Study On Spatio-temporal Evolution And Characteristics Of Cavitation Flow Field In Hydraulic Torque Converter Under Extreme Operating Conditions

The hydraulic torque converter(HTC)is a basic component widely used in the field of automatic transmission vehicle.The internal flow characteristics of the hydraulic torque converter determine its external performance,and the in-depth study of the spatio-temporal evolution of its internal flow field and flow control mechanism is of great scientific significance for optimizing the design of the cascade and improving its performance.Based on the development trend of low-carbon,lightweight and intelligent automatic transmission vehicle,the hydraulic torque converter is developing in the direction of ultra-flat,no cavitation and high power density.This will complicate the design of the cascade structure of the flat hydraulic torque converter,and induce cavitation flow dynamics problems that lead to turbulent structure instability,inefficient transfer of flow energy and high energy loss.At present,there is insufficient understanding of the unsteady multi-scale vortex flow phenomenon inside hydraulic torque converter and the complex fluid dynamics problems under extreme operating conditions.The theoretical research on the generation,maintenance and development of turbulent flow and the evolution of cavitation flow field morphological characteristics is not deep enough.There are many engineering and technical difficulties that need to be solved urgently in the aspects of cavitation phenomenon prediction and accurate identification of cavitation morphological characteristics.There has always been a phenomenon that theoretical research lags behind the development of engineering technology.Therefore,carrying out basic research on the numerical simulation of the cavitation flow field of the hydraulic torque converter and the reconstruction of the three-dimensional vortex system is of great importance for deepening the understanding of the spatio-temporal evolution of cavitation flow field and the flow control mechanism of the hydraulic torque converter under extreme operating conditions.Relying on the general project of the National Natural Science Foundation of China,"Research on Turbulent Spatio-temporal Evolution Law,Bionic Control Mechanism of Automobile Torque Converter and Coupling Design Method of Cascade Parameters",the interactive research method is adopted by means of numerical simulation and experimental verification.The three-dimensional vortex system structure inside the hydraulic torque converter is reconstructed by the vortex identification method,and the spatio-temporal evolution law of the cavitation flow field and fluid dynamics characteristics are analyzed under extreme working conditions.The research results have important scientific research significance for in-depth understanding of cavitation phenomenon in the hydraulic torque converter and revealing complex flow mechanism.There is certain engineering application value for promoting the development and improvement of modern design theory and method for the high power density non-cavitation hydraulic torque converter.The main research contents are as follows:(1)Applicability evaluation of cavitation simulation method and analysis of evolution law of cavitation flow field characteristicsThe three-dimensional model of the hydraulic torque converter is established,and the full-channel model is extracted as the fluid calculation domain.Based on Integrated Computational Engineering and Manufacturing(ICEM),a high-quality full-channel grid model is divided and grid-independent analysis is performed.In this study,different turbulence models and cavitation models are used to simulate the transient cavitation flow field,and the external characteristic data of the hydraulic torque converter under different simulation methods are obtained and the curves are drawn.From the perspective of qualitative analysis,the three-dimensional cavitation morphological characteristics in each wheel flow channel is extracted.The differences in the spatio-temporal evolution of cavitation morphological characteristics and the primary region of cavitation phenomenon predicted by different simulation methods are compared and analyzed.From the perspective of quantitative analysis,relying on the external characteristic test,the accuracy of the numerical simulation results of the four simulation methods is verified.Absolute error analysis is carried out for external characteristic parameters such as torque ratio(K),efficiency(η),and pump torque coefficient(λ_P).The reliability and applicability of different simulation methods for numerically simulating cavitation flow field are comprehensively evaluated from the qualitative analysis of the spatio-temporal evolution of cavitation morphological characteristics and the quantitative analysis of external characteristic errors.Based on the calculation result of the simulation method with higher precision,the three-dimensional flow field structure is extracted,and the flow field parameters such as the two-dimensional cross-sectional velocity field,pressure field,cavitation volume fraction,and turbulent kinetic energy are intercepted.The evolution characteristics of complex fluid dynamics is analyzed,the movement trend of cavitation flow field distribution over time is analyzed,and the evolution law of cavitation phenomenon in the hydraulic torque converter is revealed.(2)Applicability evaluation of vortex identification method and analysis of spatio-temporal evolution law of vortex structure considering cavitation phenomenonThe theoretical framework of the first-generation,second-generation,and third-generation vortex identification methods is studied.Four different vortex identification methods are adopted and the optimal threshold value is screened,finely reconstruct the three-dimensional space vortex system of the pump flow channel of the hydraulic torque converter,extracting and analyzing the morphological characteristics of the three-dimensional vortex structure inside the pump flow channel considering cavitation phenomenon.Two-dimensional cross-sections at different spatial positions are intercepted to extract the structural information of the two-dimensional vortex flow field,and the identification effect and reliability of the four vortex identification methods are comprehensively compared,analyzed and evaluated.Based on the vortex identification method with good identification effect and high reliability and its optimal threshold discrimination,the overall three-dimensional vortex flow field inside the hydraulic torque converter under the stall condition considering cavitation phenomenon is reconstructed.Relying on flow information such as velocity field and pressure field,the dynamic evolution process of the formation and development of multi-scale vortex flow field is analyzed and revealed,and the influence mechanism of the unsteady vortex flow on the formation mechanism of cavitation flow field in the hydraulic torque converter under extreme conditions is established.