Environment: Variation scheme current data

The variation scheme allows current velocity to be set based on separate horizontal and vertical schema.

Horizontal current variation

Current speed is allowed to vary with horizontal position. The variation is given as a dimensionless multiplicative factor. If it is used in conjunction with variation of current speed with depth, the factor will be applied at all depths.

A value of '~' means that there is no horizontal current variation. A numeric value (e.g. 0.5) allows you to apply a scaling factor to the vertical current speed profile.

To model current speed variation with horizontal position you must define a horizontal variation factor variable data source. The horizontal variation factor is assumed to be constant in the direction normal to the given axis.

Negative factors can be used which allow you to model reversing currents.

Vertical current variation

Current method

Can be interpolated or power law. The interpolated method uses a full depth profile with variable speed and direction. The power law method uses an exponential decay formula.

Data for interpolated method

Speed and direction

The magnitude and direction of a reference current (generally taken as the surface current). The actual current at a given Z level is then defined relative to this reference current by a current profile.

The direction is the direction in which the current is progressing – for example, 0° and 90° mean currents flowing in the $X$ and $Y$ directions, respectively.

The speed and direction can be fixed, vary with simulation time or be given by an external function.

Profile

A current profile is defined by specifying multiplicative factors and rotations at various depths, relative to the reference current. At each depth in the table the current speed is the reference current speed multiplied by the factor for that depth; the direction is the reference direction plus the rotation specified. Current speed and direction are interpolated linearly between the given depths. The current at the greatest depth given is applied to any depth below this. Similarly, the current at the least depth given is applied to any depth above this.

Negative factors may be used to model reversing currents.

If you prefer to enter current speeds and directions directly, rather than using a reference current and reference-relative profile, simply set the reference current speed to 1 and the reference direction to 0.

Data for power law method

Speed at surface and at seabed

The current speed at the still water level and at the seabed. These limiting values are applied at all horizontal positions, with exponential decay between these limits as described by the theory. As a result, vertical stretching is always applied for power law currents.

Direction

The current direction is the same at all levels. The direction specified is that in which the current is progressing – for example, 0° and 90° mean currents flowing in the $X$ and $Y$ directions, respectively.

Exponent

This determines how the current decays. With a smaller value, the decay is spread more evenly across the water depth. With a higher value, the decay mostly occurs close to the seabed.

View vertical profile graph, view vertical profile 3D view

The vertical profile graph plots Z against current speed, which can be useful to help visualise and check your vertical current speed variation.

The vertical profile 3D view displays a 3D view with a number of arrows showing the current velocity vectors at a range of depths. These vectors are non-dimensionalised: they do not show absolute current speeds, but they enable comparison of current speeds at varying depths. This view is particularly useful for visualising and checking current profile rotation data.