Pressure measurement turbulence errors

Disturbances upstream of the probe can cause large errors, in part because of the turbulence generated and its effect on the static-pressure measurement. A calming section of at least 50 pipe diameters is desirable. If this is not possible, the use of straightening vanes or a honeycomb is advisable. 

The CFD code FLUENT with the RSM turbulence model, predict very well the pressme drop in cyclones and can be used in cyclone design for any operating conditions. In the CFD numerical calculations a very small pressure drop deviation were observed, with about 3% of deviation, probably in the same magnitude of the experimental error. However behind the accuracy of the complicated RSM model it does require much expensive computational effort compared to the RNG k-c model. CFD with RNG k-e turbulence model still yield a reasonably good prediction on cyclone pressme drop with deviation of 14-18% on measured value. 

Environmental index variation The measured OPD depends directly on the refractive index of the medium through which the wavefront propagates. Turbulence adds noise, while uncertainty in the air temperature, pressure, and composition (particularly hydrocarbons) adds a time invariant component to the uncertainty. Addition of a wavelength tracker can reduce the effect of errors that are uniform across the paths of tracker and measurement axes. 

The two key properties in single-phase flow are the fluid density and the viscosity. The density is quite straightforward it is the mass per unit volume. In turbulent flow, pressure drop is directly proportional to density, so that the accuracy of the density is the accuracy of the pressure drop prediction. It is easy to get better than 1% accuracy on such values. Viscosity, on the other hand, is a more complex measurement. Low viscosity systems usually run in turbulent flow, where the viscosity has little or no effect on mixing or pressure drop. For low viscosity material the prime use of the viscosity is in calculating a Reynolds number to determine if the flow is laminar or turbulent. If turbulent, little accuracy is needed. An error in viscosity of a factor of 2 will have negligible effect. In laminar flow, however, the viscosity becomes all important and pressure drop is directly proportional to it, so that an accuracy of 10% or less is often required. For laminar processing a complete relation of stress versus strain or shear rate versus shear stress is required. See Chapter 4 for the means and type of data required. 

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