**5. Flow Field Analysis**

A Reynolds number of 21,100 and a Prandtl number of 7 has been chosen for the simulation, the operating fluid is water and the thermophysical properties are constant. To validate the simulation results, the flow field is analyzed in a range between 40 and 100 percent of the inner radius to focus on the near wall area. First, the velocity profiles calculated by the simulations are compared with the experimental data. All velocities are averaged with *U*0, where for the variance *U*<sup>2</sup> <sup>0</sup> is applied. The results in Figure 5 show that the simulation can reflect the measured values both in the area near the wall and towards the centre. For the first two velocity profiles, the values of simulation and measurement near the wall falls below zero, which is an indication of the backflow being correctly reflected by the model. The maximum velocity is 1.4 times the bulk velocity.

The investigation of the azimuthal velocity in Figure 6 also shows a good agreement with the measured values. The deviation of simulation and measurement is less than 10%. Especially at the first two measurement lines after the rib, the deviation between simulation and measurement is relatively high. Here, because of the flow recirculation, the averaging in simulation and measurement is very sensitive to pertubations and therefore susceptible to errors. Due to the detachment of the flow and the resulting backflow after the rib, the maximum velocity at the first measurement lines is lower compared to the measurement lines directly before the rib. Thus, the maximum value of the first curve is 0.069 compared to 0.143 at the last curve, which is an increase of 48%. Furthermore, in simulation and experiments, the azimuthal velocity above the ribs approaches zero. From this, it can be concluded that the described effects only occur close to the wall.

**Figure 5.** Comparison between measurement and simulation with the velocity in the flow direction.

**Figure 6.** Comparison between measurement and simulation with the velocity in the circumferential direction.

The variance <*uu*> in the modelled and measured flow shows a good agreement as shown in Figure 7. The greatest deviation is seen in the first two curves. Here the variance is significantly lower than in the experimental data. The maximum value is 43% smaller at the first curve and 13% smaller at the second curve. It is assumed that the complex structures, caused by the detachment of the flow, are responsible for this difference. Nevertheless, the measurements can be replicated with the simulation. The comparison in Figure 8 for <*uv*> also shows a good agreement in this case. Overall, the simulation is able to reproduce the measurements for both the velocity profiles and the turbulent fluctuations well. Therefore, the simulation model is able to reproduce the turbulent flow in a ribbed pipe and is suitable for further investigations of the flow.

**Figure 7.** Comparison between measurement and simulation with the velocity variations <*uu*>.

**Figure 8.** Comparison between measurement and simulation with the velocity variations <*uv*>.
