Turbulent Duct Flow Controlled with Spanwise Wall Oscillations

The spanwise oscillation of channel walls is known to substantially reduce the skin-friction drag in turbulent channel flows. In order to understand the limitations of this flow control approach when applied in ducts, direct numerical simulations of controlled turbulent duct flows with an aspect ratio of A R = 3 are performed. In contrast to channel flows, the spanwise extension of the duct is limited. Therefore, the spanwise wall oscillation either directly interacts with the duct side walls or its spatial extent is limited to a certain region of the duct. The present results show that this spanwise limitation of the oscillating region strongly diminishes the drag reduction potential of the control technique. We propose a simple model that allows estimating the achievable drag reduction rates in duct flows as a function of the width of the duct and the spanwise extent of the controlled region.     

Brownian dynamics simulation of multiphase suspension of disc-like particles and polymers

A computational model is proposed for simulating the flow of polymer nanocomposites. This model is based on a multiphase suspension of disc-like particles and polymers. The particles are represented by oblate spheroid particles that interact with each other via the Gay-Berne (GB) potential, and the polymers are modeled by finitely extensible nonlinear elastic (FENE) chains that interact with each other via the repulsive Lennard-Jones potential. The interaction between an oblate spheroid particle and a FENE chain is also considered using a modified GB potential. A Brownian dynamics simulation of the shear flows of this system was conducted to investigate the orientation behavior of disc-like particles and the rheological properties of this system. The orientation of disc-like particles was affected by polymers, and the particles in a suspension were well aligned in flows because of the flow orientation property of polymers. The predicted shear viscosity exhibited shear thinning, and the normal stress differences agree qualitatively with experimental measurements of polymer/clay nanocomposites. The simulation results suggest that the present model has the potential to be used as a computational model for polymer nanocomposites.     

Experimental results for nucleating steam flow in a two-dimensional supersonic duct and comparison with theory

Results are presented of experiments conducted in a two-dimensional duct carrying a supersonic flow of condensing steam. The measurements comprised static pressure readings along the profiled surfaces of the duct and ‘fog’ droplet sizing using a light attenuation technique. Three sets of results for dry supercooled and nucleating steam flows are presented, are are compared with the predictions of a two-dimensional numerical calculation method.     

网栅后湍流流动沿流程衰减特性的实验研究

应用ＬＤＶ测试技术对垂直方管内三种不同栅距的网所形成的湍流流动做了详细测量,网栅栅距Ｍ分别为１４ｍｍ,１０ｍｍ,６ｍｍ实验给出了管内不同位置湍流流动的各种参数沿流动方向的衰减规律,并就三种网栅进行比较分析,以探讨这种网栅流动所具有共同规律和在不同栅距对湍流结构及脉动大小的不同特点,从而对这种网栅所形成的湍流流结构较为清晰的了解。     

Combustion of premixed fuel and air downstream of a plane sudden-expansion

 Experiments have been performed to quantify the isothermal and combusting flows downstream of a plane sudden-expansion. The detailed measurements correspond to an area expansion ratio of 2.86 and a Reynolds number of 20000, and the combusting flows comprised premixed methane and air over a range of equivalence ratios with emphasis on values of 0.72 and 0.92 which gave rise to smooth and rough combustion, respectively. The results show that the extent of asymmetry of the isothermal flows was reduced by coupling the pressures between the two recirculation regions, and by imposing oscillations at the half-wave or full-wave frequency of the duct, and by combustion. Periodic variations of flame shape, velocities, acceleration, and temperature were observed in sympathy with the dominant pressure oscillation of rough combustion, and the length of the recirculation zones varied from less than 0.5 to 3 step heights. Rich and lean limits were established for combustion within the duct and, whereas the flame blew off at the lean limit, it detached from the expansion at the rich limit and stabilised on the flange at the duct exit. Within these limits, there were ranges of equivalence ratios over which the flame stabilised on one of the two steps with incomplete combustion. The imposition of oscillations narrowed the range of equivalence ratios over which the flame could be stabilised but reduced the equivalence ratio of the lean limit at which the flame could be stabilised on both steps and the effect increased with amplitude and was greatest when the frequency of the imposed oscillations corresponded to that of the half-wave in the duct. An increase in the amplitude of flow oscillations, natural or imposed, caused the concentrations of NO _x measured at the duct exit to decrease. Active control of flows with high amplitude of oscillations produced the expected reductions, but not over the entire measured range of equivalence ratio, and the imposition of pressure oscillations at the second harmonic of the half-wave frequency had a greater effect and over a wider range. Received: 14 September 1998 / Accepted: 19 February 1999     

Technical note A simple shock tunnel driver gas detector

A device has been developed to detect the arrival of the driver gas in a shock tunnel. The detector is small enough to be used in conjunction with other experiments. It works by choking a duct when the specific heat ratio is increased past a critical value. Times are given for the onset of a 7.5% contamination level in flows with freestream enthalpies ranging from 3–9 MJ/kg. These results are compared with and are shown to be in agreement with measurements made with a mass spectrometer. Results displaying the rate at which the test gas is contaminated are also given.     

环形通道内湍流旋流流动的数值模拟

本文应用一种考虑湍流－旋流相互作用及湍流脉动各向异性的新的代数Ｒｅｙｎｏｌｄｓ应力模型,对环形通道内的湍流旋流流动进行了数值模拟,研究了改主为旋流流数,进口轴向速度及半径比等参数对环形通道内湍流流动的影响,以及对强化环形通道内传热的作用。     

Prediction of Turbulent Separated Flow in a Turnaround Duct Using Wall Functions and Adaptivity

This paper presents an application of adaptive remeshing to the prediction of turbulent separated flows. The paper shows that the κ - ε model with wall functions can predict separated flows along smooth curved surfaces. Success is achieved if the wall functions exhibit values of y⁺ close to 30, and if meshes are fine enough to guarantee that wall function boundary conditions are grid converged. Adaptive remeshing proves to be a very cost effective tool in this context. The methodology is demonstrated on a problem possessing a closed form solution to establish the performance and reliability of the proposed approach. The method is then applied to prediction of turbulent flow in an annular, axisymmetric turnaround duct (TAD). Predictions from two computational models of the TAD are compared with experimental measurements. The importance of appropriate meshes to achieve grid independent solutions is demonstrated in both cases. Better agreement with measurements is obtained when partially developed profiles of u, κ, and ε are specified at the TAD inlet.     

Energy-based decomposition of friction drag in turbulent square-duct flows

The generation of friction drag in turbulent duct flows has direct connection with statistical quantities and corresponding turbulence dynamics in the duct cross-section. In this study, we generalize the RD identity (Renard and Deck, 2016) to a ‘two-dimensional’ form which we exploit to decompose the mean friction drag in turbulent square-duct flows into contributions associated with viscosity, turbulence and cross-stream convection. The friction Reynolds number of the duct flows ranges from 220 to 2000. The scaling, spatial distribution and local normalization of the contributions to friction are investigated and compared with those in pipe and channel flows. As in other canonical flows, we find logarithmic growth of the turbulent contribution in contrast to the viscous one, the former thus becoming dominant at high enough Reynolds numbers. Whereas cross-stream convection has no net effect on friction, its contribution may be locally comparable to the other two, hence may be responsible for redistribution of friction along the duct perimeter.     

Two component LDV system with dual-differential acousto-optical frequency shift and its applications

This paper describes the main properties of the two component LDV system developed with the dual-differential frequency shift technique. It has the advantages of multiple functions, better SNR and high utilization of information. So it offers as a complete technique and method for measuring 2-D complex flows. By applying this system, 2-D turbulent separated flow measurements have been made in a duct with an asymmetrical sudden expansion and in a wind tunnel flow over a 2-D rib. The Reynolds numbers were at 5900 and 4500 respectively.     

Pulsed-wire anemometry

Pulsed-wire anemometry was first developed as a tool for making velocity and turbulence measurements in the late 1960s. It has been continuously refined and its potential exploited by a number of groups who have obtained reliable data in situations where the use of other techniques would have been extremely difficult. The technique is particularly useful in flows of high turbulence intensity and has therefore been used to greatest effect in separated flows. Although its range of applicability is much more restricted than that of laser-Doppler anemometry, it is an order of magnitude cheaper, and it does not require seeding of the flow, with all the attendant uncertainties.

First the physical basis of pulsed-wire anemometry is briefly described, and the major sources of experimental error are outlined for cases in which the technique is used both remote from and close to walls. Progress in the design of probes, which have been improved in a number of ways since the early days to reduce errors, is also outlined. The author then reviews the kinds of measurements that have been successfully made and the consequent improvements in the understanding of the physics of complex flows. Examples are drawn from a wide range of work, including some unusual applications like measurements in very low velocity gas mixtures. The paper closes with a summary of the limitations of the technique and an overall assessment of the likely potential for its use in future turbulence research.     

Transition of boundary layer flows in the presence of Goertler vortices

Transition of boundary layer flows in the presence of longitudinal counter-rotating Goertler vortices was experimentally investigated on a concave surface of 1.0 m radius of curvature in a perspex (plexiglass) curved rectangular duct connected to a low speed wind tunnel for a free-stream velocity range of 5.7–11.8 m/s. Quantitative measurements were carried out using a single sensor hot-wire anemometer, while the boundary layer transitions were detected using frequency spectrum method. The results confirm that in the presence of Goertler vortices, transition is initiated at the boundary layer upwash regions, and also agree well with the predicted values obtained using the two existing empirical transition criteria for concave surface boundary layer flows.     

Sensitivity of an asymmetric 3D diffuser to plasma-actuator induced inlet condition perturbations

Experiments were conducted for the flow in a straight-walled 3D diffuser fed by a fully developed turbulent duct flow. Previous work found that this diffuser has a stable 3D separation bubble whose configuration is affected by the secondary flows in the upstream duct. Dielectric barrier discharge plasma actuators were used to produce low-momentum wall jets to determine if the separation behavior could be modified by weak forcing. Actuators producing a streamwise force along the wall where separation occurred in the baseline flow had a relatively small effect. However, spanwise acting plasma actuators that produced a pair of streamwise vortices in the inlet section of the diffuser had a strong effect on the diffuser pressure recovery. The diffuser performance could be either improved or degraded depending on the actuation parameters, including the actuator modulation frequency, duty cycle, and drive voltage. Velocity profile measurements in the diffuser inlet showed that the streamwise vortices affect the uniformity of the streamwise mean velocity accounting for some of the performance changes. However, phase-locked hotwire measurements at the diffuser exit indicate that the periodic nature of the forcing also plays an important role for cases with enhanced pressure recovery.     

On selection of reference temperature of heat transfer coefficient for complicated flows

Convective heat transfer coefficient is closely related with flow and thermal conditions. To define heat transfer coefficient, a reference temperature needs to be properly selected, which can be the fluid bulk mean temperature for internal flows or the temperature at the far field for external flows. For complicated flows, the adiabatic wall temperature is commonly adopted as the reference temperature, while other options can also be applied. This paper analyzed some of the potential selections of the reference temperature for different flow settings, including film cooling, jet impingement with cross flows, and a mixing flow in a straight duct with or without internal heat source. It is observed that heat transfer coefficient changes dramatically with selection of reference temperatures. In case of constant wall temperature, using adiabatic wall temperature as reference temperature can result in negative heat transfer coefficient, which means the heat flux has a different direction with the defined driving temperature difference. To avoid the inconsistency due to the reference temperature, an innovative method is proposed to calculate the heat transfer coefficient of complicated flows.     

Reconstruction of turbulent fluctuations for hybrid RANS/LES simulations using a Synthetic-Eddy Method

A coupling methodology between an upstream Reynolds Averaged Navier–Stokes (RANS) simulation and a Large Eddy Simulation (LES) further downstream is presented. The focus of this work is on the RANS-to-LES interface inside an attached turbulent boundary layer, where an unsteady LES content has to be explicitly generated from a steady RANS solution. The performance of the Synthetic-Eddy Method (SEM), which generates realistic synthetic eddies at the inflow of the LES, is investigated on a wide variety of turbulent flows, from simple channel and square duct flows to the flow over an airfoil trailing edge. The SEM is compared to other existing methods of generation of synthetic turbulence for LES, and is shown to reduce substantially the distance required to develop realistic turbulence downstream of the inlet.     

An Euler solver for three-dimensional turbomachinery flows

A time-marching finite volume numerical procedure is presented for three-dimensional Euler analysis of turbomachinery flows. The proposed scheme is applied to the conservative form of the Euler equations written in general curvilinear co-ordinates. A simple but computationally efficient grid is constructed. Numerical solution results for three 3D turbine cascade flows have been presented and compared with available measurements as well as with another state-of-the-art 3D Euler analysis numerical solution in order to demonstrate the accuracy and computational efficiency of the analysis method. Also, the predicted results are compared with a 3D potential flow solver and comparison is made with the analytical solution. The proposed method is an accurate and reliable technique for solving the compressible flow equations in turbomachinery geometries.     

Coupled RANS–LPT modelling of dilute,particle-laden flow in a duct with a 90° bend

A dilute, particle-laden flow in a square duct with a 90° bend is modelled using a RANS approach, coupled to a second-moment turbulence closure, together with a Lagrangian particle tracking technique, with particle dispersion modelled using a stochastic approach that ensures turbulence anisotropy. Detailed predictions of mean and fluctuating fluid and particle velocities are validated through comparisons of predictions with experimental measurements made for gas–solid flows in a vertical-to-horizontal flow configuration. Reasonable agreement between predicted first and second moments and data is found for both phases, with the consistent application of anisotropic and three-dimensional modelling approaches resulting in predictions that compare favourably with those of other authors, and which provide fluctuating particle velocities in acceptable agreement with data.     

Experimental investigation of heat transfer in a packed duct with unequal wall temperatures

Wall-to-air forced convective heat transfer has been measured in a rectangular duct with unequal wall temperatures. The measurements were taken for a developing region in laminar flow. A uniform heat flux was supplied to the front wall of the duct. Heat transfer coefficients were measured in the empty and packed duct for various airflow rates and for different diameters of Raschig ring type packings. Measurements indicate that there is a considerable increase in the value of heat transfer coefficient for a packed duct over that for a duct with an empty flow passage.     

An improved Vorticity–Potential method for three-dimensional duct flow simulations

An improved Vorticity–Potential method is presented for the numerical solutions of three-dimensional duct flow problems. The solution procedure requires first a potential solution. Then the viscous effects are added through the vorticity transport equation. By using body-fitted coordinates, the method is applied to simulate the incompressible laminar flows in a square elbow and in a twisted square elbow.