Unity Lerp Tornado Simulation

A physics-based tornado simulation combining linear interpolation remapping, animated pivot rotation, and rigidbody dynamics to create a convincing vortex with pull-push force cycling.

Objective

Implement a tornado effect built entirely from first-principles physics — no particle systems or pre-baked animations. Objects within the vortex volume experience distance-dependent forces computed via a custom interpolation remap, producing a natural spiraling motion where objects are pulled toward the centre, then ejected outward past a configurable threshold.

Methodology

The tornado effect emerges from three layered techniques working together:

1. Animated Pivot Path. An Animator drives a pivot point along a predefined trajectory, defining the tornado's movement through the scene.

2. Rotational Interpolation. A modified lerp generates a spiraling path around the moving pivot, creating the characteristic funnel geometry.

3. Rigidbody Interaction. Objects within the trigger volume are pulled in or pushed out based on their distance to the vortex centre, using the force model described below.

Force Model

Distance-Based Interpolation

For an object at distance $d$ from the vortex centre, the interpolation scalar $t$ is computed via a linear remap:

$$t = \frac{d - a}{b - a} \cdot (t_b - t_a) + t_a$$

where $a = 0$, $b = 10$, $t_a = 1$, $t_b = 0$. This produces $t \in [0, 1]$ that decreases with distance — objects closer to the centre experience stronger centripetal pull.

Direction Reversal

A threshold parameter $r$ controls the transition between pull and push behaviour:

$$\vec{F} = \begin{cases} \hat{d} \cdot F_c \cdot m \cdot t & \text{if } t \leq r \ -\hat{d} \cdot F_c \cdot m \cdot F_p & \text{if } t > r \end{cases}$$

Symbol	Meaning
$\hat{d}$	Unit direction vector from object to vortex centre
$F_c$	Centripetal force strength
$F_p$	Push (counter) force
$m$	Force multiplier
$r$	Reverse indication threshold (default: 0.8)

This creates a pull-push cycle: objects accelerate toward the centre, then get ejected outward once they cross the threshold — producing the characteristic spiraling motion of a tornado.

Parameters

Parameter	Description	Default
centripetalForce	Inward pull strength	10
push	Outward counter-force	3
forceMultiplier	Global force scalar	10
reverseIndicationValue	Distance threshold for direction flip	0.8

Controls

Action	Input
Spawn objects	Click white square buttons in Play Mode
Observe	Watch rigidbodies interact with the vortex field

Getting Started

git clone https://github.com/maybebool/Unity-Tornado.git

1. Open the project in Unity 2022.3+.
2. Open the Tornado scene.
3. Press Play and click the spawn buttons to add objects into the vortex.

Prerequisites: Unity 2022.3+.

Tech Stack

Category	Technology
Engine	Unity 2022.3+ (Rigidbody, Trigger Colliders)
Language	C# — custom interpolation and force calculations
Animation	Unity Animator — pivot path trajectory
Physics	Rigidbody force application, trigger volume detection

Limitations & Future Work

The current simulation uses a simplified radial force model. Possible extensions include:

• Vertical lift component for upward funnel motion
• Turbulence noise layered onto the base force for less uniform trajectories
• Debris fragmentation on collision with the ground plane
• Wind field visualisation using GPU particle trails
• Multiple simultaneous vortices with interaction forces
• Audio integration with wind intensity mapped to vortex proximity