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 Fig gure 4.50: Insta antaneous positio on of vorte ex core at z1. Chapter 4 The pr resence of a precessing vvortex core ( (PVC) is a vvery important characte eristic to be identified i becaause if presennt, the value oof its frequency and am mplitude will affect the exxperimental reesults in diffeerent ways. Foor example in case of profile p plots foor velocity commponents witth precession of vortex core c there will be a shift inn the radial poosition of the profile lines ffor each ti ime step/ ins stantaneous ssnapshot in case of PIV. This shift is proport tional to the amplitude a of the precession and a final teemporal averagge of all th he instantaneo ous measuremeents will resullt for example in a vortex coore size lar rger than the e actual one. . On the othher hand thee frequency of precessi ion will introd duce a bias in RMS values oof velocity commponents in thhe vortex precession p reg gion. This means that the mmeasured valuues of Reynolds stress co omponents an nd turbulent kkinetic energyy (TKE) includde a bias in thhe vortex core c region. Figure 4.50 4 shows th he instantaneoous vortex coree position forr z1 at ReA usinng the algo orithm define ed in section 44.3.6. The alggorithm is onlly applicable to instanta aneous data fo or z1 only becauuse for the othher downstreaam positions thhe instanta aneous data ca an have more than one vorrtex in the vorrtex core regioon. From figure f 4.51 it i can be seeen that for most of thee instantaneouus measure ements the vo ortex core loccation is not fixed and mooves in an area between n r/R= ± 0.2. However, H to cconclude any ffurther detailss like precessioon frequen ncy and ampli itude, high sppeed and timee resolved PIVV measuremennts are nec cessary comp pared to currrent PIV expperimentationn used in thhis experim ment. The peri iodicity in thee vortex core pprecession cann be detected bby well-def fined peaks in n the frequenncy spectra annd in case thhere is no peaak observe ed then the vo ortex precessioon is considereed to be randoom (Escudier et al., 2006 6). In orde er to find ou ut the existennce of a preecessing vorteex core and iits consequ uent frequency y, LDA measuurements are cconducted at a radial distance 102 Swirling Flow in a Pipe
Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Figur re 4.51: Vortex Precession P Frequen ncy at different flow rat tes (Schnipper, 2010). Chapter 4 of 10 mm m from the origin o and an axial distancee of 20 mm. TThe results shoow the exis stence of PVC and a linear ddependency off precession frrequency on thhe flow ra ate (Figure 4. .51). Alekseennko et al. (1999) also fouund this lineear dependency of preces ssion frequenccy with flow raate at a given sswirl number. . The eff fect of Reynol lds number iss very obviouss on the floww characteristiccs. The hig gh Reynolds number ReA exhibits commparatively larrger vortex size than lo ower ReB. The e flow at ReB is less responnsive or in othher words moore resistive e to variations s in vorticity and Reynoldds stresses as tthe swirl decaays along th he pipe. In ca ase of Reynoldds stresses by comparing coontour plots fo for ReA and d ReB, it can be seen that wwith swirl decay downstreaam, the flow at high Re eynolds numb ber has higherr tendency towwards a more uniform spatiial distribu ution of indiv vidual Reynoldds stress compponents in thhe flow domain (see section 4.2.5 and d 4.2.6). The afo orementioned effects of Reyynolds numbeer can also haave a possibiliity due to vortex core precession p by assuming voortex core preccession to be a major contributor c to o the measureed values of innstantaneous vvelocity data in the regi ion around th he cylinder axxis. This assummption will quualitatively givve some new n informat tion about tthe effect of Reynolds number on thhe precessi ion frequency y and ampliitude. For exxample if thhe vortex coore precessi ion has a lar rge amplitudee then the teemporal average of velociity compon nents will re esult in a laarge mean vvortex core ssize. From thhe experim mental results, it can probbably be concluded that, in the current experim ment, with in ncrease in Reyynolds numbber the ampliitude of vortex precessi ion increases and a with swirll decay downsstream its frequency decreasses compar red to low Rey ynolds numbeer. However, tthe effect of swwirl decay alonng the pipe e on the vortex x core precessiion frequencyy, to the knowlledge of authoor, has not t been report ted and requiires more expperiments for a detailed annd quantitative analysis. 103 Swirling Flow in a Pipe
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DTU Mechanical Engineering Section
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