Orthogonal frequency division multiplexing(OFDM) is an attractive technique to realize high data rate in light emitting diodes(LEDs)-based visible light communication(VLC). However, high peak-to-average power ratio(PA...Orthogonal frequency division multiplexing(OFDM) is an attractive technique to realize high data rate in light emitting diodes(LEDs)-based visible light communication(VLC). However, high peak-to-average power ratio(PAPR) of OFDM makes VLC-OFDM very sensitive to the nonlinearity of LEDs. In this paper, the discrete Fourier transform-spread(DFT-spread) combined with clipping method is proposed to reduce the PAPR of OFDM signal in VLC system. Combining simulation with experiment, a performance comparison is made among conventional OFDM, DFT-spread-OFDM, and clipped DFT-spread-OFDM with different clipping ratios(CRs) in a single LED-based VLC system. The experimental results show that the proposed clipped DFT-spread-OFDM method can effectively improve the system performance compared with the other two methods. At the optimum signal peak-to-peak(PTP) value, by using the clipped DFT-spread-OFDM scheme with CR at 8 dB, the bit error rate(BER) of the system can be reduced from 0.003 7 to 0.000 287.展开更多
As the traditional forging process has many problems such as low efficiency, high consumption of material and energy, large cylindrical shell rolling is introduced. Large cylindrical shell rolling is a typical rotary ...As the traditional forging process has many problems such as low efficiency, high consumption of material and energy, large cylindrical shell rolling is introduced. Large cylindrical shell rolling is a typical rotary forming technology, and the upper and lower rolls have different radii and speeds. To quickly predict the three-dimensional stresses and eliminate fishtail defect, an improved strip layer method is developed, in which the asymmetry of the upper and lower rolls, non-uniform deformation and stress, as well as the asymmetrical spread on the end surface are considered. The deformation zone is divided into a certain number of layers and strips along the thickness and width, respectively. The transverse displacement model is constructed by polynomial function, in order to increase the computation speed greatly. From the metal plastic mechanics principle, the three-dimensional stress models are established. The genetic algorithm is used for optimization calculation in an industrial experiment example. The results show that the rolling pressure, the normal stresses, the upper and lower friction stress distributions are not similar with those of a general plate rolling. There are two relative maximum values in rolling pressure distribution. The upper and lower longitudinal friction stresses change direction nearby the upper and lower neutral points, respectively. The fishtail profile of spread on the end surface is predicted satisfactorily. The reduction could be helpful to eliminate fishtail defect. The large cylindrical shell rolling example illustrates the calculation results acquired rapidly are good agreements with the finite element simulation and experimental values of previous study. A highly effective and reliable three-dimensional simulation method is proposed for large cylindrical shell rolling and other asymmetrical rolling.展开更多
The approach of getting useful information of monthly dynamical prediction from ensemble forecasts is studied. The extended range ensemble forecasts (8 members, the initial perturbations of the lagged average forecast...The approach of getting useful information of monthly dynamical prediction from ensemble forecasts is studied. The extended range ensemble forecasts (8 members, the initial perturbations of the lagged average forecast (LAF)(0000, 0600, 1200 and 1800 GMT in two consecutive days) of the 500 hPa height field with the global spectral model (T63L16) from January to May 1997 are provided by the National Climate Center of China. The relationship between the spread of ensemble measured by root–mean–square deviation of ensemble member from ensemble mean and forecast skill (the anomaly correlation or the root–mean–square distance between the ensemble mean forecast and the observation) is significant. The spread of ensemble can evaluate the useful forecast days N for the best estimate of 30 days mean. Thus, a weighted mean approach based on ensemble spread is put forward for monthly dynamical prediction. The anomaly correlation of the weighted monthly mean by the ensemble spread is higher than that of both the arithmetic mean and the linear weighted mean. Better results of the monthly mean circulation and anomaly are obtained from the ensemble spread weighted mean. Key words Monthly prediction - Ensemble method - Spread of ensemble Supported by the Excellent National State Key Laboratory Project (49823002), the National Key Project ‘Study on Chinese Short-Term Climate Forecast System’ (96-908-02) and IAP Innovation Foundation (8-1308).The data were provided through the National Climate Center of China. The authors wish to thank Ms. Chen Lijuan for her assistance.展开更多
Lateral spread of frozen ground crust over liquefied soil has caused extensive bridge foundation damage in the past winter earthquakes.A shake table experiment was conducted to investigate the performance of a model p...Lateral spread of frozen ground crust over liquefied soil has caused extensive bridge foundation damage in the past winter earthquakes.A shake table experiment was conducted to investigate the performance of a model pile in this scenario and revealed unique pile failure mechanisms.The modelling results provided valuable data for validating numerical models.This paper presents analyses and results of this experiment using two numerical modeling approaches： solid-fluid coupled finite element（FE） modeling and the beam-on-nonlinear-Winkler-foundation（BNWF） method.A FE model was constructed based on the experiment configuration and subjected to earthquake loading.Soil and pile response results were presented and compared with experimental results to validate this model.The BNWF method was used to predict the pile response and failure mechanism.A p-y curve was presented for modelling the frozen ground crust with the free-field displacement from the experiment as loading.Pile responses were presented and compared with those of the experiment and FE model.It was concluded that the coupled FE model was effective in predicting formation of three plastic hinges at ground surface,ground crust-liquefiable soil interface and within the medium dense sand layer,while the BNWF method was only able to predict the latter two.展开更多
The edge method is used to measure the source spot-size. In this paper, the measuring principle and applying range are discussed. It is shown that the method can directly be used to measure the spot-size of either lig...The edge method is used to measure the source spot-size. In this paper, the measuring principle and applying range are discussed. It is shown that the method can directly be used to measure the spot-size of either light source, or low-energy x-ray source, or x-ray source with an energy higher than 250 keV.展开更多
The goal of tomography is to reconstruct a spatially-varying image function s(x,m), where x is position and m is a finite-length vector of parameters. Many reconstruction methods minimize the total L2 error E ≡ eTe, ...The goal of tomography is to reconstruct a spatially-varying image function s(x,m), where x is position and m is a finite-length vector of parameters. Many reconstruction methods minimize the total L2 error E ≡ eTe, where individual errors ei quantify misfit between predictions and observations, to quantify goodness of fit. So-called adjoint state methods allow the gradient ∂E/∂mi to be computed extremely efficiently from an adjoint field, facilitating image reconstruction by gradient-descent methods. We examine the structure of the differential equation for the adjoint field under the ray approximation and find that it has the same form as the transport equation, whose solution involves the well-known geometrical spreading function R Consequently, as R is routinely tabulated as part of a ray calculation, no extra work is needed to compute the adjoint field, permitting a rapid calculation of the gradient?∂E/∂mi.展开更多
激光粉末床熔融(laser powder bed fusion, LPBF)增材制造技术在金属多材料结构制造方面取得了长足的进步,而异质粉末床的质量是影响成形件质量的关键之一,其中颗粒间内聚力对粉末扩散过程起着关键作用。以316L不锈钢和CuSn10合金为例,...激光粉末床熔融(laser powder bed fusion, LPBF)增材制造技术在金属多材料结构制造方面取得了长足的进步,而异质粉末床的质量是影响成形件质量的关键之一,其中颗粒间内聚力对粉末扩散过程起着关键作用。以316L不锈钢和CuSn10合金为例,使用离散元的方法来数值模拟多材料粉末床的铺展过程,评估颗粒间的内聚力对粉末层质量的影响。同时研究了内聚力对沿着铺粉方向的清晰分界线的异质粉末床的影响。研究发现:随着颗粒间的内聚力增强,粉末的铺展性越差,粒度偏析现象加重;内聚力使得粉末团聚运动,造成粉末交叉污染;聚力变大使得粉末流动性变差,休止角变大,边界质量差且陡峭。展开更多
为研究液滴撞击不同润湿性加热表面的传热传质问题,基于体积分数(volume of fluid, VOF)方法,分析了壁面润湿性和表面温度对液滴形态及传热特性的作用机理,结果表明:亲水壁面有利于液滴的铺展,疏水壁面则有利于液滴的反弹,随着接触角的...为研究液滴撞击不同润湿性加热表面的传热传质问题,基于体积分数(volume of fluid, VOF)方法,分析了壁面润湿性和表面温度对液滴形态及传热特性的作用机理,结果表明:亲水壁面有利于液滴的铺展,疏水壁面则有利于液滴的反弹,随着接触角的增大,液滴最大铺展因子减小,达最大铺展因子的时间缩短,壁面平均热流密度减小。表面温度对液滴铺展阶段的影响较小,随着表面温度的升高,液滴相变速率加快,壁面平均热流密度增大,表面温度超过临界温度时,产生Leidenfrost现象。展开更多
文摘Orthogonal frequency division multiplexing(OFDM) is an attractive technique to realize high data rate in light emitting diodes(LEDs)-based visible light communication(VLC). However, high peak-to-average power ratio(PAPR) of OFDM makes VLC-OFDM very sensitive to the nonlinearity of LEDs. In this paper, the discrete Fourier transform-spread(DFT-spread) combined with clipping method is proposed to reduce the PAPR of OFDM signal in VLC system. Combining simulation with experiment, a performance comparison is made among conventional OFDM, DFT-spread-OFDM, and clipped DFT-spread-OFDM with different clipping ratios(CRs) in a single LED-based VLC system. The experimental results show that the proposed clipped DFT-spread-OFDM method can effectively improve the system performance compared with the other two methods. At the optimum signal peak-to-peak(PTP) value, by using the clipped DFT-spread-OFDM scheme with CR at 8 dB, the bit error rate(BER) of the system can be reduced from 0.003 7 to 0.000 287.
基金Supported by National Science and Technology Major Project of China(Grant No.2011ZX04002-101)National Science and Technology Support Plan of China(Grant No.2011BAF15B02)National Natural Science Foundation of China(Grant No.51305388)
文摘As the traditional forging process has many problems such as low efficiency, high consumption of material and energy, large cylindrical shell rolling is introduced. Large cylindrical shell rolling is a typical rotary forming technology, and the upper and lower rolls have different radii and speeds. To quickly predict the three-dimensional stresses and eliminate fishtail defect, an improved strip layer method is developed, in which the asymmetry of the upper and lower rolls, non-uniform deformation and stress, as well as the asymmetrical spread on the end surface are considered. The deformation zone is divided into a certain number of layers and strips along the thickness and width, respectively. The transverse displacement model is constructed by polynomial function, in order to increase the computation speed greatly. From the metal plastic mechanics principle, the three-dimensional stress models are established. The genetic algorithm is used for optimization calculation in an industrial experiment example. The results show that the rolling pressure, the normal stresses, the upper and lower friction stress distributions are not similar with those of a general plate rolling. There are two relative maximum values in rolling pressure distribution. The upper and lower longitudinal friction stresses change direction nearby the upper and lower neutral points, respectively. The fishtail profile of spread on the end surface is predicted satisfactorily. The reduction could be helpful to eliminate fishtail defect. The large cylindrical shell rolling example illustrates the calculation results acquired rapidly are good agreements with the finite element simulation and experimental values of previous study. A highly effective and reliable three-dimensional simulation method is proposed for large cylindrical shell rolling and other asymmetrical rolling.
基金Supported by the Excellent National State Key Laboratory Project! (49823002)the National Key Project 'Study on Chinese Short
文摘The approach of getting useful information of monthly dynamical prediction from ensemble forecasts is studied. The extended range ensemble forecasts (8 members, the initial perturbations of the lagged average forecast (LAF)(0000, 0600, 1200 and 1800 GMT in two consecutive days) of the 500 hPa height field with the global spectral model (T63L16) from January to May 1997 are provided by the National Climate Center of China. The relationship between the spread of ensemble measured by root–mean–square deviation of ensemble member from ensemble mean and forecast skill (the anomaly correlation or the root–mean–square distance between the ensemble mean forecast and the observation) is significant. The spread of ensemble can evaluate the useful forecast days N for the best estimate of 30 days mean. Thus, a weighted mean approach based on ensemble spread is put forward for monthly dynamical prediction. The anomaly correlation of the weighted monthly mean by the ensemble spread is higher than that of both the arithmetic mean and the linear weighted mean. Better results of the monthly mean circulation and anomaly are obtained from the ensemble spread weighted mean. Key words Monthly prediction - Ensemble method - Spread of ensemble Supported by the Excellent National State Key Laboratory Project (49823002), the National Key Project ‘Study on Chinese Short-Term Climate Forecast System’ (96-908-02) and IAP Innovation Foundation (8-1308).The data were provided through the National Climate Center of China. The authors wish to thank Ms. Chen Lijuan for her assistance.
基金US Department of Transportation through Alaska University Transportation Centerthe State of Alaska Department of Transportation and Public Facilities(AK DOT&PF)under Project AUTC#410015
文摘Lateral spread of frozen ground crust over liquefied soil has caused extensive bridge foundation damage in the past winter earthquakes.A shake table experiment was conducted to investigate the performance of a model pile in this scenario and revealed unique pile failure mechanisms.The modelling results provided valuable data for validating numerical models.This paper presents analyses and results of this experiment using two numerical modeling approaches： solid-fluid coupled finite element（FE） modeling and the beam-on-nonlinear-Winkler-foundation（BNWF） method.A FE model was constructed based on the experiment configuration and subjected to earthquake loading.Soil and pile response results were presented and compared with experimental results to validate this model.The BNWF method was used to predict the pile response and failure mechanism.A p-y curve was presented for modelling the frozen ground crust with the free-field displacement from the experiment as loading.Pile responses were presented and compared with those of the experiment and FE model.It was concluded that the coupled FE model was effective in predicting formation of three plastic hinges at ground surface,ground crust-liquefiable soil interface and within the medium dense sand layer,while the BNWF method was only able to predict the latter two.
文摘The edge method is used to measure the source spot-size. In this paper, the measuring principle and applying range are discussed. It is shown that the method can directly be used to measure the spot-size of either light source, or low-energy x-ray source, or x-ray source with an energy higher than 250 keV.
文摘The goal of tomography is to reconstruct a spatially-varying image function s(x,m), where x is position and m is a finite-length vector of parameters. Many reconstruction methods minimize the total L2 error E ≡ eTe, where individual errors ei quantify misfit between predictions and observations, to quantify goodness of fit. So-called adjoint state methods allow the gradient ∂E/∂mi to be computed extremely efficiently from an adjoint field, facilitating image reconstruction by gradient-descent methods. We examine the structure of the differential equation for the adjoint field under the ray approximation and find that it has the same form as the transport equation, whose solution involves the well-known geometrical spreading function R Consequently, as R is routinely tabulated as part of a ray calculation, no extra work is needed to compute the adjoint field, permitting a rapid calculation of the gradient?∂E/∂mi.
文摘激光粉末床熔融(laser powder bed fusion, LPBF)增材制造技术在金属多材料结构制造方面取得了长足的进步,而异质粉末床的质量是影响成形件质量的关键之一,其中颗粒间内聚力对粉末扩散过程起着关键作用。以316L不锈钢和CuSn10合金为例,使用离散元的方法来数值模拟多材料粉末床的铺展过程,评估颗粒间的内聚力对粉末层质量的影响。同时研究了内聚力对沿着铺粉方向的清晰分界线的异质粉末床的影响。研究发现:随着颗粒间的内聚力增强,粉末的铺展性越差,粒度偏析现象加重;内聚力使得粉末团聚运动,造成粉末交叉污染;聚力变大使得粉末流动性变差,休止角变大,边界质量差且陡峭。
文摘为研究液滴撞击不同润湿性加热表面的传热传质问题,基于体积分数(volume of fluid, VOF)方法,分析了壁面润湿性和表面温度对液滴形态及传热特性的作用机理,结果表明:亲水壁面有利于液滴的铺展,疏水壁面则有利于液滴的反弹,随着接触角的增大,液滴最大铺展因子减小,达最大铺展因子的时间缩短,壁面平均热流密度减小。表面温度对液滴铺展阶段的影响较小,随着表面温度的升高,液滴相变速率加快,壁面平均热流密度增大,表面温度超过临界温度时,产生Leidenfrost现象。
