Towards Direct Numerical Simulation of Compressible Orifice Flow

Simulation methods from simple lumped parameter approaches to complex computational fluid dynamics codes have become a widely used tool in the fluid power community. Certain tasks like the predicition of flow forces on the control spools in valves or the design of port plates in axial piston pumps are usually treated by the aid of numerical simulation. Like in many other cases, the underlying principle is the control of flow by orifices. The importance of orifice flow for hydraulic systems is reflected by the vast number of publications on various aspects of orifice flow in the fluid power literature. In lumped parameter simulations, the orifice equation giving the flow rate as a square root of the pressure drop is widely used even in transient cases where it is not clear whether the flow develops fast enough to justify the assumption of stationary flow. On the other end of the model complexity spectrum computational fluid dynamcis codes are used in the fluid power community. These very complex models require a high number of parameters for the tuning of turbulence models, wall models, and the like. The quality of the results heavily dependes on a good choice for these parameters. Additionally, the vast majority of turbulent flow simulations is done with the assumption of an incompressible fluid. Very often, the results from simulations deviate heavily from measurement results and only after parameter tuning a good match between model and simulation is achieved. This paper suggests the use of direct numerical simulations for simple and prototypical geometries in order to gain a better understanding for transient orifice flows lacking the fully developed flow assumed in traditional models.

Fuzzy PID and Contour Tracking as Applied to Position Control of a Pneumatic Gantry Robot

In previous work, the performance of PID plus an adaptive neural network compensator (ANNC) was compared with the performance of a novel fuzzy adaptive PID algorithm, as applied to position control of one axis of a pneumatic gantry robot. The fuzzy PID controller was found to be superior. In this paper, a simplified non-adaptive fuzzy algorithm was applied to the control of both axes of the robot. Individual step results are first shown to confirm the validity of the simplified fuzzy PID controller. The fuzzy controller is then applied to a sinuosoidal tracking problem with and without a fuzzy PD tracking algorithm. Initial results are considered to be very promising. Future work requires developing an adaptive version of the controller in order to demonstrate robustness relative to changing tracking frequencies and changing supply pressures.

The Influence of Passive Valve Characteristics on the Performance of a Piezo Pump

In this study, a piezoelectric stack actuator is used to oscillate a piston in a single cylinder pump. The pump is intended to directly supply a hydraulic actuator for motion control, and power output of about 1kW is targeted. Flow rectification is achieved by the use of passive check valves. The valve resonant frequency is found to have a significant effect on output flow. The expected increase in pump flow rate with driving frequency has been confirmed in simulation to hold true in a certain frequency range only. In addition, check valve size and therefore orifice area has to be adequate in order not to prohibitively restrict flow. Valve spring stiffness and valve mass need to be simultaneously optimized for the area of the valve to achieve the highest flow rate. Calculations indicate that there is a power limitation due to the high current demand and also a high temperature rise for a large continuously operated piezo stack. Thus the piezo pump appears more promising for smaller scale applications, and those that require intermittent power (i.e. a low duty cycle).

Hydrostatic Transmission for Wind Turbines: An Old Concept, New Dynamics

Hydrostatic drives are commonly used in mobile machinery. A new application for this technology is the renewable energy sector, especially wind power. Despite using the same basic components the dynamics of these new drive systems are somewhat different compared to those used in mobile applications. In order to design an appropriate control system for a wind turbine it is necessary to understand these differences and how they affect the system. In this paper, the system behavior of a hydrostatic transmission for wind turbines is compared to commonly used hydrostatic drives in mobile machinery. The analysis begins by explaining that the characteristics of the loading acting on a turbine are fundamentally different to the load torque present in a standard application. Using mathematical models of both systems these differences are highlighted and discussed with special reference to how changes in system parameters can affect stability and lead to non-minimum phase behavior. These theoretical results are validated using measurements of a 1 MW hydrostatic transmission installed on a test bench.

New Designs in Fuel Dispensing System to Reduce Water Hammer

Dispensers are used for refueling vehicles at the service station. During the refueling process, the velocity of fluid changes rapidly in several working conditions, which results in a rapid pressure increasing or a water hammer effect occurring. Water hammer, often causes leakage or failure of dispensers, occurs due to pump start-up and shut-down, valves opening or closing during the refueling process. This paper experimentally characterized and theoretically calculated the impact of water hammer on the dispensers at the service station. New designs of nozzle structure and new flow-rate control modes are made to reduce the water hammer in the fuel dispensing system. Eventually, all the water hammers are reduced significantly during the refueling process under the new design modes.

STEAM: A Mobile Hydraulic System With Engine Integration

In recent years, research institutions worldwide have developed a number of new mobile hydraulic systems. Despite their improved energy efficiency, these systems have yet to gain market acceptance due to their related increase in component costs and decrease in robustness. At the Institute for Fluid Power Drives and Controls in Aachen, a new system for mobile machines, named STEAM (Steigerung der Energieeffizienz in der Arbeitshydraulik mobiler Arbeitsmaschinen), is being developed using inexpensive off-the-shelf components. The aim is to improve the total system efficiency by considering all the subsystems in the machine. This is done by integrating the internal combustion engine (ICE) into the hydraulic design process. By using a constant pressure system in combination with a low-cost fixed displacement pump the hydraulic system is designed to ensure the ICE experiences a constantly high load in a region of high efficiency, so-called point operation. To decrease the hydraulic losses incurred when supplying the linear actuators with flow, an additional intermediate pressure rail with independent metering edges is used. This enables various energy efficient discrete operating modes, including energy regeneration and recuperation.

Fault Detection of an Electrohydrostatic Actuator With the SVSF Time-Varying Boundary Layer Concept

The electrohydrostatic actuator (EHA) is an efficient type of actuator commonly used in aerospace applications. It makes use of a closed hydraulic circuit, a number of control valves, an electric motor, and a fluid pump (usually a type of gear pump). The smooth variable structure filter (SVSF) is a relatively new estimation strategy based on sliding mode concepts formulated in a predictor-corrector fashion. The SVSF offers a number of advantages over other traditional estimation methods, including robust and stable estimates, and an additional performance metric. A fixed smoothing boundary layer was implemented in an effort to ensure stable estimates, and is defined based on the amount of uncertainties and noise present in the estimation process. Recent advances in SVSF theory include a time-varying smoothing boundary layer. This method, known as the SVSF-VBL, offers an optimal formulation of the SVSF as well as a method for detecting changes or faults in a system. This paper implements the SVSF-VBL in an effort to detect faults in an EHA. The results are compared with traditional Kalman filter-based methods.

Visualization and Analysis of the Multiphase Flow in an Electromagnetically Driven Dosing Pump

Fluids with a high proportion of dissolved air lead to an increased air release in hydraulic components. Looking at the fluid flow in a piston pump, the resulting multiphase flow may affect its metering performance. To improve effects caused by cavitation, it is necessary to detect and analyze all critical flow areas in detail. This paper presents investigations of the multiphase flow in an electromagnetically driven dosing pump. This type of pump is suitable for metering any kind of liquid in motor vehicles in a very precise manner. Using high speed camera equipment and transparent components for the displacement chamber, the presented experimental work gives a comprehensive insight into the most relevant cavitation effects in the pump. In addition, the pressure inside the displacement chamber is measured with the help of a miniature pressure sensor. By combination of measuring data and visual recordings, cavitation phenomena can be determined precisely, so that a profound understanding of the flow behavior in the pump is achieved.

Study on Dynamic Behavior of a Gas Pressure Relief Valve for a Big Flow Rate

The paper investigates instable behavior of a poppet-type gas pressure relief valve operating at a big flow rate (more than 2 kg/s) under super critical pressure drop. Instability is experienced as noise and vibration and leads to severe damage of a seat and other elements. Significant and unsteady flow forces coupled with small inherent damping make it difficult to stabilize the system. In previous works, the analytical and experimental research was carried out to reveal the most essential factors influencing stability and dynamic properties of the valve. The impact of the pilot valve dynamics on the system behavior was studied for the purpose of obtaining required accuracy and stability in a wide range of flow rate. It was shown in some testing that unstable behavior of the main valve occurred when the pilot valve was stable. This paper considers inherent stability of the main valve in the gas flow. CFD software ANSYS FLUENT is employed to study the effect of the poppet geometry on aerodynamic lifting force and valve stability in axial and lateral direction. The results have been verified through comparison with experimental data.

Pneumatic Strain Energy Accumulators for Exhaust Gas Recycling

Pneumatic actuators used in devices that, by their function require light weight and small size such as orthotics, can benefit from the inclusion of accumulators to harness and recycle energy normally lost in exhausted gases. In order for an accumulator to provide benefits to these small systems, they must possess relatively high gravimetric and volumetric energy densities with adequately high efficiencies as compared to conventional accumulators, like traditional spring piston accumulators. Constructing accumulators that primarily use strain as the primary energy storage method can provide the energy storage capacities and efficiencies needed, as well as a better pressure-volume relationship than a fixed-volume accumulator. This paper outlines the behavior of an elastomeric strain accumulator constructed using natural rubber tubes as the material for an accumulator. Tube shaped accumulators fill to a preset maximum diameter, constrained by a rigid shroud so that the material’s yield strength is not approached and to control the manner in which the accumulator expands. Controlling the manner of expansion for the accumulator allows a relatively constant pressure to be used through the majority of the fill cycle. Natural rubber accumulators were experimentally evaluated and characterized for their energy storage efficiencies over a range of different parameters allowing basic design criteria to be created for use in building accumulators tailored to specific system requirements.