Air Properties

The properties of the airstream are made of the physical properties of the air plus the description of the airstream velocity direction and amplitude. The former can be expressed in different forms, while the latter are based on three-dimensional vectors which depend on multipliers.

    <arglist> ::= {
        (drive_caller) <air_density> , (scalar) <sound_speed> 
        | std , { { SI | British }
            [ , temperature deviation , <delta T> ]
            | <p0> , (drive_caller) <rho0> ,
            <T0> , <dT/dz> , <R> , <g0> , <z1> , <z2>
        } [ , reference altitude, <z0> ]
    } , (Vec3_tpl_drive_caller) <air_speed>
    [ , gust , <gust_model> ]

The first form consists in the bare input of the air density, in form of a drive caller, and of the sound celerity, e.g.:

    air properties: 1.225, 340.,
        1.,0.,0., 150.;

The second form uses standard air properties, both in the international system (SI) or in British units, possibly with a temperature deviation and an altitude offset, e.g.:

    air properties: std, SI, temperature deviation, -55,
        reference altitude, 1000.,
        1.,0.,0., 150.;

where standard properties in SI are used, with a temperature deviation of -55 K and a reference altitude of 1000 m. The air properties are computed based on the Z position of the point where the air properties are requested (plus the optional altitude offset). The last possibility lets the user input all the parameters required to compute the air properties based on the Z position of the point where they are requested, namely the reference pressure p0, the reference density rho0, the reference temperature T0, the initial temperature gradient dT/dz, the gas constant R, the initial gravity acceleration g0, the bottom and top altitudes of the null temperature gradient region z1 and z2; e.g., for SI units:

    air properties: std,
        101325.,       /* Pa */
        1.2250,        /* kg/m^3 */
        288.16,        /* K */
        -6.5e-3,       /* K/m */
        287.,          /* J/kgK */
        9.81,          /* m/s^2 */
        11000.,        /* m */
        25000.,        /* m */
        temperature deviation, -55,
        reference altitude, 1000.,
        1.,0.,0., 150.;

The asymptotic air properties are characterized by the 3D template drive of the air speed, in the global reference frame. If the optional gust keyword is used, a gust model can be added. Note that a very elementary gust model, represented by a uniform change in airstream speed and direction can be implemented by using a time-dependent airstream drive.

A more sophisticated model is currently available, and provisions are made to allow four-dimensional gust profiles, dependent on time and position. The syntax is:

    <gust_model> ::= front 1D ,
        (Vec3) <front_direction> ,
        (Vec3) <perturbation_direction> ,
        (scalar) <front_velocity> ,
        (drive_caller) <front_profile>

This model consists in a uniform front, defined as

v$$\left(\vphantom{ \boldsymbol{x}, t }\right.$$x, t$$\left.\vphantom{ \boldsymbol{x}, t }\right)$$ = ng$$\left(\vphantom{ \boldsymbol{f} \cdot \boldsymbol{x} + V_{ref} \cdot t }\right.$$f ^. x + V_ref ^. t$$\left.\vphantom{ \boldsymbol{f} \cdot \boldsymbol{x} + V_{ref} \cdot t }\right)$$

where

• v is the velocity perturbation;
• x is the position of the point whose airstream velocity is being computed;
• t is the current time;
• n is the unit vector perturbation_direction that defines the direction of the velocity perturbation;
• g$\left(\vphantom{ \cdot }\right.$ ^. $\left.\vphantom{ \cdot }\right)$ is the function front_profile that defines the gust profile;
• f is the unit vector front_direction that defines the direction of propagation of the front;
• V_ref is the velocity front_velocity of propagation of the front in direction f.

As an example, a transverse cosine-shaped gust, with a wavelength of 100 m and a peak velocity of 5 m/s moving downstream at the airstream speed, 100 m/s, in standard air, is presented:

    set: real waveLength = 100.; # m
    set: real V_inf = 100.;      # m/s
    set: real V_g = 5.;          # m/s
    air properties: std, SI,
        1.,0.,0., const, V_inf,  # reference airstream along X
        gust, front 1D,
            1.,0.,0.,            # front moving along X
            0.,0.,1.,            # gust along Z
            V_inf,               # front moving at V_inf
            cosine, 0., pi/waveLength, V_g/2., one, 0.;

Gusts can also be added later to the air properties element by using the statement

    gust : <gust_model> ;

Output

The output occurs in the .air file, which contains:

• a fake label, always set to 0
• the air density
• the sound celerity
• the three components of the reference air speed with respect to the inertial reference frame

Private Data

The following data is available:

• "vxinf" the x component of the airstream speed (without any gust contribution)
• "vyinf" the y component of the airstream speed (without any gust contribution)
• "vzinf" the z component of the airstream speed (without any gust contribution)
• "vinf" the module of the airstream speed (without any gust contribution)