Effect of wall temperature on hypersonic turbulent boundary layer

Turbulent boundary layers at Mach 4.9 with the ratio of wall temperature to recovery temperature from 0.5 to 1.5 are investigated by means of direct numerical simulation. Various fundamental properties relevant to the influence of wall temperature on Morkovin’s scaling, standard and modified strong Reynolds analogies, and coherent vortical structures have been studied. It is identified that the scaling relations proposed for cool and adiabatic wall conditions, such as Morkovin’s scaling and the modified strong Reynolds analogy, are also applicable for hot wall condition. Moreover, the relation between the density and temperature fluctuations under the second-order approximation is derived and verified to provide a reliable prediction. Based on the analysis of coherent vortical structures, it is found that the orientation of vortex core can be quantitatively determined by means of the vector with its direction and modulus using the local strain direction and the imaginary part of the eigenvalue of velocity gradient tensor, respectively. As the increase of wall temperature, the spanwise distance between the two legs of hairpin vortex increases, and the mean swirling strength and the angle of vortical structure with respect to the wall plane also increase in the inner layer. The statistical properties relevant to vortical structures are nearly insensitive to the wall temperature in the outer layer.     

The improvement of turbulence modeling for the aerothermal computation of hypersonic turbulent boundary layers

The engineering computation of turbulent flows is mainly based on turbulence modeling,however,accurate aerothermal computation of hypersonic turbulent boundary layers is still a not well-solved problem. Aerothermal computation for turbulent boundary layers on a supersonic or hypersonic blunt cone with small bluntness is done firstly by using both direct numerical simulation and BL model,and seven different cases are investigated. Then the results obtained by the two methods are compared,and the reason causi...     

Interaction between coherent structures and surface temperature and its effect on ground heat flux in an unstably stratified boundary layer

Surface layer plumes, thermals, downdrafts and roll vortices are the most prominent coherent structures in an unstably stratified boundary layer. They contribute most of the temperature and vertical velocity variance, and their time scales increase with height. The effects of these multi-scale structures (surface layer plumes scale with surface layer depth, thermals scale with boundary layer height and the resulting roll vortices scale with convective time scale) on the surface temperature and ground heat flux were studied using turbulence measurements throughout the atmospheric boundary layer and the surface temperature measurements from an infrared camera. Plumes and thermals imprint on the surface temperature as warm structures and downdrafts imprint as cold structures. The air temperature trace shows a ramp-like pattern, with small ramps overlaid on a large ramp very close to the surface; on the other hand, surface temperature gradually increases and decreases. Turbulent heat flux and ground heat flux show similar patterns, with the former lagging the latter. The maximum values of turbulent heat flux and ground heat flux are 4 and 1.2 times the respective mean values during the ejection event. Surface temperature fluctuations follow a similar power-law exponent relationship with stability as suggested by surface layer similarity theory.     

湍流边界层激励下腔体内水动力自噪声预报与测量

循环水槽中试验测量了腔体内水动力自噪声,并与模态法建立的湍流脉动压力引起腔体自噪声预报进行比较验证。透声窗振动以简支边界为条件,腔体内部声波以刚性边界为条件模态展开,通过辐射边界条件建立模态耦合振动方程。在随机湍流脉动压力作用下,推导了模态振动方程在随机力激励下的自噪声功率谱响应。对循环水槽中5 m/s和8 m/s两种流速工况下的腔体水动力自噪声和湍流脉动压力进行了测试,结合测量的脉动压力预报方法可以计算腔体水动力自噪声量值,理论预报与试验测量结果大致吻合,趋势一致,为声呐罩材料选取及声学环境控制提供了一种分析方法。     

Prediction and test of cavity's hydrodynamic self-noise induced by turbulent boundary layer

The self-noise in cavity is tested in the circling tank,prediction method of cavity's self-noise induced by turbulent boundary layer is established.The window's vibration is using the simply supported boundary condition,the sound wave in the cavity is expanded using the rigid wall boundary condition,the modal coupling vibration equation between them is established using the radiation boundary condition.The turbulent boundary layer pulsating pressure is random,the self-noise power spectrum in the cavity is solved.Test of self-noise and turbulent pressure is carried out in the circling tank when the flow velocity is 5 m/s and 8 m/s,the result verifies that the theoretical method can predict the real cavity's hydrodynamic noise approximately,the trends are similar,this provides one analytical method for sonar dome's material selection and noise control.     

The magnetocaloric effect and low temperature specific heat of SmNi

The magnetocaloric effect (MCE) in a magnetic SmNi sample was evaluated from magnetization and heat capacity measurements. The MCE phenomena in the vicinity of magnetic phase transitions in terms of magnetic entropy change, , and adiabatic temperature change, , are reported. Isothermal magnetization measurements at several temperatures around the transition were carried out and used for versusT calculations. A similar dependence of the magnetic entropy change was evaluated from heat capacity C_p(T) measurements under zero field and 5 T. The SmNi system provides magnetic refrigerants that induce an adiabatic cooling of about during the magnetization process with a field of 5 T in the temperature range of 35-45 K. The temperature dependence of C_p(T) is analyzed in terms of the magnetic and the lattice contributions.     

Reduction of turbulent boundary layer induced interior noise through active impedance control

The use of a single actuator tuned to an optimum impedance to control the sound power radiated from a turbulent boundary layer (TBL) excited aircraft panel into the aircraft interior is examined. An approach to calculating the optimum impedance is defined and the limitations on the reduction in radiated power by a single actuator tuned to that impedance are examined. It is shown that there are too many degrees of freedom in the TBL and in the radiation modes of the panel to allow a single actuator to control the radiated power. However, if the panel modes are lightly damped and well separated in frequency, significant reductions are possible. The implementation of a controller that presents a desired impedance to a structure is demonstrated in a laboratory experiment, in which the structure is a mass. The performance of such a controller on an aircraft panel is shown to be effective, if the actuator impedance is similar to but not the same as the desired impedance, provided the panel resonances are well separated in frequency and lightly damped.     

The energy and dissipation of turbulent fluctuations of wind velocity and temperature in the boundary layer of the atmosphere

The relationships between the energy of small-scale turbulence and its dissipation rate are studied based on the data of long-term high-frequency measurements of temperature and wind velocity fluctuations in urban area. It is shown that the energy of wind velocity turbulent fluctuations is linearly related to the dissipation rate ɛ. The proportionality coefficient between turbulent kinetic energy (TKE) and ɛ is dimensional and does not depend on the stratification of the atmosphere, the Richardson number, or the Monin-Obukhov scale. Measurements in different seasons show that this coefficient can be related to the mean velocity of adiabatic motions (sound speed or air temperature), which enables one to select a more universal constant, γ. A linear relationship between the temperature fluctuations variance (the characteristic of the inner energy of turbulence) and their dissipation rate is also shown. The revealed proportionality is confirmed by measurements in urban and forest conditions, as well as in the surface layer over a flat desert terrain.     

Experimental simulation of turbulent boundary layer induced vibrations by using a synthetic array

Of particular interest is the experimental feasibility of replacing wind tunnel measurements with a substitute experiment to simulate Turbulent Boundary Layer (TBL) induced vibrations. This paper describes the development of an experimental procedure to address this issue. The proposed approach is based on the concept of uncorrelated wall pressure plane waves, which is introduced as a prelude to this study. Concretely, an array of acoustic monopoles can be used to generate these wall pressure plane waves. To this end, the design parameters of the array are studied numerically. However, when dealing with experimental applications, the number of monopoles required being prohibitive, the principle of synthetic array is applied instead. This technique allows simulating the effect of an array of acoustic monopoles from sequential measurements. To assess the validity of the proposed approach, the whole procedure is applied to simulate TBL induced vibrations of a thin elastic plate. The results obtained are in good agreement with those obtained from random vibration theory.     

Effect of uniform blowing or suction on hypersonic spatially developing turbulent boundary layers

正Dear Editors,Over the past decades,the reduction of drag has attracted considerable attention for its potential applications in engineering[1-3].Direct numerical simulations(DNSs) provide accurate data that can be used to study the underlying physics of drag reduction.Inarecent DNS[4] of an incompressible spatially developing turbulentboundary layer with uniform blowing or suction applied on the wall,the drag reduction mechanism     

Analytical model of the time developing turbulent boundary layer

An analytical model for the time-developing turbulent boundary layer (TD TBL) over a flat plate is presented. The model provides explicit formulae for the temporal behavior of the wall-shear stress and both the temporal and spatial distributions of the mean streamwise velocity, the turbulence kinetic energy and Reynolds shear stress. The resulting profiles are in good agreement with the DNS results of spatially-developing turbulent boundary layers at momentum thickness Reynolds numbers equal to 1430 and 2900 [5–7]. Our analytical model is, to the best of our knowledge, the first of its kind for TD TBL. The text was submitted by the authors in English.     

Transition of the boundary layer on a flat plate at supersonic and hypersonic velocities

The transition of the boundary layer from the laminar to the turbulent state on a smooth flat plate at a zero angle of attack is studied in the range of Mach numbers M_∞ = 2–6. It is demonstrated that the results measured at the end of the transition region can be approximated by a simple dependence suitable for applications, which does not require additional measurements, is valid in the range of Mach numbers M_∞ = 2–10, and, with an error lower than 20 %, can be used to estimate the location of the transition region on a flat plate in geometrically similar wind tunnels.     

Influence of short roughness strip on the turbulent boundary layer structure

Hot-wire measurements have been undertaken in a turbulent boundary layer which is subjected to an impulse in form of a short roughness strip with the aim of examining its influence on the structure of the turbulent boundary layer. The results indicate that, while the energy containing motion is shifted from low wave number to high wave number near the wall due to the interfering of the roughness strip with the near-wall structure, the reverse is the case in the outer region. While the anisotropy at small scale changes appreciably, there is no discernable change at the large scale when distance from the wall is increased as reflected in the collapses of spectra shear correlation coefficient at the low wave number. It further shows that the roughness strip alters the flow dynamics of the boundary layer as shown in the changes in the mixing length distribution.     

Study on a neural network model for high speed turbulent boundary layer inducing optical distortions

A neural network (NN) model for high speed turbulent boundary layer inducing aero optical distortions is proposed to solve the spatial frequency problem in sparse measurements. The temperature fluctuation, a measurable flow state, is set as the NN model input, and the optical path difference (OPD) is outputted. The NN structure, its optimal inputting point number and neuron number of hidden layer are analyzed. Numerical simulation results for two flow cases show that the OPDs of simulated positions using only limited points have good agreements with the calculated ones by ray tracing from whole field. Their correlation coefficient is up to 0.95 and the relative error of OPD root mean square is within 7% for both cases, which validates this model. Besides, its accuracy performances for different cases are analyzed, and the results accord with the experimental knowledge. This research provides numerical basis for the practical application of NN in sparse measurements.     

Direct numerical simulation of a supersonic turbulent boundary layer on a flat plate and its analysis

In the past three decades, considerable progress has been made in the investigation of incompressible turbulent boundary layer through experiments, DNS and theoretical works, including: (1) the statistics characteristic and structure of turbulence; (2) the co-herent structures in turbulent flows; (3) turbulence modeling and the large eddy simula-tion (LES). In contrast, the progress was very slow for the compressible, in particular, the super-sonic turbulent boundary layer. Recent works on d…     

General specific heat dependences on temperature in neohexanol and its isomers

The Gibbs free energy, in the form resulting from Landau considerations of the first order phase transitions, is used to describe resulting temperature behaviour of the specific heat of neohexanol and its isomers given by the general formula C₆H₁₃OH. Analyses of the data give power dependences of the specific heat on temperature with exponent one half in the vicinity of the melting points and almost one and a half far below phase transformation temperatures.