Accuracy Investigations of Dynamic Characteristic Predictions of Tip Leakage Flow Using Detached Eddy Simulation

Abstract

The accurate prediction of tip leakage flow is the premise for flow mechanism analysis and compressor performance optimization. The detached eddy simulation (DES) method, which compromises cost and accuracy, has excellent potential for a high Reynolds flow, like a compressor.However, in the case of tip leakage flow, especially when there are multiple wall boundary layers and strong shear between the mainstream and leakage flow, the DES method exhibits accuracy deficiencies. This paper explores the resolution of the critical detailed structures using the DES method and its correlation with the accuracy of time-averaged aerodynamic parameter predictions. Based on this, we propose the necessary conditions for the DES method to accurately predict the leakage flow from the perspective of the detailed structure of the flow field. A simplified model is proposed to emphasize the characteristics of tip leakage flow with “multiple walls + narrow tip gap”, and the high-fidelity flow field of the WALE LES method is used as a benchmark. With the main fluctuation structures obtained by the SPOD method, it is concluded that the DES method is unable to resolve the Kelvin–Helmholtz instability at the initial position of the leakage, which leads to the generation of the secondary leakage vortex upstream of the leakage and the breakdown of the induced vortex, two critical flow structures, being incorrectly estimated. This can lead to misestimationsof the force direction on the tip leakage vortex and the main fluctuation on the flow field. As a result, the tip leakage vortex trajectory evolves toward the middle of the passage along the tangential direction and away from the upper wall downstream of the leakage compared with the LES results. Predictions of losses in the upstream and midstream regions are underestimated, whereas they are overestimated downstream of the leakage and outside the passage.Therefore, the accurate resolution of these two critical detailed structures is an essential prerequisite for the precise prediction of tip leakage flow using DES series methods.

Study on the Tip Leakage Loss Mechanism of a Compressor Cascade Using the Enhanced Delay Detached Eddy Simulation Method

“…Even though Spalart [22] proposed an improved version, DDES, in 2006 to address the issues caused by Modeled Stress Depletion (MSD) by delaying the development of LES, this version is only applicable to corner separation rather than tip leakage in compressor applications. This is because the unique flow structure of tip leakage flow presents new challenges for the DDES method [29]. As shown in Figure 2a, there is a flow along the tangential direction from the tip clearance in the compressor, forming a confined jet that shears with the mainstream.…”

“…The gray area exists in the form of a slow LES development (SLD) problem when encountering the "multiple boundary layer attachments + shear effect" typical in the case of tip leakage flow. In this situation, fluctuation cannot be resolved at the initial shear region, thereby affecting the accuracy of downstream predictions [29]. Research indicates that DES97, DDES, and IDDES methods all suffer from inaccuracies in predicting aerodynamic parameters [28], detailed structures [6], unsteady fluctuations [30], and TKE [18] in the prediction of compressor tip flow.…”