Part 1 - Compression and the Venturi Effect

Key points:

An air particle follows the path of "least resistance."
Compression is the vertical result of wind on terrain.
The takeoff area is where wind speed is strongest.

When I started learning paragliding, I felt the need to understand what was happening in this invisible element: air. How does the air mass interact with the terrain? Initially, it was for safety concerns, then to improve my performance: how to stay airborne longer and fly more often.

Many questions came to mind, and I only had partial technical answers, vaguely validated by my feelings in flight:

Why do I stay in the air?
What is compression?
Why can't I climb higher after reaching a certain altitude?
Where should I position myself to stay airborne longer in light conditions?
What should I do when the wind increases?
What are rotors?
Why are some takeoffs easier than others?
What is the impact of terrain on wind?
Why do we take off from the bottom of a dune rather than the top?
What is the best place to land if I'm being pushed backward?
What is the relationship between wind strength and altitude?

One day, I came across a simulation software, and it was a revelation. I had a real "aha moment," and that's what I want to share with you.

Disclaimer : These conclusions are my own. I am neither a paragliding instructor nor a fluid dynamics engineer, just a curious paraglider pilot.

Discovery of Flowsquare

My research led me to a fluid dynamics simulation software called Flowsquare . It's a program that simulates a digital wind tunnel in two dimensions. It's simple to use: you create a shape in a bitmap file (for example, with Paint), configure the parameters at the boundaries of the environment, and that's it. You then have a visual simulation tool for air flows.

If we can simulate an air flow around a car, we can also simulate wind interaction with a dune, a cliff, or a mountain. Let's go!

To simplify reading and avoid overloading you with technical information, I'll get straight to the point. For more information on the simulation, you can refer to the website: flowsquare.com .

"It is rarely a mysterious technique that leads us to the top, but rather a deep mastery of what might well be a set of basic skills." — Josh Waitzkin

What happens when wind meets terrain?

To understand the interaction of an air particle with an element, I created a basic terrain: a dune with a slope of about 45°, a plateau, and a break. A typical dune profile.

Simulation data:

Headwind : 6 m/s (or 21.6 km/h).
Wind tunnel : 30 m long by 10 m high tunnel.
Dune : 2.5 m high and 8 m long at its base.

The digital wind tunnel calculates the interaction between each air particle, which would represent billions of particles. To speed up and simplify calculations, we use a calculation grid of 450 points by 200 points, or 80,000 points (approximately one point every 5 cm). This approach is similar to that used for weather forecast models.

After a few minutes of calculation, here is the result:

Observations

Velocity Visualization

We observe the velocity (in m/s) of air particles when they encounter the terrain. In blue, the lowest velocities; in red, the highest.

Before encountering the terrain, the air flow is laminar. When it reaches the dune, the air is deflected, following the "path of least resistance." Like a stream flowing down a valley, the air follows the path that offers the least resistance.

The arrows on the simulation represent the velocity and direction of flow of the particles.

Velocity Components

The particle moves along two axes:

u (m/s) : velocity component on the horizontal axis (headwind).
v (m/s) : velocity component on the vertical axis.

This will be the only equation in the article. The total velocity is the result of these two components:

spd (m/s) : total velocity = √(u² + v²)

Simply remember that velocity is the combined result of horizontal and vertical velocities.

Venturi Effect

When encountering the terrain, air particles follow the shape of the dune. In our case of a gentle slope, a Venturi effect is established.

Explanation : The volume of air at the entrance of the tunnel must equal the volume of air at the exit. To maintain this constant flow despite the reduction in cross-section caused by the dune, the air velocity increases.

Implication for Paragliding

The fastest flow velocity reaches 9.3 m/s (or 33.4 km/h) at the highest point of the terrain, much higher than the initial wind of 6 m/s. This velocity is reached at the highest point of the terrain.

First conclusion:

Depending on the terrain, the wind at takeoff is stronger than the meteorological wind forecast.
The highest velocity is reached in the compression zone, where the wind is most compressed.
A decompression zone appears behind the terrain with less predictable and turbulent behaviors.

The interest of the simulation is to be able to isolate the velocity components u and v . This is when the "aha moment" occurred.

Analysis of Vertical Components ( v )

By isolating the vertical component ( v ), we discover the famous "compression zone" , which represents the intensity of ascending vertical velocities.

To better visualize, we have identified three zones:

Green zone : v = 1 m/s
Orange zone : v = 2 m/s
Red zone : v = 2.5 m/s

A paraglider naturally descends at about 1 m/s. The total vertical velocity ( V_tot ) is the sum of the paraglider's descent rate and the vertical velocity of the air mass:

V_tot = V_paraglider + V_air mass

Implication for Paragliding:

Red/orange/yellow zone : V_tot is positive (example: 2.5 m/s - 1 m/s = +1.5 m/s). The paraglider pilot gains altitude.
Green zone : V_tot is close to zero (1 m/s - 1 m/s = 0 m/s). The paraglider pilot maintains altitude ("zeroing").
Blue zone : V_tot is negative. The paraglider pilot loses altitude.

This answers several of our initial questions:

Why do I stay in the air?

Because the compression zone creates an updraft that compensates for the natural descent of the paraglider.

What is compression?

It's the increase in the vertical component of wind velocity due to the encounter with terrain.

Why do I stop climbing at some point?

Because as you move away from the terrain, the vertical component decreases and no longer compensates for the natural descent.

Analysis of Horizontal Components ( u )

Let's now examine the horizontal component of wind velocity.

Identified zones:

Zone 1 : u = 7 m/s
Zone 2 : u = 8 m/s
Zone 3 : u = 9 m/s

Let's assume our paraglider has its own horizontal velocity of 8 m/s (about 29 km/h).

The ground speed of the paraglider is the difference between its own velocity and the wind velocity:

Ground speed = V_paraglider - u

Implication for Paragliding:

Zone 1 (u = 7 m/s) : Ground speed = 8 m/s - 7 m/s = +1 m/s . The paraglider moves forward relative to the ground.
Zone 2 (u = 8 m/s) : Ground speed = 8 m/s - 8 m/s = 0 m/s . The paraglider hovers in place.
Zone 3 (u = 9 m/s) : Ground speed = 8 m/s - 9 m/s = -1 m/s . The paraglider moves backward.

This answers other questions:

What should I do when the wind increases?

The velocity distribution zones show us that we should avoid the compression zone or risk not being able to get out of it. Then we need to consider landing upstream of the terrain or moving to an area where the wind is less strong.

Why do we take off from the bottom of a dune rather than the top?

If we take off from the top of a dune, we are at the place where the wind is strongest with very little lift and a turbulent zone downstream.

If we take off from the bottom of a dune, the wind is progressive, and we easily reach the lift zone, with a safety exit upstream of the compression if needed.

The simulation with Flowsquare offers a valuable visualization of the interaction between wind and terrain. By playing with shapes and parameters, I answered so many questions that I separated the topic into several parts. I hope this first article has helped you improve your understanding of compression and the Venturi effect.

We will see in the other parts:

Part 2 - Concrete example, the Roselier site

Part 3 - Flying in light wind

Part 4 - Flying in strong wind

Part 5 - Dangers: Impact of terrain (obstacles and rotors)

Additional resources:

If you have questions, ask them on the FB group

https://www.facebook.com/groups/1766258457145440