A Numerical Investigation to Study Roughness Effects in Oscillatory Flows

2017 ◽  
Author(s):  
Chaitanya D. Ghodke ◽  
Sourabh V. Apte
Keyword(s):  
Shear Rate ◽  
Reynolds Stress ◽  
Large Diameter ◽  
Turbulent Energy ◽  
Sand Particle ◽  
Roughness Effects ◽  
Oscillatory Flows ◽  
Wake Field ◽  
Double Averaging ◽  
Acceleration Cycle

Effects of roughness on the near-bed turbulence characteristics in oscillatory flows are studied by means of particle-resolved direct numerical simulations (DNS). Two particle sizes of diameter 375 and 125 in wall units corresponding to the large size gravel and the small size sand particle, respectively, in a very rough turbulent flow regime are reported. A double-averaging technique is employed to study the nature of the wake field, i.e., the spatial inhomogeneities at the roughness length scale. This introduced additional production and transport terms in double-averaged Reynolds stress budget, indicating alternate pathways of turbulent energy transfer mechanisms. Budgets of normal components of Reynolds stress reveal redistribution of energy from streamwise component to other two components and is attributed to the work of pressure in both flow cases. It is shown that the large diameter gravel particles significantly modulate the near-bed flow structures, leading to pronounced isotropization of the near-bed flow; while in the sand case, elongated horseshoe structures are found as a result of high shear rate. Effect of mean shear rate on the near-bed anisotropy is significant in the sand case, while it is minimal for the gravel-bed. Redistribution of energy in the gravel case showed reduced dependence on the flow oscillations, while for the sand particle it is more pronounced towards the end of an acceleration cycle.

A turbulence velocity scale for curved shear flows

1975 ◽  
Vol 70 (1) ◽  
pp. 37-57 ◽  
Author(s):  
Ronald M. C. So
Keyword(s):  
Reynolds Stress ◽  
Shear Flows ◽  
Turbulent Energy ◽  
Streamline Curvature ◽  
Velocity Scale ◽  
Dimensional Boundary ◽  
Free Constant ◽  
Turbulence Velocity ◽  
Turbulence Length ◽  
Energy Balances

Assuming the turbulence length scale to be unaffected by streamline curvature, a turbulence velocity scale for curved shear flows is derived from the Reynolds-stress equations. Closure of the equations is obtained by using the scheme of Mellor & Herring (1973), and the Reynolds-stress equations are simplified by invoking the two-dimensional boundary-layer approximations and assuming that production of turbulent energy balances viscous dissipation. The resulting formula for the velocity scale has one free parameter, but this can be determined from data for non-rotating unstratified plane flows. Consequently there is no free constant in the derived expression. A single value of the constant is found to give good agreement between calculated and measured values of the velocity scale for a wide variety of curved shear flows.The result is also applied to test the validity and extent of the analogy between the effects of buoyancy and streamline curvature. This is done by comparing the present result with that obtained by Mellor (1973). Excellent agreement is obtained for the range −0·21 [les ]Rif[les ] 0·21. Therefore the present result provides direct evidence in support of the use of a Monin–Oboukhov (1954) formula for curved shear flows as proposed by Bradshaw (1969).

Effects of Low Uniform Relative Roughness on Single-Phase Friction Factors in Microchannels and Minichannels

2007 ◽  
Author(s):  
Timothy P. Brackbill ◽  
Satish G. Kandlikar
Keyword(s):  
Large Diameter ◽  
Reynolds Numbers ◽  
Transition To Turbulence ◽  
Roughness Effects ◽  
Relative Roughness ◽  
Flow Parameters ◽  
Friction Factors ◽  
Turbulent Flow Regime ◽  
Critical Reynolds Numbers ◽  
Large Diameter Pipes

Nikuradse’s [1] work on friction factors focused on the turbulent flow regime in addition to being performed in large diameter pipes. Laminar data was collected by Nikuradse, however only low relative roughness values were examined. A recent review by Kandlikar [2] showed that the uncertainties in the laminar region of Nikuradse’s experiments were very high, and his conclusion regarding no roughness effects in the laminar region is open to question. In order to conclusively resolve this discrepancy, we have experimentally determined the effects of relative roughness ranging from 0–5.18% in micro and minichannels on friction factor and critical Reynolds numbers. Reynolds numbers were varied from 30 to 7000 and hydraulic diameters ranged from 198μm to 1084μm. There is indeed a roughness effect seen in the laminar region, contrary to what is reported by Nikuradse. The resulting friction factors are well predicted using a set of constricted flow parameters. In addition to higher friction factors, transition to turbulence was observed at decreasing Reynolds numbers as relative roughness increased.

Measurement on the Structure of the Reynolds Stress for the Turbulent Boundary Layer Flow Over a Two-Dimensional Escarpment With Mild Upwind Slope

2011 ◽  
Author(s):  
Bao-Shi Shiau ◽  
Ben-Jue Tsai
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Reynolds Stress ◽  
High Speed ◽  
Slope Angle ◽  
Turbulent Energy ◽  
Quadrant Analysis ◽  
Two Dimensional ◽  
Spectrum Distribution ◽  
Layer Flow

Experimental measurement study on the structure of the Reynolds stress and turbulence spectrum for wind flows over a two-dimensional escarpment with mild upwind slope (slope angle θ = 15°) were performed in the wind tunnel. The Quadrant analysis was applied to analyze the experimental data and yield the structure of the Reynolds stress. In according to the quadrant analysis, the Reynolds stress is composed of four events of the stress components, i.e. outward interaction, ejection (low-speed fluid upward), inward interaction, and sweep (high-speed fluid downward). Measured results show that: (1) Measurements of the structure of the Reynolds stress reveal that both the sweep and ejection events are the major contributors to the Reynolds stress for flow around the two dimensional escarpment with mild upwind slope. (2) The contributions to the Reynolds stress made by ejection events and sweep events are almost the same at heights Z/Zref greater than 0.2 for different downstream distances along the mild slope of escarpment. Here Zref is the turbulent boundary layer thickness. When flow reached the top of the slope of escarpment, stress fractions of ejection event and sweep event, S2 and S4 increased significantly. (3) The he turbulent energy spectrum distribution was not found very dominant spectrum peak as winds flow over the mild upwind slope and top surface of escarpment.

scholarly journals Reynolds Stress and Turbulent Energy Production in a Tidal Channel

2002 ◽  
Vol 32 (4) ◽  
pp. 1242-1251 ◽  
Author(s):  
Tom P. Rippeth ◽  
Eirwen Williams ◽  
John H. Simpson
Keyword(s):  
Reynolds Stress ◽  
Energy Production ◽  
Turbulent Energy ◽  
Tidal Channel

The Scaling and Structure of the Estuarine Bottom Boundary Layer

2005 ◽  
Vol 35 (1) ◽  
pp. 55-71 ◽  
Author(s):  
Mark T. Stacey ◽  
David K. Ralston
Keyword(s):  
Boundary Layer ◽  
Reynolds Stress ◽  
Friction Velocity ◽  
Turbulent Energy ◽  
Buoyancy Flux ◽  
Bottom Boundary Layer ◽  
Stress Profile ◽  
Layer Height ◽  
Bottom Boundary ◽  
Boundary Layer Height

Abstract A two-week dataset from a partially and periodically stratified estuary quantifies variability in the turbulence across the tidal and spring–neap time scales. These observations have been fit with a two-parameter model of the Reynolds stress profile, which produces estimates of the time variation of the bottom boundary layer height and the friction velocity. Conditions at the top of the bottom boundary layer indicate that the dynamics governing the development of the estuarine bottom boundary layer are different on ebb tides than on flood tides. The asymmetry in the flow is explained by consideration of the strain-induced buoyancy flux, which is stabilizing on ebb tides and destabilizing on flood tides. Based on these observations, a scaling approach to estimating estuarine bottom boundary layer parameters (height and friction velocity) is presented, which includes a modified Monin–Obukhov length scale to account for the horizontal buoyancy flux created by the sheared advection. Comparison with the observations of boundary layer height and friction velocity suggests that this approach may be successful in predicting bottom boundary layer parameters in estuaries and coastal regions with significant horizontal buoyancy fluxes. Comparison between the strain-induced buoyancy flux and shear production indicates that the straining of the density field is an important contributor to the turbulent kinetic energy budget and creates an asymmetry in turbulent energy between ebb and flood tides. It appears that the structure of the turbulence, specifically the ratio of the Reynolds stress to the turbulent energy, is also modified by tidal straining, further accentuating the ebb–flood asymmetries.

scholarly journals Analysis of Trailing Vortex Structure and Turbulent Features of High-Speed Train in Vacuum Pipeline

2021 ◽  
Vol 39 (5) ◽  
pp. 1601-1608
Author(s):  
Hongjiang Cui ◽  
Shenghui Wu ◽  
Ying Guan
Keyword(s):  
Reynolds Stress ◽  
High Speed ◽  
Flow Direction ◽  
Three Dimensional ◽  
Transient State ◽  
Turbulent Energy ◽  
High Speed Train ◽  
Detached Eddy Simulation ◽  
Long Distance ◽  
Low Dissipation

Through the improved delay-detached eddy simulation (IDDES), this paper establishes a 1:1 model for a high-speed train, and simulates the transient state of the train running 600km/h in a vacuum pipeline with the pressure of 1,000Pa. The results show that, following the Ω criteria, a pair of counterrotating vortexes can be captured, which alternatively shed near the tip of the last carriage, and propagate over a long distance along the flow direction. The motion and expansion of the vortexes are clearly three-dimensional (3D). Judging by the physical meaning of vortexes, the high vorticity vortexes mainly concentrate near the tip of the last carriage, while the low vorticity vortexes scatter across the wake zone. The latter vortexes have a low dissipation rate and are dominated by rotation. The turbulent energy and Reynolds stress of the wake field are very obvious near the tip of the last carriage, and attenuate quickly along the flow direction. This means the vortexes near the tip of the last carriage face a strong shear effect, and undergo apparent dissipation. Low turbulent energy and Reynolds stress are distributed in the downstream far from the tip of the last carriage, i.e., the interaction zone between vortexes and the ground / inner pipe wall.

Structure of the Reynolds stress near the wall

1972 ◽  
Vol 55 (1) ◽  
pp. 65-92 ◽  
Author(s):  
W. W. Willmarth ◽  
S. S. Lu
Keyword(s):  
Reynolds Stress ◽  
Experimental Studies ◽  
Recovery Process ◽  
Reynolds Numbers ◽  
Turbulent Energy ◽  
Streamwise Velocity ◽  
Special Events ◽  
Individual Contributions ◽  
The Mean ◽  
Intermittency Factor

Experimental studies of the flow field near the wall in a turbulent boundary layer using hot-wire probes are reported. Measurements of the productuvare studied using the technique of conditional sampling with a large digital computer to single out special events (bursting) when large contributions to turbulent energy and Reynolds stress occur. The criterion used to determine when the productuvis sampled is that the streamwise velocity at the edge of the sublayer should have attained a certain value. With this simple criterion we find that 60% of the contribution to$\overline{uv}$is produced when the sublayer velocity is lower than the mean. This result is true at both low,Rθ= 4230, and high,Rθ= 38 000, Reynolds numbers. With a more strict sampling criterion, that the filtered sublayer velocity at two side-by-side points should be simultaneously low and decreasing, individual contributions to$\overline{uv}$as large as 62$\overline{uv}$have been identified. Additional measurements using correlations between truncateduandvsignals reveal that the largest contributions to the Reynolds stress and turbulent energy occur whenu< 0,v> 0 during an intense bursting process and the remainder of the contributions occur during a less intense recovery process. Thus, contributions to the turbulent energy production and Reynolds stress at a point near the wall are of relatively large magnitude, short duration and occur intermittently. A rough measure of the intermittency factor foruvat a point near the wall is 0·55 since 99% of the contribution to$\overline{uv}$is made during only 55% of the total time.

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