Rotation effect on near-wall turbulence statistics and flow structures

Turbulent flow in an axially rotating pipe, involving complicated physical mecha- nism of turbulence, is a typical problem for the study of rotating turbulent flow. The pipe rotation induces two effects on the flow. One is the stabilizing effect due to the centrifu- gal and Coriolis forces, which accounts for the relaminarization of the turbulence[1—3] and the reduction of the friction coefficient at the pipe wall. The behavior is also related to the wall streaks inclining to the azimuthal di…     

Effect of inlet flow turbulence on the combustion instability in a premixed backward-facing step combustor

This paper reports the effect of inlet flow turbulence intensity on the combustion instability characteristics in a backward facing step combustor. The inlet turbulence intensity is varied by a turbulence generator. Unsteady pressure measurements and OH* chemiluminescence images are recorded over a wide range of operating conditions at different inlet turbulence intensities. The study shows an early onset of instability at low turbulence level, i.e., higher turbulence postpones the onset of instability to higher Reynolds number Re and/or higher equivalence ratio Φ. The early onset of instability in the Re and Φ parameter spaces is due to the change in system parameters such as flame speed and size of the recirculation zone downstream of the step at different turbulence levels. Further, the onset is characterized as subcritical bifurcation. At low Re, the hysteresis zone width is small for low turbulence levels and it is large at higher turbulence levels; and at higher Re, the hysteresis width remains constant at all turbulence levels. Investigation of instability characteristics reveals that there are momentary slippages from limit cycle orbit into brief silent regimes in an intermittent manner. The frequency of occurrence of the momentary silent regimes increases with reduction in turbulence, indicating that higher turbulence helps in maintaining the system in a stable limit cycle orbit. High-speed chemiluminescence imaging reveals the necessity of the vortex rollup in the recirculation zone to grow up to the top wall by dilatation from the heat release for the onset of instability. Considerations of the effect of turbulence on both the flame speed and the recirculation zone size together explain all the observed bifurcation trends. These results suggest that inlet flow turbulence should not just be considered as background noise. The turbulence effects on both the flame and flow should be considered in predicting the instability characteristics.     

Influence of turbulence on train noise

The subject of this paper is the long distance propagation of train noise. The sound exposure level of train noise L_AE was measured. To describe the results of measurements, a semi-analytical model was used. It takes into account the wave-front divergence, air absorption, ground effect, and the turbulence destroying the coherent nature of the ground effect. The model contains three adjustable parameters that must be estimated at the site. To verify the model, we performed measurements of L_AE at the distance D = 450 m from the train track center. The difference between the calculated and measured mean values of L_AE equals 1.3 dB.     

On the role of anisotropic turbomachinery flow structures in inter-scale turbulence energy flux as deduced from SPIV measurements

The present study uses stereoscopic particle image velocimetry in the rotor exit of a centrifugal turbomachine to analyse anisotropy and geometrical characteristics of tensorial flow quantities. The purpose is to identify dominant topologies of turbulence stress tensor and principal directions of flow structures. The misalignment between principal directions of strain and turbulence stress tensors is more evident in the jet–wake interaction regions and questions the eddy-viscosity models which assume an exact alignment between stress/strain eigenvectors. Anisotropy analysis based on the barycentric approach shows that the disk-like structure and/or the rod-like structure limiting states of turbulence are the most frequent topologies of turbulence stress. Additionally, planar straining is the dominant deformation characteristic in the measurement area. These anisotropic behaviours considerably attribute to the turbulence energy cascade. Conditional isolation of flow structures based on inter-scale energy flux shows that a larger extent of turbulence stress anisotropy results in a larger energy flux and therefore significantly affects the dynamics of turbulent flow structures.     

Effect of inlet duct contour and lack thereof on the noise generated of an axial flow fan

This study concerns the unsteady flows in turbomachines in general, and the aeroacoustics of fans in particular. The principal objective of this paper is the determination of the influence of the upstream environment on the acoustic and aerodynamic behavior of axial fans. After analysis of the various sources of noise present in turbomachines, interest is focused on the influence of the disturbances of the velocity field at the suction. Accordingly, the effect of the presence of a contoured duct and a lack thereof at the inlet of an axial flow fan is analyzed . The results show the strong involvement of the upstream turbulence level in the generation of the noise, and in particular, of broadband noise.     

Vertical rotation effect on turbulence characteristics in an open channel flow

This paper solves the three-dimensional Navier-Stokes equation by a fractional-step method with the Reynolds number Reτ=194 and the rotation number Nτ=0-0.12. When Nτ is less than 0.06, the turbulence statistics relevant to the spanwise velocity fluctuation are enhanced, but other statistics are suppressed. When Nτ is larger than 0.06, all the turbulence statistics decrease significantly. Reynolds stress budgets elucidate that turbulence kinetic energy in the vertical direction is transferred into the streamwise and spanwise directions. The flow structures exhibit that the bursting processes near the bottom wall are ejected toward the free surface. Evident change of near-surface streak structures of the velocity fluctuations are revealed.     

Nature of mechanical instabilities and their effect on kinetic friction

It has long been recognized that kinetic friction F(k) between two solids must be due to instabilities, sudden "pops" of certain degrees of freedom. Here, such pops are studied with a focus on boundary lubrication. The pops' characteristics and consequently the friction-velocity relationship depend qualitatively on dimensionality, commensurability, and details of the lubricant wall interaction. It is found that F(k) should be small between commensurate surfaces. F(k) is large for incommensurate surfaces, unless the lubricant's motion is confined to 1D. The effects of thermal noise are discussed and computer simulations are employed to show the relevance of the predictions to less idealized models.     

基于自适应滤波器的大气湍流噪声抑制

为了解决大气湍流效应所引起的光无线通信系统中的衰落问题,建立了与大气湍流噪声及其它加性噪声有关的信号模型,实现了最大输出信噪比可提高3-20dB的匹配滤波器。根据最大似然比准则,推导了强度调制/直接探测(IM/DD)的光无线通信系统的最佳判决门限。采用自适应滤波器实现了对探测门限中所含的信号和湍流的统计值的预测,该自适应门限探测技术对大气湍流噪声有很好的抑制作用,在强湍流情况下,可降低系统的误码率到10-5以下。     

Free stream turbulence effect on the flow structure over the finite span straight wing

Flow visualization tests have been performed to examine the structure of the near-wall flow over a low-aspect ratio straight wing installed at various angles of attack a and chord Reynolds numberRe _c=U_∞c/ν=1.76×10⁵. The experiments were carried out at two free-stream turbulence levels, ε=0.1% and ε=1%, the latter one having been achieved using a baffling grid. To visualize the flow, termochromic cholesteric liquid crystals and digital processing of video images were used. At the low turbulence level and α=27°, a flow stall on the lee side of the wing was observed, with a pair of largescale vortices rotating in the wing plane. Simultaneously, no vortex structures were observed on the windward wing surface. It was found the flow patterns on either side of the wing significantly changed with increasing free-stream turbulence level. A separation bubble appeared near the leading edge on the lee side of the airfoil at ε=1%, and large-scale stationary longitudinal vortices originated over the wing windward surface. The number and sizes of the longitudinal structures were found to be dependent on the angle of attack.     

The effect of fuel inlet turbulence intensity on H2/CH4 flame structure of MILD combustion using the LES method

Large eddy simulations (LES) are employed to investigate the effect of the inlet turbulence intensity on the H₂/CH₄ flame structure in a hot and diluted co-flow stream which emulates the (Moderate or Intense Low-oxygen Dilution) MILD combustion regime. In this regard, three fuel inlet turbulence intensity profiles with the values of 4%, 7% and 10% are superimposed on the annular mixing layer. The effects of these changes on the flame structure under the MILD condition are studied for two oxygen concentrations of 3% and 9% (by mass) in the oxidiser stream and three hot co-flow temperatures 1300, 1500 and 1750 K. The turbulence-chemistry interaction of the numerically unresolved scales is modelled using the (Partially Stirred Reactor) PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influences of the turbulence intensity on the flame structure under the MILD condition are studied by using the profile of temperature, CO and OH mass fractions in both physical and mixture fraction spaces at two downstream locations. Also, the effects of this parameter are investigated by contours of OH, HCO and CH₂O radicals in an area near the nozzle exit zone. Results show that increasing the fuel inlet turbulence intensity has a profound effect on the flame structure particularly at low oxygen mass fraction. This increment weakens the combustion zone and results in a decrease in the peak values of the flame temperature and OH and CO mass fractions. Furthermore, increasing the inlet turbulence intensity decreases the flame thickness, and increases the MILD flame instability and diffusion of un-burnt fuel through the flame front. These effects are reduced by increasing the hot co-flow temperature which reinforces the reaction zone.     

The effect of forward flight on the noise and flow field of inverted profile jets

Noise and flow field measurements are reported for an inverted profile coannular jet (where the annular jet speed exceeds the center jet speed) under simulated flight conditions. The annular and center jets were cold and both were operated subsonically. Forward flight was simulated by placing the coannular jet inside a larger open jet. Acoustic measurements show the effects of inverted profile shape and simulated flight on far field directivity, total radiated power, and spectral content. Measurements of total acoustic power demonstrate that the acoustic efficiency of inverted profile jets is about 3 dB less than the efficiency of “top hat” profile jets, and that the noise decreases as the seventh power of the relative jet velocity in the limit of small flight velocity, U_f. Flow measurements demonstrate that the jet spreading parameter λ = (U_j ? U_f)/(U_j + U_f) scales the thickness of the outer shear layer and the passage frequencies of the large turbulence scales. Comparisons between the turbulence time scales and the noise spectra suggest that coherent noise sources may become more important in forward flight.     

The effect of traffic noise on sleep of young adults in their homes

The disturbance of sleep by traffic noise is a major problem area in noise pollution. Extensive laboratory tests using multichannel electroencephalograms (EEG) have been carried out by many workers to determine the general response of people when exposed to noise during sleep. An experimental technique for obtaining results in people's homes has been developed using a simplified one-channel EEG. The responses of six people in the age range 19-24 were measured. A significant correlation was found between sleep disturbance and traffic noise when the noise is expressed in EPNdB. In the home experiments the subjects appeared to be approximately 10 dB less sensitive to noises than laboratory subjects for similar noise exposure. There also appeared to be some adaptation to the noise exposure; however, only one subject was tested for a period of 20 days and was insufficient to give definite results on adaptation.     

Influence of cavity flow regimes on turbulence diffusion coefficient

Skimming, wake interference and isolated roughness flow regimes were investigated in the cavity formed between 2-d roughness elements in a laboratory water flume. The flow direction was aligned perpendicular to the roughness elements. Flow visualization and laser Doppler velocimetry were used to confirm the flow pattern and corresponding turbulence characteristics of the flow. Influence of the flow regimes on Reynolds stresses and turbulent diffusion coefficient was investigated at roof level of the cavity. Sweep events towards the wall were significantly affected by the rough bed. A relatively wider cavity demonstrated significant influence on ejection and sweep stresses because of vortex activity at upstream and downstream positions within the cavity. The turbulent diffusion coefficient increased for isolated roughness flow in comparison to skimming flow regime.     

被动在轨非连接支承侧向受力数值特性计算

为了提高被动在轨非连接支承(PODS)对侧向力的阻抗性能,减小低温贮箱通过PODS的漏热量,建立了带纤维丝的被动在轨非连接支承(PODS-IV)和无纤维丝的被动在轨非连接支承(PODS-III)结构三维模型,运用ANSYS workbench有限元软件,对带纤维丝的PODS-IV结构与无纤维丝的PODS-III结构分别进行了侧向力阻抗性能数值模拟计算,得到了侧向力作用在不同位置处两种支承结构各自所能承受的侧向力临界值,以及发生热短路时接触面位移变形云图。结果表明:带纤维丝的PODS-IV较不带纤维丝PODS-III能承受更大的侧向力而不发生热短路,同时随侧向力作用点离冷端距离增加,侧向力临界值相应增大。说明PODS-IV具有更好的支承性能及空间适用性。     

A computational study of the effect of windscreen shape and flow resistivity on turbulent wind noise reduction

In this paper, numerical simulations are used to study the turbulent wind noise reduction effect of microphone windscreens with varying shapes and flow resistivities. Typical windscreen shapes consisting of circular, elliptical, and rectangular cylinders are investigated. A turbulent environment is generated by placing a solid circular cylinder upstream of the microphone. An immersed-boundary method with a fifth-order weighted essentially non-oscillatory scheme is implemented to enhance the simulation accuracy for high-Reynolds number flow around the solid cylinder as well as at the interface between the open air and the porous material comprising the windscreen. The Navier-Stokes equations for incompressible flow are solved in the open air. For the flow inside the porous material, a modified form of the Zwikker-Kosten equation is solved. The results show that, on average, the circular and horizontal ellipse windscreens have similar overall wind noise reduction performance, while the horizontal ellipse windscreen with medium flow resistivity provides the most effective wind noise reduction among all the considered cases. The vertical ellipse windscreen with high flow resistivity, in particular, increases the wind noise because of increased self-generation of turbulence.     

Extended local balance model of turbulence and couette-taylor flow

An extended local balance model of turbulence, based on a new transport equation for the dissipation rate with a negative diffusion coefficient, is presented. Analytical solutions for the mean velocity and the dissipation rate for the turbulent Couette-Taylor problem are derived. The dependence of torque on the Reynolds number is obtained. These solutions depend only on two constants k=0.4 and C=9.5 of the turbulent boundary layer and, within the limits of a narrow channel, are reduced to the well-known von Karman's solutions for planar Couette flow. Strange attractor behavior in this limit is also observed.