Intro_Turbulence_web

Intro_Turbulence_web - ENU 4133 Introduction to Turbulence...

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Unformatted text preview: ENU 4133 Introduction to Turbulence March 7, 2011 Topics I Time Averaging I Reynolds Decomposition and RANS (Reynolds Averaged Navier-Stokes) I Law of the Wall and Velocity Profiles I Kolmogorov hypotheses and scales I Mixing length modeling of turbulence I Multi-equation models of turbulence ( k- , k- ) I Computational techniques for turbulence (including large eddy simulation LES & direct numerical simulation DNS) Example Fluctuations Sufficient time for u and p to converge couple/few seconds (White recommends 5). Time-Average Operator f = 1 T Z T fdt (1) ~ V = 1 T Z T ~ Vdt (2) p = 1 T Z T pdt (3) Rules ( f , g scalar/vector/tensor functions, h scalar variable) f = f (4) f + g = f + g (5) f g = f g (6) fg 6 = f g (7) f h = f h (8) Reynolds Decomposition Fluctuations ~ V and p : ~ V ( ~ r , t ) = ~ V ( ~ r , t )- ~ V (9) p ( ~ r , t ) = p ( ~ r , t )- p (10) Mean fluctuation: ~ V = 1 T Z T ~ V- ~ V dt (11) ~ V = ~ V- ~ V (12) ~ V = ~ V- ~ V = 0 (13) However, u v , u 2 , u p are not zero. Mean Flow Equation RANS Homework problem: use Reynolds decomposition for p and u to obtain Equation 6-21 in the text, which is the scalar equation for u D u Dt =- p x + g x + x u x- u 2 + y u y- u v + z u z- u w (14) Appearance of turbulent stress terms (turbulent convective acceleration) with no analogy in the regular N-S equation. These stresses must be modeled to solve equations for turbulent flow. Duct & Boundary Layer Case D u Dt - p x + g x + y u y- u v (15) = u y- u v (16) = lam + turb (17) = viscous + turb (18) turb vs. lam Dimensional Analysis for Wall Layer Velocity Profile u = u ( , w ,, y ) (19) Note: ignores variation in with y for duct flow. Essentially, assumes that y << L, a characteristic length of the flow . Select , w , and as repeating variables to obtain: u w = F y w (20) u w = F y w (21) u u ? = F yu ? (22) u + = F ( y + ) (23) u ? : friction/shear velocity. u + : universal velocity. Data Regions of Behavior Very near the wall y + < 5: u + = y + (24) Viscous (laminar) profile viscous sublayer . Overlap layer y + > 30: u + = A ln y + + B (25) u + = 1 ln y + + B (26) Log layer k : von K arm an constant. Value of the Theodore von K arm an Constant Must be obtained from experimental data. Results range from 0.38 to 0.43. The results of 0.40 and 0.41 are the most common. 0.41 is the most common value for single-phase, internal flows; 0.40 is more common for two-phase flows and external flows. T&K go with 0.40, White with 0.41....
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This note was uploaded on 07/14/2011 for the course ENU 4133 taught by Professor Schubring during the Spring '11 term at University of Florida.

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Intro_Turbulence_web - ENU 4133 Introduction to Turbulence...

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