., for relatively high roughness and relatively low Reynolds number. In addition, the flow behavior with respect to the wall similarity hypothesis in the case of moderately large values of the roughness height k and moderately low Reynolds number was tested. The behavior of the turbulent coherent motions appears to be similar for the two flows in the outer layer. Quantitative differences, observed mainly in the roughness sublayer, are enlightened comparing spatial velocity correlations and vortex population density and strength in the wall-normal-streamwise plane and in planes parallel to the wall. Mean flows expressed in velocity defect law, inner scaled streamwise Reynolds normal stress, and Reynolds shear stresses profiles show excellent agreement in the outer layer between the rough wall and the smooth surface. Wall normal Reynolds normal stress profiles, instead, show some small discrepancy, but within the measurement uncertainty.
41st AIAA Fluid Dynamics Conference and Exhibit, 2011
The combined impact of irregular surface roughness and moderate favorable-pressuregradient conditions (K ≈ 3 − 3.5 × 10 −7) on the structure of a turbulent boundary layer was assessed using stereo particle image velocimetry (PIV) measurements in the wall-normalspanwise plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales. These measurements were compared to measurements of smooth-wall flow under the identical favorable-pressure gradient (FPG) condition to reveal the combined impact of roughness and FPG conditions on the underlying structure of the flow. Instantaneous fields revealed low momentum and high momentum regions (LMRs and HMRs) alternating in the spanwise direction that contribute heavily to the Reynolds shear stress. In the roughwall FPG case, more intense sweeps and ejection events were observed within the LMRs and HMRs compared with the smooth-wall case. Two point correlations were analyzed to explore the average spatial structure of the flow. While two-point correlations of velocity reflected the basic signature that LMRs are bounded by HMRs, and vice-versa, correlations of velocity fields only embodying the largest spatial scales revealed an even higher degree of spanwise coherence in these motions.
Structure of Rough Wall Turbulent Boundary Layers at Relatively High Reynolds Number
The effect of two different types of surface roughness on a turbulent boundary layer was studied using 2-component LDV measurements in a relatively high speed water tunnel. One roughness consists of square bars at a streamwise spacing p equal to 2k (k is the roughness height). The other consists of cylindrical rods with p/k equal to 4. Both roughnesses are aligned in a direction transverse to the flow. Measurements of the turbulent field were carried out over a wide range of Reynolds numbers, (1 500 < R θ < 23 000) based on the momentum thickness. Comparison of the turbulent field between different surfaces is made at R θ ∼ 9 000. This study supports previous attempts to classify rough surfaces according to their turbulence characteristics, and extends them by providing measurements at high Reynolds numbers for three distinct surface conditions.
Turbulence structure in a boundary layer with two-dimensional roughness
2010
Turbulence measurements for a zero pressure gradient boundary layer over a twodimensional roughness are presented and compared to previous results for a smooth wall and a three-dimensional roughness (Volino, Schultz & Flack, J. Fluid Mech., vol. 592, 2007, p. 263). The present experiments were made on transverse square bars in the fully rough flow regime. The turbulence structure was documented through the fluctuating velocity components, two-point correlations of the fluctuating velocity and swirl strength and linear stochastic estimation conditioned on the swirl and Reynolds shear stress. The two-dimensional bars lead to significant changes in the turbulence in the outer flow. Reynolds stresses, particularly v 2 + and −u v + , increase, although the mean flow is not as significantly affected. Large-scale turbulent motions originating at the wall lead to increased spatial scales in the outer flow. The dominant feature of the outer flow, however, remains hairpin vortex packets which have similar inclination angles for all wall conditions. The differences between boundary layers over twodimensional and three-dimensional roughness are attributable to the scales of the motion induced by each type of roughness. This study has shown three-dimensional roughness produces turbulence scales of the order of the roughness height k while the motions generated by two-dimensional roughness may be much larger due to the width of the roughness elements. It is also noted that there are fundamental differences in the response of internal and external flows to strong wall perturbations, with internal flows being less sensitive to roughness effects.
Structure of transitionally rough and fully rough turbulent boundary layers
Journal of Fluid Mechanics, 1986
Structural charact er istics of transitionally rough and fully rough turbulent boundary layers are pr esented. These were measured in flows at different roughness Reynolds numbers dev eloping over uniform spheres roughness. Inner regions of the longitudinal component of normal Reynolds stress profiles and log regions of mean profiles continuously cha nge in the transitionally rough regime, as the roughness Reynolds number , R e1,;, varies. These properties asymptotically approach fully rough behaviour as Rek increa ses, and smooth behaviour at low R ek. Profiles of other Reynolds-str ess tensor components, turbulence kinetic energy, turbulence-kinetic-energy production , and the turbulence-kinetic-energy dissipation are also given, along with appropriate scaling va riab les. Fully rough, one-dimensional spectra of longitudinal velocity fluctuation s from boundary-layer inner regions are similar to smooth-wall results for k 1 y > 0.2 when non-dimensionalized using distanc e from the wally as the length sca le, and (r / p)½ as the velocity scale, where T is local shear stress, p is static den sity , and k 1 is one-dimensional wavenumber in th e flow direction. 1. Introduction Transitionally rough turbulent boundary layers exist for the rang e of roughness Reynolds numbers between smooth and fully rough flow regimes. The roughn ess Reynolds numb er , R ek, is defined as the ratio of the equivalent sandgrain roughness height , k 5 , to viscous length, v/U 7 • Rek may also be viewed as the non-dimensional sandgrain roughness height in y+ coordinat es, where y+ = yU 7 /v, with Ur equal to the friction velocity , y the normal di sta nce from the wall, and v the kinematic viscosity. Consequently, the magnitude of Re1,; may be compared to the y+ region where viscous stresses are important , which is ordinaril y at th e outer edge of the buffer zone, say y+ ~ 40. \Vhen R ek is much less than 40 (Rek < 5-10), wall roughness does not affect th e viscous-stress reg ion , the viscous sublayer is totally intact and undisturbed , and th e flow is 'smoo th '. In boundary layers where Jiek is greater than 40 (Rek > 55-90) , viscous effects are negligible and the flow is ' fully rough'. When the viscous sublayer is only partially altered by the presence of roughne ss, the flow is ' tran sitionally rough '. Her e, both bluff-body-form drag, and viscosity influence the near-wall flow, and log regions of mean-velocity profil es show dep endence on both v/Ur and k 5 , where the former quantity is more important as 's mooth ' flow is approached, and the latt er, nea r ' fully rough' flow conditions. Of studies of the structure of boundary layers dev eloping over rough surfaces, Grass P. M. Ligrani and R. J. Moffat (1971) employed flow visualization to elucidate features of the ejection-sweep cycle of events. He observed that ejection of low-speed fluid away from walls and the subsequent inrush of high-speed fluid toward walls are a common flow structure regardless of boundary-roughness condition. Differences in structure result as different dominant mod es of instability prevail for different wall roughnesses. Liu, Kline & Johnston (1966) also used flow visualization, but studied boundary-layer flow over d-type roughness consisting of square bars. Perry, Schofield & Joubert (1969) , Wood & Antonia (197 5), and others have also studied boundary layers developing over d-type roughness. The differences between d-type and k-type wall roughness ar e most apparent in pipe flows: wall roughness is d-type rather than k-type when flow properties scale on pipe diameter instead of roughness size. Pimenta, Moffat & Kays (1975) and Coleman, Moffat & Kays (1977) studied characteristics of boundary layers developing over the same k-type rough surface used in the present study: coplanar uniform spheres packed in the most dense array. Coleman focused on fully rough layers with acceleration, whereas Pimenta studied zero-pressure-gradient flows , both with and without transpiration. Pimenta showed that normalized profiles of the longitudinal component of turbulence intensity in fully rough layers had different shapes to transitionally rough profiles measured at one freestream velocity. Antonia & Luxton (1971) present energy balances for the turbulent kinetic energy and mean flow. From the former, the authors indicate that a large-eddy diffusion process may be relevant: large energy loss by diffusion from the inner layer being consistent with high turbulence intensity observed in the outer layer. Schetz & Nerney (1977) found that profiles of longitudinal-turbulence intensity normaliz ed with respect to the free-stream velocity increase with either surface roughness or injection rate in a study of flow near the surface of an axisymmetric body. Andreopoulos & Bradshaw (1981) present Reynolds-stress-tensor component profiles and tripleproduct profiles in smooth and fully rough boundary layers. According to these authors, within 3-5 roughness heights, normalized triple products are larger in fully rough flows than in flows over smooth surfaces. Recent studies of spectra measur ed in flows developing over rough surfaces have been made by Perry & Abell (1977) , Champagne (1978) , and Sabot , Saleh & Comte-Bellot (1977). Perry & Abell (1977) studied flow in pipes with hexagonal weave roughness, and showed that, for y+ > 100, rough-wall spectra can be predicted from
Effect of Roughness on Wall-Bounded Turbulence
Flow Turbulence and Combustion, 2004
Direct numerical simulation of turbulent incompressible plane-channel flow between a smooth wall and one covered with regular three-dimensional roughness elements is performed. While the impact of roughness on the mean-velocity profile of turbulent wall layers is well understood, at least qualitatively, the manner in which other features are affected, especially in the outer layer, has been more controversial. We compare results from the smooth- and rough-wall sides of the channel for three different roughness heights of h += 5.4, 10.8, and 21.6 for Re τ of 400, to isolate the effects of the roughness on turbulent statistics and the instantaneous turbulence structure at large and small scales. We focus on the interaction between the near-wall and outer-layer regions, in particular the extent to which the near-wall behavior influences the flow further away from the surface. Roughness tends to increase the intensity of the velocity and vorticity fluctuations in the inner layer. In the outer layer, although the roughness alters the velocity fluctuations, the vorticity fluctuations are relatively unaffected. The higher-order moments and the energy budgets demonstrate significant differences between the smooth-wall and rough-wall sides in the processes associated with the wall-normal fluxes of the Reynolds shear stresses and turbulence kinetic energy. The length scales and flow dynamics in the roughness sublayer, the spatially inhomogeneous layer within which the flow is directly influenced by the individual roughness elements, are also examined. Alternative mechanisms involved in producing and maintaining near-wall turbulence in rough-wall boundary layers are also considered. We find that the strength of the inner/outer-layer interactions are greatly affected by the size of the roughness elements.
Turbulent boundary layer over 2D and 3D large-scale wavy walls
Physics of Fluids, 2015
An experimental investigation of the flow over two-and three-dimensional large-scale wavy walls was performed using high-resolution planar particle image velocimetry in a refractive-index-matching (RIM) channel. The 2D wall is described by a sinusoidal wave in the streamwise direction with amplitude to wavelength ratio a/λ x = 0.05. The 3D wall is defined with an additional wave superimposed on the 2D wall in the spanwise direction with a/λ y = 0.1. The flow over these walls was characterized at Reynolds numbers of 4000 and 40 000, based on the bulk velocity and the channel half height. Flow measurements were performed in a wall-normal plane for the two cases and in wall-parallel planes at three heights for the 3D wavy wall. Instantaneous velocity fields and time-averaged turbulence quantities reveal strong coupling between large-scale topography and the turbulence dynamics near the wall. Turbulence statistics show the presence of a well-structured shear layer that enhances the turbulence for the 2D wavy wall, whereas the 3D wall exhibits different flow dynamics and significantly lower turbulence levels. It is shown that the 3D surface limits the dynamics of the spanwise turbulent vortical structures, leading to reduced turbulence production and turbulent stresses and, consequently, lower average drag (wall shear stress). The likelihood of recirculation bubbles, levels and spatial distribution of turbulence, and rate of the turbulent kinetic energy production are shown to be severely affected when a single spanwise mode is superimposed on the 2D sinusoidal wall. Differences of one and two order of magnitudes are found in the turbulence levels and Reynolds shear stress at the low Reynolds number for the 2D and 3D cases. These results highlight the sensitivity of the flow to large-scale topographic modulations; in particular the levels and production of turbulent kinetic energy as well as the wall shear stress.
PIV experiments in rough-wall, laminar-to-turbulent, oscillatory boundary-layer flows
Experiments in Fluids, 2013
Exploratory measurements of oscillatory boundary layers were conducted over a smooth and two different rough beds spanning the laminar, transitional and turbulent flow regimes using a multi-camera 2D-PIV system in a small oscillatory-flow tunnel (Admiraal et al. in J Hydraul Res 44(4):437-450, 2006). Results show how the phase lag between bed shear stress and free-stream velocity is better defined when the integral of the momentum equation is used to estimate the bed shear stress. Observed differences in bed shear stress and phase lag between bed shear stress and free-stream velocity are highly sensitive to the definition of the bed position (y = b). The underestimation of turbulent stresses close to the wall is found to explain such differences when using the addition of Reynolds and viscous stresses to define both the bed shear stress and the phase lag. Regardless of the flow regime, in all experiments, boundary-layer thickness reached its maximum value at a phase near the flow reversal at the wall. Friction factors in smooth walls are better estimated using a theoretical equation first proposed by Batchelor (An introduction to fluid dynamics. Cambridge University Press, Cambridge, 1967) while the more recent empirical predictor of Pedocchi and Garcia (J Hydraul Res 47(4):438-444, 2009a) was found to be appropriate for estimating friction coefficients in the laminar-to-turbulent transition regime. This article is part of the Topical Collection on Application of Laser Techniques to Fluid Mechanics 2012.
Journal of Fluid Mechanics, 2007
The Reynolds number dependence of the structure and statistics of wall-layer turbulence remains an open topic of research. This issue is considered in the present work using two-component planar particle image velocimetry (PIV) measurements acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility in western Utah. The Reynolds number (δu τ /ν) was of the order 10 6 . The surface was flat with an equivalent sand grain roughness k + = 18. The domain of the measurements was 500 < yu τ /ν < 3000 in viscous units, 0.00081 < y/δ < 0.005 in outer units, with a streamwise extent of 6000ν/u τ . The mean velocity was fitted by a logarithmic equation with a von Kármán constant of 0.41. The profile of u v indicated that the entire measurement domain was within a region of essentially constant stress, from which the wall shear velocity was estimated. The stochastic measurements discussed include mean and RMS profiles as well as two-point velocity correlations. Examination of the instantaneous vector maps indicated that approximately 60 % of the realizations could be characterized as having a nearly uniform velocity. The remaining 40 % of the images indicated two regions of nearly uniform momentum separated by a thin region of high shear. This shear layer was typically found to be inclined to the mean flow, with an average positive angle of 14.9 • .