Research On The Mechanism And Control Of The Underwater Suction Vortex Flow At The Inlet Of The Water Jet Propulsion Pump Device In The Shallow Water Area

At present,waterjet propulsion technology is widely used in modern amphibious tanks,large and medium-sized naval landing ships and civil yachts.This type of waterjet propulsion ship often takes a long time to sail in shallow water channels or shallow-water area.Due to the water depth of the water area where the ship is located is shallow,the water inlet of the waterjet propulsion pump device is close to the riverbed.At this time,if the ship is in the sailing state,it is very easy to form underwater suction vortex under the water inlet when the waterjet propulsion pump device is started.Therefore,in order to broaden the hydraulic stable operation range of the waterjet propulsion pump device in the shallow water area,and ensure the safe and reliable operation of the waterjet propulsion device,it is very necessary to study the mechanism and prevention measures of suction vortex under the inlet of the waterjet propulsion pump device in shallow water area.Under this background,taking model experiment as the main research means,this paper designs and establishes the water inlet hydraulic performance and thrust test system of waterjet propulsion pump device.At the same time,it realizes the visual and quantitative research on the suction vortex under the water inlet of waterjet propulsion pump device and the direct measurement of thrust.Combined with numerical simulation,the flow characteristics of suction vortex.The characteristics of instantaneous thrust fluctuation and the prevention measures of suction vortex are studied.The main achievements are as follows:(1)The influence factors of suction vortex under the inlet were studied by numerical simulation method,and the influence laws of geometric parameter and flow parameter on the suction vortex were proved.The under keel clearance is the key factor affecting the formation of suction vortex under the inlet.The increase of intake flow rate promotes the development of suction vortex.The smaller the under keel clearance,the greater the suction flow at the inlet,and the easier it is to produce suction vortex.(2)The high-speed camera technology is used to capture and analyze the suction vortex under the inlet water.The typical morphological characteristics of the suction vortex are revealed,and the necessary conditions for the formation of the suction vortex and its evolution process are clarified.Under the condition of constant under keel clearance,the shape and evolution of suction vortex under different water depth/draft ratios and rotating speed were studied.The experimental results show that the suction vortex under the inlet is the vertical axis vortex under the inlet,and the spatial motion is mainly spanwise motion.Its shape is not affected by the water depth/draft ratios and rotating speed conditions.Its shape is similar to a tornado,showing a shape of thick at the top and thin at the bottom,and is basically symmetrical along the axis of the vortex center.Using topology analysis method to analyze the formation law and conditions of the suction vortex,it is found that the existence of an inverse pressure gradient at the bottom of the riverbed,the existence of rising water flow between the riverbed and the inlet,and the surrounding environment fluid swirling are the three major conditions for the formation of the underwater suction vortex at the inlet.The underwater suction vortex of a typical inlet basically occurs,develops and annihilates at a fixed position,and its evolution process can be roughly divided into five stages:precursor,initial,development,weakening and disappearance.Based on mathematical statistics,the occurrence of underwater suction vortex at the inlet is summarized,and it is found that with the increase of the water depth draft ratios,the evolution duration of the suction vortex is significantly prolonged,and with the increase of the rotation speed,the evolution duration of the suction vortex is shortened,but the frequency of occurrence increases significantly.(3)The instantaneous thrust of the waterjet propulsion pump device is directly measured by the force sensor.The thrust generated by the waterjet propulsion system having periodic fluctuation characteristics is theoretically analyzed,and the time-frequency characteristics of the periodic fluctuation of the wateijet propulsion system are verified and obtained through experiments,and the influence law of the change of rotation speed condition on the characteristics of instantaneous thrust fluctuation is proved.The test result shows that there is a periodic fluctuation in the outlet velocity of the guide vane due to the strong dynamic and static interference between the high-speed rotating impeller and the stationary guide vane during the operation of the waterjet propulsion pump device,so the instantaneous thrust of the waterjet propulsion system has periodic fluctuation.After FFT,it turns out that the main frequency of the instantaneous thrust fluctuation is 3 times the shaft frequency,and the secondary frequency is 1 time the shaft frequency,and the low-frequency fluctuation is dominant.It shows that after the frequency is greater than 250 Hz,the thrust fluctuation amplitude is significantly weakened and approaches zero.After wavelet transform,it turns out that the wavelet transform time-frequency distribution diagram can effectively reflect the global information of the instantaneous thrust on the time and frequency scales.It shows that the high-energy area is mainly concentrated in the low-frequency area below 150Hz.In the frequency region of(80?300)Hz,there is a significant alternating light and dark texture phenomenon caused by the modulation of low-frequency signals such as shaft frequency.The characteristics of instantaneous thrust fluctuation under different rotation speed conditions are compared.The main frequency of instantaneous thrust fluctuation does not change with the change of rotation speed.The main frequency of instantaneous thrust fluctuation is 3 times the shaft frequency,and the secondary frequency is 1 time the shaft frequency.(4)Based on the V3V three-dimensional flow velocity field test technology,the three-dimensional flow velocity field of the suction vortex generation area under vortex generation and non-vortex conditions is measured and analyzed,and the law that the vorticity field changes with time in the five key stages when the suction vortex occurs is analyzed emphatically.The test result shows that streamlines and vorticity cannot be used alone as a criterion for the existence of vortices.However,the comprehensive comparative analysis based on the V3V experiment,the velocity field and the vorticity field can be used as a basis for judging whether the suction vortex occurs or not,and the key information such as the location where the suction vortex occurs and its related flow characteristics can be effectively obtained.Through the analysis of the vorticity changes in the different evolution stages of the suction vortex,it is found that as time evolves,the suction vortex first increases and then decreases,and reaches the maximum in the development stage.The circumferential velocity in the suction vortex increases first and then decreases along the radius,which is a composite vortex composed of a forced vortex and a free vortex.According to the circumferential velocity distribution curve,it is found that the suction vortex tube presents a thick top and a thin bottom shape by analyzing the radius of the vortex core.The Q criterion,?2 criterion and?ci criterion are respectively used to identify the structure of the suction vortex.The Q criterion and ?2 criterion are better for the identification of the suction vortex,but they have a strong dependence on the threshold,and the suction vortex energy is low.Therefore,the second-generation vortex identification method can cause the phenomenon of vortex breakup.Only the vertically upward suction vortex tube structure is captured at the riverbed,and the continuous vortex consistent with the experimental observation form cannot be captured intuitively.(5)The pressure fluctuation signal at the position where the suction vortex occurs under the inlet has strong nonlinear characteristics.The dynamic water pressure fluctuation data at the bottom of the riverbed are obtained by using high frequency dynamic water pressure sensor.The time-frequency characteristics of pressure fluctuation signal induced by suction vortex are proved.The results show that the occurrence of suction vortex will induce the recovery of pressure fluctuation amplitude after sudden drop.The characteristic low-frequency frequency frequency induced by suction vortex under the inlet water is 0.28Hz,and the corresponding amplitude of this frequency is much larger than that of other frequencies,and this frequency does not change with the change of water depth/draft ratio and rotating speed,indicating that the low-frequency pressure fluctuation induced by suction vortex is absolutely dominant.Based on the theory of chaotic time series analysis,the chaotic characteristics of pressure fluctuation signal induced by suction vortex are revealed.The nonlinear phase space of pressure fluctuation signal is reconstructed.The nonlinear characteristics of pressure fluctuation signal are clarified.It is found that the pressure fluctuation signals at the measuring points under the conditions of no vortex and vortex have chaotic characteristics.The three-dimensional trajectories in phase space show the characteristics of singular attractors,in which the singular attractor structure of pressure fluctuation signal induced by suction vortex is more complex and the chaotic characteristics of pressure fluctuation signal are stronger.(6)Based on numerical simulation,the prevention and control strategies of suction vortex under inlet water are proposed from two aspects:vortex control strategy and vortex prevention measures.When waterjet propulsion ships sail in shallow water,in order to avoid and reduce the occurrence of suction vortex,they should start at low speed and then cruise at high speed.It is suggested to excavate trapezoidal grooves in narrow waterways,port waterways and other coastal shallow water areas to increase the distance between the water inlet of the waterjet propulsion pump device and the riverbed.The waterjet propulsion ship starts in the trapezoidal groove area,which can effectively inhibit the generation of suction vortex.