# Dimensionless wall distance

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 March 2, 2014, 19:32 Dimensionless wall distance #1 New Member   Join Date: Nov 2011 Posts: 7 Rep Power: 14 Hi there! I would like to ask about the dimensionless wall distance. In many papers are three-dimensional domains described using ,,. I am not sure how to compute it for other axis (, ). Wall can be computed as . Where is the friction velocity at the nearest wall, y is the distance to the nearest wall and is the local kinematic viscosity of the fluid. ** Where is the wall shear stress and is the fluid density at the wall. *** Where is the dynamic viscosity, u is the flow velocity parallel to the wall and y is the distance to the wall. Is it computed for let's say as follows? but are without change? I am not sure about the wall shear stress component (gradient of u) Thank you for any hints or recommendations. Thumper likes this.

 March 3, 2014, 03:26 #2 Senior Member   Join Date: Jun 2012 Location: Germany, Bochum Posts: 230 Rep Power: 15 As far as I know the dimensionless wall distance is measured orthogonal to the wall which is usually called yplus and sometimes zplus. Never heard of xplus but it might just be dependent on how you set you coordinate system.

 March 3, 2014, 07:41 #3 New Member   Join Date: Nov 2011 Posts: 7 Rep Power: 14 Thank you. I thought as you do, but you can find , widely used in domain descriptions. For example as shown here: in LES of three-dimensional wing (http://www.iccfd.org/iccfd7/assets/p...3604_paper.pdf). It is possible to find it in other papers too. I am not sure if and are "normalized" with respect to the wall . I could not find any source with explanation. Probably, there is a trivial answer (that's why..). Any idea guys? Thank you.

 March 13, 2014, 04:33 #4 Senior Member   Rami Ben-Zvi Join Date: Mar 2009 Posts: 155 Rep Power: 17 Hi Messik, I guess there are two issues in your post requiring consideration: 1. The "u" should actually be interpreted as the velocity parallel to the wall, not the x-velocity component. 2. The "y" in wall shear stress needs be interpreted as the normal-to the wall coordinate rather than the y-coordinate. Then the and the follow naturally by merely changing components. I hope this helps, Rami

 March 15, 2014, 06:55 #5 Member   Join Date: Jun 2011 Posts: 51 Rep Power: 14 Hi messik, In 3D flows u may have the other commonents of shear stress, which IMHO makes sense to have the dimless in all three components. Although the boudary layer develops mainly normal to the wall, the resolution of the mesh in all the other two components is also important for the accuracy of your results. If you consider that u are resolving the laminar sub-layer (y+=1) it may be also important that your mesh in the other components fits within (x+/z~30). I think that u use the half of the mesh size and the velocity gradients in the right directions u can find those dimless. Regards,