Effect of an adverse pressure gradient on the streamwise Reynolds stress profile maxima in a turbulent boundary layer

It is widely accepted that in a turbulent boundary layer （TBL） with adverse pressure gradient （APG） an outer peak usually appears in the profile of streamwise Reynolds stress. However, the effect of APG on this outer peak is not clearly understood. In this paper, the effect of APG is analysed using the numerical and experimental results in the literature. Because the effect of upstream flow is inherent in the TBL, we first analyse this effect in TBLs with zero pressure gradient on flat plates. Under the individual effect of upstream flow, an outer peak already appears in the profile of streamwise Reynolds stress when the TBL continues developing in the streamwise direction. The APG accelerates the appearance of the outer peak, instead of being a trigger.

On two distinct Reynolds number regimes of a turbulent square jet

The effects of Reynolds number on both large-scale and small-scale turbulence properties are investigated in a square jet issuing from a square pipe. The detailed velocity fields were measured at five different exit Reynolds numbers of 8 × 10^3 〈 Re 〈 5 × 10^4. It is found that both large-scale properties （e.g,, rates of mean velocity decay and spread） and small-scale properties （e.g., the dimensionless dissipation rate constant A = εL/（u^2）^3/2） are dependent on Re for Re ≤ 3 ×10^4 or Reλ ≤ 190, but virtually become Re-independent with increasing Re or Reλ. In addition, for Reλ 〉 190, the value ofA = εL/（u^2）^3/2 in the present square jet converges to 0.5, which is consistent with the observation in direct numerical simulations of box turbulence, but lower than that in circular jet, plate wake flows, and grid turbulence. The discrepancies in critical Reynolds number and A = εL/（u^2）^3/2 among different turbulent flows most likely result from the flow type and initial conditions.

Modelling of pressure-strain correlation in compressible turbulent flow

Previous studies carried out in the early 1990s conjectured that the main compressible effects could be associated with the dilatational effects of velocity fluctuation. Later, it was shown that the main compressibility effect came from the reduced pressure-strain term due to reduced pressure fluctuations. Although better understanding of the compressible turbulence is generally achieved with the increased DNS and experimental research effort, there are still some discrepancies among these recent findings. Analysis of the DNS and experimental data suggests that some of the discrepancies are apparent if the compressible effect is related to the turbulent Mach number, Mt. From the comparison of two classes of compressible flow, homogenous shear flow and inhomogeneous shear flow （mixing layer）, we found that the effect of compressibility on both classes of shear flow can be characterized in three categories corresponding to three regions of turbulent Mach numbers： the low-Mr, the moderate-Mr and high-Mr regions. In these three regions the effect of compressibility on the growth rate of the turbulent mixing layer thickness is rather different. A simple approach to the reduced pressure-strain effect may not necessarily reduce the mixing-layer growth rate, and may even cause an increase in the growth rate. The present work develops a new second-moment model for the compressible turbulence through the introduction of some blending functions of Mt to account for the compressibility effects on the flow. The model has been successfully applied to the compressible mixing layers.

Delayed detached eddy simulations of fighter aircraft at high angle of attack

The massively separated flows over a realistic aircraft configuration at 40？, 50？, and 60？angles of attack are studied using the delayed detached eddy simulation（DDES）.The calculations are carried out at experimental conditions corresponding to a mean aerodynamic chord-based Reynolds number of 8.93 × 10~5 and Mach number of 0.088. The influence of the grid size is investigated using two grids, 20.0×10~6cells and 31.0 × 10~6 cells. At the selected conditions, the lift,drag, and pitching moment from DDES predictions agree with the experimental data better than that from the Reynoldsaveraged Navier–Stokes. The effect of angle of attack on the flow structure over the general aircraft is also studied, and it is found that the dominated frequency associated with the vortex shedding process decreases with increasing angle of attack.

Structural subgrid-scale modeling for large-eddy simulation: A review

Accurately modeling nonlinear interactions in turbulence is one of the key challenges for large-eddy simulation（LES） of turbulence. In this article, we review recent studies on structural subgrid scale modeling, focusing on evaluating how well these models predict the effects of small scales. The article discusses a priori and a posteriori test results. Other nonlinear models are briefly discussed, and future prospects are noted.

Tension and Drag Forces of Flexible Risers Undergoing Vortex-Induced Vibration

This paper presents the results of an experimental investigation on the variation in the tension and the distribution of drag force coefficients along flexible risers under vortex-induced vibration（VIV） in a uniform flow for Reynolds numbers（Re） up to 2.2×10^5. The results show that the mean tension is proportional to the square of the incoming current speed, and the tension coefficient of a flexible riser undergoing VIV can be up to 12. The mean drag force is uniformly and symmetrically distributed along the axes of the risers undergoing VIV. The corresponding drag coefficient can vary between 1.6 and 2.4 but is not a constant value of 1.2, as it is for a fixed cylinder in the absence of VIV. These experimental results are used to develop a new empirical prediction model to estimate the drag force coefficient for flexible risers undergoing VIV for Reynolds number on the order of 10^5, which accounts for the effects of the incoming current speed, the VIV dominant modal number and the frequency.

Effect of inertial particles with different specific heat capacities on heat transfer in particle-laden turbulent flow

The effect of inertial particles with different specific heat on heat transfer in particle-laden turbulent channel flows is studied using the direct numerical simulation（DNS） and the Lagrangian particle tracking method. The simulation uses a two-way coupling model to consider the momentum and thermal interactions between the particles and turbulence. The study shows that the temperature fields display differences between the particle-laden flow with different specific heat particles and the particle-free flow,indicating that the particle specific heat is an important factor that affects the heat transfer process in a particle-laden flow. It is found that the heat transfer capacity of the particle-laden flow gradually increases with the increase of the particle specific heat. This is due to the positive contribution of the particle increase to the heat transfer. In addition,the Nusselt number of a particle-laden flow is compared with that of a particle-free flow.It is found that particles with a large specific heat strengthen heat transfer of turbulent flow, while those with small specific heat weaken heat transfer of turbulent flow.

A Dip Structure in the Intrinsic Toroidal Rotation Near the Edge of the Ohmic Plasmas in EAST

Ion＇s toroidal velocity, vt, in both the outermost 4 cm of the confined region and the scrap-off layer of Ohmic L-mode plasmas in EAST was measured using Mach probes. At about 1 cm inside the separatrix a local minimum in vt was observed, from which a cocurrent rotation increased both inwards and outwards. The radial width of the vt dip was 1 cm to 2 cm, and both the density and electron temperature profiles exhibited steep gradients at this dip position. It was observed in both divertor and limiter configurations. To find out its origin, the toroidal torques induced by neutral friction, neoclassical viscosity, collisional perpendicular shear viscosity, ion orbit loss and turbulent Reynolds stress were estimated using the measured parameters. Our results indicate that in this particular parameter regime the neutral friction was the dominant damping force. The calculated cocurrent toroidal torque by the neoclassical viscosity dominates over those from the collisional perpendicular shear viscosity, ion orbit loss and turbulent Reynolds stress. These results are potentially important for the understanding of boundary conditions for the intrinsic toroidal momentum in tokamak plasmas.

Response of turbulent enstrophy to sudden implementation of spanwise wall oscillation in channel flow

The response of turbulent enstrophy to a sudden implementation of spanwise wall oscillation（SWO） is studied in a turbulent channel flow via direct numerical simulation. In the beginning of the application of SWO, a significant correlation is formed between ω′yand ω′z. A transient growth of turbulent enstrophy occurs, which directly enhances turbulent dissipation and drifts the turbulent flow towards a new lower-drag condition. Afterwards, the terms related to the stretching of vorticity（ωx, ω′y, and ωz）,the inclination of ω′yby ？w/？y, the turn of z by ？v′/？z, and the horizontal shear of z by ？w′/？x are suppressed due to the presence of SWO, leading to attenuation of the turbulent enstrophy.

Large-eddy simulation: Past, present and the future

Large-eddy simulation（LES） was originally proposed for simulating atmospheric flows in the 1960 s and has become one of the most promising and successful methodology for simulating turbulent flows with the improvement of computing power. It is now feasible to simulate complex engineering flows using LES. However, apart from the computing power, significant challenges still remain for LES to reach a level of maturity that brings this approach to the mainstream of engineering and industrial computations. This paper will describe briefly LES formalism first, present a quick glance at its history, review its current state focusing mainly on its applications in transitional flows and gas turbine combustor flows, discuss some major modelling and numerical challenges/issues that we are facing now and in the near future, and finish with the concluding remarks.

Flow regime and head loss in a drip emitter equipped with a labyrinth channel

Labyrinth channels are widely adopted in emitter designs to regulate the water flow.The flow regime and the head loss of labyrinth channels have significant impacts on the hydraulic performance of emitters.In this study,the flow behavior of water passing through an emitter channel is observed using the micro particle image velocimetry（PIV）,and the head loss during the flow is analyzed for an emitter with a triangular labyrinth channel.The results show that the flow regime is consistent with the classical theory of hydraulics governing straight channels,even when the cross-sectional area is very small（as small as 0.5 mm×0.5 mm）.The critical Reynolds number from laminar to turbulent flows in a labyrinth channel is approximately in a range between 43 and 94.The local head loss factor decreases as the Reynolds number increases for labyrinth channels with smaller cross-sectional areas,such as 0.5 mm×0.5 mm and 1.0 mm×1.0 mm.The local head loss factor is not related to the Reynolds number and is only a function of the boundary conditions of the labyrinth channel when the Reynolds number exceeds approximately 1 000（for cross-sectional areas of 1.5 mm×1.5 mm and 2.0 mm×2.0 mm）.The ratio of the local head loss to the total head loss total（ /）j fh h first increases and then remains nearly constant as the Reynolds number increases in the labyrinth channel.The head loss in the labyrinth channel is almost equal to the local head loss,and total（ h_j/h_（ftctal））is approximately 0.95 for cross-sectional areas of greater than 1.0 mm×1.0 mm.These results can be used for optimizing the design of emitter channels.

A new method of LES verification and validation for attached turbulent cavitating flow

The large eddy simulation(LES)is used to resolve the flow structure in the cavitating turbulent flow around the Clark-Y hydrofoil coupled with a homogeneous cavitation model.A new method is proposed in this paper to calculate the LES error of the time-averaged streamwise velocity for the LES verification and validation(V&V).From the instantaneous cavity patterns,it is demonstrated that the predicted results agree fairly well with the experimental data.With this new proposed method,the LES errors can be easily and effectively calculated with a limited mesh number,and the method might be used in the other applications of the LES V&V.Results of the LES errors obtained by the new method show that the relatively steady flow can be simulated with small errors,while the complex flow structures at the cavity shedding region might lead to an increase of errors in the LES modeling.In addition,the distributions of the resolved Reynolds stresses are used to estimate the influences of the cavitation on the turbulent fluctuations.Results indicate that the turbulent fluctuations for the cavitating flow are much larger in magnitude as compared to the cases without cavitation.