Unsteady force measurements on propeller model in cavitation tunnels

Object and purpose of research. This paper discusses longitudinal unsteady force dynamometer for cavitation tunnel tests. The purpose of the study is to improve metrological performance of the dynamometer and extend the scope of its application. Materials and methods. The study is based on metrological parameters of dynamometers and model test data available with KSRC Large Cavitation Tunnel (LCT). Main results. Development, manufacturing, certification and commissioning of longitudinal unsteady force dynamometer based on piezoceramic load cell with improved metrological performance making it applicable for model testing of not only propellers but also other types of marine propulsors. Conclusion. Dynamometer with piezoceramic load cell offers more accurate measurement of unsteady forces, wider band of measurement frequencies, as well as wider spectrum of possible applications and lower susceptibility to interference.

Predicting the Radiated Noise of a Submarine Propeller with Different Types of Control Surfaces

The objective of this paper is to predict the noise radiated from submarine propellers with different control surface types (the cross- and X-type). When the propellers are free from cavitation, such as those of submarines at a diving depth, the radiated noise dominate, due to unsteady propeller forces. A well-known submarine model (DARPA SUBOFF) is taken as the computing sample. Simulations for hydrodynamics, including stern wakes and unsteady propeller forces, are carried out by using CFD (Computational Fluid Dynamics) technology, and the results are compared with the experimental data. The accuracy of the predicted noise depends on the CFD results. Comparisons between the CFD results and the experimental data are in good agreement. The CFD results are treated as dipole strengths in the linear wave theory to predict the radiated noise caused by the unsteady forces of the propeller. It is found that, when the control surface is of the X-type, the propeller inflow is more uniform, and the radiated noise can be decreased by about 5 dB compared to the cruciform control surface. Introduction When submarines are at diving depth, the noise generated by unsteady propeller forces (i.e., dipole strengths) will dominate. Because the juncture vortex caused by the sail makes the propeller inflow more nonuniform, the dipole strength will be enhanced and the radiated noise will be more noticeable. The uniformity of the wake field at the stern should be controlled well in order to restrain the radiated noise.

Investigation of the steady and unsteady forces acting on a pair of circular cylinders in crossflow up to ultra-high Reynolds numbers

Measurements of the steady and unsteady forces acting on a pair of circular cylinders in crossflow are performed from subcritical up to ultra-high Reynolds numbers. The two cylinders with equal diameters d are arranged inline at two centre-to-centre distances: S/d = 2.8 and 4. The trend of the drag curve for the upstream cylinder $$Cd_{1}$$ C d 1 (Re) at both distances is similar to that for a single circular cylinder. The development of the drag curves $$Cd_{2}$$ C d 2 (Re, S/d = 2.8, 4) of the downstream cylinder is inverse to that of the upstream cylinder. For both cylinder spacing values, the drag on the downstream cylinder is negative for subcritical Reynolds numbers, increases abruptly to positive values at the beginning of the supercritical regime, and shows a significant dip at transcritical Reynolds numbers. This drag inversion indicates that the critical distance Sc decreases sharply in the supercritical Reynolds number range. For S/d = 2.8 at Re$$\rightarrow$$ → 10$$^{7}$$ 7 , the downstream cylinder experiences once more a thrust force. The curve of the Strouhal number St(Re) of the downstream cylinder for S/d = 4 is very close to that of a single cylinder. For Reynolds numbers of Re$$\approx$$ ≈ 1$$\times$$ × 10$$^{6}$$ 6 - 7$$\times$$ × 10$$^{6}$$ 6 , the Strouhal numbers have nearly equal values of St$$\approx$$ ≈ 0.22 - 0.24 for both distances. This is followed by a branching. For Re$$\rightarrow$$ → 10$$^{7}$$ 7 and the case S/d = 2.8, the Strouhal numbers dip at St = 0.17. However, for S/d = 4, they increase up to St = 0.27. In the supercritical range, two peaks occur in the power spectra for the large distance S/d = 4. Based on a wavelet analysis, we can conclude that the low-frequency mode, which does not occur for a single cylinder, is an interference effect. Graphic abstract

Wake optimization of ducted propeller

Object and purpose of research. This paper discusses marine ducted propeller and the ways to ensure its target performance parameters. The purpose of this study was to mitigate unsteady forces on the propeller behind the duct struts. Materials and methods. Analytical estimates of propeller parameters and in-house KSRC methods for numerical simulation of ducted propeller behaviour. Main results. Calculations of effective wake behind duct struts taking into account the flow around hull and its append-ages. Calculations of unsteady forces for a standard propeller operating in this wake. Design of a propeller with increased blade skew. Calculations of unsteady forces for the new propeller in the initial wake. Wake field parameters contributing to mitigation of unsteady forces. Calculations for the new strut shape for wake optimization. Calculations of unsteady force amplitudes for standard propeller in the new wake. Conclusion. Ducted propeller discussed in this study was meant to illustrate how propeller wake properties, like unsteady forces, can be optimized without changing propeller geometry, only by means of curved duct struts.

Investigation of Unsteady Pressure Fluctuations in a Simplified Steam Turbine Control Valve

Steam turbines are among the most important systems in the conversion of thermal into electrical power. As the amount of renewable energies increases, existing power plants are experiencing increased times at part load conditions. To control the power output of a steam turbine, the use of control valves is a widely spread method, allowing fast load gradients and a quicker response on variable power requirements. At part load, a significant amount of energy is dissipated across the valve, as the total inlet pressure of the turbine is reduced. At these conditions, the flow across the valve becomes trans- and supersonic and large pressure fluctuations appear within the downstream part of the valve. As a result, unsteady forces can trigger strong vibrations, leading to mechanical stresses and possible valve failures. A spherical valve shape is still used in smaller industrial steam turbines, in which the flow is prone to show strong flow instabilities across a wide range of operating points. Because of these known instabilities, the spherical valve shape was chosen as the experimental test geometry and the evaluation of the unsteady flow and fluid-structure-interaction within the scaled steam turbine control valve. Using numerical methods, the test valve is investigated and the time dependent pressure distribution in the downstream diffuser is evaluated. The evolution of the flow stability will be discussed for different pressure ratios. Pressure signals retrieved from the control valve test rig will be used to compare the numerical results to experimental data.

Investigation of Unsteady Pressure Fluctuations in a Simplified Steam Turbine Control Valve

Steam turbines are among the most important systems in commercial and industrial power conversion. As the amount of renewable energies increases, power plants formerly operated at steady state base load are now experiencing increased times at part load conditions. Besides other methods, the use of control valves is a widely spread method for controlling the power output of a steam turbine. In difference to other throttling approaches, the control valve enables fast load gradients as the boiler can be operated at constant conditions and allows a quicker response on variable power requirements. At part load, a significant amount of energy is dissipated across the valve, as the total inlet pressure of the turbine is decreased across the valve. At these conditions, the flow through the valve becomes trans- and supersonic and large pressure fluctuations appear within the downstream part of the valve. As a result, unsteady forces are acting on the valve structure and vibrations can be triggered, leading to mechanical stresses and possible failures of the valve. Besides more complex valve geometries, a spherical valve shape is still often used in smaller and industrial steam turbines. Because of the smooth head contour, the flow is prone to remain attached to the head surface and interact with the flow coming from the opposite side. This behaviour is accompanied by flow instabilities and large pressure fluctuations, leading to unsteady forces and possible couplings with mechanical frequencies. The spherical valve shape was therefore chosen as the experimental test geometry for the investigation of the unsteady flow field and fluid-structure-interactions within a scaled steam turbine control valve. Using numerical methods, the test valve is investigated and the time dependent pressure distribution in the downstream diffuser is evaluated. The evolution of the flow stability will be discussed for different pressure ratios. Pressure signals retrieved from the control valve test rig will be used to compare the numerical results to experimental data.

Insights into the periodic gust response of airfoils

The unsteady lift response of an airfoil in a sinusoidal gust has in the past been modelled by two different transfer functions: the first-order Sears function and the second-order Atassi function. Previous studies have shown that the Sears function holds in experiments, but recently Cordes et al. (J. Fluid Mech., vol. 811, 2017) reported experimental data that corresponded to the Atassi function rather than the Sears function. In order to clarify the observed discrepancy, the specific differences between these models are isolated analytically. To this end, data and analysis are confined to unloaded airfoils. These differences are related to physical gust parameters, and gusts with these parameters are then produced in wind-tunnel experiments using an active-grid gust generator. Measurements of the unsteady gust loads on an airfoil in the wind tunnel at Reynolds numbers ($Re_{c}$) of $2.0\times 10^{5}$ and $2.6\times 10^{5}$ and reduced frequencies between $0.09$ and $0.42$ confirm that the Sears and Atassi functions differ only in convention: the additional gust component of the Atassi problem can be scaled so that the Atassi function collapses onto the Sears function. These experiments, complemented by numerical simulations of the set-up, validate both models across a range of gust parameters. Finally, the influence of boundary-layer turbulence and the turbulent wake of the gust generator on experimental convergence with model predictions is investigated. These results serve to clarify the conditions under which the Sears and Atassi functions can be applied, and demonstrate that the Sears function can effectively model unsteady forces even when significant fluctuations in the streamwise velocity are present.

Numerical Simulation of Unsteady Moist-Air Flows Through Whole-Annulus Rotor Blade Rows in Transonic Compressor

In this study, we numerically investigated moist-air flow through the transonic compressor rotors of NASA Rotor 37, assuming whole-annulus rotor blade rows and non-uniform inlet wetness. This is an extension of our previous study, which assumed only a single passage and uniform inlet wetness. The amount of water droplets streaming into the compressor was changed in circumferentially non-uniform inlet condition. Numerical results indicated that non-uniform inlet wetness induced non-uniform temperature in the passages due to absorption of latent heat by droplet evaporation. Moreover, shock locations varied, depending on the local amount of wetness. Furthermore, turning angles of the flow and torque on the rotor blades were influenced by the wetness. Therefore, unsteady forces on the rotor blades were resultantly obtained by considering non-uniform inlet wetness conditions.