**~0.36m**and length of

**~42m**. Wall rougness is

**400micron**. Fluid is a particle laden gas flow, but let's assume just gas phase here. Initial task is connected with bends shape but the problem is the distributed pressure drop.

I started with CFX and there are no problems with CFX variants for now, but I'd like to have some proof from other software for results reliabiliry, so I used

**Saturne 7.0.2**.

I tried different settings but results look strange... First, I used SST model and mesh with inflation layers targeted to Y+=1 (meshes are identical with CFX). Resulting pressure drop is apparently very low. Then I tried k-epsilon (with other mesh, Y+ is around 120), pressure drop is, in opposite, very high. Let's look at numbers (all for clean gas without particles).

CFX, SST, roughness 400um:

**590 Pa**

CFX, SST, smooth wall:

**410 Pa**

CFX, k-epsilon, roughness 400um: around

**590...600 Pa**(oscillates)

Fluent: cannot stabilize with lots off different standard tricks, no results.

Saturne, SST, roughness 400um:

**371 Pa**(!)

Saturne, SST, smooth wall:

**317 Pa**(!)

Saturne, k-epsilon, roughness 400um:

**1153 Pa**(!)

Saturne, k-epsilon, smooth wall:

**639 Pa**

Semi-empirical correlations, rough wall 400um:

**744 Pa**

As you can see, we should have, roughly, something like

**600...700 Pa**. According to empirical formulae,

**409 Pa**is from distributed pressure drop and remaining resistance is due to bends (semi-empirical method accuonts for any Re / roughness range). But with SST I get just

**371 Pa**with 400um roughness and 317 Pa with smooth wall, while, from empirical formulae, difference must be

**141 Pa**, not

**54 Pa**(there is some level of error, but I don't think it's 2.6 times). With k-epsilon resistance is, in opposite, too high:

**1153 Pa**with roughness and

**639 Pa**with smooth wall, even more than empirical method that tends to higher values. Difference smooth/rough is

**514 Pa**(3.6 times more than with empirical method). In CFX, smooth/rough difference is

**180 Pa**that is comparable with 141 Pa in empirical method.

Fluent behavior was very strange. It can't even produce a solution, pressure oscillates and the case doesn't converge with any relaxation (down to 0.01), discretization, numeric scheme, roughness, density and viscosity. Completely disappointed with it, Saturne looks much better in this task (we use Fluent for furnaces due to it's powerful reaction mechanism support, but with just one long pipe it won't work).

**My questions are:**

1. Why there is so huge difference between SST and k-epsilon cases in Saturne?

2. What model and wall function in Saturne is more appropriate for pipe pressure drop calculations?

Here are some case features.

Gas velocity:

**20 m/s**

Gas density: 0.8 kg/m3

Gas dynamic viscosity: 2.2E-5 Pa*s

Wall function:

**none in SST smooth wall case, 1-scale or 2-scale model in other cases**(SST, k-e with rough wall 400um)

Target CFL:

**1...2**

Discretization:

**SOLU**with blend ~ 0.8 for velocity,

**Upwind**for others.

Velocity/pressure limits: yes, but only on early iterations

Velocity pressure relaxation: relaxv=

**0.1...0.3**

Mesh: tetra with max size of

**25 mm**,

**10mm**at the wall for k-e,

**50micron**at the wall (inflation first layer) for SST

Convergence control: maximum/minimum pressure stabilization + "standard" minimum iteration number

Case XML file and listing are attached for SST variant with roughness.