Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figure 2.2: Tang gential Velocity Profi iles for Rankine and a Burge ers Vortex (Alek kseenko et al., 20 007). The tan ngential veloc city profile forr the Burgerss vortex is givven in equatioon (2.3) (Escudier et al., 1982). v 1 2 r 2 2 exp r Chapter 2 (2. 3) Where is the circu ulation arounnd the vortex ccore and iis a length scaale represen nting the effec ctive size of thhe vortex core. Very clo ose to the wall there is also a steep gradieent in the veloocity profile duue to no slip condition at the wall. TThis modifiedd shape of Raankine/ Burgeers vortex profile p is clas ssified by Steeenbergen et aal. (1998) as a ‘Concentrateed Vortex’ (Figure 2.3 3). However, there is noo accurate ddefinition of a concent trated vortex and a for an ideeal fluid it is rrigorously deffined as a spacce localize ed zone with h non-zero voorticity surrouunded by a potential floow (Alekseenko et al., 2007). In otherr words, the vvorticity is concentrated inn a small tu ubular region n near the voortex center, ddecaying rapiddly outward in radial direction (La am, 1993). DDepending oon the similaarity with thhe aforeme entioned defin nition of concentrated vorrtex, a great variety of vortex flows in n nature and engineering e caan be interpreeted as concenntrated vorticees; most co ommonly are columnar or filament-typee vortex (whirrlpool in liquuid flowing g out of a con ntainer, tornaado and vortiices in flow oover delta winng under a high attack angle etc.) (AAlekseenko et al., 2007). Inn case of highhly turbulent swirling flo ows, the instanntaneous veloccity field is veery complex annd it is the e mean tangen ntial velocity profile that iss used to idenntify the type of the vort tex. 14 Swirling Flows
Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figure 2.3: Tang gential Velocity Profi ile for a Conc centrated (Stee enbergen et al., 1998) ). Fig gure 2.4: Sche ematic of Different Regi ions in Tangentia ial Velo ocity Profile for a Con ncentrated Vortex x. Mod dification of illus stration by (Islek A.A A., 2004). Chapter 2 Figure 2.4 gives a de escription of ddifferent regioons in the tanngential velociity profile of a concent trated vortex. As discussedd earlier in caase of Rankinne vortex, the vortex cor re has a forcedd vortex profile until a radiial distance ‘ r a ’ where the t maximum m tangential veelocity is obseerved. In real swirling flowws, the radial position ‘ r a ’ in figure 22.4 is determiined by tangeential velocitiees, viscosity y, turbulence, and/or the introduction of non-rotating fluid at thhe vortex center c (Vanyo o, 1993). The ‘wall layer’ hhas very largee gradients annd flow in n this region, , in cylindriccal enclosures, is probably y influenced bby Görtler vortices (Escu udier et al., 22006). Görtler vortices are ssecondary flowws that app pear in a boun ndary layer floow along a conncave wall see Saric (1994) fo for details. Further, in the t near wall region, vorticcity is generatted due to skkin friction n factor acting as an externall force to a verry thin fluid laayer attached to the wall (Ebrahimi et t al., 2007). Thhe region in bbetween the foorced vortex annd wall lay yer is ‘Annular region’. Thhe flow in thhis region shoould ideally bbe irrotatio onal (based on n description oof Rankine voortex). However, in real flowws this reg gion has a very y low vorticity. . The visualizaation results byy Escudier et aal. (1982) show s ring-like e large vorticees in the outer region and appear to be of Taylor-G Görtler type; distorted by the axial floow and also responsible ffor uniform mly distributin ng the small amount of voorticity from highly vorticcal vortex core. c Alekseen nko et al. (19999) also founnd out that thhe experimenttal 15 Swirling Flows
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DTU Mechanical Engineering Section
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