The experimental research on zero-net-mass-flux jet closed-loop active control was conducted in the wind tunnel.The mu-level method successfully detected burst events of the coherent structures. The streamwise velocit...The experimental research on zero-net-mass-flux jet closed-loop active control was conducted in the wind tunnel.The mu-level method successfully detected burst events of the coherent structures. The streamwise velocity signals in the turbulent boundary layer were measured by HWA. The drag reduction rate of 16.7% is obtained comparable to that of the open-loop control and saves 75% of the input energy at the asynchronous 100 V/160 Hz control case, which reflects the advantages of the closed-loop control. The experimental findings indicate that the intensity increases in the near-wall region.The perturbation of the PZT vibrators on the skewness factor is concentrated in the region y+< 60. The generation of highspeed fluids is depressed and the downward effect of high-speed fluids weakens. The alteration of energy distribution and the discernible impact of modulation between structures of varying scales are observed. The correlation coefficient exhibits a strong positive correlation, which indicates that the large-scale structures produce modulation effect on small-scale ones.The occurrence of burst events is effectively suppressed. The disturbance has the characteristics of stable periodicity,positive and negative symmetry, low intermittency, and high pulsation strength. The conditional phase waveform shows that the fluctuation amplitude increases, indicating amplitude modulation effects on coherent structures.展开更多
Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional co- herent structures in the logarithmic region of the turbulent boundary layer in a water tunnel. The Reynolds number ...Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional co- herent structures in the logarithmic region of the turbulent boundary layer in a water tunnel. The Reynolds number based on momentum thickness is Reo = 2 460. The in- stantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low speed fluid, which is flanked on either side by high- speed ones. Statistical support for the existence of hairpins is given by conditional averaged eddy within an increasing spanwise width as the distance from the wall increases, and the main vortex characteristic in different wall-normal re- gions can be reflected by comparing the proportion of ejec- tion and its contribution to Reynolds stress with that of sweep event. The pre-multiplied power spectra and two-point cor- relations indicate the presence of large-scale motions in the boundary layer, which are consistent with what have been termed very large scale motions （VLSMs）. The three dimen-sional spatial correlations of three components of veloc- ity further indicate that the elongated low-speed and high- speed regions will be accompanied by a counter-rotating roll modes, as the statistical imprint of hairpin packet structures, all of which together make up the characteristic of coherent structures in the logarithmic region of the turbulent boundary layer （TBL）.展开更多
Si-based multilayer structures are widely used in current microelectronics. During their preparation, some inhomogeneous residual stress is induced, resulting in competition between interface mismatching and surface e...Si-based multilayer structures are widely used in current microelectronics. During their preparation, some inhomogeneous residual stress is induced, resulting in competition between interface mismatching and surface energy and even leading to structure failure. This work presents a methodological study on the measurement of residual stress in a multi-layer semiconductor heterostructure. Scanning electron microscopy（SEM）, micro-Raman spectroscopy（MRS）, and transmission electron microscopy（TEM） were applied to measure the geometric parameters of the multilayer structure. The relationship between the Raman spectrum and the stress/strain on the [100] and [110] crystal orientations was determined to enable surface and crosssection residual stress analyses, respectively. Based on the Raman mapping results, the distribution of residual stress along the depth of the multi-layer heterostructure was successfully obtained.展开更多
The spatial-temporal evolution of coherent structures （CS） is significant for turbulence control and drag re- duction. Among the CS, low and high speed streak structures show typical burst phenomena. The analysis wa...The spatial-temporal evolution of coherent structures （CS） is significant for turbulence control and drag re- duction. Among the CS, low and high speed streak structures show typical burst phenomena. The analysis was based on a time series of three-dimensional and three-component （3D-3C） velocity fields of the flat plate turbulent boundary layer （TBL） measured by a Tomographic and Time-resolved PIV （Tomo TRPIV） system. Using multi-resolution wavelet transform and conditional sampling method, we extracted the intrinsic topologies and found that the streak structures appear in bar-like patterns. Furthermore, we seized locations and velocity information of transient CS, and then calculated the propagation velocity of CS based on spatial-temporal cross-correlation scanning. This laid a foundation for further studies on relevant dynamics properties.展开更多
When using a miniature single sensor boundary layer probe, the time sequences of the stream-wise velocity in the turbulent boundary layer （TBL） are measured by using a hot wire anemometer. Beneath the fully develope...When using a miniature single sensor boundary layer probe, the time sequences of the stream-wise velocity in the turbulent boundary layer （TBL） are measured by using a hot wire anemometer. Beneath the fully developed TBL, the wall pressure fluctuations are attained by a microphone mechanism with high spatial resolution. Analysis on the statistic and spectrum properties of velocity and wall pressure reveals the relationship between the wall pressure fluctuation and the energy-containing structure in the buffer layer of the TBL. Wavelet transform shows the multi-scale natures of coherent structures contained in both signals of velocity and pressure. The most intermittent wall pressure scale is associated with the coherent structure in the buffer layer. Meanwhile the most energetic scale of velocity fluctuation at y＋ = 14 provides a specific frequency f9 ≈ 147 Hz for wall actuating control with Ret = 996.展开更多
The present experimental work focuses on a new model for space-time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wail. A turbulent boundary l...The present experimental work focuses on a new model for space-time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wail. A turbulent boundary layer flow at Reo = 2460 is measured by tomographic particle image velocimetry （tomographic PIV）. It is demonstrated that arch, cane, and hairpin vortices are dominant in the logarithmic layer. Hairpins and hairpin packets are responsible for the elongated low-momentum zones observed in the instantaneous flow field. The conditionally-averaged coherent structures systemically illustrate the key roles of hairpin vortice in the turbulence dynamic events, such as ejection and sweep events and energy transport. The space-time correlations of instantaneous streamwise fluctuation velocity are calculated and confirm the new elliptic model for the space-time correlation instead of Taylor hypothesis. The convection velocities derived from the space-time correlation and conditionally-averaged method both suggest the scaling with the local mean velocity in the logarithmic layer. Convection velocity result based on Fourier decomposition （FD） shows stronger scale- dependency in the spanwise direction than in streamwise direction. Compared with FD, the proper orthogonal decomposition （POD） has a distinct distribution of convection velocity for the large- and small-scales which are separated in light of their contributions of turbulent kinetic energy.展开更多
Forward-/backward-facing steps in boundary-layer flows are often seen in engineering applications, and they have potential impacts on laminar-turbulent transition through scattering of the oncoming instability modes(e...Forward-/backward-facing steps in boundary-layer flows are often seen in engineering applications, and they have potential impacts on laminar-turbulent transition through scattering of the oncoming instability modes(e.g., Tollmien-Schlichting(T-S) waves). This issue is studied in the present paper by applying a local scattering framework, which is a rather generic mathematical framework on describing the mode scattering process. In this framework, a high-Reynolds-number triple-deck formalism is employed, and a transmission coefficient, defined as the ratio of the asymptotic amplitude of the instability mode downstream of the step to that upstream, is introduced. Through the systematical study, it has been found that both the forward-and backward-facing steps have a destabilizing effect on the oncoming T-S waves in subsonic boundary layers, this effect increases with the height of the step and/or the frequency of the T-S wave, and a backward-facing step(BFS) always has a greater impact than a forward-facing step(FFS). These facts agree with most of the previous investigations.However, one numerical study(WORNER, A., RIST, U., and WAGNER, S. Humps/steps influence on stability characteristics of two-dimensional laminar boundary layer. AIAA Journal, 41, 192–197(2003)), which was based on an ad-hoc configuration, showed an opposite impact of an FFS. Through the investigation on the specific configuration, it is revealed that the wrong conclusion was drawn by misinterpreting the numerical results.展开更多
With a piezoelectric （PZT） oscillator, the predetermined controls of the turbulent boundary layer （TBL） are effective in reducing the drag force. The stream-wise velocities in the TBL are accurately measured downs...With a piezoelectric （PZT） oscillator, the predetermined controls of the turbulent boundary layer （TBL） are effective in reducing the drag force. The stream-wise velocities in the TBL are accurately measured downstream of the oscillator driven by an adjustable power source. The mean velocity profiles in the inner and outer scales are reported and the skin friction stresses with different voltage parameters are compared. Reduction of integral spatial scales in the inner region below y＋ of 30 suggests that the oscillator at work breaks up the near-wall stream-wise vortices responsible for high skin friction. For the TBL at Reo of 2183, the controls with a frequency of 160Hz are superior among our experiments and a relative drag reduction rate of 26.83% is exciting. Wavelet analyses provide a reason why the controls with this special frequency perform best.展开更多
The streamwise velocity components at different vertical heights in wall turbulence were measured. Wavelet transform was used to study the turbulent energy spectra, indicating that the global spectrum results from the...The streamwise velocity components at different vertical heights in wall turbulence were measured. Wavelet transform was used to study the turbulent energy spectra, indicating that the global spectrum results from the weighted average of Fourier spectrum based on wavelet scales. W＇avelet transform with more vanishing moments can express the declining of turbulent spectrum. The local wavelet spectrum shows that the physical phenomena such as deformation position in the boundary layer, and the or breakup of eddies are related to the vertical energy-containing eddies exist in a multi-scale form. Moreover, the size of these eddies increases with the measured points moving out of the wall. In the buffer region, the small scale energy-containing eddies with higher frequency are excited. In the outer region, the maximal energy is concentrated in the low-frequency large-scale eddies, and the frequency domain of energy-containing eddies becomes narrower.展开更多
The deformation process of the microstructure in 2205 duplex stainless steel(DSS)under thermo-mechanical coupling at 250℃was investigated using digital image correlation(DIC).A thermal tension test of duplex stainles...The deformation process of the microstructure in 2205 duplex stainless steel(DSS)under thermo-mechanical coupling at 250℃was investigated using digital image correlation(DIC).A thermal tension test of duplex stainless steel(2205DSS)with a banded structure was carded out to observe the initial deformation of the microstructure.It was found that inhomogeneous strain fields occurred primarily in austenite.The maximum normal strain in austenite was almost pos-itive,while that in ferrite was almost negative.In addition,a thermal cyclic-loading test was conducted,and the strain field was characterized by e11.Strain heterogeneities were induced after 400 cycles,which spread within the austenite and at the phase boundaries with the load increasing.The high tensile-strain regions were always located adjacent to regions of intense compressive strain.Based on the strain matrix sum vs.cycle number,we found that hardening occurred in the early cycles followed by softening.展开更多
The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e.,...The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory（LST） for two-dimensional（2D） disturbance on one-dimensional（1D） basic flow, comparison with LST for three-dimensional（3D） disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence（FST）. Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien–Schlichting（TS） waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous＇ in secondary instability.展开更多
Abstract Experiments were conducted in a water tunnel by tomographic time-resolved particle image velocimetry （Tomo-TRPIV）. The Reynolds number Reo is 2 460 on the base of momentum thickness. According to the physic...Abstract Experiments were conducted in a water tunnel by tomographic time-resolved particle image velocimetry （Tomo-TRPIV）. The Reynolds number Reo is 2 460 on the base of momentum thickness. According to the physical mechanism of the stretch and compression of multi-scale vortex structures in the wall-bounded turbulence, the topological characteristics of turbulence statistics in logarithmic layer were illustrated by local-averaged velocity structure function. During coherent structures bursting, results reveal that the topological structures of velocity gradients, velocity strain rates and vorticities behave as antisymmetric quadrupole modes. A three-layer antisymmetric quadrupole vortex packet confirms that there is a tight relationship between the outer layer and the near-wall layer.展开更多
An experimental investigation was performed for active control of coherent structure bursting in the near-wall region of the turbulent boundary layer. By means of synchronous and asynchronous vibrations with double pi...An experimental investigation was performed for active control of coherent structure bursting in the near-wall region of the turbulent boundary layer. By means of synchronous and asynchronous vibrations with double piezoelectric vibrators, the influence of periodic vibration of the double piezoelectric vibrators on the mean velocity profile, drag reduction rate, and coherent structure bursting is analyzed at Reo = 2766. The case with 100 V/160 Hz-ASYN is superior to other conditions in the experiment and a relative drag reduction rate of 18.54% is exciting. Asynchronous vibration is more effective than synchronous vibration in drag reduction at the same voltage and frequency. In all controlled cases, coherent structures at large scales are regulated while the small-scale structures are stimulated. The fluctuating velocity increases significantly. A periodic regulating effect on the coherent structure can be seen in the ASYN control conditions at the frequency of 160 Hz.展开更多
Tendril-bearing climbing plants must recur to the tendril helices with chiral perversion or dual chirality for climbing and to obtain sun exposure. Despite researchers' prolonged fascination with climbing tendrils...Tendril-bearing climbing plants must recur to the tendril helices with chiral perversion or dual chirality for climbing and to obtain sun exposure. Despite researchers' prolonged fascination with climbing tendrils since Darwin's time and even earlier, why the soft and slender tendrils can bear heavy loads such as the self-weight of a plant or additional load caused by rain remains elusive. In this paper, we take towel gourd tendrils as an example and investigate the macroscopic and microscopic mechanical behaviors of tendrils through experiments and simulations. Our study indicates that the tendril filament exhibits rubber-like hyperelastic behaviors and can particularly endure large elongation, which is mainly attributed to the superelasticity of the cellulose fibril helix contained in the cell wall. Combination of the tendril helical structure with dual chirality or chiral perversion at a macroscale and a cellulose filament helix at a subcellular level creates superior elasticity for biological species relying on support and climbing. This study provides deep insight into the structure-property relationship of climbing tendrils, and the relationship is useful for the bioinspired design of composite systems with superior elasticity.展开更多
Lithium-ion batteries suffer from mechano–electrochemical coupling problems that directly determine the battery life. In this paper, we investigate the electrode electrochemical performance under stress conditions, w...Lithium-ion batteries suffer from mechano–electrochemical coupling problems that directly determine the battery life. In this paper, we investigate the electrode electrochemical performance under stress conditions, where seven tensile/compressive stresses are designed and loaded on electrodes, thereby decoupling mechanics and electrochemistry through incremental stress loads. Four types of multi-group electrochemical tests under tensile/compressive stress loading and normal package loading are performed to quantitatively characterize the effects of tensile stress and compressive stress on cycle performance and the kinetic performance of a silicon composite electrode. Experiments show that a tensile stress improves the electrochemical performance of a silicon composite electrode, exhibiting increased specific capacity and capacity retention rate, reduced energy dissipation rate and impedances, enhanced reactivity, accelerated ion/electron migration and diffusion, and reduced polarization. Contrarily, a compressive stress has the opposite effect, inhibiting the electrochemical performance. The stress effect is nonlinear, and a more obvious suppression via compressive stress is observed than an enhancement via tensile stress. For example, a tensile stress of 675 k Pa increases diffusion coefficient by 32.5%, while a compressive stress reduces it by 35%. Based on the experimental results, the stress regulation mechanism is analyzed. Tensile stress loads increase the pores of the electrode material microstructure, providing more deformation spaces and ion/electron transport channels. This relieves contact compressive stress, strengthens diffusion/reaction, and reduces the degree of damage and energy dissipation. Thus, the essence of stress enhancement is that it improves and optimizes diffusion, reaction and stress in the microstructure of electrode material as well as their interactions via physical morphology.展开更多
Active control of a fully developed turbulence boundary layer(TBL) over a flat plate has been investigated with a statistical view. The piezoelectric(PZT) oscillator is employed to produce periodic input into the inne...Active control of a fully developed turbulence boundary layer(TBL) over a flat plate has been investigated with a statistical view. The piezoelectric(PZT) oscillator is employed to produce periodic input into the inner region of the TBL.A wall probe is fixed upstream of the oscillator to identify the high-or low-speed fluctuations as the detecting signals.Then, the impact of the detecting signals on the small-scale bursting process is investigated based on the data acquired by the traversing probe downstream of the oscillator. The results indicate that the small-scale bursting intensity is restrained more apparently at high-speed detecting fluctuations but less impacted at low-speed detecting fluctuations. Furthermore, the perturbed-scale fluctuations arrange the small-scale bursting process in the near-wall region. The detecting signals have an obvious impact on this arrangement, especially the high-intensity regions of the small-scale bursting events: the vibration enhances the intensity at high-speed detecting signals but weakens it at low-speed detecting signals in these regions, which gives a direct evidence on how detecting signals interfering the small-scale bursting process.展开更多
An improved expansion of the parabolized stability equation(iEPSE) method is proposed for the accurate linear instability prediction in boundary layers. It is a local eigenvalue problem, and the streamwise wavenumber ...An improved expansion of the parabolized stability equation(iEPSE) method is proposed for the accurate linear instability prediction in boundary layers. It is a local eigenvalue problem, and the streamwise wavenumber α and its streamwise gradient dα/dx are unknown variables. This eigenvalue problem is solved for the eigenvalue dα/dx with an initial α, and the correction of α is performed with the conservation relation used in the PSE. The i EPSE is validated in several compressible and incompressible boundary layers. The computational results show that the prediction accuracy of the i EPSE is significantly higher than that of the ESPE, and it is in excellent agreement with the PSE which is regarded as the baseline for comparison. In addition, the unphysical multiple eigenmode problem in the EPSE is solved by using the i EPSE. As a local non-parallel stability analysis tool, the i EPSE has great potential application in the eNtransition prediction in general three-dimensional boundary layers.展开更多
In this work, the mechanical behavior of a block of soft material subject to large deformation from a series of wedge-shaped indenters is evaluated. Data fields acquired from digital image correlation （DIC） are comp...In this work, the mechanical behavior of a block of soft material subject to large deformation from a series of wedge-shaped indenters is evaluated. Data fields acquired from digital image correlation （DIC） are compared with the existing theoretical models. The slope angles of the wedges vary from 5° to 73.5°, and the minimum measure- ment uncertainties of the DIC system are established in advance to define the accuracy. It is concluded that the assumptions underpinning the analytical theory make it difficult to characterize large deformation of soft materials during contact. The strain fields are also obtained from the measured displacement field and verify the previously postulated existence of two deformation sectors, namely, a so-called shrinkage sector symmetric to the loading axis and an expansion sector, which become smaller with the increasing load and decreasing wedge angle.展开更多
Flow visualization and hot-wire measurement techniques were combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation mechanism. Streamwise vortices generat...Flow visualization and hot-wire measurement techniques were combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation mechanism. Streamwise vortices generated by the tabs of different sizes and numbers were observed from the flow visualization images. Combined with flow visualization, hot-wire measurement gave a quantitative insight of the effect of various tabbed jet flows. Instantaneous two-component velocity signals (longitudinal and transverse velocity components) at different cross sections along radius direction and streamwise direction with different tabbed jet nozzles were measured using hot-wire anemometer. Average flow field parameters of tabbed jet flow such as mean velocity, tur-bulence intensity, vorticity were analyzed and the effects of tabs with different sizes and numbers were compared with that of circular no-tab jet flow. It is revealed that the generation of a series of counter-rotating quasi-streamwise vortices, azimuthal vortices and double-row azi-muthal vortex are the reasons for mixing enhancement of tabbed turbulent jet flow.展开更多
Experimental measurement of hypersonic boundary layer stability and transition on a sharp cone with a half angle of 5° is carried out at free-coming stream Mach number 6 in a hypersonic wind tunnel. Mean and fluc...Experimental measurement of hypersonic boundary layer stability and transition on a sharp cone with a half angle of 5° is carried out at free-coming stream Mach number 6 in a hypersonic wind tunnel. Mean and fluc- tuation surface-thermal-flux characteristics of the hypersonic boundary layer flow are measured by Pt-thin-film thermocouple temperature sensors installed at 28 stations on the cone surface along longitudinal direction. At hypersonic speeds, the dominant flow instabilities demonstrate that the growth rate of the second mode tends to exceed that of the low-frequency mode. Wavelet-based cross-spectrum technique is introduced to obtain the multi-scale cross-spectral characteristics of the fluctuating signals in the frequency range of the second mode. Nonlinear interactions both of the second mode disturbance and the first mode disturbance axe demonstrated to be dominant instabilities in the initial stage of laminar-turbulence transition for hypersonic shear flow.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12202309,1233000165,12172242,and 12272265)Science and Technology Program of Gansu Province of China(Grant No.22JR5RA304)Tianjin Research Innovation for Postgraduate Students(Grant No.22KJ049)。
文摘The experimental research on zero-net-mass-flux jet closed-loop active control was conducted in the wind tunnel.The mu-level method successfully detected burst events of the coherent structures. The streamwise velocity signals in the turbulent boundary layer were measured by HWA. The drag reduction rate of 16.7% is obtained comparable to that of the open-loop control and saves 75% of the input energy at the asynchronous 100 V/160 Hz control case, which reflects the advantages of the closed-loop control. The experimental findings indicate that the intensity increases in the near-wall region.The perturbation of the PZT vibrators on the skewness factor is concentrated in the region y+< 60. The generation of highspeed fluids is depressed and the downward effect of high-speed fluids weakens. The alteration of energy distribution and the discernible impact of modulation between structures of varying scales are observed. The correlation coefficient exhibits a strong positive correlation, which indicates that the large-scale structures produce modulation effect on small-scale ones.The occurrence of burst events is effectively suppressed. The disturbance has the characteristics of stable periodicity,positive and negative symmetry, low intermittency, and high pulsation strength. The conditional phase waveform shows that the fluctuation amplitude increases, indicating amplitude modulation effects on coherent structures.
基金supported by the National Natural Science Foundation of China (10832001 and 10872145)the State Key Laboratory of Nonlinear Mechanics,Institute of Mechanics,Chinese Academy of Sciences
文摘Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional co- herent structures in the logarithmic region of the turbulent boundary layer in a water tunnel. The Reynolds number based on momentum thickness is Reo = 2 460. The in- stantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low speed fluid, which is flanked on either side by high- speed ones. Statistical support for the existence of hairpins is given by conditional averaged eddy within an increasing spanwise width as the distance from the wall increases, and the main vortex characteristic in different wall-normal re- gions can be reflected by comparing the proportion of ejec- tion and its contribution to Reynolds stress with that of sweep event. The pre-multiplied power spectra and two-point cor- relations indicate the presence of large-scale motions in the boundary layer, which are consistent with what have been termed very large scale motions （VLSMs）. The three dimen-sional spatial correlations of three components of veloc- ity further indicate that the elongated low-speed and high- speed regions will be accompanied by a counter-rotating roll modes, as the statistical imprint of hairpin packet structures, all of which together make up the characteristic of coherent structures in the logarithmic region of the turbulent boundary layer （TBL）.
基金supported by the National Basic Research Program of China (Grant 2012CB937500)the National Natural Science Foundation of China (Grants 11422219, 11227202, 11372217, 11272232)+1 种基金the Program for New Century Excellent Talents in University (Grant NCET-13)China Scholarship Council (201308120092)
文摘Si-based multilayer structures are widely used in current microelectronics. During their preparation, some inhomogeneous residual stress is induced, resulting in competition between interface mismatching and surface energy and even leading to structure failure. This work presents a methodological study on the measurement of residual stress in a multi-layer semiconductor heterostructure. Scanning electron microscopy（SEM）, micro-Raman spectroscopy（MRS）, and transmission electron microscopy（TEM） were applied to measure the geometric parameters of the multilayer structure. The relationship between the Raman spectrum and the stress/strain on the [100] and [110] crystal orientations was determined to enable surface and crosssection residual stress analyses, respectively. Based on the Raman mapping results, the distribution of residual stress along the depth of the multi-layer heterostructure was successfully obtained.
基金supported by the National Natural Science Foundation of China(11332006,11272233,and 11411130150)the National Basic Research Programm of China(2012CB720101)
文摘The spatial-temporal evolution of coherent structures （CS） is significant for turbulence control and drag re- duction. Among the CS, low and high speed streak structures show typical burst phenomena. The analysis was based on a time series of three-dimensional and three-component （3D-3C） velocity fields of the flat plate turbulent boundary layer （TBL） measured by a Tomographic and Time-resolved PIV （Tomo TRPIV） system. Using multi-resolution wavelet transform and conditional sampling method, we extracted the intrinsic topologies and found that the streak structures appear in bar-like patterns. Furthermore, we seized locations and velocity information of transient CS, and then calculated the propagation velocity of CS based on spatial-temporal cross-correlation scanning. This laid a foundation for further studies on relevant dynamics properties.
基金Project supported by the National Basic Research Program of China(Grant Nos.2012CB720101 and 2012CB720103)the National Natural Science Foundation of China(Grant Nos.11272233,11332006,and 11411130150)
文摘When using a miniature single sensor boundary layer probe, the time sequences of the stream-wise velocity in the turbulent boundary layer （TBL） are measured by using a hot wire anemometer. Beneath the fully developed TBL, the wall pressure fluctuations are attained by a microphone mechanism with high spatial resolution. Analysis on the statistic and spectrum properties of velocity and wall pressure reveals the relationship between the wall pressure fluctuation and the energy-containing structure in the buffer layer of the TBL. Wavelet transform shows the multi-scale natures of coherent structures contained in both signals of velocity and pressure. The most intermittent wall pressure scale is associated with the coherent structure in the buffer layer. Meanwhile the most energetic scale of velocity fluctuation at y＋ = 14 provides a specific frequency f9 ≈ 147 Hz for wall actuating control with Ret = 996.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11332006 and 11272233)the National Key Basic Research and Development Program of China(Grant No.2012CB720101)
文摘The present experimental work focuses on a new model for space-time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wail. A turbulent boundary layer flow at Reo = 2460 is measured by tomographic particle image velocimetry （tomographic PIV）. It is demonstrated that arch, cane, and hairpin vortices are dominant in the logarithmic layer. Hairpins and hairpin packets are responsible for the elongated low-momentum zones observed in the instantaneous flow field. The conditionally-averaged coherent structures systemically illustrate the key roles of hairpin vortice in the turbulence dynamic events, such as ejection and sweep events and energy transport. The space-time correlations of instantaneous streamwise fluctuation velocity are calculated and confirm the new elliptic model for the space-time correlation instead of Taylor hypothesis. The convection velocities derived from the space-time correlation and conditionally-averaged method both suggest the scaling with the local mean velocity in the logarithmic layer. Convection velocity result based on Fourier decomposition （FD） shows stronger scale- dependency in the spanwise direction than in streamwise direction. Compared with FD, the proper orthogonal decomposition （POD） has a distinct distribution of convection velocity for the large- and small-scales which are separated in light of their contributions of turbulent kinetic energy.
基金supported by the National Natural Science Foundation of China(Nos.11472189 and11772224)
文摘Forward-/backward-facing steps in boundary-layer flows are often seen in engineering applications, and they have potential impacts on laminar-turbulent transition through scattering of the oncoming instability modes(e.g., Tollmien-Schlichting(T-S) waves). This issue is studied in the present paper by applying a local scattering framework, which is a rather generic mathematical framework on describing the mode scattering process. In this framework, a high-Reynolds-number triple-deck formalism is employed, and a transmission coefficient, defined as the ratio of the asymptotic amplitude of the instability mode downstream of the step to that upstream, is introduced. Through the systematical study, it has been found that both the forward-and backward-facing steps have a destabilizing effect on the oncoming T-S waves in subsonic boundary layers, this effect increases with the height of the step and/or the frequency of the T-S wave, and a backward-facing step(BFS) always has a greater impact than a forward-facing step(FFS). These facts agree with most of the previous investigations.However, one numerical study(WORNER, A., RIST, U., and WAGNER, S. Humps/steps influence on stability characteristics of two-dimensional laminar boundary layer. AIAA Journal, 41, 192–197(2003)), which was based on an ad-hoc configuration, showed an opposite impact of an FFS. Through the investigation on the specific configuration, it is revealed that the wrong conclusion was drawn by misinterpreting the numerical results.
基金supported by the National Natural Science Foundation of China(Grant Nos.11332006,11272233,and 11411130150)the National Basic Research Program of China(Grant Nos.2012CB720101 and 2012CB720103)
文摘With a piezoelectric （PZT） oscillator, the predetermined controls of the turbulent boundary layer （TBL） are effective in reducing the drag force. The stream-wise velocities in the TBL are accurately measured downstream of the oscillator driven by an adjustable power source. The mean velocity profiles in the inner and outer scales are reported and the skin friction stresses with different voltage parameters are compared. Reduction of integral spatial scales in the inner region below y＋ of 30 suggests that the oscillator at work breaks up the near-wall stream-wise vortices responsible for high skin friction. For the TBL at Reo of 2183, the controls with a frequency of 160Hz are superior among our experiments and a relative drag reduction rate of 26.83% is exciting. Wavelet analyses provide a reason why the controls with this special frequency perform best.
基金supported by the National Natural Science Foundation of China (Nos. 10832001 and10872145)the Program for New Century Excellent Talents in Universities of Education Min-istry of China
文摘The streamwise velocity components at different vertical heights in wall turbulence were measured. Wavelet transform was used to study the turbulent energy spectra, indicating that the global spectrum results from the weighted average of Fourier spectrum based on wavelet scales. W＇avelet transform with more vanishing moments can express the declining of turbulent spectrum. The local wavelet spectrum shows that the physical phenomena such as deformation position in the boundary layer, and the or breakup of eddies are related to the vertical energy-containing eddies exist in a multi-scale form. Moreover, the size of these eddies increases with the measured points moving out of the wall. In the buffer region, the small scale energy-containing eddies with higher frequency are excited. In the outer region, the maximal energy is concentrated in the low-frequency large-scale eddies, and the frequency domain of energy-containing eddies becomes narrower.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11472187 and 11602166)the National Basic Research Program of China(Grant No.2014CB046805)the Natural Science Foundation of Tianjin,China(Grant No.16JCYBJC40500)
文摘The deformation process of the microstructure in 2205 duplex stainless steel(DSS)under thermo-mechanical coupling at 250℃was investigated using digital image correlation(DIC).A thermal tension test of duplex stainless steel(2205DSS)with a banded structure was carded out to observe the initial deformation of the microstructure.It was found that inhomogeneous strain fields occurred primarily in austenite.The maximum normal strain in austenite was almost pos-itive,while that in ferrite was almost negative.In addition,a thermal cyclic-loading test was conducted,and the strain field was characterized by e11.Strain heterogeneities were induced after 400 cycles,which spread within the austenite and at the phase boundaries with the load increasing.The high tensile-strain regions were always located adjacent to regions of intense compressive strain.Based on the strain matrix sum vs.cycle number,we found that hardening occurred in the early cycles followed by softening.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.1120214711332007+2 种基金11172203and 91216111)the Specialized Research Fund for the Doctoral Program of Higher Education of China(Grant No.20120032120007)
文摘The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory（LST） for two-dimensional（2D） disturbance on one-dimensional（1D） basic flow, comparison with LST for three-dimensional（3D） disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence（FST）. Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien–Schlichting（TS） waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous＇ in secondary instability.
基金supported by the National Natural Science Fundation of China (11272233)National Basic Research Program (973 Program) (2012CB720101)2013 Opening Fund of LNM,Institute of Mechanics,Chinese Academy of Sciences
文摘Abstract Experiments were conducted in a water tunnel by tomographic time-resolved particle image velocimetry （Tomo-TRPIV）. The Reynolds number Reo is 2 460 on the base of momentum thickness. According to the physical mechanism of the stretch and compression of multi-scale vortex structures in the wall-bounded turbulence, the topological characteristics of turbulence statistics in logarithmic layer were illustrated by local-averaged velocity structure function. During coherent structures bursting, results reveal that the topological structures of velocity gradients, velocity strain rates and vorticities behave as antisymmetric quadrupole modes. A three-layer antisymmetric quadrupole vortex packet confirms that there is a tight relationship between the outer layer and the near-wall layer.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11332006,11732010,11572221,and 11502066)
文摘An experimental investigation was performed for active control of coherent structure bursting in the near-wall region of the turbulent boundary layer. By means of synchronous and asynchronous vibrations with double piezoelectric vibrators, the influence of periodic vibration of the double piezoelectric vibrators on the mean velocity profile, drag reduction rate, and coherent structure bursting is analyzed at Reo = 2766. The case with 100 V/160 Hz-ASYN is superior to other conditions in the experiment and a relative drag reduction rate of 18.54% is exciting. Asynchronous vibration is more effective than synchronous vibration in drag reduction at the same voltage and frequency. In all controlled cases, coherent structures at large scales are regulated while the small-scale structures are stimulated. The fluctuating velocity increases significantly. A periodic regulating effect on the coherent structure can be seen in the ASYN control conditions at the frequency of 160 Hz.
基金the National Natural Science Foundation of China (Grants 11872273, 11472191, 11602163, and 11672297)the Major Program of the National Science Foundation of China (Grant 11890683)+1 种基金the Opening Fund of State Key Laboratory of Nonlinear Mechanicsthe Australian Endeavour Research Fellowship.
文摘Tendril-bearing climbing plants must recur to the tendril helices with chiral perversion or dual chirality for climbing and to obtain sun exposure. Despite researchers' prolonged fascination with climbing tendrils since Darwin's time and even earlier, why the soft and slender tendrils can bear heavy loads such as the self-weight of a plant or additional load caused by rain remains elusive. In this paper, we take towel gourd tendrils as an example and investigate the macroscopic and microscopic mechanical behaviors of tendrils through experiments and simulations. Our study indicates that the tendril filament exhibits rubber-like hyperelastic behaviors and can particularly endure large elongation, which is mainly attributed to the superelasticity of the cellulose fibril helix contained in the cell wall. Combination of the tendril helical structure with dual chirality or chiral perversion at a macroscale and a cellulose filament helix at a subcellular level creates superior elasticity for biological species relying on support and climbing. This study provides deep insight into the structure-property relationship of climbing tendrils, and the relationship is useful for the bioinspired design of composite systems with superior elasticity.
基金Project supported by the Major Program of the National Natural Science Foundation of China(Grant No.11890680)the National Natural Science Foundation of China(Grant No.12022205)。
文摘Lithium-ion batteries suffer from mechano–electrochemical coupling problems that directly determine the battery life. In this paper, we investigate the electrode electrochemical performance under stress conditions, where seven tensile/compressive stresses are designed and loaded on electrodes, thereby decoupling mechanics and electrochemistry through incremental stress loads. Four types of multi-group electrochemical tests under tensile/compressive stress loading and normal package loading are performed to quantitatively characterize the effects of tensile stress and compressive stress on cycle performance and the kinetic performance of a silicon composite electrode. Experiments show that a tensile stress improves the electrochemical performance of a silicon composite electrode, exhibiting increased specific capacity and capacity retention rate, reduced energy dissipation rate and impedances, enhanced reactivity, accelerated ion/electron migration and diffusion, and reduced polarization. Contrarily, a compressive stress has the opposite effect, inhibiting the electrochemical performance. The stress effect is nonlinear, and a more obvious suppression via compressive stress is observed than an enhancement via tensile stress. For example, a tensile stress of 675 k Pa increases diffusion coefficient by 32.5%, while a compressive stress reduces it by 35%. Based on the experimental results, the stress regulation mechanism is analyzed. Tensile stress loads increase the pores of the electrode material microstructure, providing more deformation spaces and ion/electron transport channels. This relieves contact compressive stress, strengthens diffusion/reaction, and reduces the degree of damage and energy dissipation. Thus, the essence of stress enhancement is that it improves and optimizes diffusion, reaction and stress in the microstructure of electrode material as well as their interactions via physical morphology.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11972251,11732010,11572221,11502066,and 11872272)。
文摘Active control of a fully developed turbulence boundary layer(TBL) over a flat plate has been investigated with a statistical view. The piezoelectric(PZT) oscillator is employed to produce periodic input into the inner region of the TBL.A wall probe is fixed upstream of the oscillator to identify the high-or low-speed fluctuations as the detecting signals.Then, the impact of the detecting signals on the small-scale bursting process is investigated based on the data acquired by the traversing probe downstream of the oscillator. The results indicate that the small-scale bursting intensity is restrained more apparently at high-speed detecting fluctuations but less impacted at low-speed detecting fluctuations. Furthermore, the perturbed-scale fluctuations arrange the small-scale bursting process in the near-wall region. The detecting signals have an obvious impact on this arrangement, especially the high-intensity regions of the small-scale bursting events: the vibration enhances the intensity at high-speed detecting signals but weakens it at low-speed detecting signals in these regions, which gives a direct evidence on how detecting signals interfering the small-scale bursting process.
基金Project supported by the National Natural Science Foundation of China(Nos.11332007,11402167,11672205,and 11732011)the National Key Research and Development Program of China(No.2016YFA0401200)
文摘An improved expansion of the parabolized stability equation(iEPSE) method is proposed for the accurate linear instability prediction in boundary layers. It is a local eigenvalue problem, and the streamwise wavenumber α and its streamwise gradient dα/dx are unknown variables. This eigenvalue problem is solved for the eigenvalue dα/dx with an initial α, and the correction of α is performed with the conservation relation used in the PSE. The i EPSE is validated in several compressible and incompressible boundary layers. The computational results show that the prediction accuracy of the i EPSE is significantly higher than that of the ESPE, and it is in excellent agreement with the PSE which is regarded as the baseline for comparison. In addition, the unphysical multiple eigenmode problem in the EPSE is solved by using the i EPSE. As a local non-parallel stability analysis tool, the i EPSE has great potential application in the eNtransition prediction in general three-dimensional boundary layers.
基金Project supported by the National Basic Research Program of China(No.2012CB937500)the National Natural Science Foundation of China(No.11127292)
文摘In this work, the mechanical behavior of a block of soft material subject to large deformation from a series of wedge-shaped indenters is evaluated. Data fields acquired from digital image correlation （DIC） are compared with the existing theoretical models. The slope angles of the wedges vary from 5° to 73.5°, and the minimum measure- ment uncertainties of the DIC system are established in advance to define the accuracy. It is concluded that the assumptions underpinning the analytical theory make it difficult to characterize large deformation of soft materials during contact. The strain fields are also obtained from the measured displacement field and verify the previously postulated existence of two deformation sectors, namely, a so-called shrinkage sector symmetric to the loading axis and an expansion sector, which become smaller with the increasing load and decreasing wedge angle.
基金National Natural Science Foundation of China (No.10472081)Program for New Century Excellent Talents in Universities of Minis-try of Education of China and Plan of Tianjin Science and Technology Development (No.06TXTJJC13800)
文摘Flow visualization and hot-wire measurement techniques were combined to investigate the influence of the size and number of tabs on jet flow field and vortex structure generation mechanism. Streamwise vortices generated by the tabs of different sizes and numbers were observed from the flow visualization images. Combined with flow visualization, hot-wire measurement gave a quantitative insight of the effect of various tabbed jet flows. Instantaneous two-component velocity signals (longitudinal and transverse velocity components) at different cross sections along radius direction and streamwise direction with different tabbed jet nozzles were measured using hot-wire anemometer. Average flow field parameters of tabbed jet flow such as mean velocity, tur-bulence intensity, vorticity were analyzed and the effects of tabs with different sizes and numbers were compared with that of circular no-tab jet flow. It is revealed that the generation of a series of counter-rotating quasi-streamwise vortices, azimuthal vortices and double-row azi-muthal vortex are the reasons for mixing enhancement of tabbed turbulent jet flow.
基金Supported by the National Natural Science Foundation of China under Grant No 10472081, the New Century Excellent Talent Project （NCET） of the Ministry of Education of China, and Plan of Tianjin Science and Technology Development under Grant No 06TXTJJC13800.
文摘Experimental measurement of hypersonic boundary layer stability and transition on a sharp cone with a half angle of 5° is carried out at free-coming stream Mach number 6 in a hypersonic wind tunnel. Mean and fluc- tuation surface-thermal-flux characteristics of the hypersonic boundary layer flow are measured by Pt-thin-film thermocouple temperature sensors installed at 28 stations on the cone surface along longitudinal direction. At hypersonic speeds, the dominant flow instabilities demonstrate that the growth rate of the second mode tends to exceed that of the low-frequency mode. Wavelet-based cross-spectrum technique is introduced to obtain the multi-scale cross-spectral characteristics of the fluctuating signals in the frequency range of the second mode. Nonlinear interactions both of the second mode disturbance and the first mode disturbance axe demonstrated to be dominant instabilities in the initial stage of laminar-turbulence transition for hypersonic shear flow.
