A SOURCE OF PROPELLER EXCITED BROADBAND VIBRATION ON A HIGH-SPEED TWIN SCREW SHIP

The cause of the severe propeller excited broadband vibration on a twin screw ship’s stern was investigated in a cavitation tunnel using conventional modelling methods. At first sight the results did not indicate anything untoward about the stern and propeller designs tested, apart from some unusual cavitation patterns in the propellers’ slipstreams. The fluctuating pressure levels on the model hull varied considerably depending on the propeller designs and loading conditions used in the tests, but these did not provide an explanation for the vibration on the ship. Some unusual patterns in the relative levels of the harmonic pressures on the stern were noticed in addition to the presence of a large cavitation disturbance in a propeller trailing vortex that was captured in a single frame of a video recording. This latter observation led to a plausible explanation for the broadband vibration on the ship’s stern.

Pressure Fluctuations Characteristics of the Stilling Basin with a Negative Step

Fluctuating pressure is the main cause of the floor fatigue of the stilling basin with a negative step. Despite investigations of stilling basin with a negative step conducted by many researchers, there is not enough information about the influence of the geometric parameters on fluctuating pressure on the floor. In the present study, fluctuating pressure on the floor of the stilling basin with a negative step was systematically investigated by a total of 85 model tests. The results show that the fluctuating pressure coefficient Cp’ has a process of rapid increase and decrease, and then decreases slowly until it becomes stable, and the maximum fluctuating pressure coefficient Cp’max lies in the reattachment zone rather than in the jet impingement area for Type II-jump. The dominant frequency of the fluctuating pressure on the floor shows a decreasing trend along stilling basin. With the increase of the step height, the Cp’max presents decreasing trend but X*0 where the Cp’max occurs increasing trend. While there has on obvious regularity between incident angle and Cp’. Finally, according to the fitting of test data, an empirical formula to calculate Cp’max is developed. These research results provide reference for the design of stilling basin with a negative step in engineering applications.

Fluctuating Characteristics of the Stilling Basin with a Negative Step Based on Hilbert-Huang Transform

The violent fluctuation of hydrodynamic pressure in stilling basins is an important factor threatening the safety of the bottom plates of stilling basins, and plays an important role in the safe operation of stilling basins. In order to deeply understand the fluctuating characteristics of stilling basins, the fluctuating pressure signal of a stilling basin bottom plate is processed by the Hilbert-Huang transform method through a hydraulic model test. In this paper, three signal decomposition methods are used to decompose the pulsating pressure signal. A Hilbert transform is used to select the component with the best decomposition effect. The time-frequency-amplitude diagram of the pulsating pressure signal is obtained by Hilbert transform, and its time-frequency characteristics are discussed in depth. The analysis results are as follows: (a) the decomposition results from the CEEMD method are orthogonal and complete. The HHT method is suitable for processing fluctuating pressure signals. (b) With an increase in IMF decomposition order, the signal frequency band becomes narrow, the Hilbert spectrum amplitude decreases and the pulsating pressure energy decreases. The decomposition of the fluctuating pressure signal into components of different scales shows that the turbulence is composed of multiple scales of vortices, reflecting the vortex structure in the turbulence. (c) The jet impingement zone of the drop bucket stilling basin is near x/L = 0.075. The dominant frequency and marginal spectrum energy of the jet impingement zone are very prominent, and the marginal spectrum energy is mostly concentrated within 5.0 Hz. (d) At different drop height and different flow energy ratio, the fluctuation in the dominant frequency of fluctuating pressure decreases, the dominant frequency of the head of the stilling basin is larger, the dominant frequency of the middle and rear parts tends to be stable, and the dominant frequency is finally stabilized at about 1.0 Hz. This paper attempts to use the HHT method to process the fluctuating pressure signal, and the results provide a new discussion method for exploring the fluctuating pressure characteristics of hydraulic structures.

Parallel Calculation of Random Vibration for Reentry Vehicles in Fluctuating Pressure Environment

The reentry vehicle is in an unconstrained free-flight state during the reentry process. The pressure of air fluctuations acts on the external surface of the vehicle and induces a random vibration environment with a wide-frequency range, which makes great influences on the structural dynamic properties of vehicles. This paper aims at the parallel calculation of the wide-frequency random vibration problems for the free body under multipoint fluctuating pressure loads. The calculation method is based on the self-developed JAUMIN framework and PANDA platform. The novel algorithm based on the modal superposition method reduces the computational complexity, realizes the parallel calculation, and achieves the maximum numerical simulation calculation capacity of 1.045 billion degrees of freedom (DOFs) while improving the calculation efficiency and the accuracy of the results. Finally, this article also uses typical examples to verify the correctness and parallel scalability of algorithms and programs.

Time-Frequency Characteristics of Fluctuating Pressure on the Bottom of the Stilling Basin with Step-Down Floor Based on Hilbert–Huang Transform

Fluctuating pressure is an important feature of the bottom of a stilling basin with step-down floor. To analyze the frequency domain characteristics and energy distribution of this fluctuating pressure, the Hilbert–Huang transform (HHT) method is used. First, empirical mode decomposition is performed on the pressure fluctuation signal to obtain a number of intrinsic mode functions (IMFs), and then the Hilbert transformation is performed on each IMF to obtain the Hilbert spectrum and marginal spectrum for characterizing the pressure fluctuation signal. The results show that the fluctuating pressure signal of the stilling basin with step-down floor has obvious characteristics of low frequency and large amplitude. The dominant frequencies of the head and tail of the stilling basin are very prominent, and most of the energy is concentrated below 5.0 Hz; with the increase in the relative position of the measuring point, the energy distribution in stilling basin with step-down floor changes from high-frequency component to low-frequency component. The fluctuating pressure signal of the stilling basin with step-down floor has random amplitude modulation and frequency modulation. The marginal spectrum obtained by the HHT method can obtain the local characteristics of the signal more accurately and is more suitable for processing nonlinear and nonstationary signals.

Cavitation and Induced Excitation Force of Ice-Class Propeller Blocked by Ice

The presence of broken ice in the flow field around a propeller causes severe blade erosion, shafting, and hull vibration. This study investigates the performance of the propeller of a ship sailing in the polar regions under the propeller–ice non-contact condition. To this end, we construct a test platform for the propeller-induced excitation force due to ice blockage in a large circulating water channel. The hydrodynamic load of the propeller, and the cavitation and propeller-induced fluctuating pressure, were measured and observed by varying the cavitation number and ice–propeller axial distance under atmospheric pressure and decompression conditions. The results show that the fluctuation range of the blade load increases with a decrease in cavitation number and ice–propeller axial distance. The decrease in the cavitation number leads to broadband characteristics in the frequency-domain curves of the propeller thrust coefficient and blade-bearing force. Under the combined effects of ice blockage and proximity, propeller suction, the circumfluence zone around the ice, and the Pirouette effect, propeller–hull vortex cavitation is generated between the ice and propeller. The decrease in cavitation number leads to a sharp increase in the amplitude of the high-order frequency of the propeller-induced fluctuating pressure.

Experimental investigation of the fluctuating static pressure in a subsonic axisymmetric jet

Measuring the fluctuating static pressure within a jet has the potential to depict in-flow sources of the jet noise. In this work, the fluctuating static pressure of a subsonic axisymmetric jet was experimentally investigated using a 1/8” microphone with an aerodynamically shaped nose cone. The power spectra of the fluctuating pressure are found to follow the -7/3 scaling law at the jet centerline with the decay rate varying as the probe approaches the acoustic near field. Profiles of skewness and kurtosis reveal strong intermittency inside the jet shear layer. By applying a continuous wavelet transform (CWT), time-localized footprints of the acoustic sources were detected from the pressure fluctuations. To decompose the fluctuating pressure into the hydrodynamic component and its acoustic counterpart, two techniques based on the CWT are adopted. In the first method the hydrodynamic pressure is isolated by maximizing the correlation with the synchronously measured turbulent velocity, while the second method originates from the Gaussian nature of the acoustic pressure where the separation threshold is determined empirically. Similar results are obtained from both separation techniques, and each pressure component dominates a certain frequency band compared to the global spectrum. Furthermore, cross-spectra between the fluctuating pressure and the turbulent velocity were calculated, and spectral peaks appearing around Strouhal number of 0.4 are indicative of the footprint of the convecting coherent structures inside the jet mixing layer.

MANAGEMENT OF INTRACRANIAL HIGH PRESSURE ON STROKE: A NARRATIVE REVIEW

Intracranial pressure (ICP) is a dynamic and fluctuating pressure within the cranial vault influenced by cerebrospinal fluid (CSF), brain tissue, and blood. Any increase in the volume of its contents will increase the pressure within the cranial vault. As a response to ICP, compensation begins by the movement of CSF from the ventricle to cerebral subarachnoid space and increases CSF absorption. Increased ICP is a state of neurological emergency caused by various neurological injuries. It is associated with poor outcomes, including traumatic brain injury, large acute ischemic stroke, intracerebral hemorrhage, or even death. This article aims to review the symptoms and signs of increased ICP, particularly on infarction stroke and hemorrhagic stroke condition, how the therapeutic goals of the treatment strategies, and pharmacological or non-pharmacological management to mitigate the elevation of ICP on stroke.