Dryden Wind Turbulence Model (Discrete)

Generate discrete wind turbulence with Dryden velocity spectra

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  • Aerospace Blockset / Environment / Wind

  • Dryden Wind Turbulence Model (Discrete) block

Description

The Dryden Wind Turbulence Model (Discrete) block uses the Dryden spectral representation to add turbulence to the aerospace model by using band-limited white noise with appropriate digital filter finite difference equations. This block implements the mathematical representation in the Military Specification MIL-F-8785C, Military Handbook MIL-HDBK-1797, and Military Handbook MIL-HDBK-1797B. For more information, see Algorithms.

Limitations

The 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 aircraft's 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)

Ports

Input

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Altitude, specified as a scalar, in selected units.

Data Types: double

Aircraft speed, specified as a scalar, in selected units.

Data Types: double

Direction cosine matrix, specified as a 3-by-3 matrix.

Data Types: double

Output

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Turbulence velocities, returned as a three-element signal, in specified units.

Data Types: double

Turbulence angular rates, specified as a three-element vector, in radians per second.

Data Types: double

Parameters

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Units of wind speed due to turbulence, specified as:

UnitsWind VelocityAltitudeAir Speed
Metric (MKS) Meters/secondMetersMeters/second
English (Velocity in ft/s) Feet/secondFeetFeet/second
English (Velocity in kts) KnotsFeetKnots

Programmatic Use

Block Parameter: units
Type: character vector
Values: 'Metric (MKS)' | 'English (Velocity in ft/s)' | 'English (Velocity in kts)'
Default: 'Metric (MKS)'

Military reference, which affects the application of turbulence scale lengths in the lateral and vertical directions, specified as MIL-F-8785C, MIL-HDBK-1797, or MIL-HDBK-1797B.

Programmatic Use

Block Parameter: spec
Type: character vector
Values: 'MIL-F-8785C' | 'MIL-HDBK-1797' | 'MIL-HDBK-1797B'
Default: 'MIL-F-8785C'

Select the wind turbulence model to use:

Continuous Von Karman (+q -r)

Use continuous representation of Von Kármán velocity spectra with positive vertical and negative lateral angular rates spectra.

Continuous Von Karman (+q +r)

Use continuous representation of Von Kármán velocity spectra with positive vertical and lateral angular rates spectra.

Continuous Von Karman (-q +r)

Use continuous representation of Von Kármán 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 Discrete Dryden selections conform to the transfer function descriptions.

Programmatic Use

Block Parameter: model
Type: character vector
Values: 'Continuous Von Karman (+q +r)' | 'Continuous Von Karman (-q +r)' | 'Continuous Dryden (+q -r)' | 'Continuous Dryden (+q +r)' | 'Continuous Dryden (-q +r)' | 'Discrete Dryden (+q -r)' | 'Discrete Dryden (+q +r)' | 'Discrete Dryden (-q +r)'
Default: 'Discrete Dryden (+q +r)'

Measured wind speed at a height of 20 feet (6 meters), specified as a real scalar, which provides the intensity for the low-altitude turbulence model.

Programmatic Use

Block Parameter: W20
Type: character vector
Values: real scalar
Default: '15'

Measured wind direction at a height of 20 feet (6 meters), specified as a real scalar, which is an angle to aid in transforming the low-altitude turbulence model into a body coordinates.

Programmatic Use

Block Parameter: Wdeg
Type: character vector
Values: real scalar
Default: '0'

Probability of the turbulence intensity being exceeded, specified as 10^-2 - Light, 10^-1, 2x10^-1, 10^-3 - Moderate, 10^-4, 10^-5 - Severe, or 10^-6. Above 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 intensity being exceeded.

Programmatic Use

Block Parameter: TurbProb
Type: character vector
Values: '2x10^-1' | '10^-1' | '10^-2 - Light' | '10^-3 - Moderate' | '10^-4' | '10^-5 - Severe' | '10^-6'
Default: '10^-2 - Light'

Turbulence scale length above 2000 feet, specified as a real scalar, which is assumed constant. 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.

Programmatic Use

Block Parameter: L_high
Type: character vector
Values: real scalar
Default: '533.4'

Wingspan, specified as a real scalar, which is required in the calculation of the turbulence on the angular rates.

Programmatic Use

Block Parameter: Wingspan
Type: character vector
Values: real scalar
Default: '10'

Noise sample time, specified as a real scalar, at which the unit variance white noise signal is generated.

Programmatic Use

Block Parameter: ts
Type: character vector
Values: real scalar
Default: '0.1'

Random noise seeds, specified as a four-element vector, which are used 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.

Programmatic Use

Block Parameter: Seed
Type: character vector
Values: four-element vector
Default: '[23341 23342 23343 23344]'

To generate the turbulence signals, select this check box.

Programmatic Use

Block Parameter: T_on
Type: character vector
Values: 'on' | 'off'
Default: 'on'

Algorithms

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According to the military references, turbulence is a stochastic process defined by velocity spectra. For an aircraft flying at a speed V through a frozen turbulence field with a spatial frequency of Ω radians per meter, the circular frequency ω is calculated by multiplying V by Ω. The following table displays the component spectra functions:

 MIL-F-8785CMIL-HDBK-1797 and MIL-HDBK-1797B
Longitudinal

Φu(ω)

2σu2LuπV11+(LuωV)2

2σu2LuπV11+(LuωV)2

Φp(ω)

σw2VLw0.8(πLw4b)131+(4bωπV)2

σw22VLw0.8(2πLw4b)131+(4bωπV)2

Lateral

Φv(ω)

σv2LvπV1+3(LvωV)2[1+(LvωV)2]2

2σv2LvπV1+12(LvωV)2[1+4(LvωV)2]2

Φr(ω)

(ωV)21+(3bωπV)2Φv(ω)

(ωV)21+(3bωπV)2Φv(ω)

Vertical

Φw(ω)

σw2LwπV1+3(LwωV)2[1+(LwωV)2]2

2σw2LwπV1+12(LwωV)2[1+4(LwωV)2]2

Φq(ω)

±(ωV)21+(4bωπV)2Φw(ω)

±(ωV)21+(4bωπV)2Φw(ω)

The variable b represents the aircraft wingspan. The variables Lu, Lv, Lw represent the turbulence scale lengths. The variables σu, σv, σw represent the turbulence intensities.

The spectral density definitions of turbulence angular rates are defined in the references as three variations, which are displayed in the following table:

pg=wgy

pg=wgy

pg=wgy

qg=wgx

qg=wgx

qg=wgx

rg=vgx

rg=vgx

rg=vgx

The variations affect only the vertical (qg) and lateral (rg) turbulence angular rates.

Keep in mind that the longitudinal turbulence angular rate spectrum, Φp(ω), is a rational function. The rational function is derived from curve-fitting a complex algebraic function, not the vertical turbulence velocity spectrum, Φw(ω), 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:

VerticalLateral

Φq(ω)

Φq(ω)

−Φq(ω)

−Φr(ω)

Φr(ω)

Φr(ω)

To generate a signal with the correct characteristics, a unit variance, band-limited white noise signal is used in the digital filter finite difference equations.

The following table displays the digital filter finite difference equations:

 MIL-F-8785CMIL-HDBK-1797 and MIL-HDBK-1797B
Longitudinal

ug

(1VLuT)ug+2VLuTσuσηη1

(1VLuT)ug+2VLuTσuσηη1

pg

(12.6LwbT)pg+(22.6LwbT)(0.952Lwb23σηη4)σw

MIL-HDBK-1797

(12.62LwbT)pg+(22.62LwbT)(1.92Lwbσηη4)σw

MIL-HDBK-1797B

(12.6V2LwbT)pg+(22.6V2LwbT)(1.92Lwbσηη4)σw

Lateral

vg

(1VLuT)vg+2VLuTσvσηη2

(1VLuT)vg+2VLuTσvσηη2

rg

(1πV3bT)rgπ3b(vgvgpast)

(1πV3bT)rgπ3b(vgvgpast)

Vertical

wg

(1VLuT)wg+2VLuTσwσηη3

(1VLuT)wg+2VLuTσwσηη3

qg

(1πV4bT)qg±π4b(wgwgpast)

(1πV4bT)qg±π4b(wgwgpast)

Divided into two distinct regions, the turbulence scale lengths and intensities are functions of altitude.

References

[1] U.S. Military Handbook MIL-HDBK-1797B, April 9, 2012.

[2] U.S. Military Handbook MIL-HDBK-1797, December 19, 1997.

[3] U.S. Military Specification MIL-F-8785C, November 5, 1980.

[4] Chalk, Charles, T.P. Neal, T.M. Harris, Francis E. Pritchard, and Robert J. Woodcock. "Background Information and User Guide for MIL-F-8785B(ASG), Military Specification-Flying Qualities of Piloted Airplanes." AD869856. Buffalo, NY: Cornell Aeronautical Laboratory, August 1969.

[5] Hoblit, Frederic M., Gust Loads on Aircraft: Concepts and Applications. Reston, VA: AIAA Education Series, 1988.

[6] Ly, U., Chan, Y. "Time-Domain Computation of Aircraft Gust Covariance Matrices," AIAA Paper 80-1615. Presented at the Atmospheric Flight Mechanics Conference, Danvers, Massachusetts, August 11-13, 1980.

[7] McRuer, D., Ashkenas, I., Graham, D., Aircraft Dynamics and Automatic Control. Princeton: Princeton University Press, July 1990.

[8] Moorhouse, David J. and Robert J. Woodcock. "Background Information and User Guide for MIL-F-8785C, `Military Specification-Flying Qualities of Piloted Airplanes'." ADA119421, Flight Dynamic Laboratory, July 1982.

[9] McFarland, R. "A Standard Kinematic Model for Flight Simulation at NASA-Ames." NASA CR-2497. Computer Sciences Corporation, January 1975.

[10] Tatom, Frank B., Stephen R. Smith, and George H. Fichtl. "Simulation of Atmospheric Turbulent Gusts and Gust Gradients." AIAA Paper 81-0300, Aerospace Sciences Meeting, St. Louis, MO, January 12-15, 1981.

[11] Yeager, Jessie, "Implementation and Testing of Turbulence Models for the F18-HARV Simulation." NASA CR-1998-206937. Hampton, VA: Lockheed Martin Engineering & Sciences, March 1998.

Extended Capabilities

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Introduced before R2006a