Surface Roughness Effects in Near-Bed Turbulence: Implications to Sediment Entrainment

Size: px

Start display at page:

Download "Surface Roughness Effects in Near-Bed Turbulence: Implications to Sediment Entrainment"

Cecily Pearson
5 years ago
Views:

1 Surface Roughness Effects in Near-Bed Turbulence: Implications to Sediment Entrainment by A.N. Papanicolaou, P. Diplas, C. L. Dancey, and M. Balakrishnan Journal of Engineering Mechanics, Vol. 127, No. 3,

Introduction The existing methods for predicting sediment flux with shear stress are mainly

They are good only when the range of flow and bed configuration is within those originally

They are developed for densely packed bed configurations, which corresponds to skimming flow regimes.

2 Introduction The existing methods for predicting sediment flux with shear stress are mainly empirical, this causes problems because: 1.They are good only when the range of flow and bed configuration is within those originally calibrated; 2.They are developed for densely packed bed configurations, which corresponds to skimming flow regimes. Different flow regimes due to packing density Isolated flow Wake interference Skimming flow 3. Various roughness geometries may produce the same effect on the mean velocity, but the turbulent characteristics, which account for sediment entrainment, may be different.

3 Introduction (cont d) Different Bed Roughness Geometry Turbulence Structure Sediment Entrainment Ejection Inward Outward Sweep Bursting: cycles of outward, ejection, inward, and sweeps Sweeps cause the initiation of bed-load motion in a stream; ejections are primarily responsible for the particles suspension motion. (e.g. Clever and Yates, 1976) Outward interactions are also responsible for bed-load transport. (Nelson et al., 1995) 2 2 Normal Reynolds stresses ( u, w ) may be more important in sediment transport than the shear stress component ( uw). (Sterk et al., 1998) The companion paper of the author investigated the second part of this study and provided qualitative assessment of the turbulent flow structures triggering sediment entrainment. (Papanicolaou et al. 1999)

4 Introduction (cont d) Different Bed Roughness Geometry Turbulence Structure Sediment Entrainment The aim of this paper is focused on the first part of the study: to provide the evidence of the influence of roughness geometry on the turbulence structure. The study is done through an experimental approach. Three bed roughness configuration is constructed to create isolated, wake interference, and skimming flow regimes. Turbulence was recorded using 3D LDV. Normal and shear Reynolds stresses were calculated and quadrant analysis was performed under various roughness geometries.

Experiment Setup Experiment descriptions

8mm in diameter - packing configurations:

measurement: 3D LDV, 20 measurements/sec

5 Experiment Setup Experiment descriptions - flume size: 20.5m(L) x 0.6m(W) x 0.3m (H) - test section: 3m(L) x 0.4m(W) - roughness particles: lead spheres 8mm in diameter - packing configurations: 2% (isolated flow) - velocity measurement: 3D LDV, 20 measurements/sec - other parameters: 50% (wake interference) 70% (skimming flow)

6 Reynolds Stress Distribution Results - Differences are found among various roughness configuration/flow regime at near bed flows. - The sign change of Reynolds shear stress for 2% roughness configuration implies that secondary flow effects are significant for isolated flow regime only.

7 Results (cont d) Reynolds and Normal Stresses - Values of U 2 is at least 6-7 times higher than those of W 2 and UW and may serve as a better predictor for sediment entrainment. (Clifford et al., 1991; Nelson et al., 1995; William et al., 1989) - Due to flow separation of the isolated flow regime, the frequency of the events is relatively low in the case of the 2% roughness configuration.

8 Results (cont d) Quadrant Analysis - The tilting of the joint frequency distribution shows different trend as roughness geometry changes. - For isolated flow regime (2%), the inward and outward interactions occupy the highest percentage of time within a bursting cycle. This agrees previous studies from Kaftori et al. (1998) and Bennett and Best (1995).

9 Conclusions The results from this study show that the roughness geometry affects the turbulence structure and flow characteristics. The magnitude of U 2 suggest that normal stress may be a more relevant component to sediment entrainment than the Reynolds shear stress. The quadrant analysis shows that the roughness geometry affects the frequency of the events within a bursting cycle. This suggests the underprediction of the sediment transport base on empirical relations in low-density packing cases.

Quadrant method application to the study of the beginning of sediment motion of sedimentary particles

V Conferência Nacional de Mecânica dos Fluidos, Termodinâmica e Energia MEFTE 214, 11 12 etembro 214, Porto, Portugal APMTAC, 214 Quadrant method application to the study of the beginning of sediment motion