Numerical Simulation of Multi-Cluster Fracture Propagation In Horizontal Wells With Limited-Entry Designs

The effective propagation of multi-cluster fractures in horizontal wells is the key to the development of unconventional reservoirs. Due to the influence of pressure drops at perforating holes and the stress shadow effect, it is difficult to predict the fracturing fluid distribution and fracture dimensions in a fracturing stage. In this paper, a two-dimensional fluid-solid coupling model for simultaneous propagation of multiple fractures is established, and fluid distributions and dimensions of multiple fractures are studied with respect to different perforation designs. The model combines the User Amplitude Curve Subroutine (UAMP) in ABAQUS and the cohesive zone model (CZM) to calculate the perforating friction, fluid distribution and fracture propagation behaviors. After the accuracy of this model is verified by the analytical solution, a group of simulation is conducted to compare fracture propagations when the conventional limited-entry method (CLE) and extreme limited-entry method (less than 5 perforations per cluster, XLE) are used. Simulation results show that the edge and sub-central fractures in CLE cases almost get all the fluid and effectively propagate; central fractures receive little fluid and hardly propagate. In XLE cases, the fluid distribution of each fracture is relatively uniform, but the fracture lengths within one fracturing stage is still uneven; however, only reducing numbers or radii of perforation holes cannot achieve the uniform fracture propagation, where diverters might be further needed. Findings of this study provide a reference for the perforation optimization of multi-cluster horizontal wells in the field.

Investigation of the nonlinear phenomena of a Langevin ultrasonic transducer caused by high applied voltage

In the field of power ultrasound, Langevin ultrasonic transducers (LUTs) usually operate at a large displacements output power by applying high voltages. However, empirically, a LUT exhibits nonlinearities such as amplitude jumping and peak hysteresis for high voltages in actual operations. The nonlinearities would reduce the efficiency and output accuracy of an LUT. In this research, the burst-mode method was used to measure the longitudinal vibration velocity of the LUT, which gradually decreased with time after the excitation voltage was turned off. The equivalent mechanical losses and equivalent spring constants were determined using the velocity attenuation rate and resonant frequency and they were found to be the linear functions of velocity, helping to develop a novel nonlinear model. This model contained two quadratic nonlinear terms based on the linear model. Furthermore, the developed nonlinear model was analyzed using the Lagrangian method as well as the multiscale method, which confirmed that the model was effective in describing the nonlinear behavior. It was also found that the frequency-amplitude curve bent when the nonlinear term was taken into account, which resembled the nonlinear phenomenon tested experimentally. From a physical point of view, this bending was meaningful because it led to the formation of multi-valued response regions with jumping phenomena. Additionally, according to the obtained results, the maximum value of the system response was independent of the degree of nonlinearity of the system.

Effect of Modulated Signals on Vibrational Resonance in a Harmonically Trapped Potential System

We consider a harmonically trapped potential system driven by modulated signals with two widely different frequencies ω and Ω, where Ω >> ω. The forms of modulated signals are amplitude modulated (AM) and frequency-modulated (FM) signals. An amplitude-modulated external signal is consisting of a low-frequency (ω) component and two high-frequencies (Ω + ω) and (Ω − ω) whereas the frequency modulated signal consisting of the frequency components such as f sinωt cos(g cosΩt) and f sin(g cosΩt) cosωt. Depending upon the values of the parameters in the potential function, an odd number of potential wells of different depths can be generated. We numerically investigate the effect of these modulated signals on vibrational resonance (VR) in single-well, three-well, five-well and seven-well potentials. Different from traditional VR theory in the present paper, the enhancement of VR is made by the amplitudes of the AM and FM signals. We show the enhanced response amplitude (Q) at the low-frequency ω, showing the greater number of resonance peaks and non-decay response amplitude on the response amplitude curve due to the modulated signals in all the potential wells. Furthermore, the response amplitude of the system driven by the AM signal exhibits hysteresis and a jump phenomenon. Such behavior of Q is not observed in the system driven by the FM signal.

Dynamic Analysis of Ball Screw Feed System with the Effects of Excitation Amplitude and Design Parameters

In this paper, a nine degree-of-freedom dynamic model of the ball screw feed system considering the contact nonlinearity between balls and raceways is established to analyze the vibration characteristics. The position relationship between raceway centers for the ball screw and bearings is determined by using the homogeneous coordinate transformation, and then the restoring force functions along the axial and lateral directions are derived. The dynamic equations of the feed system are solved by using Newmark method, and the proposed model is verified by the experimental method. Furthermore, the effect of the excitation amplitude on the axial vibration of the feed system is investigated by the frequency-amplitude curve and 3-D frequency spectrum. With the increase of excitation amplitude, the dynamic response of the feed system exits the softening, hardening type nonlinearity and jump phenomenon. Additionally, the effects of the initial contact angle, length of screw shaft and number of loaded balls on the axial vibration of the feed system in the resonance region are discussed. The results show that the dynamic model established in this paper is suitable for improving the machining accuracy and stability of the ball screw feed system.

COVID-19-ASSOCIATED ACUTE LOWER LIMBS ISCHAEMIA IN ELDERLY AND SENILE PATIENTS: TREATMENT PROSPECTS (PRELIMINARY ANNOUNCEMENT)

Summery. The aim — analysis of preliminary experience of treatment of COVID-19-associated acute lower limbs ischemia in elderly and senile patients. Materials and methods. We examined 48 patients (age 60- 84 years). First group — 16 patients with COVID-19-associated acute lower limbs ischemia, second group — 32 patients with chronic limb ischemia without coronavirus infection. . Results and discussion. First group’s patients had extended blood clotting time (more than 13-15 min). Other blood clotting indicators changed slightly. An isoline or a low-amplitude curve was recorded during photoplethysmography. After thrombectomy (6 patients) 3 patients had fatal thrombotic complications (PE, myocardial infarction, stroke), 1 had progressive ischemia. Pharmacotherapy was performed in 10 patients (in 2 patients later than 12 hours from the onset of ischemia). The effectiveness of timely started pharmacotherapy reached 67 %. All patients of group II had arteriosclerosis of the tibial arteries. Blood clotting indicators were normal. A high-amplitude curve was recorded during photoplethysmography. Thus, all elderly and senile patients has arteriosclerosis of the tibial arteries. On this basis, there was a fatal coincidence (coagulopathy, a sharp slowdown in blood flow, endothelial dysfunction) in case of coronavirus infection (COVID-19). Prospects for further developments are the improvement of anticoagulant therapy (for example, the simultaneous use of unfractionated and low molecular weight heparin) and improving of collateral circulation (for example, nerve block and forced intraarterial infusion or lavage). Conclusion. Currently, the effectiveness of thrombectomy in elderly and senile patients with COVID-19-associated acute lower limbs ischemiа is 33 %, the effectiveness of timely started pharmacotherapy Is 67 %.

Four-Cable-Plane Spatial Cable Stayed Bridge with Two-Amplitude Curved Deck for Canyon-River

On the basis of the original design of Ruck Bridge by Mr. Lin Tongyan, this paper modifies the single-amplitude curved cable-stayed bridge of Ruck Bridge to be an elliptical ring main beam of two-amplitude curved deck, with a duck-egg-shaped arch tower and a spatial cable net with four cable planes, to form a spatial four-cable-plane two-amplitude-curve cable-stayed bridge for the canyon-river topography, so as to improve the structural stress performance of the curved-beam cable-stayed bridge, promote the traffic function and improve the landscape. Combined with the 400m-span New Ruck Cable-stayed Bridge, engineering parameters are designed, a Midas finite element analysis model is established, and the dynamic modal analysis is carried out to verify the structural superiority of this new four-cable-plane spatial cable-stayed bridge with two-amplitude curved deck.

About mixed forced, parametric and self-oscillations by limited excitation and delayed elasticity

Mixed forced, parametric, and self-oscillations are considered if there is a delay in the elastic force in the system. A dynamic model is a friction self-oscillation system describing the frictional self-oscillations that occur in many technical systems for various purposes (metal-cutting machines, textile equipment, brakes and a number of other engineering objects). The operation of the system is supported by the energy source of limited power. For the analysis we used the method of straight linearization which is easier than the known methods of analysis of nonlinear systems, has no time-consuming and complex approximations of different orders, provides an opportunity to obtain the final design ratios regardless of the specific type and degree of nonlinearity, thus reducing labor costs and time by several orders of magnitude. By using this method, we obtained solutions of a nonlinear system of differential equations describing the system's motion. The equations of non-stationary and stationary movements are derived. To analyze the stability of stationary movements, the stability conditions based on the Routh-Hurwitz criteria are compiled. Calculations were performed to obtain information about the effect of delay on the oscillation modes. It is shown that the delay affects both the magnitude of the amplitude and the location of the amplitude-frequency curve in the frequency range depending on the magnitude of the delay, the amplitude curve is shifted to the region of lower frequencies. The stability of stationary oscillations depends both on the energy source characteristics and lag value. The interaction of the oscillating system and the energy source leads to a number of effects, both in the presence and absence of the lag. However, their course may be different depending on the lag value.

Experimental Investigation and Constitutive Modeling of the Uncured Rubber Compound Based on the DMA Strain Scanning Method

Existing research tends to focus on the performance of cured rubber. This is due to a lack of suitable testing methods for the mechanical properties of uncured rubber, in particular, tensile properties. Without crosslinking by sulfur, the tensile strength of uncured rubber compounds is too low to be accurately tested by general tensile testing machines. Firstly, a new tensile stress testing method for uncured rubber was established by using dynamic thermomechanical analysis (DMA) tensile strain scanning. The strain amplitude was increased under a set frequency and constant temperature. The corresponding dynamic force needed to maintain the amplitude was then measured to obtain the dynamic force-amplitude curve observed at this temperature and frequency. Secondly, the Burgers model is usually difficult to calculate and analyze in differential form, so it was reduced to its arithmetic form under creep conditions and material relaxation. Tensile deformation at a constant strain rate was proposed, so the Burgers model could be modified to a more concise form without any strain terms, making mathematical processing and simulating much more convenient. Thirdly, the rate of the modified Burgers model under constant strain was in good agreement with the test data, demonstrating that the elastic stiffness was 1–2 orders of magnitude less than the tensile viscosity. In the end, it was concluded that large data dispersion caused by the universal tensile test can be overcome by choosing this model, and it may become an effective way to study the tensile modeling of uncured rubber compound.

Measurement of the DC current based on magnetic fluids with an orthogonal magnetic field

Measurement of DC current is critically important in a wide range of applications, such as in photovoltaic power generation and DC power transmission. In this paper, the DC current measuring method based on magnetic fluids with orthogonal magnetic fields is studied. Firstly, the induced voltage of measurement coil, which contains a series of harmonics and the amplitude of its fundamental component is 2.5 times larger than that of other components, is derived and analyzed. Then, influence factors of the induced voltage are systematically studied. As a result, with a DC component injection into the exciting current, the frequency of the induced voltage becomes a half of that without a DC component, and when the DC component reaches to or exceeds the amplitude of the AC component, the induced voltage nearly has no distortion. Finally, the input-output characteristic of the measurement method is examined, and the accuracy, measuring range, and sensitivity are discussed, indicating that the linearity of the foundamental component root mean square (RMS) value curve is better than the one of the voltage amplitude curve under the same DC current range, and the sensitivities of induced voltage amplitude and the fundamental component RMS value are 5.948 and 3.717 mΩ, respectively.