Semi-Confined Flows

Underlying Flow Regime 3-34

Description

Introduction

The UFR in question is characterized by several complex flow phenomena. They include APGinduced separation of the turbulent boundary layer from a smooth surface, reattachment of the separated shear layer, and relaxation of the reattached turbulent boundary layer farther downstream. All these phenomena are known to be very difficult to predict with RANS models as well as with scale-resolving approaches to turbulence representation. The UFR is of significant industrial importance and is directly related to several Application Challenges included in the ERCOFTAC Knowledge Base Wiki, e.g., AC 1-05 (Ahmed body), AC 1-08 (L1T2 3 element airfoil), AC 4-01 (wind environment around an airport terminal building), and AC 4-03 (Air flows in an open plan air conditioned office). Hence it is of high interest for the ground transportation, aerospace, civil engineering, and other industries. The 2DWMH flow includes all the major flow features of the UFR listed above and, therefore, is a representative example of the considered UFR. This makes it an attractive test case (TC) for an objective evaluation of capabilities of existing modelling/simulation approaches and CFD techniques in terms of predicting the UFR in question. The purpose of this document is to provide a comprehensive summary of results of a systematic study of this TC with the use of enhanced hybrid RANS-LES methods undertaken in the framework of the EU Research Collaborative Project “Grey Area Mitigation for Hybrid RANS-LES Methods” (Go4Hybrid) [‌5 and a comparison with computations performed recently by Uzun and Malik [‌6] who computed the flow with the use of much more expensive Wall-Resolved LES (WRLES). This and, also, a brief overview of the results of RANS computations of this flow presented in detail on the NASA Turbulence Modeling Resource web site https://turbmodels.larc.nasa.gov/nasahump_val.html [&#8204 7], give a clear idea of capabilities and restrictions of different turbulence modelling/simulation strategies with regard to the considered UFR. It should be noted also that an accurate prediction of the separation point in the 2DWMH flow is not that difficult thanks to a rather abrupt variation of the downstream part of the hump geometry and, therefore, sudden appearance of the APG. On the one hand, this somewhat decreases its value but, on the other hand, it allows to “isolate” the issues associated with prediction of separation and reattachment. The document is organized as follows. In section 2 a brief review is presented of studies of this UFR and a rationale is given of the choice of the specific TC. Then, in Sections 3, 4 an outline is performed of the experiments [1], [2]. After that, in Sections 5, 6 a summary of CFD methods used, results of the simulations performed, and comparison of the results with each other and with the experimental data are presented. First, an overview of the RANS studies of the TC is given, based on the database accumulated in [7]. Then, a more detailed analysis is provided of the studies carried out in the course of the Go4Hybrid project [8], [9] within non-zonal and zonal hybrid RANS-LES approaches and, also, in the recent work [6] where the flow is thoroughly investigated with the use of WRLES. Finally, in Section 7 some practical advices on computing the considered UFR are given based on the performed analysis.

Brief Review of UFR Studies and Choice of Test Case

Considering both fundamental and industrial importance of the UFR in question, it is not surprising that a variety of flows related to this UFR have been investigated in a huge number of experimental and numerical studies. As mentioned in the introduction, this document focuses on one of these flows, namely, on the 2DWMH TC [1], [2]. Despite a relatively simple (nominally 2D) geometry, this flow includes major challenging features of the considered UFR (see Section 1), thus presenting a relevant test case for assessment of CFD capability of reliably predicting these features. Other than that, the experiments have been specially designed for CFD validation. As a result, the experimental database ensures the possibility of the formulation of a corresponding computational problem which reasonably adequately represents the experimental setup. Moreover, the data are well documented and are available in digital form at web sites [3] and [7]. The last but not least rationale for the choice of exactly this TC is that it has been used in the framework of several national and international initiatives directed at the evaluation of different CFD techniques and turbulence modelling/simulation strategies. In particular, it has been included by NASA in a set of test cases aimed at verification of the implementation and at validation of a wide range of RANS turbulence models (NASA Turbulence Modeling Resource portal [7]), and also thoroughly investigated in the framework of the recent EU Project Go4Hybrid [8], [9] with the use of enhanced hybrid RANS-LES methods and in the work of

�Uzun and Malik [6], who performed WRLES of the 2DWMH flow on extremely large (up to 850 million points) grids and addressed different computational aspects of the simulations.

BRIEF DESCRIPTION OF THE STUDY TEST CASE

A 3D sketch of the experimental setup is shown in Fig. 1. The configuration presents a GlauertGoldschmied type body consisting of a relatively long fore body and a relatively short concave ramp comprising the aft part of the model mounted between two glass endplate frames with both leading edge and trailing edges faired smoothly with a wind tunnel splitter plate.

Figure 1: 3D sketch of experimental setup [1], [2]

Major geometric and flow parameters of the TC are presented in Fig.2 and summarized in Table 1 (note that in the “baseline” experiment considered here the slot shown in Fig.2 was closed).

Figure 2: Schematic of TC geometry of the flow parameters Table 1: Major geometrical and flow parameters Parameter

Notation

Value

Free stream velocity

U

34.6 m/s

Hump chord

c

0.42 m

Crest height

h

0.0537 m

Reynolds number

Re=Uc/ν

936, 000

Mach number

M

0.1

The experimental data are available at http://cfdval2004.larc.nasa.gov/case3expdata.html (Case 3), at https://turbmodels.larc.nasa.gov/nasahump_val.html, and also enter the ERCOFTAC

�Classic Collection http://cfd.mace.manchester.ac.uk/ercoftac under number C.83. The data set includes: streamwise distributions of the surface pressure and skin-friction coefficients, C P and C f , and mean velocity and Reynolds stresses fields in the tunnel center-plane, roughly covering the region 0.63 < x/c < 1.39.

TEST CASE EXPERIMENTS

A photo of the experimental rig is presented in Fig.3.

Figure 3: Photo of the experimental rig

The experiments were performed in the NASA Langley 20′′×28′′ shear flow tunnel. The flow was nominally 2D, although with side-wall effects (3D flow) expected near the endplates. The reference “chord” length of the model, c, is defined as the length of the hump on the wall and is equal to 420mm. The maximum thickness of the hump, h, is equal to 53.7mm. The model was equipped with 153 centre-span static pressure ports and 20 dynamic pressure ports in the vicinity of the separated flow region. Sixteen spanwise pressure ports were located on the fore body (x/c = 0.19) and on the ramp at (x/c = 0.86). Two-dimensional PIV data were acquired in a plane, along the model centreline and normal to the surface, starting from right upstream of the slot and ending well beyond the reattachment location at x/c ≈ 1.4. Stereoscopic PIV (3D) data were acquired in planes perpendicular to the flow direction, arranged to intersect the 2D plane from x/c = 0.7 to 1.3 in steps of approximately 0.1. Oil-film interferometry was used to quantify the skin friction over the entire model, from the region upstream of the hump to beyond the reattachment location. Two-dimensionality of the flow in the separated and reattachment region was thoroughly assessed via three methods: by considering the spanwise pressures on the ramp in the separated region, performing 3D PIV measurements in planes perpendicular to the flow direction, and by means of the surface oil-film flow visualization. Spanwise distribution of the surface pressure was also measured on the fore body of the model. It was shown that the spanwise variations of the flow parameters are small (e.g., at the test condition, the pressure variation over the central half of the model (–0.25 ≤ z/c ≤ 0.25) is ∆CP = ±0.005) and that departures from twodimensionality are observed mainly near the wall. At the inflow location (x/c = –2.14), pitot-probe and hot-wire anemometer data were compared with 2D and 3D PIV. Inflow skin friction was also documented using oil-film interferometry. Resulting inflow velocity profile which was used in the numerical studies as a benchmark for imposing boundary conditions at the inflow of the computational domain (see the next section) is shown in Fig.4.

�Figure 4: Experimental profile of streamwise velocity at x/c = -2.14 (momentum thickness Re=7,200)

Experimental uncertainties reported in the original experimental papers are

Contributed by: E. Guseva, M. Strelets — Peter the Great St. Petersburg Polytechnic University (SPbPU)