Numerický model zavzdušňovacího ventilu pro výpočet jednorozměrného proudění

The paper is focused on a numerical simulation of unsteady flow in a pipeline. The special attention is paid to a numerical model of an air valve, which has to include all possible regimes:critical/subcritical inflow and critical/subcritical outflow of air. Thermodynamic equation of subcritical mass flow was simplified to get more friendly shape of relevant equations , which enables easier solution of the problem.Článek je zaměřen na numerickou simulaci nestacionárního proudění v potrubí. Zvláštní pozornost je věnována numerickému modelu zavzdušňovacího ventilu, který musí obsáhnout všechny možné režimy: kritické/podkritické sání a kritický/podkritický výfuk vzduchu. Termodynamická rovnice podkritického proudění plynu byla zjednodušena, takže její tvar je mnohem jednodušší pro další řešení

Solution of Non-Linear Hydraulic Networks

V práci je popsán způsob řešení nestacionárního proudění v hydraulických systémech, které mají dominantní jednu složku rychlosti. Tyto systémy mohou být libovolně strukturované a nejsou omezeny počtem prvků. Samotný výpočet probíhá metodou Lax-Wendroff a umožňuje uvažovat proměnnou rychlost zvuku v závislosti na změnách statického tlaku i na vlastnostech potrubí, takže řešená hydraulická soustava může být velmi různorodá. Dále je v práci podrobně popsána samotná numerická metoda a její citlivost na velikost časového a délkového kroku, nikoliv z pohledu stability, ale z pohledu numerického útlumu a jeho porovnání s druhou viskozitou kapaliny. Na uvedených numerických postupech byl sestaven počítačový software s pracovním názvem Ráz, který umožňuje v poměrně krátkém čase namodelovat libovolnou potrubní síť a vypočítat proudění kapaliny v ní.Thesis deals about solution of non-steady flow in hydraulic systems, which have one dominant component of velocity. Such systems can be arbitrarily structured and they are not limited by number of elements. Computation is based on Lax-Wendroff method and enables considering of variable sound peed as function of static pressure and properties of pipe material. It means, that hydraulic system can be very various. Numerical method is described in detail and description is also focused on sensitivity of method for time step and length step. It can be very imported for evaluation of numerical viscosity, which is compared with second viscosity of fluid. Hammer is working title of software, which was developed on the basis of written numerical procedures. This software enables fast computation of flow in pipe-line systems.

Influence of Second Viscosity on Pressure Pulsation

A mathematical model of pulsating flow is proposed in the paper. The model includes more accurate description of energy dissipation, so it allows, for example, better stability analysis of water power plant control and more effective operation. Flow in a pipeline system is usually treated as a one-dimensional flow. This is also applied for more difficult cases of the Newtonian and non-Newtonian liquids simulations in the rigid or flexible pipes. Computational simulations of pressure pulsations in pipelines often predict lower damping than what the experimental results show. This discrepancy can be caused by neglecting one of the important damping mechanisms. The second viscosity describes the energy losses due to the compressibility of the liquid. Its existence and use in the computations specifies the real pulsations damping descriptions and predictions. A frequency dependent model of pressure pulsations including second viscosity is introduced. The second viscosity is determined from the system eigenvalue. The experiments were performed with water for low frequencies (from 0.1 to 1 kHz). This area is not fully covered by the current available research results

Investigation and numerical simulation of a water hammer with column separation

The paper describes an investigation into a water hammer with column separation. Special attention is given to the volume of cavity as a result of a pressure drop. Its volume has been determined by Gibson’s method and using images taken by a high-speed camera. The water hammer was then addressed numerically as a one-dimensional flow. Air bubbles affecting the speed of sound in the system were considered. The pipeline’s flexibility and attenuation were taken into account in the computation.Web of Science1413art. no. 0401408

Evaluation of pump characteristic from measurement of fast deceleration

Article describes an experiment where a pump connected to the simple hydraulic circuit is decelerated. Since the deceleration is fast enough the operating point of the machine moves from the initial steady position to the breaking zone, turbine zone and back to the new steady position. A dependence of the specific energy and the torque on the flow rate was evaluated from the measurement of the input and output pressure, torque and rotational speed recorded during the deceleration. Obtained characteristic is much wider than curves obtained from regular measurement of steady state

Evaluation of pump characteristic from measurement of fast deceleration

Article describes an experiment where a pump connected to the simple hydraulic circuit is decelerated. Since the deceleration is fast enough the operating point of the machine moves from the initial steady position to the breaking zone, turbine zone and back to the new steady position. A dependence of the specific energy and the torque on the flow rate was evaluated from the measurement of the input and output pressure, torque and rotational speed recorded during the deceleration. Obtained characteristic is much wider than curves obtained from regular measurement of steady state

Experimental analysis of static and dynamic properties of the check valves

The check valves play an important role in safety and reliability of many technologies using liquid or gas flow. Check valves are elements of the hydraulic system enabling the one-way flow without a significant pressure loss and preventing the reverse flow. The ideal check valve should have an acceptable pressure loss and, when closed, there should be no undesirable dynamic phenomena like water hammer or slam. The static properties describe energy losses of the check valve, and the producers provide them as a standard. The static properties can be defined either in an experimental way or using the CFD analysis. The dynamic properties of the check valve express a relation between the maximum reverse velocity and water column deceleration. Unfortunately, the definition of dynamic properties can be obtained only experimentally, and for small pipe diameters. It is very demanding, time consuming and expensive to define the dynamic characteristic so it is why the producers do not offer it as a standard. The article deals with static and dynamic properties of different check valves with different closing principles. There was built a testing hydraulic circuit with the pipe diameter DN 100, that enables the simulation of check valve slam during the flow reversing. The attention was paid to the definition of flow deceleration, which is rather problematic and usually defined in a different way for each lab

Druhá viskozita vody v akustické modální analýze a v experimentu

Bulk viscosity is an important factor in the damping properties of fluid systems and exhibits frequency dependent behaviour. A comparison between modal analysis in ANSYS Acoustics, custom code and experimental data is presented in this paper. The measured system consists of closed ended water-filled steel pipes of different lengths. The influence of a pipe wall, flanges on both ends and longitudinal waves in the structural part were included in measurement evaluation. Therefore, the obtained values of sound speed and bulk viscosity are parameters of the fluid. A numerical simulation was carried out only using fluid volume in a range of bulk viscosity. Damping characteristics in this range were compared to measured values. The results show a significant influence of sound speed and subsequently, the use of sound speed value regressed from experimental data yields a better fit between the measurement and the computation