# FAQ 004731 | Why does RWIND Simulation not converge to the preset minimum pressure difference, but oscillates about a higher limit value?

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#### Question

Why does RWIND Simulation not converge to the preset minimum pressure difference, but oscillates about a higher limit value?

#### Answer

In fluid mechanics, a distinction is made between laminar and turbulent flows of liquids and gases.

Laminar flow is characterized by the fact that no eddies are formed perpendicular to the flow velocity in the transition area between two different flow velocities. In this case, the medium flows around the model in layers and does not mix with each other.

In contrast, turbulent flow in the flow field seems to be randomly varied with clear mixing of the medium.

The Reynolds number, which expresses the ratio of force of inertie to viscous forces, is used to describe the flow behavior of geometrically similar bodies.

$$Re=ρ·c·dη=c·dν$$

If the model geometry and the medium property remain the same, the flow changes with the increasing flow velocity from laminar to turbulent flow. In this case, the laminar flow is characterized by a low Reynolds number and the turbulent flow by a high Reynolds number.

The transition from laminar to turbulent flow passes through the following basic stages for simple bodies:

1. At low Reynolds numbers, the medium flows around the body in a laminar way. This behavior occurs at very low velocity or high viscosity. The medium divides in front of the body and flows behind it together again. In this case, we talk about stationary flow.

2. In the case of slightly increased Reynolds numbers, you can see that the symmetric vortex pair is formed directly on the back of the body in a flow. This type of flow is still considered as stationary.

3. With a further increase of the Reynolds number, a Kármá vortex street is formed behind the body in a flow. In this flow diagram, the right and left vortex separate from the back of the body out of phase. As of this moment, the stationary flow becomes a temporally periodic flow form.

4. At high Reynolds numbers, the eddies decay into smaller elements and form a turbulent boundary layer. In this area, the medium is highly turbulent and hardly predictable. The medium is no longer stationary at this stage.

If RWIND Simulation converges with the pressure difference below the specified minimum value, you can usually assume the stationary flow (see Points 1 and 2). If the solution process oscillates about a higher differential value, the program does not find a stable state of the flow.

The oscillation is an indication of the periodic vortex shedding (see Point 3). As of this point, the result is influenced by the flow varying in time, and a time-dependent transient calculation is required. Since RWIND Simulation can only determine a temporally independent stationary solution at present, the result obtained is ambiguous and must be interpreted accordingly. In some cases, it is helpful to check the drag force: If this force remains constant over the different iterations, the result can be used with limitations although the minimum pressure difference has not been reached.

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• Updated 10/14/2020

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