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Energies 2019, 12, x FOR PEER REVIEW Energies 2019, 12, 2791 Energies 2019, 12, x FOR PEER REVIEW o37 17 of 22 17 of 22 gure 21. Comparison of the absolute velocity contours of the outflow and inflow turbines at ΔP0 = F1i4g0u0rPea2a1n. dCΔoPm0 =pa1r1i,s0o0n0 Poaf .the absolute velocity contours of the outflow and inflow turbines at Figure 21. Comparison of the absolute velocity contours of the outflow and inflow turbines at ΔP0 = ∆P0 = 1400 Pa and ∆P0 = 11,000 Pa. 1400 Pa and ΔP0 = 11,000 Pa. 1000 18000 68000 46000 24000 200 -2000 -4-2000 -400 0 2000 4000 6000 8000 10000 12000 Inflow Outflow Inflow Outflow Δ0 (Pa) 1000 Inflow Outflow Inflow Outflow 1000 500 500 -500 0 -500 -1000 -1000 -1500 -1500 -2000 0 -20000 2000 4000 6000 8000 10000 12000 Δ0 (Pa) 0 2000 4000 6000 8000 10000 12000 0 2000 4000 6000 8000 10000 12000 Δ0 (Pa) (a) Δ0 (Pa) (b) (a) Figure 22.. Comparison of the energy transfer terms of tthe outtflflow and iinflflow tturrbiines.. (a): Term 2 (b) Figure 22. Comparison of the energy transfer terms of the outflow and inflow turbines. (a): Term 2 (changeiinblladespeed));;((b)):: Terrm3((changeiinrreellaattiiveeveellocciitty)).. (change in blade speed); (b): Term 3 (change in relative velocity). 6.3. Unsteady Performance Evaluation of the Optimum Outflow Turbine 6.3. Unsteady Performance Evaluation of the Optimum Outflow Turbine 6.3. Unsteady Performance Evaluation of the Optimum Outflow Turbine As mentioned in Section 3, the optimisation study was performed using a steady computational As mentioned in Section 3, the optimisation study was performed using a steady computational modelAtsomrednuticoentehdeitnimSecatinodnc3o,mthpeuotpattiimonisaaltcionsts.tHudoywwevaesrp, ethrfeoarmctuedalucsoingdiatisotneaidsyuncostmeapduytastinocneal model to reduce the time and computational cost. However, the actual condition is unsteady since thmeocdoemltpourtaedtiuoncealthgeotimeetraynidncloumdepsurtaotiaotninaglcdoosmt.aHinosw.eTvheur,st,haetraacntusaielnctomndoidtieoln(TisRu)nwstaesaudsyedsintoce the computational geometry includes rotating domains. Thus, a transient model (TR) was used to cothnetrcoolmthpeuretalatitoivnealmgoetoiomneotfrythienrcolutodreisnraotpautirneglyduonmstaeiandsy. fTahsuhiso,naatnradntsoienvtalmuaotdeetlh(eTaRc)cuwracsyuosfedtheto control the relative motion of the rotor in a purely unsteady fashion and to evaluate the accuracy of ocbotanitnroedltehffiecreielantciyveremsuoltisoinothfethoeprtoimtoirsaintioanpsuturedly.uTnhsetesatedaydfyasmhoiodnelainsdcatloleedvacaluseat1e,twhehiacchcwurascyseotf the obtained efficiency results in the optimisation study. The steady model is called case 1, which was utpheuosibntgaianemdoevfifnicgiernecfeyrreenscueltfsrainmteh(eMoRptFi)maipsaptrioancshtuadndy.wThase vstaelaidaytemdoidneSleisctciaolnle4d. case 1, which was set up using a moving reference frame (MRF) approach and was validated in Section 4. set uInp uthseintgranmsieonvtinmgordefeelr, esnixcerefrvaomlueti(oMnRs Fo)fatphperpoearcihodanicddwoamsavinaliwdaetredsimn Suelactieodna4t. a rotational In the transient model, six revolutions of the periodic domain were simulated at a rotational −6 speedIonf tωhe=t1ra20nsriaedn/ts,mgoivdienlg, saixtortaelvtoilmuteionf s0.o0f12t3h2e sp. eTrhioedriecsidoumalasinwewrerseetsitmo 1u0late,danadt a triomtaetsiotenpal speed of ω = 120 rad/s, giving a total time of 0.01232 s. The residuals were set to asstudityabwleastipmeerfsotremp,edancdonitsiwdearsinlegssthtrheaendoifnferefonrt ecascehs.cCelol utorahnatvneunmubmere(rCicFaLl )stwabaisliutytil[i5se1d,52to].cFhiorosts,e