Cfd what is y

Re x : Reynolds number. Note: -1 indicates an input error. How Do You Define a Good Grid Computational fluid dynamics CFD is essentially a series of approximations; the role of the practitioner is managing the magnitude of the errors in these approximations. How to Get More Information About Pointwise and CFD Sign up for our newsletter or register for one of our interactive forums that encourage discussions about various mesh generation topics.

This means at. Also, another more obvious observation is that at. Now since we defined our integration limits to an arbitrary , we can let it be :. This is a very useful result since it helps us get rid of the pressure term in equation 1. Substituting the results into equation 1 :. After all these manipulations and latex typing , we finally get an equation for our turbulent flow in a channel.

As you can already guess, the first thing we can do is to neglect the term when we are close to the wall. This still does not give us anything useful. At the wall, our velocities go to zero. So we can also assume. This leaves us with:. Dividing both sides by and introducing the nondimensional velocities and wall distances as defined by the terms in the parenthesis :.

Integrating to an arbitrary as we did before:. Finally, a useful result we get from all the math we did. What does this simple expression mean? Trackbacks are Off. Pingbacks are On. Refbacks are On. Forum Rules. All times are GMT The time now is Add Thread to del. Recent Entries. Best Entries. Best Blogs. Search Blogs. Can someone explain the Y plus value. User Name. Remember Me. Members List. Mark Forums Read. Page 1 of 2. Thread Tools.

May 27, , Re: Can someone explain the Y plus value. May 31, , John C. June 3, , Philip vishnujbr , nasa55 , Yakamoz and 17 others like this. June 29, , July 1, , July 2, , July 4, , April 5, , Response to andimiller. Hello David Creech. December 17, , December 27, , In most high-Reynolds-number flows, the wall function approach substantially saves computational resources, because the viscosity-affected near-wall region, in which the solution variables change most rapidly, does not need to be resolved.

The wall function approach is popular because it is economical, robust, and reasonably accurate. It is a practical option for the near-wall treatments for industrial flow simulations. The wall function approach, however, is inadequate in situations where the low-Reynolds-number effects are pervasive in the flow domain in question, and the hypothesis underlying the wall functions cease to be valid. Such situations require near-wall models that are valid in the viscosity-affected region and accordingly integrable all the way to the wall.

From above discussion it is clear that the placement of the first node in our near-wall inflation mesh is very important. Reference : Leap CFD. From the above image we need to be careful to ensure that our values are not so large that the first node falls outside the boundary layer region.

If this happens, then the Wall Functions used by our turbulence model may incorrectly calculate the flow properties at this first calculation point which will introduce errors into our pressure drop and velocity results. Let us see how to calculate first cell height.

Firstly, we should calculate the Reynolds number for our model based on the characteristic scales of our geometry. From the definition of , we know: where is shear velocity. The target value and fluid properties are known a priori, so we need to calculate the frictional velocity as given above. The wall shear stress, can be calculated from skin friction coefficient, is such that:. Thus to calculate we need to know, there are empirical formulae to calculate which are given as:.

For internal flows - For external flows -. Thus with all these inputs we can insert the values in the above equation to calculate. When considering simple flows and simple geometry, we might find this correlation to be highly accurate. However, when considering complex geometry, refinement in the boundary layer may be required to ensure the desired value is achieved.

In such cases re-mesh has to be done or else mesh adaption techniques has to be used to achieve the required value across the entire model. Thus we have learnt that the wall function approach and value required is determined by the flow behavior and the turbulence model being used.

If we have an attached flow, then generally we can use a Wall Function approach, which means a larger initial value, smaller overall mesh count and faster run times. If one expects to have flow separation and knows that the accurate prediction of the separation point will be having an impact on the result, then he would be advised to resolve the boundary layer all the way to the wall with a finer mesh. Unfortunately, as the value is dependent on the local fluid velocity which varies across the wall significantly for most industrial flow applications, it is not possible to know the exact prior to running an initial simulation.

Hence, it is important for one to get into the habit of checking the values as part of his normal post-processing so that one can make sure to fall in the valid range for the flow physics and turbulence model selection.