Between two limits: Flow separation behind a bluff body close to a wall E. Ulrichs, H. Herwig Abstract There are two classical cases of a considerable flow separation: A body in a flow without limits is leading to a separation behind the body, a wall with a remarkable change of contour is leading to a separation region at the wall. Here, both situations are considered for the two- dimensional case of a flow past a cylinder with a triangle cross section. The free flow case and the flow at a cylinder directly placed at a wall is investigated. An intermediate situation can be observed, when the cylinder is placed close to the wall. The occurring complex separation region can be interpreted in that way, that both separation regions, body and wall related, are existing. Zwischen zwei Grenzen: Stro ¨mungsablo ¨sung hinter einem stumpfen Ko ¨rper in Wandna ¨he Zusammenfassung Es gibt zwei ‘‘klassische’’ Fa ¨lle der massiven Stro ¨mungsablo ¨sung: An einem frei umstro ¨mten Ko ¨rper entsteht ein Ablo ¨segebiet hinter dem Ko ¨rper, an einer Wand mit starker Kontura ¨nderung entsteht ein Ablo ¨segebiet an der Wand. Beide Situationen werden fu ¨r den zweidimensionalen Fall der Umstro ¨mung eines Zylinders mit dreieckegem Querschnitt gezeigt. Dabei handelt es sich einmal um die freie Umstro ¨mung und zum anderen um die Stro ¨mung la ¨ngs einer Wand, auf der dieser Zylinder angebracht ist. Eine Situation zwischen diesen Grenzfa ¨llen entsteht, wenn der Zylinder nahe der Wand positioniert wird. Das komplexe Ablo ¨segebiet kann dann so interpretiert werden, daß beide Ablo ¨segebiete, d.h. das zylinder- und das wandgebundene Gebiet vorhanden sind. 1 Introduction Flow separation undoubtedly is an important phenome- non in many flows of technical importance. Often it is unwanted since due to flow separation lift (of streamlined bodies like airfoils) is limited and drag (of bluff bodies like circular cylinders) is increased considerably. Since flow separation is such a fundamental flow phenomenon a large number of experimental and numerical studies address the problem of flow separation behind bluff bodies in a free stream or flow separation at obstacles on a wall. Typical examples are separation behind a circular cylinder e.g. [1, 2], and separation at a backward facing step, e.g. [3–5]. Very little, however, is known about the separation behaviour of bluff bodies in the vicinity of a wall though such geometries often occur, for example in heat exchangers, mixers or more general in all interior flow devices with component parts close to a wall. In Sect. 2 we classify two-dimensional flow fields with respect to its separation behaviour qualitatively. In Sect. 3 we show Laser Doppler Velocimeter (LDV)measurements and analyse them on the background of this classification scheme. The bluff body shape we used is that of a right-angled triangle at, or close to an adjacent wall. 2 Classification of separation regions In Fig. 1 two basically different separated flows are sket- ched. In Fig. 1a a classical bluff body flow in a free stream (h/H ¼1) is shown. The two-dimensional separation re- gion has two separation points and one saddle point. The second flow in Fig. 1b is that over a triangular shaped obstacle at the wall (h/H = 0) with a wall-bounded separation region between the separation point and the reattachment point. If, however, the obstacle is mounted close to the wall with a distance h between it and the wall the flow field will be something ‘‘between’’ that of Fig. 1a and Fig. 1b. Figure 1c shows what is expected. The mass flux under- neath the obstacle will separate a bluff body separation region from the wall bounded one, i.e. there will be two separation regions and as many separation, reattachment and saddle-points as in Fig. 1a and b together. If h is increased the wall bounded separation region eventually disappears and Fig. 1a is reached. If h is decreased the mass flux between the two separation regions shrinks and the wall bounded separation region forces back the bluff body separation region so that finally (h/H ¼ 0) Fig. 1b is reached. In Sect. 3 measured flow fields of kind 1b and 1c will be shown and discussed. 3 Wind tunnel experiments In a low speed wind tunnel at the TU Hamburg-Harburg 2D incompressible flow in the vicinity of a surface Forschung im Ingenieurwesen 68 (2003) 36 – 38 Ó Springer-Verlag 2003 DOI 10.1007/s10010-003-0105-8 36 Received: 19 December 2002 E. Ulrichs, H. Herwig (&) Technische Universita ¨t Hamburg-Harburg, Technische Thermodynamik, Arbeitsbereich 6-08, 21071 Hamburg The authors would like to thank Dr. A. Moschallski, W. Borelius, W. Michelson and J. Scheel for their support within this project.