Dryden Wind Turbulence Model (Continuous)翻译.docx

Aerospace BlocksetDryden Wind Turbulence Model (Continuous)Generate continuous wind turbulence连续风紊流 with Dryden velocity spectra德莱顿速度谱LibraryEnvironment/WindDescriptionThe Dryden Wind Turbulence Model (Continuous) block uses the Dryden spectral representation德莱顿谱表示 to add turbulence to the aerospace model by passing band-limited white noise through appropriate forming filters将带限白噪声通过适当的成形滤波器. This block implements the mathematical representation数学表达 in the Military Specification MIL-F-8785C and Military Handbook MIL-HDBK-1797.According to the military references, turbulence is a stochastic process defined by velocity spectra紊流是由速度谱定义的随机过程. For an aircraft flying at a speed V through a frozen turbulence field with a spatial frequency of radians per meter,the circular frequency is calculated by multiplying V by . The following table displays the component spectra functions:The variable b represents the aircraft wingspan b为翼展. The variables Lu , Lv , Lw represent the turbulence scale lengths标尺长度. The variables u , v , w represent the turbulence intensities紊流强度.The spectral density definitions of turbulence angular rates紊流角速率的谱密度定义 are defined in the specifications as three variations, which are displayed in the following table:The variations affect only the vertical (qg) and lateral (rg) turbulence angular rates.Keep in mind that the longitudinal turbulence angular rate spectrum is a rational function. The rational function is derived from curve-fitting曲线拟合 a complex algebraic function, not the vertical turbulence velocity spectrum, w(), multiplied by a scale factor. Because the turbulence angular rate spectra contribute less to the aircraft gust response than the turbulence velocity spectra, it may explain the variations in their definitions.The variations lead to the following combinations of vertical and lateral turbulence angular rate spectra:VerticalLateralq()q()q()r()r()r()To generate a signal with the correct characteristics, a unit variance, band-limited white noise signal is passed through forming filters. The forming filters are derived from the spectral square roots of the spectrum equations.The following table displays the transfer functions:Divided into two distinct regions, the turbulence scale lengths and intensities are functions of altitude紊流的标尺长度和强度被分成两个不同的区域，都是高度的函数.Note The military specifications result in the same transfer function after evaluating the turbulence scale lengths. The differences in turbulence scale lengths and turbulence transfer functions balance offset.Low-Altitude Model (Altitude 2000 feet)For medium to high altitudes the turbulence scale lengths and intensities are based on the assumption that the turbulence is isotropic各向同性的. In the military references, the scale lengths are represented by the following equations:MIL-F-8785CMIL-HDBK-1797Lu = Lv = Lw = 1750 ftLu = 2Lv = 2Lw = 1750 ftThe turbulence intensities are determined from a lookup table 查表that provides the turbulence intensity as a function of altitude and the probability of the turbulence intensity being exceeded. The relationship of the turbulence intensities is represented in the following equation: u = v = w .The turbulence axes orientation in this region is defined as being aligned with the body coordinates.Between Low and Medium/High Altitudes (1000 feet Altitude 2000 feet)At altitudes between 1000 feet and 2000 feet, the turbulence velocities and turbulence angular rates are determined by linearly interpolating between the value from the low altitude model at 1000 feet transformed from mean horizontal wind coordinates to body coordinates and the value from the high altitude model at 2000 feet in body coordinates.Dialog BoxUnitsDefine the units of wind speed due to the turbulence.UnitsWind VelocityAltitudeAirspeedMetric (MKS)Meters/secondMetersMeters/secondEnglish (Velocity in ft/s)Feet/secondFeetFeet/secondEnglish (Velocity in kts)KnotsFeetKnotsSpecificationDefine which military reference to use. This affects the application of turbulence scale lengths in the lateral and vertical directions.Model typeSelect the wind turbulence model to use.Continuous Von Karman (+q -r)Use continuous representation of Von Krmn velocity spectra with positive vertical and negative lateral angular rates spectra.Continuous Von Karman (+q +r)Use continuous representation of Von Krmn velocity spectra with positive vertical and lateral angular rates spectra.Continuous Von Karman (-q +r)Use continuous representation of Von Krmn velocity spectra with negative vertical and positive lateral angular rates spectra.Continuous Dryden (+q -r)Use continuous representation of Dryden velocity spectra with positive vertical and negative lateral angular rates spectra.Continuous Dryden (+q +r)Use continuous representation of Dryden velocity spectra with positive vertical and lateral angular rates spectra.Continuous Dryden (-q +r)Use continuous representation of Dryden velocity spectra with negative vertical and positive lateral angular rates spectra.Discrete Dryden (+q -r)Use discrete representation of Dryden velocity spectra with positive vertical and negative lateral angular rates spectra.Discrete Dryden (+q +r)Use discrete representation of Dryden velocity spectra with positive vertical and lateral angular rates spectra.Discrete Dryden (-q +r)Use discrete representation of Dryden velocity spectra with negative vertical and positive lateral angular rates spectra.The Continuous Dryden selections conform to the transfer function descriptions.Wind speed at 6 m defines the low altitude intensityThe measured wind speed at a height of 6 meters (20 feet) provides the intensity for the low-altitude turbulence model.Wind direction at 6 m (degrees clockwise from north)The measured wind direction at a height of 6 meters (20 feet) is an angle to aid in transforming the low-altitude turbulence model into a body coordinates.Probability of exceedance of high-altitude intensityAbove 2000 feet, the turbulence intensity is determined from a lookup table that gives the turbulence intensity as a function of altitude and the probability of the turbulence intensitys being exceeded.Scale length at medium/high altitudes (m)The turbulence scale length above 2000 feet is assumed constant, and from the military references, a figure of 1750 feet is recommended for the longitudinal turbulence scale length of the Dryden spectra.Note An alternate scale length value changes the power spectral density asymptote and gust load.WingspanThe wingspan is required in the calculation of the turbulence on the angular rates.Band-limited noise sample time (sec)The sample time at which the unit variance white noise signal is generated.Noise seedsThere are four random numbers required to generate the turbulence signals, one for each of the three velocity components and one for the roll rate. The turbulences on the pitch and yaw angular rates are based on further shaping of the outputs from the shaping filters for the vertical and lateral velocities.Turbulence onSelecting the check box generates the turbulence signals.Inputs and OutputsInputDimension TypeDescriptionFirstContains the altitude, in units selected.SecondContains the aircraft speed, in units selected.ThirdContains the direction cosine matrix.OutputDimension TypeDescriptionFirstThree-element signalContains the turbulence velocities, in the selected units.SecondThree-element signalContains the turbulence angular rates, in radians per second.Assumptions and LimitationsThe frozen turbulence field assumption is valid for the cases of mean-wind velocity and the root-mean-square turbulence velocity, or intensity, is small relative to the aircrafts ground speed.The turbulence model describes an average of all conditions for clear air turbulence because the following factors are not incorporated into the model: Terrain roughness Lapse rate Wind shears Mean wind magnitude Other meteorological factions (except altitude)ExamplesSee the Airframe subsystem in the aeroblk_HL20 demo for an example of this block.ReferencesU.S. Military Handbook MIL-HDBK-1797, 19 December 1997.U.S. Military Specification MIL-F-8785C, 5 November 1980.Chalk, C., Neal, P., Harris, T., Pritchard, F., Woodcock, R., Background Information and User Guide for MIL-F-8785B(ASG), Military Specification-Flying Qualities of Piloted Airplanes, AD869856, Cornell Aeronautical Laboratory, August 1969.Hoblit, F., Gust Loads on Aircraft: Concepts and Applications, AIAA Education Series, 1988.Ly, U., Chan, Y., Time-Domain Co