Hello every1,
I am simulating a LPT cascade in fluent. I have run a simulation using Trans-SST model. The problem is that when i plot mean skin friction coefficient directly in the fluent it matches with the expt. very well but it does not go negative. when i plot the stream line there is clearly a separation bubble near the trailing edge so there skin friction should be negative since the velocity there is negative.
My question is that how fluent calculate skin friction, which velocity it uses? is it the magnitude or x-velocity. as far as i think it should be the tangential velocity.
there are x-wall shear stress and y-wall shear stress also what are they? are they the components of shear stress.
one more thing the pressure loss at 40% downstream of the trailing edge is coming very high as compared to the experiments. when i ran with k-kl-w the pressure loss matches exactly with the experiments. why is it so? is the Trans SST is not a good model for study wake behavoiur.
Hi,
The skin friction coeff. is defined as: C_f = tau_w/(0.5*rho_ref*vel_ref^2) where tau_w= wall shear stress ( tau_w = mu*(d(u)/d(y) u= the velocity parallell to the wall ) and rho_ref & vel_ref reference density & velocity.
Regarding the turbulence models, this is far more complex as each model has its own wall-resolution dependence and thus applicability. While Fluent manual offers some guidance in choice of model vs grid resolution, the ERCOFTAC and other literature data bases offers best-practise-advice for wider range of applications.
Kind Regards,
Erdzan
Thanks for ur answer but i wanna know which velocity its uses to calculate shear stress. Ideally it should be the tangential velocity (i think) ? but when i find tou using the x-velocity available in the fluent. the cf is not matching with the one calculated by the fluent.
see the contribution of the form factor of the naca profile to make corrections
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