Mean Skin Friction Coefficient Research Articles

Evaluation of hybrid RANS/LES models for prediction of flow around surface combatant and Suboff geometries

Predictive capabilities of hybrid RANS/LES models are compared for single-phase flow over a surface combatant at Re=5.3×106 and an appended DARPA Suboff model at Re=1.2×107. The turbulence models used in the study are: k–ω shear stress transport (SST)-URANS; Spalart Allmaras based detached eddy simulation (SA-DDES); k–ω based improved delayed detached eddy simulation (KW-IDDES); and a dynamic hybrid RANS/LES (DHRL) model coupling SST and implicit LES. For the surface combatant case, both SA-DDES and KW-IDDES predicted <1% resolved turbulence, whereas DHRL predicted up to 70% resolved turbulence. The SST-URANS, SA-DDES, KW-IDDES and DHRL predictions compared within 4.23%, 4.14%, 5.88% and 3.88% of the experimental data, respectively. For the Suboff case, both the SA-DDES and KW-IDDES predicted only limited resolved turbulence in the sail wake and downstream of the fins, whereas DHRL predicted resolved turbulence levels comparable to LES. The averaged error for the mean velocity profile at propeller plane for SST-URANS, SA-DDES, KW-IDDES and DHRL predictions was 6.4%, 8.9%, 13.1% and 3.0%, respectively. The corresponding errors for the turbulence variables were 44%, 70%, >100% and 28%, respectively. Overall, the DHRL model performed best among the turbulence models tested, and KW-IDDES performed worst. The study indicates that the DHRL approach has the potential to provide accurate mean flow predictions while resolving small-scale turbulent structures. Results also highlight the importance of the wall function formulation for accurately resolving mean skin friction coefficient, especially over smooth regions of the hull.

0640 円管内オリフィス下流域における壁面せん断応力測定(OS6-8 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

In the present study, the distributions of time-averaged and fluctuated wall shear stresses downstream from an orifice were measured using a Flow Vector Sensor of MEMS. The air flow was supplied to a circular pipe from a blow-down wind tunnel. The diameter of the pipe is D = 51 mm. The diameter ratio are β(=d/D;d,diameter of orifice) = 0.41, 0.5 and 0.62. Near the wall in the recirculating region, the minimum mean skin friction coefficient was located at approximately x/h = 5, and the maximum RMS intensity skin friction coefficient was located at approximately x/h = 7.5. The minimum mean skin friction coefficient decreased with increasing in Reynolds number. The maximum RMS intensity skin friction coefficient increased with increasing in the Reynolds number. The similarity of mean and RMS intensity skin friction coefficients with Reynolds number based a height of orifice and a bulk velocity at orifice was found.

Statistical estimates for channel flows driven by a pressure gradient

We present rigorous estimates for some physical quantities related to turbulent and non-turbulent channel flows driven by a uniform pressure gradient. Such results are based on the concept of stationary statistical solutions, which is related to the notion of ensemble averages for flows in statistical equilibrium. We provide a lower bound estimate for the mean skin friction coefficient and improve on a previous upper bound estimate for the same quantity; both estimates are derived in terms of the Reynolds number. We also present lower and upper bound estimates for the mean rate of energy dissipation, the mean longitudinal bulk velocity (in the direction of the pressure gradient), and the mean kinetic energy in terms of various physical parameters. In particular, we obtain an upper bound related to the energy dissipation law, namely that the mean rate of energy dissipation is essentially bounded by a non-dimensional universal constant times the cube of the mean longitudinal bulk velocity over a characteristic macro-scale length. Finally, we investigate the scale-by-scale energy injection due to the pressure gradient, proving an upper bound estimate for the decrease of this energy injection as the scale length decreases.

A numerical study of developing slurry flow in the entrance region of a horizontal pipe

In this paper, a simplified 3-D algebraic slip mixture (ASM) model was introduced to obtain the numerical solution in sand–water slurry flow. In order for the study to obtain the precise numerical solution in fully developed turbulent flow, the RNG k– ε turbulent model was used with the algebraic slip mixture model. An unstructured (block-structured), non-uniform grid was chosen to discretize the entire computation domain, and a control volume finite difference method (CVFDM) was applied to solve the governing equations. The mean pressure gradients from the numerical solution in the fully developed turbulent flow were compared with the authors’ experimental data and that in the open literature to validate the simulation results. The solutions were found to be in good agreement with the experimental data. The numerical investigations have focused mainly on illustrating and comparing the developing processes of volume fraction and density distributions, mean velocity profiles, and mean skin friction coefficient distributions in the entrance region. It was found that the difference of friction factors between single- and double-species slurries increases along flow direction in most part of the entrance region.

A MEMS skin-friction sensor for time resolved measurements in separated flows

A MEMS surface fence sensor for skin-friction measurements in separated flows has been developed and tested successfully in low-speed flows. The new sensor is able to distinguish between forward and reverse flow and features a temporal resolution of up to 1 kHz. Calibrations of the sensor have been obtained in the range of –0.7 N m-2≤τw≤0.7 N m-2 with a resolution of 0.02 N m-2. Comparative measurements with the wall-pulsed wire technique in a reverse-flow region show excellent agreement with respect to the mean skin-friction coefficient cf but also reveal some discrepancies for the fluctuating part cf′.

Numerical investigations of liquid–solid slurry flows in a fully developed turbulent flow region

In this paper, a simplified 3D algebraic slip mixture (ASM) model is introduced to obtain the numerical solution in sand–water slurry flow. In order for the study to obtain the precise numerical solution in fully developed turbulent flow, the RNG K– ε turbulent model was used with the ASM model. An unstructured (block-structured) non-uniform grid was chosen to discretize the entire computational domain, and a control volume finite difference method was used to solve the governing equations. The mean pressure gradients from the numerical solutions were compared with the authors’ experimental data and that in the open literature. The solutions were found to be in good agreement when the slurry mean velocity is higher than the corresponding critical deposition velocity. Moreover, the numerical investigations have displayed some important slurry flow characteristics, such as volume fraction distributions, slurry density, slip velocity magnitude, slurry mean velocity distributions, and slurry mean skin friction coefficient distributions in a fully developed section, that have never been displayed in the experiments.

Some effects of oscillation waveform and amplitude on unsteady turbulent shear flows

Some physical features of several unsteady separating turbulent boundary layers are presented for practical Reynolds numbers and reduced frequencies such as for helicopter and turbomachinery flows. The effects of unsteadiness amplitude and waveform are examined for flows along the floor of a converging and diverging wind tunnel test section. At the end of the converging portion, the mean skin friction coefficient normalized on the mean dynamic pressure is independent of the waveform and amplitude within low experimental uncertainties. In the detaching and detached portions of the flow, wall values of the fraction of time that the flow moves downstream of gamma sub pu, which is a separated flow state variable, shows that oscillation waveform and amplitude strongly influence the detached flow behavior. Distributions of gamma sub pu during a cycle indicate hysteresis within the detached flow and the effects of the higher harmonics of pressure gradient and velocity.

Forces On A Rough Bed In Oscillatory Flow

Results are presented of numerical and analytical solutions for the drag and lift forces exerted in laminar flow on a two-dimensionally rough bed by wave action. As far as is known there are no other results, either experimental or theoretical, for the lift forces under these conditions and only two measurements, both at very high Reynolds numbers, of the drag. The mean skin-friction coefficient is also calculated but the agreement with experimental results is poor. The reasons for this disagreement are discussed.