FLUENT Fluent Software Trainin Ncct⊙tLD TRN-99-003 Modeling turbulent Flows c Fluent Inc. 2/20/01
D1 © Fluent Inc. 2/20/01 Fluent Software Training TRN-99-003 Modeling Turbulent Flows
FLUENT Fluent Software Trainin Ncct⊙tLD TRN-99-003 What is Turbulence? Unsteady, aperiodic motion in which all three velocity components fluctuate mixing matter, momentum, and energy Decompose velocity into mean and fluctuating parts U1=U1+l1( l1) U1(0) Similar fluctuations for pressure, temperature, and species concentration values C Fluent Inc. 2/20
D2 © Fluent Inc. 2/20/01 Fluent Software Training TRN-99-003 u Unsteady, aperiodic motion in which all three velocity components fluctuate ’ mixing matter, momentum, and energy. u Decompose velocity into mean and fluctuating parts: Ui (t) º Ui + ui (t) u Similar fluctuations for pressure, temperature, and species concentration values. What is Turbulence? Time U i (t) Ui ui (t)
FLUENT Fluent Software Trainin Ncct⊙tLD TRN-99-003 Why model Turbulence? Direct numerical simulation of governing equations is only possible for simple low-Re flows Instead, we solve Reynolds averaged Navier-Stokes(RANS equations aU Op aU aR p k ai ardr y(steady, incompressible flow +u w/o body forces where Rii=- pu;ul (Reynolds stresses) Time-averaged statistics of turbulent velocity fluctuations are modeled using functions containing empirical constants and information about the mean flow Large Eddy simulation numerically resolves large eddies and models c Fluent Inc. 2/20/01
D3 © Fluent Inc. 2/20/01 Fluent Software Training TRN-99-003 Why Model Turbulence? u Direct numerical simulation of governing equations is only possible for simple low-Re flows. u Instead, we solve Reynolds Averaged Navier-Stokes (RANS) equations: where (Reynolds stresses) u Time-averaged statistics of turbulent velocity fluctuations are modeled using functions containing empirical constants and information about the mean flow. u Large Eddy Simulation numerically resolves large eddies and models small eddies. (steady, incompressible flow w/o body forces) Rij = - ruiu j j ij j j i k i i k x R x x U x p x U U ¶ ¶ + ¶ ¶ ¶ + ¶ ¶ = - ¶ ¶ 2 r m
FLUENT Fluent Software Trainin Ncct⊙tLD TRN-99-003 Is the flow turbulent? External flows where Re,s pUl Re25×103 along a surface L=xDD. etc Re>20000 around an obstacle Other factors such as free-stream turbulence surface conditions and Internal flows disturbances may cause earlier Ren22300 transition to turbulent flow Natural convection Ra≥103-100 where Ra≡ gBaTLp c Fluent Inc. 2/20/01
D4 © Fluent Inc. 2/20/01 Fluent Software Training TRN-99-003 Is the Flow Turbulent? External Flows Internal Flows Natural Convection 5 ³5´10 Rex along a surface around an obstacle where m rUL Re where L º Other factors such as free-stream turbulence, surface conditions, and disturbances may cause earlier transition to turbulent flow. L = x, D, Dh , etc. ³ 2,300 Dh Re 8 10 Ra ³10 -10 ma b r 3 g TL Ra D º ³ 20,000 ReD
FLUENT Fluent Software Trainin Ncct⊙tLD TRN-99-003 How Complex is the flow ◆ Extra strain rates Streamline curvature Lateral divergence Acceleration or deceleration Swirl Recirculation (or separation) Secondary flow ◆3 D perturbations Transpiration(blowing/suction Free-stream turbulence g Shear layers C Fluent Inc. 2/20
D5 © Fluent Inc. 2/20/01 Fluent Software Training TRN-99-003 How Complex is the Flow? u Extra strain rates l Streamline curvature l Lateral divergence l Acceleration or deceleration l Swirl l Recirculation (or separation) l Secondary flow u 3D perturbations u Transpiration (blowing/suction) u Free-stream turbulence u Interacting shear layers