Layered structure of turbulent plane wall jet

This paper investigates the layered structure of a turbulent plane wall jet at a distance from the nozzle exit. Based on the force balances in the mean momentum equation, the turbulent plane wall jet is divided into three regions: a boundary layer-like region (BLR) adjacent to the wall, a half free jet-like region (HJR) away from the wall, and a plug flow-like region (PFR) in between. In the PFR, the mean streamwise velocity is essentially the maximum velocity, and the simplified mean continuity and mean momentum equations result in a linear variation of the mean wall-normal velocity and Reynolds shear stress. In the HJR, as in a turbulent free jet, a proper scale for the mean wall-normal flow is the mean wall-normal velocity far from the wall and a proper scale for the Reynolds shear stress is the product of the maximum mean streamwise velocity and the velocity scale for the mean wall-normal flow. The BLR region can be divided into four sub-layers, similar to those in a canonical pressure-driven turbulent channel flow or shear-driven turbulent boundary layer flow. Building on the log-law for the mean streamwise velocity in the BLR, a new skin friction law is proposed for a turbulent wall jet. The new prediction agrees well with the correlation of Bradshaw and Gee (1960) over moderate Reynolds numbers, but gives larger skin frictions at higher Reynolds numbers.     

Numerical solution of the problem of the mixing of the boundary layers shed from the trailing edge of a wing

During the mixing of viscous incompressible flows with different velocities, in the vicinity of a trailing edge an interaction region with a three-layer structure is formed, similar to that in the case of symmetric shedding with equal velocities. The boundary layers developing on the upper and lower sides of the airfoil form a viscous mixing layer, or vortex sheet, which separates the flows downstream of the trailing edge. The boundary value problem corresponding to the flow in the viscous sublayer in the vicinity of the trailing edge of a flat plate is solved for high Reynolds numbers using an efficient numerical method for solving the equations of asymptotic interaction theory.     

可渗透壁对翼型声涡相互作用下流场及声场的影响

由仿生学原理构建的可渗透翼型对湍流气动噪声抑制作用已展现良好的应用前景。对NACA 0012可渗透翼型和实体翼型进行了数值计算,得到了声涡相互作用下气动噪声声场和流场,分析了可渗透壁对翼型流场和声场的影响。研究表明,相对实体翼型,可渗透壁通过减小声源强度降低了主纯音噪声声压级幅值和远场总声压级,消除了高阶离散纯音,但对噪声的指向性没有较大改变。进一步的流场分析表明,可渗透壁对翼型气动性能影响不大的情况下能够降低边界层扰动和翼型后缘大尺度涡旋强度,并推迟分离泡转捩和再附位置。     

An investigation into the interaction of closely-spaced starting jets

The transient, three-dimensional scavenging flow inside a novel two-stroke engine has been investigated both experimentally in a scaled water model as well as numerically using a commercial CFD code incorporating an unsteady Reynolds averaged Navier–Stokes (URANS) formulation. The scavenging flow consists of 16 round jets in close proximity of each other and the cylinder wall, developing from the top of the combustion chamber down towards the exhaust ports located along the wall at the bottom of the cylinder. Flow visualization of the scavenging flow was performed using a scaled fixed-piston water model and was used as a means of validating the URANS simulations themselves. The flow visualization experiments provided insight into the complex jet–jet and jet–wall interactions within the engine cylinder. These interactions were not as well predicted by the CFD simulations. In fact, the CFD simulations were found to significantly under-predict the turbulent mixing between the jets. This suggests that unsteady-RANS formulations are incapable of reproducing the large-scale and unsteady mixing structures associated with the vortex shedding between the closely-spaced jets.     

Large-eddy simulation of a turbulent boundary layer with blowing

The modifications of a turbulent boundary layer induced by blowing through a porous plate were investigated using large-eddy simulation. The Reynolds number (based on the length of the plate) of the main flow was about 850000. Large-eddy simulations of such a boundary layer needs a turbulent inflow condition. After a review of available turbulent inflow, we describe in details the condition we developed, which consisted of recycling the velocity fluctuations. Then we show the necessity for this inflow to be non-stationary and to be three dimensional with respect to the mass conservation equation. If these properties are not achieved, we found that the velocity fluctuations do not grow as expected along the domain. Finally, the results of simulations of the boundary layer submitted to blowing are compared with experimental measurements. The good agreement obtained validate our turbulent inflow conditions and also the blowing model used. PACS 47.27.Eq, 47.27.Te, 44.20.+b     

DNS,LES and RANS of turbulent heat transfer in boundary layer with suddenly changing wall thermal conditions

The objectives of this study are to investigate a thermal field in a turbulent boundary layer with suddenly changing wall thermal conditions by means of direct numerical simulation (DNS), and to evaluate predictions of a turbulence model in such a thermal field, in which DNS of spatially developing boundary layers with heat transfer can be conducted using the generation of turbulent inflow data as a method. In this study, two types of wall thermal condition are investigated using DNS and predicted by large eddy simulation (LES) and Reynolds-averaged Navier–Stokes equation simulation (RANS). In the first case, the velocity boundary layer only develops in the entrance of simulation, and the flat plate is heated from the halfway point, i.e., the adiabatic wall condition is adopted in the entrance, and the entrance region of thermal field in turbulence is simulated. Then, the thermal boundary layer develops along a constant temperature wall followed by adiabatic wall. In the second case, velocity and thermal boundary layers simultaneously develop, and the wall thermal condition is changed from a constant temperature to an adiabatic wall in the downstream region. DNS results clearly show the statistics and structure of turbulent heat transfer in a constant temperature wall followed by an adiabatic wall. In the first case, the entrance region of thermal field in turbulence can be also observed. Thus, both the development and the entrance regions in thermal fields can be explored, and the effects upstream of the thermal field on the adiabatic region are investigated. On the other hand, evaluations of predictions by LES and RANS are conducted using DNS results. The predictions of both LES and RANS almost agree with the DNS results in both cases, but the predicted temperature variances near the wall by RANS give different results as compared with DNS. This is because the dissipation rate of temperature variance is difficult to predict by the present RANS, which is found by the evaluation using DNS results.     

Time-resolved PIV measurements of the interaction of polystyrene beads with near-wall-coherent structures in a turbulent channel flow

Time-resolved PIV measurements were performed in a dilute particle-laden flow tracking near-neutrally buoyant polystyrene beads and the velocity field of a near wall turbulent boundary layer. Data were taken in a vertical light sheet aligned in the streamwise direction at the center of a horizontal, closed loop, transparent square water channel facility. In addition, low speed measurements were performed characterizing the effects of the dispersed phase on mean and turbulence flow quantities. Reynolds shear stress slightly differed from clear water conditions whereas fluid mean and rms values were not affected. A case study for several beads revealed a clear relation between their movement and near-wall coherent structures. Several structures having 2D vorticity signatures of near-wall hairpin vortices and hairpin packets, directly affected bead movement. A statistical analysis showed that the mean streamwise velocity of ascending beads lagged behind the mean fluid velocity and bead rms values were higher than fluid ones. Particle Reynolds numbers based on the magnitude of the instantaneous relative velocity vector peaked near the wall; values not exceeding 100, too low for vortex shedding to occur. Quadrant analysis showed a clear preference for ascending beads to reside in ejections while for descending beads the preference for sweeps was less.     

Effect of tube diameter on Preston tube calibration curves for the measurement of wall shear stress

The effect of tube diameter (d) on Preston tube calibration curves for the measurement of wall shear stress (τ_w) in a zero pressure gradient turbulent boundary layer has been investigated. Five different outside diameter tubes of 1.46, 1.82, 3.23, 4.76 and 5.54 mm, corresponding to (d/δ) of 0.022, 0.027, 0.048, 0.071 and 0.082 were used to measure τ_w at Reynolds numbers based on momentum thickness (R_θ) of 2800–4100. The calibration curves of Patel (V.C. Patel, J. Fluid Mech. 23 (part I) (1965) 185–208) and Bechert (D.W. Bechert, AIAA J. 34 (1) (1995) 205–206) are both dependent on the tube diameter. The maximum difference in the τ_w measurements from the different tubes using Patel's calibration is about 8%, while Bechert's calibration gives a maximum difference of approximately 18%.     

Numerical evaluation of passive control of shock wave/boundary layer interaction on NACA0012 airfoil using jagged wall

Shock formation due to flow compressibility and its interaction with boundary layers has adverse effects on aerodynamic characteristics, such as drag increase and flow separation. The objective of this paper is to appraise the prac-ticability of weakening shock waves and, hence, reducing the wave drag in transonic flight regime using a two-dimensional jagged wall and thereby to gain an appropriate jagged wall shape for future empirical study. Different shapes of the jagged wall, including rectangular, circular, and triangular shapes, were employed. The numerical method was validated by experimental and numerical studies involving transonic flow over the NACA0012 airfoil, and the results presented here closely match previous experimental and numerical results. The impact of parameters, including shape and the length-to-spacing ratio of a jagged wall, was studied on aerodynamic forces and flow field. The results revealed that applying a jagged wall method on the upper surface of an airfoil changes the shock structure significantly and disinte-grates it, which in turn leads to a decrease in wave drag. It was also found that the maximum drag coefficient decrease of around 17%occurs with a triangular shape, while the max-imum increase in aerodynamic efficiency (lift-to-drag ratio) of around 10%happens with a rectangular shape at an angle of attack of 2.26?.     

Large-eddy simulation of low-frequency unsteadiness in a turbulent shock-induced separation bubble

The need for better understanding of the low-frequency unsteadiness observed in shock wave/turbulent boundary layer interactions has been driving research in this area for several decades. We present here a large-eddy simulation investigation of the interaction between an impinging oblique shock and a Mach 2.3 turbulent boundary layer. Contrary to past large-eddy simulation investigations on shock/turbulent boundary layer interactions, we have used an inflow technique which does not introduce any energetically significant low frequencies into the domain, hence avoiding possible interference with the shock/boundary layer interaction system. The large-eddy simulation has been run for much longer times than previous computational studies making a Fourier analysis of the low frequency possible. The broadband and energetic low-frequency component found in the interaction is in excellent agreement with the experimental findings. Furthermore, a linear stability analysis of the mean flow was performed and a stationary unstable global mode was found. The long-run large-eddy simulation data were analyzed and a phase change in the wall pressure fluctuations was related to the global-mode structure, leading to a possible driving mechanism for the observed low-frequency motions.      

Low Reynolds number hydrodynamic interaction of a solid particle with a planar wall

The slow viscous flow problem of an arbitrary solid particle in motion near a planar wall is recast into a boundary integral formulation. The present formulation employs the Green function appropriate to the planar wall problem and is developed in sufficient generality to allow calculations for arbitrary particles in any base flow which satisfies Stokes equations and no-slip on the wall. The resulting integral equations are easily discretized and solved for the particle surface tractions. Calculations are performed for axisymmetric motions of a variety of ellips?ids near the planar wall. Agreement with existing theory is excellent.     

Dynamics of vortical structures in cylinder/wall interaction with moderate gap ratio

The interaction between the wake of a transverse circular cylinder and the underlying flat-plate boundary layer with a moderate gap ratio G/D=1.0 is investigated using both hydrogen-bubble-based and PIV-based visualization techniques. The spanwise rollers in the cylinder wake are found to be capable of inducing secondary vortices in the near-wall region. The mutual induction from the counter-clockwise rollers, which are closer to the wall, plays a primary role, so that these secondary vortices present linear lift-up motion at first. Their subsequent evolution dominantly determines the characteristics of the wake/boundary-layer interaction. Two different vortex interaction scenarios are observed: the secondary vortices can be either entrained into the rollers or pushed down towards the wall. This leads to a rapid three-dimensional destabilization process, through which streamwise vortices are generated. And it is suggested that these streamwise vortices are the dominant structures to promote the following boundary layer transition.     

Experimental study of rotation effect on film cooling over the flat wall with a single hole

A new rotating test rig was set up to investigate the rotation effect on the film cooling over the flat wall. A simple flat blade with an inclined 30° film hole, which is parallel to the hot mainstream, was installed. And different rotation orientations were selected to simulate the blade pressure or suction side of a turbine blade. A steady liquid crystal technique was applied to obtain detailed distribution of the temperature over the blade surface. And the average adiabatic film cooling effectiveness of the area adjacent to the film hole was selected to evaluate the cooling effect. Five different rotational speeds, i.e., 0, 300, 500, 800, 1000 r/min, were considered. Experimental results indicate that the film trajectory could bend under the rotating condition. With the increase of the rotational speed, on the pressure side, the film trajectory inclines centripetally firstly and then centrifugally; whereas, on the suction side the film trajectory bends centrifugally. On the other hand, as the rotational speed increases, the cooling effect is improved firstly and then worsened when Ω > 500–600 r/min on the pressure side. On the suction side, however, the cooling effect is not sensitive to the rotational speed.     

Heat transport mechanisms of low Mach number turbulent channel flow with spanwise wall oscillation

Large eddy simulation （LES） of low Mach num- ber compressible turbulent channel flow with spanwise wall oscillation （SWO） is carried out. The flow field is analyzed with emphases laid on the heat transport as well as its rela- tion with momentum transport. When turbulent coherent structures are suppressed by SWO, the turbulent transports are significantly changed, however the momentum and heat transports change in the same manner, which gives the evi- dence of inherently consistent transport mechanisms between momentum and heat in turbulent boundary layers. The Reynolds analogies of all the flow cases are quite good, which confirms again the fact that the transport mechanisms of momentum and heat are consistent, which gives theoreti- cal support for controlling the wall heat flux control by using the drag reducing techniques.     

A hybrid numerical approach to predict the vibrational responses of panels excited by a turbulent boundary layer

In this work, a hybrid numerical approach to predict the vibrational responses of planar structures excited by a turbulent boundary layer is presented. The approach combines an uncorrelated wall plane wave technique with the finite element method. The wall pressure field induced by a turbulent boundary layer is obtained as a set of uncorrelated wall pressure plane waves. The amplitude of these plane waves are determined from the cross spectrum density function of the wall pressure field given either by empirical models from literature or from experimental data. The response of the planar structure subject to a turbulent boundary layer excitation is then obtained from an ensemble average of the different realizations. The numerical technique is computationally efficient as it rapidly converges using a small number of realizations. To demonstrate the method, the vibrational responses of two panels with simply supported or clamped boundary conditions and excited by a turbulent flow are considered. In the case study comprising a plate with simply supported boundary conditions, an analytical solution is employed for verification of the method. For both cases studies, numerical results from the hybrid approach are compared with experimental data measured in two different anechoic wind tunnels.