Mean Skin Friction Research Articles

Control of turbulent channel flow using a plasma-based body force

A steady streamwise co-flow body force originated by plasma actuators was used to control wall turbulence. The effects of such plasma-based body force on channel flow were investigated by direct numerical simulations. We found such body force can suppress turbulence fluctuations. The underlying mechanism was proposed in the discussion of Reynolds-stress budget and turbulent structures. To explore the possibility of drag reduction, we examined the skin-friction coefficient Cf in the controlled flow. Two opposite effects were identified to account for the behavior of Cf. The first is large streamwise velocity gradient in the normal direction at wall induced by the body force, thus large increase in Cf occurs over the forcing region. The second is reduction in the Reynolds stresses, which will contributes to the decrease in Cf. Away from the forcing region, the skin-friction coefficient rapidly decreases, even less than that of the baseline. Furthermore, force strength and spacing were considered in the parametric study. The results showed that the plasma-based body force can reduce mean skin-friction drag by 13.4%.

A DPIV study on the effects of separation distance upon the vortical behaviour of jet–cylinder impingements

An experimental investigation based on laser-induced fluorescence (LIF) and digital particle image velocimetry (DPIV) was conducted to provide a better understanding into the effects of jet–cylinder separation distance on the vortical structures and dynamic behaviour resulting from jet impingements upon convex cylinders. Separation distances of 1, 2 and 4 jet diameters were investigated, while cylinder–jet diameter ratio was maintained at 2 throughout. LIF and DPIV results show that jet ring-vortex initiation, wall-separated vortex initiation, vortex dipole formation and vortex separation occur further away from the impingement point as the separation distance decreases. Varying the separation distance also influences the recirculating wake region size at the cylinder lee sides, as well as producing two distinct flow modes between adjacent vortex dipoles along the cylinder straight edges. Mean and instantaneous skin friction coefficient distributions determined from DPIV results not only showcase the different effects of separation distance on the wall shear stress, but also illustrate how the flow dynamics influence the local wall shear stress levels. On the other hand, wall-separated vortex initiation, vortex dipole formation and separation events incur little distinctive changes upon the local wall shear stress distributions that may ease their unique identification.

Sustainable drag reduction in turbulent Taylor-Couette flows by depositing sprayable superhydrophobic surfaces.

We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at Re=8.0×10(4). We show that the mean skin friction coefficient C(f) in the presence of the superhydrophobic coating can be fitted to a modified Prandtl-von Kármán-type relationship of the form (C(f)/2)(-1/2)=Mln (Re(C(f)/2)(1/2))+N+(b/Δr)Re(C(f)/2)(1/2) from which we extract an effective slip length of b≈19 μm. The dimensionless effective slip length b(+)=b/δ(ν), where δ(ν) is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as b(+)∼Re(1/2) in the limit of high Re.

Turbulence and skin friction modification in channel flow with streamwise-aligned superhydrophobic surface texture

Direct numerical simulations of turbulent flow in a channel with superhydrophobic surfaces (SHS) were performed, and the effects of the surface texture on the turbulence and skin-friction coefficient were examined. The SHS is modeled as a planar boundary comprised of spanwise-alternating regions of no-slip and free-slip boundary conditions. Relative to the reference no-slip channel flow at the same bulk Reynolds number, the overall mean skin-friction coefficient is reduced by 21.6%. A detailed analysis of the turbulence kinetic energy budget demonstrates a reduction in production over the no-slip phases, which is explained by aid of quadrant analysis of the Reynolds shear stresses and statistical analysis of the turbulence structures. The results demonstrate a significant reduction in the strength of streamwise vortical structures in the presence of the SHS texture and a decrease in the Reynolds shear-stress component ⟨R12⟩ which has a favorable influence on drag over the no-slip phases. A secondary flow which is set up at the edges of the texture also effects a beneficial change in drag. Nonetheless, the skin-friction coefficient on the no-slip features is higher than the reference levels in a simple no-slip channel flow. The increase in the skin-friction coefficient is attributed to two factors. First, spanwise diffusion of the mean momentum from free-slip to no-slip regions increases the local skin-friction coefficient on the edges of the no-slip features. Second, the drag-reducing capacity of the SHS is further reduced due to additional Reynolds stresses, ⟨R13⟩.

Large eddy simulation of unsteady transitional flow on the low-pressure turbine blade

The aim of this paper is to predict the phenomenon of laminar separation, transition and reattachment in a low-pressure turbine (LPT). Self-developed large eddy simulation program of compressible N-S equations was used to describe the flow structures of T106A LPT blade profile at Reynolds number of 1.1×105 based on the exit isentropic velocity and chord length. The computational results show the distributions of time-averaged wall-static pressure coefficient and mean skin-friction coefficient on the blade surface. The locations of laminar separation and reattachment points occur around 87% and 98% axial chord, which agree well with experiment data. The two-dimensional shear layer is gradually unstable along the downstream half of the suction side as a result of the spanwise fluctuation and the roll up of shear layer via Kelvin-Helmholtz (KH) instability. Three-dimensional motions appear near 84% axial chord which later triggers spanwise vortexes and streamwise vortexes, leading to transition to turbulence in the separation bubble. Through introducing the concept of dissipation function, the high loss mainly comes from the places where strong shear layer and intense fluctuation exist. Furthermore, the separation region is only an accumulation center of the low-energy fluid rather than an area of loss source.

Flow and heat transfer measurements in a planar offset attaching jet with a co-flowing wall jet

Measurements were performed to characterize the flow, mean skin friction, and heat transfer rate in flows consisting of a turbulent planar jet with Reynolds number of approximately 42,000 offset one jet height above a wall and a co-flowing wall jet with an initial height 0.18 times the offset distance (Hs). The results were compared to previous measurements for flows with similar offset jets and a co-flowing wall jet with initial height 0.5 times the offset distance. Low velocity wall jets in both geometries were quickly entrained into the offset jet increasing the attachment length of the offset jet and reducing the turbulent fluctuations and the heat transfer rate in the near field of the attaching jet. These changes increased with the mass flow rate of the wall jet as less mass is recirculated upstream from the attachment region. Higher velocity wall jets in both geometries initially remained attached to the wall causing a recirculation region to form between the jets that induce periodic motions that significantly increase the wall normal turbulent fluctuations in the flow. The periodic motions in the flows with the smaller wall jets significantly increased the pressure fluctuations on the wall below the near field flow and enhanced the heat transfer in this region. This differed from the results for the offset jet with the larger co-flowing wall jet where the flow fluctuations did not have a significant effect on the wall pressure fluctuations and the heat transfer below the flow. The difference in the effect of the periodic motions for flows appears to be due to differences in the forcing of the offset jet by the periodic motions and differences in the interaction between the recirculation region and the wall due to the height and mass flux of the wall jets.

Free Convection Flow along an Infinite Vertical Plate with Time-Varying Suction

Free convection flow up a vertical infinite plate subjected to a periodic suction is discussed when the plate temperature oscillates with respect to time over a constant mean. In particular, the response of the mean velocity and temperature profiles to the fluctuating components of the suction velocity and the plate temperature is analysed. For small amplitudes, the mean skin friction increases with the frequency while the mean heat transfer at the plate is independent of it but varies linearly as the product of the amplitudes. For Prandtl number P=1, solutions are also presented when the suction velocity in any arbitrary function of time provided it changes slowly.

Simulation and validation of a spatially evolving turbulent boundary layer up to [formula omitted

Results of a finely resolved large-eddy simulation (LES) of a spatially developing zero-pressure-gradient turbulent boundary layer up to a Reynolds number of Reθ=8300 are presented. The very long computational domain provides substantial assessment for suggested high Reynolds number (Re) trends. Statistics, integral quantities and spectral data are validated using high quality direct numerical simulation (DNS) ranging up to Reθ=4300 and hot-wire measurements covering the remaining Re-range. The mean velocity, turbulent fluctuations, skin friction, and shape factor show excellent agreement with the reference data. Through utilisation of filtered DNS, subtle differences between the LES and DNS could to a large extent be explained by the reduced spanwise resolution of the LES. Spectra and correlations for the streamwise velocity and the wall-shear stress evidence a clear scale-separation and a footprint of large outer scales on the near-wall small scales. While the inner peak decreases in importance and reduces to 4% of the total energy at the end of the domain, the energy of the outer peak scales in outer units. In the near-wall region a clear k-1 region emerges. Consideration of the two-dimensional spectra in time and spanwise space reveals that an outer time scale λt≈10δ99/U∞, with the boundary layer thickness δ99 and free-stream velocity U∞, is the correct scale throughout the boundary layer rather than the transformed streamwise wavelength multiplied by a (scale independent) convection velocity. Maps for the covariance of small scale energy and large scale motions exhibit a stronger linear Re dependence for the amplitude of the off-diagonal peak compared to the diagonal one, thereby indicating that the strength of the amplitude modulation can only qualitatively be assessed through the diagonal peak. In addition, the magnitude of the wall-pressure fluctuations confirms mixed scaling, and pressure spectra at the highest Re give a first indication of a −7/3 wave number dependence.

Multiscale Fluid Mechanics and Modeling

In recent years, there has been a tremendous growth of activity on multiscale modeling and computation. In particular, the multiscale hybrid numerical methods are those that combine multiple models defined at fundamentally different length and time scales within the same overall spatial and temporal domain. For examples, a framework of hybrid continuum and molecular dynamics multiscale method has been developed to simulate micro- and nanoscale fluid flows, which combines the continuum computational fluid dynamics (CFD) or the mesoscopic lattice Boltzmann method for the bulk flow region and the atomistic molecular dynamics for the interface region. The similar idea of constrained variation has also been used in developing multiscale fluid turbulent models for constrained dynamic subgrid-scale stress model, Reynolds stress constrained large eddy simulation (RSC-LES) for wall-bounded turbulent flows with massive separation and heat flux constrained large eddy simulation. For RSC-LES, our model is able to solve the traditional log-layer mismatch problem in RANS/LES approaches and can predict mean velocity, turbulent stress and skin friction coefficients more accurate than pure dynamic large eddy models and traditional detached eddy simulation using the same grid resolution. Our results demonstrate the capability of multiscale simulation methods for complex fluid systems and the necessity of physical constraints on the multiscale methods.

A211 円管内オリフィス板下流域の流れ場と壁面せん断応力 : レイノルズ数による影響(OS2 保全設備診断技術(3))

In the present study, the distributions of time-average and fluctuated wall shear stresses were measured using a flow vector sensor of MEMS and velocity downstream from an orifice using laser Doppler velocimetry. The air flow was supplied to a circular pipe from a blow-down wind tunnel. The diameter of the pipe were D = 51 mm, 100 mm and 194 mm. The diameter ratio were β (= d/D; d, diameter of orifice) = 0.41, 0.50 and 0.62. Near the wall in the recirculating region downstream from the orifice, the minimum mean skin friction coefficient was located at approximately x/h = 5, and the maximum RMS intensity skin friction coefficient at approximately x/h = 7. The absolute value of the minimum mean skin friction coefficient and the maximum RMS intensity skin friction coefficient decreased with increasing in Reynolds number. The similarity between skin friction coefficients of RMS and the FAC rate was confirmed.

Turbulent Heat Transfer From a Slot Jet Impinging on a Flat Plate.

The flow field and heat transfer of a plane impinging jet on a hot moving wall were investigated using one point closure turbulence model. Computations were carried out by means of a finite volume method. The evolutions of mean velocity components, vorticity, skin friction coefficient, Nusselt number and pressure coefficient are examined in this paper. Two parameters of this type of interaction are considered for a given impinging distance of 8 times the nozzle thickness (H/e = 8): the jet-surface velocity ratio and the jet exit Reynolds number. The flow field structure at a given surface-to-jet velocity ratio is practically independent to the jet exit Reynolds number. A slight modification of the flow field is observed for weak surface-to-jet velocity ratios while the jet is strongly driven for higher velocity ratio. The present results satisfactorily compare to the experimental data available in the literature for Rsj ≤ 1.The purpose of this paper is to investigate this phenomenon for higher Rsj values (0 ≤ Rsj ≤ 4). It follows that the variation of the mean skin friction and the Nusselt number can be correlated according to the surface-to-jet velocity ratios and the Reynolds numbers.

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.

Effect of MHD and Thermal Diffusion on Natural Convection Oscillatory Flow Past Plate with Viscous Heating

This communication investigates the unsteady two-dimensional oscillatory flow of an incompressible viscous fluid past an infinite vertical, porous flat plate with the effect of magnetic field. It is assume that free stream velocityoscillates in times about a constant mean. Assuming the periodic heat flux at the plate with the effect of viscous dissipation, the set of non-linear coupled differentialequations is solved by regular perturbationtechnique. The approximate solutions are obtained for velocity and temperature field. The effect of various parameters on mean flow velocity, transient velocity, mean skin friction and transient temperature are discussed and shown graphically.

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.

Reynolds-stress-constrained large-eddy simulation of wall-bounded turbulent flows

AbstractIn the traditional hybrid RANS/LES approaches for the simulation of wall-bounded fluid turbulence, such as detached-eddy simulation (DES), the whole flow domain is divided into an inner layer and an outer layer. Typically the Reynolds-averaged Navier–Stokes (RANS) equations are used for the inner layer, while large-eddy simulation (LES) is used for the outer layer. The transition from the inner-layer solution to the outer-layer solution is often problematic due to the lack of small-scale dynamics in the RANS region. In this paper, we propose to simulate the whole flow domain by large-eddy simulation while enforcing a Reynolds-stress constraint on the subgrid-scale (SGS) stress model in the inner layer. Both the algebraic eddy-viscosity model and the one-equation Spalart–Allmaras (SA) model have been used to constrain the Reynolds stress in the inner layer. In this way, we improve the LES methodology by allowing the mean flow of the inner layer to satisfy the RANS solution while small-scale dynamics is included. We validate the Reynolds-stress-constrained large-eddy simulation (RSC-LES) model by simulating three-dimensional turbulent channel flow and flow past a circular cylinder. Our model is able to predict mean velocity, turbulent stress and skin-friction coefficients more accurately in turbulent channel flow and to estimate the pressure coefficient after separation more precisely in flow past a circular cylinder compared with the pure dynamic Smagorinsky model (DSM) and DES using the same grid resolution. Furthermore, the computational cost of the RSC-LES is almost the same as that of DES.

On the Reliability of Eddy Viscosity Based Turbulence Models in Predicting Turbulent Flow past a Circular Cylinder Using URANS Approach

Turbulent flow past circular cylinder at moderate to high Reynolds number has been analysed employing an second-order time accurate pressure-based finite volume method solving two-dimensional Unsteady Reynolds Averaged Navier Stokes (URANS) equations for incompressible flow, coupled to eddy-viscosity based turbulence models. The major focus of the paper is to test the capabilities and limitations of the present turbulence model-based 2D URANS procedure to predict the phenomenon of Drag Crisis, usually manifested in reliable measurement data, as a sharp drop in the mean drag coefficient around a critical Reynolds number. The computation results are compared to corresponding measurement data for instantaneous aerodynamic coefficients and mean surface pressure and skin friction coefficients. Turbulence model-based URANS computations are in general found to be inadequate for correct prediction of the mean drag coefficients, the Strouhal number and also the coefficients of maximum fluctuating lift over the range of flow Reynolds number varying from 104 to 107.

An Experimental Study of a Turbulent Wall Jet on Smooth and Transitionally Rough Surfaces

The effect of surface roughness on the mean velocity and skin friction characteristics of a plane turbulent wall jet was experimentally investigated using laser Doppler anemometry. The Reynolds number based on the slot height and exit velocity of the jet was approximately Re = 7500. A 36-grit sheet was used to create a transitionally rough flow (44 &lt; ks+ &lt; 70). Measurements were carried out at downstream distances from the jet exit ranging from 20 to 80 slot heights. Both conventional and momentum-viscosity scaling were used to analyze the streamwise evolution of the flow on smooth and rough walls. Three different methods were employed to estimate the friction velocity in the fully developed region of the wall jet, which was then used to calculate the skin friction coefficient. This paper provides new experimental data for the case of a plane wall jet on a transitionally rough surface and uses it to quantify the effects of roughness on the momentum field. The present results indicate that the skin friction coefficient for the rough-wall case compared to a smooth wall increases by as much as 140%. Overall, the study suggests that for the transitionally rough regime considered in the present study, roughness effects are significant but mostly confined to the inner region of the wall jet.

Experimental study of near-wall turbulent characteristics in an open-channel with gravel bed using an acoustic Doppler velocimeter

This experimental study investigated the mean velocity profiles, skin friction and turbulent characteristics of a gravel bed over a wide range of roughness using an acoustic Doppler velocimeter (ADV). The median diameter of bed material ranged from 2 to 40 mm, and the normalized roughness heights ranged from 47 to 4,881 mm. The flow regime was fully developed turbulence with a Reynolds number in the range of 4.2 × 104–9.86 × 104. All velocity curves exhibited logarithmic distributions, and the log-law region was influenced greatly by both the roughness and the Reynolds number. Moreover, the roughness of the gravel bed exerted a strong effect on Reynolds stress, and the turbulence tended towards isotropic with increasing roughness. Using statistical analyses, the third-order turbulence moments, sweep, and ejection motions were also examined. The results of this experimental analysis present a contrast to the classical wall similarity hypothesis.

Accuracy of Direct Skin-Friction Measurements in High-Enthalpy Supersonic Flows

Measurement of skin friction and a survey of airflow momentum were conducted in a rectangular straight duct with airflow Mach number, unit Reynolds number, and stagnation enthalpy at the nozzle exit of 2.5, 2.7 x 10 7 m -1 , and 1.1 MJ/kg, respectively. Skin fiction was measured by a direct skin-friction sensor at the test section that was connected to the duct exit, with the state of airflow there also being surveyed. From the result of the measurement and survey at two streamwise locations with different lengths, mean skin friction was calculated by two different and independent methods. The mean skin friction calculated by the directly measured skin friction was compared with that calculated by the momentum conservation law in order to examine the accuracy of direct skin-friction measurement. The accuracy of skin-friction measurement using the direct skin-friction sensor in the test was found to be ±5% (root-sum-square value).

A Hybrid-Filter Approach to Turbulence Simulation

Germano (Theor Comput Fluid Dyn 17:225–331, 2004) proposed a hybrid-filter approach, which additively combines an LES-like filter operator (F) and a RANS-like statistical operator (E) using a blending function k: H = kF + (1 − k)E. Using turbulent channel flow as an example, we first conducted a priori tests in order to gain some insights into this hybrid-filter approach, and then performed full simulations to further assess the approach in actual simulations. For a priori tests, two separate simulations, RANS (E) and LES (F), were performed using the same grid in order to construct a hybrid-filtered field (H). It was shown that the extra terms arising out of the hybrid-filtered Navier–Stokes (HFNS) equations provided additional energy transfer from the RANS region to the LES region, thus alleviating the need for the ad hoc forcing term that has been used by some investigators. The complexity of the governing equations necessitated several modifications in order to render it suitable for a full numerical simulation. Despite some issues associated with the numerical implementation, good results were obtained for the mean velocity and skin friction coefficient. The mean velocity profile did not have an overshoot in the logarithmic region for most blending functions, confirming that proper energy transfer from the RANS to the LES region was a key to successful hybrid models. It is shown that Germano’s hybrid-filter approach is a viable and mathematically more appealing approach to simulate high Reynolds number turbulent flows.