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UDF to calculate phaseaveraged turbulence variables in a periodic flow 

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October 26, 2011, 23:48 
UDF to calculate phaseaveraged turbulence variables in a periodic flow

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Carl
Join Date: Aug 2010
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Hi,
I'm new to Fluent UDF. But I need to write a UDF to calculate phaseaverage turbulence variables in a 3D timeperiodic flow. I describe it in detail as follow: I'm simulating a full 3D stirred vessel via both RANS and LES method. Comparisons between turbulence variables will be made. It's reported that direct ensembleaveraged result will not work in this case, due to pseudoturbulence (contribution from the periodic flow). I found some hints from the post by Lionel Larcheveque (http://www.cfdonline.com/Forums/mai...intensity.html). For convenience, I copied that original post below: Suppose you have a periodic signal (for instance pressure signal). You can then define a period duration D. Then divide this duration into n equal intervals of time. At any given time t, you will be able to determine in which subinterval i you are, i.e. : A + B*D + (i1)*D/n < t < A + B*D + i*D/n with A a constant and B the number of periods since the beginning of the experiment or the calculation. For each subinterval 1 <= i <= n : *** compute classical statistics Mi(u)=U_i, Mi(v)=V_i, Mi(u^2) and Mi(v^2) (M means mean, _i for phase i) *** phase fluctuations u'_i and v'_i are given by : u'_i=sqrt(Mi(u^2)U_i^2) v'_i=sqrt(Mi(v^2)V_i^2) *** phase averaged turbulent intensity may be defined for instance by the ratio : sqrt(u'_i^2+v'_i^2)/sqrt(U_i^2+V_i^2) In practice, you have to store arrays of size n for the sums of u, v, u^2 and v^2 and for the number of samples. At each time, identify the phase number i, and then add instantaneous u, v, u^2 and v^2 data to the i th element of the related arrays and increase by 1 the number of samples for the i th phase. At the end, for each i, divide the element i by the number of samples of the i th phase to obtain phase averaged statistics. Since I'm new UDF user, could anybody kindly help me with that or give me some advice (which macros & functions should I use)? Many Thanks!!! ShuangqingXU 

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phaseaverage, turbulence, udf 
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