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Numerical error evaluation for tip clearance flow calculations in a centrifugal compressor

机译:离心压缩机叶尖间隙流量计算的数值误差评估

摘要

Since globally mesh independent solution are still beyond available computer resources for industrial cases, a method to quantify locally the numerical error is proposed. The design of experiments method helps selecting mesh parameters that influence the tip clearance solution, so that additional meshes are computed to evaluate the numerical error on the shroud friction coefficient.udIn the field of CFD applied to turbomachinery, this study results from a partnership between ENSICA, Liebherr-Aerospace Toulouse and Numeca International. This paper focuses on numerical error evaluation for RANS simulations, applied to centrifugal compressor flow field calculations. CFD is now commonly used for centrifugal compressor design optimization, but, as Hutton and Casey develop in [1], there is an urging demand for improved quality and trust in industrial CFD. Indeed, this stresses the need for comprehensive and thorough numerical error evaluation, namely the process of verification, as defined for example by Oberkampf and Trucano in [2]. Unfortunately, 3D turbulent calculations for turbomachinery components are still very demanding in computational resources and, to the knowledge of the author, there is no published result concerning comprehensive verification of the entire flow field in centrifugal compressors. As a first step on the way to achieve that, this paper presents a method aiming at the obtention of a numerical solution that can be regarded as locally mesh-independent. In other words, the objective is to compute the flow field on a grid such that the solution obtained has a specific region where the numerical error is negligible. It has long been recognized that the tip clearance of a centrifugal compressor is of paramount importance for aerodynamic performances, which means that accurately predicting the flow field in this region is crucial for accurate prediction of performances by means of CFD codes. Numerous studies have been published that compare numerical and experimental results in the tip region. However, in these studies, numerical error still remains an issue; for instance Basson and Lakshminarayana [3] show excellent comparisons with experiments, but they attribute the remaining discrepancies to insufficient grid resolution. Indeed, accurate predictions of global effects, such as efficiency, require a fine description of flow details. Therefore, friction at the shroud endwall is the concern of the study, since it is a very sensitive indicator of the quality of the velocity profile’s prediction at the wall.
机译:由于全局网格无关的解决方案仍然超出工业案例可用的计算机资源,因此提出了一种本地量化数值误差的方法。实验方法的设计有助于选择影响叶尖间隙解决方案的网格参数,以便计算其他网格以评估护罩摩擦系数的数值误差。 ud在应用于涡轮机械的CFD领域中,这项研究是由于ENSICA,利勃海尔-图卢兹航空航天公司和Numeca International。本文着重于对RANS模拟的数值误差评估,并将其应用于离心压缩机流场计算。 CFD现在通常用于离心压缩机的设计优化,但是,随着Hutton和Casey在[1]中提出,迫切需要提高质量和对工业CFD的信任。确实,这强调了对全面而彻底的数值误差评估的需求,即由Oberkampf和Trucano在[2]中定义的验证过程。不幸的是,对涡轮机械部件的3D湍流计算仍然非常需要计算资源,并且据作者所知,还没有公开的结果涉及对离心压缩机整个流场的全面验证。作为实现该目标的第一步,本文提出了一种旨在获得数值解的方法,该方法可以被视为与局部网格无关。换句话说,目标是计算网格上的流场,以使获得的解具有特定区域,在该区域中数值误差可以忽略。长期以来,人们已经认识到,离心式压缩机的叶尖间隙对于空气动力性能至关重要,这意味着准确预测该区域的流场对于通过CFD代码准确预测性能至关重要。已经发表了大量研究,比较了尖端区域的数值和实验结果。但是,在这些研究中,数值误差仍然是一个问题。例如,Basson和Lakshminarayana [3]与实验进行了很好的比较,但他们将剩余的差异归因于网格分辨率不足。确实,对诸如效率之类的全局影响的准确预测需要对流细节进行详细描述。因此,在护罩端壁处的摩擦是研究的重点,因为它是壁处速度分布的预测质量的非常敏感的指标。

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