Abstract:

Objective In the air-jet weaving, the weft yarn is dragged by the supersonic airflow in a specially shaped reed, and the weft could be broken while clashing with the wall of the special shaped reed, leading to decreased insertion efficiency. Due to the strong coupling effect between auxiliary jet and the weft yarn, the movement of the weft yarn shows strong nonlinear and non-stationary characteristics. The transient velocity fluctuation of the air flow directly affects the fluctuation amplitude of the weft movement. This paper aims to analyze the velocity fluctuation characteristics of transient jet generated by the relay nozzle based on the variational modal decomposition (VMD) method.

Method Three types of auxiliary nozzles were designed, i.e., conical inlet, arc inlet and cylindrical inlet, respectively. Firstly, the transient flow field of the auxiliary nozzle was simulated using large eddy simulation (LES). Two monitor points were settled in the jet potential core and tail area, where the original instantaneous velocity signal was obtained from LES dataset. In order to verify the correctness of the LES simulation, both a grid independence verification and an experiment measurement were conducted. Then, the original velocity signal decomposed into a series of the intrinsic mode functions (IMFs) based on the VMD, each IMF representing a different frequency mode inherent in the original velocity signal. The fluctuations of the IMF captured the movement information of the weft. Finally, the fluctuation characteristic of IMF2 was evaluated according to the mean absolute error (MAE), the root mean square error (RMSE) and the mean absolute percentage error (MAPE). In addition, in order to explore the weft movement, the bidirectional fluid-structure interaction method was adopted to explore the radial fluctuation of the yarn, and the weft fluctuation amplitude was predicted by comparing the radial fluctuation and the IMF2 signal law.

Results In order to learn the velocity fluctuation of the auxiliary nozzle with different structures, the transient velocity contour diagram of the auxiliary nozzle with different structures was shown, and it can be seen that at 0.2 s, the three jet velocities remained basically stable, among which the core velocity of the conical jet was the largest (338 m/s), followed by the arc type (326 m/s), and the lowest core velocity of the cylindrical jet was 323 m/s. In the special-shaped reed, the potential core (monitoring point A) determines the maximum velocity, in fact, the potential tail region of the auxiliary jet (monitoring point B) is in contact with the weft, its transient velocity fluctuation will determine the motion displacement of the weft yarn head, and the results show the comparison of the transient velocity original signals of the three auxiliary jets at monitoring points A and B, respectively. In order to find out the fluctuation law behind it, the transient velocity signals of monitoring points A and B were decomposed by VMD, and the intrinsic mode components IMF1, IMF2 and IMF3 were obtained. The results show the comparison of the IMFs of the two monitoring points, respectively, and find that IMF1 is a stable velocity curve, while IMF2 and IMF3 are sinusoidal velocity curves. In order to verify and predict the trend of weft fluctuation, the flexible weft yarn fluctuates radially in the coupled flow field composed of three different structures of auxiliary nozzles and special-shaped reeds, among which the maximum radial offset of the conical auxiliary nozzle is 0.33 mm, the second is 0.73 mm for cylindrical type, and the radial offset value of the arc auxiliary nozzle is the largest, which is 1.35 mm.

Conclusion After the three auxiliary jets enter the free space from the nozzle, the transient velocity presents the characteristics of non-stationary signal, and the amplitude gradually decays. The velocity signals of the conical, arc and cylindrical auxiliary jet potential core region (monitoring point A) and potential tail (monitoring point B) were decomposed by VMD, and the intrinsic mode function IMF1 of the two monitoring points was the largest and stable, which can be used as the average velocity of the auxiliary jet. The obvious fluctuation of IMF2 can be used as the main basis for predicting the weft shift. However, the IMF3 amplitude is small and the frequency is high, which indicates that there is a local disturbance in the flow field, which can be filtered out as a noise signal. Further analysis of the IMF modes in the potential tail region of the three auxiliary jets, and the results of the fluctuations of MAE, RMSE and MAPE show that the amplitude fluctuation of IMF2 of the conical auxiliary jet is the smallest, and its airflow fluctuation is the most stable. Considering the two-way coupling effect of the synthetic flow field and the flexible weft yarn in the special-shaped reed, the numerical simulation shows that the radial offset of the weft yarn head is the smallest in the synthetic flow field of the conical auxiliary spray, followed by the displacement of the head end in the cylindrical auxiliary spray, and the tip displacement generated in the arc auxiliary injection synthesis gas flow is the largest.

Key words: air jet loom, auxiliary nozzle jet, transient flow field, large eddy simulation, variational modal decomposition, fluctuation of weft yarn